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Two-photon absorption spectra for a range of bis(acceptor)-substituted bis(dibutoxythienyl)ethene and bis(N-hexylpyrrolyl)ethene chromophores have been recorded using Z-scan and white-light-continuum pump-probe techniques. All the chromophores studied show strong near-infrared two-photon absorption with cross sections in the range of 2400-5900 GM (1 GM = 1 × 10-50 cm 4 s/photon) at photon wavelengths between 1.0 and 1.3 μm; cross sections as high as 10000 GM can be accessed close to the one-photon absorption edge. Quantum-chemical calculations reproduce the experimentally observed variations of the two-photon properties with the chemical structure. © 2007 American Chemical Society.

The bandgap and wavelength scaling of three-photon absorption (3PA) is studied in several semiconductors by the Z-scan technique. The 3PA coefficient is found to vary as Eg-7as predicted by theory.

We generate high energy, high spatial quality supercontinua in Krypton gas for Z-scan measurements of nonlinear absorption spectra and the dispersion of nonlinear refraction in optical materials throughout the visible. This eliminates the need for tunable sources (e.g., optical parametric devices) for nonlinear measurements.

A farewell message from the founding Editor-in-Chief of Optical Materials Express and a look back at the first five years of the Journal.

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.

We compare two-photon absorption spectra of polymethine, squaraine and tetraon dyes experimentally. Both squaraine and tetraon dyes display peak two-photon cross-sections considerably larger (>8×103GM) than polymethine.

We studied the nonlinear refraction due to bound electrons in InSb using femtosecond pulses in the range of 8-13 μm and show that the nonlinear refraction coefficient changes sign at a wavelength around 10 μm.

We compare two-photon absorption spectra of polymethines and squaraines experimentally and quantum-chemically. Squaraines display two-photon cross-sections considerably larger (7times103GM) than polymethines, due to an extra band introduced by the squaraine structure in the conjugated chain.

The spectrum of two-photon absorption in undoped InSb is measured by temperature dependent transmittance studies using a nanosecond CO2 laser and by pump-probe and Z-scan methods using tunable infrared picosecond and femtosecond sources.

We use white-light continuum Z-scan to measure the degenerate two-photon absorption spectrum and associated dispersion of the nonlinear refraction, n/sub 2/, of ZnSe/spl middot/n/sub 2/ changes sign at around 0.7 of the bandgap energy as predicted.

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.

Three new dipolar chromophores based on a diaklyaminophenyl donor, a pyrrole auxiliary donor, a thiazole auxiliary acceptor, and strong heterocyclic acceptors have been synthesized. For one of these compounds we have measured a very large non-degenerate two-photon cross section of ca. 1500 GM in the near-IR telecommunications range using a pump-probe technique. Calculations indicate the cross section for degenerate two-photon absorption is likely to be ca. 60% of this value.

We present 2PA spectral measurements of new fluorene-based molecules with electron-withdrawing nitro groups measured using femtosecond Z-scans and compare them to picosecond Z-scan results and 2-photon fluorescence. We observe what looks like one-photon resonance enhancement.

The degenerate two-photon absorption spectra for a series of symmetric and asymmetric polymethine and squarylium dyes were measured by femtosecond Z-scan and upconverted fluorescence methods. Structure-property relationships were revealed and explained with quantum-chemical analysis.

We report degenerate and non-degenerate two-photon absorption spectra of II-VI semiconductor quantum-dots. We find that these spectra depend on quantum dot size and size distribution. We propose a theory for explaining the non-degenerate case.

We use a white-light continuum (WLC) as the source in a standard Z-scan technique to measure the degenerate two-photon absorption (2PA) spectrum and the nonlinear refraction dispersion in optical materials.

Nondegenerate two-photon absorption spectra of organic molecules have been taken using our femtosecond continuum nonlinear spectrometer. This data shows interesting structure/property relations in addition to resonance enhancement of the third-order nonlinearity which is consistent with simple models.

The degenerate and nondegenerate two-photon absorption (2PA) spectra for a symmetric and an asymmetric fluorene derivative were experimentally measured in order to determine the effect of intermediate state resonance enhancement (ISRE) on the 2PA cross section delta. The ability to tune the individual photon energies in the nondegenerate 2PA (ND-2PA) process afforded a quantitative study of the ISRE without modifying the chemical structure of the investigated chromophores. Both molecules exhibited resonant enhancement of the nonlinearity with the asymmetric compound showing as much as a twentyfold increase in delta. Furthermore, the possibility of achieving over a one order of magnitude enhancement of the nonlinearity reveals the potential benefits of utilizing ND-2PA for certain applications. To model ISRE, we have used correlated quantum-chemical methods together with the perturbative sum-over-states (SOS) expression. We find strong qualitative and quantitative correlation between the experimental and theoretical results. Finally, using a simplified three-level model for the SOS expression, we provide intuitive insight into the process of ISRE for ND-2PA.

