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We performed a detailed experimental investigation and quantum-chemical analysis of a new series of near IR polymethine dyes with 5-butyl-7,8-dihydrobenzo[ cd ]furo[2,3- f ]indolium terminal groups. We also synthesized and studied two neutral dyes, squaraine and tetraone, with the same terminal groups and performed a comparison of the spectroscopic properties of this set of “near IR” dyes (polymethine, squaraine, and tetraone) with an analogous set of “visible” dyes with simpler benzo[ e ]indolium terminal groups. From these measurements, we find that the dyes with dihydrobenzo[ cd ]furo[2,3- f ]indolium terminal groups are characterized by a remarkably large shift ≈300 nm (≈200 nm for tetraone) of their absorption bands towards the red region. We discuss the difference in electronic structure for these molecules and show that the “near IR” dyes are characterized by an additional weak fluorescence band from the higher lying excited states connected with the terminal groups. Absorption spectra for the longest polymethines are solvent-dependent and are characterized by a broadening of the main band in polar solvents, which is explained by ground state symmetry breaking and reduced charge delocalization within the polymethine chromophore. The results of these experiments combined with the agreement of quantum chemical calculations moves us closer to a predictive capability for structure–property relations in cyanine-like molecules.

We performed a detailed experimental investigation and quantum-chemical analysis of linear, excited state absorption (ESA) spectra, and lifetime dynamics of a new series of near IR symmetrical cationic polymethine dyes, with 5-butyl-7,8-dihydrobenzo[cd]furo[2,3-f]indolium terminal groups. Additionally, two neutral dyes, squaraine and tetraone, were synthesized with the same terminal groups. We compare the nonlinear absorption properties of these three dyes with an analogous set of three “visible” dyes with simpler benzo[e]indolium terminal groups. Measurements are performed using laser systems with femtosecond and picosecond pulsewidths. We find that the dyes with dihydrobenzo[cd]furo[2,3-f]indolium terminal groups are characterized by a remarkably large red shift (200–300 nm) of their linear and excited state absorption bands. The absorption bands for these dyes can now be shifted systematically to the near IR region by a contribution from their terminal group that strongly increases the total length of conjugation. ESA spectra in the “near IR” set of molecules have strong absorption bands with excited state absorption cross sections increasing from 2 × 10−16 cm2 for the shortest polymethine chain (trimethine cyanine) to 9 × 10−16 cm2 for the longer chain (heptamethine cyanine). The peak ESA magnitude for heptamethine cyanine is approximately twice as large compared to its peak ground state absorption, but the oscillator strengths of the two bands are almost equal. This observation indicates that the ESA oscillator strengths can be as large as those of ground state transitions. The results of these experiments combined with agreement of the quantum-chemical calculations move us closer to a predictive capability for molecular structure–nonlinear properties relations of cyanine-like molecules.

We performed a detailed experimental investigation and quantum-chemical analysis of two-photon absorption (2PA) spectra of symmetrical cationic polymethine and neutral squaraine and tetraone dyes having nearly identical terminal groups and similar conjugation length. Squaraine and tetraone structures differ from polymethines in that their conjugated backbone includes an electron accepting bridge. Frequency degenerate 2PA spectra of these molecules are measured by two-photon fluorescence spectroscopy and the Z-scan technique. Comparing 2PA spectra of polymethine with squaraine and tetraone structures, we find that we can access considerably larger 2PA cross-sections (P8000 � 10 � 50 cm 4 s/photon) in the squaraine and tetraone molecules due to an increase in the density of final states introduced by the electron accepting bridge in the conjugated chain. We discuss the difference in electronic structure for these molecules and introduce a link between 2PA and excited-state absorption spectra. The results of these experiments combined with agreement of quantum-chemical calculations moves us closer to a predictive capability for the relation between molecular structure and nonlinear optical properties.

