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Linearly polarized light beam can rotate the trans-azobenzene molecules into cis-form resulting in inducing photoanisotropy. This property is used to control the transmission of nanosecond and picosecond laser pulses and also for holographic data storage applications. Studies were performed using four commercially available azobenzene dyes in different solvents and also in custom prepared nanoporous azobenzene Dendron films. Nanopores are created in a silica matrix and azobenzene molecules are attached to the inner walls of these pores to mimic solution like environment for the azobenzene molecules such that they can undergo photoinduced trans-cis-trans isomerization process at a faster rate.

We studied the linear and nonlinear optical properties of halloysite nanotubes using Z-scan technique. Halloysite is alumina silicate clay rolled into 50 nm diameter hollow cylinders, where the silica layer is at the outer surface of the tube and alumina layer is in the inner surface. Optical absorption spectra show an absorption peak around 600 nm. Open aperture Z-scan measurements using 3 ns laser pulses at 532 nm reveal two-photon induced absorption. The closed aperture Z-scan indicates a positive nonlinear refractive index. As these nanotubes are nontoxic and biocompatible, they have advantages over conventional carbon nanotubes for biomedical applications.

In this paper we demonstrated enhancement of photoinduced polarization rotation for improved transmission of laser light at low input intensities through a crossed polarized system. It is achieved by utilizing two azobenzene doped polymer thin films in tandem. As the input beam propagates through the films, its polarization rotation induced by each film is additive. We obtained polarization rotation of as much as 24° resulting in enhanced transmission compared to a single-film approach. In addition, this novel design is promising for use as a broadband nonlinear transmission system.

Real-time medical image processing through nonlinear optical Fourier filtering and transient Fourier holography is demonstrated using bio-organic polymer films of Bacteriorhodopsin (bR). In nonlinear Fourier filtering, the photo-controlled light modulation characteristics of bR films are exploited for early detection of microcalcifications in analog (screen film) as well as digital mammograms. bR films are well known for light modulation between the two states: the initial stable B state with broad absorption band with maximum at 570 nm and the relatively long lived M state with absorption maximum at 412 nm. The spatial frequency information carried by a blue probe beam is selectively manipulated in the bR film by changing the position and intensity of a yellow control beam. In transient Fourier holography, photoisomerizative gratings are recorded and reconstructed in bR films. The laser output from an Ar–Kr ion is expanded and split into two beams. One of them, the object beam is Fourier transfo...

Rationale and Objectives To exploit the spectral phase characteristics of digital or digitized mammograms for early detection of microcalcifications, shape, and sizes of suspected lesions and to demonstrate its use for training radiologists to discriminate signal features in different spatially varying backgrounds. Materials and Methods We propose two algorithms: in the phase-only image (POI) reconstruction algorithm the spectral phase of the digital mammogram is extracted from its Fourier spectrum. This is coupled with unit magnitude and inverse Fourier transformed to reconstruct the POI thus enhancing the features of interest such as microcalcifications, shape, and sizes of suspected lesions. In the algorithm for image reconstruction from a priori phase-only information, spectral phase is used to extract signal features of the digital mammogram and then this is combined with spectral magnitude that is extracted and averaged over an ensemble of unrelated digital mammograms. Results The results for several digital phantoms and mammograms show that POI reconstructs only high spatial frequencies related to the features such as microcalcifications, shape, and size of masses like cysts and tumors. The results on image reconstruction from a priori phase-only information demonstrate the changes in the visibility of signal features when buried in a wide variety of real world mammogram backgrounds with different densities. Conclusion The POI can aid radiologists in early detection of microcalcifications, lesions, and other masses of interest in digital mammograms. This reconstruction method is self-adaptive to changes in the background. The image reconstruction from a priori phase-only information can help the radiologist as a training tool in his decision-making process. Preliminary experiments indicate the potential of the techniques for early diagnosis of breast cancer. Clinical studies on these algorithm procedures are in progress for application as a diagnostic CAD tool in digital mammography. These methods can in general be applied to other medical images such as CT and MRI images.

