Valery S. Cherkassky

Classic holography, tomography and speckle metrology using a high-power terahertz free electron laser and real-time image detectors

Experiments with in-line and two-beam holography as well as speckle photography and speckle interferometry using highpower monochromatic radiation of free electron laser have been carried out. An experimental-and-theoretical approach to tomography with monochromatic terahertz sources is presented. All measurements were carried out using real-time image sensors: a microbolometer focal plane array and a luminescence-quenching thermal image plate with intensified CCD camera.

Fresnel reflection in optical components and detectors for the terahertz frequency band

Plane-parallel dielectric and semiconductor windows, beam splitters and radiation absorbers, as well as reflecting metal surfaces, are important components used in experiments with monochromatic terahertz radiation. The wavelengths of this radiation are longer than the wavelengths of visible light; therefore, multibeam interference becomes important in relatively transparent components with a thickness of the order of 1 mm, which is typical for optical systems. This interference causes the characteristics of optical components and detectors to be dependent on the wavelength. It has been found that bolometric power meters with a crystal used as an absorber of terahertz radiation and pyroelectric detectors with a germanium entrance window demonstrate individual periodic variations in sensitivity under variations in the wavelength. Basic characteristics of the windows and beam splitters manufactured from germanium, polypropylene, and Dacron (the materials that are of the greatest interest for terahertz-band spectroscopy) are calculated in the wave band 30–300 µm. The phase shift of s- and p-polarized terahertz waves reflected from metal surfaces are calculated as functions of the wavelength and the angle of incidence. Using the Fresenel formulas for processing experimental data on the reflection coefficient of terahertz radiation incident onto a K8 glass specimen, it is shown that, in the terahertz frequency band, this coefficient is equal to 16% at normal incidence and the absorption coefficient is equal to 860 cm−1.

Novosibirsk terahertz free electron laser: status and survey of experimental results

During the past year, generation characteristics of the Novisibirsk terahertz free electron laser were growing up. It generates now coherent radiation tunable in the range 120-170 /spl mu/m at the repetition rate of 2.8-11.2 MHz. Maximum average output power reaches 400 W at 11.2 MHz. Laser radiations are transmitted through a 14-m optical beamline to the user stations. Experiments on chemistry, biology, spectroscopy, imaging and holography with employment of terahertz radiation are in progress.

Obtaining spectrally selective images of objects in attenuated total reflection regime in real time in visible and terahertz ranges

An imaging attenuated total reflection (ATR) spectrometer for the terahertz range is created for the first time. The spectrometer uses a powerful free-electron laser. Images are recorded with a microbolom-eter detector array as a source of frequency tunable monochromatic radiation. Recording spectrally selective images of dynamic objects at a rate of 20 frames per second in the visible and terahertz ranges is demonstrated. In the terahertz range, images of the interdiffusion of liquids with strongly differing optical constants are obtained. Optimal configurations for the operation of the ATR spectrometer are found. Merits and demerits of the method are considered, as well as ways of improving the quality of image.

The Talbot effect in the terahertz spectral range

The Talbot effect in the terahertz spectral range has been first observed and used in metrology applications: measurement of distance, determination of radiation wavelength, and real-time detection of wavefront distortion.

Introscopy of solids at Novosibirsk terahertz free electron laser

High-power Novosibirsk free electron laser, generating coherent monochromatic tunable radiation within the spectral range from 120 to 240 micrometers, is used for the shadwography of semi-transparent objects. Toepler and holography systems, which are under construction, will enable spectrally-selective imaging at the laser beam diameter of about 10 cm.

Quasi-Talbot effect with vortex beams and formation of vortex beamlet arrays

We study diffraction of Bessel vortex beams with topological charges of ±1 and ±2 and a wavelength of 130 µm on two-dimensional amplitude periodic gratings. Results of simulations and experiments at the Novosibirsk Free Electron Laser facility show that there appear periodic patterns in the planes corresponding to the classical main and fractional Talbot planes, but instead of self-images of the holes, there are observed periodic lattices of annular vortex microbeams with topological charges corresponding to the charge of the beam illuminating the grating. The ring diameters are the same for beams with different topological charges, but they are proportional to the grating period and inversely proportional to the diameter of the beam illuminating the grating.

Single-channel- and real-time imaging attenuated total reflection spectrometers for THz range

Two attenuated total reflection (ATR) spectrometers for study of substances highly-absorbing in the terahertz range have been developed. First system is a home-made single-reflection silicon ATR sampling module with an isolated sample cell to VECTOR-30 and IFS 66v/S Bruker spectrometers. This module was used for the examination of biological objects, including human blood, grounded rat bone, and other tissues. Second system is an imaging terahertz spectrometer using a tunable 100-W terahertz free electron laser as a source and a 160times120 microbolometer focal plane array camera (repetition rate up to 90 frames/s) as a recorder.

Feasibility of real-time terahertz speckle metrology

A 100-W terahertz free electron laser and a 160times120 microbolometer focal plane array were used to record speckled images of different objects. First- and second-order statistics of the speckle patterns obey classical dependencies. Using a set of speckle photographies, recorded with a repetition rate of 41 frames/s, amplitude and logarithmic decrement of damped rotational oscillations of a diffuse reflector were determined.

Novosibirsk free electron laser: Second stage construction and new results at the terahertz user stations

Progress in the construction of the second stage of Novosibirsk free electron laser and main results obtained during the year at the terahertz user stations are described.