Ed Stutz Air Force Research Laboratory
Ingrid Wilke Rensselaer Polytechnic Institute
Kenneth Kreischer Northrop Grumman Corporation
Qing Hu Massachusetts Institute of Technology
Air Force of Scientific Research, Army Research Laboratory, Materials Research Society
Tuesday PM, April 10, 2007
Room 3014 (Moscone West)
9:00 AM - CC1.1
Terahertz-assisted Forster Transfer Between Coupled Quantum Dots.
Lev Mourokh 1 2 3 , Anatoly Smirnov 2 3 , Igor Kuskovsky 1 Show Abstract
1 Physics, Queens College of CUNY, Flushing, New York, United States, 2 , RIKEN, Wako-shi Japan, 3 , Quantum Cat Analytics, Brooklyn, New York, United States
Forster process is the non-radiative exciton transfer from one quantum dot to another due to Coulomb interaction between them. This transfer exhibits strong dependence on the energy detuning of the two dots and it is maximal in the resonant case. In this work, we examine a feasibility to compensate this detuning by an additional terahertz irradiation. We show that in contrast with phonon-assisted Forster transfer, when the energy of the donor should be larger than the energy of acceptor, the up-conversion process is also possible.The signature of successful Forster processes can be found experimentally in the time-resolved photoluminescence signal or in the photon correlation data. Our results can be used for a new type of terahertz detector and for the control of energy transfer in the biological systems.This work is supported by the NSF, grant ECS-0609146.
9:15 AM - CC1.2
Terahertz Detection by Coupled Quantum Dots.
Lev Mourokh 1 2 3 , Anatoly Smirnov 2 3 , Jonathan Bird 4 Show Abstract
1 Physics, Queens College of CUNY, Flushing, New York, United States, 2 , Frontier Research System, RIKEN, Wako-shi Japan, 3 , Quantum Cat Analytics, Brooklyn, New York, United States, 4 Electrical Engineering, University at Buffalo, SUNY, Buffalo, New York, United States
We have examined the terahertz- (THz) related transport properties of a pair of coupled quantum dots, that are connected by a barrier thick enough to prevent inter-dot tunneling in the absence of radiation. Each dot features a ground-state energy level and a common upper level that is separated from the first one by a THz-range energy gap. In the presence of resonant THz irradiation, electrons from the first dot can absorb photons and be excited into to the common upper level. Due to the applied source-drain bias, the first dot is populated from the source and the second dot is effectively emptied by the drain, so an electron in the upper level will relax to the second dot and so contribute to the current. By solving the equations of motion derived from the Hamiltonian for this system, we have obtained the THz-induced source-drain current and corresponding photoconductance. Our results indicate a marked change in conductance in the presence of THz radiation which persists clearly up to 20 K.This problem will eventually be generalized to the system of two coupled quantum point contacts, in which control of the coupling potential is used to achieve detector tuning. With care it should be possible to resolve a photocurrent of ~10 pA. According to the results of our numerical calculations, such a conductance change should arise for a THz power of ~100 pW.This work is supported by the NSF, grant ECS-0609146.
9:30 AM - CC1.3
New Type of Sensitive Detectors of Terahertz Radiation
Dmitry Khokhlov 1 Show Abstract
1 Physics Department, M.V.Lomonosov Moscow State University, Moscow Russian Federation
Doping of the lead telluride and related alloys with the group III impurities results in appearance of unique physical features of a material, such as persistent photoresponse, enhanced responsive quantum efficiency (up to 100 photoelectrons/incident photon), radiation hardness and many others. We present physical principles of operation of photodetecting devices based on the group III-doped IV-VI including possibilities of fast quenching of the persistent photoresponse, construction of a focal-plane array, new readout technique, and others. Strong persistent photoconductivity in Pb0.75Sn0.25Te(In) under the action of monochromatic Terahertz radiation at wavelengths of 176 and 241 microns has been observed. The red cut-off wavelength exceeds the upper limit of 220 microns observed so far for the quantum photodetectors in the uniaxially stressed Ge(Ga). It is possible that the photoconductivity spectrum of Pb1-xSnxTe(In)covers all the Terahertz wavelength range. The advantages of Terahertz photodetecting systems based on the group III-doped IV-VI in comparison with the modern photodetectors are summarized.
9:45 AM - CC1.4
Picosecond Polarisation Detector for Infrared and Terahertz Radiation
Sergey Ganichev 1 , Sergey Danilov 1 , Wolfgang Weber 1 , Dieter Schuh 1 , Christian Gerl 1 , Werner Wegscheider 1 , D. Bougeard 2 , G. Abstreiter 2 , Wilhelm Prettl 1 Show Abstract
1 Department of Physics, Universität Regensburg, Regensburg Germany, 2 Walter Schottky Institute, TU Munich, Garching Germany
We report on a room temperature detector of infrared/terahertz laser radiationallowing to measure and characterized the radiation polarization state with picosecond time resolution. The ellipticity of radiation is analyzed applying three photoelectric phenomena: circular photogalvanic effect (CPGE), linear photogalvanic effect (LPGE), and photon drag effect . Each of these effects is monitored by different detector units stacked together in one single detector. In the infrared/THz range these detector units are practically transparent and therefore the units can be mounted one behind the other for illumination with the same laser beam.The experimental access to radiation helicity is provided by CPGE in quantum wells (QWs). The photocurrent is proportional to the radiation helicity and is measured in unbiased structures via the voltage drop across a 50Ω loadresistor in a closed circuit. The signal changes its strength by variation of the helicity Pcirc from right- to left-handed circularly polarized radiation and vanishes for linearly polarized radiation.In the optical set-up where polarization is modified from linear to circular using a Fresnelrhomb or λ/4 plates the signal is proportional Pcirc = sin 2φ, where φ corresponds to the angle between the initial plane of polarization and theoptical axis of the polarizer. For this detector unit we use (113)-grown SiGe or GaAs QWs in which CPGE is allowed at normal incidence. While this detector unit is sufficient for distinguishing between right- and left-handed circularly polarized radiation, parameters of elliptically polarized radiation can not completely be obtained by CPGE becausesin 2φ is symmetric around π/4 and 3/4 π for 0 < φ < π/2 and π/2 < φ < π, respectively. To reconstructthe whole polarization state we use again a (113)-grown SiGe or GaAs QWs but with other parameters. We find thatin these structures the signal is due to an admixture of the CPGE and LPGE and is given by j = j1 sin 2φ + j2 \sin 4φ. Measuring simultaneously the signals from both detector units we obtain the exact characteristic of the polarization state.Samples for these units have sizes of about 5 × 5 mm2. The sample edges are oriented along [1-10]-direction and [33-2]-direction in the QW plane, currents are measured along [1-10]-direction.The last detector unit is used for determination of the total power of the laser beam which is needed for calibration of the polarization sensitive detectors. Thisis a photon drag detector. We use the longitudinal effect in cylindrical germanium crystals irradiated along the -crystallographic axis.The detector units are connected to an analytical part, which converts the measured signals into clear information. S.D. Ganichev, and W.~Prettl, Intense Terahertz Excitation of Semiconductors,Oxford University Press, (2006).
10:00 AM - **CC1.5
Metal/semiconductor Heterostructures for Terahertz Applications.
Arthur Gossard 1 , Micah Hanson 1 , Joshua Zide 1 , Jeramy Zimmerman 1 , Seth Bank 1 , Elliott Brown 2 , Mark Rodwell 2 Show Abstract
1 Materials Department, UCSB, Santa Barbara, California, United States, 2 ECE Department, UCSB, Santa Barbara, California, United States
CC2: Spectroscopy and Characterization I
Tuesday PM, April 10, 2007
Room 3014 (Moscone West)
11:00 AM - **CC2.1
Selection of Optimum Nonlinear Crystals for Efficient Parametric Generation and Sensitive Detection of Monochromatic ns THz Pulses.
Yujie Ding 1 Show Abstract
1 ECE, Lehigh University, Bethlehem, Pennsylvania, United States
11:30 AM - CC2.2
Electric-Field Induced Tuning of the Terahertz Properties of SrTiO3 Single Crystals.