New Retinal nano-ceramic materials with pillared hybrid micro-structures are fabricated for potential applications in optical holographic data storage. We observe that the Schiff bases have substantial effect on optical properties of Retinal nano-ceramic films as well as temporal response and diffraction efficiency for holographic storage. These inexpensive synthetic biological nano-ceramic thin films are good candidates for holographic storage and offer a simple procedure to solve the problems of volatility of optical storage in Retinal proteins. In addition, our study indicates feasibility of optimizing optical properties of nano-ceramic clay systems using Schiff bases for a variety of photonic applications.

We demonstrate here that large area periodic arrays of well-aligned carbon nanotubes can be fabricated inexpensively on Ni dots made by the process of self-assembly nanosphere lithography. These periodic arrays appear colorful due to their efficient reflection and diffraction of visible light. In addition, due to their honeycomb lattice structure, these arrays can act as photonic band gap crystals in the visible frequency range. In this report, we present the initial exploration of the optical properties of such arrays. Here we show that these potential 2D photonic band gap crystal arrays might find very important applications in optoelectronics.

Optical image processing techniques are inherently fast in view of parallel processing. A self-adaptive optical Fourier processing system using photoinduced dichroism in a bacteriorhodopsin film was experimentally demonstrated for medical image processing. Application of this powerful analog all-optical interactive technique for cancer diagnostics is illustrated with two mammograms and a Pap smear. Microcalcification clusters buried in surrounding tissue showed up clearly in the processed image. By playing with one knob, which rotates the analyzer in the optical system, either the microcalcification clusters or the surrounding dense tissue can be selectively displayed. Bacteriorhodopsin films are stable up to 140 degrees C and environmentally friendly. As no interference is involved in the experiments, vibration isolation and even a coherent light source are not required. It may be possible to develop a low-cost rugged battery operated portable signal-enhancing magnifier.

X-ray mammography is the current gold standard for breast cancer screening. Microcalcifications and other features which are helpful to the radiologist for early diagnostics are often buried in the noise generated by the surrounding dense tissue. So image processing techniques are required to enhance these important features to improve the sensitivity of detection. An innovative technique is demonstrated for recording a hologram of the mammogram. It is recorded on a thin polymer film of Bacteriorhodopsin (bR) as photo induced isomerization grating containing the interference pattern between the object beam containing the Fourier spatial frequency components of the mammogram and a reference beam. The hologram contains all the enhanced features of the mammogram. A significant innovation of the technique is that the enhanced components in the processed image can be viewed by the radiologist in time scale. A technician can record the movie and when the radiologist looks at the movie at his convenience, freezing the frame as and when desired, he would see the microcalcifications as the brightest and last long in time. He would also observe lesions with intensity decreasing as their size increases. The same bR film can be used repeatedly for recording holograms with different mammograms. The technique is versatile and a different frequency band can be chosen to be optimized by changing the reference beam intensity. The experimental arrangement can be used for mammograms in screen film or digital format.

We demonstrate an all-optical switch using molecular states in a bacteriorhodopsin (bR) thin film. All-optical logic gates are implemented with wild-type and chemically stabilized films of bacteriorhodopsin using a two-color backward degenerate four-wave mixing geometry. The films are observed to be stable over a period of four years. The sensitivity of each bR film is markedly different due to the large differences in the saturation intensity which can be controlled by different means. Red light is used to form a grating due to the B to M transition and blue light is used to form a grating due to the fast photochemical transition from M to B. Each of the two wavelengths in the experimental system acts as an input to the all-optical gate and the phase conjugate signal beam bears the output of the gate.

