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Analytical, numerical, and experimental studies of the nonlinear transmission of a Pt:ethynyl compound have been carried out. Based on a model for the electronic transitions derived from earlier work, optical limiting behavior that is both broadband across the visible and effective over a range of pulse lengths, is predicted. Detailed experimental results are presented which exhibit many of the theoretically predicted characteristics, including a broadband nonlinear response from 450 to 700 nm.

A theoretical model describing the fluence-dependent absorption of Gaussian shaped laser beams by optically thick nonlinear media is applied to the interpretation of Z-scan measurements with thick samples of chloro-aluminium-phthalocyanine. The results obtained from the measurements and the modeling have produced an enhanced understanding of the nonlinear processes operating within this type of material, as well as the role of the relaxation mechanisms and saturation effects in shaping the nonlinear absorption, and the optical limiting capability of this type of absorber. An exact first-order analysis of a five-level molecular model has been developed and explored for the case where the incident beam possesses a flat temporal profile. The predictions of the model are in excellent agreement with experimental observations and with results produced from a numerical modeling technique, previously developed, where a simple "relaxation-free" and "population-saturation-free" three-level model is used to predict the transmission properties of a thick five-level system.

We report pulsed laser studies of nonlinear luminescence in a platinum ethynyl complex which, from 560 to 700 nm, shows strong two-photon absorption from the ground singlet state to an excited singlet state. The spectral dependence of the photoluminescence peaks indicates that in the region from 560 to 700 nm the two-photon absorption cross section is independent of excitation wavelength. Through the use of a simple calibration we have been able to estimate the number of states (NS1) formed from two-photon absorption, and have found good agreement with theory using the value of β=0.34 cm/GW measured previously.

The Z-scan technique is a popular method for measuring degenerate (single frequency) optical nonlinearities using a single laser beam. In order to perform reliable measurements, it is necessary to carefully characterize and control a number of experimental parameters, such as the beam quality, the power and temporal characteristics of the laser, the collection aperture size and position, the sample reflectivity, sample thickness and imperfections in the sample. Failure to control these parameters leads to inaccurate determinations of the nonlinearities. In this paper, we review the theory of Z-scan and examine each of these issues from experimental and theoretical viewpoints. This work will be of interest to anyone who performs Z-scan experiments and to those interested in optical power limiting and nonlinear optical propagation.

A device which acts as both a simple cw laser warner and anti-dazzle protection device has been designed and tested. The design is based on a single stage, double element, tunable liquid crystal Lyot filter. Laboratory tests have shown that the detection sensitivity of the device for monochromatic cw radiation is approximately 43×10−9 W/cm 2 and that the achievable contrast ratio is greater than 100:1 Automatic detection and filtering of a cw laser source has been demonstrated. Results indicate that the algorithm to automatically filter the laser radiation works well and that the extinction obtained is good. Factors which affect the contrast ratio are discussed and improved device design suggested.

We review many of the optical devices exploiting nonlinear optical and electro-optic effects which fall within the general categories of switches, limiters and discriminators.

Numerical studies of nonlinear prism coupling into thin-film organic material waveguides have been carried out. Results are presented showing that, for an undiluted, low absorption material, the electronic response dominates at pulse widths of tens of nanoseconds. The result of an increase in the absorption is that shorter pulses are necessary for access to the Kerr law nonlinearity alone. Guidelines are provided for distinguishing between electronic and thermal responses.

A method for attenuating a laser beam based on the excitation of a surface plasmon is described. This attenuator is continuously variable, has a dynamic range of 10(2), has a spectral range covering the visible, does not steer the beam or distort its profile, and can handle powers of at least 1-W cw. Some preliminary results obtained on a crude prototype are presented; final device design and predicted performance are discussed.

A double-element broad-band liquid crystal tunable filter consisting of a high-order quartz wave plate in series with a very-low-order electrooptically tunable liquid crystal phase plate and placed between crossed polarizers is described. A possible application for such a device is protection against intrusive laser radiation. The advantage of this double-element filter over a single-element low-order LC filter is that it increases the wavelength resolution of the device, which, when operated in the protect mode, allows continued sensing of the scene. Experimental and theoretical studies were made of the contrast ratios obtainable with such a filter. >

Broadband electro-optically tunable filters have uses in applications such as wavelength de-multiplexing and laser light detection and filtering. The materials requirements for such devices are that they must have a very low absorption over the wavelength region of interest, have a large electro-optic coefficient and be easily engineered into optical components. Liquid crystals satisfy these criteria.

