Organic semiconductors are attractive as an active medium for the study of strong exciton-photon coupling due to large exciton oscillator strengths and binding energies. These properties permit the robust observation of exciton-polaritons at room temperature and under electrical excitation. In addition, organic single crystals have been used to demonstrate polariton lasing. To date, most investigations of strong coupling in organic semiconductors have focused on embedding organic thin films or crystals in Fabry-Perot-type microcavities. In this work, the active material for strong coupling is instead embedded in an all-organic, one-dimensional photonic crystal, in which the coupling is between the exciton and a Bragg mode of the structure. The photonic crystal consists of a high index of refraction light-absorbing active layer periodically distributed in a low index of refraction non-absorbing background medium. In order to probe the degree to which strong coupling extends throughout the entire structure, the photon-mediated exciton hybridization of multiple absorbing species is examined as a function of the spatial arrangement of the active layers in the photonic crystal. It is found that the strongly coupled state in this structure is shared between multiple active layers separated by distances longer than the wavelength of light in the structure.

Photonic crystals have led to numerous technological advances in areas ranging from optical communications to solid-state lighting manipulating the flow of light through nanoscale variation in refractive index. Recently, exploration has begun to focus on a more generalized form of photonic crystal with independent variation of the real and imaginary refractive index components. Such ‘complex index modulation&’ (CIM) has recently become the focus of intense theoretical interest because it forms the basis for parity-time (PT) symmetric optical potentials that enable one-way waveguides as well as more exotic effects such as unidirectional invisibility. It has nevertheless been challenging to realize CIM materials to date because of the added difficulty in controlling both refractive index components at subwavelength scale.
Here, we demonstrate PT symmetric organic thin films at a design wavelength of lambda;=532 nm by co-evaporating small molecules with different complex refractive indices (~n =n+ik) and continuously varying the relative proportion of each during growth. Teflon and tris-(8-hydroxyquinoline)aluminum [Alq3] are used to vary the real index component (Δn) while 4-(dicyanomethylene)-2-methyl-6-(4-dimethyleaminostyryl)-4H-pyran [DCM] is used to vary the imaginary index component (Δk). The index variation profile is made symmetric with respect to its complex conjugate as a function of position, x, along the film growth direction, resulting in the PT symmetry condition, Δ~n (x)= Δ~n * (-x). The resulting films are characterized by variable angle ellipsometry and reflectivity measurements, where we observe PT symmetry breaking through a maximum left/right asymmetry in normal incidence reflectivity at the exceptional point Δn= Δk, consistent with numerical simulation. These results open up a new opportunity for organic materials to serve as a platform in studying PT symmetry in optics as well as an avenue for realizing integrated photonic devices such as unidirectional waveguides, coherent perfect absorbers, and all-optical switches.

Spontaneous buildup of giant surface potential (gSP) in vacuum-evaporated films of tris-(8- hydroxyquinolinato)Alminium(III) (Alq3) and its derivatives is attracting attentions as an interesting self-organizing phenomenon.
Origin of the gSP is noncentrosymmetric (polar) molecular orientation in the films, and such spontaneous molecular orientation is applicable to control energy level alignment in electronic devices and to activate 2nd-order optical nonlineality for photonic devices. However, mechanism of the spontaneous noncentrosymmetric molecular orientation was not elucidated. We have reported that the orientation does not depend on the substrate or evaporation condition, therefore interfacial interaction between the substrate and the molecule is not a dominant factor for the polar molecular orientation. However, gSP was significantly affected by the ligand modification. For example, Al(7-Prq)3, having ligands with propyl substitution at position 7, presented negative gSP, the polarity opposite to that of an Alq3 thin film.
Considering there results, we are proposing asymmetric dice model (ADM), a simple geometrical model in which a molecule is regarded as an asymmetric polyhedral dice rolled on the surface. Asymmetric molecular shape results in asymmetric distribution of the posture on the surface, and therefore the averaged surface normal projection of the molecular dipole moment can be non-zero. Monte Carlo simulation was performed for various Alq3 derivatives, and it was shown that polarity and (to some extent) magnitude of the averaged molecular dipole moment was consistent with the experimental results. Order parameter of polar molecular orientation was estimated to be on the order of 0.001 to 0.01, which coincides with the experimental results.
Utilizing ADM, we can design the ligand of an Alq3 derivative to control gSP and polar molecular orientation, thus can optimize energy level alignment at the organic-metal interface or nonlinear optical coefficient.

A strongly-coupled microcavity is a structure in which cavity photons and semiconductor excitons undergo coherent coupling, resulting in the emergence of new states termed cavity polaritons [1]. Despite significant interest in inorganic-semiconductor based cavities, the physics of cavities containing organic semiconductors is much less well-understood. Here, we study the ultrafast processes happening in an organic semiconductor microcavity containing J-aggregates of a cyanine dye operating in the strong-coupling regime. Using broadband ultrafast pump-probe spectroscopy we are able to resolve the decay dynamics of the particles population of the upper polariton (UPB), exciton-reservoir (EX) and lower polariton branch (LPB). We use a series of rate equations to model the experimental decays. We provide direct evidence that upper-branch cavity-polaritons undergo an ultra-fast scattering process that populates incoherent states in the exciton reservoir with a rate of (150 fs)-1 a result in good agreement with theoretical predictions [2-4]. We also find that reservoir excitons scatter into lower-branch polaritons with a time constant of around 3.2 ps[5]. We also experimentally find evidence of Rabi oscillations in the upper and lower polariton branches, using the up-conversion technique to resolve spectrally and temporally the transmission through the cavity. Understanding such processes could be important in the design and optimization of polariton-based devices in which excitons are initially created following charge-injection or structures in which two semiconductors are strongly coupled to the same optical mode.
References
[1] Skolnick, M. S., Fisher, T. A. & Whittaker, D. M. Semicond. Sci. Technol. 13, (1998) pp645-669.
[2] Litinskaya, M., Reineker, P., & Agranovich, V. M.. J. Lumin. 119-120, (2006), pp 277-282.
[3] Agranovich, V. M. & La Rocca, G. C. Sol. State Commn. 135, (2005), pp544-553.
[4] Michetti, P. & La Rocca, G. C.. Phys. Rev. B 71, (2005), pp115320-115326.
[5] Virgili T. et al. Phys. Rev. B 83, 245309 (2011)

The rise of organic photonics in applications ranging from optical communications to spectroscopy and imaging will be driven by attributes such as mechanical versatility, capacity for new form factors, synthetic materials freedom and low cost that complement, rather than attempt to replace, existing inorganic technologies. Here, we discuss two such avenues relevant to organic laser prospects based on the direct generation of non-diffracting optical vortex beams from microdisk resonators and the construction of organic composite films in which the real and imaginary refractive index components are varied independently with position. This latter effort, referred to as complex index modulation, is uniquely enabled by the capability to deposit organic materials in arbitrary and spatially varying proportion without lattice matching concerns and enables us to explore the creation of parity-time (PT) symmetric films that enable unidirectional mode conversion and broadband optical isolation.