The diketones 4,4′-di[3,4-bis(n-dodecyloxy)styryl]benzil and 4,4′-di[3,4-bis(n-dodecyloxy)phenylethynyl]benzil have been synthesized from 4,4′-dibromobenzil using Heck coupling with the appropriate styrene and Sonogashira coupling with the appropriate phenylalkyne, respectively. Treatment of the distyrylbenzil derivative with phosphorus pentasulfide followed by nickel(II) chloride gave the corresponding extended nickel bis(dithiolene) derivative which shows strong near-infrared two-photon absorption with a cross-section of over 440 GM (1 GM = 10−50 cm4 s photon−1 molecule−1) throughout the telecommunications range and a peak cross-section of 5300 GM at ca. 1.2 μm.

A polarization-resolved beam deflection technique is used to separate the bound-electronic and molecular rotational components of nonlinear refractive transients of molecular gases. Coherent rotational revivals from N(2), O(2), and two isotopologues of carbon disulfide (CS(2)), are identified in gaseous mixtures. Dephasing rates, rotational and centrifugal distortion constants of each species are measured. Polarization at the magic angle allows unambiguous measurement of the bound-electronic nonlinear refractive index of air and second hyperpolarizability of CS(2). Agreement between gas and liquid phase second hyperpolarizability measurements is found using the Lorentz-Lorenz local field correction.

Organic π-conjugated materials have been widely used for a variety of nonlinear optical (NLO) applications. Molecules with negative real components Re(γ) of the third-order polarizability, which leads to nonlinear refraction in macroscopic systems, have important benefits for several NLO applications. However, few organic systems studied to date have negative Re(γ) in the long wavelength limit, and all inorganic materials show positive nonlinear refraction in this limit. Here, we introduce a new class of molecules of the form X(C6H5)4, where X = B(-), C, N(+), and P(+), that have negative Re(γ). The molecular mechanism for the NLO properties in these systems is very different from those in typical linear conjugated systems: These systems have a band of excited states involving single-electron excitations within the π-system, several of which have significant coupling to the ground state. Thus, Re(γ) cannot be understood in terms of a simplified essential-state model and must be analyzed in the context of the full sum-over-states expression. Although Re(γ) is significantly smaller than that of other commonly studied NLO chromophores, the introduction of a new molecular architecture offering the potential for a negative Re(γ) introduces new avenues of molecular design for NLO applications.

We present data indicating two-photon gain using extremely nondegenerate (END) photons in optically excited GaAs. These results are consistent with our demonstration of END two-photon absorption enhancement and points a possible way to two-photon lasing.

We present a theoretical study of extremely nondegenerate two-photon absorption in direct-gap quantum well semiconductors. We predict large enhancement in nondegenerate two-photon absorption for TM-TM polarized light over bulk semiconductors.

Nanophotonic devices offer an unprecedented ability to concentrate light into small volumes which can greatly increase nonlinear effects. However, traditional plasmonic materials suffer from low damage thresholds and are not compatible with standard semiconductor technology. Here we study the nonlinear optical properties in the novel refractory plasmonic material titanium nitride using the Z-scan method at 1550 nm and 780 nm. We compare the extracted nonlinear parameters for TiN with previous works on noble metals and note a similarly large nonlinear optical response. However, TiN films have been shown to exhibit a damage threshold up to an order of magnitude higher than gold films of a similar thickness, while also being robust, cost-efficient, bio- and CMOS-compatible. Together, these properties make TiN a promising material for metal-based nonlinear optics.

We present theoretical calculations and experimental measurements of the nondegenerate three-photon absorption coefficient of GaAs, which is enhanced by >3 orders of magnitude over the degenerate case.

We use the beam-deflection method to measure nondegenerate nonlinear refractive indices of ZnO and ZnSe and show, in agreement with theory, extremely nondegenerate nonlinear refraction is significantly larger than in the degenerate or near-degenerate case.

We use nonlinear beam deflection to separate the bound-electronic and nuclear-rotational responses in gas-phase CS2. This allows a direct comparison of the second hyperpolarizability to our liquid CS2 measurements considering local-field effects.

Transparent ceramics gain much attention as an alternative medium for high power ultra-short lasers because of its superior thermal properties over single crystals. Measurement of nonlinear refractive index is essential to understand the limit of such material for ultra-short laser generation. Dual arm Z-scan technique was employed to measure n2 for single crystal and ceramic at the same time to avoid any uncertainty due to different measuring times.

