Symposium EP9 : Materials and Processes for Nonlinear Optics

The use of a two-photon excitation offers new perspectives for photodynamic therapy (PDT) of cancer, especially for the treatment of small areas, such as small solid tumors. Indeed two-photon absorption (2PA) offers several advantages, which include the ability for highly selective excitation in biological media and intrinsic three-dimensional resolution as well as increased penetration depth in tissues. We have developed organic two-photon photosensitizers, combining large 2PA cross-sections in the biological spectral window, a full linear transparency in this spectral range, and exhibiting both significant fluorescence and suitable photosensitization efficiency.[1] These photosensitizers were covalently incorporated within organic and inorganic nanoparticles (NPs), the surface of which was post-functionalized with mannose moieties or with a mannose-6-phosphate analog (allowing efficient targeting of lectins over-expressed by cancer cells). The photophysical properties of the individual chromophores are maintained and sometimes enhanced when confined within NPs. These NPs were found to be efficient for two-photon fluorescence imaging, and active in inducing cancer cell death upon short two-photon excitation, but inactive under daylight exposure for several hours. They hold promise for theranostic applications, and were successfully used in two-photon excited PDT experiments, in vitro on several human cancer cell lines [2], ex vivo on prostate cancer tissues [3], and in vivo on nude mice bearing tumor xenografts [2].

[1] C. Rouxel, M. Charlot, Y. Mir, C. Frochot, O. Mongin, M. Blanchard-Desce, New J. Chem. 2011, 35, 1771-1780.
[2] M. Gary-Bobo, Y. Mir, C. Rouxel, D. Brevet, I. Basile, M. Maynadier, O. Vaillant, O. Mongin, M. Blanchard-Desce, A. Morère, M. Garcia, J.-O. Durand, L. Raehm, Angew. Chem. Int. Ed., 2011, 50, 11425-11429.
[3] O. Vaillant, K. El Cheikh, D. Warther, D. Brevet, M. Maynadier, E. Bouffard, F. Salgues, A. Jeanjean, P. Puche, C. Mazerolles, P. Maillard, O. Mongin, M. Blanchard-Desce, L. Raehm, X. Rébillard, J.-O. Durand, M. Gary-Bobo, A. Morère, M. Garcia, Angew. Chem. Int. Ed., 2015, 54, 2015, 54, 5952 –5956.

Applications related to two-photon absorption (2PA) are numerous and concern different fields. We will develop in this presentation our work in the fields of biology and defense.
Two-photon excited luminescence, and photodynamic therapy are becoming promising tools for biological applications like imaging, diagnostic or even therapy. In this context, we designed chromophores, with excellent quantum yield of luminescence, or exhibiting enhanced efficiency for singlet oxygen generation, and featuring optimized biphotonic cross-section (1-3).
Over the past decade, laser sources in the NIR range (1100 -1660 nm) have been widely used in optical telecommunications, which has stimulated the design of new systems for optical power limiting. We will present properties of heptamethine cyanines and of aza-bodipy derivatives for this type of applications (4); properties will be discussed on the basis of 2PA and of excited states properties (5).

(1) (a) Picot A., D’Aléo A., Baldeck P. L., Grishine A., Duperray A., Andraud C., Maury O. J. Am. Chem. Soc., 2008, 130, 1532; (b) Bourdolle, Brustlein A. S., Fauquier T., Grichine A., Duperray A., Baldeck P. L., Andraud C., Brasselet S., Maury O. Angew. Chem. Int. Ed. 2012, 51, 1
(2) a) Lanoe P. H., Gallavardin T. Dupin A., Maury O.; Baldeck P. L., Lindgren M., Monnereau C., Andraud C. Org. Biomol. Chem. 2012, 10, 6275-6278. b) Gallavardin T., Armagnat C., Maury O. Baldeck P. L., Lindgren M., Monnereau C., Andraud C. Chem. Comm. 2012, 48, 1689.
(3) (a) Massin J., Charaf-Eddin A., Appaix F., Bretonniere Y., Jacquemin D., van der Sanden B., Monnereau C., Andraud C. Chem. Sci. 2013, 4, 2833. (b) Monnereau C., Marotte S., Lanoe P.-H., Maury O. Baldeck P., Kreher D., Favier A., Charreyre M.-T., Marvel J., Leverrier Y., Andraud, C. New J. Chem. 2012, 36, 2328.
(4) Pascal S., Haefele A., Monnereau C., Charaf-Eddin A., Jacquemin D., Le Guennic B. , Andraud C., Maury O. J. Phys. Chem. A, 2014, 118, 4038
(5) (a) Bouit P.-A., Wetzel G., Berginc G., Loiseaux B., Toupet L., Feneyrou P., Bretonnière Y., Kamada K., Maury O., Andraud C. Chem. Mat. 2007, 19, 5325; (b) Bouit P.-A., Kamada K., Feneyrou P., Berginc G., Toupet L., Maury O., Andraud C. Adv. Mat. 2009, 21, 1151; (c) Bellier Q., Makarov N. S., Bouit P.-A., Rigaut S., Kamada K., Feynerou P., Berginc G., Perry J.W., Maury O., Andraud C. Phys. Chem. Chem. Phys., 2012,14, 15299 ; D. Château, Q. Bellier, F. Chaput, P. Feneyrou, G. Berginc, O. Maury, C. Andraud, S. Parola J. Mater. Chem. C , 2014, 2 , 5105.

Cellular imaging has become very relevant in such important fields as biophysics, diagnostics and therapeutics. The advances of nonlinear optics for this imaging modality gradually become obvious. Two-photon fluorescence induced by near-infrared light has brought a number of advantages, such as higher resolution and intrinsically confocal imaging, and less scattering and absorption, leading to reduced phototoxicity and longer photostability.
More recently, it has become obvious that an additional imaging modality is possible with the same pulsed laser en microscope equipment. Second-harmonic imaging only requires detection at a different wavelength, but provides additional information, different from one- or two-photon fluorescence. Because these are odd-order processes (either first or third order) they are not sensitive to symmetry. Second harmonic imaging, being an even order nonlinear process, is symmetry-sensitive and will provide structural information based on the symmetry requirements for the superstructural features of the probe, while fluorescence will only provide information about the where-about of the probe.
We will report on the design and engineering of new molecular probes for this combined two-photon fluorescence and second-harmonic imaging of cellular structure. These can be molecular probes with combined advanced structural (amphiphilic properties) and optical functions (second- and third-order optical nonlinearities), based on a number of different strategies. We will also report on our approach to screen fluorescent proteins for second harmonic imaging and our effort to rationally design a new fluorescent protein for this new function of second harmonic imaging.