The large oscillator strengths and binding energies of Frenkel excitons make organic semiconductors highly attractive candidates for polariton condensation. To date, however, the only reported demonstration of a polariton condensate was made in a microcavity containing an anthracene single crystal.
In this work, we will discuss those experimental results and the theoretical understanding of lasing in that material set. We will then show that condensation in organic materials is not limited to crystals such as anthracene and show experimental results on the role of polariton-polariton interactions in organic microcavities.
Finally, we will discuss potential applications of the ultrastrong coupling regime and demonstrate microcavities with Rabi splittings on the order of 1 eV, a value comparable in strength to uncoupled excitonic energy.

Coherent exciton interactions in organic materials are responsible for a number of optical phenomena including exciton-polariton condensation, anomalous second order phase transitions, and more recently photon Bose-Einstein condensation. However, until now, exciton coherence has not been harnessed to control and improve organic optoelectronic device performance. Here we demonstrate that cooperative interaction between excitons in a microcavity containing a solid-state organic gain layer leads to superradiant emission, manifested as a 10-fold reduction in lasing threshold, resulting in a record low threshold for an organic planar microcavity of 400 nJ/cm^2. The threshold reduction is achieved by utilizing ultra-short excitation pulses, which generates a high density of excitons within the dephasing time. We show that superradiance is the mechanism responsible for coherence in these cavities using time-resolved luminescence measurements and by comparison to conventional laser theory. While great emphasis has been placed on polariton lasers as a route to low-threshold coherent emission, our system shows that coherent interactions between excitons, rather than strong coupling with the photon field, is a route towards low-threshold lasing.

In the present study we show strong coupling at room temperature in a Fabry-Pérot microcavity filled with a ladder-type polymer exhibiting a coupling strength of 2Omega; = 116 meV and a detuning of the cavity mode to the electronic transition of Δ = -77 meV. The emission from the lower polariton shows a relaxation bottleneck at low temperature. At higher temperatures we obtain a Boltzmann-like energy and momentum distribution. Power-dependent measurements with incoherent off-resonant picosecond laser pulses show above a threshold of (500+-200) mu;J/cm^2 a blue-shifted emission peak at zero in-plane momentum, accompanied by a nonlinear increase in the emission intensity and a sudden drop of the linewidth. We attribute this behavior to the onset of non-equilibrium Bose-Einstein condensation (BEC) of exciton-polaritons where the nonlinearity is a signature of the stimulated scattering and the blue-shift arises from the repulsive polariton-polariton interaction. In contrast to conventional lasing, we find a strong increase in threshold when decreasing the temperature, which can be explained by the peculiar polariton thermalization properties. Spontaneous symmetry breaking across the phase transition can manifest itself in the polarization of the emitted light. While the emission of the lower polariton branch is unpolarized, we observe that the polarization of the BEC is pinned to the one of the excitation beam. Additional evidence for BEC stems from the emission dynamics which are monitored by time-resolved photoluminescence spectroscopy. Below threshold we observe a single exponential decay with a time constant of 40 ps close to the one of the polymer without cavity. Above threshold a short emission burst appears (~ 4-8 ps) followed by a red-detuned emission with a time constant reflecting the lifetime of the exciton reservoir. Further evidence for BEC comes from the observation of long range phase coherence. Spatially-resolved measurements with a Michelson interferometer show a macroscopic phase relation over almost the whole spot size of 100 mu;m. The fringe pattern exhibits non-flat phase fronts and fork-like dislocations, indicating a large number of vortices and excitations. Furthermore, we are able to obtain time-resolved single-shot measurements of the interference patterns.

Strong coupling of light and matter can give rise to a multitude of exciting physical effects through the formation of hybrid states. Organic molecules have been increasingly used for the study of strong coupling since their large transition dipole moment permits the observation of vacuum Rabi splitting approaching 1 eV at room temperature. Such large modifications in the energy levels have significant implications for organic devices and material sciences. Our recent research on this topic will be presented.

Bose-Einstein condensation, the macroscopic ground state accumulation of particles with integer spin (bosons) at low temperature and high density, has been observed in several physical systems, including cold atomic gases and solid state physics quasiparticles. However, the most omnipresent Bose gas, blackbody radiation (radiation in thermal equilibrium with the cavity walls) does not show this phase transition. The photon number is not conserved when the temperature of the photon gas is varied (vanishing chemical potential), and at low temperatures photons disappear in the cavity walls instead of occupying the cavity ground state. Here I will describe an experiment observing a Bose-Einstein condensation of photons in a dye-filled optical microcavity [1]. The cavity mirrors provide a trapping potential and a non-vanishing effective photon mass, making the system formally equivalent to a two-dimensional gas of trapped massive bosons. Thermalization of the photon gas is reached in a number conserving way by repeated absorption-emission cycles in the dye molecules. Recently, we have furthermore investigated the number statistics of photon Bose-Einstein condensates [2]. The current experimental status of the Bonn photon gas experiment will be reported.
[1] J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, Nature 468, 545 (2010).
[2] J. Schmitt, T. Damm, D. Dung, F. Vewinger, J. Klaers, and M. Weitz, to be published.

Light-emitting, solution-processable organic and inorganic semiconductor materials are compatible with a wide range of lithography techniques and permit the fabrication of high optical gain photonic structures on a variety of substrates. These materials therefore have an important role to play in the development of mechanically-flexible and conformable photonics - an ‘upcoming&’ technology that promises to bring the capabilities and benefits of photonics to new systems and applications that require non-planar formats - where they could underpin development of compact lasers and related devices.
In this context, we are developing and studying mechanically-flexible, optically-pumped visible lasers - with typical cavity volume ranging from 100 mu;m³ to 50,000 mu;m³ - made with pi-conjugated polymers, mono-dispersed organic macromolecules and colloidal quantum dots, respectively, for the gain region. The specific laser structures we present are based on patterned thin-films of gain materials and fabricated by a combination of soft and hard lithography at the micro- and nano-scale. We will show examples of such lasers with all-organic devices emitting in the blue (~430nm, oligofluorene truxenes) and the green (~530nm, PPV derivative, BBEHP-PPV) and hybrid lasers emitting in the orange-red (~615nm, CdSe/ZnS quantum dots). The blue and green organic devices have threshold density down to 10kW/cm² or below when pumped in the nanosecond regime and therefore laser diode and/or LED pumping is envisaged. Mechanically-flexible arrays of all-organic lasers where the cavity size of each device is entirely physically constrained by the fabricated features in order to improve the spectral purity will also be presented and discussed.
One of our research focuses is the usability of organic lasers in practical applications. Consequently, we aim to obtain devices with both low oscillation threshold and high photostability without compromising their applicability. We will show that coating the aforementioned lasers with a PVA-film significantly extends their operating lifetime under ambient conditions with no trade-off on the laser threshold. Finally, flexible lasers are attractive for biomedical applications and we will report on the demonstration of label-free, specific detection using an oligofluorene truxene-based laser.