We measure the temporal dynamics of nonlinear absorption (NLA) and refraction (NLR) in crystalline and hydrogenated-amorphous silicon at 1550 nm. The effect of relative polarization on NLA and NLR are also reported.

We demonstrate a scanning 3-D IR imaging technique using extremely nondegenerate two-photon absorption in an uncooled GaN photodiode, and obtain ∼2 μm depth resolution at a wavelength of ∼5 μm in buried semiconductor structures.

Unsymmetrical fluorenyl-based chromophores, flanked on either side by electron-acceptor (A) and/or electron-donating (D) groups, separated by conjugated (π) moieties, were synthesized. Heck coupling and Wittig reactions were utilized to prepare target fluorene derivatives that possess very high solubility in organic solvents, high thermal stability (up to 395 °C), and generally high fluorescence quantum yields (0.6−0.9). The systematic alteration of the structural design allowed for investigation of numerous factors that affect the optical nonlinearity, as well as probing effects of molecular symmetry, solvent polarity, strengths of electron donating and/or withdrawing end groups, and π-conjugation length. A detailed spectroscopic study of these molecules, including absorption, fluorescence emission, excitation, and excitation anisotropy, was conducted. Two-photon absorption (2PA) cross sections (δ) were determined by a nonlinear transmission method employing a femtosecond white-light continuum (WLC) pump...

Novel alkene and alkyne branched structures have been synthesized, and their two-photon absorption (2PA) properties are reported. This series of alkene and alkyne trimer systems tests the mechanistic approach for enhancing the 2PA process which is usually dictated by the pi-bridging, delocalization length, and corresponding charge transfer on the 2PA cross sections. The results suggest that alkene branched systems have higher 2PA cross sections. While steady-state absorption and emission measurements were not successful in predicting the observed trend of 2PA cross sections, time-resolved measurements have explained the trends observed. It was found that, upon photoexcitation, there is an ultrafast charge localization to an intramolecular charge-transfer (ICT) state, followed by the presence of a solvent and conformationally relaxed ICT state in these branched systems.

One- and two-photon anisotropy spectra of a series of symmetrical and asymmetrical polymethine (PD) and fluorene molecules were measured experimentally and discussed theoretically within the framework of three-state and four-state models. For all the molecules discussed in this paper, the experimental two-photon anisotropy values, r2PA, lie in the relatively narrow range from 0.47 to 0.57 and remain almost independent of wavelength over at least two electronic transitions. This is in contrast with their one-photon anisotropy, which shows strong wavelength dependence, typically varying from � 0 to 0.38 over the same transitions. A detailed analysis of the twophoton absorption (2PA) processes allows us to conclude that a three-state model can explain the 2PA anisotropy spectra of most asymmetrical PDs and fluorenes. However, this model is inadequate for all the symmetrical molecules. Experimental values of r2PA for symmetrical polymethines and fluorenes can be explained by symmetry breaking leading to the deviation of the orientation of the participating transition dipole moments from their ‘‘classical’’ orientations. � 2005 Elsevier B.V. All rights reserved.

The optical absorption properties of biological protein bacteriorhodopsin (bR) under the illumination of different intensity and pH environment were investigated. It is observed that the wild type bR film offers more than 30% effective UV protection. The UV protection effect can be further enhanced to 90% in some specific UV region by genetically or chemically modifying bR molecules or by the illumination of visible wavelengths ranging from green to red wavelengths. Preliminary results indicate that bR could be a promising material for UV protection.

Two-photon absorbing polymers with alternating fluorenyl units as a p-conjugated bridge and bisbenzothiazole or thiazolothiazole as electron-acceptors were synthesized and characterized by IR, elemental analysis, absorption and fluorescence spectroscopy, and thermogravimetric analysis (TGA). Improvement on the solubility was observed in comparison with their phenyl based analogies. The linear absorption and emission spectra were recorded and high fluorescence quantum yields were found. Relatively large two-photon absorption cross-section per repeat unit (420GM) were measured under fs-pulsed laser excitation. Copyright # 2005 John Wiley & Sons, Ltd.