Petr Kuzel 1 , Filip Kadlec 1 , Gregor Panaitov 2 , Eugen Hollmann 2 , Norbert Klein 2 Show Abstract
1 , Institute of Physics, Czech. Acad. Sci., Prague 8 Czech Republic, 2 , Forschungszentrum Jülich GmbH, Jülich Germany
Ferroelectric materials which are characterized by a high permittivity up to the THz range offer a possibility of tuning of their dielectric properties by means of temperature or external electric field . Strontium titanate (STO) is an incipient ferroelectric material: on the one hand, its low-frequency dielectric behavior is fully controlled by the soft mode dynamics and on the other hand, it remains paraelectric down to the lowest temperatures due to quantum fluctuations.The electric-field tunability of strontium titanate (STO) has been extensively studied in view of its applications in microwave tunable devices. However, these studies have been mostly limited to low temperatures (20–90 K) and to low frequencies (kHz–GHz range) [2,3,4]. Recently, we have reported a room temperature voltage-induced tuning of STO thin film in the THz range at room temperature .In this contribution we will characterize the tuning capabilities of electric-field driven STO bulk single crystals at THz frequencies in a broad temperature range. The variation of the complex THz permittivity was studied in two geometries: (i) a transverse one where the bias electric field is applied using an interdigited micro-electrode structure such that it is parallel to the electric field of the probing THz radiation and (ii) a longitudinal arrangement where the bias electric field is parallel to the wave vector of the THz wave. Appreciable effects are observed even close to the room temperature. Namely the second mentioned geometry seems to be promising for applications such as THz tunable filters  or modulators.A. K. Tagantsev, V. O. Sherman, K. F. Astafiev, J. Venkatesh, and N. Setter, J.Electroceram. 11, 5 (2003).H.-M. Christen, J. Mannhart, E. J. Williams, and Ch. Gerber, Phys. Rev. B 49, 12095 (1994).J. Dec, W. Kleemann, and B. Westwanski, J. Phys.: Condens. Matter 11, L379 (1999).A. Eriksson, A. Deleniv, and S. Gevorgian, J. Appl. Phys. 93, 2848 (2003).P. Kuzel, F. Kadlec, H. Němec, R. Ott, E. Hollmann, and N. Klein, Appl. Phys. Lett. 88, 102901 (2006).H. Nemec, P. Kuzel, L. Duvillaret, A. Pashkin, M. Dressel, and M. Sebastian, Opt. Lett., 30, 549 (2005).
11:45 AM - CC2.3
III-VI Chalcogenides and Ternary Semiconductor Crystals for Broadband Tunable THz Sources and Sensors.
Krishna Mandal 1 , Sung Kang 1 , Michael Choi 1 , R. Rauh 1 , Nils Fernelius 2 Show Abstract
1 Advanced Materials Division, EIC Laboratories, Inc., Norwood, Massachusetts, United States, 2 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio, United States
Layered chalcogenides, GaSe and GaTe, have been grown under various crystal growth conditions for optimum performance of tunable THz wave generation and broadband THz detection. Low temperature photoluminescence (PL), Raman spectroscopy, optical absorption/transmission, anisotropic electrical charge transport properties, and terahertz time domain spectroscopy (THz-TDS) have been used to characterize the grown crystals. It is observed that In-doping enhances hardness of the grown GaSe crystals which is very useful to process and fabricate large-area devices. GaSe crystals have demonstrated promising characteristics with good optical quality (absorption coefficient ≤0.1 cm-1 in the spectral range of 0.62-18 μm), high dark resistivity (≥109 Ω cm), wide band gap (2.01 eV at 300K), good anisotropic (∥ and ⊥) electrical transport properties (μe/h, τe/h, and μτe/h) and long term stability. The THz-TDS measurements have shown that the GaSe crystals are highly efficient for broadband tunable THz sources (up to 40 THz) and sensors (up to 100 THz). Additionally, new terahertz wave generations (0.1-3 THz) have been observed for the first time from an anisotropic binary and two ternary semiconductor crystals. Details of characterization as well as optimum crystal growth conditions including simulation and computer modeling will be presented.
12:00 PM - CC2.4
Investigation of Far-infrared Dynamics in Organic Semiconductors.
Hynek Nemec 1 , Fengling Zhang 2 , Han-Kwang Nienhuys 3 , Petr Kuzel 4 , Filip Kadlec 4 , Villy Sundstrom 1 Show Abstract
1 Chemical Physics, Lund University, Lund Sweden, 2 Department of Physics, Linköping University, Linköping Sweden, 3 , Institute for Atomic and Molecular Physics, Amsterdam Netherlands, 4 , Institute of Physics, Academy of Sciences of the Czech Republic, Prague Czech Republic
12:15 PM - **CC2.5
Coherent Source THz Spectroscopy of Solids.
Martin Dressel 1 , Boris Gorshunov 1 2 , Joris van Slageren 1 Show Abstract
1 1. Physikalisches Institut, Universitat Stuttgart, Stuttgart Germany, 2 , Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow Russian Federation
Optical spectroscopy on solids in the energy range of μeV and meV remains a challenge since P. Drude and others were dreaming about it more than hundred years ago. Utilizing backward-wave oscillators as powerful cw radiation sources, coherent and tunable between 30 and 1500 GHz, we were able to perform quasi-optical experiments on a variety of organic and inorganic materials in the temperature range from 0.5 K to 300 K and in magnetic fields up to 8 Tesla. We are able to extract the real and imaginary parts of the dielectric or magnetic properties, like conductivity, permittivity, or permeability.We will discuss the advantages and versatility of this technique and give an overview on the numerous applications in molecular and solid state spectroscopy. Broken symmetry ground states, like density waves, exhibit collective excitations in the low-energy range due to the pinning of the condensate to lattice defects. The corresponding pinned mode resonance is nicely observed around 135 GHz by optical transmission through thin Rb0.3MoO3 films. Upon entering the superconducting state the BCS energy gap opens at 2Δ=3.53kBTc which typically falls into the THz range and was nicely observed in Nb, for instance, but also in the organic conductor αt-(BEDT-TTF)2I3. In high-Tc materials, like La1-xSrxCuO4, a pronounced plasma edge is observed around 0.7 THz for E∥c due to the Josephson coupling of the superconducting layers. Two distinct insulating layers in the unit cell (as obtained by SmLa1-xSrxCuO4) form two different Josephson junctions and thus two longitudinal plasmons. Necessarily a transverse plasma mode shows up in between. In general electronic interactions are a low-energy phenomena which can only be observed at low temperatures and frequencies. Recently indications of a correlation gap in the THz range has been found in heavy fermions and also the RKKY interaction causes partial charge localization. It is well known that ferromagnetic and antiferromagnetic resonances can fall within the THz range of frequency, depending on the coupling and external field. In additioncoherent THz radiation from BWOs has proven to be an excellent tool for probing electron paramagnetic resonances; thus we have built up a frequency-domain magnetic resonance spectrometer to probe the zero-field splitting and magnetic excitations in fields up to 8 Tesla. Quite spectacular are the resonance spectra of the molecular magnet Mn12ac, where the excitations within the crystal-field-split multiplets are observed, but also the dynamics of the quantum tunneling of the magnetization can be watched in situ: within the absorption spectra a tunnel dip is observed which can be considered as magnetic hole burning. Finally, without an applied external magnetic field, molecular magnets cause a giant Faraday rotation of 150°/mm in the THz range near the magnetic resonance at 300 GHz.
12:45 PM - CC2.6
Photoionization Mechanisms of Atmospheric Gases Probed by Terahertz Pulses.
Zoltan Mics 1 , Filip Kadlec 1 , Petr Kuzel 1 , Pavel Jungwirth 2 Show Abstract
1 , Institute of Physics, Czech. Acad. Sci., Prague Czech Republic, 2 , Institute of Organic Chemistry and Biochemistry, Czech Acad. Sci., Prague Czech Republic
We show that optical pump - terahertz (THz) probe spectroscopy is a suitable experimental tool for exploring ultrafast laser induced ionization and plasma formation in gases. The plasma is produced by ionization of gaseous oxygen or nitrogen using focused amplified 55-110 fs pulses. Picosecond THz pulses in frequency range 0.2-2 THz are used to measure the THz dielectric function of the photoionized gas.The ionization process is much faster than the THz probe pulse length. Shortly after photoionization the response of the free electron plasma dominates the transient THz spectra. Screening of carriers is observed which dramatically influences their motion and leads to transverse plasma oscillations at ωp/√2 (where ωp is the plasma frequency). The measured spectra can be fitted using a Drude-Lorentz model which yields the electron scattering times (400 fs for oxygen and 200 fs for nitrogen) and the free electron density. The free electron density is found to be strongly dependent on the wavelength of the pump pulse (400 and 800 nm were used), on its polarization state (linear and circular) and on its intensity (3-300 TW/cm2). By varying these parameters multiphoton and strong-field processes of photoionization are identified and characterized. The free electron densities observed lie within the range 1013 - 1016 cm-3; higher densities are observed for oxygen which has a lower ionization potential than nitrogen.By varying the delay between the optical pump and THz probe pulse the plasma dynamics is explored. The transient THz signal decreases within several hundreds of picoseconds both for oxygen and nitrogen. We conclude that the diffusion of free electrons is responsible for this decrease and we determine the effective velocity of the plasma expansion.Finally, as an evidence of formation of other species (O3 and probably also O3+) during the experiments with oxygen, we have observed corresponding rotational lines in the THz transmission spectra.