In this talk we will present the design and development of polymer microlasers pumped by light-emitting diodes. Compact distributed feedback resonators, fabricated by UV nanoimprint lithography, are used in combination with a PPV-derivative polymer to achieve very low threshold nanoimprinted polymer lasers. Using mixed-order and substructured distributed feedback resonators, optically-pumped laser thresholds below 50 W/cm2 are demonstrated. These lasers are combined with nanosecond-pulsed InGaN LEDs to create very compact hybrid polymer laser systems. The LED-pumped lasers are found to exhibit an unusual two-stage turn on of the laser emission, which is related to the dynamics of laser action under pulsed pumping of 30 to 50 ns duration. Finally we will show how these sources can be used as miniature sensors for explosive vapours.

Organic semiconductor lasers (OSLs) are being pursued for a variety of potential applications that promise to benefit from their broad wavelength tunability, temperature independent performance and mechanical versatility. Many OSL resonator geometries have been demonstrated to date, including Fabry-Perot cavities, vertical microcavities, distributed feedback gratings, and microdisks/rings. Microdisk and ring OSLs in particular have the potential to sustain high-Q whispering gallery modes (WGMs) and attain correspondingly low pump thresholds, however, interest in this geometry is limited because emission is typically out-coupled omnidirectionally instead of in a well-defined beam.
Here, we explore an evanescently-coupled microdisk OSL architecture that yields collimated, cylindrical vector Bessel beams with azimuthal polarization and high orbital angular momentum (OAM). The microdisk OSLs are based on the gain medium tris(8-hydroxyquinoline)aluminum (Alq3) doped with 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) and have diameters ranging from 50-200 mu;m that support few-mode lasing at thresholds <100 mu;J/cm2. We show that quasi-transverse electric WGMs result in an azimuthally polarized laser ring and that different azimuthal modes conserve OAM in the output beam by Fourier transforming its helical phase front. This architecture may open up new OSL opportunities that exploit the ‘self-healing&’ nature of Bessel modes or the wide range of OAM-anticipated applications in microscopy, free-space communication and quantum information.

In the past years, solid-state organic semiconductor lasers have attracted a lot of attention. Organic gain materials can be deposited by low-cost fabrication methods while they also offer tailoring the emission wavelength due to their spectrally broad gain spectra. Distributed feedback (DFB) lasers can be integrated into photonic systems as coherent light sources and are therefore interesting for a variety of sensing applications. An appealing aspect of such lasers is the possibility of additive manufacturing into functional systems.
The additive definition of grating structures inducing the distributed feedback can be realized by a laser assisted embossing process. With this versatile approach the local generation of laser resonators into various plastic substrates is possible. To fabricate localized organic semiconductor DFB lasers from solution, we used ink-jet printing to deposit the active conjugated polymer onto the predefined DFB grating regions. With precise control of jetting duration, frequency, slew rate and firing voltage, the spatial accuracy of printing and the laser emission from the device can be improved. We demonstrate the accurate lateral positioning of low threshold ink-jet printed laser pixels on a polymer substrate with 500 µm × 500 µm grating areas. Possible lab-on-chip sensing applications are discussed.

Whispering gallery mode (WGM) micro-resonators are promising building blocks for the integrated photonics as well as their importance for the fundamental study of light matter interactions. Add-drop filters, low threshold lasers and extremely sensitive biological sensors are some applications of WGM resonators emerged during last decade. Surface tension induced micro-resonators, especially micro-spheres and micro-toroids, have the highest quality factors up to 100 million, due to the atomically flat surface, and very small mode volume reducing threshold for parametric and nonparametric phenomena such as Raman lasing, third harmonic generation and four wave mixing. Although high quality factor and small mode volume are the advantages of symmetrical WGM micro-cavities when compared to that of Fabry-Perot resonators, they suffer from low output coupling efficiency, due to isotropic emission. Lack of emission directionality in WGM resonator based lasers has been recognized as a major barrier before their technological exploitation. Evanescent couplers such as high index prisms and tapered fibers are necessary to efficiently couple light into and out of whispering gallery modes. Another approach to increase the directionality of emission is breaking the rotational symmetry of the cavity by a physical deformation. Quadrupole, stadium, and limaccedil;on cavities are some known two-dimensional asymmetric resonant cavities (ARCs).
Here, we report a method for the production of chalcogenide WGM micro-resonators with sphere, spheroidal and ellipsoidal boundaries, and the results of optical characterizations. First step of cavity production starts with formation of spherical cavities embedded in polyethersulfone (PES) fiber. Thermal treatment of an As2Se3 single core PES fiber induces the Plateau-Rayleigh instability resulting in the breakup of the continuous core into spherical particles orderly arranged in the cladding. As a second step, applying a pressure on the fiber sandwiched between two glass plates and controlling the deformation of the spherical cavities at a temperature above the glass transition temperature of the materials, we produced high quality factor WGM ARCs with various sizes and shapes. Surface-tension-driven instability results in extremely smooth cavity surfaces. In addition, use of chalcogenides as a cavity material is important due to their advantageous nonlinear properties including high Kerr nonlinearity, high Raman gain, and low two photon absorption.

Organic solid state lasers, especially those which employ a distributed feedback resonator, garnered a lot of attention from the research community in the recent years. These devices can be tailored to emit any wavelength in the visible spectrum with low cost fabrication methods. Also integrating multiple of these laser sources with different wavelengths into chip based micro-optical systems is less elaborate than with inorganic semiconductor lasers. Possible applications are integrated analytical systems for use in medicine, drug discovery or environmental monitoring especially in a point-of-care scheme.
A unique feature of organic lasers is their versatility. It allows for fiber coupled and online tunable benchtop systems as well as several hundred microns small on-chip integrated devices, whereas both are based on the same technology and material platform.
Up to today a vast number of analytical methods still rely on the use of a white light or metal vapor line sources combined with optical filters which limits the potential for further miniaturization. Organic lasers may be a solution for this problem, especially with the recently available bench top tunable organic laser systems which allow seamless application development through all stages towards miniaturization without changing the light source technology.
We present such a development approach towards miniaturization. We utilized a tunable organic laser and fiber optics to set up a two-beam spectrophotometer. With this system we are able to conduct absorption spectroscopy. In particular we are capable of determining human serum albumin at all clinically relevant concentration levels. The tunable laser source allows for an easy adaption of the measurement wavelength for additional blood analysis methods thus marking the first step towards a miniaturized mobile blood analysis device based on organic microlasers.

