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Materials with vanishing real part of permittivity, also known as epsilon-near-zero (ENZ), have emerged as a new paradigm to obtain large optical nonlinearities. Transparent conducting oxides such as indium tin oxide (ITO) naturally exhibit an ENZ condition in the near-infrared and close to their plasma frequency. The peculiarity of ENZ materials is their highly dispersive nature explained by Drude model, allowing to obtain reduced group velocity at frequencies where the phase velocity is enhanced. This condition can dramatically increase the phase sensitivity caused by any given changes in material permittivity. To accurately address the NLO mechanism in such materials, we present a generalized formalism for the nonlinear phase shift for a dispersive material, where it includes contributions from both group velocity and phase velocity. We experimentally demonstrate that the nonlinear phase can reach to the values close to π/2 when the probe beam is at ENZ, while the index change in the conventional definition is not enhanced necessarily. In this work, we present nonlinear optical measurements using the nondegenerate Beam Deflection (BD) methodology at normal incident together with cross-phase modulation experiments. BD is a pump-probe method that directly characterizes the ultrafast response of the nonlinear phase-shift, hence nonlinear refraction. We see no polarization dependence proving that bound electronic third-order nonlinearities are negligible compared to the sub-picosecond fast carrier effects. We also present that frequency shift is highly sensitive to the temporal dynamics of the nonlinear phase shift suggesting that short pulses may help to improve the magnitude of frequency shift.

Materials with vanishing real part of permittivity, also known as epsilon-near-zero, have emerged as a new paradigm to obtain large optical nonlinearities. In this region, light-matter interaction enhances significantly, which gives rise to an unprecedentedly large nonlinear phase shift. The phase sensitivity enhancement is due to the strong dispersion in this spectral region. We present nonlinear optical measurements via the Beam Deflection technique, to directly measure the spectral dependence of the ultrafast nonlinear phase shift of an Indium Tin Oxide thin film. We present simultaneously cross-phase modulation induced frequency changes that depend on how fast the nonlinear phase changes.

Here, we present our development of several experimental methods, which, when applied together, can provide a thorough characterization of the nonlinear refraction and absorption properties of materials. We focus mainly on time-resolved methods for studying both transient absorption and refraction that reveal molecular dynamics including excited-state absorption, singlet-triplet transfer, instantaneous electronic nonlinear refraction, and molecular reorientation. In particular, we will describe our recent studies of new materials including organometallic compounds and organic solvents such as Tetrachloroethylene (C2Cl4).

We present beam deflection measurements to study the nondegenerate nonlinear refraction of highly doped semiconductors at epsilon-near-zero (ENZ) for several different pump and probe polarizations. Beam deflection is sensitive to induced optical path length as small as 1/20,000 of a wavelength, which enables us to resolve NLR in the presence of large nonlinear absorption backgrounds. The optically induced index changes in these materials can be both very large (on the order of unity) and fast (on the order of 100 fs). Our results show that carrier redistribution effects dominate the nonlinear refraction, and by independently tuning the pump and probe wavelengths, we find that the strong wavelength dependence of nonlinearities around the ENZ point is different for pump and probe waves. These nonlinear optical properties, where the ultrafast index change can be larger than the linear index, offer new paradigms for dynamically switchable diffractive elements that respond to structured light, allowing manipulation of optical beams in transmission and reflection not only along the two spatial dimensions but also in time. This is a revolutionary change in the field of nonlinear optics allowing a myriad of potential applications, ranging from rapid all-optical beam steering and switching, to spectral scanning, spatial mode conversion, as well as pulse shaping and suppression, all on sub-picosecond time scales.

We present measurements of nonlinear refraction and absorption in transparent conductive oxides (TCO), which are essentially highly-doped semiconductors, at Epsilon-Near-Zero (ENZ). In this spectral region, where the real part of the permittivity crosses zero, materials demonstrate interesting nonlinear properties such as enhanced harmonic generation, nonlinear absorption (NLA), and nonlinear refraction (NLR). We find that induced changes in the refractive index of Indium Tin Oxide, can be very large with respect to the initial index. This means that the even the Fresnel coefficients are highly irradiance-dependent. Therefore, the nonlinear transmission, reflection, and absorption of the material will be significantly different from conventional materials which means that it is challenging to use conventional methods such as Z-Scan and time-resolved transmission and reflection techniques to accurately determine the underlying nonlinear optical coefficients. We have studied optical nonlinearities of TCOs using the Beam-Deflection (BD) method to independently characterize the temporal dynamics and polarization dependence of the NLR. This method enables us to resolve NLR in the presence of large NLA backgrounds. In addition, we can also study the dependence on relative polarization and incidence angle of excitation and probe waves to characterize the enhancement mechanism and the physical origin of the nonlinear response. We conduct these BD measurements in conjunction with Z-Scan and transient reflection and transmission at different wavelengths, incidence angle, and polarization. Our measurements reveal that there is a strong wavelength dependence of nonlinearities around the ENZ point. We find that the wavelength dependence is quite different for excitation and probe waves.

