N. C. Das

Spectroscopic ellipsometry of TiO2 layers prepared by ion-assisted electron-beam evaporation

Abstract Single layer TiO 2 films deposited on BK7 glass substrates by the ion-assisted electron-beam evaporation technique have been characterized by phase modulated spectroscopic ellipsometry. The ellipsometry spectra were recorded in the wavelength range of 300–1000 nm. The measured spectra were then fitted with theoretically simulated curves generated assuming different model sample structures. Analyses were carried out separately in three different wavelength regimes, the transparent regime with no dispersion of refractive index (650–1000 nm), the transparent regime with dispersion of refractive index (400–650 nm) and the absorbing regime (300–450 nm). Modeling has been attempted with both homogeneous and inhomogeneous sample structures. For the inhomogeneous structure, both linear and non-linear variation of refractive index along the depth of the sample were used. Refractive indices of the samples were determined separately from the best-fit ellipsometric data in the above three wavelength regimes. Finally, variation of refractive index with the variation in ion beam currents has been studied.

Reactive electron beam evaporation of gadolinium oxide optical thin films for ultraviolet and deep ultraviolet laser wavelengths

Availability of ultraviolet optical thin film materials, especially high index refractory oxides that transmit down to deep ultraviolet (approx. 0.2 mm) is very much limited. The present article discusses some of the research optimization studies on gadolinium oxide (Gd O ), a novel lanthanide sesquioxide material, for such challenging spectral requirements. Optical and 23 topographical properties of single layer films have been studied using phase modulated spectroscopic ellipsometry, spectrophotometry and multimode atomic force microscopy. Films deposited at lower substrate temperatures have shown higher band gaps and higher substrate temperatures yielded higher refractive indices. Multilayer high reflection filters have been developed for ultraviolet laser wavelengths such as ArF (193 nm), KrCl (222 nm), KrF (248 nm) and Nd-Yag-III (355 nm), using this material at lower substrate temperature conditions and successfully tested for their performances. 2003 Elsevier Science B.V. All rights reserved.

Characterization of a multilayer highly reflecting mirror by spectroscopic phase-modulated ellipsometry

The characterization of optical multilayer coatings has been a challenging task for thin-film scientists and engineers because of the various complex, interdependent layer parameters that exist in the system. Spectroscopic phase-modulated ellipsometry has some advantages in the postanalysis of the layer parameters of such multilayer coatings because it suitably models the layer structure with respect to the ellipsometric measurements. An algorithm to characterize multilayer optical coatings with large numbers of layers has been described by spectroscopic ellipsometry by use of a discrete spectral zone fitting approach. A 23-layer multilayer highly reflecting mirror has been characterized by this technique in the wavelength range 280-1000 nm. The ellipsometric spectra (? and D versus wavelength) have been fitted separately in three wavelength regimes. Fitting the ellipsometric spectra in the wavelength regime of 700-1000 nm permitted the sample structure to be determined. The data were then fitted in the wavelength range 280-340 nm, i.e., near the fundamental absorption edge of TiO(2), to yield the dispersion relation for the optical constants of TiO(2). Finally, the data were fitted in the wavelength range 340-700 nm, and the true dispersion of the refractive index of TiO(2), along with the best-fitting sample structure, was obtained.

Spectroscopic ellipsometry of multilayer dielectric coatings

Abstract Multilayer dielectric coatings deposited by e-beam evaporation have been characterised by the phase modulated spectroscopic ellipsometer (PMSE). Measurements have been done on various multilayer thin films devices e.g., high reflectivity mirror, narrow band filter, beam combiner, beam splitter, etc. consisting of several bilayers of TiO 2 /SiO 2 . Results have been shown here for the first two samples. The measured Ellipsometry spectra are fitted with theoretical spectra generated assuming appropriate models regarding the sample structures. Optical constants of the substrates and the SiO 2 films have been supplied and trial dispersion relations have been used for the optical constants of the TiO 2 layers. The fittings have been done by minimising the squared difference ( χ 2 ) between the measured and calculated values of the ellipsometric parameters ( ψ and Δ) and accurate information have been derived regarding the thickness and refractive indices of the different layers.