A comparison is made of three device configurations which can be used as tunable filters: a Fabry-Perot etalon used in transmission (FPT), a Fabry-Perot etalon used in reflection (FPR), and a variable birefringence filter used in transmission (VBF). Initial conditions are defined for the achievement of a contrast ratio of 100:1, and the implications of these results on material choice discussed. The effect of nonuniform device thickness on contrast ratio is analyzed for all three cases. Analytic expressions are derived which predict the behavior of the contrast ratio under differing device conditions and which give rule-of-thumb values for the maximum acceptable thickness variation for a given contrast ratio. Using these expressions it is shown that the configuration least sensitive to variations in thickness is the VBF, with the FPT coming second and the FPR last. Experimental studies were carried out on three devices-a piezoelectric tunable air-spaced Fabry-Perot etalon, an electrooptically tunable liquid-crystal Fabry-Perot etalon, and an electrooptically tunable liquid-crystal variable birefringence filter. >

The Global Hawk system is a high altitude endurance unmanned aerial vehicle developed under the United States Air Force Advanced Concept Demonstrator program primarily as a reconnaissance system for use against fixed and mobile targets. The Global Hawk system deployed to Australia in April 2001 for six weeks and in this period conducted 11 missions with the focus on maritime surveillance. The Australian deployment was the culmination of two years collaboration between the United States and Australia that included modifications to the radar sensor, system control and exploitation to support a surveillance focus. This paper presents aspects of the Australian contribution to the Global Hawk deployment including the rationale behind the sensor modifications and employment that achieved a surveillance capability with a system primarily designed for land reconnaissance.

This work reports the results of nonlinear absorption and nonlinear fluorescence studies carried out on a 0.8 M solution of the complex in THG in the spectral range 450 to 700 nm. Nonlinear absorption results demonstrate that this material exhibits optical limiting behavior over this entire spectral band when excited with nanosecond pulses. Results of nonlinear fluorescence and excite probe studies show that at wavelengths longer than 530 nm, the nonlinear absorption is a consequence of the pathway: two-photon absorption from the S/sub 0/ to the S/sub 1/ state, fast intersystem crossing in less than 300 ps from S/sub 2/ to T/sub 1/, followed by excited state absorption from T/sub 1/ to T*. Fluorescence studies as a function of the excite wavelength show that the two-photon absorption coefficient has little variation with wavelength in the region 550 to 700 nm. Detailed studies of the nonlinear absorption behavior at 595 nm reveal that the nonlinear absorption has a significant contribution from the triplet excited state absorption, resulting in an optical limiting performance that is not strongly dependent on pulse length.

The Global Hawk high altitude endurance unmanned aerial vehicle was deployed to Australia in April 2001 for six weeks. The system conducted eleven missions with the focus on maritime surveillance. This was the culmination of two years collaboration between the United States and Australia which included development of three new sensor modes, sensor control and exploitation techniques to efficiently perform surveillance with a system primarily developed for the reconnaissance role. The paper describes the Australian contribution to the Global Hawk deployment. It describes the sensor, system integration, system control, mission management and ground element challenges presented by shifting the operational focus from reconnaissance to surveillance.

Complete ZnSe interference filters exhibiting optical bistability have been fabricated using ultrahigh-vacuum molecular-beam techniques. The wavelength dependence of the critical switching power wa...

The mechanism of multi-layer build up of Langmuir-Blodgett films has been studied using the liquid crystal imaging technique. Two materials were studied w-tricosenoic acid and cadmium arachidate. Differences in the morphologies of different numbers of layers is discussed and results on the behaviour of defects throughout the film thickness are presented.