An interesting concept for manipulation of DNA is that of attaching to it suitably engineering photochromic molecules. The phenomenon of photochromism can then be employed for forcing changes in the DNA either by one-photon or by multiphoton absorption by the photochrome. We report on linear and nonlinear optical properties measurements of photochromic molecules, including water-soluble azobenzene derivatives and materials consisting of spiropyran and the merocyanine photoproduct. The studies of nonlinear absorption and refraction properties of the molecules dissolved in chloroform or in heavy water were performed with the Z-scan technique, using femtosecond pulses, in a wide range of the wavelengths. Maxima in the two-photon absorption spectra were found and determined for the resting and photosonverted forms of the studied molecules. The processes of three-photon absorption or absorption by some intermediate species were also observed in the presented materials. The discussion on the use of such molecules for effective DNA manipulation will be presented.

Organic materials that exhibit large real third-order optical nonlinearities, |Re(χ⁽³⁾)|, and that also have low linear and nonlinear losses at telecommunication wavelengths (1300–1500 nm) may prove useful for a range of all-optical signal-processing (AOSP) applications. Based on their linear and nonlinear optical characteristics in solution, polymethine dyes in general, and chalcogenopyrylium-terminated heptamethines in particular, are a promising class of materials for AOSP,¹ but the translation of their microscopic nonlinearity to device-relevant materials is hindered by significant aggregation of the molecules in high-chromophore density films. An approach to minimize aggregation of these dyes, in which bulky and rigidly out-of-plane groups are introduced both in the center of the polymethine bridge and on the heterocyclic end groups, has been developed² and can lead to thin-film materials exhibiting combinations of large |Re(χ⁽³⁾)|, large two-photon figure-of-merit, and low linear loss that are suitable for AOSP, although the synthesis of dyes is generally a complex multi-step procedure. An alternative approach has subsequently been developed in which the bulky Pd(PPh₃)₂Cl group is introduced into the center of dyes in the final stage in the synthesis. This approach also leads to more solution-like optical properties in high-chromophore density films and, thus, to materials that are potentially suitable for AOSP. The effect of Pd(PPh₃)₂Cl functionalization on the solid-state optical properties can be rationalized in some cases by examining the interchromophore π interactions revealed by single-crystal X-ray diffraction. The reaction is applicable to a wide range of cationic polymethines, to anionic polymethines, and to neutral merocyanines. Related insertions of bulky metal-phosphine species into C–Hal bonds may prove useful in other areas of organic photonics and electronics where control of aggregation is required. Indeed the tendency of perylene- and naphthalene-diimide materials to aggregate can largely be supressed by oxidative addition of bromo derivatives to Pd(PPh₃)₄.

[1] Hales J. M., Matichak J., Barlow S., Ohira S., Yesudas K., Brédas J.-L., Perry J. W. and S. R. Marder, Science, 2010, 327, 1485.
[2] Barlow S., Brédas J.-L., Getmanenko Y.A., Gieseking R.L., Hales J.M., Kim H., Marder S.R., Perry J.W., Risko C., Zhang Y., Materials Horizons, 2014, 1, 17

Polymethine dyes have been shown to have promising molecular third-order nonlinear optical (NLO) properties but have not shown adequate figures-of-merit (FOM) for all-optical signal processing (AOSP). This is largely due to the strong attractive interactions between molecules, which leads to aggregation and alterations of the electronic energy level structure that deleteriously affect the optical nonlinearity and optical loss. We have employed a molecular design strategy to achieve AOSP that is based on substitution of polymethines with bulky groups that prevent aggregation leading to high-number-density films with macroscopic NLO close to those needed for AOSP with exceptionally high two-photon FOM (Re| c⁽³⁾|/ Im| c⁽³⁾|) and a large value of n₂. The use of bulky groups on polymethine compounds results in dilute-solution like properties in 50wt.% blend films with amorphous polycarbonate and, remarkably, for neat molecular films. These thio- or seleno-pyryllium based polymethines exhibit a negative Re| c⁽³⁾| that can be useful in the compensation of self phase modulation (spectral broadening) that is associated with the propagation of strong optical signals in optical fibers. We demonstrate that the use of a 50 cm liquid core optical fiber filled with a 20 mM solution of a thiopyryllium based polymethine was able to reverse the spectral broadening due to self-phase modulation from propagation of the pulses through a solution of CCl₄ and CS₂. Finally, we will report on the observation of a high-order excited state refraction in polymethine dyes.

Singlet diradicaloid is a class of molecules and has intermediate electronic character between the two limits: the closed shell (covalent) and open shell (diradical). It can be understood as a molecule having a semi-breaking chemical bond in which the electrons are bounded weakly to the nuclei forming the molecular structure. The weakly bounded electrons fluctuate easily to the external filed, such as laser light, so sensitive optical responses, particularly nonlinear optical (NLO) one, can be expected. Nakano et al. predicted that the third-order optical nonlinearity of singlet diradicaloids. Their theoretical studies clarified that the NLO response is greatly enhanced by orders of magnitude for diradicaloids molecules that have an intermediate value of diradical character y, defined as a measure of degree of diradical between 0 (meaning perfect covalent) and 1 (meaning perfect diradical) [1].
Two-photon absorption (TPA) is the imaginary part of the third-order NLO process and the same behavior is, therefore, expected against y. In this paper, the TPA properties of diradicaloids molecules are reviewed based on the experimental works. The first example demonstrated experimentally was the polycyclic aromatic hydrocarbon (PAH) consisting of two phenalenyl groups, which consisting of three benzene rings fused together in triangle manner [2]. Thermodynamic stabilization among the resonance structures makes the molecule stable and allowed the NLO measurement by the Z-scan method. The diradicaloids PAH was found to show a large TPA cross section of thousands GM at near IR wavelengths, nearly two magnitude higher than that of the covalent PAH [3]. This success encouraged us to unveil the correlation between the theoretical and experimental results, and found that the value of y, which had been considered as a theoretical entity, can be obtained from experimentally accessible values [4]. In addition, with the other family of the stable diradicaloids, which is kinetically stabilized, switching between the diradicaloid and closed-shell states of a bis(acridine) dimer through a redox reaction gives 300-fold contrast in the TPA cross section [5].
These results show that the TPA properties of singlet diradicaloids can be understood with and tuned by diradical character y and that singlet diradicaloid is a promising class of nonlinear optical materials [6].
[1] M. Nakano et al., J. Chem. Phys. 2006, 125, 074113; ibid., 2009, 131, 114316.
[2] T. Kubo et al., Angew. Chem. Int. Ed. 2005, 44, 6564.
[3] K. Kamada et al., Angew. Chem. Int. Ed. 2007, 46, 3544.
[4] K. Kamada et al., J. Phys. Chem. Lett. 2010, 1, 937.
[5] K. Kamada et al., J. Am. Chem. Soc. 2013, 135, 232.
[6] C. Lambert, Angew. Chem. Int. Ed. 2011, 50, 1756.

