Changes In Complex Refractive Index Research Articles

Highly sensitive ultrafast all-optical light modulation by complex refractive-index changes in guided mode geometry composed of a photoresponsive polymer and a low-refractive-index polymer

Highly sensitive ultrafast all-optical light modulation is proposed on the basis of photoinduced complex refractive-index changes in composite guided wave mode (GWM) geometry composed of a low-refractive-index polymer and a photoresponsive polymer. Both calculated and observed results indicated that reflectance, which was unity at an extinction coefficient k=0, decreased rapidly with increasing k until a certain value (kc), and increased again after that. The kc-value and the GWM dip width decreased with increasing the thickness of both polymer layers. The real-part changes shifted the GWM dip very sensitively. The low-refractive polymer contributed great improvements, such as high sensitivity and durability against a repeated femtosecond laser. The present device can give highly enhanced changes of probe light as compared with simple transmission-type devices or the GWM geometry on a metal thin film. A photoelectrochromic polymer was employed in this geometry to demonstrate ultrafast reflectance control upon femtosecond laser excitation.

Measurement of electrically induced change in complex refractive index of nonlinear polymer by improved attenuated-total-reflection technique

Based on improved attenuated-total-reflection technique, the electrically induced complex refractive index change of electrooptic polymer film was measured by employing the data obtained from different guided wave resonance dip. Meanwhile, an accurate measurement of electrooptic coefficient in active polymer absorption region could also be achieved.

ALL-OPTICAL REFLECTANCE CONTROL BASED ON PHOTOINDUCED COMPLEX REFRACTIVE INDEX CHANGES IN GUIDED MODE THIN FILMS CONTAINING INDIUM OR GALLIUM PHTHALOCYANINES

Tetrasubstituted indium or gallium phthalocyanines and their dimers bridged with various ligands were dispersed in a polymer thin film, which was spin-coated on silver thin film vacuum-evaporated on a glass slide. All-optical reflectance control was achieved by complex refractive index changes upon photoexcitation of phthalocyanines by nanosecond laser in such a guided mode geometry. They gave rise in less than ns pulse width, and a few to a few tens of microseconds decay characteristic to the lifetime of the excited triplet state. Repeated and reversible reflectance changes were achieved. Axially bridged phthalocyanine dimers showed almost the same photoresponses as monomers.

Large-scale resonance amplification of optical sensing of volatile compounds with chemoresponsive visible-region diffraction gratings.

Micropatterning of the vapochromic charge-transfer salt, [Pt(CNC6H4C10H21)4][Pd(CN)4], on transparent platforms yields transmissive chemoresponsive diffraction gratings. Exposure of the gratings to volatile organic compounds (VOCs) such as chloroform and methanol leads to VOC uptake by the porous material comprising the grating lattice or framework, and a change in the material's complex refractive index, ñ. The index change is accompanied by a change in the degree of index contrast between the lattice and the surrounding medium (in this case, air), and a change in the diffraction efficiency of the grating. When a monochromatic light source that is not absorbed by the lattice material is employed as a probe beam, only changes in the real component of ñ are sensed. Under these conditions, the grating behaves as a nonselective, but moderately sensitive, sensor for those VOCs capable of permeating the porous lattice material. When a probe color is shifted to a wavelength coincident with the vapochromic charge-transfer transition of the lattice material, the sensor response is selectively amplified by up to 3.5 orders of magnitude, resulting in greatly enhanced sensitivity and some degree of chemical specificity. On the basis of studies at four probe wavelengths, the amplification effect is dominated by resonant changes in the imaginary component of the refractive index. The observed wavelength- and analyte-dependent amplification effects are quantitatively well described by a model that combines a Kramers-Kronig analysis with an effective-medium treatment of dielectric effects.

Ultrafast all-optical spatial light modulation based on complex refractive index changes of copper phthalocyanine-doped polymer film upon photoexcitation in guided mode geometry

Ultrafast all-optical reflectance control was achieved by photoinduced complex refractive index changes in thin poly(vinyl alcohol) (PVA) films containing water-soluble phthalocyanine at the incident angle corresponding to a sharp guided mode resonance upon excitation with a ns pulsed laser as a writing beam. The rise time of reflectance was less than 20 ns and the fall time was controlled by the lifetime of the excited triplet phthalocyanine, 31 ns for copper phthalocyanine (CuPcS). The spatial resolution was determined to be better than about 3 μm by using a photochromic dye. The present system based on photoexcitation of CuPcS will allow all-optical spatial light modulation upto about 30 MHz.

Ultrafast Reflection Control based on Photoinduced Complex Refractive Index Changes in Guided Mode Thin Films Containing Indium Phthalocyanine

Ultrafast all-optical reflection control was achieved by complex refractive index changes upon photoexcitation of indium phthalocyanine in a guided mode geometry device. It gave a few microseconds response mostly characteristic to the lifetime of the excited triplet state.

