Yu. Yu. Choporova

Generation of Terahertz Surface Plasmon Polaritons Using Nondiffractive Bessel Beams with Orbital Angular Momentum.

Bessel vortex beams with topological charges of l=±1 and l=±2 were produced in the terahertz spectral range from a free electron laser Gaussian beam (λ=141  μm) transformed using silicon binary diffractive optical elements. The spatial characteristics of the beams were obtained using a microbolometer array. A radius to path length ratio of 1:100 was achieved for nondiffractive beams with the average power of 30 W. Surface plasmon polaritons (SPPs) on gold-zinc-sulphide-air interfaces were generated due to diffraction of vortex beams on a sample edge. A new effect, a dependence of the efficiency of SPP generation on the direction of the azimuthal component of incident-radiation Poynting vector, was revealed.

Silicon diffractive optical elements for high-power monochromatic terahertz radiation

This paper presents a fabrication technique and results of studies of silicon binary diffractive optical elements (DOEs): a diffractive lens and a 1 : 2 diffractive beam splitter with an aperture diameter of 30 mm for the terahertz spectral range. The elements were fabricated in two versions: with and without an antireflection coating of parylene C. The DOE characteristics were investigated in the beam of the Novosibirsk free electron laser at a wavelength of 141 μm. The results are given of a study of the radiation resistance of the coating, which remained intact upon exposure to an average radiation power density of 4 kW/cm2; the peak power in a 100 ps pulse was almost 8 MW/cm2. Experimental estimates of the diffraction efficiency of the elements coated with the antireflection coating are in good agreement with theoretical estimates.

A terahertz ellipsometer

A terahertz ellipsometer, based on the Novosibirsk free electron laser (FEL), is described. The device operates using the dynamic photometric “polarizer-sample-analyzer” scheme with a rotating analyzer. Sources of systematic errors, attributed to imperfections of optical elements, the accuracy of their alignment, and the influence of random errors, which are caused by measuring-section noises, were analyzed. The whole device and its separate units were tested. From results of measurements on tested samples, the operation accuracy of the ellipsometer was determined: δΨ ⩽ 0.3° and δ(cosΔ) ⩽ 0.01. The measurement data obtained at a wavelength of 147 μm on the thickness and reflective index of blood films, which were deposited on a silicon substrate, are presented. In this case, the measurement accuracy of the reflective index is ±0.05 and thickness is 0.2 μm.

Optical elements for focusing of terahertz laser radiation in a given two-dimensional domain

Binary silicon-based diffractive optical element (DOE)—Gaussian-to-Square focuser (diameter of aperture is 30 mm) for the terahertz spectral range has been designed and characterized using terahertz radiation of the Novosibirsk Free Electron Laser (NovoFEL) at the wavelength of 141 μm. The preliminary experiments have demonstrated feasibility of application of binary silicon DOE for focusing of terahertz radiation into pre-given focal domain.

Diffractive lenses for high-power terahertz radiation beams

Techniques for manufacturing silicon binary (two-level) diffractive lenses and polypropylene kinoform diffractive lenses for the terahertz spectrum range are described. The elements are 1 and 0.8 mm thick, respectively. The silicon lens is manufactured in two versions: with no coating and with a parylene C (polyparaxylylene) antireflection coating. Characteristics of the diffractive optical elements are studied in the beam of a pulse-periodic free electron laser at a radiation wavelength of 141 μm and a repetition rate of 5.6MHz. The radiation resistance of the parylene coating, tested on the Novosibirsk free electron laser, was not impaired when the coating was exposed to an average power density of 4 kW cm−2, the peak power in a 100-picosecond pulse being almost 8 MW cm−2.

A terahertz scanning near-field optical microscope with an attenuated total internal reflection module

The first terahertz scanning near-field optical microscope with an attenuated total internal reflection module and a free-electron laser (FEL) as the radiation source was developed. A scanning system with positioning using a confocal sensor with chromatic coding and a surface-subwavelength probe touch sensor were developed and tested. A new technique for sensing the distance between the probe and a conducting surface via corona-discharge current measurement was developed. A specific lock-in system for detection of probe-scattered pulse-periodic radiation, which includes a hot-electron superconducting bolometer and an electronic signal-storage circuit, was developed to operate with the Novosibirsk terahertz FEL. All elements of the microscope were tested, and their working capacity was demonstrated. Experiments on the detection of microscope-probe-scattered terahertz radiation have been initiated.

Low-temperature intracenter relaxation times of shallow donors in germanium

The relaxation times of localized states of antimony donors in unstrained and strained germanium uniaxially compressed along the [111] crystallographic direction are measured at cryogenic temperatures. The measurements are carried out in a single-wavelength pump–probe setup using radiation from the Novosibirsk free electron laser (NovoFEL). The relaxation times in unstrained crystals depend on the temperature and excitation photon energy. Measurements in strained crystals are carried out under stress bar S > 300, in which case the ground-state wavefunction is formed by states belonging to a single valley in the germanium conduction band. It is shown that the application of uniaxial strain leads to an increase in the relaxation time, which is explained by a decrease in the number of relaxation channels.

Spectrum of spatial frequency of terahertz vortex Bessel beams formed using phase plates with spiral zones

This paper presents the first numerical and experimental investigation into the angular spectrum of terahertz Bessel beam with orbital angular momentum generated by a phase plate with spiral zones. The plate was exposed to a Gaussian beam of the Novosibirsk free electron laser. The Bessel beam formed was passed through a collecting lens. The distribution of the intensity of radiation with a wavelength of 141 microns before and after the focusing lens was recorded by a microbolometer array, which was moved along the optical axis by a motorized translation stage. The experimentally measured intensity distributions over the beam cross section recorded along the optical axis are in good agreement with numerical calculations.