Fabry-Perot Interference Fringes Research Articles

Porous silicon-polyvinylidene fluoride-carbon dots based portable temperature sensor

Optical sensors are an attractive alternative to semiconductor-based sensors for temperature measurement in harsh environments. Based on Porous Silicon (PSi) Fabry-Perot interferometer, this work presents a novel way of sensing temperature with PSi hybrids using a polymeric nanocomposite of polyvinylidene fluoride (PVDF) and carbon dots (CDs). The operation is based on the temperature-induced changes in the refractive index of PVDF, which in turn changes the effective refractive index of the PSi hybrid. The dual temperature sensing was based on the simultaneous monitoring of the peak wavelength redshift of the Fabry-Perot interference fringes, and the measurement of the intensity of the reflectance spectrum at a specific wavelength. Both measurements showed excellent linear responses, with coefficients of determination greater than 0.99, revealing the highest sensitivity of 0.2 nm/°C for the sample with the highest content of CDs. Apart from the enhanced temperature dependence of the dielectric constant of PVDF, which improved the sensitivity of the proposed PSi/PVDF-CDs based sensors, the hybrid structure facilitates the formation of β phase up to 91.8%. The proposed silicon-based temperature sensor being very large-scale integration (VLSI) compatible, open the way towards the possible formation of free-standing PSi/PVDF-CDs films with a possible application as biocompatible micro temperature sensors.

Optical phonons and magnetoelastic coupling in the ionic conductor AgCrSe2

${\mathrm{AgCrSe}}_{2}$ is an example of a superionic conductor that has recently attracted attention for its low thermal conductivity. Here we investigate the optical properties of ${\mathrm{AgCrSe}}_{2}$ in the ordered phase between 14 K and 374 K using reflectivity experiments and compare them to the optical conductivity obtained from electronic structure calculations. The far infrared optical response is dominated by three phonon modes, while six interband transitions are observed in the visible range. We find good agreement between the experimental interband transitions and the calculated optical transitions. From our analysis we find that the phonon parameters display an interesting temperature dependence around the N\'eel temperature, pointing to a small magnetoelastic coupling. In addition, the lifetimes of the modes indicate that three-phonon processes dominate and the optical phonons decay into low-energy acoustic modes involved in the superionic transition. Finally, we detect a small free charge carrier response through the analysis of Fabry-Perot interference fringes in our reflectivity data.

Electronic sputtering of solid N[formula omitted] by swift ions

Most sputtering yield measurements for solid N2 are reported for stopping powers lower than 10−13eV cm2/molecule. We measured the sputtering yield for solid N2 at stopping powers, in the electronic regime, above 10−12eV cm2/molecule, extending the range of such measurements by more than an order of magnitude, using a 33 MeV 58Ni9+ swift heavy ions beam. The evolution of the thin N2 ice films was monitored in-situ by mid-infrared spectroscopy (FTIR) during irradiation. As N2 is only weakly infrared active, and can be hardly monitored directly via an infrared absorption mode in such experiments, we use the Fabry–Perot interference fringes of the ice film to evaluate, via an optical model, the erosion of the N2 film as a function of ion fluence. A sputtering model including several sputtering crater shapes is developed and tested against experimental data. We derive the sputtering yield for a semi-infinite N2 ice film and its dependence with the ice thickness for thin film conditions, monitoring the N2 ice sputtering depth. We combine the results with previous measurements at lower stopping powers to derive the electronic sputtering of solid N2 over a large stopping power range.

Highly sensitive optical fiber temperature sensor based on the thermal shift of the liquid–air interface

A highly sensitive optical fiber temperature sensor based on the thermal shift of the liquid–gas interface is proposed. A section of silica capillary tube (SCT) was spliced to a single mode fiber, and a small quantity of liquid polydimethylsiloxane was filled into the innermost part of SCT by using the offset capillarity method. Then, the end of the SCT was sealed by solidified UV curable adhesive. As a result, a liquid Fabry–Perot (FP) cavity, an air FP cavity, and a liquid–gas interface were formed simultaneously in the SCT. The lengths of the two cavities changed synchronously and complementarily along with the thermal shift of the liquid–air interface, which resulted in the opposite wavelength shifts of the corresponding FP interference fringes. The temperature sensitivities are 1.67 and −0.32 nm / ° C, respectively. A differential sensitivity of 1.99 nm/°C can be achieved by monitoring the two kinds of fringes synchronously.

Mesoporous anodic α-Fe2O3 interferometer for organic vapor sensing application.