The third order nonlinear optical susceptibilities of cast films of diacetylene-containing polyamides, copolyamides and polyesters, were measured using degenerate four-wave mixing with picosecond pulses at 532 nm. The blue films of polyamides which contain aromatic polydiacetylene had χ(3) values of the order of 10-9–10-10 esu, while the orange films of polyesters having aliphatic polydiacetylenes showed χ(3) values of the order of 10-10–10-11 esu.

We present detailed theoretical evaluation and thorough experimental investigation of quantitative phase imaging using our previously demonstrated dual-plane in-line digital holographic microscopy technique [Opt. Lett.35, 3426 (2010)10.1364/OL.35.003426OPLEDP0146-9592]. This evaluation is based on the recording of two interferograms at slightly different planes and numerically reconstructing the object information. The zero-order diffracted wave is eliminated by using the method of subtraction of average intensity of the entire hologram, and the twin-image diffracted wave is removed by Fourier domain processing of the two recorded holograms. Experiments are performed using controlled amplitude and phase objects and human muscle cells to demonstrate the potential of this technique.

Thinfilms of bacteriorhodopsin are used to manipulate amplitude, phase, polarization of the incident light for a variety of applications like all-optical switching, logic gates, power limiting, optical Fourier processing for breast cancer diagnostics, slow/fast light.

We have developed a common-path multimodal optical microscopy system that is capable of using a single optical source and a single camera to image amplitude, phase, and fluorescence features of a biological specimen. This is achieved by varying either contrast enhancement filters at the Fourier plane and/or neutral density/fluorescence filters in front of the CCD camera. The feasibility of the technique is demonstrated by obtaining brightfield, fluorescence, phase-contrast, spatially filtered, brightfield+fluorescence, phase+fluorescence, and edge-enhanced+fluorescence images of the same Drosophila embryo without the need for image registration and fusion. This comprehensive microscope has the capability of providing both structural and functional information and may be used for applications such as studying live-cell dynamics and in high throughput microscopy and automated microscopy.

We present a microscope that uses single source and single detector, and capable of imaging multiple features like brightfield+fluorescence, phase+fluorescence, and edge enhanced+fluorescence of the biological specimen without the need of image registration and fusion.

We observed enhancement of photoinduced polarization rotation, as much as 24°, when the input laser beam propagates through azobenzene doped polymer thin films that were placed in tandem.

We observed superluminal light in aqueous solution of the protein complex bacteriorhodopsin (bR) at 647.1 nm wavelength where it exhibits reverse saturable behavior, exploiting the technique of coherent population oscillations (CPO). With a modulation frequency of 10 Hz, the signal pulse through a 1 cm path cell is ahead by 3 msec relative to the reference pulse, corresponding to a group velocity of -3.3 m/sec. Following our early work on slow light in the same sample at the saturable wavelength 568.2 nm, we now explicitly observed the narrow spectral hole in the absorption band of the stable B state and further, demonstrated a close correlation between the profile of the hole and the corresponding pulse delay for various modulation frequencies. A similar behavior is observed for superluminal light versus antihole blown in the absorption band.

This paper briefly reviews the basics of optical Fourier techniques (OFT) and applications for medical image processing as well as phase contrast imaging of live biological specimens. Enhancement of microcalcifications in a mammogram for early diagnosis of breast cancer is the main focus. Various spatial filtering techniques such as conventional 4f filtering using a spatial mask, photoinduced polarization rotation in photosensitive materials, Fourier holography, and nonlinear transmission characteristics of optical materials are discussed for processing mammograms. We also reviewed how the intensity dependent refractive index can be exploited as a phase filter for phase contrast imaging with a coherent source. This novel approach represents a significant advance in phase contrast microscopy.

Transient optical modulation properties of a disperse-red-1–doped polymer film are investigated by use of four-wave mixing with two-color light. Interesting behavior is observed for the transient modulation on variation of the two-color light intensities because of competition between the two mechanisms of one-photon (reorientation) and biphoton (isomerization) gratings. Positive and negative modulation as well as double-frequency modulation relative to blue-light input is observed in the experiment. The results may have applications in optical information processing.