CC3: Vacuum Electronics and Waveguides
Tuesday PM, April 10, 2007
Room 3014 (Moscone West)
2:30 PM - CC3.1
THz-Near-Field Micro-Spectroscopy with Backward-Wave Oscillators and a Photo-Induced Aperture
Bruno Gompf 1 , Hanna Heer 1 , Nadine Gebert 1 , Martin Dressel 1 Show Abstract
1 Physikalisches Institut, Universitat Stuttgart, Stuttgart Germany
We have developed a near-field spectrometer operating in the THz-range between 30 GHz and 1.5 THz using backward-wave oscillators (BWO) as continuous-wave sources and a photo-induced aperture as near-field probe. THz-imaging and spectroscopy with sub-wavelength resolution are performed without a complicated probe-sample distance control and with a continuously adjustable aperture. In general, the long wavelength (0.1 – 3mm) of THz-radiation strongly limits the resolution in the far-field. In principle near-field techniques can overcome this problem, but one has to pay for the better resolution with low intensities. Especially for biological and medical samples, which exhibit only very small differences in their optical properties, it is hard to find a good compromise between resolution and intensity. Also the interpretation of the image contrast is often not clear, hence additional spectroscopic information is needed. A high power continuously tunable cw-source in combination with a freely adjustable aperture solves most of the above mentioned difficulties. BWO supply highly monochromatic and coherent radiation with an output power of up to 300mW. The emitted radiation is focused to a diffraction limited spot on the near-field stage by a hyperbolic lens. Through its center a hole is drilled on which a small optical lens is glued with the same focal length. Using a small mirror, a laser beam is superimposed to the THz beam so that both are focused at the same spot on a 100µm thick Si-wafer. But whereas the THz-beam can only be focused down to about 1mm, the laser can be focused down to 1µm. At the Si-wafer the laser beam creates a electron-hole plasma, which is opaque for THz radiation. This µm sized opaque region acts as effective scatterer for the THz-radiation. Using lock-in technique we get information only from this area: thus by scanning the sample we transfer the resolution achievable in the visible to the THz range. The Si-wafer acts as a sample holder. As long as the wafer is thin compared to the THz-wavelength, a sample prepared on the backside is in the near-field of the opaque scatterer and can be imaged with a resolution comparable with the laser spot size. A Golay cell is utilized for the detection of the near-field signal. We demonstrate a resolution of about λ/8 depending only on the laser spot size and not on the THz-wavelength. The resolution is mainly limited by the diffusion length of the photo-induced free carriers. Higher doped Si-samples improves the resolution. The BWO radiation is strongly polarized. This leads to additional polarization effects in the near-field images, which opens up the possibility to image anisotropic sample properties on spots much smaller than λ. In the near-field images recorded at 250 GHz the quasi-one-dimensional conductor (TMTTF)2PF6 shows a strongly polarization dependent contrast due to the anisotropic conductivity, a property which cannot be seen in the visible or infrared region.
2:45 PM - CC3.2
Materials Issues for Terahertz Vacuum Electron RF Sources.
R. Ives 1 , John Booske 2 , Carol Kory 1 , Michael Read 1 , Sean Sengele 2 , Hongrui Jiang 2 , Dan Van der Weide 2 , Steve Schwartzkopf 3 , Ron Witherspoon 3 Show Abstract
1 , Calabazas Creek Research, Inc., San Mateo, California, United States, 2 , University of Wisconsin, Madison, Wisconsin, United States, 3 , Ron Witherspoon, Inc., Campbell, California, United States
Vacuum Electron RF sources include devices that convert the energy in electron beams propagating through a vacuum into RF power. Submillimeter and terahertz devices include backward wave oscillators (BWOs), traveling wave tubes (TWTs), and gyrotrons. Electrons propagating through a vacuum do not encounter mobility issues present for solid state devices. Because the size of the structures is dependent on the wavelength, these sources, which are simple to construct at low frequencies, become problematic above 100 GHz. Calabazas Creek Research, Inc. (CCR) and the University of Wisconsin (UoW) are developing several RF sources in this frequency regime. Non traditional fabrications techniqes are under investigation to create the micron size structures for the RF circuits. These include micro electro discharge machining, deep reactive ion etching (DRIE), and laser ablation of chemically vapor deposited (CVD) diamond. The particular fabrication technique depends on several factors, including the frequency of operation, thermal requirements, surface finish, dimensional tolerance, conductivity, and strength. In general, the mechanical, thermal, and electrical characteristics of the materials control applicability to the source development. Electron beams are emitted from cathodes and typically guided through the circuit by an external magnetic field. Current density requirements can be quite high, depending on the device and required power level. This places significant demands on the cathode and magnetic materials. In general, cathodes in this frequency range should operate at high current densities to avoid excessive beam compression. Cathode materials research is in progress to develop high current density emitters for terahertz souces, including reservoir cathodes and field emission devices.This presentation will report on materials issues affecting RF source development at CCR and UoW. The presentation will address the specific material characteristics that impact sources developed and in development.
3:00 PM - **CC3.3
Microfabrication of CVD Diamond Structures for THz Vacuum Electron Devices
James Dayton 1 , Gerald Mearini 1 , Christopher Bower 2 Show Abstract
1 , Genvac Aerospace Corp., Cleveland, Ohio, United States, 2 , RTI International, Research Triangle Park, North Carolina, United States
The ideal medium for the interaction of electrons with electromagnetic waves is one where electrons have unlimited mobility, there are no ohmic losses, and permittivity is minimal. It may be out of place within the context of an MRS meeting, but that condition is met only when there is no material present, that is, in a vacuum. One of the earliest sources of THz radiation was a vacuum electron device, the backward wave oscillator (BWO). However, the exacting tolerances required for the THz BWO are very difficult to achieve using conventional materials and fabrication techniques. We have obtained the required tolerances by utilizing the microfabrication techniques pioneered in the manufacture of solid state devices. Further improvements in performance are achieved by utilizing CVD diamond, a material not previously applied to vacuum electron devices. The result is a new class of efficient, light weight BWOs to meet the demands of modern applications.Because of its unsurpassed thermal conductivity, relatively low permittivity, low loss tangent and high dielectric strength, natural diamond has been recognized for decades as a highly desirable material for use in the high power density environment of high frequency vacuum electron devices. However, because of the expense and difficulty of shaping natural diamond, it has been used, although with great effectiveness, in only a few experimental devices. We overcome these obstacles by employing a patented method for the fabrication of intricate CVD diamond structures with dimensions, on the scale of a few microns. The resulting structures are component parts of a new class of BWOs operating over a range from 0.27 to 0.65 GHz. The first step is to utilize deep reactive ion etching to create a silicon mold that is the negative of the desired diamond structure. The desired diamond structure is produced by chemical vapor deposition into the mold, which is subsequently removed chemically. The diamond structure is selectively metallized to complete the slow wave circuit and the device is assembled borrowing technology previously developed for the manufacture of liquid crystal displays. These fabrication procedures will be described and examples of the structures fabricated will be presented.This work has been sponsored by DARPA and by NASA JPL.
3:30 PM - CC3.4
A Micromachining Technology for High Performance THz Waveguides.
Robert Mihailovich 1 , Kenneth Kreischer 2 , David Gallagher 2 , Jack Tucek 2 Show Abstract
1 , Teledyne Scientific, Thousand Oaks, California, United States, 2 , Northrop Grumman Electronic Systems, Rolling Meadows, Illinois, United States
One challenge for THz systems is the manufacturing of high performance waveguide components, such as waveguides used traveling wave tube (TWT) sources. Requirements for performance THz waveguides include complex geometries, small features, high aspect ratios, accurate/uniform dimensions, and very smooth surfaces. Various microfabrication approaches have been proposed for making such waveguides. However, few approaches have been developed sufficiently to allow waveguide component demonstration in actual THz systems. We present a demonstrated microfabrication technology, based on Si deep reactive ion etching (DRIE), for manufacturing high-performance THz waveguides. The technology involves the microfabrication of (mirrored) waveguide halves, formed by DRIE and metalization of Si substrates, which are aligned and bonded to form the waveguide structure. Our implementation of the DRIE technology allows the fabrication of waveguides meeting very demanding specifications. Such capabilities have been demonstrated in the performance of a THz TWT source, built using a DRIE microfabricated waveguide. The DRIE microfabrication technology consists of three main manufacturing operations. First, a trench is etched into the Si substrates, using a DRIE process. Trench depth is approximately half the waveguide design height. Next, the trench is coated with a blanket film of a low-resistivity metal. Metal thickness exceeds a few multiples of the THz signal skin depth. Finally, mirrored substrates are aligned and bonded to form the waveguide cavity. The bonded wafers are robust to subsequent dicing and assembly processes. THz waveguides microfabricated with this technology can achieve very challenging specifications. Capabilities include waveguide aspect ratios exceeding 8:1, height accuracy of 5%, height uniformity better than 1%, sidewall roughness of 50nm, and two halve alignment accuracy of 2um. Such capabilities have been demonstrated in the performance of a prototype 650GHz TWT source designed and built using a DRIE-microfabricated waveguide. The TWT source met or exceeded design targets for operating frequency, RF power and stability characteristics. This work was conducted with the support of DAPRA MTO, under the TIFT program.
3:45 PM - CC3.5
Material Considerations for the Development of THz Regime Waveguides.