Platinum(II) organometallic complexes with π-conjugated organic chromophores have received considerable interest for application in non-linear absorption and as active materials for organic/polymer solar cells. The talk will highlight recent work from the author’s laboratory in which the properties and applications of organometallic chromophores and polymers are discussed. Incorporation of platinum into π-conjugated electronic systems leads to considerable enhancement in the efficiency of singlet → triplet intersystem crossing, and the involvement of the triplet state in the non-linear absorption process is described. Recent work has also explored incorporation of ortho-metallated platinum(II) centers into DPP- and isoindigo-type donor-acceptor conjugated polymers, and these materials have been explored as active materials in polymer solar cells.

Studies assessing the impact on optical nonlinearity of various molecular structural changes have resulted in the development of structure – nonlinear optical (NLO) property relationships for organometallic and coordination complexes.^[1] We have been studying the nonlinear optical (NLO) properties of metal alkynyl complexes, extending our studies with small complexes^[2] to molecular trees (dendrimers).^[3] The two-photon absorption cross-sections of these complexes are similar or superior to benchmark organic molecules, whether compared in absolute terms or following appropriate scaling. We have recently found that zero-generation dendrimers with extended “arms” (“stars”) exhibit nonlinear absorption resulting from a higher-order intensity dependence, displaying significant three-photon absorption at telecommunications wavelengths and possess appreciable four-photon absorption cross-sections at longer wavelengths. The results from these studies will be presented.

References
1. M. G. Humphrey, T. Schwich, P. J. West, M. P. Cifuentes, M. Samoc. In Comprehensive Inorganic Chemistry II, J. Reedijk, K. Poeppelmeier, Eds; Elsevier: Oxford, UK, 2013, 8 (V. W. W. Yam Ed.), ch. 8.20, pp 781-835.
2. B. Babgi, L. Rigamonti, M. P. Cifuentes, T. C. Corkery, M. D. Randles, T. Schwich, S. Petrie, R. Stranger, A. Teshome, I. Asselberghs, K. Clays, M. Samoc, M. G. Humphrey, J. Am. Chem. Soc., 131 (2009) 10293-10307.
3. T. Schwich, M. P. Cifuentes, P. A. Gugger, M. Samoc, M. G. Humphrey, Adv. Mater., 23 (2011)1433-1435.

Long-lived triplet excited state with broad and strong absorption in the visible and near-infrared wavelength region is desired in material development for reverse saturable absorption (RSA). Increasing the π-conjugation of the ligands in transition-metal complexes may lead to more ligand-localized ³π,π^* character to the lowest triplet excited states of the complexes and prolong the lowest triplet excited-state lifetime. However, ground-state absorption may also be red-shifted due to the increased π-conjugation of the ligands. This could reduce the effectiveness of RSA due to an increased ground-state absorption cross section(σ_g) at the interested visible wavelengths. In this work, derivatives of pyrazino[2,3-f][1,10]phenanthroline (PyPhen) and 2,3-diphenylpyrazine (dpp) were synthesized and used in cationic bis-cyclometalated Ir(III) complexes as diimine and cyclometalating ligand, respectively. Photophysical studies were carried out to understand how the extended π-conjugation of the diimine and the cyclometalating ligands influences the ground-state and excited-state absorption as well as the triplet excited-state lifetime. It is found that when the cyclometalating ligand is fixed as 1-phenylisoquinoline (piq), the extended π-conjugation of the diimine ligand along the direction of pyrazine ring does not influence the ground-state and excited-state absorption pronouncedly. Their triplet lifetimes also keep the similar magnitude. Thus the RSA of the complexes remains essentially the same. In contrast, when the diimine ligand is fixed as benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (dppn), extending the π-conjugation of the cyclometalating ligand via fusion of phenyl ring(s) to the pyrazine ring of the dpp ligand alters the ground-state absorption and the triplet lifetime dramatically, which leads to pronouncedly different RSA. An Ir(III) complex with quite broad but weak ground-state absorption extending to ~830 nm, broad and strong excited-state absorption in the visible and NIR region, and long-lived triplet excited state (~15 μs in CH₃CN) was obtained, which is a very promising broadband reverse saturable absorber.

The development of organic and organometallic nonlinear optical materials has been an ongoing area of research at the Air Force Research Laboratory for quite some time. The effects of high concentration on the photophysical properties of a nonlinear material have been of interest for some time in our group and it is well known in the literature that for a nonlinear absorbing dye to be the most effective, high concentrations are needed. Recently efforts have been made to study the effects of incorporating a dye into a solid matrices consisting of polyurethane, epoxy, or sol gel at high concentration to better understand the constraints this environment has on a given material. Two series of samples were prepared containing the previously reported two photon chromophores AF455 and E1-BTF.[1]^,[2] In these systems studies were done looking at a given dye as a function of concentration to elucidate the effects of increased concentration on performance. Results reveal the formation of excimers (excited state dimers) with an increase in concentration. Excimers may form from either the singlet excited state or the triplet excited state depending on the molecule and provide a competitive pathway for deactivation of the excited state back to the ground state. Determining the rate of formation of the excimers has been done for both dye systems. For example in a neat sample of AF452-6OH in polyurethane we observe a quenching effect that results in 92% of the singlet excited states forming excimers. This is a significant quenching effect that will reduce nonlinear performance of this material. Understanding and mitigating excimer formation is an active area of research in our laboratory and will be the focus of this presentation.


[1] Rogers, J.E.; Slagle, J.E.; McLean, D.G.; Sutherland, R.L.; Sankaran, B.; Kannan, R.; Tan, L.-S.; Fleitz, P.A. J. Phys. Chem. A 2004, 108, 5514.

[2] Rogers, J.E.; Slagle, J.E.; Krein, D.M.; Burke, A.R.; Hall, B.C.; Fratini, A.; McLean, D.G.; Fleitz, P.A.; Cooper, T.M.; Drobizhev, M.; Makarov, N.S.; Rebane, A.; Kim, K.-Y.; Farley, R.; Schanze, K.S. Inorg. Chem. 2007, 46, 6483.

Sol-gel glasses doped with platinum(II) acetylides have previously been shown to possess high optical quality and act as broadband nonlinear absorbers for short pulsed lasers[1]. But there is also an increasing need for nonlinear absorbing materials for CW lasers. AuNP:s in polycarbonate films has previously been shown to possess this quality[2]. AuNP:s has also been shown to enhance the nonlinear absorption of platinum(II) acetylides for nanosecond laser pulses[3].

Sol-gel glasses with smaller pore sizes has less oxygen in the pores which can quench a doped dye giving long-lived triplet excited states[1]. It is expected that smaller pore size glasses are more effective against CW-lasers.

Different sol-gel glasses doped with platinum(II) acetylide and co-doped with AuNP:s are investigated for nonlinear absorption of CW lasers. The nonlinear absorption is investigated both statically and time resolved.