La mesure de la puissance-seuil d'autofocalisation d'un faisceau laser permet d'obtenir la susceptibilite optique non lineaire d'ordre 3 d'un polymere en solution; application au cas du poly(dihexylsilane). La methode est sensible et permet l'etude des transitions conformationnelles des polymeres conjugues en solution

Spatial light modulation by use of a biphotonic holographic grating has been demonstrated in an azobenzene-doped polymer film. The mechanism of incoherent-to-coherent optical conversion originates from the photoinduced trans--cis isomerization of azobenzene molecules by two color lights. It is also found that when the intensity of the blue beam is varied the output replica of the input information changes sign.

Novel Fourier phase contrast microscope is developed exploiting monochromaticity, intensity and phase coherence of laser and photo-induced birefringence of liquid crystals. As condenser annulus - phase plate is not required the images are free from artifacts.

Photo-controlled absorption and polarization of Bacteriorhodopsin (bR) films are exploited to perform spatial filtering for screen film as well as digital mammograms. The results offer useful information to radiologists for early detection of breast cancer.

We demonstrated novel all-optical self-adaptive continuous band-pass spatial filtering system which exploits any nonlinear transmission mechanism. As intensity is increased above threshold, low spatial frequencies are blocked resulting in edge-enhanced images containing high spatial frequencies.

Real-time nonlinear optical Fourier filtering for medical image processing is demonstrated, exploiting light modulating characteristics of thin films of the biophotonic material bacteriorhodopsin (bR). The nonlinear transmission of bR films for a 442 nm probe beam with a 568 nm control beam and vice versa is experimentally studied in detail. The spatial frequency information carried by the blue probe beam is selectively manipulated in the bR film by changing the position and intensity of the yellow control beam. The feasibility of the technique is first established with different shapes and sizes of phantom objects. The technique is applied to filter out low spatial frequencies corresponding to soft dense breast tissue and displaying only high spatial frequencies corresponding to microcalcifications in clinical screen film mammograms. With the aid of an electrically addressed spatial light modulator (SLM), we successfully adapt the technique for processing digital phantoms and digital mammograms. Unlike conventional optical spatial filtering techniques that use masks, the technique proposed can easily accommodate the changes in size and shape of details in a mammogram.

Spectroscopic observations are presented for carbon nanotubes grown on silicon and quartz substrates in a hexagonal honeycomb configuration using self-assembly nanosphere lithography and plasma enhanced chemical vapor deposition method. A white light source is used as an incident beam and light reflected from the surface of the carbon nanotubes results in a distinctive signature in the reflected spectrum. A comparison of non-periodic arrays and periodic arrays of carbon nanotubes show that the reflectance signature is only observed when the carbon nanotubes are oriented in a periodic array. Further observations regarding the light antenna effect observed in nonperiodic arrays are also reported. Theoretical curves show good agreement to experimentally observed phenomena. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicate that these materials may find many uses in the field of optoelectronics.

We report a convenient method to obtain all‐optical light modulation in bacteriorhodopsin films using a degenerate four‐wave mixing geometry. Chemically stabilized films of bacteriorhodopsin in a polymer matrix for which the lifetime of the excited M state is tens of seconds were used to demonstrate all‐optical light intensity modulation. The films are observed to be stable over a period of 4 years. Due to the sensitivity of the films, small intensities of order microwatt/ cm2 are required in the modulation experiments. Furthermore the fast photochemical transition from M to B permit reasonably fast modulation speeds independent of the slow thermal M and B relaxation time. The experimental system also acts as an all‐optical switch where a low power blue pulse turns on a signal red beam.