Sean Sengele 1 , Benjamin Yang 1 , Amy Marconnet 2 , Neville Dias 1 , Hongrui Jiang 1 , Irena Knezevic 1 , John Booske 1 , Daniel van der Weide 1 , Alan Betterman 1 , Steve Limbach 1 , Nicola Ferrier 2 Show Abstract
1 Electrical and Computer Engineering, University of Wisconsin - Madison, Madison, Wisconsin, United States, 2 Mechanical Engineering, University of Wisconsin - Madison, Madison, Wisconsin, United States
As a precursor to the development of a THz regime folded-waveguide traveling wave tube, we are using bulk micromachining techniques to investigate the design and fabrication of THz regime waveguides. The current design requires the waveguides to be fabricated in halves on two separate silicon wafers using deep reactive ion etching. The two wafers are then metallized and bonded together to form the final structure. In order to implement this design, significant effort has been put into finding an appropriate diffusion barrier and bonding technique to produce both a structurally and electrically-sound device.In conjunction with the development of the waveguides, we have also developed methods for maximizing electromagnetic coupling efficiency for the waveguides. This coupling technique takes the form of tapered silicon tips made by wet chemical etching. When these tips are placed in a waveguide they act as both a dielectric rod antenna as well as a dielectric window. Starting from a square silicon rod, the tapered tip is fabricated by slowly pushing the rod’s tip into an etch bath. By monitoring the electrolytic current in the etchant and controlling the submersion depth of the rod via robotic controls, a dimensional tolerance of less than one micron is achieved.Finally, we are also investigating how THz regime radiation interacts with metallic thin films. An exploration of quantum electron transport in these materials will provide us with a fundamental understanding of ohmic dissipation within the regime and should provide insight into how certain parameters, such as surface roughness, affect surface conductivity at THz frequencies. We have developed a coupled Monte Carlo-Maxwell equation solver to model wave propagation at THz frequencies in metallic thin films while incorporating the surface roughness scattering at the interfaces. Using our microfabricated THz regime waveguides, we can experimentally determine the ohmic loss per unit length by measuring the throughput power for waveguides of different lengths. This provides the necessary experimental platform for validating the model.This presentation will discuss the latest developments in all of these research efforts namely the fabrication of THz regime waveguides, coupling techniques and our investigation into electron transport in metallic thin films at THz frequencies.
CC4: Emitters I
Tuesday PM, April 10, 2007
Room 3014 (Moscone West)
4:30 PM - CC4.1
Growth and Characterization of Orientation-Patterned GaAs for THz Generation.
Candace Lynch 1 , David Bliss 1 , Timothy Zens 1 , Konstantin Vodopyanov 2 , Joe Schaar 2 , Martin Fejer 2 , James Harris 2 Show Abstract
1 , Air Force Research Laboratory, Hanscom AFB, Massachusetts, United States, 2 , Stanford University, Stanford, California, United States
We have demonstrated the use of orientation-patterned GaAs (OP-GaAs) for efficient, room temperature generation of THz radiation by quasi-phase-matched parametric down-conversion. In OP-GaAs, quasi-phase-matching is accomplished by varying the sign of the nonlinear optical susceptibility periodically across the crystal. Since GaAs is polar, the nonlinear optical susceptibility can be modulated by reversing the orientation of the crystal. This reversal can be created epitaxially, using a non-polar Ge layer to invert the GaAs orientation, and lithography and etching to periodically reveal the substrate, forming a grating of alternating crystallographic orientation. The grating must be regrown thick enough to accommodate the pump laser beam, which enters at the side and propagates across the structure. Thick growth of homoepitaxial GaAs is conducted via hydride vapor phase epitaxy (HVPE) in a custom-built reactor at the Air Force Hanscom Research Site. Using HVPE we can attain growth rates over 150 microns per hour; however, careful control over the gas supersaturation and furnace temperature profile is necessary to maintain this growth rate for many hours, while preventing parasitic GaAs deposition on the reactor walls upstream of the sample. We achieve thicknesses of up to 800 µm in a single 8 hour growth run and over 1 mm total after multiple runs. For efficient THz generation, we require low optical loss, low free carrier concentration, and propagation of the patterned grating through the thick epitaxial growth. Electronic defects have been characterized using Hall measurement, CL imaging, and PL; the free carrier concentration can be controlled via a secondary HCl flow to minimize absorption in the THz. Faceting of the surface during thick growth can lead to lateral overgrowth and a failure of the initial periodic structure to propagate vertically through the epilayer. We will discuss strategies to mitigate this overgrowth, such as alignment of the grating walls with the substrate tilt. Recent results have demonstrated the use of this material in a broadly tunable THz source based on cascaded optical down-conversion of a cw-modelocked picosecond Nd:VAN laser pumping a nearly-degenerate Type-II synchronously-pumped OPO. Using thick orientation-patterned GaAs samples, tuning from 0.8-3.5 THz was obtained with greater than 1 mW average THz power at 2.9 THz.
4:45 PM - **CC4.2
Temperature-dependence of Terahertz Emission from Dilute Magnetic Semiconductors.
Hui Zhan 1 , Jason Deibel 1 , Jonathan Laib 1 , Chanjuan Sun 1 , Junichiro Kono 1 , Hiro Munekata 2 , Daniel Mittleman 1 Show Abstract
1 ECE Department, Rice University, Houston, Texas, United States, 2 Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, Yokohama Japan
Dilute magnetic semiconductors (DMS) based on III-V compounds, which were first produced in 1989, have inspired much interest in recent years. III-V DMS are attractive candidates for studying dynamical processes in magnetically ordered systems, due to the clear distinction between mobile carriers and localized spins in the III-V semiconductors. They also show great potential for use in spintronic applications. Such applications will require a detailed understanding of the carrier dynamics, spin magnetism, and in particular their interactions in these materials. Terahertz (THz) emission spectroscopy is a powerful technique for studying ultrafast charge carrier dynamics in semiconductors. Many studies have examined THz emission from a variety of III-V semiconductors surface, including InAs, InSb, InP, and GaAs. In particular, magnetic field-induced enhancement of THz radiation from nonmagnetic semiconductor surfaces has been investigated thoroughly. Recently, THz emission from ferromagnetic metal films and from the dilute ferromagnetic semiconductor GaMnAs have also been demonstrated. However, so far there has been no studies of InMnAs, one of the first III-V DMS ever grown.
Here, we report the first THz emission from InAs-based DMS samples, building on the large body of literature related to THz emission from InAs. We have observed a temperature-induced polarity reversal of the THz radiation from both p-InMnAs and n-InMnAs, in contrast to what has been previously observed in p-InAs. We attribute the observed polarity reversal of the THz radiation to the competition between two oppositely-directed sources of photo-induced current: the surface field-induced current and the photo-Dember effect. Due to the presence of the Mn dopant, the photo-Dember effect in InMnAs is much weaker than in InAs. As a result, the dominant THz radiation mechanism in InMnAs switches from the surface field current at high temperature to the photo-Dember effect at low temperature, leading to a change in the polarity of the emitted THz field. This result is reminiscent of the competition between displacement and transport currents in bulk GaAs, which manifests itself as a polarity reversal as a function of pump laser wavelength.
5:15 PM - CC4.3
Efficient THz Generation Limited by Two-photon Absorption in GaP Wafer.
Yuliya Zotova 1 , Xiaodong Mu 2 , Yujie Ding 2 , Darnell Diggs 3 , Nils Fernelius 3 , F. Hopkins 3 Show Abstract
1 , ArkLight, Center Valley, Pennsylvania, United States, 2 ECE, Lehigh University, Bethlehem, Pennsylvania, United States, 3 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States
5:30 PM - CC4.4
Structural Dependence of THz-radiation from BiFeO3 Thin Films.
Dhanvir Rana 1 , K. Takahashi 1 , Krushna Mavani 1 , I. Kawayama 1 , H. Murakami 1 , M. Tonouchi 1 Show Abstract
1 Institute of Laser Engineering, Osaka University, Osaka, Osaka-fu, Japan
Multiferroic BiFeO3 emits THz-radiation upon illumination of femto-second laser pulses (wavelength – 400 nm) on an optical switch fabricated on thin films of this compound . The pulsed laser deposited BiFeO3 thin films on (LaAlO3)0.3(Sr2AlTaO6)0.7 (0 0 1) substrate crystallize in different structures depending on the film thickness (t); the coherently strained tetragonal films with t≤75 nm relax partially to bulk like rhombohedral phase at t≥110 nm, via a coexistence of these two phases in the range 80nm≤t≤110nm . In the present work, we have studied the structural dependence of THz-radiation emitted from a dipole-type optical switch fabricated on these varying thickness films. We observe the following salient features. 1) The intermediate thickness films with coexisting phases display larger THz-radiation emission efficiency compared to the thicker films (200nm) and the thinner films (t≤75nm). Furthermore, amongst the films with coexisting phases, the THz-emission efficiency decreases when both the phases have comparable fractions. 2) All the films display a linear dependence of THz-emission on the laser power. 3) The THz-radiation dependence on the electric-field (THz-E) results in a hysteresis loop which is bent towards the electric-field axis. This hysteretic effect is reminiscent of the electric polarization versus electric-field hysteresis loop. The coercivity and the shape of THz-E hysteresis loop of these films depend on their structure. A coercivity of ~70 kV/cm for 200nm and t≤75nm films increases to ~100-140 kV/cm for intermediate thickness (80nm≤t≤110nm) films. Furthermore, the THz-E loops for 200 nm and t≤75nm films saturate at around 120 kV/cm while those for 80nm≤t≤110nm films remain unsaturated up to 200 kV/cm and are more bent towards electric-field axis. 4) The THz-imaging around the optical switch also showed signatures thickness/structural dependence of a photo-assisted polarization switching. On the basis of these THz-emission and THz-imaging results, the variations in THz-radiation effects in these varying thickness/structure films most plausibly seem to originate from both the leakage current and the lattice effects. The leakage current that would be inherent from the defects and dislocation from coexisting structures, on one hand, increases the THz-radiation efficiency but, on the other hand, results in enhanced electric coercivity, non-saturation and larger bending of THz-E hysteresis loops in the intermediate thickness films.. K. Takahashi, N. Kida, and M. Tonouchi, Phys. Rev. Lett. 96, 117402 (2006).. D.S. Rana et al., (Unpublished).