[1] Denis Chateau et al., ACS Applied Materials & Interfaces vol. 4 p. 2369, 2012.
[2] Maria Chiara Frare et al., Proc. of SPIE vol. 8901 890113, 2013">8901 890113, 2013.
[3] To Be Published

We describe a series of octahedral iridium(III) complexes in which strong spin-orbit coupling induced by the iridium ion enables what we believe are both singlet and triplet metal-to-ligand and ligand-to-ligand charge transfer (^1,3MLCT/^1,3LLCT) transitions from the ground state. These give rise to a weak absorption tail between 500 and 750 nm which displays a slight “bump” in range 600-650 nm (in dichloromethane solution). The bump serves to broaden into the red the region over which single-photon absorption is sufficiently strong to populate the excited states, effectively extending the spectral range over which the complexes exhibit reverse saturable absorption (RSA). We discuss the design of the new chromophores, present their ground- and excited-state absorption spectra, report their photophysical parameters as determined from nanosecond and femtosecond transient difference absorption measurements and nanosecond and picosecond open-aperture Z scans, and employ these results to infer structure-property relationships for the new materials.

Octahedral d⁶ Ru(II) polypyridyl complexes have been extensively investigated for their applications in the field of dye-sensitized solar cells, photo catalysis, sensing, displays, and bio-technology. Their third-order nonlinear susceptibilities have been reported as well. However, very limited reports were focused on the nonlinear absorption of the ruthenium complexes. In an effort to develop long-lived broadband reverse saturable absorbers (RSA), we designed and synthesized a series of Ru(II) complexes containing 3,8-bis(benzothiazolylfluorenyl)-1,10-phenanthroline and different diimine ligands with varied degree of p-conjugation.
The absorption spectra of these complexes feature intense ligand-localized ¹p,p* transitions below 350 nm, a structureless ¹ILCT (intraligand charge transfer) / ¹p,p* band at ca. 410 nm, and ¹MLCT (metal-to-ligand charge transfer) / ¹LLCT (ligand-to-ligand charge transfer) transitions at ca. 470 nm. Above 600 nm, these complexes exhibit broad, very weak absorption (e < 800 L.mol^-1.cm^-1) tails that probably arise from the spin-forbidden transitions to the triplet excited states. With the increased p-conjugation of the ancillary diimine ligand, the absorption of this tail becomes stronger and red-shifted and extends to almost 900 nm. On the other hand, four of these complexes possess broad positive absorption bands at 440 nm, 507 nm, and 720 nm in their triplet transient absorption (TA) spectra. With the increased p-conjugation of the two identical ancillary ligands, the intensity of the 440 nm band increases, while the intensities of the 507 and 720 nm decreases. In addition, their triplet lifetimes decrease from ~2 ms to ~ 900 ns. In contrast, the TA of the complex with the most p-conjugated benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (dppn) ancillary ligands is drastically different from the other four complexes, with two positive absorption bands at 350-410 nm and 450-650 nm, and a very long triplet lifetime of ~ 41 ms. These features suggest that the excited state giving rise to the observed TA spectrum is the dppn ligand-localized ³p,p* state for this complex, rather than the predominant charge transfer triplet excited states for the other four complexes. The broad and very weak ground-state absorption between 500 and 900 nm, and the broad and long-lived triplet excited-state absorption in this spectral region make these complexes very interesting broadband reverse saturable absorbers. The RSA of these complexes at 532 nm using ns laser pulses is demonstrated.

Absorption characteristics of materials generally do not depend on the intensity of incident light. However, an interesting phenomenon of increased light absorption upon an increase in incident optical intensity exists, and is referred to as reverse saturable absorption (RSA). Carbon black suspensions, fullerenes, metallophthalocyanines, nanotubes, metalloporphyrins, π-conjugated polymers, metal and semiconductor nanoparticles, metal nanowires, Pt(II) acetylides, and graphenes have all been reported to exhibit high RSA with fast response under high-power pulsed laser irradiation. Generally, they respond only to large-scale, expensive, intense light sources such as pulsed neodymium-yttrium-aluminum-garnet lasers. If materials with high RSA in response to continuous visible light with a power of less than 100 mWcm^-2 could be realized, the phenomenon would readily occur under irradiation with compact, inexpensive, low power incoherent light sources such as light-emitting diodes (LEDs), and even with solar light.
One promising method to obtain RSA upon exposure to weak continuous incoherent light is the efficient accumulation of an excited state population, and this can be accomplished by the formation of long-lived room temperature (RT) triplet excitons. Recently, we reported that aromatic carbon materials, dispersed as guests in amorphous hydroxyl steroidal host matrices, exhibit very long-lived (>1 s) RT triplet excitons in air, and these are >1000 times longer than the triplet exciton lifetimes of conventional organic materials. In these materials, the number of aromatic guest compounds in their lowest triplet excited state (T₁ state) should increase upon their continuous exposure to a weak excitation source such as LED or solar light because of their very long T₁ lifetime at RT. Host-guest systems with long-lived RT triplet excitons are promising candidates for high RSA under low irradiance.
Here, we present RSA materials activated by weak continuous incoherent light and demonstrate a large optical limiting phenomenon upon weak continuous irradiation in transparent thin films of these materials. To obtain high RSA under low power irradiation two kinds of guest compounds were dispersed in β-estradiol as an amorphous hydroxyl steroidal host matrix. One of the guest compounds is a transition metal complex, which is used as a donor for photosensitization. The other compound is an aromatic species that behaves as an acceptor on which the efficient accumulation of the T₁ state occurs. Transparent thin films of the RSA materials consisting of an optimized combination of the donor and acceptor showed a considerable increase in absorption upon low irradiance. The transmittance of the thin films gradually decreased from 60% to less than 30% because of the high RSA upon irradiation with blue or green continuous light in the intensity range between 1.0 mWcm^-2 and 1.0 Wcm^-2.

Reverse saturable absorption (RSA) is a nonlinear optical phenomenon in which the absorptivity of the material increases with increased incident fluence because of stronger excited-state absorption than the ground-state absorption. An ideal RSA material should have the following characteristics: large ratio of excited-state to ground-state absorption cross section; high quantum yield of excited state; and long-lived excited states. Therefore, understanding the excited-state properties of the RSA material is the key. Recently, octahedral d₆ Ir(III) complexes have attracted extensive interest due to the strong spin–orbit coupling induced by the Ir(III) ion, which enhances the intersystem crossing (ISC) from S1 to T1 states and thus make it feasible to access to the long-lived triplet manifold. We synthesized a series of cationic iridium(III) complexes bearing one N^N ligand and two C^N ligands with different degrees of π-conjugation on different position and direction. We aim to understand the effects of extended π-conjugation of N^N and C^N ligand on the photophysics and nonlinear absorption of the cationic Ir(III) complexes. It is found that extending the π-conjugation of the N^N ligand along the axis of the N^N ligand could lower the N^N ligand localized LUMO, and thus red-shift the charge-transfer absorption band and introduce more metal-to-ligand charge transfer character to the lowest triplet excited state. In contrast, extending the π-conjugation of the N^N ligand along the vertical direction of the ligand has an opposite effect on the ground-state charge-transfer band and the nature of the lowest triplet excited state. On the other hand, extending the π-conjugation of the C^N ligand along the phenyl ring or the pyridine ring has different effects on the ground-state charge transfer band and the triplet excited-state lifetime as well. All these changes influence the RSA of these complexes significantly.