The determination of the spectral position of the excited states and orientation of the transition dipole moments of polymethine molecules is experimentally measured using two methods: the steady-state fluorescence anisotropy method, and a two-color polarization-resolved pump–probe method. This novel use of the pump–probe method is described in detail and a comparison to the fluorescence method is given. Quantum-chemical modeling on the effects of the bridge structure in the polymethine chromophore on the linear absorption spectrum is also discussed.

A detailed experimental investigation and quantum-chemical analysis of symmetric cyanines of different conjugation lengths have been performed with the goal of understanding the nature of the electronic transitions in molecules that possess a long chromophore. The nature of electronic transitions in cyanines with a relatively short chromophore (inside the cyanine limit) has already been investigated and many properties of these molecules are well understood. However, little is known about the nature of the transitions beyond the cyanine limit. Their unusual properties, which were proposed by Tolbert and Zhao to be connected with symmetry breaking, still remain unexplored. The analysis of the spectral data in various solvents and results of femtosecond pump– probe saturable absorption measurements enable us to conclude that an increase in the length of the chain leads to a symmetry breaking and the appearance of two forms with symmetrical and asymmetrical distributions of the charge density in the ground state. For thiacarbocyanines, symmetry breaking is predicted and observed for a pentacarbocyanine dye. Quantum-chemical calculations provide additional proof of this hypothesis. The excited-state absorption properties of a pentacarbocyanine in the visible region are also reported. For the first time we have observed an excited-state cross-section that is larger (� 3·) than the ground state cross-section at the peak spectral position. � 2004 Elsevier B.V. All rights reserved.

The synthesis of a series of four new compounds containing fluorenyl chromophores is presented, along with the results of spectroscopic and photochemical studies aimed at understanding the two-photon absorption properties and energetics of their electronically excited states. The molecular structures of the compounds were systematically varied to allow comparison of molecules possessing high and low molecular symmetry, short and long alkyl chains, and a fluorenyl conjugated π-system. Solvent-dependent absorption and emission were investigated along with π-conjugation length. Preliminary measurements of two-photon absorption (2PA) using a two-photon fluorescence method indicate that these chromophores exhibit high two-photon absorptivity. A symmetrical molecule (3), possessing a relatively large π-conjugated system, flanked on either side by electron-withdrawing groups (benzothiazole), exhibited a peak 2PA cross section (δ) of 6000 × 10-50 cm4 s photon-1 molecule-1 at 600 nm. Excitation anisotropy studies ...

A series of linear, symmetrical, diphenylaminofluorene-based materials are reported. The series investigated was model 9,9-didecyl-2,7-bis(N,N-diphenylamino)fluorene (1), oligomer 9,9-didecyl-N,N-bis(9,9-didecyl-7-N,N-diphenylaminofluoren-2-yl)-N,N-diphenyl-fluorene-2,7-diamine (2), and poly(9,9-didecyl-2,7-diphenylaminofluorene) (3). Structural characterization and photophysical properties, including linear absorption, quantum yields, single photon fluorescence, and two-photon absorption (2PA) spectra, were studied in polar and nonpolar solvents. 2PA spectra were determined by two independent methods: a nonlinear transmission method employing a femtosecond white-light continuum (WLC) and a two-photon fluorescence (2PF) method. Polymer 3, a low-molecular-weight polymer with fourteen fluorene units, exhibited a very large two-photon absorption cross-section of 17 200 and 6800 × 10-50 cm4 s photon-1 molecule-1 as estimated from the WLC and 2PF methods, respectively.