Recently, a novel type of localized mode of the electromagnetic field (Tamm plasmon) were predicted theoretically [1] and subsequently demonstrated experimentally [2]. Tamm plasmons (TP) are localized at the interface of a specially designed Bragg reflector and a metal and are the analogues to Fabry-Perot cavity modes. Despite metallic mirrors being the most commonly used type of light reflectors in normal life, metallic optical components have not found wide use in optoelectronics. The main obstacle preventing the application of metallic mirrors in optoelectronics is the optical loss and heating of the metal due to optical absorption which leads to a catastrophic degradation of the mirrors and surrounding materials. A peculiarity of TP localized on thin metal films of sub-wavelength thickness embedded into a microcavity is the occurrence of a node of electric field at the metal. Thus, absorption in such structure is substantially reduced, and makes possible the experimental observation of macroscopic optical coherence in a microcavity with single metallic layer. For an implementation of the efficient electrical pumping of a microcavity, the two metallic layers should be inserted into the structure. On each interface between silver layer and Bragg reflectors, TP can be localized. Another Fabry -Perot can be localized between two silver layer. When the frequencies the three modes are matched, three hybrid modes appears, the energy splitting of these three modes is defined by a thickness of intra-cavity metallic layers. Each mode have different field distribution distribution. The mode of interest, which will potentially support lasing, should possess the following properties: i) minimized overlapping with metallic layers (to reduce absorption); ii) maximized overlapping with active area (to increase amplification of light). We can vary the field profile of eigen-mode by tuning the phases gained by light propagating through various layers. The electric field of optimized mode have the node at the centre of active area and on metallic layers. Since metal at the node, absorption is reduced. It should noted, that for two other eigen-mode field is not localized upon based active area. Insertion of metallic layers into the nodes of the electric field of an eigen-mode can be used for construction of the structures, where not with just two, but with many metallic layers in the active area of the laser. Since the metallic layers in such structure are adjacent to the active area, there the ohmic resistance of the structure will be reduced, and the wall-plug efficiency of the electrically pumped laser will be increased. Also, contacts of this type can provide uniform distribution of pumping current over the active area of a large lateral size.
[1] M. Kaliteevski, et al. Phys.Rev. B 76, 165415 (2007)
[2] M.E.Sasin, et al, Appl. Phys. Lett, 92(25) 251112 (2008)

Organic microcavities offer fascinating prospects for studying the interactions of light and matter and various coherence phenomena at room temperature. Such observations are made possible by the large oscillator strengths and exciton binding energies provided by organic semiconductors. However, for many applications electrically driven devices have to be developed. One way of realization is to implement highly conductive electrodes. For this purpose, metallic electrodes are under study, but the large absorption properties of metals are generally to prevent optical coherence.
In this talk, we will review the process in small-molecule vertical cavity surface emitting lasers. In a further step, we discuss the impact of metallic structures implemented into these microcavities with the prospect of being used as electrodes. Optimizing the structural design and introducing a higher order confinement are beneficial to maintain optical coherence at low thresholds.
At first, a homogeneous silver layer of 40nm thickness is implemented into a high quality organic microcavity, where an active half-wavelength layer of the organic host:guest system Alq3:DCM (2 wt.%) is embedded between two dielectric mirrors (DBRs). By exciting this structure non-resonantly, the transition from a single photonic cavity mode to two coupled Tamm states is observed [1,2]. Optimizing the design reduces the overall optical losses and increases the Q factor to 650 allowing for the observation of coherent emission from cavity photons and Tamm plasmons simultaneously and at room temperature [3].
A lateral confinement of the optical modes can be introduced by generating an optical gain grating or by structuring the metal layer by means of photo-lithography. The optical approach enables a dynamic manipulation of the optical mode and its far field emission pattern according to the spatial gain distribution. The lateral extent of the coherent optical mode is only limited by the size of the pumped area. Structuring the metal layer into periodically placed stripes leads to a further reduction of the absorption. Studying the dispersion pattern in the far-field reveals Bragg-scattered surface plasmon polaritons and Bloch-like photonic bands. Above the lasing threshold, macroscopic coherence is observed occurring on different Bloch states localized either at the center or at the edges of the resulting Brillouin zone [4].
[1] R. Brückner, M. Sudzius, S. I. Hintschich, H. Fröb, V. G. Lyssenko, and K. Leo, Phys. Rev. B 83, 033405 (2011).
[2] M. Kaliteevski, I. Iorsh, S. Brand, R. A. Abram, J. M. Chamberlain, A. V. Kavokin, and I. A. Shelykh, Phys. Rev. B 76, 165415 (2007).
[3] R. Brückner, A. Mischok, A. A. Zakhidov, S. I. Hintschich, H. Fröb, V. G. Lyssenko, M. A. Kaliteevski, and K. Leo, submitted (2013).
[4] R. Brückner, A. A. Zakhidov, R. Scholz, M. Sudzius, S. I. Hintschich, H. Fröb, V. G. Lyssenko, and K. Leo, Nature Photon. 6, 322-326 (2012).

Vertical External Cavity Surface Emitting Organic Lasers (VECSOLs) are the counterparts of VECSELs with organic thin-film gain materials.
They offer the promise to combine in a single device the advantages of thin-film organic lasers (low cost, ease of fabrication, broad emission spectra, easy chemical tuning and high gain) with those of inorganic semiconductor VECSELs, i.e., high conversion efficiency, diffraction-limited beam quality, power scaling capability, and a high versatility offered by the open cavity, which includes the possibility to easily tune the laser wavelength or to achieve frequency conversion to reach new spectral domains.
The basic VECSOL structure consists of a plane highly-reflective mirror onto which a thin film of dye (e.g. Rhodamine 640)-doped PMMA layer is spin-cast, and a concave output coupler closing the cavity. The laser is pumped by the second or third harmonic of a Nd:YAG laser (532 or 355 nm, 25 ns, 10 Hz). With a mode-matched short (mm) cavity, we achieved a record conversion efficiency of 60% with a diffraction-limited output at 620 nm.
Dynamical numerical simulations based on Statz-DeMars rate equations can be used to accurately describe how laser performance varies with all the experimental parameters. As an illustration, these simple models and experimental results agree quite well to explain why the conversion efficiency of VECSOLs is optimized for long (> 10 ns) pump pulses in short cavities, since basically the oscillation buildup time has the same order of magnitude as the gain lifetime.
The open cavity is a key element which clearly distinguishes VECSOLS from other classical organic laser architectures such as DFB or VCSEL structures. Besides the fact that it guarantees a single transverse mode operation, it can be used to perform intracavity frequency doubling and obtaining a deep-UV laser continuously tunable from 309 to 322 nm, with 2% efficiency and high-energy pulses (µJ), in a very compact setup (1-cm long). The photodegradation rate is the same as the red emitting material upon green (532 nm) pumping, which is far better than deep-UV organic emitters, which may not in any case be able to emit at so low wavelengths.
The open cavity is also the key to enable an extended and well-controlled tunability of the laser. With a simple 2-µm-thick free-standing PMMA film used as an intracavity etalon, we achieved continuous tuning across the visible spectrum (440-670 nm) with five different dye-doped PMMA samples. In order to avoid parasitic etalon filtering due to the thin film itself, the latter was spin-cast onto a glass slide and set at Brewster angle inside the cavity. The organic material thus becomes a very cheap disposable object that can be replaced easily. This kind of tunable source may find interesting applications in spectroscopy and sensing.