A series of organic optical materials related to iridium complexes containing
b-diketonate ligands have been designed and synthesized with reverse saturable
absorption (RSA) properties. The relationship of molecular structure and their
RSA properties were found by preliminary studies of their photophysical
properties, such as high inter system crossing quantum yield, ratio of
intensity of singlet absorption and triplet absorption obtained through nano
second Z-scan studies. This finding should lead to more efficient synthetic
design of better organic optical materials with RSA properties for many
related applications.

Linear response (LR) time-dependent density functional theory (TDDFT), combined with the fragment molecular orbital (FMO) method is used to predict nonlinear optical properties of room temperature ionic liquids. The calculations were performed on 1-Butyl-3-methylimidazolium hexafluorophosphate. The calculations were made possible by the massively parallel capability of the FMO method. The FMO/LR-TDDFT results are compared with those obtained from full LR-TDDFT calculations

Much attention has recently been paid to the experimental characterization of materials with appreciable
two-photon absorption (TPA) properties. These efforts have been paralleled by computer–
aided design of TPA-active molecules and simulations of their spectra. Our aim is to present a
comprehensive survey of recent developments in simulations of two-photon absorption spectra of
molecules including the effect of environment, with an emphasis on a contribution from our group.
In particular, we will describe a new relationship between the metric of charge transfer excitation
and the two-photon absorption probability, developed to reinforce the rational design of TPA-active
molecules and molecular materials [1]. We will also summarize our recent efforts directed toward
simulations of one– and two–photon absorption of molecules in solution [2, 3] and in heterogeneous
environments [4]. Ramifications of the findings for general band shape modeling, including
nonempirical estimation of inhomogeneous broadening and the role of non–Condon effects, will
also be discussed.

References
[1] N. H. List, R. Zale'sny, N. A. Murugan, J. Kongsted, W. Bartkowiak, H. Ågren, J. Chem.
Theory Comput. 11 (2015) 4182–4188.
[2] R. Zale'sny, N. A. Murugan, F. Gel’mukhanov, Z. Rinkevicius, B. O'smialowski, W.
Bartkowiak, H. Ågren, J. Phys. Chem. A 119 (2015) 5145.
[3] R. Zale'sny, N. A. Murugan, G. Tian, M. Medved’, H. Ågren, J. Phys. Chem. B, submitted.
[4] X. Li, Z. Rinkevicius, H. Ågren, J. Chem. Theory Comput. 10 (2014) 5630–5639.

Co-crystallization is one of the techniques by which unique properties can be achieved through judicious combination of two or more compounds. However, multitude of different combinations and structures are possible while developing novel multicomponent systems. The ability of functional materials to tune properties by various external stimuli (e.g., electric field, stress) opens up the potential for a wide range of applications. In this study we will primarily focus on non-linear optical (NLO) response estimated for charge-transferred (CT), hydrogen or halogen bonded co-crystals. Using density functional theory (DFT) we will present the second order electrical susceptibility (c⁽²⁾) of selected co-crystal systems, and compare the data to the computed static c⁽²⁾ values of Urea and 4-N, N-dimethylamino-4-N-methyl-stilbazolium tosylate (DAST). We will discuss our results on estimating dynamic c⁽²⁾ using computational approach, which allows a direction comparison with experimental c⁽²⁾ values. In addition to the NLO properties, we will present other functionalities achieved in these co-crystals.

This material is based upon work supported by the U. S. Army Research Laboratory and the U. S. Army Research Office under contract/grant number W911NF-13-1-0387.

We present design methods for generating structures, such as organic molecules, inorganic nanowires, and any other quasi-1D structure supporting electron dynamics that have branched charge transfer pathways. The methods are based upon a new observable parameter, phase disruption, a quantifiable and localized disturbance in the shapes of the eigenfunctions near the Fermi level. Phase disruption in a quasi-1D system is the interruption of the slope of an eigenfunction along the principle axis of the structure by an alternate path for charge along one or more short side structures. The side structures, or prongs (wires, side groups), are necessarily located away from the ends of the main structure. Phase disruption is tuned by design to produce large changes in dipole moments while maintaining large spatial overlap for transition moments contributing to the nonlinear optical response. The designs do not require donor-acceptor groups or any specific modulation of the background potential along the main chain or wire. Using quantum graphs in a large-scale, multielectron simulation, we show how these structures achieve intrinsic first and second hyperpolarizabilities approaching the quantum limits. Phase disruption helps us to understand the large gap between theory and experiment for intrinsic nonlinearities, as well as positing new methods for closing the gap in molecular and nanostructured systems.

A pseudo-symmetric two-photon absorbing dye (AF391) containing a central piperazine unit substituted with (benzothiazol-2-yl)-9,9-diethylfluoren-2-yl pendant groups (Figure 1) has been synthesized and characterized. The molecule has a two-photon-absorption cross-section of σ₂ = 672 GM in tetrahydrofuran and shows significant solvatochromism in the excited-state fluorescence spectra. The emission spectra broaden and the maxima bathochromically shift from 411 nm to 524 nm in n-hexane and acetonitrile, respectively. Moreover, the central piperazine moiety serves as a potential chelation site for ions. Addition of copper(I) hexafluorophosphate and zinc(II) triflate in acetonitrile indicate ground-state complexation with a shift in the emission maximum from 524 nm to 489 nm and 487 nm, respectively. Interestingly, the newly formed Cu and Zn complexes are more strongly emissive than the free dye. Finally, addition of p-toluenesulfonic acid in tetrahydrofuran also blue-shifts the emission maximum, but the intensity is quenched. Due to the photophysical changes induced by addition of metal ions and protons, the dye shows promise as a potential sensor.

Organic compounds^[1] and organometallic complexes^[2] with dendritic structures can possess large two-photon absorption (2PA) cross-sections. Organometallic complexes in particular offer a range of advantageous properties such as being easily synthetically modified, being thermally and chemically robust, and containing redox-active metal centers, the last-mentioned being potentially important for optical switching. Systematically-varied octupolar phenyl-cored oligo(phenyleneethynylene)-bridged ruthenium alkynyl complexes have been synthesized via a convergent approach. The nonlinear optical behaviour of these “star-shaped” complexes was explored using the Z-scan technique.^[3] The importance of measuring spectral dependencies of the third-order NLO properties has recently been emphasized,^[4] so the dispersion of the real and imaginary parts of the second hyperpolarizabilities of the title complexes has been established. Strong NLO responses have been observed, with maximal values competitive with those found for higher generation dendrimers,^[5,6] outcomes that are maintained when a variety of different scaling schemes^[7] are applied.



