Vanadium dioxide exhibits a semiconductor to metallic phase transition at a temperature of about 68 °C. This can be exploited in the form of optical thin film structures which will exhibit non-linear behaviour when exposed to pulsed infra-red radiation. Since the phase transition is structural in nature, it is of interest to explore the temporal and spatial properties when irradiated with pulsed laser sources. Fast CMT detectors have been used to resolve nanosecond temporal detail on an experimental basis and the spatial charateristics have been explored using a simple adiabatic heating model. The dynamic transmission values measured for VO2 devices are complex combinations of the temporally and spatially varying characteristics of the film.

Thin-film vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) undergo shear phase transformations at 340K and 120K, respectively. The electrical and infrared optical properties of these materials are dramatically impacted by this phase transformation. Below the phase transition temperature, both VO2 and V2O3 are “poor” electrical insulators and have minimal absorption in the infrared spectral region. Above the phase transition temperature, both VO2 and V2O3 are poor conductors and are opaque in the infrared. The infrared thermochromic properties of these materials make them interesting candidates for several optical applications.

Utilization of either VO2 or V2O3 necessitates an accurate knowledge of the optical properties both above and below the phase transition temperature. This paper reports on some recent infrared ellipsometric measurements on the optical properties of these materials above the transition temperature. These measurements found that both are strongly dispersive through the 3–12 micron spectral region. In addition, the temperature dependence of the optical properties of VO2 above the transition temperature can be related to the film morphology and stoichiometry.

Large values of the infrared nonlinearity make semiconductors important in many optical applications. Along with the two-photon absorption coefficient and free-carrier absorption cross-section, the values of the decay rates of the generated charge carriers control the nonlinear optical performance of semiconductors. In this paper, calculations of the limits of the charge carrier decay rates for five infrared materials are provided and some experimental results, performed in the long-wave and the mid-wave infrared wavelengths on three of these materials are presented. To the authors' best knowledge, this is the first report of measurement of optical nonlinearity of Hg0.65Cd0.35Te in the 4 -5 jtm wavelength range. Comparisons of room temperature and low temperature nonlinear optical behavior of all the materials studied are presented.

A variety of approaches have been used for optical limiting in the past [1–6].A relatively new approach, an electro-optic power limiter (EOPL) has a simple and compact design, wavelength agile sensitivity, the ability to respond to coherent as well as incoherent radiation, low threshold and a large field of view (up to 15°). EOPL devices allow the transmission of low intensity background radiation while simultaneously blocking the damaging high intensity beam. We have demonstrated EOPL in the past using the II-VI semiconductors CdTe:In [7,8], CdTe:V [9] and ZnTe [10]. In this paper, we are reporting on EOPL using a new ternary II-VI semiconductor Cdl-xMnxTe:V. Cd0.55Mn0.45Te is capable of limiting optical radiation from visible (0.630.tm) to near infrared (1.5pm) wavelengths, which covers the wavelength ranges of both CdTe and ZnTe combined. Additionally, crystals of Cd0.55Mn0.45Te are relatively easier to grow than ZnTe. We report our preliminary results of optical limiting in Cd0.55Mn0.45Te:V at 0.780μm, 0.895μm, and 1.06μm, and we discuss the materials issues involved in improving and optimizing Cd1-xMnxTe for electro-optic power limiting.

Electro-optical properties of thin films of hydrated transition metal oxides, WO3 2(H2O) and WO3 1/3(H2O), have been investigated in situ using photoelectrochemical methods. These films can reversibly change their light absorption in the near IR region due to the double charge injection upon the applied electric field in electrolyte media. Taking into account the high stability and fast switching time, these hydrated tungsten oxide films can be utilized for efficient light modulation in the near infrared region of the spectrum.

