Yu. G. Goncharov

Glassy dynamics in mono-, di- and tri-propylene glycol: From the α- to the fast β-relaxation

We present a thorough characterization of the glassy dynamics of three propylene glycols (mono-, di- and trimer) by broadband dielectric spectroscopy. By covering a frequency range of more than 15 decades, we have access to the entire variety of dynamic processes typical for glassy dynamics. These results add three more molecular glass formers to the sparse list of materials for which real broadband spectra, including the region of the fast β-process, are available. Some first analyses of the various observed dynamic processes are provided.

Polypropylene-substrate-based SRR- and CSRR-metasurfaces for submillimeter waves

In this paper it is presented the fabrication of low loss millimeter wave metamaterials based on patterning on polypropylene substrates by conventional contact photolitography. We study numerically and experimentally the transmission and reflection properties of two dimensional arrays of split ring resonators (SRRs), or metasurfaces, and their complementary structure (CSRRs) for co- and cross-polarization excitations up to submillimeter frequencies under normal incidence conditions. The obtained results suggest the possibility of scaling them at terahertz frequencies based on this substrate where other lossy substrates degrade the resonators quality. Left-handed metamaterials derived from these SRRs and CSRRs metasurfaces could be feasible.

Development and Characterization of Quasi-Optical Mesh Filters and Metastructures for Subterahertz and Terahertz Applications

We review our recent results on development of passive quasi-optical selective devices based on metallized subwavelength microstructure arrays designed for controlling radiation beams at frequencies from a few tens GHz up to ten THz: filters, polarizers, metasurfaces, ET-metamaterial lenses. The methods of electromagnetic simulation, technological implementation and (sub)THz-characterization of microstructure devices, as well as their applications, are discussed.

Scaling of terahertz conductivity at the metal-insulator transition in doped manganites

Magnetic field and temperature dependence of the Terahertz conductivity and permittivity of the colossal magnetoresistance manganite Pr_{0.65}Ca_{0.28}Sr_{0.07}MnO_3 (PCSMO) is investigated approaching the metal-to-insulator transition (MIT) from the insulating side. In the charge-ordered state of PCSMO both conductivity and dielectric permittivity increase as function of magnetic field and temperature. Universal scaling relationships between the changes in permittivity and conductivity are observed in a broad range of temperatures and magnetic fields. Similar scaling is also seen in La_{1-x}Sr_xMnO_3 for different doping levels. The observed proportionality points towards the importance of pure ac-conductivity and phononic energy scale at MIT in manganites.

Phase-sensitive terahertz spectroscopy with backward-wave oscillators in reflection mode

In this article we describe a method which allows accurate measurements of the complex reflection coefficient r = absolute value(r) x exp(i phi(R)) of a solid at frequencies of 1-50 cm(-1) (30 GHz-1.5 THz). Backward-wave oscillators are used as sources for monochromatic coherent radiation tunable in frequency. The amplitude of the complex reflection (the reflectivity) is measured in a standard way, while the phase shift, introduced by the reflection from the sample surface, is measured using a Michelson interferometer. This method is particular useful for nontransparent samples, where phase-sensitive transmission measurements are not possible. The method requires no Kramers-Kronig transformation in order to extract the samples electrodynamic properties (such as the complex dielectric function or complex conductivity). Another area of application of this method is the study of magnetic materials with complex dynamic permeabilities different from unity at the measurement frequencies (for example, colossal-magnetoresistance materials and metamaterials). Measuring both the phase-sensitive transmission and the phase-sensitive reflection allows for a straightforward model-independent determination of the dielectric permittivity and magnetic permeability of such materials.

Universal relationship between the penetration depth and the normal-state conductivity in YBaCuO

The absolute values of the conductivity in the normal state σn and of the low-temperature penetration depths λ(0) were measured for a number of different samples of the YBaCuO family. We found a striking correlation between σn and λ−2 regardless of doping, oxygen reduction or defects, thus providing a simple method to predict the superconducting penetration depth and to have an estimate of the sample quality by measuring the normal-state conductivity.

A unified terahertz radiation source based on a backward-wave tube

A unified terahertz radiation source based on backward-wave tubes is described. The device is intended for performing spectroscopic studies of condensed media, gases, and plasma in the frequency range 0.18–1.50 THz. The advantage of this design is the actual development of a terahertz radiation source with continuously tuned oscillation frequency in a wide range. No analogues of this instrument exist. Its schematic design is presented and individual functional units are described. The output radiation power is 0.5 mW–0.5 W, and the frequency range is 0.18–1.50 THz.

Double effective band-pass submm-filters based on anisotropic resonant meshes

We present the results of development of original band-pass filters for 4-channel radiometric diagnostics of hot plasma in the frequency range 260-400GHz. The filters constitute multilayer interference structures consisting of specially designed thin anisotropic 2D resonant metal meshes separated by polypropylene layers 150-170 microns thick. Due to anisotropy of meshes the transmission band position of the filters strongly depends on polarizations of the incident radiation. It enables to use only two filters for realizing a 4-channel spectral system that reduces costs of its production. Numerical simulations of frequency response of developed filters are compared with the results of their characterization at a submillimeter BWO-spectrometer. The technological aspects of filter fabrication are described.

Temperature evolution of dielectric response spectra of stillwellite-like glasses in the terahertz and infrared ranges

The reflectance and transmittance spectra of the PrBGeO5 glass were measured in the terahertz and infrared ranges. At low temperatures, an increase in the permittivity and the appearance of narrow resonance-type lines in the terahertz range were revealed. The complex permittivity spectra were calculated from the Fresnel and Kramers-Kronig formulas.

Phase-sensitive terahertz spectroscopy with BWOs in reflection mode

We report on a method, which allows accurate measurements of complex reflection coefficient, r∘ = |∘| exp(iϕR), of a solid at frequencies 1 to 50 cm-1 (30 GHz–1.5 THz). Backward-wave oscillators (BWOs) are used as sources of monochromatic coherent radiation, tunable in frequency. The amplitude of the complex reflection (the reflectivity) is measured in a standard way, while the phase shift, introduced by the reflection from the sample surface, is measured using a Michelson interferometer. This method is particular useful for not-transparent samples, where phase-sensitive transmission measurements are not possible. The method requires no Kramers-Kronig transformation in order to extract the samples electrodynamic properties (such as complex dielectric function or complex conductivity). Another area of application of this method is the study of magnetic materials with complex dynamic permeability different from unity at the measurement frequencies (for example, metamaterials). Measuring both, the phase-sensitive transmission and the phase-sensitive reflection, would allow a straightforward model-independent determination of the dielectric permittivity and magnetic permeability of such materials.