Acknowledgements: We thank the Australian Research Council for support.

References
[1] M. Drobizhev, A. Karotki, A. Rebane, C. W. Spangler, Opt. Lett. 26 (2001) 1081.
[2] M. P. Cifuentes, C. E. Powell, J. P. Morrall, A. M. McDonagh, N. T. Lucas, M. G. Humphrey, M. Samoc, S. Houbrechts, I. Asselberghs, K. Clays, A. Persoons, T. Isoshima, J. Am. Chem. Soc. 128 (2006) 10819.
[3] M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan, E. W. Van Stryland, IEEE J. Quant. Electron. 26 (1990) 760.
[4] C. E. Powell, J. P. Morrall, S. A. Ward, M. P. Cifuentes, E. G. A. Notaras, M. Samoc, M. G. Humphrey, J. Am. Chem. Soc. 126 (2004) 12234.
[5] M. Samoc, J. P. Morrall, G. T. Dalton, M. P. Cifuentes, M. G. Humphrey, Angew. Chem. Int. Ed. 46 (2007) 731.
[6] R. L. Roberts, T. Schwich, T. C. Corkery, M. P. Cifuentes, K. A. Green, J. D. Farmer, P. J. Low, T. B. Marder, M. Samoc, M. G. Humphrey, Adv. Mater. 21 (2009) 2318.
[7] T. Schwich, M. P. Cifuentes, P. A. Gugger, M. Samoc, M. G. Humphrey, Adv. Mater. 23 (2011) 1433.

Abstract
Owing to their fluidity and large birefringence that extends from optical to very long wavelength regime, particularly nematic liquid crystals (NLC) have become a favorite material for incorporation into various photonic structures including photonic crystal inverse opals, channel waveguide and micro-ring resonator, nanostructured plasmonic and metamaterials [1-3]. By virtue of the easy susceptibility of the crystalline alignment to external field (e.g. optical field), and their organic molecular constituents, NLC possess extraordinarily large nonlinear optical responses with wide-ranging temporal characteristics. Nevertheless they possess some inherent limitations. In particular, their polarization selectivity naturally imposes restrictive conditions on practical device configurations, especially in nano- or complex- photonic structures. Also, the need to impose strong boundary surface anchoring to fabricate cells invariably creates an immobile layer that diminishes the overall response such as tunable birefringence and effective optical nonlinearities [1,2].
In this talk, we present a critical review of recent studies of another unique phase of chiral nematic liquid crystals, namely, the so-called Blue-Phase Liquid Crystals (BPLC) which are capable of circumventing some of these nematic limitations, and possess comparable optical nonlinearities to realize many nonlinear optical processes. In particular, pristine BPLC have been utilized in our studies of optical self-limiting effects in BPLC-cored fiber arrays, and MR-dye doped BPLC have been shown to possess large intensity dependent nonlinear index that enable transient and persistent holographic formation. Applications of such dye-doped BPLC to all-optical tuning of resonator modes in micro-ring resonators cladded with BPLC, and coherent optical image processing have also been reported [3-6]. We also report the results of a series of optical wave mixing experiments in BPLC doped with other nano-particles such as fullerene C₆₀ and carbon nanotubes, and also possible enhanced nonlinearity with the application of AC/DC bias field. Using some aggregate-induced-emission (AIE) molecules as gain dopant, we have also observed amplified spontaneous emission and possible lasing action in unbiased BPLC.
1. I. C. Khoo, Physics Report 471, pp. 221-267 [2009].
2. I. C. Khoo, Progress in Quantum Electronics, 38, Issue 2, pp. 77–117 (2014).
3. I. C. Khoo et al Optics Express Vol. 21 Issue 4, 4319-4327 (2013).
4. I. C. Khoo et al, Mole. Cryst. Liq. Cryst. 594:1, 31-41, [2014].
5. I. C. Khoo, Opt. Lett. 40, pp. 60-63 (2015)
6. J. Ptasinski, I. C. Khoo, Y. Fainman, Optics Letts. 39 Pages: 5435-5438 (2014)

Easy and industrially viable nanofabrication techniques that consistently produce well-defined features much smaller than the wavelength of the visible light and that can even facilitate production of features smaller than the diffraction limit are in great demand. In recent years, femtosecond (fs) laser direct writing (LDW) has been touted as an excellent technique owing to its ease of operation and its use for advanced applications.Weare going to present our recent results on fabrication of laser induced sub-wavelength periodic structures (LIPSS) also known as ripples on various materials like diamond, Si, Ge,Ti, Al, and Ag by fs LDW technique. Several theories have been proposed in literature to explain the observed sub-wavelength features and we take a look at some of these explanations and see how they support our results. These sub-wavelength surface patterns form due to coupling of the fs laser with the surface plasmons. This interference causes a periodic intensity variation on the surface and gives rise to LIPSS. Spatial periodicity (Λ) of LIPSS is significantly smaller than laser wavelength (λ ) and two different kinds of periodic gratings have been detected for the range of laser fluences, number of irradiated pulses and surrounding medium. The first one corresponds to periods less than 320nm which was assigned as high spatial frequency LIPSS (HSFL) or deep sub-wavelength ripples where the ratio of Λ to the λ, is lower than 0.4. On the other hand LIPSS with periods between 320 and 800nm, namely low spatial frequency LIPSS (LSFL) or near sub-wavelength ripples appear with Λ/λ ratio between 0.4 and 1.0. These surfaces were coated with Au and applied for surface enhanced Raman scattering (SERS) studies.

When two molecules with different transition energies are strongly coupled electrostatically, they often exhibit quantum beats in an ultrafast transient absorption experiment. These quantum beats have a frequency at the energy difference between the new eigenstates of the dimer molecule. It was previously thought that no quantum beats can be detected when those molecules are uncoupled, unless additional vibronic structure of the molecule is invoked. This is the main essence of 2D spectroscopy: cross peaks, associated with quantum beats, indicate whether optical transitions are near each other, yielding a way to characterize systems structurally, from NMR to the visible. In this talk, I'll explain how in transient absorption experiments, when the probe reaches the single-photon limit, spurious quantum beats emerge, the latter of which are associated with optical transitions in completely non-interacting molecules. This is a purely quantum mechanical effect of the electromagnetic vacuum. Furthermore, this phenomenon scales quadratically as the concentration of molecules in the cuvette, a rather surprising superradiant phenomenon which is often ignored due to the standard experimental conditions at which these experiments are performed. I will speculate on the applications of this observation, namely, that a simple solution of noninteracting molecules which does not exhibit nonlinear optical effects, all of a sudden, can exhibit them by invoking quantum fluctuations of the electromagnetic field and a superradiant effect of having many molecules interact with the same field. We anticipate this as a probe of weak photon intensities.