We have investigated the potential for using photorefractive phenomena in optical limiting devices. The major obstacle preventing the photorefractive limiter from reaching this potential has been speed of response. In this paper we report on several techniques which are successful in enhancing the photorefractive beam fanning response time. In particular, we demonstrate and characterize the use of applied electric fields, gratings to seed the beam fan, and cylindrical focusing. We also report the first demonstration of a prototype beam fanning optical limiter encompassing many of the proposed improvements.

Optical limiting has been suggested as a possible device application of high gain photorefractive (PR) polymers. The principle of operation relies on the existence of amplified scattering (beam fanning) to deplete the intensity of the incident coherent optical beam. The newest photorefractive polymers show a combination of high gain and reasonable speed, and we show how these materials behave as a beam fanning optical limiter and as a limiter based on self-pumped phase conjugation. A prototype cascaded beam fanning device is demonstrated and its salient features are discussed.

We present two-wave mixing results obtained with CdSSe:V crystals. A large photorefractive gain of 0.24 cm-1 was observed at 633 nm with an optical intensity of 60 mW/cm2 and a grating period of 1.6 μrm. At this wavelength, we measured the photorefractive gain as a function of the grating period and incident optical intensity. To our knowledge, this is the first observation of the photorefractive effect in vanadium doped CdSSe crystals. The CdSSe crystals were grown by physical vapor transport (PVT) and doped with 150 ppm (nominal) vanadium for creating trap centers. The grown crystal has a large crystal size, good optical quality and a medium resistivity of 10-5 - 108 Ω-cm. Room temperature absorption and low temperature photoluminescence spectroscopy measurements are also discussed. With a significant photorefractive effect, the CdSSe:V crystals are promising for many device applications based on photorefractive effect, including optical limiting devices in the visible region.

Computer simulations on Fabry-Perot liquid crystal tunable filters are performed in the 3–5 and 8–12 μm bands and some discrete laser lines. Effects of transmittance, contrast ratio and response time on liquid crystal birefringence, layer thickness, absorption coefficient and reflectivity of mirrors are analyzed. Absorption spectra and responsible mechanisms of liquid crystals in the 2.5–20 μm region are studied. The fluorinated tolane and diphenyl-diacetylenes and dialkyl diphenyldiacetylenes are found to possess low absorption in the mid-IR range. Mixtures of these compounds will find useful applications for the proposed IR tunable filters.

The nonlinear optical effects of chlorophyll ‘a’ have been determined using an array of measurement techniques. Regions of spectral interest exhibiting induced excited state absorption were identified with transient absorption spectroscopy measurements. At specific wavelengths within these regions, the dye was characterised using the Z-scan technique. Additional measurements were made to complement the latter results including a focused aperture experiment in which thermal defocusing and scattering mechanisms were identified.

The effects of adding various chemical surfactants to the prepolymer syrup on the electro-optical switching properties of Bragg gratings recorded in polymer dispersed liquid crystals (PDLC's) have been studied. The gratings were holographically recorded in prepolymer recipes substituting hexanoic, heptanoic, propylpentanoic and octanoic acids as surfactants in the recipe. A small percentage of monomer containing an attached long chain alkyl group (vinyl neononanoate) was also used instead of a surfactant. The addition of surfactants resulted in lowering the required switching field from 17 V/μm with no surfactant to 1.5 to 8 V/μm at a concentration of about 6.7% by weight, depending on the surfactant. Field on response times for electrical switching decreased with the addition of surfactant, and off times increased.

Several liquid phase liquid crystals and liquids are found to possess broadband nonlinear absorption characteristics suitable for optical limiting applications. Using these liquids as fiber waveguiding cores, we have developed fiber arrays that provide excellent limiting performance characteristics against frequency agile visible laser pulses in the nanosecond - picosecond time scale. These fiber arrays are compact, low cost/weight, and are capable of transmitting high quality images throughout the entire visible spectrum.