In the past two decades, there is great interest in developing an electrically-pumped organic semiconductor laser (OSL), as it would provide a new class of convenient, easy-to-manufacture, and multi-coloured lasers. The optically-pumped OSL were demonstrated as early as in the late 60`s and 70`s. On the other hands, electrically-pumped OSL have not been demonstrated yet at present. Huge number of exciton density is necessary to be generated and accumulated in organic semiconductor to achieve lasing. Many efforts have been made to surpass the laser threshold energy, either by new material invention or by developing new devices. Also attempts to reduce the required threshold energy have been performed by the introduction of optical resonators, i.e. DBR microcavity and DFB resonator onto light-emitting devices, including ambipolar light-emitting transistor (LET). No clear indication that the laser threshold has been surpassed by electrical pumping to date. There is a big question in dispute on how high the energy should be given to the devices [1], and on whether we should estimate the threshold current density as the real metric.
Here we show a comparison between the exciton density values generated by optical pumping and those generated by electrical pumping. Recently, current-driven spectral narrowing with a clear threshold has been reported from modified ambipolar LETs of BP3T single crystals operated at very high current density (>10 kA/cm^2) [2]. However, it is still unclear whether the observed phenomena in the ambipolar LET indicates the early stage of ASE or lasing. We introduce a new parameter as a metric, which is so called the volumetric exciton density. In optical pumping, the exciton density is estimated by taking account on how much the actual absorbed photon per volume is. While, in the case of the electrical pumping by ambipolar LET, both the recombination zone width [3] and the thickness of the charge carrier accumulation should be taken account [4-5]. Through this method, it is found that the value of the exciton density at the narrowing threshold in the LET is up to 5.78x10^16 /cm3. On the other hands, the threshold exciton density from optical pumping shows 1.45x10^17 /cm3, indicating that the LETs were short by only one order magnitude before it can achieve ASE or lasing. These results clearly show that LET with embedded simple resonator and a proper material choice has been very close to achieve ASE or lasing. Also, the volumetric exciton density become the most relevant metric to compare the laser threshold measured by optical pumping and the exciton density value that should be achieved by any electroluminescence devices.
Ref: [1] E. Namdas, et al. Nature Photon. 3, 546 (2009); [2] S. Z. Bisri, et al. Scientific Reports 2, 985 (2012); [3] S. Z. Bisri, et al. Adv. Mater. 23, 2753 (2011); [4] T. Takenobu, et al. Phys. Rev. Lett. 100, 066601 (2008); [5] K. Sawabe, et al. Appl. Phys. Lett. 97, 043307 (2010).

Since the first demonstration of lasing in organic thin films, the field of organic solid state lasers (OSL) has seen impressive progress. Broad optical gain spectra combined with relatively simple processing seed the prospect for a cost effective, easy to integrate and widely tunable laser source.[1] While electrical operation has not yet been demonstrated, highly efficient organic gain materials combined with high-Q resonator geometries (distributed feedback (DFB), VCSEL, etc.) now allow for OSLs, optically pumped by simple inorganic laser diodes or even LEDs.[2] In spite of this success, continuous wave (cw) operation of OSLs remains a challenge. Typically, the emission of an OSL terminates within 10 ns even when pumped with substantially longer pulses. To understand the underlying photo-physical mechanisms that impede cw operation, and to ultimately find strategies to overcome them, careful assessment of losses related to the generation of triplet excitons and polarons in the organic gain medium is required. We will show that both triplet state absorption and triplet singlet annihilation are indispensable to fully explain the dynamics of OSLs.[3] We will discuss case studies of low threshold organic lasers based on neat and dye doped polyfluorene gain media. We present measurements of the density, the spectral absorption and the lifetime of triplet excitons in these systems at room temperature. We evidence that especially in low-threshold guest-host systems, the lifetime of triplets may be significantly increased due to a reduced mobility as a result of trapping on the dopant molecules.[4] To overcome triplet related losses, various concepts of triplet management can be considered.[5] Alternatively, we show that there are polyfluorene based gain media with separated optical gain and triplet-related absorption spectra, which pave the way towards cw organic lasers.[6]
[1] P. Görrn, M. Lehnhardt, W. Kowalsky, T. Riedl, S. Wagner, Adv Mater 2011, 23, 869; J. Clark, G. Lanzani, Nat Photon 2010, 4, 438.
[2] T. Riedl, T. Rabe, H. H. Johannes, W. Kowalsky, J. Wang, T. Weimann, P. Hinze, B. Nehls, T. Farrell, U. Scherf, Appl Phys Lett 2006, 88, 241116; Y. Yang, G. A. Turnbull, I. D. W. Samuel, Appl Phys Lett 2008, 92, 163306.
[3] M. Lehnhardt, T. Riedl, T. Weimann, W. Kowalsky, Phys Rev B 2010, 81, 165206.
[4] M. Lehnhardt, T. Riedl, T. Rabe, W. Kowalsky, Org Electron 2011, 12, 486.
[5] S. Schols, A. Kadashchuk, P. Heremans, A. Helfer, U. Scherf, ChemPhysChem 2009, 10, 1071; S. Kéna-Cohen, A. Wiener, Y. Sivan, P. N. Stavrinou, D. D. C. Bradley, A. Horsfield, S. A. Maier, Acs Nano 2011, 5, 9958; Y. Zhang, S. R. Forrest, Phys Rev B 2011, 84, 241301.
[6] M. Lehnhardt, T. Riedl, U. Scherf, T. Rabe, W. Kowalsky, Org Electron 2011, 12, 1346.

Lasers are omnipresent in our world today and play an integral role in various technologies ranging from data storage and communications to materials processing, sensing and biomedicine. However, lasing has remained an artificial, man-made phenomenon and to our knowledge lasing is non-existent in natural, biological systems.
Here, I will present examples of a new class of lasers where coherent light is generated by biological materials or in biological systems.[1-3]
In the cell laser, mammalian cells are transfected with vectors encoding for expression of the fluorescent protein eGFP and the cells are then filled into a micro-cavity and pumped with ns pulses of blue light. In this configuration single cells produce laser light at sub-nJ pump thresholds. Interestingly, these single cell lasers generate complex spatial emission patterns that are associated with their transversal mode structure and contain information about cell morphology and size. I will also discuss high-throughput analysis of single cell lasers using a microfluidic cell-delivery platform (“lasing cytometer”). Cells from a large population are one-by-one delivered to a micro-resonator and optically pumped with a low-cost diode laser. The output is recorded by an optically triggered fast detection system and statistically analyzed.
In another example, fluorescent proteins like eGFP and tdTomato are used as laser medium in their solid-state form. The unique molecular structure of fluorescent proteins prevents concentration quenching and allows bright fluorescence of the pure material in the solid state. I will discuss high performance protein micro-lasers, self-assembly of fluorescent proteins into micro-resonators and sensing applications based on biocompatible, solid state protein devices.
Besides the fundamental significance of being able to generate laser light with biologically produced materials and in biological systems, these bio-lasers may become enabling tools for new biomedical applications.
[1] Malte C. Gather, Seok Hyun Yun, “Single-cell biological lasers”, Nature Photonics 5, 406-410 (2011).
[2] Malte C. Gather, Seok Hyun Yun, “Lasing from Escherichia coli bacteria genetically programmed to express green fluorescent protein”, Optics Letters 36, 3299-3301 (2011).
[3] Sedat Nizamoglu, Malte C. Gather, and Seok Hyun Yun, “All-biomaterial laser using vitamin and biopolymers”, Advanced Materials (accepted)