Recent interests in and applications of two-photon absorption (2PA) induced photopolymerization have afforded advanced opportunities to perform three-dimensionally resolved polymerization, resulting in intricate microfabrication and imaging. Many of the reported 2PA-induced polymerizations make use of commercially available photoinitiators, and a key parameter to consider is the two-photon absorption cross-section (δ) of the initiator. To date, there has been no comprehensive investigation of two-photon absorptivity of commercial photoinitiators, though a few studies presenting novel photoinitiators for two-photon polymerization have appeared. Herein, we report the 2PA properties of common, commercially available photoinitiators typically utilized in conventional radiation curing science and technologies, and often used in 2PA-based polymerizations. Z-scan and white-light continuum (WLC) pump–probe techniques were utilized to obtain two-photon absorption cross-sections ( δ). The results for most compounds were found to yield good agreement between the two methods. Most of the photoinitiators studied possess low δ, except Irgacure OXE01, indicating a need for the development of new photoinitiators with improved properties optimized for 2PA applications. A compound prepared in our laboratories exhibits high 2PA and was useful as a two-photon free-radical photoinitiator. © 2004 Elsevier B.V. All rights reserved.

Polarization-resolved excitation-probe measurements are performed for a new series of polymethine dyes in several solvents and a polyurethane acrylate elastopolymer. We describe our experimental studies and give an analysis of the nature of the rotational motions of excited molecules and orientation of the excited-state transitions relative to transitions from the ground state.

We present an experimental technique along with the method of data analysis to give nondegenerate two-photon absorption (2PA) spectra. We use a femtosecond pump pulse and a white-light continuum (WLC) probe to rapidly generate the 2PA spectra of a variety of materials. In order to analyze data taken with this method, the spectral and temporal characteristics of the WLC must be known, along with the linear dispersion of the sample. This allows determination of the temporal walk-off of the pump and probe pulses as a function of frequency caused by group-velocity mismatch. Data correction can then be performed to obtain the nonlinear losses. We derive an analytical formula for the normalized nonlinear transmittance that is valid under quite general experimental parameters. We verify this on ZnS and use it for the determination of 2PA spectra of some organic compounds in solution. We also compare some of the data on organics with two-photon fluorescence data and find good agreement.

We report the two-photon-induced photoisomerization of 3-[1-(1,2-dimethyl-1H-indol-3yl)-ethylidene]-4-isopropylidene-dihydrofuran-2,5-dione (1), a photochromic compound with Imax ) 385 nm, using 775-nm femtosecond pulsed laser irradiation. The resulting photoisomer had Imax ) 582 nm. The kinetic rate constant for the two-photon-induced electrocyclic isomerization reaction was measured at two different intensities (two different powers), showing a quadratic dependence with respect to the pump intensity. Results of pump-probe solution phase experiments and guest/host polymer thin film interferometric imaging studies are reported. A two-photon absorption molecular cross section U2 ) 10.3 10 -48 cm 4 ‚s/photon was measured using Z-scan, further supporting a two-photon-induced isomerization process. Two-photon-induced interferometric recording in a fulgide-containing polymer film was demonstrated.

There is much interest in enhancement of the absorbance performance of nonlinear absorber solid-state filters. In this work we present an advanced reversible nonlinear filter based on a dye-doped sol-gel matrix. The absorbance enhancement was achieved by using a combination of two absorption mechanisms in the same molecule; a photochromic absorption which is induced by 2-photon absorption (2PA). The 2PA serves as the trigger for initiating the photochromism through Forster-resonance-energy-transfer (FRET) between the fluorescent donor and the photochromic acceptor. We synthesized a new bifunctional-chromophore that incorporated a carbazole-derived 2PA fluorescent donor and a chromene-derived photochromic acceptor, covalently linked together in a single molecule by a ~6 A carboxyl group or oxygen bridge. The bifunctional-chromophore was doped in an inorganic-organic hybrid matrix prepared by the fast-sol-gel process. These materials solidify without shrinkage or formation of cracks and present promising properties as optical matrices for smart filters. The dye-doped sol-gel disc presents high transparency in the visible region ("colorless"), which under UV-irradiation (one-photon absorption in the photochromic part of the molecule), transforms into a strongly absorbing filter ("dark colored"), due to the conversion of the photochromic moiety to its "open" absorbing form. We have demonstrated that this ring-opening can also be induced by visible-light (620 nm) using the 2PA carbazole-derived moiety of the molecule. We have studied the fabrication routes and optical performance of these filters. We present studies of the 2PA mechanism of the carbazole derivative, FRET efficiency of the combined-molecule as well as in solutions of the individual moieties, and reversible dynamics of the photochromic moiety.

