G. A. Komandin

Optical properties of BiFeO3 ceramics in the frequency range 0.3–30.0 THz

Reflection and transmission infrared spectra of BiFeO3 ceramic samples have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 5 to 1000 cm−1 at temperatures in the range from 10 to 500 K. New resonant modes (probably, magnetic in nature) with the eigenfrequencies decreasing with an increase in the temperature have been recorded in the range 10–30 cm−1 by IR spectroscopy for the first time. An additional absorption with a fairly large dielectric contribution has been revealed in the range 30–60 cm−1. It has been demonstrated that the corresponding oscillators couple with both the lowest frequency phonon mode and the magnetic subsystem.

Magnetic and dielectric response of cobalt-chromium spinel CoCr2O4 in the terahertz frequency range

The nature of the phonon and magnon modes in the CoCr2O4 multiferroic with a cubic spinel structure has been studied using submillimeter spectroscopy and infrared Fourier spectroscopy. This paper reports on the first measurement of the evolution with temperature of the exchange optical magnon in the ferrimagnetic (TC = 94 K) and two low-symmetry (TS ≈ 26 K, Tlock-in = 14.5 K) phases of CoCr2O4 down to T = 5 K in zero magnetic field. It has been shown that the detected magnon is not a ferrimagnetic order parameter and originates, most probably, from spin precession in the cobalt sublattices. At the points of the magnetic phase transitions, the oscillator parameters of the two lowest-frequency phonon modes reveal an anomalous temperature behavior, thus evidencing the presence of significant interaction between the magnetic and phonon subsystems. The increase by 25% of the damping parameter of the phonon mode originating from vibrations of the CoO4 tetrahedra during the transition of CoCr2O4 to the multiferroic state (T < TS) suggests structural changes in the lattice involving loss of spatial central symmetry of the medium.

BWO Generators for Terahertz Dielectric Measurements

For many years, the terahertz frequency domain has been neglected compared to neighboring parts of the spectrum. However, the situation has changed with the development of backward-wave oscillator (BWO) and BWO-based spectrometers. Packaged (30-180 GHz) and unpackaged (0.18-1.50 THz) BWOs are now being produced. The 0.18-1.50 THz frequency range is of greatest interest for spectroscopic research. To work with unpackaged BWOs, a universal magnetic focusing system on a permanent magnet with a protective housing shielding the magnetic field was developed. The stable generation of radiation with a frequency of up to 2.1 THz without geometry change was achieved. A universal automated power-supply unit with high-speed BWO frequency radiation scanning and a high degree of BWO electronic security was also designed for generating unpackaged BWOs through a heating and high cathode voltage.

Terahertz dielectric spectra of (Ba,Sr)TiO3 thin films

Reflection and transmission spectra of Ba0.7Sr0.3TiO3 films with thicknesses of 36 and 800 μm on the MgO substrate are measured in the frequency range from 8 to 1000 cm−1 at room temperature. The dielectric parameters of the film material are derived using the simulation method. The dependence of the permittivity on the film thickness is determined. It is found that the absorption spectrum of the substrate strongly affects the calculated dielectric spectra of the film material. It is demonstrated that the simulation should be performed with the inclusion of the dielectric losses in the substrate in the subphonon frequency range.

Multiphonon absorption in a MgO single crystal in the terahertz range

The reflection and transmission spectra of a MgO single crystal at frequencies of 10–1000 cm−1 in the temperature range 10–300 K have been measured using submillimeter and infrared Fourier spectros-copy. The evolution of anharmonic absorption with variations in temperature has been investigated in the frequency range below the frequency of the transverse optical phonon. The parameters of the dispersion model that adequately describes the lattice absorption of terahertz radiation in single-crystal magnesium oxide have been determined.

Dielectric spectra of Bi0.98Nd0.02FeO3.00 multiferroic thin films in the terahertz frequency range

Transmission and reflection spectra of Bi0.98Nd0.02FeO3.00 multiferroic thin films on MgO single-crystal substrates have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 8 to 1000 cm−1 at room temperature. The complex permittivity spectra of the films have been calculated in terms of the layered medium model. It has been revealed that a decrease in the film thickness leads to a considerable increase in the losses in a range of 30 cm−1 and the corresponding fivefold increase in the static permittivity (to 500 for a film 32 nm thick). This phenomenon has been discussed in the framework of the phenomenological theory of phase transitions.

Soft polar modes and phase states of Ca1−xPbxTiO3 solid solutions

Dielectric spectra ɛ′(ν) and ɛ″(ν) of Ca1−xPbxTiO3 ceramic samples (x=0, 0.15, 0.2, 0.4) have been studied in the frequency range ν=7–1000 cm−1 at temperatures from 5 to 300 K using IR Fourier spectroscopy and submillimeter-range techniques. In the low-frequency range, polar phonons were established to undergo temperature-induced evolution. The results obtained are discussed in terms of the Landau theory of second-order phase transitions for coupled soft modes. The existence of one (or several) phase state(s) in the intermediate concentration region of Ca1−xPbxTiO3 solid solutions is tentatively assumed.

Effect of BiFeO3 ceramics morphology on electrodynamic properties in the terahertz frequency range

The nature of the contribution (additional to the phonon one) to the permittivity of bismuth ferrite ceramics has been studied using submillimeter and Fourier transform infrared spectroscopy. It has been shown that its numerical value depends on morphological features of samples, and the nature is determined by the defectness and crystallite interfaces. The effect of structure-phase features of bismuth ferrite ceramics on antiferromagnetic resonance line frequencies is indicated.

Mechanisms of loss formation in nonlinear optical crystals ZnGeP2 in the terahertz frequency range

Physical mechanisms of formation of radiation losses in the terahertz range in ZnGeP2 crystals have been experimentally studied in the wave number range of 5–350 cm−1 at temperatures of 10–300 K. The dominant contribution of two-phonon difference processes to the loss formation in the given frequency range has been shown.

Dielectric response of (Ba,Sr)TiO3 thin films in a terahertz and IR ranges

Dependences of the dielectric response spectra on the thickness of the films based on the (Ba,Sr)TiO3 (BST) solid solutions, which were deposited on single-crystal MgO substrates, have been analyzed. Using the Lorentz and Lyddane-Sachs-Teller models, the mutual correspondence of longitudinal optical (LO) and transverse optical (TO) phonon frequencies has established. The giant LO-TO splitting of the lowest-frequency vibration and inversion of LO and TO frequencies of other phonons in the IR spectrum of BST films has been found for the first time.