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.

By systematically altering the structural properties of various fluorene-based molecules, we determine how these changes affect their two-photon absorbing capabilities. Here we emphasize the effects of conjugation extension, however other structural motifs were also investigated.

Saturation limits excited-state absorption (ESA) in many organic molecules. We studied polymethines using continuum spectroscopy to determine ESA spectra, two-color pump-probe anisotropy for transition dipole moment orientations, picosecond and nanosecond z-scans and nonlinear transmission measurements.

We measured nondegenerate two-photon absorption spectra of single-crystal polydiacetylene (PTS) with white-light continuum nonlinear optical spectroscopy. We observed an intermediate state resonance enhancement in qualitative agreement with a three-level model

Utilizing a type II interaction for second‐harmonic generation in a crystal of potassium titanyl phosphate, we experimentally demonstrate the all‐optical action of a light modulator with both signal and output at the same optical wavelength. This modulator is controlled by the intensity of the injected signal, a characteristic that makes it a suitable candidate for all‐optical transistor action and ultrafast analog processing in transparent networks for telecommunications.

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.

Summary form only given. Over the past several years our efforts have been directed toward a systematic investigation of nonlinear properties of a new series of polymethine dyes (PDs) and squarylium dyes (SDs) in liquid solutions and polymeric media. These studies led to the development of several new dyes showing strong excited-state absorption (ESA) in the visible region with the ratios of excited-to ground-state absorption cross-sections up to 200 at 532 nm. One of these dye's (labeled PD2338) chemical structure is shown along with its singlet energy-level diagram. The study of the excited state transition moments will lead to a deeper understanding of the nature of ESA in organic molecules and will aid in the quantum-chemical calculations for the prediction of their nonlinear properties. To measure the orientation of the transition moments we use a pump-probe geometry in which the probe polarization is fixed and the pump polarization can be rotated using a halfwave plate. In a pump-probe experiment an intense pump beam induces a nonlinear response in the medium that is then sensed by the weak probe beam. This experimental procedure may be used to determine population decay kinetics and rotational relaxation times. These decay times can be measured by conducting three measurements for the pump polarization-parallel, perpendicular, and at the magic angle (57.3 degrees) to the probe. The results for PD2338, using the second harmonic of a picosecond (25 ps FWHM) Nd:YAG laser are shown.

Summary form only given. Two-photon absorption (2PA) has achieved appreciable interest in a wide array of fields including all-optical switching, two-photon fluorescence microscopy, and optical power limiting. Consequently, it is a goal to find materials that exhibit large 2PA cross-sections. Organic materials are strong candidates for these applications because their material properties can be tailored through molecular engineering. This not only allows organics to be utilized in a wide variety of applications, but also provides a means to analyze structure-property relationships for a class of samples. We present a method for rapid characterization of 2PA materials using a femtosecond white-light continuum (WLC) pump-probe nonlinear spectrometer. In this experimental instrument, a strong pump beam induces a nonlinear response in the system under investigation and a weak, broad-band WLC beam probes the induced nonlinearity. The 2PA process requires simultaneous absorption of one photon from both the pump and probe beams and the outcome is the nondegenerate 2PA spectrum of the sample. We have synthesized a group of fluorene derivatives in order to investigate the structure-property relationships of these dyes.

Summary form only given. Measurements of the nonlinear absorption spectra and the dispersion of the nonlinear refraction are of interest in modeling nonlinear optical materials and determination of structure/properties relations. In the past, most of these experiments were performed at a single wavelength. Tuning over a broad spectral region can provide with a comprehensive information but, often times, one has to perform calibration of the experiment at each wavelength. An alternative is to use a white-light continuum produced by focusing short pulsed lasers into suitable media (e.g. sapphire) and using a pump-probe geometry where the probe is the continuum and the pump is an intense pulse that induces the nonlinearity. We have developed a femtosecond continuum spectrometer to measure nondegenerate spectra from 300 nm in the UV to 1.7 /spl mu/m in the IR. The pump beam is the output of a regeneratively amplified fiber ring oscillator at 775 nm or the output of a subsequent optical parametric generator/amplifier producing wavelengths from 500 nm to 1650 nm. The continuum is produced by focusing /spl sim/2 /spl mu/J of different frequencies from these two sources. Dual diode arrays for the IR and visible are used to measure the spectral changes in the transmittance against a reference beam. The spectra can be obtained as a function of time delay between pump and probe pulses. This method is applied for measuring nonlinear absorption spectra of organic materials.

Summary form only given. A variety of experimental techniques for measuring nonlinear refractive and absorptive coefficients has been invented and developed during the last two decades. Here we concentrate on perhaps the simplest of these methods-the so-called Z-scan technique. By varying the position of a sample with respect to the focal plane of an input lens and detecting the energy transmitted through an aperture placed in the far field, one can determine the nonlinearly induced phase shift. In the absence of the aperture the nonlinear loss is measured. From these measurements, and knowledge of the underlying physics describing the nonlinear optical (NLO) response of the material, the nonlinear refractive index and absorption coefficient can be obtained.