Nonlinear refractive index coefficient n2 of bacteriorhodopsin suspensions in water is measured by the Z-scan technique with a low power continuous wave laser at 647.1 manometer wavelength. Our results indicate that both the magnitude and the sign of n2 depend strongly on the light intensity. Negative values for n2 are obtained for on axis laser irradiance at the focus above 3 W/cm2. The observed self-defocusing phenomena can be attributed to the index change due to the light induced transition between the photochromic states. The results elucidate the origin of n2 and offer a plausible explanation for the differences in the reported n2 measurements.

Carbon nanotubes (CNT) have been grown in a honeycomb configuration on silicon substrates using nanosphere self-assembly and plasma enhanced chemical vapor deposition. The optical properties of the arrays were also studied. Diffraction efficiency was found to be a function of the wavelength, angle of incidence and state of polarization of incident light. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicates that these materials may find many uses in the field of optoelectronics. In addition to their optical properties, periodic CNT arrays have a host of other unique electromagnetic and mechanical properties that may be exploited for numerous applications. Polarization measurements indicate that the intensity of both the diffracted light and diffusely scattered light is dependent on wavelength and angle of incidence. These arrays not only reflect and diffract light, but can also have a photonic band gap in, or around, the visible frequency range. The precise frequency location and size of this gap can be controlled by the structural and material parameters of the arrays.

We study the power-limiting properties of photoanisotropic azobenzene films with low-power lasers. The trans-cis photoisomerization and molecular reorientation of azobenzene molecules induced by polarized laser beams result in intensity-dependent anisotropic effects. Consequently, the transmittance of the input beam that passes through the film between two crossed polarizers becomes enhanced at low intensities and clamped at high intensities. The limiting threshold is adjustable by changing the intensity of excitation beam.

We demonstrated a novel filtering technique to enhance the features of micro-calcifications in mammograms. In our hybrid optoelectronic processing architecture (figure 1) programmed gray scale filters are displayed on the electrically addressed filter-SLM placed at the Fourier plane while digitally acquired mammograms are displayed on electrically addressed input-SLM at the input plane in a 4f-system configuration.

A comparison is made of two, laser-induced-damage assessment techniques. The first technique monitors the sample for changes in linear transmission after high-energy laser illumination. With the second technique, an image is transmitted through the sample, after high-energy laser illumination, at the position of incidence. Both single and multiple shot data are considered. Results show the imaging technique to be an efficient method by which to unambiguously discern the onset of image-degrading laser- damage, regardless of detector noise, shot-to-shot variations and sample inhomogeneities. Practically speaking, the imaging technique is relatively easy to incorporate into a laser-based experimental system and is particularly relevant to the assessment of optical systems for imaging.

The optical transmission processes for a nonlinear optical (NLO) material are influenced by the properties of its environment. NLO properties such as intersystem crossing rates may be altered by characteristics of the host such as polarity. The effects of the host material on the optical transmission of the NLO material is investigated using numerical laser beam propagation modeling (LBPM) techniques. Numerical simulations are reported for the optical transmission for zinc meso-tetra(p-methoxyphenyl)tetrabenzporphyrin (ZnTMPTBP) in liquids of differing polarity such as toluene, tetrahydrofuran (THF), dichloromethane (DCM), acetone, and pyridine. In addition to investigating the effect of the solvent on transmission, these calculations explore the effect on transmission of two different singlet lifetimes which have been reported in the literature. Transmission curves are calculated using separately determined parameters obtained from curve fitting using zscan data. Calculated results are compared with experimental data for all cases. Z-scans are also calculated for several cases and the results compared to experimental data. Numerical simulations provide a valuable tool to study the optical transmission behavior of NLO materials such as ZnTMPTBP.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report direct observations of all‐optical mirrorless bistability associated with saturable absorption in three kinds of bacteriorhodopsin (BR) samples: wild‐type BR in water solution and dispersed in thin films of a polymer matrix as well as water solution of the genetically engineered mutant BRD96N. The experiments are carried out with picosecond pulses at 532 nm. The values measured for the saturation intensity are explained in terms of the relaxation of the excited M state population to the B state of the BR photocycle for the three kinds of samples studied.