Here we present a new method for creating wavelength-tunable solid-state organic VCSELs in which conventional solution-processed laser dyes such as Rhodamine 6G can be incorporated as the gain material. In most studies of organic VCSELs, the gain layer is prepared either by vacuum thermal evaporation or by spin-coating. These approaches do not work well for many conventional laser dyes, which cannot be thermally evaporated because they are salts and also tend to lose their brightness due to concentration quenching when spin-casted. Spin-coating is also not very amenable to smooth deposition of a second “top” dielectric mirror which is a prerequisite for creating a high-Q cavity. Instead, we developed a new idea for creating VCSELs by dissolving the laser dye in a polymer precursor solution, then flowing the mixture into the cavity “void” between two dielectric Bragg reflectors (DBR&’s) that are separated by nanometric thickness spacer layers, and finally solidifying the organics via chemical crosslinking. We create the microcavity optical resonator by sandwiching two substrates that each has a DBR coating. A wedge-like cavity with variable separation between the mirrors is obtained by growing metallic spacer layers of two different thicknesses (200 nm and 500 nm respectively) on either end of the DBR coating and then cold-welding the substrates together. For Rhodamine, methyl methacrylate (MMA) was mixed with azobisisobutyronitrile (AIBN) and used to dissolve the laser dye. The entire mixture was entered into the cavity “void” and was polymerized via gentle heating to 80^omicron;C for 1 hour. Using our technique, we achieved lasing with a threshold of 1.2 mJ/cm². The lasing wavelength was tunable over the range of 585 nm to 610 nm by optically exciting a different section of the substrate.

A laser primarily consists of a cavity and a gain material that emits monochromatic, coherent light; however, the size of the laser is constrained by the diffraction limit. By using a metal nanoparticle as the cavity and a semiconducting conjugated polymer as the gain medium, a sub-wavelength analogue of a laser could be created that produces surface plasmons instead of photons. Unlike photonic resonances, surface plasmon resonances are localized on sub-wavelength dimensions, and the resulting device, termed a SPASER (Surface Plasmon Amplification by Stimulated Emission of Radiation), would have a size less than the diffraction limit [1]. The use of a semiconducting conjugated polymer as the gain medium is advantageous for several reasons. These materials are more stable than the small organic laser dye molecules that are used to dope the silicon shell that is traditionally used as the gain medium. They also exhibit large gain cross sections, do not undergo concentration quenching in the solid state, and undergo amplified spontaneous emission [2, 3, 4, 5]. Here, we will present a procedure for coating gold nanorods that support surface plasmon resonances in a conjugated polymer through a miniemulsion/solvent evaporation procedure.
To create the polymer nanoparticles, a conjugated polymer (F8BT) is dissolved in dichloromethane and the solution is then injected drop-wise into a solution of water and polyethylene glycol. After stirring and sonication, the nanoparticle solution is spin-coated on a silicon substrate. From bright- and dark-field optical images and scanning electron microscope images, it has been determined that F8BT nanoparticles were present, with diameters between 150 and 600 nm. UV-Vis spectroscopy of the F8BT nanoparticle solution reveals peaks at 340 nm and 480 nm, while UV-Vis spectroscopy of the F8BT nanoparticle/gold nanorod heterostructures demonstrates that the two F8BT peaks are decreased in intensity and shifted to longer wavelengths, a result of overlapping with the transverse and longitudinal surface plasmon resonances of the gold nanorods at 515 nm and 580 nm, respectively. Epifluorescence images of the F8BT nanoparticles and heterostructures also indicate that the gold nanorods partly quench the fluorescence of the F8BT. Future work will characterize scattered light and fluorescence emission spectra from individual F8BT nanoparticle/gold nanorod heterostructures as a function of nanorod size and light polarization to elucidate the degree of exciton-plasmon coupling in the structures.
References:
1. M. A. Noginov, G. Zhu, et al. Nature Letters, 460, 1110-1112 (2009).
2. R. Xia, G. Heliotis, Y. Hou, D.D.C Bradley, Org. Electron. 4, 165 -177 (2003).
3. C. Rothe, F. Galbrecht, U. Scherf, A. Monkman. Adv. Mater. 18, 2137-2140 (2006).
4. F. Hide, et al., Sci. 273, 1833-1836 (1996).
5. N. Tessler et al., Nature 382, 695-697 (1996).

Among organic devices, ambipolar transistors are very unique device, in which both electrons and holes are equally mobile, and we are able to emit light through the recombination of them [1,2]. One of the advantages of such ambipolar light-emitting transistors (LETs) based on several organic single crystals is the coexistance of high current density and high luminescent efficiency, which might be possible route to the electrical-driven organic laser [3,4]. However, in these ambipolar LETs, many issues are still necessary to overcome. Here, I will present recent improvement in ambipolar LETs.
One of the open issues in ambipolar transistors is the basic understanding of junction structure between p-type and n-type region. We fabricated single-crystal ambipolar transistors using a wide variety of molecules and tried to clarify junction structure [5,6]. Although it was understood as the simple pn junction before, we finally found the existence of non-doped area between them, which size is decided by the density of trap states. Based on this knowledge, we reduced the effect of trap states by the optimization of fabrication process and dielectric layer. Particularly, using the high-capacitance electric-double layer of ion gel, we realized extremely low-voltage operation of ambipolar transistors [7].
We feedback these basic understandings to improve LETs. The reduction of trap states leads to the high current density, which is necessary for bright light-emitting devices, and the luminescent properties under high current density were investigated. In particularly, the special current confinement structure realized extremely high current density [4] and these results strongly suggest the necessity of optical resonator for the electrical-driven organic laser. Therefore, we found the novel device structure to introduce naturally grown Fabry-Perot cavity into LETs [8]. Moreover, the micro-fabrication of optical resonators opens the new roots to functionalization of ambipolar transistors. These recent developments in ambipolar single-crystal LETs will be presented.

The organic field-effect transistor (OFET) offers an elegant device structure for efficient and high-intensity light emission [1]. The current densities can be even high enough to reach the lasing threshold. Unfortunately the highest efficiency is obtained in the ambipolar regime, where the current is inherently low. Contradictorily the highest current density is obtained in the unipolar regime, where the efficiency is inherently low. This fundamental problem might be solved by adding a second gate. The resulting ambipolar dual-gate transistor allows for the creation of two unipolar accumulation layers of opposite polarity. The question is if the accumulated holes and electrons can vertically be forced together to recombine efficiently. The answer to this question is negative.
To this end we investigated experimentally and theoretically dual gate ambipolar field-effect transistors as a function of semiconductor layer thickness. For thick semiconductor films two accumulation layers can be formed, each controlled by its own gate. However, they are physically and electrically separated from each other. For thin films only a single accumulation layer can be formed that is simultaneously controlled by both gates. Either two accumulation layers are physically and electrically separated from each other or it is not possible to simultaneously create two accumulation layers of opposite polarity. Hence, independent hole and electron accumulation is mutually exclusive with efficient vertical recombination and intense light emission.
[1] M.C. Gwinner et al, Adv. Func. Mat. 24, 20 (2012)

In an asymmetric three- slab metal/organic (/air) active waveguide structure, the thickness of the organic layer can be controlled at the critical (cutoff) thickness where only TM1 or TE0 mode is just allowed at peak wavelength of the organic material. At that case, the optical field profile mainly lies in the organic layer, and the edge emission largely narrows (up to several nanometers) due to the sensitive mode-selecting effect of the thickness of the organic layer, thus it can be considered a type of lambda;-tunable organic laser. These lasers generally have 1) a sub-wavelength scale thickness due to the strong optical confinement of the metal film, 2) a sufficiently long cavity along the edge emission direction without causing the current injection problem, and 3) a very low loss compared to the plasmonic mode at the metal/organic interface. In this study, we have numerically and experimentally studied the TE0 and TM1 modes in the Au / AlQ (x nm) and Au/AlQ: DCM (x nm) structures. The narrowed (~ 6 nm) and polarized photoluminescence spectra are observed when pumped at a low intensity. The calculated optical loss is below 100 cm-1, much less than the optical gain of the typical organic material.

