V. Skoromets

Electric-field-tunable defect mode in one-dimensional photonic crystal operating in the terahertz range

A one-dimensional photonic crystal possessing an electric-field-tunable defect mode in the lowest forbidden band is demonstrated. The compact photonic structure consists of two symmetric Bragg mirrors made of alternate quarter-wave layers of SiO2 and CeO2 separated by a defect layer of an incipient ferroelectric SrTiO3 with electrodes transparent for terahertz radiation on its both sides. The applied bias is then perpendicular to the layer and modifies the in-plane dielectric function, which is probed by the transverse terahertz wave. The observed tunable behavior is in agreement with the model of the ferroelectric soft mode behavior in SrTiO3 single crystals. The defect-mode frequency tunability is proportional to that of the soft mode: we achieved a relative tunability of 6.5% at 105 K under an electric bias of 60 kV/cm.

Ferroelectric phase transition in polycrystalline KTaO3 thin film revealed by terahertz spectroscopy

KTaO3 single crystal is an archetypal incipient ferroelectric in which a long-range ferroelectric order does not establish at low temperatures owing to quantum fluctuations. We report on a strong evidence of the ferroelectric phase transition near 60 K revealed by terahertz spectroscopy and microwave permittivity measurements of a polycrystalline KTaO3 thin film on (0001) sapphire substrate prepared by chemical solution deposition. The soft mode behavior is clearly observed in the terahertz (THz) spectra with a minimum frequency at 60 K. At the same temperature microwave permittivity maximum appears. The THz spectra strongly resemble that of strained epitaxial SrTiO3/DyScO3 films: the ferroelectric soft mode is linearly coupled to a central peak which is silent in the paraelectric phase and it becomes coupled to the polarization below the ferroelectric transition temperature with a progressively increasing bare strength.

Lattice dynamics and broad-band dielectric properties of the KTaO3 ceramics

High-density KTaO3 ceramics were synthesized and studied by means of microwave, terahertz, infrared, and Raman spectroscopies. The results were analyzed together with recently published radio-frequency data. [S. Glinsek et al., J. Am. Ceram. Soc. 94, 1368 (2011)] Three polar modes expected for the cubic structure were observed. As in single crystals, the lowest-frequency TO1 mode (soft mode) strongly softens on cooling, while the TO2 and TO4 mode frequencies do not change with temperature. The permittivity does not show any significant dispersion below the soft mode frequency and its value in the kHz and GHz range is mainly given by the intrinsic polar lattice modes contribution. The soft mode frequency agrees with the values found in single crystals; this indicates a negligible influence of the grain boundaries on the dielectric response in KTaO3 unlike in other ferroelectric or incipient ferroelectric perovskite ceramics.

Tunable dielectric properties of KTaO3 single crystals in the terahertz range

Electric-field tunability of the dielectric properties of potassium tantalate single crystal was studied by terahertz spectroscopy in a broad temperature range (40-250 K). Complex-valued terahertz transmission spectra of samples were measured with an external electric field perpendicular to the sample surface and parallel to the terahertz wave-vector. We found that the ferroelectric soft mode hardening is fully responsible for the observed electricfield-induced changes in the spectra and no signature of a central mode was detected. We determined the anharmonic properties of the soft-mode potential in the mean field approximation. The observed behavior was compared with that previously reported for SrTiO3 single crystals.

Thin film polycrystalline Si solar cells studied in transient regime by optical pump–terahertz probe spectroscopy

We used time-resolved terahertz spectroscopy to study ultrafast photoconductivity of polycrystalline thin-film silicon solar cells. We selected a series of samples, which exhibited variable conversion efficiencies due to hydrogen plasma passivation under various technological conditions. The decay of the transient terahertz conductivity shows two components: the fast one is related to the charge recombination at interfaces, while the slow nanosecond one is attributed to the trapping of photocarriers by defects localized at grain boundaries or at dislocations in the polycrystalline p− layer of the structure. We observed a clear correlation between the open-circuit voltage and the nanosecond-scale decay time of the transient terahertz conductivity of the solar cells. Thus, the terahertz spectroscopy appears to be a useful contactless tool for inspecting the local photoconductivity of solar cells including, in particular, various nanostructured schemes.

Electric field tuning of hard polar phonons in strained SrTiO3 films

We investigate 100 nm thick epitaxial monolayer of SrTiO3 strained by 1% in biaxial tension and grown on (110) DyScO3. Spectra within the whole far-infrared spectral range are obtained as a function of temperature and an electric bias. An appreciable tunability of high frequency polar phonons is observed upon bias. The splitting of TO4 mode clearly observed below 220 K, which is further increased upon bias, is a signature of the ferroelectricity of the film at these temperatures.