Dispersive Fourier-transform Spectroscopy Research Articles

Accurate, high-resolution dispersive Fourier-transform spectroscopy with undetected photons

We present dispersive Fourier-transform spectroscopy with undetected photons using a nonlinear interferometer based on spontaneous parametric down-conversion. The interferometer transfers mid-infrared information of sample interaction to the near-infrared, which allows using low-noise silicon-based detectors, enabling accurate measurements with extremely low light exposure. We demonstrate a dispersion measurement of gaseous methane with an accuracy greater than 1 part in 106, which is limited by random phase noise. The Fourier-transform approach allows high resolution, rotational-line resolving spectroscopy of both, the absorptive and dispersive properties of a sample in a simple and robust measurement concept.

A THz Spectroscopy System Based on Coherent Radiation from Ultrashort Electron Bunches

A spectroscopy system will be discussed for coherent THz transition radiation emitted from short electron bunches, which are generated from a system consisting of an RF gun with a thermionic cathode, an alpha-magnet as a magnetic bunch compressor, and a linear accelerator for post-acceleration. The THz radiation is generated as backward transition radiation when electron bunches pass through an aluminum foil. The emitted THz transition radiation, which is coherent at wavelengths equal to and longer than the electron bunch length, is coupled to a Michelson interferometer. The performance of the spectroscopy system employing a Michelson interferometer is discussed. The radiation power spectra under different conditions are presented. As an example, the optical constant of a silicon wafer can be obtained using the dispersive Fourier transform spectroscopy (DFTS) technique.

An efficient biomimetic coating methodology for a prosthetic alloy

The combination of the load-bearing metallic implants with the bioactive materials in the design of synthetic implants is an important aspect in the biomaterials research. Biomimetic coating of bioinert alloys with calcium phosphate phases provides a good alternative to the prerequisite for the continual replacement of implants because of the failure of bone-implant integration. We attempted to accelerate the biomimetic coating process of stainless steel alloy (316L) with biomimetic apatite. In addition, we investigated the incorporation of functioning minerals such as strontianite and smithsonite into the deposited layer. In order to develop a highly mature apatite coating, our method requires soaking of the pre-treated alloy in highly concentrated synthetic body fluid for only few hours. Surface characterizations were performed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Also, the deposited apatitic layers were analysed by powder diffraction X-ray analysis (XRD). 316L surface showed the growth of highly crystalline, low carbonated hydroxyapatite, after only 6h of the whole soaking process.

Dispersive Fourier-transform spectroscopy of surface plasmons in the infrared frequency range

The optical scheme and the principle of operation of an asymmetric Mach-Zehnder interferometer for dispersive spectroscopy of surface IR plasmons are discussed. A thin-layer sample deposited on a conducting substrate, which guides surface plasmons, is placed in one of the arms of the interferometer. This makes it possible, by applying a complete Fourier transform to the interferogram, to obtain the spectrum of the complex refractive index of surface plasmons and, therefore, of the complex dielectric permittivity of the sample or the substrate material.

Statically scanned single and tandem low-coherence interferometers

Statically scanned single and tandem Michelson interferometer configurations are compared for the remote measurement of thermally induced group delay change in optically dispersive glass samples. A broadband tungsten filament bulb was used to illuminate the single interferometer, and a much narrower spectral bandwidth superluminescent diode (SLD) was used to illuminate the tandem interferometer. For a BK7 glass sample, measurements of thermally induced group delay changes were made with <0.5 fs root mean square error for optical path delay (OPD) scan lengths of only 260 µm when applied to low-coherence interferograms with signal-to-noise ratios as low as 6.5 dB. These results demonstrate the power of dispersive Fourier transform spectrometry (DFTS) applied to noisy, dispersion distorted low-coherence interferograms captured with non-mechanical, short path length scans. Further, following experimentally observed source bandwidth-induced measurement resolution limitations between the different illuminating sources, simulations were performed to examine this feature.

Measurement of transmittance and permittivity of dielectric materials using dispersive Fourier transform spectroscopy

AbstractTransmittance and permittivity of several dielectric materials such as plexiglas, acrylic, and polypropylene were measured using dispersive Fourier transform spectroscopy (DFTS). The measured results showed that the transmittance of plexiglas and acrylic specimens decreases with the increase of frequency in the range of 50–380 GHz. Accurate spectra of complex permittivity and loss tangent of these specimens in this frequency range are also given. Very good agreement was seen in the overlapped frequency range between the measured data and the published data, measured using other techniques such as the W‐band spectrometer with backward wave oscillator and the free‐space measurement. This technique can be used for measurements of various dielectric materials, provided that flat specimens are available. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 27–30, 2003

Determination of the optical constants of thin films in the visible by static dispersive Fourier transform spectroscopy