In this work, we reported the utilization of mesoporous α-Fe2O3 films as optical sensors for detecting organic vapors. The mesoporous α-Fe2O3 thin films, which exhibited obvious Fabry–Perot interference fringes in the reflectance spectrum, were successfully fabricated through electrochemical anodization of Fe foils. Through monitoring the optical thickness of the interference fringes, three typical organic species with different vapor pressures and polarities (hexane, acetone and isopropanol) were applied as probes to evaluate the sensitivity of the α-Fe2O3 based interferometric sensor. The experiment results showed that the as-synthesized mesoporous α-Fe2O3 interferometer displayed high reversibility and stability for the three organic vapors, and were especially sensitive to isopropanol, with a detection limit of about 65 ppmv. Moreover, the photocatalytic properties of α-Fe2O3 under visible light are beneficial for degradation of dodecane vapor residues in the nano-pores and refreshment of the sensor, demonstrating good self-cleaning properties of the α-Fe2O3-based interferometric sensor.

Measurement of principal refractive indices of birefringent wafer by analysis of Fabry-Perot interference fringes.

We developed an efficient method for measuring the principal refractive indices and thickness of an optically anisotropic wafer that involves the analysis of Fabry-Perot interference fringes. Utilizing the birefringence of the medium, the 2π phase ambiguity was readily resolved in single-wavelength measurements of the birefringent medium index. Although the accuracy of the index measurements is limited due to the innate ambiguity, our analysis method overcame this limit and could determine the principal refractive indices and thickness with an uncertainty of 10(-5). Our method was validated against measurements of a lithium niobate wafer for which the values of the indices are precisely known.

Determination of the glass transition temperature of thin unsupported polystyrene films using interference fringes

Controversy exists as to why nanometer-sized polymer films have thermal properties that differ from bulk samples. This paper presents a simple method of determining the glass transition temperature ( T g) of unsupported ultrathin polymer films based on the determination of the refractive index from Fabry–Perot interference fringes observed in the infrared or UV–visible regions. T g is extrapolated from a plot of the variation of refractive index with temperature. The plot of T g as a function of the thickness of polystyrene films presents a sharp decrease when the thickness reaches values below 60nm. Results also show variations in the slope of the glassy line, α g, with decreasing thickness, supporting the hypothesis that an interface with a different mobility does exists close to the polymer–air interface. However, this decrease was not as sharp as expected indicating that the decrease in T g with thickness could not be solely explained by the presence of an interface of different mobility.

Polarity inversion of type II InAs∕GaSb superlattice photodiodes

The authors demonstrated the realization of p-on-n type II InAs∕GaSb superlattice photodiodes. Growth condition for high quality InAsSb layer lattice matched to GaSb was established for the use of an effective n-contact layer. By studying the effect of various GaSb capping layer thicknesses on the optical and electrical performances, an optimized thickness of 160nm was determined. In comparison to as grown n-on-p superlattice photodiodes, this inverted design of p on n has shown similar quality. Finally, by analyzing Fabry-Perot interference fringes in the front side illuminated spectral measurement, the refractive index of the superlattice was determined to be approximately 3.8.

On the problem of microcavity effects on the top emitting OLED with semitransparent metal cathode

Recent interest in the top emission organic light emitting devices (TEOLEDs) has been stimulated by their potential application for the development of full color flat panel displays. We have fabricated the bottom emitting, ITO/α-NPD(50 nm)/Alq3(35 nm)/BCP(10 nm)/Ca(10 nm)/Ag(120 nm) device with a microcavity length of L = 95 nm and three top emitting structures, namely Ni/α-NPD(50 nm)/Alq3(35 nm)/ BCP(10 nm)/Ca(10 nm)/Ag(10 nm) (L = 95 nm), Ni/α-NPD(50 nm)/Alq3(35 nm)/BCP(5 nm)/Ca(10 nm)/ Ag(10 nm) (L = 90 nm), and Ni/α-NPD(35 nm)/Alq3(50 nm)/BCP(5 nm)/Ca(10 nm)/Ag(10 nm) (L = 90 nm). These devices were characterized by electroluminescence (EL) and current–voltage (I–V) measurements. Results on the top and the bottom emitting devices have been compared. In Ni/α-NPD(50 nm)/ Alq3(35 nm)/BCP(10 nm)/Ca(10 nm)/Ag(10 nm) structure, the EL yield is lowest and Fabry-Perot interference fringes are observed, due to microcavity effects because of the presence of the residual reflection at the semitransparent Ca/Ag cathode. The turn-on voltage is significantly high (7 V), about two times compared to the bottom emitting structure. The device performance could be improved by properly optimizing the microcavity length. No interference fringes are noticed in TEOLEDs with the microcavity length of L = 90 nm. Enhanced EL and very low threshold voltage (2.75V) are reported in the Ni/α-NPD(50 nm)/Alq3(35 nm)/BCP(5 nm)/Ca(10 nm)/Ag(10 nm) TEOLED. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Deriving wavelength spectra from fringe images from a fixed-gap single-etalon Fabry-Perot spectrometer.