We will describe several education programs that serve the growing optics industry in the Central Florida region, including the School of Optics’ graduate programs and two-year and four-year programs in Photonics Technology taught by collaboration between the University of Central Florida and local community colleges.

The results presented here give a demonstration of the potential of all-optical switching using X/sup 2/ nonlinearities. The switching irradiance scales as [X/sup 2/]/sup -2/ so that new organic crystals with X/sup 2/ more than ten times that of KTP will significantly reduce switching irradiances. In addition, waveguide geometries will lead to further reductions in the switching energy. >

This paper will discuss in general the so-called third-order nonlinear optical susceptibility X(3). Values for this quantity are often quoted that vary wildly in magnitude depending on the material and source used, in particular the source pulsewidth. It would be "nice" if X(3) were constant rather than depending on the illumination parameters.

We have developed a nonlinear spectrophotometer that can measure nondegenerate spectra from 1.7 /spl mu/m in the IR to 200 nm in the UV. Dual diode arrays for the IR and visible are used to measure the spectral changes in the transmittance against a reference beam. The pump beam is the output of a regeneratively amplified, frequency doubled, femtosecond fiber ring oscillator.

The limit in measuring the Raman gain at small frequency detunings using traditional techniques is determined by the requirement that the DC pump beam must be removed from the detection system. Low-frequency Raman gain measurements have been reported down to 6 cm/sup -1/ in glass fibers with interaction lengths of several hundred meters. Obviously this method is not applicable to other materials. Here we propose a novel method for measuring the Raman gain at low frequency mm sample lengths based on two beam coupling of chirped pulses. The frequency difference between the pump and probe beams in a pump-probe experiment thus depends on the time delay between the pulses, which in turn determines the two beam coupling. The experiment essentially consists of a noncollinear pump-probe system. The two-beam coupling between the pump and probe beams is due to a frequency difference between the beams.

We measure the optical limiting behavior of carbon black suspensions in various viscosity solvents by using a 10-Hz repetition rate, 532-nm, 5-ns pulsed laser. We found that, for common solvents used in the past such as water and ethanol, the limiting behavior ceases after a few laser firings and a turnover in the limiting curve appears. This can be explained by depletion of the carbon black within the focal volume. This turnover shifts to lower energies as the viscosity of the solvent becomes greater. However, for low viscosity liquids, such as carbon disulfide or pentane, the limiting is unaffected by the repetition rate, at least for frequencies up to 10 Hz, because of diffusion of the carbon black particles. This diffusion allows fresh material to replace the irradiated volume within the time between pulses.

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.

A cascaded-focus optical limiter using carbon suspensions has been demonstrated to clamp the output near 1 microJoule for input pulses at several wavelengths throughout the visible spectrum. The device was tested up to 60 mJ without damage.

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.

Summary form only given. We have shown that the nonlinear refraction in CS/sub 2/ for nanosecond pulses is due to a combination of the molecular reorientation (Kerr) effect and electrostriction. The latter works best for tightly focused beams and longer pulses. Modeling indicates that without the electrostrictive effect, the CS/sub 2/ would not be able to protect the PbPc cell from optical damage. We have also studied other mechanisms that might further improve the CS/sub 2/ limiting, including solutions of two-photon absorbers in the CS/sub 2/ and the suspension of carbon black particles in the CS/sub 2/. Carbon black suspensions (CBS) are known to produce strong limiting due to their nonlinear scattering properties. We show how the addition of CBS to the CS/sub 2/ enhances the limiting effect at low to moderate energies with a small penalty in linear transmittance.

We describe optical limiters for nanosecond pulses based on a new cascaded-focus geometry. Nonlinear materials in the second focal plane provide low-energy limiting, while the nonlinear liquid CS2 placed in the first focal plane provides a higher energy limiting threshold, to protect the material in the second focal plane from damage. The element in the second focal plane may be a reverse saturable absorber or a carbon black suspension (CBS) which exhibits nonlinear scattering. Using CBS, the limiter has a broad wavelength response, and clamps the maximum encircled output energy below 1 μJ for nanosecond and even longer pulses. The measured figure of merit of the device is approximately 20,000.We describe optical limiters for nanosecond pulses based on a new cascaded-focus geometry. Nonlinear materials in the second focal plane provide low-energy limiting, while the nonlinear liquid CS2 placed in the first focal plane provides a higher energy limiting threshold, to protect the material in the second focal plane from damage. The element in the second focal plane may be a reverse saturable absorber or a carbon black suspension (CBS) which exhibits nonlinear scattering. Using CBS, the limiter has a broad wavelength response, and clamps the maximum encircled output energy below 1 μJ for nanosecond and even longer pulses. The measured figure of merit of the device is approximately 20,000.