For bioimaging purposes, non-cytotoxic fluorescent labels, stable in biological media and capable of site-specific labeling are in upsurging demand. However, many fluorescent dyes with promising properties, such as fluorescent organic dyes, are poorly water-soluble and often lose their properties in water, limiting their use for bioapplications. The weak signals, poor photobleaching resistance, short fluorescence life time and low chemical stability are continuous challenges for the development of optimal fluorescent probes. The use of aqueous colloidal structures, such as nanovesicles, as nanocarriers for the loading of the organic dyes offers a promising strategy to overcome this limitation. As a matter of fact, we have engineered a new class of fluorescent organic nanoparticles (FONs) using thermodynamically stable nanovesicles named quatsomes (QS) [1-4]. QSs are new class of exceptionally stable small unilamellar vesicles with sizes smaller than 100nm, and formed by the self-assembly of sterols and quaternary ammonium surfactants. [5,6] Dye-loaded QSs can be prepared by a one-step method using compressed CO2, named depressurization of expanded liquid organic solution-suspension (DELOS-susp).[7] Indeed, it is a green technology leading to a formation of a highly homogenous dispersion of nanovesicles stables in an aqueous environment. The loading of water-insoluble carbocyanines dyes into QSs, has successfully proved the photostability of the obtained FONs in aqueous solutions, as well as, their valuable brightness [6]. They show excellent colloidal stability and structural homogeneity along with superior optical properties, in comparison with the fluorophores in solution. Dye-loaded QSs have enhanced optical properties, demonstrating the absence of non-fluorescent aggregates due to the Aggregation Caused Quenching (ACQ) effect, neither the formation of J- and H-aggregates was produced, in contrary to the well-known tendency of cyanines to aggregate [8]. Different methods for obtaining dye loaded nanoparticles were compared pointing the advantages of dye-loaded QS over other dye-based FONs, in terms of both, optical and colloidal properties. The effect of the dye loading on the physicochemical properties were studied as well as the brightness, showing higher brightness when dyes were located at the QS membrane. Moreover, experimental results were supported by molecular dynamics (MD) simulations which give information on the configuration of the dyes within the membrane. The potential of dye-loaded QSs for biological imaging was studied using a superresolution microscopy technique, the stochastic optical reconstruction microscopy (STORM). This study determine that dye-based QS are remarkable candidates as nanostructured probes for biological imaging, not only because of their photophysical properties but also, for their capabilities to be precisely decorated at their surfaces with targeting groups3 and to integrate small drugs or large biomolecules. All such benefits represent a certainly promising probe for bioimaging and, especially, for theragnostic nanomedicine. 1. Elizondo, E. et al. Influence of the Preparation Route on the Supramolecular Organization of Lipids in a Vesicular System. J. Am. Chem. Soc. 134, 1918–1921 (2012). 2. Ferrer-Tasies, L. et al. Quatsomes: Vesicles Formed by Self-Assembly of Sterols and Quaternary Ammonium Surfactants. Langmuir 29, 6519–6528 (2013). 3. Cabrera, I. et al. Multifunctional Nanovesicle-Bioactive Conjugates Prepared by a One-Step Scalable Method Using CO2-Expanded Solvents. Nano Lett. 13, 3766–3774 (2013). 4. Grimaldi, N. et al. Lipid-based nanovesicles for nanomedicine. Chem. Soc. Rev. 45, 6520–6545 (2016). 5. Silbaugh, D. A. et al. Highly Fluorescent Silicon Nanocrystals Stabilized in Water Using Quatsomes. Langmuir 33, 14366–14377 (2017). 6. Antonio, A. et al. Nanostructuring Lipophilic Dyes in Water Using Stable Vesicles, Quatsomes, as Scaffolds and Their Use as Probes for Bioimaging. Small 14, 1703851 (2018). 7. Ventosa, Nora; Veciana, Jaume; Sala, Santiago; Cano, M. Method for Obtaining Micro- and Nano-disperse systems. (2006). doi:WO/2006/079889 8. Gadde, S., Batchelor, E. K. & Kaifer, A. E. Controlling the Formation of Cyanine Dye H- and J-Aggregates with Cucurbituril Hosts in the Presence of Anionic Polyelectrolytes. Chem. – A Eur. J. 15, 6025–6031 (2009).

Materials where the real part of the permittivity is near zero are known to have interesting nonlinear optical properties such as enhanced harmonic generation and large nonlinear refraction (NLR). In particular, the NLR of highly doped semiconductors such as Indium Tin Oxide and Aluminum doped Zinc Oxide is enhanced in the near-infrared spectral regions, where the real part of the permittivity crosses zero, the precise wavelength of which can be tuned by controlling the doping level.. This is also known as the epsilon near zero (ENZ) regime, although the imaginary part of the permittivity is not necessarily small at this wavelength. In order to characterize these nonlinearities, we use the Beam-Deflection (BD) method to directly characterize the temporal dynamics and polarization dependence of the nondegenerate NLR and nonlinear absorption of doped semiconductors at ENZ. BD has sensitivity to induced optical path length as small as 1/20,000 of a wavelength, which enables us to resolve NLR in the presence of large nonlinear absorption backgrounds. The BD technique also allows separation of instantaneous bound electronic nonlinearities from non-instantaneous mechanisms such as the carrier redistribution effects that dominate in ENZ materials,. We can also study the dependence on relative polarization and incidence angle of excitation and probe waves. Our method also reveals the effect of tuning the wavelength of excitation or probe waves through ENZ separately and we find that that the strong wavelength dependence of nonlinearities around the ENZ point is quite different for pump and probe waves.

Materials exhibiting near zero refractive index are shown to have interesting nonlinear optical properties such as enhanced second and third harmonic generation, and large nonlinear refraction (NLR) due to their unique interplay between linear and nonlinear optical features. In particular, the NLR of highly doped semiconductors such as Indium Tin Oxide and Aluminum doped Zinc Oxide is enhanced in the near-infrared spectral regions, where the real part of the permittivity crosses zero with the advantage of having a tunable zero crossover frequency by controlling the doping level. This is also known as the epsilon near zero (ENZ) regime, where the refractive index is very small. We have used the Beam-Deflection (BD) method to directly characterize the temporal dynamics and polarization dependence of the nondegenerate (ND) NLR of doped semiconductors at ENZ. The origin of the nonlinear optical response of these materials is different than for the case of bound electronic nonlinearities which depend upon the third-order susceptibility. The ND BD technique has the potential to study the dependence on relative polarization of excitation and probe waves to accurately determine the instantaneous electronic nonlinearities separately from the non-instantaneous mechanisms such as carrier redistribution effects, however, the carrier nonlinearities are dominant in such materials. This method also reveals the effect of tuning the wavelength of excitation or probe waves through ENZ separately. BD has sensitivity to induced optical path length as small as 1/20,000 of a wavelength, which enables the possibility to resolve NLR in the presence of large nonlinear absorption backgrounds.