A series of azulenic compounds are synthesized and their third-order nonlinear optical propertiesare determined. The compounds are studied in solution using degenerate four wave mixing andz-scan at 532 nm with 30 ps pulses. Extrapolated values for the third-order nonlinear opticalsusceptibility of the compounds are in the rangel09 to 10 esu. Z-scan results indicate reverse saturable absorption. Optical power limiting with a f/S system is demonstrated for all ofthe compounds studied. Key words: Azulenic Polymethine dyes, third order susceptibility, Degenerate Four Wave Mixing, Z-scan, Optical Power limiting.Introduction In a previous paper we investigated the third-order nonlinear optical properties ofa series of azulertic donor/acceptorcompounds. These chromophores were designed to include donors and acceptors of differing strengths as well asconjugation lengths. Despite the fact that their intended use was for their second order properties1 the compoundsexhibited relatively high values2. in the previous study a range of two orders of magnitude was reported insusceptibility (3x1010 to 3xlO esu) for the five compounds2. This finding suggested their suitability for a study instructure property relationships with the intention of optimizing excited state and NLO properties for the eventualfabrication of an NLO device . Also, in the previous study it was found that four of the compounds saturated whenexposed to picosecond pulses and one compound exhibited reverse saturable absorpfion2. The reverse saturableabsorber was the only polymethine dye of that particular series.The current study is a continuation of the previous effort We now report results of a series of azulenic polymethinedyes. This class of third-order NLO chromophores contains no hetero atoms. The compounds are completely

We report optical power limiting in undoped single crystals of the wide band-gap semiconductor Zinc Oxide (ZnO). The ZnO crystals were grown at the Sensors Directorate of the Air Force Research laboratory using the hydrothermal method. The crystals are of high optical quality and are transparent in the visible part of the spectrum. The contributions of the real and imaginary parts of the third-order nonlinear optical susceptibility were studied using the Z-scan technique. The Z-scan experiments were carried out using picosecond pulses at 532 nm wavelength with the electric field polarization oriented orthogonal to the crystal c- axis. The nonlinear refractive index of the crystals is found to be negative. The ZnO samples exhibit two photon absorption and high damage thresholds. Our results indicate that the ZnO crystals may be good candidates for applications in optical power limiting and pulse stabilization.