Design, fabrication and testing of elliptical crystal bender for the EXAFS beam-line at INDUS-II synchrotron source

An extended X-ray absorption fine structure (EXAFS) beam-line for X-ray absorption studies using energy dispersive geometry and position sensitive detector is being developed for the INDUS-II synchrotron source. The optical design of the beam-line has been completed based on the working principle that a single crystal bent in the shape of an ellipse by a crystal bender would act as a dispersing as well as focusing element. The heart of the beam-line is the crystal bender which has been designed on the basis of the principle of four-point bending and has been fabricated indigenously. The crystal bender is capable of producing pre-defined elliptical curvature on a crystal surface by applying different couples at the two-ends of the crystal which has variable width along its length. The focusing property of the crystal bender has been tested using a laser source and has been compared with the theoretically simulated results.

Aberration properties of a Czerny–Turner spectrograph using plane-holographic diffraction grating

In this paper I discuss the construction and the aberration properties of plane-holographic diffraction gratings in a Czerny-Turner mounting. A ray-tracing scheme has been formulated for computing the aberrations of the system. It has been found that in the area near the recording wavelength for the holographic grating, the system has better resolution than does a conventional grating system. The design parameters of a medium-sized holographic grating spectrograph are specified. The performance of the spectrograph is evaluated by plotting spot diagrams, which show that astigmatic defects are much reduced.

Phase modulated spectroscopic ellipsometry of dielectric multilayer beam combiner

Abstract An algorithm to characterise multilayer optical coatings with large numbers of layers by spectroscopic ellipsometry, using a discrete spectral-zone fitting approach, has been demonstrated by characterising a 27-layer TiO2/SiO2 multilayer beam combiner in the wavelength range of 280–1200 nm. The ellipsometric spectra are fitted first in the wavelength regime of 700–1200 nm and the sample structure was determined. TiO2 and SiO2 layers have been assumed to be transparent in this wavelength regime with dispersion-less refractive indices. The data were then fitted in the wavelength range of 280–340 nm to find the dispersion relation for the optical constants of TiO2. Finally, the fitting has been done in the wavelength range of 340–700 nm and the true dispersion of refractive index of TiO2 along with the best fit sample structure have been obtained.

Simulation of temperature distribution by finite element analysis on different components of the EXAFS beamline at INDUS-II synchrotron source

An extended X-ray absorption fine structure (EXAFS) beamline is being developed for the INDUS-II synchrotron source. Several optical and mechanical components of the beamline are exposed to high intensity synchrotron radiation while in operation. The temperature rise on different components of the beamline on exposure to the synchrotron beam has been simulated by finite element analysis. Design of the cooling mechanism for each of these components has been carried out and estimation of the temperature rise has also been done incorporating the cooling mechanism.

Design of the Extended X‐ray Absorption Fine Structure (EXAFS) Beam‐Line at INDUS‐II Synchrotron Source

An Extended X‐ray Absorption Fine Structure (EXAFS) beam line for X‐ray absorption studies using energy dispersive geometry and position sensitive detector is being designed for the INDUS‐II Synchrotron source. The optical design of the beam line has been completed based on the working principle that a single crystal bent in the shape of an ellipse by a crystal bender would act as a dispersing as well as focusing element. The mechanical design of the beam line including the crystal bender has also been completed and the components are being developed indigenously. Calculations have been done to determine the temperature profile on the different components of the beam line under exposure to synchrotron radiation and proper cooling channels have been designed to bring down the heat load on the components.

A Simple Method of Calibration for Multichannel Spectrograph Using Charge-Coupled Device (CCD) as a Detector for Raman Spectroscopy

A wavelength calibration procedure has been devised for measuring the wavelength in the multichannel spectrograph using a single wavelength of radiation for calibration which can be obtained from commonly available lamps such as mercury spectral lamp, cadmium lamp, sodium lamp and thallium lamp. This wavelength calibration procedure has been incorporated in a multichannel Czerny-Turner type of spectrograph using CCD as a detector developed indigenously. The spectrograph uses a plane grating of frequency 1200 grooves/mm, concave spherical mirror of focal length 0.523 m as a collimating element and concave spherical mirror of focal length 0.5 m as a focusing element. The linear dispersion of the spectrograph varies from 2.519 pixel/ Å to 2.686 pixel / Å. The spectrum is recorded on the CCD array of pixels 1024 × 256. The pixel size is 26 micron × 26 micron. The accuracy of wavelength measurement is found to be about 1 Å. The instrument works in the wavelength region from 4000 Å to 7000 Å. Raman spectrum of benzene has been recorded by irradiating the sample with an intense beam of a diode-pumped green laser (λ= 532 nm) of power variable from 1 mW to 200 mW. The measured values of wavelengths of Raman lines of benzene using the multichannel spectrograph agree well with the values of the Raman lines reported in the literature. The multichannel spectrograph is found to be useful for Raman spectroscopic applications.