We are working on utilizing the 103 enhancement observed for no degenerate two-photon absorption in semiconductors to obtain two-photon gain in inverted media. Experiments along with analysis for GaAs show potential for gain greater than losses.

We observe large enhancement of nondegenerate two-photon absorption compared to the degenerate case. We use this to sensitively detect and image mid-IR light with simple uncooled photodiodes. The enhancement also applies to 2-photon emission and gain.

All optical switching (AOS) applications require materials with a large nonlinear refractive index (n2) but relatively small linear and nonlinear absorption loss. The figure-of-merit (FOM), defined as the ratio between the real and imaginary parts of the second hyperpolarizability (γ), is widely used to evaluate the operating efficiency of AOS materials. By using an essential-state model, we describe the general dispersion behavior of γ of symmetric organic molecules and predict that the optimized wavelength range for a large FOM is near its linear absorption edge for cyanine-like dyes. Experimental studies are normally performed on organic solutes in solution which becomes problematic when the solvent nonlinearity dominates the total signal. This has been overcome using a Dual-arm Z-scan methodology to measure the solution and solvent simultaneously on two identical Z-scan arms and discriminating their small nonlinear signal difference. This technique significantly reduces the measurement uncertainty by correlating the excitation noise in both arms, leading to nearly an order-of-magnitude increase in sensitivity. Here we investigate the n2 and two-photon absorption (2PA) spectra of several classes of cyanine-like organic molecules and find that the results for most molecules agree qualitatively and quantitatively with the essential-state model. Many cyanine-like molecules show a relatively small FOM due to the presence of large 2PA bands near the linear absorption edge; however, an exception is found for a thiopyrylium polymethine molecule of which the maximum FOM can be < 400, making it an excellent candidate for AOS.

FRET within a donor-acceptor composite photochromic molecule was investigated in a dye-doped sol-gel matrix using both 1 and 2-Photon absorption. The energy transfer efficiency was estimated from UV-excitation photokinetic measurements by applying a 2-level model.

We apply the beam deflection technique to measure the transient nonlinearity of air well below filamentation threshold with unprecedented sensitivity at 1atm. Rotational contributions of N2 and O2 were resolved over 300 ps of delay.

Nonlinear refractive dynamics of CS2 are measured with the beam deflection technique. A response function model is fit, from which the pulse width dependent n2,eff is calculated and compared to Z-scan measurements.

Time-resolved excitation-probe polarization measurements are performed for polymethine and squarylium dyes in ethanol and an elastopolymer of polyurethane acrylate (PUA). These molecules exhibit strong excited-state absorption in the visible, which results in reverse saturable absorption (RSA). In pump–probe experiments, we observe a strong angular dependence of the RSA decay kinetics upon variation of the angle between pump and probe polarizations. The difference in absorption anisotropy kinetics in ethanol and PUA is detected and analyzed. Anisotropy decay curves in ethanol follow a single exponential decay leading to complete depolarization of the excited state. We also observe complete depolarization in PUA, in which case the anisotropy decay follows a double exponential behavior. Possible rotations in the PUA polymeric matrix are connected with the existence of local microcavities of free volume. We believe that the fast decay component is connected with the rotation of molecular fragments and the slower decay component is connected with the rotation of entire molecules in local microcavities, which is possible because of the elasticity of the polymeric material.

We present a new, wavelength-independent optical limiter based on the Babinet compensator principle. The initial polarization change of an incident optical wave passing through a nematic liquid crystal placed between two parallel polarizers is compensated with a second identical sample, with its director vector perpendicular to that of the first cell. If there is some small absorption in just one of the cells, there is a thermally induced index change that produces an irradiance-dependent retardance. The resulting polarization change causes a decrease in the transmittance by more than one order of magnitude as the input energy is increased. Experiments were performed using a frequency-doubled 7 ns pulsed Nd-YAG laser, at a 10 Hz repetition rate as a pump, and a cw He-Ne as a probe to monitor the pump laser generated limiting effect.