5:45 PM - CC4.5
Temperature Dependent THz Radiation from Charge-ordered Nd0.50Ca0.48Ba0.02MnO3 Manganite Thin Film: The Correspondence with Magnetic and Electronic Properties.
Krushna Mavani 1 , Dhanvir Rana 1 , K. Takahashi 1 , I. Kawayama 1 , H. Murakami 1 , M. Tonouchi 1 Show Abstract
1 Institute of Laser Eng., Osaka University, Osaka, Osaka-fu, Japan
We have observed terahertz (THz) radiation from charge-exchange type charge-ordered antiferromagnetic Nd0.50Ca0.48Ba0.02MnO3(NCBMO) manganite by illuminating femtosecond laser pulses to an optical switch fabricated on highly oriented thin film. The film was deposited on single crystal MgO substrate (100) using pulsed laser deposition technique. An optical switch coupled with bow-tie antenna was fabricated on this thin film using conventional photolithographic and lift-off techniques. The Ti:Sapphire laser was used to generate 80 fs optical pulses with λ = 800 nm for the THz radiation experiments. The THz radiation shows strongly temperature dependent characteristics, which correspond well with the temperature dependent magnetic and electronic properties of this manganite thin film. The charge-ordering transition at ~220 K (TCO) is observed by the maximum THz amplitude whereas a broad minimum is observed around Néel temperature in the temperature dependent THz radiation. Also, the TCO is clearly observed as a hump in the magnetization of the thin film. Below 100 K, ferromagnetic-metallic phase manifests in the background of long range charge-ordered antiferromagnetic-insulating state in NCBMO thin film. It is found that the metallic fraction is induced by a cation disorder which is caused by 2% Ba2+ (1.47 Å) doping for Ca2+ (1.18 Å) in this ABO3 type manganite. A comparison of these results with the earlier reports on the THz radiation from manganite films  suggests that the ferromagnetic-metallic fraction below 100 K in this film affects the THz radiation. Furthermore, a change in THz radiation is observed corresponding to an onset of a glass-like magnetic phase at 50 K. The results will be discussed in the context of correlated effects of cation disorder on magnetic, electronic and THz radiation properties of Nd0.50Ca0.48Ba0.02MnO3 manganite. K. R. Mavani and P. L. Paulose, Appl. Phys. Lett. 86, 162504 (2005). Noriaki Kida, Masayoshi Tonouchi, Appl. Phys. Lett. 78, 4115 (2001); 82, 3412 (2003).
Ed Stutz Air Force Research Laboratory
Ingrid Wilke Rensselaer Polytechnic Institute
Kenneth Kreischer Northrop Grumman Corporation
Qing Hu Massachusetts Institute of Technology
Air Force Office of Scientific Research, Army Research Laboratory, Material Research Society
CC5: Negative Index and Metamaterials
Wednesday AM, April 11, 2007
Room 3014 (Moscone West)
9:00 AM - CC5.1
XPS Studies of a High-temperature Defect-induced Crystal Structure Transition in Single Crystals of Magnetoelectric Multiferroic RMnO3.
Shishir Ray 1 , Mark Williamsen 1 , Adam Guenther 1 , Samrat Ghosh 1 , Marshall Onellion 2 1 , Prasenjit Guptasarma 1 Show Abstract
1 Department of Physics, University of Wisconsin Milw, Milwaukee, Wisconsin, United States, 2 Dept of Physics, University of Wisconsin Madison, Madison, Wisconsin, United States
9:15 AM - CC5.2
Photonic Crystal Based opto-THz Modulator.
Ladislav Fekete 1 , Hynek Němec 2 , Filip Kadlec 1 , Petr Kuzel 1 Show Abstract
1 , Institute of Physics, Czech. Acad. Sci., Prague 8 Czech Republic, 2 Chemical Center, Lund University, Lund Sweden
Generation and control of pulsed and continuous-wave terahertz radiation have received considerable attention during last years. As more efficient THz sensors and sources become available, there will be increasingly more research emphasis placed on manipulation of freely propagating THz beams for future technology. An area of particular interest is that of all-optical devices allowing transfer of information from the optical to the THz spectral band.High-resistivity semiconductors show a great potential for the opto-THz coupling. On the one hand, in their ground state, they are transparent and virtually dispersion-free for the THz radiation. On the other hand, optically excited semiconductors exhibit a strong interaction with the THz light mediated by photo-excited carriers. A fine tuning of the strength of interaction by the intensity of optical excitation then leads to a number of interesting phenomena exploitable for the THz-light modulation and switching. A broadband opto-THz switch based on a photoexcitation of the surface of a thick GaAs wafer has been reported recently . A drawback of this element is a relatively high carrier concentration required for an efficient switching in the THz range. In this contribution we report on an agile opto-THz modulator consisting of a thin GaAs platelet enclosed in a one-dimensional photonic crystal structure. Due to the electric field enhancement in the GaAs layer this device allows us to achieve efficient modulation of the THz light at substantially lower photoexcitation fluences. The experiments were performed with a structure composed of a multilayer stack of crystalline quartz and MgO thin wafers (as low- and high-index materials), where the central MgO layer is replaced by a GaAs platelet (acting as a defect). The structure displays a forbidden band between 460 GHz and 710 GHz with a defect mode at 607 GHz with a FWHM of 16 GHz. The transmittance of the structure in equilibrium at the defect mode frequency is 90%. Upon photoexcitation the transmittance is temporarily inhibited by photocarriers generated at the surface of GaAs. We show that a 50% modulation of the power transmittance is achieved at 1016 cm-3 carrier concentration; the response time of the element is about 120 ps.  L. Fekete, J. Y. Hlinka, F. Kadlec, P. Kuzel and P. Mounaix, Opt. Lett. 30, 1992 (2005).
9:30 AM - **CC5.3
Double-Negative, Single-Negative, and Negative-Index Metamaterials in Far Infrared and THz Regimes.
Nader Engheta 1 , Andrea Alu 1 Show Abstract
1 Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
The field of matematerials has seen a growing interest in recent years due to the exciting electromagnetic properties of these materials and their wide range potential applications. The ability to tailor the effective material parameters, such as the permittivity, permeability and refractive index, in order to attain values that are not readily available in nature, such as negative and near-zero values, has opened exciting possibilities for the functionalities and applications of these special materials. Most of the recent efforts in this area have been concentrated in the microwave regimes, while the interest in the optical domain has gained significant momentum in recent years. It is therefore of particular interest now to explore the ideas that may lead to the feasibility of having such metamaterials in the far-infrared and the THz regimes. In our group, we have investigated theoretically several methods for developing metamaterials in the optical domain, namely, the possibility of designing nanorings of plasmonic particles with magnetic resonance at optical frequencies (A. Alù, A. Salandrino, and N. Engheta, Optics Express, Vol. 14, No. 4, pp. 1557-1567, Feb 2006), and the one-, two- and three-dimensional nanotransimssion lines in the optical domains (A. Alù and N. Engheta, J. Opt. Soc. Am. B. Vol. 23, No. 3, pp. 571-583, March 2006; A. Alù and N. Engheta, “Theory of linear chains of metamaterial/plasmonic particles as sub-diffraction optical nanotransmission lines,” Physical Review B, in press, online at: http://arxiv.org/abs/physics/0609061; A. Alù and N. Engheta, “Three-dimensional nanotransmission lines at optical frequencies: a recipe for broadband negative-refraction optical metamaterials,” submitted for publication, under review, available online at: http://arxiv.org/abs/cond-mat/0609625). Under proper conditions and with proper plasmonic constituent materials (like noble metals, polar dielectrics and some semiconductors), these methods may be applied to the far-infrared and THz regimes as well. This may lead to the possibility of tailoring metamaterials with single-negative, double-negative, and negative-refractive-index properties at far-IR and THz frequencies. In this talk, we will present some of our analytical and numerical results in this area, and will discuss some of the features and potential applications of these materials at different frequencies.