We studied the third-order nonlinear optical properties of five azulenic donor-acceptor compounds. The compounds were studied in solution in Tetrahydrofuran using backward degenerate four-wave mixing and Z-scan with 30 picosecond pulses at 532 manometer wavelength. Extrapolated values for the third-order nonlinear optical susceptibility ((chi) (3)) of the solute molecules were obtained from concentration dependent measurements. The compounds possess large values of (chi) (3) in the range 3 X 10-10 esu to 3 X 10-8 esu. The two orders of magnitude change in the magnitude of the susceptibility that we observe for this series of donor-acceptor azulenic compounds makes them interesting candidates for the study of structure property relationships in nonlinear optical chromophores. The compounds also exhibit large solvatochromism and may also prove to be useful in second-order nonlinear optical applications.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The photochromic protein bR is receiving much attention as an interesting alternative to conventional inorganic semiconductor based materials for a variety of applications in optical recording and information processing. Recent advances in genetic engineering techniques for tailoring molecular properties without degrading of its inherent mechanical and thermal stability make it an outstanding photonic material. Some of the photonic applications already demonstrated by our group are reviewed. We exploit the parallel processing of optics and the real-time nature of bR films for the implementation of several photonic applications. We reported a convenient method to obtain all-optical light modulation in bacteriorhodopsin films using a degenerate four-wave mixing geometry. Chemically stabilized films of bacteriorhodopsin in a polymer matrix for which the life time of the excited M state is tens of seconds are used to demonstrate all-optical light intensity modulation. Small intensities of order mW/cm2 are used in the modulation experiments. The fast photochemical transition from M to B permits reasonably fast modulation speeds independent of the slow thermal M to B relaxation time. The experimental system can also be viewed as an all-optical switch, that uses molecular states in a bacteriorhodopsin thin film, where a low power blue pulse turns on a signal red beam. All-optical logic gates are implemented with wild- type and chemically stabilized films of bacteriorhodopsin using a two-color backward degenerate four- wave mixing geometry. The sensitivity of each bR film is markedly different due to the large differences in the saturation intensity. Red light is used to form a grating due to the B to M transition and blue light is used to form a grating due to the fast photochemical transition from M to B. Each of the two wavelengths in the experimental system acts as an input to the all-optical gate and the phase conjugate signal beam bears the output of the gate. To establish the origin of the nonlinearity we studied the intensity dependence of the self-focusing and self-defocusing properties of wild-type bR in water solution using the Z-scan technique with low power cw lasers at two wavelengths on either side of the absorption band. Our measurements indicate that the sign of the nonlinearity depends on the wavelength and the magnitude depends on the fluence of the incident laser beam. The observed self-defocusing and focusing is not due to the intrinsic electronic nonlinearity. The observations can be explained in terms of the Kramers-Kronig dispersion relation that relates the real and imaginary parts of the complex index of refraction to the absorption spectrum.We developed an optical pattern recognition system using Optical Fourier Transform with bR films. The photoinduced dichroism and its dependence on light intensity allows one to use the bR film as a real-time self-adaptive spatial filter at the FT plane. We also demonstrate flow disturbance visualization and beam shaping.

We constructed an optical system for performing 2D logic operations. We demonstrated the logic operations of OR, NOR, XOR, AND. The photoinduced dichroism of Bacteriorhodopsin (bR) is the physical mechanism exploited in the optical system. In its normal state with no light illumination a bR doped polymer film is isotropic with random distribution of bR molecules. When excited by linearly polarized light, only those bR molecules whose transition dipole moments for absorption lie in or near the direction of the electric field ar bleached at the actinic light wavelength due to the B to M transition. If a probe beam is incident in the regions illuminated by the actinic light it will no longer be interacting with an isotropic film. The film is now anisotropic and dichroic. Due to dichroism the actinic light illumination produces an angular rotation of the plane of polarization of the probe beam. In the experiments the bR film is kept between two crossed polarizers to get zero output from the probe beam at a screen when no actinic light is incident on the bR film. The two polarizers and the screen are kept in the path of the probe beam. There are two actinic light beams that induce dichroism and they are made orthogonally polarized with respect to each other by the use of a polarization rotator. The polarization rotator can be arranged so that their polarizations are made parallel for some of the logic operations. The plane of polarization of these two actinic beams are at 45 degrees to the pane of polarization of the probe beam. The combination of rotation of the analyzer and the polarization rotation of one of the actinic beams allows for many logic operations to be performed. No interference recordings are involved in the experiments and hence vibration isolation systems are not required. A coherent source is not a requirement either since a white light source with an appropriate wavelength filter can induce photoanisotropy in the bR film.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have fabricated Polycarbonate/ Porphyrin blends. The mixtures were prepared with the use of solvents suitable to both the Porphyrins and the Polycarbonate. Thin films were prepared by evaporation of the blends on a Teflon substrate. The samples exhibit reverse saturable absorption. We present results of optical power limiting experiments for the composites of films sandwiched between Polycarbonate sheets. The composites have high impact strength and remarkable mechanical robustness.