Miniature light sources based on biocompatible organic material have a potential to enable a novel approach in photomedicine. Here, I discuss this possibility.

Organic materials have shown enormous potential in applications like organic light emitting diodes and transistors. Since the demonstration of lasing in organic solid-state thin films of both small molecular weight molecules and polymers many efforts have been devoted to this field. Dye doped matrices which are based on the Förster energy transfer from a host to a guest molecule have shown to be excellent organic laser materials. These guest host systems exhibit high efficiency and low threshold due to large stokes shifts and low waveguide losses. Since the Förster energy transfer depends strongly on the doping concentration a high variation of the gain properties can be observed. Here we present the results of waveguide measurements using the variable stripe length method providing information about waveguide losses and optical gain in different guest host systems. In such a guest host system the occupation of the upper laser level (dopant side) is due to Förster energy transfer. This energy transfer strongly depends on the donor acceptor distance and hence on the concentration of the laser dye. Therefore, the doping concentration is varied and the gain coefficients are measured at various excitation densities to analyze the stimulated emission cross section. Additionally small molecules systems and polymer based materials are compared concerning their properties like waveguide loss, material parameter and cross section determined by waveguide measurements. E.g. conjugated polymers are very suitable as laser active materials but with the help of waveguide measurements we show how the material treatment before the film preparation affects optical loss and gain in a thin film. And finally it discussed how the spectral position within the gain spectrum determined by waveguide measurement influences the threshold of an organic DFB laser.

The conversion of incoherent, spectrally broad solar energy into a coherent monochromatic energy beam opens the science and technology of solar energy to the field of nonlinear optics. The ideal optical gain media would efficiently absorb spectrally broad sunlight in a small mode volume for low solar pump concentration to achieve population inversion. By reducing the mode volume to reduce inversion threshold, the efficiency of pump absorption, for traditional gain media such as rare earths, is also reduced. These two parameters must be decoupled for low inversion threshold and efficient solar pumping of optical gain media. This can be done by adding an intermediate step between absorption and emission. Unlike rare earth materials, nanocrystal materials can absorb spectrally broad light in small volumes and have high photoluminescence efficiency with a spectrally narrow emission compared to sunlight. When the nanocrystal thin film is optically coupled to the gain media, it will act as a waveguide for the nanocrystal photoluminescence. The photoluminescence of the nanocrystal can then be tuned to the absorption peaks of the gain medium via the particle size for an indirect optical pump. This photoluminescence tuning and waveguiding allows long interaction distances between gain media and photoluminescence which results in sunlight absorption and energy transfer in small mode volumes. This novel solar pumped gain media scheme is realized through a 1064nm AR/AR coated Nd:YAG slab with a thickness of 750µm, width of 2mm, and length of 43mm. The slab is coated with a 100µm thick film of PLMA doped with 2% wt. CdSe/ZnS nanocrystals. The nanocrystal film absorbs broadly in the visible spectrum and emits at 580nm where neodymium has a strong absorption peak. This gain media is placed in a hemispherical cavity with a planar mirror of R = 99.999% and output mirror of radius 15cm and R = 98% to realize a solar powered laser. The nanocrystal coating is pumped with a 18W 532nm quasi-CW laser as a high intensity photon source. An equivalent solar threshold of approximately 1000 suns has been achieved. Integration of this solar gain media into an optical amplifier for solar energy applications will also be discussed.

Optical concentration is a powerful and near-term strategy to lower the cost of electricity produced from established solar cell technologies because it reduces the cell area needed to generate a given amount of power. Whereas passive geometric optical concentrators are invariably bound by the sine limit, luminescent solar concentrators (LSCs) can in principle achieve high concentration ratio (>100x) without tracking the Sun. These devices traditionally consist of a luminescent slab that absorbs sunlight and emits it into modes confined by total internal reflection, where it is subsequently absorbed by photovoltaic cells attached to the edges.
We are exploring opportunities to improve LSC performance and diversify their application by photonically controlling the luminescent etenduacute;e. Leveraging highly directional luminescence together with a generalized nonimaging optical framework, we demonstrate routes to both increase concentration ratio for photovoltaics and alternatively to optimize the distribution of light within closed photobioreactors for enhanced algal biofuel productivity. Our approach, which is based on the use of macroscopic freeform waveguides, leads to the formation of optical pseudo-potentials that act like in-plane graded index variations, channeling the flow of light and localizing it at particular points to dramatically boost concentration ratio.

Using our recently developed non-adiabatic excited-state dynamics simulations framework, we study ultrafast dynamics and exciton transport in several large molecular systems. These simulations reveal a fascinating interplay of conformational vibrational dynamics and internal conversion followed photoexcitation, which has specific spectroscopic signatures and can be observed using time-resolved pump-probe spectroscopies. Observed relationships between spatial extent/properties of electronic wavefunctions and resulting electronic functionalities allow us to understand and to potentially manipulate excited state dynamics and energy transfer pathways in a number of organic molecular materials suitable for a variety of technological applications.

In the frame of developing new active materials for laser and light amplifier applications large interest has been devoted in the last decades to organic conjugated polymers. These molecules are particularly interesting as they can combine high optical gain, good charge mobility, and good film forming properties when deposited from solution.
Amplified Spontaneous Emission (ASE) and lasing has been demonstrated in several classes of luminescent polymers, thus proposing these molecules as potential active materials for low cost light amplifying waveguides and solid state lasers.
Large research efforts have been devoted to date to the increase of the active material optical gain, in order to decrease the ASE and lasing threshold, recently allowing the remarkable demonstration of organic lasers pumped by inorganic Light Emitting Diodes, that evidences how close to real laser applications are organic lasing systems.
In the last years we focused our interest in the investigation of the physical aspects that affect the final ASE properties of active polymeric waveguides, beyond the intrinsic gain properties of the active molecule.
In this work we will present our recent results relative both to effects with a direct applicative interest, and to effects more related to the basic photophysics of the active molecules.
In particular we will investigate the extrinsic elements affecting the ASE threshold and the operational stability, beyond the intrinsic active material gain cross section and photostability.
Moreover we will investigate by confocal laser spectroscopy the correlation between the mascroscopic ASE properties and the microscopic non uniformities of the active layer emission properties and morphology.
Our results indicate, on one side, that some care as to be used in order to be able to directly relate the ASE properties of a polymeric thin film to the intrinsic emission and gain properties of the active molecules.
On the other side we will show that many parameters, when properly controlled, can be used to modify the ASE properties of a system, thus allowing the optimization of the sample emission for specific device configuration.