One of the main challenges in nonlinear plasmonics is to search for novel materials and/or novel nano-/meso-structures that display drastically enhanced nonlinear optical effects. As a surface-plasmon-dictated nonlinear optical process, surface-enhanced hyper-Raman scattering (SEHRS) provides a convenient and application-oriented technique for exploring, studying and optimizing nonlinear plasmonic materials and structures. In this work, we report the fabrication of a series of pattern-controlled Ag mesostructures, including dendrite, fractal and dense-branching morphology (DBM), by an approach that combines the conventional photolithography with various electrochemical deposition techniques. The effects of electrochemical deposition potential, deposition time, concentrations of both the precursor salt and the supporting electrolyte were studied systematically, based on which a morphological “phase” diagram of electrodeposition for hierarchical Ag mesostructures was constructed. We show that the size of three morphological patterns, dendrite, fractal and DBM, can be tuned by controlling the deposition time under potentiostatic condition. The mechanism of morphological selection and transition under non-equilibrium growth conditions was rationalized with the help of interfacial dynamics (anisotropy and instability) and thermodynamics (Nernst-Planck equation). It was found that the growth of dendrite with branch symmetry could be ascribed mainly to the anisotropy and instability introduced under the conditions of low electrical deposition potential, high precursor concentration and low supporting electrolyte concentration, that the growth of diffusion-limited aggregate (DLA) or DLA-like fractal was determined mainly by the precursor diffusion driven by concentration gradient, and that the DBM was controlled mainly by the precursor migration driven by applied electric field under the conditions of high electrical deposition potential, low precursor concentration and high supporting electrolyte concentration. Furthermore, the as-fabricated Ag mesostructures display an ultra-broadband linear plasmonic property, as shown by their UV-vis-NIR extinction spectra and multi-wavelength excited surface-enhanced Raman scattering (SERS). Among three different hierarchical Ag mesostructures, the fractals display an excellent nonlinear plasmonic property, as revealed by the multi-wavelength excited SEHRS.

The current inorganic NLO crystals can be classified to three categories according to their transparent regions: NLO materials in visible region, in ultra-visible (UV) region and in infrared (IR) region. There have been quite a number of excellent NLO crystals in either visible region such as KDP (KH₂PO₄), KTP (TiOPO₄) and LN (LiNbO₃) or UV region such as BBO (b-BaB₂O₄), LBO (LiB₃O₅) and KBBF (KBe₂BO₃F₂). The NLO crystals used in IR region such as AgGaS₂, AgGaSe₂ and ZnGeP₂, however, exhibit very low laser damage threshold (LDT) so that their applications are badly limited. To search for new NLO materials in IR region with high LDT has become one of the great challenges in the area of nonlinear optical materials.

It is believed that the narrow band gaps of the current IR NLO crystals are the inherent reason for their low LDT. So we have pursued the research work to search for new IR NLO crystals from insulating halides. In this talk we shall summarizes our research effort and progress in this issue.

To understand the relationship between composition, crystal structure and various properties in halides, we systematically studied how the comprehensive properties of the halides can be influenced by variety of compositions in the halides, including the influence of four different halogen anions (F, Cl, Br and I), existence or absence of A-site cation, selection of the central metallic cations, and the simple or mixed halides. Based on the knowledge learned from the above study, we have done the rational molecular design, synthesis, crystal structure study and characterization of various properties as well as preliminary crystal growth. As the results, we have found several new IR NLO materials that exhibit high LDT and other good comprehensive performances, such as strong NLO effect, wide transparent window in IR region, high environmental stability and good crystal growth habit. These materials include NaSb₃F₁₀, b-HgBrCl, CsHgBr₃, K₂SbF₂Cl₃, Cs₂HgCl₂I₂, Rb₂CdBr₂I₂ among the others.

Acknowledgment: This work was supported by the National Key Fundamental Research Program of China and the Natural Science Foundation of China.

We report on the second harmonic generation of green light through a three-dimensional (3D) nonlinear waveguide beam splitter, which has been fabricated by direct femtosecond laser writing of a KTP crystal. The laser pulses induce damage tracks with reduced refractive index. Periodically arrayed laser-induced tracks serve as depressed cladding structure, which can be used for waveguiding of light. Through such structure, the light propagation can be engineered via the track-confined refractive index profiles, achieving tailored 3D output beam distributions. With a certain designs, the photonic-lattice-like structure can be used for beam splitting, which contains a guiding core surrounded by a number of laser-induced tracks with negative index modifications. In this work, we apply this technique in a nonlinear KTP crystal to implement 1x4 3D beam splitting. Two different kinds of splitters are designed with the input cross sectional dimension of 50x45 and 30x30μm², respectively. The structures show excellent guiding properties at TE and TM polarization for both visible and infrared bands. With Type II phase matching of the fundamental wavelength (@1064 nm) to second harmonic waves (@ 532 nm), the frequency doubling has been achieved through this three-dimensional beam splitter. Under continuous-wave pump, the conversion efficiency of guided-wave second harmonic generation at 532 nm is up to ~ 14%/W.

Since the invention of the laser, nonlinear optics has become a popular and efficient routine for expanding the frequency window of lasers from ultraviolet to visible, infrared, and terahertz bands, and for generating broadband coherent light sources and ultrafast pulse lasers. However, it’s still difficult to realize high-harmonic generation (HHG) in a single crystal, because many nonlinear up-conversion processes must be simultaneously adopted with phase matching. Instead, a chain of two or more individual nonlinear crystals with cascaded second-harmoni generation (SHG) and sum-frequency generation (SFG) is used, but the experimental setup is complicated and the conversion efficiency is low.
In recent years, our group makes many efforts to accomplish high-efficiency frequency conversion by quasi-phase-matching (QPM) technology. We have realized multi-direction high-efficiency second harmonic generation ^[1] and simultaneous broadband generation of second and third harmonics ^[2] by nonlinear photonic crystals.
Very recently, our group realized simultaneous 5th–8th harmonic generation (HG) from a single chirped periodic poled lithium niobate (CPPLN) nonlinear crystal ^[3]. The CPPLN crystal offers a series of broad QPM bands with a considerably large effective nonlinear susceptibility χ_eff and freely designated spectral position to support SHG and SFG in the different wavelength bands that are necessary for various-order HHG. Upon illumination of a mid-IR femtosecond pulse laser, we have observed the generation of an ultrabroadband visible white light beam corresponding to 5th–8th HG with a record high conversion efficiency of 18%.
The CPPLN nonlinear crystal with HHG opens up a new avenue for greatly expanding the power to engineer high-order nonlinear interactions in solid state materials and can find application in supercontinuum generation, ultrafast lasers, frequency combs, large-scale laser displays, and short-wavelength laser sources.