Nondegenerate nonlinear refraction (ND-NLR) in semiconductors is greatly enhanced over the degenerate case and exhibits strong nonlinear dispersion, which provides the potential to greatly modify the refractive dispersion. This nondegenerate enhancement arises from the resonance of the small photon energy with the intraband self-transition, and the larger photon energy with the interband transition. Our earlier theory predicts the dispersion of ND-NLR in semiconductors from a Kramers-Kronig transformation of a nonlinear absorption spectrum, which considers nondegenerate two-photon absorption (ND-2PA), electronic Raman, and quadratic Stark effect. Experimentally, the dispersion of ND-NLR is measured using our Beam-Deflection technique, and the coefficient, n2, is determined over a broad spectral range with various degrees of nondegeneracy. In the extremely nondegenerate case, n2 is greatly enhanced near the onset of ND-2PA, which rapidly switches sign to negative near the bandgap over a very narrow wavelength range. The data suggests larger figure-of-merits by using ND-NLR for all-optical switching. Near and within the ND-2PA regime, strongly dispersive ND-NLR can significantly alter the dispersion of a material, and provides the possibility to optically modify the group index, and group velocity dispersion (GVD) properties. From their irradiance dependence, the nonlinear group index, n2,g, and the nonlinear GVD factor, D2, are calculated from the first and second order derivatives of n2, which show even greater nondegenerate enhancement. Also the nonlinear group index is maximized where there is no two-photon absorption. Potential applications, including nondegenerate all-optical switching based on n2,g, and ultrafast all-optical pulse shaping based on D2, are discussed.

Many materials exhibit nonlinear refraction (NLR) and absorption (NLA) that has multifaceted temporal dynamics. As a result, measurements at one laser pulse width may not be fully predictive of the behavior at other pulse widths. We have recently developed a method, Nonlinear Beam Deflection, (BD) that allows sensitive time-resolved measurement of nonlinear refraction (NLR) and absorption (NLA) by using an excitation beam to create an index gradient deflecting a probe beam onto a quad-cell detector. The method has a demonstrated sensitivity to induced phase changes as small as 1/20,000 of a wavelength, which is sensitive enough to measure NLR in gases. By changing the relative polarization of the beams we can separate the bound-electronic response from the slower and different-symmetry nuclear contributions. In gases and liquids where reorientational nonlinearities are important, measurements at the magic angle allow isolation of the ultrafast nonlinearities. In isotropic solids the bound-electronic symmetry dictates a ratio of 1/3 for parallel to perpendicular polarizations which measurements confirm. This method also allows for measurements of nonlinearities using very different wavelengths for the excitation and probe. We have used this method to characterize the impulse response function for third-order nonlinearities in many transparent organic solvents. This allows accurate prediction of the nonlinear refraction for any pulse width longer than that used for the BD characterization. The method proves to be very useful in organic materials that may show strong nonlinear absorption, as it is able to resolve NLR in the presence of strong NLA better than other methods, such as Z-scan.

We have shown both experimentally and theoretically that the effect of intermediate-state resonance enhancement causes highly nondegenerate 2-photon absorption, 2PA, to be strongly enhanced in direct-gap semiconductors. Calculations indicate an additional 10x increase in this enhancement is possible for quantum-well semiconductors. This enhancement leads to interesting applications of 2PA, such as mid-infrared detection, where uncooled, large-gap photodiodes can rival the sensitivity of cooled MCT detectors (for short pulses). Additionally, mid-IR imaging and tomography based on this effect have been shown. Even larger enhancement of 3PA is calculated and observed. In the case of optically-pumped semiconductors, we have now demonstrated that the complementary process of nondegenerate 2-photon stimulated emission can be observed. Theoretically, this results in 2-photon gain (2PG) that is enhanced as much as 2PA, leading to the possibility of large gap devices with tunable mid-infrared gain. However, the effect of nondegenerate enhancement of 3PA can be detrimental to the observation of this gain. Additionally, by causality, Kramers-Kronig relations predict that the enhancement of 2PA is accompanied by an enhancement of the nonlinear refractive index, n2, which is very highly dispersive in the region of 2PA. Our latest experimental results confirm this enhancement and strong dispersion.

We extend our recently developed Beam-Deflection (BD) method which we used to determine the sign, magnitude, and ultrafast dynamics of bound electronic and nuclear nonlinear optical responses, to measure elements of the third-order nonlinear optical susceptibility tensor with longitudinal field components. In these measurements, in contrast to the conventional BD technique where excitation and probe beams are nearly collinear, the interaction geometry involves orthogonal propagation of excitation and probe beams. This cross-propagating BD method enables probing with the electric field polarized parallel to the wavevector of the excitation beam as well as conventional parallel and perpendicular polarizations as in most other experimental methods. This technique may be of use in detecting possible magneto-electric contributions to the nonlinear susceptibility. The ratio between parallel and perpendicular bound-electronic responses in this method is shown to be larger than the factor of 3 predicted by theory and measured in the conventional configuration.