10:00 AM - CC5.4
Metamaterial Based Terahertz Functional Devices.
Hou-Tong Chen 1 , John O'Hara 1 , Antoinette Taylor 1 , Richard Averitt 1 3 , Willie Padilla 2 , Joshua Zide 4 , Arthur Gossard 4 , Clark Highstrete 5 , Mark Lee 5 Show Abstract
1 MPA-CINT, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Department of Physics, Boston University, Boston, Massachusetts, United States, 2 Department of Physics, Boston College, Chestnut Hill, Massachusetts, United States, 4 Materials Department, Unversity of California - Santa Barbara, Santa Barbara, California, United States, 5 , Sandia National Laboratories, Alburquerque, New Mexico, United States
Electromagnetic metamaterials have shown promise properties for the realization of exotic phenomena that is not possible or extremely difficult to demonstrate with natural materials. They are geometrically scalable, which makes them operational over a large range of frequencies and is of importance for the technological relevant terahertz frequency regime due to the lack of terahertz functional devices. We design, model, fabricate, and characterize planar electric metamaterial structures operating at terahertz frequencies. Terahertz time domain spectroscopy is used to characterize their transmission as well as reflection properties arising from electric resonances of the sub-wavelength metamaterial elements. The experimental data also permits extraction of the frequency dependent complex dielectric functions. Further consideration of substrate properties and embedding environment enables dynamic or active switch and tunability to construct terahertz functional devices such as filters, switches and modulators. The results have shown significant improvement over existing state of the art and make metamaterial based terahertz functional devices approach practical applications.
10:15 AM - CC5.5
Multiferroic Manganates grown from Low Temperature Solution Routes.
Samrat Ghosh 1 , Shishir Ray 1 , Mark Williamsen 1 , Prasenjit Guptasarma 1 Show Abstract
1 Department of Physics, University of Wisconsin Milw, Milwaukee, Wisconsin, United States
Intrinsic magnetoelectric multiferroics exhibit ferroelectricity, coupled with a ferromagnetic or an antiferromagnetic ground state: simultaneously, and in the same chemical phase. Magnetoelectrics with nearby electric and magnetic dipole resonances in thefrequency domain, or overlapping regions of negative permittivity and permeability, hold apromise for realizing intrinsic far-infrared negative index materials [1,2]. While we are far from a fundamental understanding of coupled intrinsic magnetoelectricity, a materials-related question is whether such ‘coupling’ occurs in the same phase or due to the coupling of adjacent grains of different phases. With an intent to sort out such questions, and to explore new phases as well as systematic variations of ferroelectric and magnetic properties, we have developed a series of solution-derived low temperature techniques to synthesize phase-pure multiferroic manganates of the ABO3 type: RMnO3 (R=Dy, Tb, Yb, Y) along with substitutions (e.g (Tb, Gd) MnO3) at the A and B sites. We have used two different techniques: a polyhydroxy-carboxylate-gel technique to synthesize homogeneous fine particles (a soft-chemistry low temperature synthesis route), and the floating-zone technique to grow large single crystals; and will compare results. The soft chemistry synthesis process helps us stabilize, for example meta-stable phases  of RMnO3 at ambient pressure, which otherwise requires high pressure synthesis . The solution-based method also helps improve the homogeneity of the dopant in the doped system and achieve higher control over substituent stoichiometry. References:  The Role of Multiferroics in the Negative Index of Refraction., D.W. Ward, E. Statz, K.J Webb, Keith A. Nelson, http://arxiv.org/abs/cond-mat/0401046;  The Electrodynamics of Substances with Simultaneously Negative values of epsilonand mu., V.G. Veselago, Soviet Physics Uspekhi 10 (4) 509 (1968);  H.W. Brinks, et al., J.Sol.St.Chem. 129 334-340 (1997);  Y. H. Huang, Chem. Mater. 18 (8), 2130 -2134, 2006.
11:00 AM - CC5.6
Epitaxial BaTiO3/SrTiO3 and BaO/SrTiO3 Superlattices for THz Phonon Bragg Mirrors and Cavities
Arsen Soukiassian 1 , N. Lanzillotti-Kimura 2 , A. Bruchhausen 2 , A. Fainstein 2 , W. Tian 1 , D. Tenne 1 5 , H. Sun 3 , X. Pan 3 , A. Cros 4 , A. Cantarero 4 , R. Uecker 6 , P. Reiche 6 , X. Xi 1 , D. Schlom 1 Show Abstract
1 Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 , Centro Atómico Bariloche & Instituto Balseiro, S. C. de Bariloche Argentina, 5 Physics, Boise State University, Boise, Idaho, United States, 3 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 4 Materials Science Institute, University of Valencia, Valencia Spain, 6 , Institute for Crystal Growth, Berlin Germany
We discuss the design and material parameters of BaTiO3/SrTiO3 and BaO/SrTiO3 heterostructures relevant for novel phonon devices, including mirrors, filters, and cavities for coherent THz phonon generation and control. The first step to phonon confinement structures is to grow a λ/4 thick planar periodic stack of two different materials with different acoustic impedance, which will work as a Bragg reflector with a stop band around λ and functions as a phonon mirror. The second step is the construction of a phonon cavity by enclosing a spacer of thickness dc = m λc/2 between two phonon mirrors, where λc is the acoustical phonon wavelength at the center of the phonon minigap and m is an integer. The advantages of using these ferroelectric superlattices include that they have an enormous stop band compared to the GaAs/AlAs superlattices previously reported for this application and that there can be greatly amplified light-sound interaction in these oxide materials. Using reactive molecular-beam epitaxy (MBE) we have grown BaTiO3/SrTiO3 superlattices on TiO2-terminated (001) SrTiO3, (110) DyScO3, and (110) GdScO3 substrates. With the aid of RHEED, precise one monolayer doses of BaO, SrO, and TiO2 were deposited sequentially to create BaTiO3/SrTiO3 superlattices. Structural characterization by XRD and TEM revealed that the samples are of high quality with nearly atomically abrupt interfaces. BaTiO3/SrTiO3 superlattices grown on (110) DyScO3 and (110) GdScO3 substrates demonstrate the highest structural perfection ever reported for any oxide superlattices. This is due to the excellent structural perfection of these ReScO3 substrates and their close lattice match to the superlattice materials grown. UV Raman results (the first ever reported for thin ferroelectric films) show that BaTiO3 is tetragonal and SrTiO3 is polar due to strain in these BaTiO3/SrTiO3 superlattices. We have observed folded acoustic phonons at the expected energy with the expected selection rules using UV Raman spectroscopy. The observed results are in excellent agreement with theoretical predictions.
11:15 AM - **CC5.7
Micromachining for Terahertz Artificial Materials.
Andrew Gallant 1 , Mikhail Kaliteevski 2 , Peter Swift 2 , Stuart Brand 2 , David Wood 1 , Richard Abram 2 , Dagou Zeze 1 , Mike Petty 1 , Martyn Chamberlain 2 Show Abstract
1 School of Engineering, Durham University, Durham United Kingdom, 2 Department of Physics, Durham University, Durham United Kingdom
CC6: Spectroscopy and Characterization II
Wednesday PM, April 11, 2007
Room 3014 (Moscone West)
11:45 AM - **CC6.1
Low-Terahertz Resonance Spectroscopy of Biological Materials.
Tatiana Globus 1 , Alexei Bykhovski 1 , Xiaowei Li 1 , Boris Gelmont 1 , Dwight Woolard 2 Show Abstract
1 ECE, University of Virginia, Charlottesville , Virginia, United States, 2 , U.S. Army Research Laboratory, Army Research Office, Research Triangle Park, North Carolina, United States
Terahertz (THz) resonance spectroscopy, as an emerging technique for biological materials characterization, detects low-frequency internal molecular vibrations involving the weakest hydrogen bonds and other weak interactions. The discovery of resonance character of interaction between THz radiation and biological materials that is sensitive to macromolecular structure opens many possible applications for THz spectroscopy technique in biological sensing and biomedicine using multiple resonances as distinctive spectral fingerprints. A review will be presented on low THz-frequency transmission (absorption) results for biological materials from Fourier Transform Infrared (FTIR) spectroscopy during the last few years [1-6]. The choice of the spectral range for the most practical application from 10 to 25 cm-1 is determined by the fast reducing of water absorption with reducing radiation frequencies. Several techniques to collect data from material in the form of thin solid films, gells or in aqueous form will be described. The examples of vibrational spectra of DNAs, RNAs, proteins and bacterial and tissue cells in the sub-THz spectral range will be demonstrated. The important new result from THz spectroscopy application for materials characterization is detecting hydrogen bonds in liquid water. Vibrational spectra of water in the sub-THz range will be demonstrated. The theoretical approach for computer simulation of THz absorption spectra from biological macromolecules will also be discussed. Two different approaches are used to simulate atomic vibrations of bio macromolecules at THz. One of them involves normal mode analysis and the other approach relies on molecular dynamics. There are several available packages to simulate a 3-D structure and dynamic processes of biological molecules and their vibrational characteristics (Amber, Charmm). However the choice of parameters for the simulation depends on understanding of realistic physical models including the interaction of biological molecules with environment. Several examples of modeling results will be presented. The experimental spectra will be compared with the theoretical predictions.1. T.Globus, D. Woolard, T. W. Crowe, T. Khromova, B. Gelmont, J. Hessler, J. Phys. D: Appl. Phys. 39, 3405-3413 (2006).2. T. Globus, T. Khromova , B. Gelmont , D. Woolard , and L. K. Tamm, Proc. SPIE, V. 6093-7 (2006).3. T. Globus, D. Theodorescu, H. Frierson , T. Kchromova , D. Woolard, Progress in Biomedical Optics and Imaging, Vol 6, No7, pp. 233-240 (2005).4. A. Bykhovski, X. Li , T. Globus, T. Khromova , B. Gelmont , D. Woolard , A. C. Samuels, and J. O. Jensen, Proc. SPIE, V. 5995, pp. 59950N-1:10 (2005).5. X. Li, A. Bykhovski, B. Gelmont, T. Globus, D. Woolard, M. Bykhovskaia, “IEEE-NANO” 2005, IEEE PRESS BOOKS, vol. 1, 221 – 224 (2005).6. R. Parthasarathy, T. Globus, T. Khromova, N. Swami, D. Woolard, J. Appl. Phys. Lett., v. 87, 11 (2005).