Spatial Distribution of Complex Refractive Index Change in Guest–Host Liquid Crystals Determined by a Phase-Sensitive Interferometric Method

The spatial distributions of the complex photothermal refractive index change in the guest-host liquid crystals were determined by a phase-sensitive optical interferometric method using a Mach-Zehnder interferometer. The Gaussian profile of the pump laser beam reflects the spatial distribution of the laser-induced index change and the radius of the distribution was slightly larger than that of the intensity distribution of the pump beam. In addition, the radius of the distribution of the extraordinary index change is larger than that of the ordinary one. The real part of the extraordinary refractive index change was negative, while that of the ordinary one was positive. Although the absorption coefficient was negligibly small at the wavelength of the probe laser beam, the imaginary part could not be ignored. The imaginary parts of both the extraordinary and ordinary indices changes were negative. These results can be explained by the photothermal effects of the guest-host liquid crystal.

High-speed light modulation using complex refractive-index changes of electro-optic polymers

We propose a promising type of light modulator using complex refractive-index changes of electro-optic polymers in the wavelength region of absorption. The material used in this study is a high-glass-transition-temperature active-polyimide thin film. The wavelength dependence of electrically induced complex refractive-index changes and the modulation efficiency are evaluated. A modulation efficiency of 33.8% has been realized with 141 V rms applied potential at a wavelength of 633 nm. Very fast modulation at over 25 MHz has been demonstrated.

Read-Out Signal Simulation of an Optical Disk Having an Oxide Super-Resolution Film

The read-out signal simulation of an optical disk having a cobalt-oxide super-resolution film was performed to find the optimum condition for a higher recording density of the super-resolution disk. A simple model of the complex refractive index change in a super-resolution film due to its temperature was applied, which can explain experimental data. It was found that this type of super-resolution disk must increase its reflectivity as the incident laser-beam intensity rises. To confirm this experimentally, a new super-resolution disk with an added Ge–N layer was prepared, and its read-out signals were measured, which agreed well with the calculated results.

Stress-analysis method for optical waveguides composed of elastically anisotropic materials and its application to strain-induced optical waveguides

In this paper we propose a new stress analysis method that can evaluate the residual thermal stress distribution in an optical waveguide with an elastically arbitrary anisotropy. We analyze the optical waveguide by taking into account the refractive index change obtained from stress analysis. A rigorous evaluation of the propagation characteristics in a strain-induced optical waveguide is now possible. Since the finite-element method is used for both the stress analysis and the optical waveguide analysis, the complex refractive index change obtained from stress analysis can be faithfully reflected in the optical waveguide analysis. Strain-induced optical waveguides with a SiO2/GaAs structure and a SiO2/LiNbO3 structure are chosen. The stress and strain in the waveguide and the change of the refractive index profile are obtained numerically and their effects on the transmission characteristics of the optical wave are studied. To the best of our knowledge, this is the first time that an optical waveguide analysis has been carried out from stress analysis taking into account an anisotropy. © 1998 Scripta Technica, Electron Comm Jpn Pt 2, 81(5): 16–23, 1998

Ultrafast wide range all-optical switch using complex refractive-index changes in a composite film of silver and polymer containing photochromic dye

An all-optical ultrafast switch which operates at a wide wavelength range has been demonstrated based on photoinduced changes of real and/or imaginary parts of complex refractive index in a composite thin film of silver and polymer containing photochromic spiropyran dye. Very fast switching ON and OFF of a reading beam within 20 ns in a 400–800 nm range was achieved at the incident angle of guided-wave mode using a photochromic spiropyran-doped polystyrene thin film and a ns pulsed UV or visible laser as a writing beam. These responses were caused by the changes of complex refractive index due to photochromism. No applied power was required to hold the switched state. An image was also written to the probe beam by irradiation through a pattern.

Complex photothermal refractive index change in host-guest liquid crystals determined with a novel interferometric method

A complex photothermal refractive index change in host-guest liquid crystals was determined with a novel interferometric method. The complex refractive index change was proportional to the pump beam intensity. The absolute value of the real part was about ten times as large as that of the imaginary one. The absolute value of the extraordinary refractive index change was about nine times as large as that of the ordinary one. The real part of the extraordinary refractive index change was negative, while that of the ordinary one was positive, and the imaginary part of both the extraordinary and ordinary refractive index change was negative. These results can be explained by the photothermal effects in the host-guest liquid crystals and dyes.

Novel all optical light modulation based on complex refractive index changes of organic dye-doped polymer film upon photoexcitation

A novel all-optical light modulation has been proposed based on photoinduced changes of an imaginary part of complex refractive index in guided wave geometry. Very fast and repeated modulation of a reading beam (reflected intensity) was demonstrated at the incident angle corresponding to a sharp guided wave resonance using phthalocyanine-doped poly(vinyl alcohol) film as a photoresponsive layer and a nanosecond pulsed laser as a writing beam. Such responses were mainly due to the formation and decay of excited triplet phthalocyanine. The rise and fall time was markedly improved as compared with previous systems based on photothermal changes of the refractive index.

