The quasi-optical segmented mirror for a formation of the target irradiation field was manufactured and investigated. It was designed for the high power THz beam of the free electron laser (FEL) using as a target a dust particle cloud, simulating cosmic dust. Numerical calculation of the beam shape and its low power laboratory measurements was made in the spectral region 1 - 3 THz of the first phase of the Novosibirsk FEL construction. The theoretical calculations of the diffraction effects reveal a speckle structure of a target spot, which was confirmed by the laboratory experiment. The beamformer technology was adapted for manufacturing and such device could be widely used for a concentration of powerful terahertz radiation.

Памяти Сергея Петровича Капицы, Андреев А.Ф., Белоцерковский О.М., Богомолов Г.Д., Быков В.П., Каган Ю.М., Карлов Н.В., Луганский Л.Б., Питаевский Л.П., Прозорова Л.А., Рыжов Ю.А., Фортов В.Е., Ципенюк Ю.М.

The problem of the eigenmodes of a 2D open resonator is solved. Two polarizations are considered. Eigenmodes in resonators with various mirror shapes are investigated. The results of a rigorous approach are compared to the results obtained with the help of various approximate asymptotic techniques.

The problem on the eigenmodes of a 2D open resonator is solved with the help of the method of extended boundary conditions. A modification of this method that makes it possible to substantially enhance the accuracy of computation is proposed. The results obtained by means of approximate techniques are compared to the solution obtained with the use of the singular integral equation method.

A technique and results of solution of the problem on plane-wave excitation of an open quasioptic resonator and a quasi-optic resonator located in an oversize waveguide are presented. Structures with various shapes of the mirror are investigated. The results obtained with the help of the proposed technique are compared with the results obtained by means of an approximate approach.

A technique and results of the solution of the problem on excitation of an open mirror resonator coupled with a waveguide via a slot in one of the mirrors are presented. The effect of the coupling slot on the radiation loss is analyzed. The results obtained with the help of a rigorous approach and approximate techniques are compared.

Mesh band-pass filters for 300-, 450-, 600-, and 750-GHz central frequencies are designed and manufactured. Copper and aluminum foil and foil-clad Teflon filters were made by chemical and ionic etching methods, and an aluminum film evaporated on a kapton film was formed by the lift-off lithography method. A gold layer was electrolytically applied on copper mesh filters. Characteristics of the filters were measured in the millimeter, submillimeter, and infrared (IR) ranges. Foil filters demonstrate better characteristics at lower frequencies, while filters with evaporated films have better characteristics at upper frequencies. The smallest transmission loss was 0.13 dB. For a combination consisting of four filters, this loss was 0.9 dB. The IR radiation attenuation in a 2.5- to 25.0-μm wavelength region was no less than 11 dB per filter.

Diffraction of a plane wave by a strip grating on a grounded dielectric slab is calculated on the basis of a rigorous description of the field inside strip conductors. The resonance regime of plane-wave reflection with substantial absorption is investigated. It is shown that, in this regime, the structure under study can be used, for example, for measurements of low loss in dielectric films, investigations of microwave properties of semiconductors, additional selection of oscillations in open resonators, and enhancement of the selectivity of detectors in the terahertz band. The results of this study are compared to the those obtained with the use of various approximate methods, and the limits of the applicability of these methods are established. A new algorithm involving equivalent boundary conditions is proposed. The algorithm provides for calculations that can be performed within a wide range of the parameters of the problem with the accuracy acceptable in practice.

Geodesic elements (prisms, lenses, and beam splitters) are suggested to control (focus, deflect, and split) beams of terahertz (THz) surface plasmons (SPs). A geodesic deflector made in the form of conical trench crossing a SP beam can be effectively used not only for deflection of the beam but also for separation of surface and bulk electromagnetic waves. Formulae for calculating the angle of SP beam deflection with a geodesic prism as well as the angle of divergence of SP beams at the output of the geodesic splitter have been obtained. Schemes of THz SP absorption sensor and interferometer based on geodesic elements are discussed as well.

A new method has been proposed for determining the complex refractive index of terahertz surface plasmons. This method implements the concept of asymmetric static interferometry in the planar version, where the interference pattern is formed by surface rather than bulk waves. The information about the real part of the refractive index is carried by the interference pattern period, while its imaginary part can be found from the illumination magnitudes in the pattern extrema.

Людмила Андреевна Прозорова (к 80-летию со дня рождения), Абрикосов А.А., Андреев А.Ф., Богомолов Г.Д., Гантмахер В.Ф., Герштейн С.С., Капица С.П., Кведер В.В., Крейнес Н.М., Марченко В.И., Питаевский Л.П., Смирнов А.И., Халатников И.М.

The optical scheme and functioning of a static Fourier-transform (FT) spectrometer employing surface electromagnetic waves (SEW) is discussed. SEW are excited by the probing radiation on the sample placed in one shoulder of the asymmetric interferometer. Interference pattern (interferogram) carrying information about the surface is registered by the detector array, disposed on the ellipse surface. This way of detectors positioning leads to linearization of the phase shift function on the pixels coordinates projection on the ellipse large axis. There are no moving parts in the instrument. That is why it can work as “one pulse spectrometer”. The length of SEW path determines the limiting spectral resolution, while time resolution depends on detector characteristics. The spectrometer provides the possibility of obtaining not only absorption (emission) spectra, but the spectra of complex refraction index as well.

The optical scheme and functioning of a static Fourier-transform (FT) spectrometer employing surface electromagnetic waves (SEW) at terahertz (THz) frequencies [A.A. Balashov et al., 2007] is discussed. The spectrometer enables one to obtain optical constants spectra of smooth metal surfaces as well as those of super thin (up to nm thick) films on their surfaces. Ordinary for IR spectroscopy a band, with 3000 cm-1 width, including IR and THz ranges, could be obtained with femto-second laser pulse colored in 1600 cm-1 for very fast processes spectroscopic studies of surface with fs time resolution (in principal).

The optical scheme and functioning of a static Fourier-transform (FT) spectrometer employing surface electromagnetic waves (SEW) is discussed. SEW are excited by the probing radiation on the sample placed in one shoulder of the asymmetric interferometer. Interference pattern (interferogram) carrying information about the surface is registered by the detector array, disposed on the ellipse surface. This way of detectors positioning leads to linearization of the phase shift function on the pixels coordinates projection on the ellipse large axis. There are no moving parts in the instrument. That is why it can work as “one pulse spectrometer”. The length of SEW path determines the limiting spectral resolution, while time resolution depends on detector characteristics. The spectrometer provides the possibility of obtaining not only absorption (emission) spectra, but the spectra of complex refraction index as well.

The effect of a thin dielectric cladding layer of a metal on the absorption of surface plasmons (SPs) in the terahertz frequency range is studied experimentally and numerically. It is found that, as the radiation wavelength increases, the attenuation of SPs caused by the cladding layer can increase by a factor of ∼104 as compared to the absorption of SPs propagating along the unperturbed metal-air interface. Data obtained in experiments with germanium-cladded aluminum specimens using radiation from a terahertz free-electron laser (v = 90 cm−1) confirm that application of a dielectric cladding on the metal surface causes the SP absorption to increase.