Two-photon absorption, 2PA, in semiconductors is enhanced by two orders of magnitude due to intermediate-state resonance enhancement, ISRE, for very nondegenerate (ND) photon energies. Associated with this enhancement in loss is enhancement of the nonlinear refractive index, n2. Even larger enhancement of three-photon absorption is calculated and observed. These large nonlinearities have implications for applications including ND two-photon gain and twophoton semiconductor lasers. Calculations for enhancement of ND-2PA in quantum wells is also presented showing another order of magnitude increase in 2PA. Potential devices include room temperature gated infrared detectors for LIDAR and all-optical switches.

We present measurements of the temporal and polarization dependence of the nonlinear optical (NLO) response of selected organic solvents using our beam deflection (BD) method. These measurements allow us to separately determine the bound-electronic and nuclear responses which then determines the NLO response function. With this NLO response function the outcome of other experiments such as Z-scan as a function of pulse-width can be predicted. By performing similar measurements on the gas phase of these solvents we can compare the hyper-polarizabilities in the two phases.

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.

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.

Understanding the nonlinear optical processes in semiconductor nanostructures leads to possible applications in areas including laser amplifiers, optical switches, and solar cells. Here we present a study of the frequency degenerate twophoton absorption (2PA) spectrum of a series of PbS and PbSe quantum dots (QDs). The influence of the quantum confinement is analyzed using a four-band model which considers the mixing of valence and conduction bands. In contrast to our observations of CdSe QDs, the present results point to an increase of the 2PA cross-section (normalized by the QD volume) as the quantum dot size is made smaller. This is explained by the symmetry between the valence and conduction bands which allows the density of states to remain high even for small QDs. A study of the ultrafast carrier dynamics of the PbS quantum dots is also presented. Through nondegenerate femtosecond pump-probe experiments we show evidence of multi-exciton generation with quantum yield (number of excitons generated per absorbed photon) up to 170% for excitation with hω > 3 Eg (where Eg is the bandgap energy).

We have been developing tools for nonlinear spectroscopy aimed toward the ultimate goal of building a nonlinear spectrophotometer analogous to the ubiquitous linear spectrophotometer where a sample is placed in the instrument, a button is pushed, and the absorption spectrum is obtained sometime later. This paper describes our progress toward this goal, describing many difficulties and complications as well as opportunities. We also show spectroscopic data and analysis of a variety of materials that we have taken with preliminary nonlinear spectroscopic instrumentation we have already developed. One of the more interesting observations obtained along this research path is the realization that linear dispersion theory can also be applied to nonlinear systems when formulated properly such that Kramers-Kronig relations can be used to connect the dispersion of nonlinear refraction to the spectrum of nonlinear absorption. In some circumstances this can be more easily applied to nonlinear systems than to linear systems since the nonlinear absorption spectrum can be limited in wavelength. In addition, we have developed tools that can simultaneously give the spectrum of nonlinear absorption as well as the dispersion of the nonlinear refraction over an octave spectral range from 400nnm to 800 nm, the so called White-Light-Continuum Z-scan. Much of the research on nonlinear optical materials has been a collaborative effort requiring the skills and expertise of organic chemists and materials manufacturers. The goals of this part of the research are to determine predictive structure-property relation capabilities. The database needed for this research makes the nonlinear spectrophotometer a necessity.

We present the design, fabrication and characterization of the optical properties of one-dimensional metal-organic photonic bandgaps (MO-PBGs) composed of a tetraphenyldiaminobiphenyl-based polymer and ultrathin electrically continuous Cu layers. The fabricated MO-PBGs achieve a peak transmission of around 44% at 620 nm combined with very large spectral, around 120 nm FWHM, and angular, more than 120° field-of-view, bandwidths. Using 140 fs pulses at various wavelengths we have found up to 10 × enhancements in the nonlinear optical (NLO) properties of the MO-PBGs when compared with the NLO response of ultrathin electrically continuous Cu layers.

We present degenerate and nondegenerate two-photon absorption spectra in a series of CdSe and CdTe quantum dots. The measurements show that the two-photon absorption (2PA) spectrum is strongly dependent on the quantum dot size and that the 2PA coefficient decreases as the quantum dot size decreases, and it is larger for the frequency nondegenerate process. Previously we had shown a theoretical analysis of these results based on a simple model using the effective mass approximation. Although this model works well for larger quantum dots, it fails for the smaller ones. Here we use the more (formula available in manuscript) model for the band structure and consider the hole band mixing in quantum dots to describe our data. This theory better describes the spectral structures for smaller quantum dots and also predicts the decrease of the 2PA coefficient with the decrease of quantum dot size. This is due to the reduction of the number of possible transitions and the blue shift of the optical bandgap from quantum confinement. This theory predicts the reduction of the 2PA coefficient with size, although our experimental results show an even stronger reduction.

The nonlinear optical performance of several Fluorene-based molecules was studied using different measurement methods and pulse durations. We used picosecond pulses at 532 nm and, femtosecond pulses tunable from 532-810 nm for performing open and closed aperture Z-scans, and we used femtosecond 570-930 nm pulses for two-photon induced fluorescence (2PF) spectroscopy. The observed nonlinear losses were compared using the three methods. The results exhibit much stronger nonlinear absorption with picosecond pulses due to the additional excited-state absorption processes involved. Also the nonlinear refractive index was found to be higher for the picosecond measurements. In addition using a femtosecond white-light continuum (WLC) pump-probe method we showed that by proper tuning of pump and probe wavelengths an intermediate state resonance enhancement (ISRE) of the 2PA can be observed yielding the same overall nonlinear absorption observed using picosecond pulses.

