Far-infrared absorption of PbSe nanorods

Measurements of the far-infrared absorption spectra of PbSe nanocrystals and nanorods are presented. As the aspect ratio of the nanorods increases, the Fro?hlich sphere resonance splits into two peaks. We analyze this splitting with a classical electrostatic model, which is based on the dielectric function of bulk PbSe but without any free-carrier contribution. Good agreement between the measured and calculated spectra indicates that resonances in the local field factors underlie the measured spectra.     

Far-infrared properties of hybrid plasmonic geometries

Plasmonic structures made of periodically arranged metallic rings integrated into subwavelength holes are investigated at the far-infrared terahertz frequencies. The emergence and the interplay of various resonances sustained by such plasmonic samples are elucidated. To reveal a coherent physical picture, relevant dimensions of the samples are modified and their impact on the resonance properties is analyzed. The experimental work is fully supported by numerical simulations. The understanding of the interplay of various resonances will foster applications which require plasmonic substrates to exhibit simultaneously resonances at well-defined frequencies and line widths.     

Far-infrared spectroscopy of protein higher-order structures

Far-infrared (FIR) spectroscopy in the spectral region of 50-450 cm(-1) was used to study a series of protein higher-order structures constructed using β-lactoglobulin and polyomavirus capsid protein VP1. There were marked differences in the spectra for β-lactoglobulin monomer and dimer and between untreated β-lactoglobulin and heat-induced gels formed at neutral pH. Untreated β-lactoglobulin and heat-induced gels formed at acidic pH exhibited little difference in their spectra. Assembly of the quaternary structure of polyomavirus virus-like particles also caused large changes in the FIR spectra. These findings suggest that FIR spectroscopy may prove useful in studying some protein quaternary and higher-order structures. There was evidence of detection of β-lactoglobulin dimerization, intermolecular disulfide bonding in heat-induced neutral gels, and polyomavirus virus-like particle assembly but no evidence that FIR could detect β-lactoglobulin fibrils with their polymeric structure and hydrogen-bonded intermolecular β-pleated sheeting.     

Far-infrared,Waveguide and Distributed-feedback Gas Lasers

A survey is given of the history and the development of far-infrared and waveguide gas lasers. These lasers were the first to bridge the power gap between the thermal infrared and the microwave region, giving new impetus to spectrometry of gases, condensed matter and astronomical objects, as well as to plasma diagnostics. The basic types of far-infrared gas lasers are reviewed: electrically and transversely excited lasers, optically pumped lasers, waveguide lasers, distributed-feedback and helical-feedback lasers.     

Fourier-transform phase-shifting interferometry

Phase-shifting interferometry is a preferred technique for high-precision surface form measurements, but the difficulty in handling the intensity distortions from multiple-surface interference has limited the general use of the technique to interferometer cavities producing strict two-beam interference. I show how the capabilities of phase-shifting interferometry can be extended to address this problem using wavelength tuning techniques. The basic theory behind the technique is reviewed and applied specifically to the measurement of parallel plates, where surfaces, optical and physical thickness, and homogeneity are simultaneously obtained. Basic system requirements are derived, common error sources are discussed, and the results of the measurements are compared with theory and alternative measurement methods.     

Three-color differential interferometry

It is shown that differential interferometry using a Wollaston prism and a three-color laser source is an optical technique that has all the advantages of differential interferometry in polarized white light and of classical monochromatic interferometry. The interference fringe pattern obtained is very large and colored and presents a central white fringe that enables easy identification of the zero order of the interferogram. The three-color source is obtained by filtering the unwanted lines of the ionized laser (mixed argon and krypton) and balancing the three red, green, and blue lines by a technique that involves placing birefringent plates between the polarizer and the analyzer, the thickness of which has been calculated to create a natural filter. The unsteady aerodynamic flow downstream of a diamond shape airfoil has been visualized with this technique, which shows that the power of the light source is sufficient to record the interferograms at a high rate.     

External phase-modulation interferometry

We analyze and test a laboratory benchtop version of a compound interferometric phase sensor, a Michelson interferometer whose output is combined coherently with a phase-modulated local oscillator beam tapped off the Michelson input beam. This configuration models a whole class of external-modulation interferometers designed to shift signals, obscured by low-frequency intensity noise of the light source, into a shot-noise-limited region of the photocurrent spectrum. We find analytically that the shot-noise-limited sensitivity achievable with this system is comparable with that obtained by using internal phase modulation, with both schemes suffering (for different reasons) approximately a 22% sensitivity penalty compared with ideal shot-noise-limited direct detection. Experimentally we achieve true shot-noise-limited sensitivity, and we investigate trade-offs necessitated by commonly encountered nonideal features in any external-modulation system. Our analytic model, which specifically accounts for Michelson fringe contrast, electronic receiver noise, phase-modulation depth, and the local oscillator tap-off fraction, is sufficiently accurate to predict the absolute sensitivity of our benchtop instrument to within 0.5 dB.     

Automating extensometer interferometry

Translated from Izmeritel'naya Tekhnika, No. 7, pp. 43–44, July, 1989.     

Vibration-resistant phase-shifting interferometry

A method to reduce the sensitivity of phase-shifting interferometry to external vibrations is described. The returning interferogram is amplitude split to form two series of interferograms, taken simultaneously and with complementary properties, one with high temporal and low spatial resolution and the other with low temporal and high spatial resolution. The high-temporal-resolution data set is used to calculate the true phase increment between interferograms in the high-spatial-resolution data set, and a generalized phase-extraction algorithm then includes these phase increments when the topographical phases in the high-spatial-resolution data set are calculated. The measured topography thereby benefits from the best qualities of both data sets, providing increased vibration immunity without sacrificing high spatial resolution.     

Improved vertical-scanning interferometry

We describe a method that combines phase-shifting and coherence-peak-sensing techniques to permit measurements with the height resolution of phase-shifting interferometry without the interval-slope limitation of lambda/4 per data sample of phase-shifting interferometry. A five-frame algorithm is used to determine both the best-focus frame position and the fractional phase from the best-focus frame of the correlogram acquired through vertical scanning. The two surface profiles retrieved from the phase and the modulation contrast of the correlograms are compared in the phase-unwrapping process to remove fringe-order ambiguity.