Conventional dispersive Fourier transform spectrometry (DFTS) is a powerful tool for determining optical constants of materials. However, the refined and intrinsically high-cost mechanically scanned interferometers that are necessary are not well suited to use in hostile environments or for time-resolved operation. We describe here a novel approach to DFTS that employs a combination of a Wollaston prism and a linear detector array. It is ideally suited to the precision characterization of thin films with physical thicknesses of up to about 1000 wavelengths or typically about 1 mm. The simplicity and optical efficiency of conventional DFTS are combined with the inherent robustness, superior time resolution, and high repeatability of spatial interferometry. The technique offers an optical throughput that is an order of magnitude higher than spectrophotometry or spectral ellipsometry while accuracies of 1 part in 104 and repeatability of 1 part in 105 are possible for the measurements of the real part of the refractive index. The imaginary component of the refractive index of thick transparent samples has been measured with an absolute error of less than 2×10−4. The technique may be readily applied from the vacuum ultraviolet to the mid infrared. We present proof-of-principle measurements of optical constants at wave numbers between 9000 and 25 000 cm−1 for a self-supporting film of Melinex and for a thin film of ZnSe grown by molecular beam epitaxy onto a glass substrate.

Geometric representation of errors in measurements of strain and temperature

Many optical fiber sensors, designed to recover quasistatic strain fields in the presence of significant temperature changes, have been reported in recent years. A number of recent publications have attempted to devise a method for assessing the relative performances of such sensing schemes. Here we report an analysis that represents the data recovery process from a geometrical standpoint and provides useful insight into the physical differences between measurement schemes. The performance of methods based on Bragg grating sensors, polarization-maintaining Fabry-Perot interferometers, combined dualmode interference-polarimetry sensors, and dispersive Fourier-transform spectroscopy measurements are contrasted.

Dispersive Fourier Transform Spectroscopy Of Free-Standing Porous Silicon Films

AbstractWe have investigated optical constants of free-standing porous silicon films by dispersive Fourier transform spectroscopy (DFTS) in the NIR-VIS range. This allows the spectral variation of both the absorption coefficient and the refractive index of a material to be determined from the measurements of the attenuation and phase shift imposed on an electromagnetic wave by its interaction with a specimen. Using these optical constants, we have studied the complex dielectric function and the complex conductivity. To avoid the additive error in the absorption spectra arising from the pseudocoherence, we measured the transmission spectra by conventional Fourier transform spectroscopy (FTS). Using the refraction spectrum derived from the DFTS measurements, we have corrected for reflection losses in calculation of the absorption spectrum from the FTS transmission spectrum. The changes in the absorption coefficient and the refractive index due to oxidation, which is the most common aging phenomenon in porous silicon, have been studied using samples with different types of oxidization.

Determination of the far infrared optical constants of η-doped bulk CdxHg1−xTe (CMT) by dispersive fourier transform spectroscopy

Far infrared phase and amplitude reflectivity measurements have been made on two bulk CdxHg1−xTe mixed crystals with composition x=0.29 and x=0.22 by dispersive Fourier transform spectroscopy (DFTS). The results have been used to calculate the real and imaginary parts of the dielectric function (ɛ′, ɛ″) from the Fresnel relations. A plasma contribution is observed in the spectra in addition to the phonon response. For both samples a broad but weak reflection band around 95–105 cm−1 is observed as well as the expected two-oscillator response from the HgTe-like and CdTe-like optical phonons. This feature is attributed to absorption due to phonon combination bands, but it is too broad to enable assignments to be made. There is no evidence of additional features in the CdTe region due to clustering.

Dispersive Fourier transform spectroscopy

Abstract Dispersive Fourier transform spectroscopy (DFTS) is a technique for determining the optical constants of solids, liquids and gases from direct measurements of both the amplitude and phase of either their reyection or transmission coeficients. Measurements are usually made at normal incidence with the specimen in one arm of a Michelson interferometer. The technique has been widely used for measurements in the far-infrared region of the spectrum, where the phase measurement is comparatively straightforward. Recent developments have established the feasibility of extending the spectral range to the visible and ultraviolet. In this article the principles of DFTS are briefly reviewed, and a selection of results presented to illustrate the scope and potential of the technique.

Far-IR spectroscopy of bulk and surface phonon-polaritons on epitaxial layers of CdTe deposited by plasma MOCVD on GaAs substrates

Far-IR measurements by dispersive Fourier transform spectroscopy (DFTS) and attenuated total reflection (ATR) spectroscopy have been made on semiconductor samples consisting of epitaxial layers of CdTe on GaAs substrates. The two techniques have enabled all bulk and surface phonon modes and guided waves propagating in the system to be investigated. Both the CdTe and GaAs reststrahlen bands, which lie in well separated spectral regions, are observed by DFTS. ATR measurements have revealed a p polarized surface polariton in the CdTe restsrahlen band and both s and p polarized guided waves in the GaAs reststrahlen band. In each case the measured results are in good agreement with calculations based on standard multilayer optics techniques, and each technique provides a sensitive probe of the layer thickness.