We have tested a series of Ytterbium doped large core fibers operating near 10Kpps and producing pulses of approximately 1ns. We have achieved 0.85mJ/pulse resulting in peak powers in excess of 2MW with 0.4ns pulses and near diffraction limited beams. In another fiber, we have achieved over 1.5mJ/pulse with pulses of 900ps corresponding to 1.65MW of peak power and M2 of 2.5. In the latter case, wall-plug efficiencies, excluding cooling of the pump diode lasers, in excess of 15% were also achieved. This fiber amplifier has operated for 2 months without any degradation or observed optical damage.

Graduate degrees specializing in optics have been offered at the University of Central Florida since 1987, with stand-alone Optics degrees being offered since 1998. In 2002, the Optics Ph.D. core was radically changed to allow students to take the PhD qualifying examination earlier in their studies, while still providing a broad and rigorous grounding in optics. This involved the creation of several new courses. We describe how this new system has worked over the first three years. We also discuss results of a study on how well typical admission criteria such as GRE exam results, grade point average, etc. predict student performance in our program.

N-on-1 LIDT measurements were performed on ytterbium doped preforms used to make high peak power fiber amplifiers. Damage measurements were complicated by large index of refraction changes across the preforms. These difficulties were overcome by monitoring the beam profile before and after the samples and by only taking data where the transmitted beam was not significantly distorted. Single and 1000 shot data suggest slight laser conditioning of the preforms and rule out laser fatigue in the doped cores and surrounding fused silica. At 1064 nm, inside the emission spectra, there seemed to be little influence of the Yb dopant concentration on the measured LIDT.

Both degenerate and nondegenerate two-photon absorption (2PA) spectra are studied in two different samples of CdTe quantum-dots in borosilicate glass hosts. One sample (CdTe-600) contains quantum-dots of radius 3.2 ± 0.2 nm and has its absorption edge at 600nm. The other sample (CdTe-750) contains quantum-dots of radius 6.6 ± 0.9 nm and absorption edge at 750nm. CdTe-600 contains quantum-dots with a narrower size distribution than CdTe-750. Consequently, the peaks corresponding to discrete transitions are more clearly visible in CdTe-600 than in CdTe-750. Both nondegenerate and degenerate spectra for these samples show a marked difference from bulk CdTe. In CdTe-750 the two-photon absorption spectrum has a shape similar to that for bulk solids but for CdTe-600 the 2PA spectrum is somewhat different from that expected for the bulk. In the Z-scan measurements we also observed a photo-darkening effect, which is accompanied by an increase in the measured effective 2PA coefficient. All results suggest that 2PA cannot be predicted by the bulk theory especially near to the 2PA edge, that the 2PA in quantum dots is generally smaller than would be expected for the same volume of bulk semiconductor with the same band edge, and that the quantum-dot size and size distribution play important roles in the 2PA spectral behavior and magnitude.

Fluorene-based molecules were studied in detail as candidates for two-photon absorption (2PA) applications. Previous studies were carried out over the wavelengths range 560 - 900 nm. In this work we have extended our studies to shorter wavelengths in order to determine behavior closer to the 2PA resonance peak. We used picosecond pulses at 532 nm for performing open and closed aperture Z-scan measurements, in order to obtain both the real part of the third order nonlinearity (X r (3) ), corresponding to nonlinear refraction (n 2 ) and the imaginary part of the third order nonlinearity (X i (3) ), corresponding to nonlinear absorption (β) due to 2PA. We studied several fluorene-based molecules, which differ by length of conjugation system and substitution moieties. From open aperture Z-scan measurements we obtained the imaginary part of the third order nonlinearity (X i (3) ). We measured 2PA cross-sections, δ, varying from 1,000 up to 10,000 GM at 532 nm. The influence of the molecular structure on δ was examined. For some of the dyes we also measured n 2 via closed aperture Z-scan measurements. By subtracting the contribution of the solvent from the observed value for the solution, we obtain the contribution to n 2 due to the solute.

Organic compounds that undergo strong nonlinear, multiphoton absorption have been gaining greater interest, mainly in the developing fields of multiphoton fluorescence imaging, optical data storage, 3-D microfabrication, and photodynamic therapy. Systematic studies have shown that conjugated organic molecules with large delocalized π electron systems show very large nonlinear optical effects. Two-photon absorbing chromophores have also been incorporated into dendrimers to increase two-photon absorptivity. A cooperative enhancement of two-photon absorption (2PA) has been observed, such as in the linkage of branched chromophores through a common amine group and chromophore-metal complexes. This enhancement may be related to extensive two-dimensional π-delocalization in these molecules. Herein, we describe the synthesis, structural characterization and photophysical study of a series of compounds (model, oligomer, and polymer) with symmetric molecular structure of the D-π-D motif and branched D-π-D dendrimeric structures based on substituted fluorene derivatives. Femtosecond 2PA cross sections were very large for some derivatives (over 10,000 GM) and often exhibited substantial solvent effects.

Fluorescent dyes and probes are key components in multiphoton based fluorescence microscopy imaging of biological samples. While many commercially available fluorescent dyes have sufficed, most exhibit relatively low two-photon absorption (2PA) cross-section values in the tunability range of Ti:sapphire lasers commonly used in multiphoton microscopy imaging. Furthermore, available fluorophores may be plagued with either low fluorescence quantum yields and/or the additional problem of rapid photobleaching upon exposure to the high peak powers provided by fs laser sources. In order to address the demand for better performing dyes for two-photon based imaging, we have prepared a new series of reactive fluorophores tailored for multiphoton imaging. These fluorophores are based upon the fluorene ring system, known to exhibit high fluorescence quantum yields, typically > 0.7, and possess high photostability. They have been functionalized with moieties to act, e.g., as efficient amine-reactive fluorescent probes for the covalent attachment onto, e.g., proteins and antibodies. The synthesis and the single-photon spectral characteristics, as well as measured two-photon absorption cross sections of the reactive fluorophores in solution will be presented. Spectral characterizations of bovine serum albumin (BSA) conjugated with the new reactive probe will also be presented.