Two-photon absorption, excited state absorption, nonlinear refraction and scattering are the processes explored for optical power limiting applications. We report a novel approach for power limiting in polyphenylquinoxalines (PPQ) solutions based on two-photon induced cluster formation. The samples of PPQ were measured in solution in chloroform. The solutions are completely transparent at 532 nm. The contributions of the real and imaginary parts of the susceptibility were studied using the Z-scan technique. The nonlinear index of refraction of the samples is found to be positive. Open aperture Z-scan experiments indicate two photon absorption induced cluster formation and scattering. Values for the effective nonlinear absorption coefficient and the nonlinear index of refraction were obtained. Though two-photon absorption is responsible for triggering the process of cluster formation, optical power limiting in this case is due both to two-photon absorption and to scattering of light by the molecular clusters. The process of two- photon absorption and induced scattering yield a very attractive figure of merit of 10.3 for a solution sample of 2 mg/ml concentration compared to values reported in the recent literature. PPQ is an interesting candidate for applications in optical power limiting.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We studied the nonlinear absorption properties of tetrabenzporphyrins (TBP). Two dimensional delocalized pi- electron systems such as the macrocyclic dyes tetrabenzporphyrins exhibit nonlinear absorption. Their nonlinear absorption and refraction are due mainly to a mechanism that involves the formation of large populations of excited states. This takes place in the spectral regions between B and Q bands. The relevant time frame in which the nonlinear absorption takes place is determined by the dipole dephasing times in the case of the excited singlet states (which is of order picosecond). TBPs have been proposed as candidates for applications in optical power limiting. We have monitored the nature of the nonlinear absorption of TBP samples with picosecond laser pulses at 532 nm wavelength. Open aperture picosecond Z-scan measurements obtained with solutions of TBP in tetrahydrofuran (THF) exhibit reverse saturable absorption (RSA). Our experiments also reveal RSA for a thin film of TBP in a polymethyl methacrylate (PMMA) matrix. On the other hand we observed that for films deposited using the Langmuir-Blodgett (LB) technique the RSA is decreased. A simple extrapolation of solution measurements to thin film samples might result in erroneous conclusions. We investigated the nonlinear absorption of TBP in THF-water mixture to elucidate the role of molecular aggregates in the nonlinear properties of TBP samples. The RSA observed in the case of THF solutions is lost in the mixture.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The photochromic protein bacteriorhodopsin (bR) obtained from the purple membrane of the halocbacterium halobium has attracted considerable amount of interest recently. Several applications in photonics technology have already been proposed. bR lends itself to structural alterations at the molecular level by use of bioengineering and chemical synthesis methods. It is very stable under hard environmental conditions of temperature, salinity, etc. The films that were used in our experiments are stable over a period of four years. The lifetimes of some of the intermediate states of bR photocycle can be altered over a wide range by genetic manipulations of some of the amino acids that compose the bR membrane or by controlling the pH of the host materials. All of this makes bR a very attractive material for applications in optical computers and information processing. We present a technique that utilizes the molecular states of a bR thin film to implement an all-optical switch and all-optical logic AND and Or gates. A two-color backward degenerate four-wave mixing geometry with wild-type and chemically stabilized films of bacteriorhodopsin constitute the experimental setup. The saturation intensity, sensitivity and excited state lifetime (M state lifetime) of the films are very different. We use red light to form a holographic grating,due to the B to M transition and blue light to form a grating due to the fast photochemical transition from.M to B. Each of the two wavelengths in the experimental system acts as an input to the all-optical gates and the phase conjugate signal beam is the output of the gates.

Prior studies demonstrated that the properties of chemically synthesized polyaniline could be modified by the type of synthesis - electrochemical, chemical, or potential cycling methods (1–5). In addition the optical properties of polyaniline could be controlled by the substituents on the nitrogen or aromatic ring (6,7). Enzyme-catalyzed polymer syntheses in organic solvents with different amounts of water were described in earlier publications (7–10), and nonlinear optical properties of some of these polymers were reported (11). This paper describes the horseradish peroxidase-catalyzed synthesis of polyaniline and the evaluation of its optical properties to determine differences, if any, between this polyaniline and those chemically synthesized.