A new method is described for inferring wavelength spectra from two-dimensional images of Fabry-Perot interference fringes. This new method addresses the practical difficulties that have been previously encountered in determining the fringe image's center, magnification, and distortions accurately enough to fully exploit the spectral resolution provided by the etalon. The method proceeds in two steps. First, the instrument's mapping of image position to interference order is characterized by use of images of a scene illuminated uniformly by a highly monochromatic laser. Then this information is applied to resample two-dimensional images of unknown radiation sources down to sets of one or more one-dimensional wavelength spectra. Discrete cross-correlation techniques are used at both stages of anal-

Optical Characterization of Iv-VI Mid-Infrared Vcsel

AbstractPbSe/PbSrSe multiple-quantum-well (MQW) structures and PbSrSe thin films were grown on BaF2 (111) substrates by molecular beam epitaxy (MBE) and characterized by Fourier transform infrared (FTIR) spectrometer. Strong photoluminescence without Fabry-Perot interference fringes was observed even at room temperature from the MQW structures. The peak energies for the MQW structures with different well widths shifted to high energy with increasing temperature. The absorption edge of PbSrSe layer was determined by transmission spectra. Meanwhile, we designed and fabricated λ=4.1 μm MQW vertical cavity surface emitting laser (VCSEL). A power output of 40 mW was obtained at room temperature. The room temperature threshold pump density is 200 kW/cm2.

Above-room-temperature continuous-wave mid-infrared photoluminescence from PbSe/PbSrSe quantum wells

Strong photoluminescence between 3 and 4 μm was observed at temperatures as high as 55 °C from PbSe/PbSrSe multiple-quantum-well structures grown on BaF2 (111) substrates by molecular-beam epitaxy. Fabry–Perot interference fringes dominated the spectra, indicating that the luminescence was primarily due to stimulated emission processes. Peak emission energies were determined by fitting Gaussian functions to the spectra, and they showed that emission energies at 25 °C decreased from 402 to 312 meV as quantum-well thickness increased from 40 to 200 Å. The temperature tuning coefficient was also observed to decrease from 0.400 meV/K for a 200 Å multiple-quantum-well sample to 0.313 meV/K for a 40 Å multiple-quantum-well sample.

Development of a Porous Silicon Based Biosensor

AbstractIn this paper we demonstrate that porous silicon (PS) can be used as an immobilization matrix and a transducer for biosensor applications. Thin layers of PS were fabricated showing fine structure in their reflection spectra, characteristic of longitudinal optical cavity modes, or Fabry-Perot interference fringes. The PS surface was modified by covalently bonding streptavidin to a heterobifunctional linker immobilized to the surface using common silane chemistry. The mode spacing and wavelength in the interference spectrum was modified, by displacing buffer and introducing proteins into the PS layer. Protein-protein interactions between immobilized Streptavidin and biotinylated Protein A followed by Protein A and IgG were detected. The surface was regenerated during the course of the experiment showing reversibility of the sensor at the third layer.

Modeling of absorption data complicated by Fabry–Perot interference in germanosilicate thin-film waveguides

Complex absorption spectra obtained from thin films at normal incidence can be difficult to interpret owing to the appearance of Fabry–Perot interference fringes in the data. We describe a technique for modeling such spectra so that true absorption features can be identified and evaluated separately from the overlying fringes. The technique is used to interpret data obtained from photosensitive germanosilicate solgel films on fused-silica substrates but may be easily extended to analysis in other material systems.

Two-Photon Resonant Raman Scattering in Red-HgI2

The two-photon resonant Raman scattering (TRRS) process via the excitonic molecule (EM) state in red-HgI 2 is studied at 1.6 K. The intensity of the TRRS line is enhanced with FWHM 0.7 meV at 2.3324 eV which corresponds to a half of the EM energy. From the analysis of the TRRS data, the dispersion curve of the transverse exciton polariton is determined: The transverse exciton energy and its effective mass relative to the electron rest mass are obtained as 2.3348 ±0.0001 eV and 1.3 ±0.1, respectively. The background dielectric constant, ε ∞ is determined to be 10.5 ±0.5 from the Fabry-Perot interference fringes of a thin single crystal.

Time-Resolved Spectroscopy of Ruby Laser Emissions*

The time variations of Fabry–Perot interference fringes produced by the coherent light from a ruby laser have been recorded with an image converter camera. The recorded variations indicate that: (a) the mode of oscillation of the ruby can change from one relaxation–oscillation burst of emission, or laser spike, to another; (b) a monotonic progression of wavelengths frequently occurs in a sequence of spikes; (c) the oscillation mode can switch within a single spike; and (d) several modes of oscillation can occur simultaneously in a single spike. The multiplicity of modes was observed to increase with an increase of pumping power. The wavelength separation between simultaneous modes corresponds closely to an integral difference in the number of half-wavelengths between the silvered end faces of the ruby.