For achieving organic solid-state lasers, current-injected narrow linewidth emissions (NLEs) are indispensable because NLEs are thought to be a precursor phenomenon. For the laser devices, a combination of organic materials with high optical gain and optical cavities is prerequisite. We chose thiophene/phenylene co-oligomers (TPCOs) [1, 2] as the organic semiconductor materials, and observed optically-excited laser oscillations from crystals of various TPCOs with parallel crystal facets acting as a Fabry-Pérot resonator [3-5]. Thus TPCOs are regarded as one of the promising materials for the organic solid-state laser devices.
We have fabricated light-emitting field-effect transistors (LEFETs) using the TPCO crystals as an emitting layer. We equipped these LEFETs with diffraction gratings. These gratings functioned as parts of the gate insulators as well. We engraved these gratings by a focused ion beam [6] or fabricated them by using the interference exposure [7] or the nanoimprint lithography [8, 9]. The TPCO crystals were laminated or grown directly on the diffraction gratings. When we operated these devices with the alternating gate voltage method [10], the devices indicated current-injected NLEs. The full-widths at half-maxima of these narrow lines were less than ~10 nm.
[1] S. Hotta, H. Kimura, S.A. Lee, T. Tamaki, J. Heterocycl. Chem. 37 (2000) 281.
[2] S. Hotta, T. Katagiri, J. Heterocycl. Chem. 40 (2003) 845.
[3] T. Yamao, K. Yamamoto, Y. Taniguchi, T. Miki, S. Hotta, J. Appl. Phys. 103 (2008) 093115.
[4] S. Hotta, Y. Sakurai, Y. Okuda, T. Miki, K. Matsunaga, F. Hirato, T. Yamao, H. Jinnai, J. Nanosci. Nanotechnol. 10 (2010) 440.
[5] T. Yamao, Y. Okuda, Y. Makino, S. Hotta, J. Appl. Phys. 110 (2011) 053113.
[6] T. Yamao, Y. Sakurai, K. Terasaki, Y. Shimizu, H. Jinnai, S. Hotta, Adv. Mater. 22 (2010) 3708.
[7] Y. Makino, T. Hinode, A. Okada, T. Yamao, N. Tsutsumi, S. Hotta, Physics Procedia 14 (2011) 177.
[8] Y. Makino, A. Okada, S. Hotta, T. Yamao, Mol. Cryst. Liq. Cryst. 566 (2012) 8.
[9] A. Okada, Y. Makino, S. Hotta, T. Yamao, Phys. Status Solidi C 9 (2012) 2545.
[10] T. Yamao, Y. Shimizu, K. Terasaki, S. Hotta, Adv. Mater. 20 (2008) 4109.

Vacuum deposition of para-sexiphenyl (p-6P) on muscovite is known to yield linear aggregates of co-oriented nanocrystals, usually referred to as nanofibers [1-3], having precise epitaxial relationships to the muscovite substrate [4]. These epitaxial nanofibers exhibit important optical properties, such as highly polarized blue luminescence and laser action [5], which could be exploited for applications in photonic and sensing technologies. Recent advances in the field include (i) the achievement of highly polarized, red-green-blue (RGB) emission from epitaxially aligned bilayer nanofibers obtained upon deposition of sexithiophene (6T) on p-6P/muscovite templates, and (ii) the development of a roll-printing technique for high-yield and massive transfer of nanofibers from the native substrate to a receiving substrate for device applications.
We report successful tuning of laser wavelength from ~420 nm to ~600 nm in epitaxially aligned nanofibers grown by periodic deposition of p-6P and 6T on p-6P/muscovite templates. The nanofibers were photoexcited by subpicosecond pulses tuned to the lowest p-6P absorption band, and the emission of 6T, whose coverage was kept in the submonolayer regime, was efficiently sensitized through resonance energy transfer [6]. 6T lasing was achieved at room temperature with threshold fluences as low as 10 mu;J/cm2 per pulse. Transient photoluminescence measurements with picosecond resolution showed that at these pump fluences the decay dynamics of 6T emission is independent of the excitation density, thereby demonstrating the attainment of room-temperature monomolecular lasing from epitaxially oriented 6T submonolayer aggregates. Main lasing properties remained unaltered upon direct photoexcitation of 6T below the p-6P absorption edge [7].
[1] H. Yanagi, and T. Morikawa, Appl. Phys. Lett. 75, 187 (1999)
[2] A. Andreev et al., Adv. Mater. 12, 629 (2000)
[3] F. Balzer, and H.-G. Rubahn, Appl. Phys. Lett. 79, 3860 (2001)
[4] C. Simbrunner et al., J. Am. Chem. Soc. 133, 3056 (2011)
[5] F. Quochi et al., Appl. Phys. Lett. 84, 4454 (2004)
[6] C. Simbrunner et al., ACS Nano 6, 4629 (2012)
[7] F. Quochi et al., Adv. Opt. Mater 1, Adv. Opt. Mater. 1, 117 (2013)

The illumination of a film of a polymer containing azobenzene moieties by means of linearly polarized light, in the UV/Visible wavelengths region, leads to the orientation of the azobenzene units perpendicularly to the light polarization direction. This results, after many trans-cis-trans isomerization cycles of the azo-unit, from the reduction of the transition probability when the rod-like trans-isomer is perpendicular to the electric field vector. This mechanism has been proposed for holographic data storage on this polymeric support. However, in 1995 a new phenomenon has been observed, the material-displacement (mass-migration) on the free surface of azobenzene-containing polymers.
Here we report about a new feature in patterning azo-polymers deriving by light-matter interaction where the film is sensitive to the phase information carried out by optical vortexes of different topological charges.
In fact, we have found the unexpected experimental observation of spiral-shaped relief patterns on the surface of an azopolymer that has been illuminated with a vortex laser beam, that is a beam having a helical wavefront. The spiral handedness of the polymer pattern is determined by the vortex one. This result is quite surprising because the common understanding hitherto was that these surface patterns respond to the light intensity distribution and its gradients. The intensity pattern of a vortex beam is shaped as a “doughnut” and carries no information whatsoever about the vortex handedness. We found an explanation for our observations that links them to a peculiar interference effect occurring between longitudinal and transverse field components of the vortex beam. Furthermore, we have found out that the main features of the observed phenomenon can be predicted by a phenomenological theory that does not rely on a specific microscopic model and therefore, in this sense, is a model-independent interpretation.
Our finding [1,2] will benefit the development of new lithography schemes as well as the interpretation of the phenomenon driving the material-displacement and the imaging of phase-related information.
[1] A. Ambrosio, L. Marrucci, F. Borbone, A. Roviello, P. Maddalena, Light-induced spiral mass transport in azo-polymer films under vortex-beam illumination, Nature Communications, 3:989 (2012)
[2] A. Ambrosio, P. Maddalena, L. Marrucci, Molecular Model for Light-Driven Spiral Mass Transport in Azopolymer Films, Physical Review Letters 110, 146102 (2013)