A study has been made of the transient response of a Fabry-Perot cavity exhibiting bistability of the optical output with applied voltage. In this system the electro-optic and optical nonlinear effects act cooperatively to produce the switch, and the transient response was shown to have characteristics associated with both mechanisms. The switch was found to be composed of three distinct regions, a purely electro-optic part, followed by a slow change in the cavity intensity brought about by the electro-optic and nonlinear effects, with a final fast switch up to the high transmission state. The relative magnitude of the time duration of regions 1 and 2 was found to be dependent upon the applied voltage, initial dc biasing and optical input power. In particular, critical slowing down was evident in region 2 of the switch for values of applied voltage close to the critical value. In this case region 2 was the dominant part of the switching time. Overdriving the system resulted in the time duration of region 2 decreasing and region 1 becoming more dominant. Qualitative agreement was achieved between the experimental results and theoretical predictions of the transient response of a cavity undergoing a slow change in some external parameter.

A method has been developed which allows the characterisation of Langmuir-Blodgett films. The technique utilises the liquid crystal aligning properties of the films and may be applied to films comp...

Optical studies were carried out at 647 nm in a Fabry-Perot etalon containing the liquid crystal mixture 3/5/7 PCH. The electro-optic and nonlinear effects were brought together in two sets of experiments. Firstly, optical bistable loops were obtained at different initial cavity tunings imposed by the applied voltage through the electro-optic effect. Secondly, bistable switching was achieved by fixing the input optical power and varying the voltage to obtain “voltage bistable” loops. The behaviour of these loops was found to follow the trend predicted by theory.

Some initial studies of the liquid crystal aligning properties of LB films of u-tricosenoic acid, cadmium arachidate and merocyanine dyes have been carried out. These particular materials have been found to induce homeotropic, or tilted homeotropic, alignment in liquid crystal layers. The tilt angle has been measured for alignment on u-tricosenoic acid and an example of an electrically switched liquid crystal cell using these aligning layers has been demonstrated. Differences have been noted in the uniformity of aligning properties of multilayers of these materials dependent on the packing density of the molecules within the films. In addition it has been found possible to change the alignment of the liquid crystal on u-tricosenoic acid that has been subject to e-beam irradiation.

This paper reviews the different types of optical bistability in II–VI semiconductor compounds with particular emphasis on experimental results obtained in CdHgTe, CdTe and ZnSe. Results of optical bistability in an entirely molecular beam deposited ZnSe filter are presented for the first time.

Some preliminary studies of monolayers of ω-acid using the liquid crystal characterisation technique have been carried out. Observations have revealed a crystalline nature to the films with the size of the crystalline domains varying between approximately 30 and 100 microns. In addition these crystallites were seen to form well defined patterns which are reminiscent of flow. Photographs are presented which illustrate these features.

The Shubnikov-de Haas effect has been studied in InSb to elucidate some remaining problems. A range of samples was produced by neutron transmutation doping to provide high-homogeneity impurity distributions. Double-differentiation techniques were used to improve the peak resolution. The Dingle temperature obtained from the low-field mobility was lower by a factor of three than that deduced from Shubnikov-de Haas measurements. This difference is attributed to a difference in the relaxation mechanisms as the quantum limit is approached. The small amplitude of the higher-field peaks of the spin-split longitudinal doublets is explained in terms of the predominance of small-angle scattering. It is concluded that the small 0- longitudinal peak reported by most previous workers is real and not a transverse residue due to misalignment, g-values in good agreement with theory were obtained from spin-splitting measurements.

Semiconductor interference filters based on II-VI semiconductors show promise as large area, room temperature, optically bistable devices for applications in digital optical circuits using visible light [11]. These active filters, operating by opto-thermally induced changes in material optical constants, have generally been fabricated by thermal evaporation of multilayers of ZnSe or ZnS and some low index material. Evaporated filters are limited in thickness and tend to suffer from drift of operating point during use. In this work we have studied Fabry-Perot filters deposited entirely in an ultra-high vacuum (UHV) system using molecular beam techniques (MBE). UHV/MBE fabrication produces a highly dense film structure, free of porosity and with low contamination levels which should improve the stability of these devices.

Ultrahigh vacuum and molecular beam deposition techniques have been used to fabricate ZnSe interference filters. These demonstrate optical bistability with critical switching power being measured at five wavelengths between 521 and 676 nm. These filters demonstrate considerably improved long term operation stability over that offered by samples deposited using conventional thermal evaporation. Both dispersive and absorptive bistability could be demonstrated in the same sample.