Determination of the Optical Constants of Solids in the Visible by Dispersive Fourier Transform Spectroscopy: 1. Reflection Measurements on an Opaque Solid

Abstract A Michelson interferometer has been developed for determining the optical constants of solids in the visible and ultraviolet by dispersive Fourier transform spectroscopy. Systematic phase errors in amplitude reflection spectroscopy arising from factors such as non-flatness and imperfect alignment of the specimen are discussed in detail, and it is shown that they can be eliminated by following a simple measuring procedure. The performance of the instrument is demonstrated with measurements in the visible region of the optical constants of thin films of Cu and Au, vacuum deposited on optically polished fused quartz substrates.

Pseudocoherence in dispersive fourier transform spectroscopy—II. Reflection DFTS

Pseudocoheren ce provides a major source of possible systematic error in measurements of the optical constants of heavily absorbing solids by reflection Dispersive Fourier Transform Spectroscopy. It arises when the reflecting surface of a specimen is not perfectly flat. An analytic model of its effects on the measured absorption and refraction spectra in such a measurement is presented. This allows the tolerances required for the flatness of the reflecting surface of the specimen to be derived. These are proportional to wavelength. This demonstrates that while the replacement method of reflection DFTS is only well-suited to FIR studies, the switched field-of-view method can be used in all spectral regions for which FTS is practicable.

Computation of Dispersive Fourier Interferograms for Gases in the Visible Wavenumber Range

Abstract Simulation techniques are used to study the capability of dispersive Fourier transform spectroscopy (DFTS). Dispersive interferograms in the visible are calculated numerically in a simple way using the model of an ideal two-beam interferometer with the sample gas of complex refractive index [ncirc](σ) in the fixed arm. Non-absorbing as well as artificially absorbing nitrogen (N2) is considered as a model gas. The non-absorbing species is investigated under different ‘experimental’ conditions (different spectral apparatus functions and different wavenumbers σm of the modulating light). In addition, the inverse Fourier transformation of dispersive interferograms is discussed with reference to common transform techniques (apodization, fast Fourier transform) and ‘experimental’ disturbances of the interferograms.

Refractive index and derived molecular properties of some gases determined by dispersive fourier transform spectroscopy (DFTS) in the visible wave number range

Dispersive Fourier transform spectroscopy (DFTS) is used to determine the dispersion of the real refractive indexn(σ) of the gases CH4, C2H4, C2H6, C3H8 and cyclo-C3H6 in the visible wave number range. Some molecular properties, e.g. oscillator strength sums, are derived from these measurements and compared with available data of literature.

Far infrared study of optical phonons in K1?xRbxI mixed crystals

The farinfrared properties of mixed alkalai halide crystals, K1−xRbxI, x=0.25 and 0.50, have been studied at room temperature. Dispersive Fourier transform spectroscopy (DFTS) has been used to obtain the complex dielectric function of these samples. Infrared-active phonon modes are observed at frequencies close to those of KI and RbI. This two-mode behavior is in agreement with theoretical predictions.

Millimeter-wave dielectric measurement of materials

It is no longer necessary to use extrapolated microwave dielectric values when designing millimeter-wave components and systems. Very recently, highly accurate millimeter-wave (5- to 1/2-mm) data on complex dielectric permittivity and loss tangent have become available to engineers for a variety of materials such as common ceramics, semiconductors, crystalline, and glass materials. One quasi-optical measurement method has proved to be most accurate and reproducible, namely, dispersive Fourier transform spectroscopy (DFTS) applied to a polarizing interferometer. The openresonator method and the Mach-Zehnder-IMPATT spectrometer will also be described and compared. The fact that the dielectric loss increases with frequency in the millimeter, unlike the microwave, is an important feature of these data. Reliable measurements also reveal that the methods of preparation of nominally identical specimens can change the dielectric losses by a factor of three.

Dispersive Fourier transform spectroscopy with gases in the visible region

The method of dispersive Fourier transform spectroscopy (DFTS) with gases in the visible wavenumber range is described in detail and compared with the method of scanning-wavelength interferometry (SWI). Measurements of the dispersion of the complex refractive index of gases can be performed successively in several minutes using the same apparatus and gas sample conditions for both methods. In the reported experiments with CH 4 a very simple mirror drive was used. Nevertheless, reasonable results are obtained for the dispersion of the real refractive index of CH 4 in the wavenumber range 16,000 ⩽ σ ⩽ 23,000 cm −1.

Optical constants of some commercial microwave materials between 90 and 1200 GHz

The optical constants of three commercial microwave materials have been determined between 90 and 1200 GHz at 290 K by the techniques of dispersive Fourier-transform spectroscopy. The materials studied were a cast microwave absorber known as Ferroflow, a machinable glass ceramic known as Macor and a substrate material for planar microwave circuitry known as Epsilam-10. The former two of these have been shown to be potentially suitable for use as thermal sources for the absolute calibration of radiometric instrumentation. The glass ceramic has also been shown to have a broad absorption band centred on 9 cm−1 that is obscured in the absorption spectrum by its rapid increase to high wave number, but that is clearly seen in the refraction spectrum.