Reference
1. Bao-Qin Chen, Chao Zhang, Rong-Juan Liu, and Zhi-Yuan Li, “Multi-direction high-efficiency second harmonic generation in ellipse structure nonlinear photonic crystals”, Appl. Phys. Lett. 105, 151106 (2014).
2. Bao-Qin Chen, Ming-Liang Ren, Rong-Juan Liu, Yan Sheng, Bo-Qin Ma, Chao Zhang, and Zhi-Yuan Li*, “Simultaneous Broadband Second and Third Harmonic Generation in Chirped Nonlinear Photonic Crystal”, Light: Science & Application 3, e189 (1-6) (2014)
3. Bao-Qin Chen, Chao Zhang, Chen-Yang Hu, Rong-Juan Liu, and Zhi-Yuan Li*, “High-Efficiency Broadband High-Harmonic Generation from a Single Quasi-Phase-Matching Nonlinear Crystal”, Phys. Rev. Lett. 115, 083502 (2015).

We have experimentally determined two-photon absorption, 2PA, spectra and the nonlinear refractive dispersion of several organic molecules. By approximating the molecules as centrosymmetric and using a simplified essential state model (quasi 3-level model), we are able to predict the nonlinear refraction from the experimentally determined 2PA spectra and find good agreement. This approach also allows predictions of the figure-of-merit (FOM) defined as the ratio of nonlinear refractive phase shift to the 2PA fractional loss. This ratio determines the usefulness of such molecules for photonic device applications. The predicted FOMs also agree well with the model; however, care needs to be taken for the inclusion of the so-called N-term or virtual saturation.

Nonlinear absorption and refraction, particularly excited state induced absorption are important optical processes in phthalocyanines and porphyrins.The strong excited state absorption found in these materials arises from their rigid hetero-aromatic ring structure.These structures most often contain differing central metals, some of which also incorporate axial substituents, both of which can highly influence the electronic properties of the macrocycles. In addition, the periphery of the ring structure can be modified to promote desirable physical properties such as high solubility in both commons solvents and in polymers, as well as control of the degree of aggregation and even the physical phase of the material.

Measurements of the photophysical mechanisms and properties of phthalocyanines incorporated into polymers at high concentration, in pure liquid phthalocyanines and supramolecular porphyrins were carried out using ultrafast transient absorption spectroscopy.From these measurements the nonlinear absorption cross section spectra and excited state lifetimes were deduced. fs Z-scan measurements of some of these samples were also carried out to establish their two-photon absorption coefficients. Nonlinear transmission measurements were performed using an f/6.5 tunable nanosecond laser system.

After establishing the photophysical mechanisms, several of these materials were incorporated into devices which enhance the nonlinear optical properties. One of the devices utilizes nanolayered polymers whose layers thicknesses are controlled to yield a bandgap in the visible. The nanostructure enhances the nonlinear performance through induced excited state refraction.A second device incorporated the materials into a waveguides and waveguide arrays. The nonlinear optical performance is enhanced by increasing the interaction length. In supramolecular porphyrins, this leads to two-photon pumped excited state absorption in the NIR.

Characterization of properties of new potential nonlinear optical materials needs to be carried out using low repetition rate femtosecond laser pulses. The Z-scan technique is capable of providing wide spectral range information on nonlinear refraction and on two-photon and multi-photon (n>2) absorption cross sections as well as on processes of absorption saturation, however, obtaining such data is often tedious. We have been examining features of a modification of that technique, called f-scan, where an electrically focus-tunable lens is used to rapidly scan a sample instead of moving it along the beam path. The obvious advantage is reduction of the duration of a single scan (from minutes, in the case of a typical Z-scan setup, to a few seconds), which makes the technique suitable for studying materials with relatively poor stability. On the other hand, the need for realignment of the system after each wavelength change is still a major obstacle for speeding up the process of obtaining nonlinear absorption and refraction spectra.
The talk will present several examples of the use of f-scan icluding those of organic molecules, plasmonic nanoparticles and graphene. We find that analysis of the results obtained on some materials often needs to involve assumptions of competition of absorption saturation processes with those of multiphoton absorption and excited state absorption. Those various nonlinear absorption contributions may also lead to clearly seen presence of higher-order nonlinear refraction effects.

We present the experimental evidence for a degenerate frequency two beam coupling (TBC) in two photon absorbing (2PA) organic solutions. It has been well established that the two critical requirements for TBC are a nonlinear refractive index with a finite lifetime and that the interacting fields must have non-degenerate frequencies. However, degenerate frequency coupling has been shown for fields containing a time-dependent phase, i.e. a frequency chirp. This chirp can either be intrinsic to the fields or induced by self- and cross- phase modulation (S/XPM). For nanosecond pulses, the relatively small intrinsic chirp of the fields is negligible compared to the strong cumulative effects of population redistribution which generates large S/XPM. A S/XPM-mediated theoretical treatment for degenerate frequency TBC is presented along with numerical simulations using known nonlinear optical parameters to model the experimental results.

In recent years great interest has been shown in nonlinear optical materials due to their use in applications such as frequency conversion, ultrafast lasers, and sensor protection [1]. A low power continuous laser does not have enough power to induce instantaneous electronic nonlinearities in a material, but it is often sufficient to generate thermal nonlinearities with response times of a few milliseconds. Thermal effects in absorbing materials such as noble metal colloids, porphyrins and dyes are known to result in substantial refractive nonlinearities when the medium is exposed to a few milliwatts of laser power. In the present work, by using a low power cw He-Ne laser beam (0.3 to 3.5 mW, 632.8 nm), we have measured the thermo-optical refractive nonlinearity of Styryl 8 (a laser dye) and Copper Sulphate (CuSO₄) in different solvents, employing the closed aperture Z-scan technique. Beam diffraction due to spatial self-phase modulation could be observed at relatively higher powers (3.5 - 4.0 mW) in the far field while translating the sample through the focal point of the focused Gaussian beam. The Z-scan data has been analyzed using both the Sheik-Bahae formalism and the Thermal lens model. From the goodness of fit with the experimental data, the limits of validity for these models are determined. While Sheik-Bahae formalism agrees with the transmittance data only for lower values of the on-axis phase shift, thermal lens model fits well with all experimental data measured below the onset power for self-phase modulation ring pattern formation. The nonlinear optical parameters calculated from numerical fits to the measured data show that Styryl 8 and CuSO₄ are potentially useful for designing optical power limiters for the low power excitation regime.
References
[1] R.L. Sutherland, Handbook of Nonlinear optics, Marcel Dekker, New York (1996)