Two-photon fluorescence microscopy is a prominent tool in biological imaging analysis. Many commercially available fluorescent dyes currently being used have sufficed for multiphoton based imaging of biological samples. While measured two-photon cross-sections (in Goppert Meyer, GM units) of some of the dyes are available, many exhibit relatively low two-photon cross-section values in the tunability range of Ti:sapphire lasers commonly used in multiphoton microscopy imaging. For example, Bodipy FL exhibits a maximum GM unit of 18 at 925 nm, compared to a range of 2-4 GM units from 775 - 875 nm. Furthermore, available fluorophores may be plagued with either low fluorescence quantum yield and/or the additional problem of rapid photobleaching upon exposure to the high peak power provided by the fs laser source. In order to address the demand for better performing dyes for two-photon based imaging, we have prepared a new series of reactive fluorophores tailored for multiphoton imaging. These fluorophores are based upon the fluorene ring system, known to exhibit high fluorescence quantum yields, typically > 0.7, and possess high photostability. They have been functionalized with various moieties to act, e.g., as efficient amine-reactive fluorescent probes for the covalent attachment onto amine-containing biomolecules. Single-photon spectral characteristics, as well as measured two-photon cross sections of a reactive fluorophore and its model conjugate in solution, as well as spectral characterizations of a bovine serum albumin (BSA) conjugate are presented.

We have performed nonlinear spectroscopic measurements to investigate the chemical structure/nonlinear optical property relations for a set of alkyl fluorene derivatives. The characterization method we have utilized is a femtosecond white-light continuum (WLC) pump-probe spectrometer that can rapidly characterize an organic sample’s nondegenerate two-photon absorption (2PA) spectrum. The nature of these experiments requires sophisticated data analysis. In particular, the relative group velocity mismatch between the pump and probe, which are at different frequencies, makes these pulses walk through each other within the thickness of the sample. For widely different frequencies, this can severely diminish the 2PA signal strength. However, given careful analysis, we have found good agreement with well-known semiconductor samples. Confidence in this method has allowed us to investigate the effects of solvism, electron-withdrawing character, conjugation length, and symmetry on the two-photon absorbing properties of these molecules. We have found an optimum solvent polarity as well as electron-withdrawing character which serves to maximize the strength of the 2PA in these materials. Different synthesis avenues have provided us with two different methods of extending the conjugation length that increases the nonlinearity as well. Finally, investigations of molecules with disparate symmetry have allowed us to identify the symmetry of the excited states. In addition, we present the first experimental study of the intermediate state resonance enhancement of nondegenerate 2PA in organic molecules. Using a simplified sum-over-states expression, we make comparisons between experiment and theory.

We report image formation via single and two-photon photoinduced fluorescence changes in a polymeric medium with two-photon fluorescence readout of multiplayer structures. Photoinduced acid generation in the presence of a two-photon fluorescent dye possessing strongly basic functional groups (7-benzothiazolyl-9,9-didecyl-2,2-(N,N- diphenylamino)fluorene underwent protonation upon exposure with UV or near-IR (740 nm fs pulses). Solution studies demonstrate formation of monoprotonated and diprotonated species upon irradiation, each resulting in distinctly different absorption and fluorescence properties. The fluorescence of the original, neutral, fluorophore is quenched upon monoprotonation with a concomitant increase in fluorescence at longer wavelengths due to the monoprotonated form. Hence, two channel two-photon fluorescence imaging provides 'positive' or 'negative' image readout capability. Results of solution and solid polymer thin films experiments are presented.

The realization of all-optical switching schemes is mostly hindered by the lack of suitable materials with a refractive index change that is large and fast enough. The characterization of the linear and nonlinear optical properties of potential materials is therefore of prime importance. Various characterization methods have been proposed and are employed, yielding different parameters of the nonlinear optical response at the involved laser frequencies. However, in most techniques the resulting nonlinearities are measured only at one point in the spectral dispersion. To generate the whole nonlinear spectra, the laser source has to be tuned over the desired wavelength range and consecutive measurements have to be taken. We propose and demonstrate here a novel technique to measure the nonlinear optical response for a broad wavelength region in a pump-probe scheme that requires no laser tuning. In order to detect the two-photon absorption at several wavelengths simultaneously, we use a white-light-continuum as the probe beam. As the pump beam is held constant, the Kramers-Kroenig transformation can be used to calculate the dispersion of the nonlinear refractive index from the two-photon spectra. By delaying the probe beam relative to the pump beam, the temporal behavior of the nonlinearity can be obtained.

We report the two-photon induced photoisomerization of 3-[1-(1,2- dimethyl-1H-indol-3-yl)-ethylidene]-4-isopropylidene-dihydro-furan- 2,5-dione (1), a photochromic compound with (lambda) maxequals385nm, using 775 fs pulses. The resulting photoisomer has a (lambda) maxequals582 nm. The kinectic rate contant for the isomerization reaction w as 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 shown. A two-photon absorption molecular cross-section (sigma) 2equals10.3 x 10- 45 cm4 s/photon was measured using Z-scan, supporting a two photon induced isomerization process.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Wide applications of lasers have stimulated a great interest in development of optical limiting devices. These devices can be used to protect optical sensor form laser-induced damage because their transmission is high when they are exposed to low-power laser light, and their transmission becomes low when irradiated by intense laser beams. Here we report such a device based on nonlinear optical effects in two neutral nickel complexes with multi-sulfur 1,2 dithiolene ligands, 1 and 2. The limiting device consisted of a focusing setup and a 1-mm-thick cell, which contained a benzene solution of one of the complexes. THe limiting properties were investigated by both nanosecond and picosecond laser pulses. At 532 nm, the limiting thresholds of complexes 1 and 2 measured by the picosecond laser pulses. At 532 nm, the limiting threshold of complexes 1 and 2 measured by the picosecond laser pluses with a focusing geometry were determined to be approximately 0.3 J/cm 2 . The linear absorption spectra of the to complexes also indicated that their limiting response should cover the visible and near-IR region. All these result show their limiting performance is superior to the limiting effecting C 60 . Picosecond time-resolved pump-probe and Z-scan experiments revealed that the observed limiting effects should originate from excited-state absorption and refraction.