Optical Limiters which operate by exploiting nonlinear absorption and nonlinear refraction are most effective towards an incoming laser beam when the active material is somewhat thicker than one Rayleigh length. A variety of thick optical limiter configurations are described and modeled by appropriately integrating the formulas for thin nonlinear media with gaussian-shaped beams. The attenuating effects of linear losses are allowed for, and the approach is exact to the first order in optical power or irradiance. Irradiance-dependent and fluence-dependent absorbers are considered. For a gaussian-shaped input beam, we find that the combined effects of diffraction and nonlinear absorption produce, in the thick-medium limit, a transmitted beam which retains a gaussian-shaped profile at the far field, but which is now uniformly attenuated according to the magnitudes of the nonlinear and linear absorption parameters. This implies that the normalized transmittance is independent of the size of the receiving aperture. As the optical beam power increases the limiting behaviour saturates. We have discovered a simple saturation formula which agrees well with numerically calculated transmittances for thick irradiance-dependent absorbers within the strongly nonlinear regime. Under certain conditions this formula also describes the behaviour of refractive limiters, providing a simple and accurate technique for modeling limiters over a broad parameter range.

We study in detail the linear and nonlinear optical properties of a heavy metal substituted naphthalocyanine reverse saturable absorber (RSA). The excited states involved in the RSA process under nanosecond pulsed laser excitation in the visible are fully characterized. We then demonstrate that this material can be used as an efficient optical limiter for protecting silicium CCD arrays from destruction due to nanosecond-pulsed laser irradiation in the visible.

A user-friendly beam propagation program has been developed for use over the worldwide- web to aid the optical limiting community in modeling the transmission of light through nonlinear refractive and absorptive media having local intensity-dependent or nonlocal fluencedependent mechanisms.

The first experimental investigation of an optical limiting device based on a suspension of spherical particles in a surrounding liquid is presented. The expected property is the non-linear light scattering based on a refractive index mismatch between the two components that appears at high intensity. Several experiments performed at 532 nm with nanosecond laser pulses provide good indication that such non-linear scattering is observed.

Time-resolved imagery is presented showing the changes that occur in the focal volume of dilute liquid/particle suspensions following the arrival of single, Q-switched, frequency-doubled, Nd:YAG laser pulses. Limiting data and corresponding imagery at 21, 84, 244, 790, and 2900 nanoseconds following the laser pulse are presented for a carbon particle suspension consisting of used (carbonized) 1OW-15 motor oil and for a suspension of the inorganic metallic cluster molecule Mo2Ag4S8 [PPh3]4. The images in conjunction with the accompanying limiting data show that the reduction in transmission, observed as the input pulse energy is increased, results from scattering from bubbles augmented by plasma absorption. Keywords: laser induced breakdown, plasma, suspension, bubble, nonlinear materials, shadowgraph, imagery

Although Carbon-Black suspensions (CBS) have been studied extensively for optical limiting applications, the microscopic mechanisms responsible for their limiting behavior still remain unclear. To study the mechanisms leading to the nonlinearity, we have performed a pump-probe experiment coupled with plasma emission measurements. Time resolved probe transmission have permitted to differentiate bubbles from plasma effects in the case of Carbon-Black in CS2. Other effects are discussed in terms of limiting performance.

The nonlinear optical effects of chlorophyll ‘a’ have been determined using an array of measurement techniques. Regions of spectral interest exhibiting induced excited state absorption were identified with transient absorption spectroscopy measurements. At specific wavelengths within these regions, the dye was characterised using the Z-scan technique. Additional measurements were made to complement the latter results including a focused aperture experiment in which thermal defocusing and scattering mechanisms were identified.

he Effective Fragment Potential (EFP) model of solvation can now be extended beyond aqueous systems due to the development of transferable exchange repulsion potentials. EFPs for methanol and chloroform have been developed, and calculations with these new EFPs agree well with full ab initio calculations. Ab initio calculations have been carried out on zinc tetraphenyl-octobromyl-porphyrin both with and without the EFP solvation model. While the aqueous calculation, which had its geometry optimized, gave good results, the single-point calculations carried out with the two new solvent models indicate the need for geometry optimization.