Subpicosecond transient absorption measurements were performed for several polymethine and squarylium dyes in ethanol solution and a polymeric host over the spectral range 400-1500 nm. A variety of nonlinear effects including saturable absorption, reverse saturable absorption, and gain were observed and analyzed. We observe strong excited-state absorption (ESA) in all dyes in the range 450-600 nm. We also report the first prediction and observation of additional ESA bands in the near-infrared range. The predictions were based on quantum chemical calculations and the ESA experiments were performed with femtosecond pump-continuum probe techniques. For polymethine dye 2-[2-[3-[(1,3-dihydro-3,3-dimethyl-1-phenyl-2H-indol-2-ylidene) ethylidene]-2-phenyl-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-phenylindolium perchorate, an additional ESA band was detected near 1250 nm, and for squarylium dye 1,3-Bis-[(1,3-dihydro-1-butyl-3,3-dimetyl-2H-benzo[e]indol-2-ylidene)methyl]squ araine, two additional ESA bands were found around 870- and 1380-nm, respectively. To further study the nature of these transitions, the steady-state excitation anisotropy was also studied and compared with predictions. The relationship between ESA spectra of organic dyes and their molecular structure is discussed.

Experiments measuring the transmission of 5 nanosecond (FWHM), 532 nm pulses through an optical limiter using two different nonlinear materials at two separate focal planes show by far the highest dynamic-range measured to date. This cascaded focus, f/5 optical limiter combines self focusing and consequent nonlinear scattering in CS 2, with reverse saturable absorption in lead-phthalocyanine (PbPc). Both the AC Kerr effect and electrostriction contribute to the total refractive index change in CS 2. Our beam propagation code is used to model the propagation of light through the 2 cm thick CS 2 cell placed at the first focus, which protects a second 0.1 mm cell containing a solution of PbPc placed at the second focus. An aperture is placed in the focal plane of a final focusing lens to measure the "encircled" energy, defined as the energy passing through a 1.5 mrad diameter focal-plane aperture. Experiments and modeling show that the strong self-focusing in the CS 2 keeps the energy at the second cell below its damage threshold. This combination of nonlinearities clamps the maximum encircled energy below 1 μ J for input energies up to 14.5 mJ at a 10 Hz repetition rate. This corresponds to a dynamic range of at least 7500 while it is possibly much greater since no damage occurred to any device components. Here we present details of the operation of this device.

We report the observation of optical-limiting effects in benzene solutions of two neutral nickel complexes with multi-sulfur 1,2 dithiolene ligands, [Ni(medt)2] 1 (medt = 5,6-dihydro-6-methyl-1,4-dithiin-2,3-dithiolate) and [Ni(phdt)2] 2 (phdt = 5,6-dihydro-5-phenyl-1,4-dithiin-2,3-dithiolate). The limiting effects were observed with nanosecond and picosecond laser pulses. The limiting thresholds of complexes 1 and 2 were ≈0.3 J/cm2, measured with the ps laser pulses at 532 nm. The linear absorption spectra of the two complexes indicated that their limiting response should cover the visible and near-infrared region (up to 900 nm). Both ps time-resolved pump-probe and Z-scan experiments revealed that the limiting effects should originate from excited-state absorption and refraction

Efficient Pd-catalyzed Heck coupling methodology was employed to provide two new fluorene derivatives with phosphonate (2) and nitro (3) electron-withdrawing functionalities. Both derivatives exhibit two-photon absorption (2PA), as determined by nonlinear absorption measurements using a femtosecond pump/white light continuum probe “NLO spectrometer”. Both fluorene derivatives have high 2PA cross sections (650 and 1300 × 10-50 cm4 s photon-1 molecule-1 for compounds 2 and 3, respectively).