Frequency chirp in optical reflection modulators

The chirping behaviour of reflective, resonant electroabsorptive optical modulators is analysed for the first time and an equivalent to the chirp parameter for these devices derived. Using the example of a Franz-Keldysh Effect based device the frequency chirp behaviour of a matched resonator is investigated and compared to the non resonant case. The chirp is found to display oscillatory behaviour which is explained by the interaction of the electric field induced changes in complex refractive index with the cavity. The effect of a front face reflectivity lower than that for matching is shown to be poorer modulation and chirping performance. If the front face reflectivity is greater than that needed to match then negative chirp is possible but the insertion loss is greater than 8 dB. By fine tuning the length of the device it is shown that this can be reduced to 3 dB producing a contrast ratio of 4.5 dB and −500 MHz chirp for 400 ps, 15 V pulses.

Measurement of carrier-induced changes of the complex refractive index in InGaAsP near the band edge

A new method has been developed to measure the complex refractive index changes of a thin film medium, using a probe light perpendicularly transversing the surface and a small interference-ellipsometry bridge. Carrier-induced changes of the refractive index and absorption have been measured in InGaAsP near the band edge using this method. Experimental results are compared with calculations.

Prediction of scattering effect by ash polydispersion on spectral emission from coal-fired MHD combustion gas

Scattering effects by ash polydispersion on spectral emission from the coal-fired MHD combustion gas are predicted numerically by means of the first ( P 1) and third ( P 3) spherical harmonics approximations of a radiative transfer equation in a rectangular cross section of the MHD power generation channel, with consideration of variation in optical properties across a boundary layer, particle diameter distribution of ash polydispersion and wavelength change of ash complex refractive index. Results of P 1 approximation agree well with those of P 3 approximation when an optical thickness is greater than about 2. Scattering effects can be neglected for a large optical thickness and a small scattering albedo, but precise calculation is needed to get the radiation intensity distribution when the absorption coefficient of combustion gas is of the same order of magnitude as the scattering of absorption coefficient of the ash polydispersion. The self-resonant absorption of potassium 4P-4S spectrum measured through an observation port in the presence of a cold boundary layer is not predicted by the P 1 approximation with an emitting and absorbing slag wall.

A new ellipsometric model of wet and dry etched GaAs surfaces

In this paper we describe the application of ellipsometry to study (100)GaAs surfaces. Investigations concern the surfaces after polishing, cleaning, wet and dry etching of GaAs substrates and the surfaces of liquid phase epitaxy- and molecular beam epitaxy-grown GaAs epilayers. A new model is proposed to interpret the ellipsometric data. It assumes the presence of a transitional layer characterized by spherical disturbances situated between the bulk GaAs and the native oxide layer. The ellipsometric parameters are analysed as a function of the size and density of the spherical disturbances and the transitional and native oxide layers. Different angles (70:75 °) of the incident light beam are used at a wavelength of 546.1 nm. A refractive index of 1.81 for the oxide film and (4.05–i0.304) for the GaAs substrate are assumed.Our results show that the thickness of the native oxide layer changes from 11Åto 35Ådepending on the surface treatment. The thickness of the transitional layer reaches the radius of the spherical disturbances. Their radius ranges from 10Åto 30Å, while the complex refractive index changes from (4.05–i0.34) to (4.70–i1.30).

Femtosecond laser excitation of the semiconductor-metal phase transition in VO2

We have measured the subpicosecond optical response of a solid-state, semiconductor-to-metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of VO2 thin films by making pump-probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties of the high-temperature metallic state being attained within 5 ps. The ultrafast change in complex refractive index enables ultrafast optical switching devices in VO2.

Analysis of lossy optical materials possessing graded refractive index profiles

Analytical expressions for reflectance, transmittance, and bilinear transformed reflectance of processed materials containing complex graded refractive index profiles are presented. Oblique incidence is considered. Substantial changes in complex refractive index are modeled by means of a second-order theory, whereas moderate refractive index changes are described by means of a much simpler first-order approximation. An ellipsornetric expression for inhornogeneous layers is also derived using this theory. The performance of the integral formulation is compared to that of the matrix technique by simulations and analysis of experimental data. Simulations using our first-order formulas have a time savings benefit of more than a factor of 10 over reflectance curves calculated by the well-known matrix method. Experimental infrared reflectance of semi-insulating (100) GaAs samples implanted with 280-keV beryllium ions is also analyzed and is compared with our theoretical results obtained from a first-order theory. The Fourier spectrum in the optical thickness domain yielded the position of the peak carrier concentration accurately.

Electromodulation of Light Scattering in Single Crystals of Anthracene

Abstract Modulation of light scattering by electric field in anthracene crystals is reported. Scattered light was that obtained from a He-Ne laser. Effective modulation of scattered light was possible only when concentration of trapped charge carriers was changed by an external electric field under space-charge-limited conditions. External voltage characteristics of electromodulation signal were structured, the signal height was up to ten percent. The observed effect is discussed in terms of the modulation of the population of the filled up charge trapping domains responsible for the light scattering. The change in the local complex refractive index due to the charge captured by a spatially extended defect (macrotrap) is considered as the most probable microscopic mechanism of the effect.