12:15 PM - **CC6.2
Molecular Systems Operating at Terahertz Frequencies.
Jorge Seminario 1 Show Abstract
1 Department of Chemical Engineering, Texas A&M University, College Station, Texas, United States
We have developed a scenario for the implementation of molecular circuits and sensors operating at terahertz frequencies. Under this scenario, not only the detection but also logic and communication is performed using molecular systems. Transfer of information and processing is performed by a strategic combination of molecular potentials and molecular vibrations. Our progress towards these goals will be presented, which includes our efforts to read information from molecules constructing a nano-micro interface that will interconnect the molecular systems to the standard microelectronics; these efforts include the development of molecular amplifiers and signal enhancers.References:127. Intensity enhancement of the vibrational spectrum of oxygen when attached to a platinum nanocluster, J. Seminario, L. Saenz, J. Chem. Phys., vol. 125, 174302 (2006)122. Encoding and transport of information in molecular and biomolecular systems, J. M. Seminario, L. Yan, Y. Ma, IEEE Trans. Nanotechnology, vol. 5 (5) 436-440 (2006)119. Terahertz Signal Transmission in Molecular Systems, L. Yan, Y. Ma, and J. M. Seminario, Int. J. High Speed Electronics Syst., vol 16, 669-675 (2006)118. Molecular-based processing and transfer of information in the terahertz domain for military and security applications, Y. Ma, L. Yan, and J. M. Seminario, Proc. SPIE vol. 6212, 621204:1-8 (2006).114 Moletronics Modeling towards Molecular Potentials, L. Yan and J. M. Seminario, Int. J. Quantum Chem., vol. 106, pp. 1964-1969, (2006)112. Encoding Information using Molecular Vibronics, L. Yan, Y. Ma, and J. M. Seminario, J. Nanoscience and Nanotechnology, 5, 675-684 (2006).110 nano-Detectors using molecular circuits operating at THz frequencies, J. M. Seminario, L. Yan, and Y. Ma, Proc. SPIE, vol. 5995, 230-244 (2005)109 Transmission of Vibronic Signals in Molecular Circuits, J. M. Seminario, L. Yan, and Y. Ma, J. Phys. Chem., Vol 109, 9712-9715 (2005)108. Scenarios for Molecular–Level Signal Processing, J. M Seminario, L. Yan, and Y. Ma, Proc. IEEE Vol 10, 1753-1764 (2005)106. Encoding and transport of information in molecular and biomolecular systems, J. M. Seminario, L. Yan, Y. Ma, Proc. IEEE Nanotech. Conf, Vol. 5 (2005)105. Terahertz Molecular Electronics Devices and Systems, Y. Ma, L. Yan, J. M. Seminario, Proc. SPIE, Vol. 5790, 206-218 (2005)103. Molecular Dynamics Simulations of a Molecular Electronics Device: The NanoCell, J. M. Seminario, P. A. Derosa, L. E. Cordova, and B. H. Bozard , Comp. Chem.: Rev. Curr. Trends, Singapore, World Scientific, Vol. 9, 85-119 (2005) 101. Vibrational Study of a Molecular Device using Molecular Dynamics Simulations, J. M. Seminario, P. A. Derosa, B. H. Bozard, and K. Chagarlamudi., J. Nanoscience and Nanotechnology, 5(2), 1-11 (2005).
12:45 PM - CC6.3
Excitation of Coherent Phonons in Crystalline Bi: Theory for Driving Atomic Vibrations by Femtosecond Pulses.
Davide Boschetto 2 , Eugene Gamaly 1 , Andrei Rode 1 , Thomas Garl 2 , Antoine Rousse 2 , Barry Luther-Davies 1 , Davide Glijer 2 , Olivier Albert 2 , Jane Etchepare 2 Show Abstract
2 Laboratoire d’Optique Appliquée, ENSTA/Ecole Polytechmique, Palaiseau France, 1 Laser Physics Centre, RSPhysSE, The Australian National University, Canberra, Australian Capital Territory, Australia
CC7: Emitters II
Wednesday PM, April 11, 2007
Room 3014 (Moscone West)
2:30 PM - **CC7.1
Carbon Nanotube FETs for High Frequency Electronics
Hong Zhang 1 Show Abstract
1 , Northrop Grumman, Lithicum , Maryland, United States
It is known that carbon nanotube field effect transistors (CNT FETs) are potential candidates for high frequency electronics applications. CNT FETs have demonstrated the highest carrier mobility at room temperature of any known material and high electrical and thermal conductivity, along with their small size and low parasitic resistances, making them ideal candidate for mm-wave and terahertz active devices for many emerging systems applications including space-based radar, mobile communications, and man-portable THz imagers. A top-gated CNT FET was fabricated on a quartz substrate using a single nanotube grown by CVD and a sputtered SixNy gate dielectric. Measurements of the mixing products produced by two closely spaced microwave input signals applied to the gate of the FET circumvented the problems associated with measuring high impedance RF devices in 50 ohm systems. The frequency-independent performance of a CNT FET, at frequencies as high as 23 GHz, was demonstrated for the first time. This observed operating frequency represents a significant breakthrough in the realization of carbon nanotube-based electronics for high frequency applications.
3:00 PM - **CC7.2
Progress in Long Wavelength Terahertz Quantum Cascade Lasers.
Jerome Faist 1 , Giacomo Scalari 1 , Christoph Walther 1 , Milan Fischer 1 Show Abstract
1 Physics, Univ. of Neuchatel, Neuchatel Switzerland
3:30 PM - CC7.3
Antireflection Coating for External-Cavity Quantum Cascade Laser Near 5 THz.
David Fenner 1 , J. Hensley 1 , M. Allen 1 , Jihua Xu 2 , A. Tredicucci 2 Show Abstract
1 , Physical Sciences Inc, Andover, Massachusetts, United States, 2 NEST-INFM, Scuola Normale Superiore, Pisa Italy
The practical utility of quantum cascade lasers (QCL) operating in the very far infrared will be enhanced by the development of methods for tuning over wide bandwidths. One potential solution, external cavity QCL (EC-QCL), provides wide-range tuning in the mid-IR but will require low-loss emission of THz radiation from one facet of the GaAs-based QCL structures. Attempts with various THz antireflection (AR) strategies have been reported such as adhesive-bonded elements of silicon, quartz, or parylene. Traditional AR coatings of deposited quarter-wave films are a mature technology in the visible and conventional IR but present optical materials challenges in the THz range and for cryogenic QCL operation. Here, a coating was designed for AR on GaAs to function near the atomic oxygen line at ~158 wavenumbers, or ~4.75 THz. At room temperature the index of semi-insulating GaAs is relatively constant across the THz range and falls 1-2% when cooled to cryogenic temperatures. At the design wavelength, n ~3.7 for GaAs and hence the ideal single-layer, quarter-wave AR film would have index ~1.9 and thickness ~8.8 microns. Multilayer AR coatings would likely provide a wider bandwidth, but due to the materials difficulties involved a single-layer AR was used for this first-generation EC-QCL. Quartz has long been used in far infrared optics and deposited silica films have similar index, n =1.9-2.1, and high transparency. Calculated minimum reflectivity is below 0.001 for an AR film index within 2% of the ideal AR index. Silica films were evaporated with substrates mounted to a water-cooled plate. Test coatings onto Si and GaAs wafer pieces were characterized with a far-IR spectrophotometer and deposition process tooling parameters obtained. Minimum reflectivity observed was close to the calculated design, and coatings survived repeated rapid cooling to liquid nitrogen temperature. Facets of fabricated QCL were also coated without affecting the electrical performance of the devices. These QCL were observed to only lase when an external mirror was placed near the AR-coated facet confirming that the minimum THz reflectivity is below 1% .  Hensley and Tredicucci et al, in preparation. Work supported by NASA through SBIR contract NNA05BE72C.