We have developed a femtosecond continuum spectrometer to measure nonlinear absorption spectra from 300 nm in the UV to 1.7 micrometers in the IR. This method is applied for measuring NLA spectra of semiconductor, organic and polymeric materials. The pump-probe nature of the experiment also allows the temporal response to be determined, thus helping in the determining of the underlying physical mechanisms for the nonlinearity. We describe studies of two-photon absorption in a series of alkyl fluorenes and excited state absorption dynamics in a series of polymethines using this spectrometer.

We describe studies of optical limiting by thermally induced nonlinear light scattering in highly porous solgel glasses. We impregnate the porous glass with a solution of reverse saturable absorber in a solvent that is index-matched to the solgel matrix at room temperature. We observe a strong enhancement of the limiting properties at high energies in the porous glass, compared to the performance of the reverse saturable absorber alone. We attribute this effect to nonlinear scattering. However, the properties of this scattering are substantially different to what was originally expected. We provide evidence that the observed effect is due to formation of gas bubbles in the focal region of the laser beam. These bubbles are trapped in the host matrix, allowing the effect to accumulate over several laser shots.

We present our recent advances toward the development of high-performance solid-state optical limiting devices using reverse saturable absorption (RSA) dyes doped into optical host materials. Femtosecond transient absorption spectroscopy was employed to determine both the spectral regions of strong RSA, and the singlet-triplet excited-state dynamics. The optical limiting in the visible spectrum in both metallo-phthalocyanines and metallo-porphyrins is due to a combination of singlet and triplet RSA. Optical limiting performance was studied for RSA dyes in dual tandem limiters (both in solution and solid-state). Our best results in the solid-state yielded an attenuation of 400X, and a damage threshold of up to several mJ at f/5 focusing. The optical limiting at f/5 is further enhanced, particularly in the solid-state, by self-defocusing thermal nonlinearities.

We describe two methods for the spectral measurement of nonlinear absorption and refraction in reverse-saturable absorber materials. In the first, we use a picosecond optical parametric oscillator to perform Z-scan at many different wavelengths to measure excited state refraction and absorption cross sections throughout the visible. The second methods uses a chirped-pulse amplification scheme to produce 100 fs pulses at 840 nm. Focusing these into sapphire generates a white light continuum that is used as a probe in an excite-probe experiment. The excitation beam is derived from the second harmonic of the remaining 840 nm light. By measurement of the transmission spectrum of the probe as a function of excite- probe delay time, we can determine the spectral dependence of the excited-state absorption cross section. Moreover, by use of Kramers-Kronig relations, the excited state refraction can also be extracted from this data. We describe our measurements using both methods in a Zn:tetrabenzporphyrine derivative (TBP). The fact that both methods give excellent agreement not only verifies the utility of continuum measurements, but also reveals some interesting properties of the excited states of TBP.

We developed a set of computer codes organized in a software package that allows us to model high-energy laser pulse propagation through bulk nonlinear optical media. Nonlinearities included in the model are two-photon absorption, the electronic Kerr effect, excited-state and free-carrier absorption along with the associated electrostrictive and photo-acoustic refractive index change. The propagation of CW beams, nsec/psec pulses and picosecond pulse trains is determined for various spatial distributions of the input beam. We used a cylindrically symmetric spatial geometry typical for laser outputs to reduce the CPU and memory requirements making modeling a real-time task on PC's, even for significant propagation paths (many Rayleigh ranges). The validity of the numerical outputs was tested against known results for a large range of parameters. In particular the codes are being used to investigate and design optical limiting devices with single or multi-element geometries, as well as limiters using stepped or graded density of nonlinear material. The linear propagation module integrated with the nonlinear beam propagation method (BPM) codes allow the simulation of typical experiments such as Z-scan (sample position changes in a focused beam) and limiting experiments (different input energy values).

