Vasily V. Gerasimov

Novosibirsk Free Electron Laser—Facility Description and Recent Experiments

The design and operational characteristics of the Novosibirsk free electron laser facility are described. Selected experiments in the terahertz range carried out recently at the user stations are surveyed in brief.

Diffraction of a surface wave on a conducting rectangular wedge

Diffraction of a surface wave on a rectangular wedge with impedance faces is studied using the Sommerfeld-Malyuzhinets technique. An analog of Landaus bypass rule in the theory of plasma waves is introduced for selection of a correct branch of the Sommerfeld integral, and the exact solution is given in terms of imaginary error function. The formula derived is valid both in the near-field and far-wave zones. It is shown that a diffracted surface wave is completely scattered into freely propagating electromagnetic waves and neither reflected nor transmitted surface waves are generated in case of bare metals which have positive real part of surface impedance. The scattered waves propagate predominantly at a grazing angle along the direction of propagation of the incident surface wave and mainly in the upper hemisphere regarding the wedge face. The profile of radiated intensity is nonmonotonic and does not resemble the surface wave profile which exponentially evanesces with the distance from the wedge face. Comparison with experiments carried out in the terahertz spectral range at Novosibirsk free electron laser has shown a good agreement of the theory and the experiments.

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.

A way to determine the permittivity of metallized surfaces at terahertz frequencies

A method for determining the dielectric permittivity of metal surfaces at terahertz (THz) frequencies has been suggested and tested. The method implies excitation of surface plasmons (SPs) on the sample surface and measurements of the SPs propagation length and field penetration depth in air. The technique was tested on evaporated gold with Novosibirsk free-electron laser at the wavelength of 130 μm. The method suggested paves the way for refractometry of metallized surfaces at THz frequencies.

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.

Growth of terahertz surface plasmon propagation length due to thin-layer dielectric coating

Here we consider a longstanding problem: why terahertz (THz) surface plasmons (SPs) do not run so far as the Drude theory predicts. We experimentally demonstrated that the main cause for this paradox was a drastic rise in the SP radiative losses at THz frequencies, not violation of the theory. Analysis of SPs induced with the radiation of the Novosibirsk free electron laser (λ=130  μm) showed that a thin dielectric layer (≈λ/250) on a metal surface can reduce the losses substantially and hence enlarge the SP propagation length several times. Furthermore, it was found that there was an optimal layer thickness corresponding to the minimum total SP energy losses; the cause of this phenomenon is discussed.

Experimental investigations into capability of terahertz surface plasmons to bridge macroscopic air gaps.

Results of experimental and theoretical studies of the capability of terahertz surface plasmons (SPs) to cross macroscopic air gaps in a substrate (or between substrates) with admissible losses are presented. SPs were launched with quasi-cw free-electron laser radiation with 130 μm wavelength (λ). We managed to detect SPs passing across gaps as wide as 100 mm (or about 10(3)⋅λ), which is very promising for development of terahertz SP circuitry. The phenomenon was harnessed for splitting an SP beam into two new ones, guided by their own individual plane-surface substrates.

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.

The excitation of terahertz-range surface plasmons by frustrated total internal reflection through the substrate

A method of exciting surface plasmons in the terahertz range using the Kretschmann layout of the frustrated total internal reflection method is proposed and tested. The method involves making the illuminated part of the metallic layer transparent, with its thickness smoothly increasing in the direction in which the radiation propagates. This makes it possible to excite terahertz-range surface plasmons without the incident radiation simultaneously generating parasitic diffraction satellites of these plasmons. The SNR in terahertz surface spectroscopy and plasmon sensor devices is increased by using this method. Experiments to test the method have been carried out on gold, using the radiation of the Novosibirsk free-electron laser.