We study nonlinear absorption in a series of ten polymethine dyes and two squarylium dyes using Z-scan, pump-probe and optical limiting experiments. Both picosecond and nanosecond characterization were performed at 532 nm, while picosecond measurements were performed using an optical parametric oscillator (OPO) from 440 to 650 nm. The photophysical parameters of these dyes including cross sections and excited-state lifetimes are presented both in solution in ethanol and in an elastopolymeric material, polyurethane acrylate (PUA). We determine that the dominant nonlinearity in all these dyes is large excited-state absorption (ESA), i.e. reverse saturable absorption. For several of the dyes we measure a relatively large ground-state absorption cross section, σ 01 , which effectively populates an excited state that possesses an extremely large ESA cross section, σ 12 . The ratios of σ 12 / σ 01 are the largest we know of, up to 200 at 532 nm, and lead to very low thresholds for optical limiting. However, the lifetimes of the excited state are of the order of 1 ns in ethanol, which is increased to up to 3 ns in PUA. This lifetime is less than optimum for sensor protection applications for Q-switched inputs, and intersystem crossing times for these molecules are extremely long, so that triplet states are not populated. These parameters show a significant improvement over those of the first set of this class of dyes studied and indicate that further improvement of the photophysical parameters may be possible. From these measurements, correlations between molecular structure and nonlinear properties are made. We propose a five-level, all-singlet state model, which includes reorientation processes in the first excited state. This includes a trans – cis conformational change that leads to the formation of a new state with a new molecular configuration which is also absorbing but can undergo a light-induced degradation at high inputs.

Chromophores in which vinylene units and either pyrrole or dialkoxythiophene groups form the bridge between two diarylaminophenyl groups exhibit two-photon cross-sections of 1000-5000 GM at 600-650 nm. © The Royal Society of Chemistry.

The nonlinear absorption and refraction of octasubstituted metalophthalocyanines (R 8 PcM: M = 2H, Cu) in solution is measured in the picosecond regime by the Z-scan technique at a wavelength of 1.064 μm. The results at different pulsewidths show that the nonlinear absorption process is dominated by two-photon absorption in the metal-free compound, and by excited state absorption in the Cu-substituted molecule.

Two different approaches to increase intersystem crossing rates in polymethine-like molecules are presented: traditional heavy-atom substitution and molecular levels engineering. Linear and nonlinear optical properties of a series of polymethine dyes with Br- and Se-atom substitution, and a series of new squaraine molecules, where one or two oxygen atoms in a squaraine bridge are replaced with sulfur atoms, are investigated. A consequence of the oxygen-to-sulfur substitution in squaraines is the inversion of their lowest-lying ππ* and nπ* states leading to a significant reduction of singlet-triplet energy difference and opening of an additional intersystem channel of relaxation. Experimental studies show that triplet quantum yields for polymethine dyes with heavy-atom substitutions are small (not more than 10%), while for sulfur-containing squaraines these values reach almost unity. Linear spectroscopic characterization includes absorption, fluorescence, quantum yield, anisotropy, and singlet oxygen generation measurements. Nonlinear characterization, performed by picosecond and femtosecond laser systems (pump-probe and Z-scan measurements), includes measurements of the triplet quantum yields, excited state absorption, two-photon absorption, and singlet and triplet state lifetimes. Experimental results are in agreement with density functional theory calculations allowing determination of the energy positions, spin-orbital coupling, and electronic configurations of the lowest electronic transitions.

We detect both pulsed and CW IR light using uncooled GaN or GaAs photodiodes using extremely-nondegenerate two-photon-absorption (2PA) which shows large enhancement over degenerate 2PA. Urbach-tail absorption limits the signal-to-noise ratio for CW detection.

We report nonlinear refraction and absorption of organic (polymethine) and semiconductor (ZnO) thin films on thick substrates using the dual-arm Z-scan method to remove the substrate signals, thus allowing the discrimination of very small signals.

Using a dual-arm Z-scan to increase the signal-to-noise, we measure the dispersion of the electronic third-order nonlinearity of symmetric polymethines and squaraines and find good agreement with the essential-state model including CS 2 .

A 4-band Kane model gives good agreement for three-photon absorption spectra of GaAs and other semiconductors, showing quantum interference between pathways; however, 2-band scaling rules showing Eg-7 scaling with bandgap give useful estimates.

We perform degenerate pump-probe experiments at 1700 nm on GaAs with an additional 780 nm excitation pulse generating free carriers. An increased normalized transmission of the probe is observed, indicative of doubly-stimulated two-photon emission.