Thenonlinear absorption of a series of polymethine dyes (PD's) have been investigated, and some of them are identified as potential candidates for optical limiting applications. We have demonstrated that very low optical limiting thresholds can be obtained by using polymethine dyes with a large ratio of the absorption cross section ofthe excited to the ground-state: a ratio as large as 200 has been measured in one of the PD's. This value is significantly higher than any previously reported value, while, at the same time, maintaining a large ground-state cross section. It has been shown that nonlinear absorption of PD's in polyurethane acrylate (PUA) polymer is as large as in ethanol solution. In addition, photostability of the PD's and initial measurements ofoptical limiting in these samples are reported.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report measurements of the nonlinear transmission spectra of organic dye solutions. The reverse saturable absorption (RSA) of these organic dyes in the visible and near IR was measured using a 200 fs 425 nm excitation and a temporally delayed 200 fs white light continuum probe. These RSA materials have attracted attention due to their potential usefulness in optical limiting devices. Knowledge of the dynamics of the nonlinear response along with the spectral dependence is important, in order to determine the range of operation of a given material and to properly model the level structure and lifetimes. We developed an optical source based on an argon ion pumped, Kerr lens modelocked Ti:sapphire oscillator followed by a Cr:LiSAF regenerative amplifier producing millijoule level, 200 fs pulses (FWHM) around 850 nm. A single pulse is then split to generate a second harmonic (SH) at 425 nm and a femtosecond continuum that are used as the pump and probe respectively in a standard pump-probe geometry. The SH is produced in a thin BBO crystal and the continuum is produced by focusing the 850 nm light into a water cell. This results in up to 50 microjoules of pump and 3 microjoules of probe in the spectral range from 200 nm to 800 nm. These pulses have been used to temporally resolve the nonlinear spectra of several organic solutions including zinc tetra (p-methoxyphenyl) tetrabenzporphyrin, lead phthalocyanine, and silicon naphthalocyanine up to delays of several nanoseconds.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe a series of experiments on solutions of the inorganic metal cluster molecules Mo2Ag4S8(PPh3)4 and compare them with data on a suspension of carbon particles in liquid (ink). The optical limiting behavior is measured using both single picosecond 532 nm pulses and nanosecond long trains of these picosecond pulses. Both materials show reduced transmittance for increasing fluence (energy per unit area). We also perform picosecond time-resolved pump-probe measurements. We find that the observed pump-probe data is nearly identical for the metal cluster solution and the carbon particle suspension (CBS), and we conclude that the nonlinear mechanisms are the same for the two materials. Our previous studies have shown that the nonlinear losses are due to scattering and absorption by microplasmas formed after thermionic emission from heated particles of carbon or inorganic clusters.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Transient energy transfer or two-beam coupling is demonstrated in CS2 and other transparent Kerr liquids using frequency chirped, 17 picosecond (HW1/eM) 532 nm pulses with several polarization combinations. As the temporal delay between pulses in a standard pump-probe geometry is varied within the coherence time, the first pulse always loses energy while the second pulse gains this energy. Scattering from phase gratings can lead to coherent energy coupling only if the nonlinearity has a finite relaxation time. This two-beam coupling in Kerr media such as CS2 is associated with stimulated Rayleigh-wing scattering (SRWS). The frequency difference needed for beam coupling can be achieved with chirped pulses or with short pulses in nonlinear materials if irradiance dependent phase shifts are being developed during the laser pulse due to self and cross-phase modulation. Here we consider the interaction between linearly chirped pulses obtained from our modelocked, Q-switched Nd:YAG laser. This leads to an energy transfer linearly proportional to irradiance, so that the signal can be observed at irradiances lower than those needed for induced phased modulation. The measurements are performed on CS2 but the results are valid for any Kerr liquid that has a nonlinear index of refraction with a relaxation time on the order of the laser pulse width. We demonstrate that the interaction follows the polarization dependence of SRWS. The only parameters needed for the theoretical fittings are the nonlinear index n2, its relaxation time and the linear chirp of the laser pulse. The first two are well known for CS2 and the laser chirp is independently measured using first and second order autocorrelations.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe several methods for optimizing optical limiters, including a modification to existing geometries called the stepped limiter. We show that a stepped limiter may have a performance close to that of a fully optimized limiter with a graded molecular density. Given the difficulty in making a graded molecular density, the stepped limiter may be an attractive approach toward making practical devices. We also discuss the importance of damage threshold of the nonlinear limiting material on the limiter design and performance. Liquids have high damage thresholds and for this reason we may use tandem limiting geometries for devices based on nonlinear absorber molecules in liquid solutions. With currently available materials, this is still the best approach. Our experimental results on a tandem limiter based on Zn:tetra((rho) -methoxyphenyl) tetrabenzporphyrin show the best limiting performance to date.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The damage threshold of five different types of quartz glass obtained from NSG Precision Cells, Inc. was determined using nanosecond pulses at 532 nm. It was found that the damage threshold of one of the glasses is more than twice that of others.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have measured n2 and two-photon absorption coefficients in type IIa diamond at (lambda) equals 1064, 532, 355, and 266 nm using Z-scan technique and picosecond pulses from a modelocked Nd:YAG laser. The observed despersion is compared to the two-band theory of semiconductors.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We discuss the characterization of nonlinear optical processes that give rise to changes in the absorption coefficient and refractive index. We primarily concentrate on methods for determining the dominant nonlinearities present in condensed matter and the responsible physical mechanisms. In extensive studies of a wide variety of materials we have found that there is seldom a single nonlinear process occurring. Often several processes occur simultaneously, sometimes in unison, sometimes competing. It is necessary to experimentally distinguish and separate these processes in order to understand and model the interaction. There are a variety of methods and techniques for determining the nonlinear optical response, each with its own weaknesses and advantages. In general, it is advisable to use as many complementary techniques as possible over a broad spectral range in order to unambiguously determine the active nonlinearities. Here we concentrate on the use of nonlinear transmittance, Z-scan and degenerate four-wave mixing experiments as applied to polycrystalline and single crystal semiconductors and dielectric materials.

Photoelectron spectroscopy of coordinatively unsaturated organometallic anions can provide a means to probe the ground and low lying excited states of the corresponding neutral radicals. We report results for the early 3D transition metal monocarbonyls VCO and CrCO, and for the late metal complexes FeCO, CoCO and NiCO. Each spectrum displays a transition to the ground state of the neutral complex, and to an excited state whose spin multiplicity differs by two from that of the ground state. For a given complex, these states share nominally the same electron configuration but differ in the spin coupling of the metal 4s electron. There is a reversal in the state ordering as one proceeds across the transition series, from a high spin ground state for VCO (6(Sigma) +) and CrCO (7(Sigma) +) to a low spin ground state for FeCO (3(Sigma) -), CoCO (2(Delta) ) and NiCO (+1)(Sigma) )+). The measured state splittings and vibrational frequencies provide insight into the factors that determine the ordering and bonding properties of these states. Recent results for the linear H-M-CO isomers of Fe and Co are also reported.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.