4:15 PM - **CC7.4
Sensor Applications of THz QCL's.
Mark Allen 1 Show Abstract
1 , Physical Sciences Inc., Andover, Massachusetts, United States
The advent of QCL's at wavelengths beyond 60 microns has enabled mature diode laser sensor platforms for in-situ and remote-sensing applications to be extended to the Far-IR or THz spectral region. This presentation will highlight recent results developing broadly tunable, external-cavity stabilized THz QCL's for absorption spectroscopy of fine-structure transitions in atomic gases, heterodyne receivers for ultra-sensitive room-temperature detectors, and differential reflectance measurements for remote sensing of crystalline solids. The spectroscopy of both target and interfering compounds will be emphasized, providing guidance for development of future sources
4:45 PM - **CC7.5
Intense Terahertz Wave Generation in Gaseous Media
Masashi Yamaguchi 1 , Yunqing Chen 1 , Xi-Cheng Zhang 1 Show Abstract
1 Department of Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States
Terahertz (THz) technology has shown the rapid development in last a few decades,  and it offers innovative sensing and imaging technology which is unavailable by using other portion of electromagnetic spectrum. The advancement of the THz technology will impact wide variety of the potential applications including improvised explosive device (IED) detections, chemical and biological sensing, medical diagnostics to microelectronics. Many of the current applications of THz technology are still limited by the power of THz radiation source. However, many of conventional THz emitters tend to be damaged or saturated when they are pumped with high-energy laser pump pulses in mJ range, and therefore they are not suitable for the generation of high power terahertz field. The generation of intense THz radiation in air has attracted recent attention of researchers.[2-4] The mechanism of the generation is attributed to the four-wave rectification process in the laser-induced plasma. This type of the emitter is particularly suitable for the generation of the intense THz field over 10 kV. I will present abut the recent results of intense terahertz wave generation in various types of gases and molecular vapors. In our study, wide spectrum range over 7 THz has been shown in selected gases and also strong THz emission compatible with the most commonly used ZnTe crystals emitter. Particularly Xe gas at 760 Torr showed 6 times stronger than ZnTe crystals with 1 mm thickness. Furthermore the ionization energy and the terahertz generation efficiency (amplitude per molecule per pump power) is not necessarily correlated for organic vapors while the strong correlation was found for inert gases.  B. Ferguson, X.-C. Zhang,“Materials for terahertz science and technology”, Nature Materials, 1, 26-33 (2002). T. Löffler, M. Kress, M.Thomson, T.Hahn, N.Hasegawa and H.G Roskos, “Comparative performance of terahertzemitters in amplifier-laser-based systems”,Semicond. Sci. Technol. 20, S134–S141 (2005).  X.Xie, J.Dai, and X.C.Zhang, “Coherent control of THz wave generation in ambient air”, Phys.Rev.Lett., 96, 075005(2006). T.Bartel, P.Gaal, K.Reimann, M.Woerner, and T.Elsaesser, Opt.Lett., 30, 2805(2005
5:15 PM - CC7.6
1.55 µm Photomixing in Nipnip THz-emitters
Stefan Malzer 1 , Sascha Preu 1 , Gottfried Döhler 1 , Lijun Wang 1 , Micah Hanson 2 , Art Gossard 2 , Elliott Brown 3 Show Abstract
1 Institute of Optics, Information, and Photonics, University of Erlangen, Erlangen Germany, 2 Materials Department, UCSB, Santa Barbara, California, United States, 3 Department of Electrical and Computer Engineering , UCSB, Santa Barbara, California, United States
In the development of compact, tunable, room temperature operating, semiconductor CW potomixing THz-emitter, transit time limited devices (pin-diodes) have taken over carrier lifetime limited devices (low temperature grown (LT)-GaAs photomixers). In properly designed (InGaAs/InP) pin-diodes THz-powers in the range of 10 µW at 1 THz have been achieved , in very good agreement with theoretical estimates. However, the power could be even higher if these devices would not suffer from transit- and the RC-time roll-off, resulting in a f^-4 power roll-off with frequency. The trade-off between optimizing transit time or RC-roll-off by varying the thickness d_i of the i-layer for a fixed device area can be overcome by dividing an "RC-optimized" pin-diode into a stack of N nano-pin diodes with a i-layer width suitable for ballistic transport. The individual nano-pin diodes are connected by LT-GaAs or ErAs enhanced np-recombination diodes allowing for a continous recombination of the photocurrent. We have demonstrated with our nipnip concept  that utilizing the ballistic transport in GaAs, the 3 dB frequency for the transit time roll-off can be shifted to about 1 THz. With an external bias voltage the electric field in the device can be kept at the optimum value for a wide range of optical power. Even more promising results are expected in the lower bandgap material InGaAs as the maximum ballistic velocity is expected to be twice as high as in GaAs and a higher energy can be gained by the carriers before being scattered into side-valleys. In this paper, we present first results on InGaAs/InAlGaAs nipnip structures with ErAs-enhanced recombination diodes on InP substrate. While the theoretical expected THz output power and measured values diverged by a factor of 80 in the formerly investigated GaAs samples , this discrepancy was lowered in the InGaAs samples to about a factor of 10 and a THz-power of 1 µW was measured at 400 GHz with a log-periodic antenna at a photocurrent of 3.8 mA. A substantial improvement was gained by reducing the contact resistance with a semitransparent Au top contact coating. From the RF-measurements we deduce that the coupling the devices to the antenna needs to be improved. Though the estimated transit times have not yet been confirmed we find a better efficiency for high frequencies compared to the pin-diodes similar to that used in .  H. Ito, F. Nakajima, T. Furuta, and T. Ishibashi, "Continuous THz-wave generation using antenna-integrated uni-travelling-carrier photodiodes",Semicond. Sci. Technol. 20, 191 (2005). G.H. Döhler, F. Renner, O. Klar, M. Eckardt, A. Schwanhäusser, S. Malzer, D. Driscoll, M. Hanson, A.C. Gossard, G. Loata, T. Löffler, and H. Roskos, THz-photomixer based on quasi-ballistic transport", Semicond. Sci. Technol. 20, 178 (2005).
5:30 PM - CC7.7
Optically Illuminated GaN based THz IMPATT Device
Moumita Mukherjee 1 2 , Nilratan Mazumder 1 2 Show Abstract
1 Department of Applied Physics, International Institute of Information Technology, Kolkata, West Bengal, India, 2 , Visva Bharati University, Santiniketan, West Bengal, India
The terahertz (THz) regime (0.3-10 THz) is rich with emerging possibilities in remote sensing, imaging and communications, with unique applications for detecting hidden biological weapons and explosives. During last decade significant efforts were devoted to search of semiconductor sources for high power generation in the THz frequency range. IMPATT devices are found to be the most efficient solid state sources that can deliver highest RF power even at 300 GHz. Although the conventional IMPATT diodes fabricated on GaAs (Gallium Arsenide) and Si (Silicon) are found to be reliable, these are limited by power and operating frequencies due to the fundamental limitations of the material parameters. On the other hand, wide band gap, III-V semiconductor GaN (Gallium Nitride) has excellent material properties that can be explored to develop high power IMPATT devices. Again considerable progress in the growth of nitrides during last five years makes this a good material for electronic devices. The authors have therefore designed and studied the small-signal characteristics of a p+nn+ SDR (Single Drift Region) IMPATT diode based on Wz phase GaN at 0.7 THz for the first time and the results are reported here. The diode design parameters are as follows: epilayer doping (n region) = 1x1024 m-3, epilayer width = 0.10 μm, current density = 1.6x109 Am-2, operating temperature = 600 K. The authors have designed and studied this diode through a generalized computer simulation technique. The temperature dependent values of the material parameters viz realistic field dependence of carrier ionization rates, saturated drift velocities of charge carriers and carrier mobility in Wz GaN have been incorporated in the analysis. The output data of the simulated diodes are: Maximum field = 3.3x108 Vm-1, Breakdown voltage (VB)= 32.0 volt, Efficiency (η) = 20%, Avalanche resonance frequency (fa) = 0.52 THz, Optimum frequency (fP) = 0.72 THz, Peak negative conductance (-GP) = 19.6x108Sm-2, Quality factor (QP) = 0.8, RF power (PRF) = 2.0 W, assuming a diode chip of diameter 8 μm. Optical control of microwave properties of the device is also studied. When an optical photon of appropriate energy is absorbed at the edges of the active layer of an IMPATT diode, additional electron-hole pairs are generated within the active region of the diode, which, in turn, enhance the existing thermal leakage current in the device. The enhancement of leakage current is manifested as the lowering of electron current multiplication factor, Mn. It is found that -GP decreases (~7%), while, fP shifts upwards (~5 GHz) and QP increases gradually with increasing leakage current. These studies thus suggest that GaN SDR IMPATT is highly radiation sensitive THz power generator. Our findings on the optical control of GaN THz IMPATT may be utilized for realizing optically integrated THz modules for applications in interstellar explorers.