THz lasing of shallow donors in stressed silicon crystals

Results of experimental and theoretical study of terahertz stimulated emission from optically excited group-V donors (phosphor P, antimony Sb, arsenic As, bismuth Bi) in uniaxially stressed and liquid helium cooled silicon crystal are summarized and discussed. It is shown that compressive force of 1-1.5 kbar for P, Sb and of 2-3 kbar for As, Bi applied along {100} crystallographic orientations results in the remarkable enhancement of the laser gain as well as THz emission efficiency and laser threshold intensity is decreased by the order of magnitude or even more. For As and Bi donors it is accompanied by a switching of the emission line because of the upper laser state change. The effect of uniaxial stress on donor lasing originates from energy shift of the conduction band valleys of silicon which split donor states changing their eigen-values and eigenfunctions. According to the calculations of phonon-assisted relaxation rates appropriate stress-induced donor modification increases the lifetime and makes pump efficiency of the upper laser states better. Thus THz laser performance of donors in silicon can be substantially improved by host crystal deformation

Terahertz silicon lasers

Stimulated donor and Raman Stokes emission has been achieved under intracenter excitation and photoionization of shallow donor centers in silicon. A pulsed laser emission of up to a few mW of peak power in the 1 - 7 THz frequency range has been obtained at low temperatures ( < 30 K). Thorough laser characterization and further optimization of the laser threshold and efficiency has been carried out

Low-threshold terahertz Si:As laser

The optical threshold of terahertz intracenter arsenic-doped silicon lasers has been reduced by two orders of magnitude by applying a compressive force to the laser crystal. The Si:As lasers were optically excited with radiation from a CO2 laser operating at a wavelength of 10.59 µm. The lowest threshold intensity of 8 kW/cm2 was realized at about 3×108 Pa stress applied along the [001] crystal axis. The uniaxial stress breaks the resonant interaction of electrons bound to donors with intervalley f phonons. This changes the upper laser state from 2p± to 2p0, lowers the laser threshold, and increases the output power. ©2007 American Institute of Physic

Quantitative characterization of local thermal properties in thermoelectric ceramics using “jumping‐mode” scanning thermal microscopy

Thermoelectric conversion may take a significant share in future energy technologies. Oxide-based thermoelectric composite ceramics attract attention for promising routes for control of electrical and thermal conductivity for enhanced thermoelectric performance. However, the variability of the composite properties responsible for the thermoelectric performance, despite nominally identical preparation routes, is significant, and this cannot be explained without detailed studies of thermal transport at the local scale. Scanning thermal microscopy (SThM) is a scanning probe microscopy method providing access to local thermal properties of materials down to length scales below 100 nm. To date, realistic quantitative SThM is shown mostly for topographically very smooth materials. Here, methods for SThM imaging of bulk ceramic samples with relatively rough surfaces are demonstrated. "Jumping mode" SThM (JM-SThM), which serves to preserve the probe integrity while imaging rough surfaces, is developed and applied. Experiments with real thermoelectric ceramics show that the JM-SThM can be used for meaningful quantitative imaging. Quantitative imaging is performed with the help of calibrated finite-elements model of the SThM probe. The modeling reveals non-negligible effects associated with the distributed nature of the resistive SThM probes used; corrections need to be made depending on probe-sample contact thermal resistance and probe current frequency

First terahertz-range experiments on pump – probe setup at Novosibirsk free electron laser

A single-color pump-probe system has been commissioned at the Novosibirsk free electron laser. The laser emits a tunable monochromatic terahertz radiation. To prove the proper system operation, we investigated the time-resolved absorption of a sample of n-type germanium doped with antimony, which was previously investigated at the FELBE facility, in the temperature range from 5 to 40 K. The measured relaxation time amounted to about 1.7 ns, which agreed with the results obtained at the FELBE. The results of pump-probe measurements of non-equilibrium dynamics of hot electrons in the germanium crystal at cryogenic temperatures are presented for wavelengths of 105, 141 and 150 μm

Anomalous thermal conductivity of alkaline-earth-metal-substituted EuTiO3 induced by resonant scattering

We have investigated thermal conductivities (κ) of polycrystalline Eu1–xAxTiO3 (A = Ca, Sr, Ba, 0 ≤ x ≤ 0.8) bulk materials in the temperature range of ∼ 2 K < T < 1173 K. The EuTiO3 demonstrates anomalous glass-like κ(T) behavior at low temperatures. Partial substitutions with Sr2+ and Ba2+ do not cause a significant change in the κ(T) behavior, while a κ(T) peak, which looks like a manifestation of a typical crystalline solid, appears only in the Ca-substituted samples with an orthorhombic structure when x ≥ 0.4. After excluding the magnetic effects on κ and discussing the possible phonon scattering mechanisms in depth, together with heat capacity Cp measurements and high-resolution X-Ray diffraction characterization, we find that the unusual low κ at low temperatures is attributed to resonant scattering induced by the intrinsic disordered local structure and lattice instability in EuTiO3. Due to the different lattice dynamics of ATiO3, the lattice structure of Eu1–xCaxTiO3 can be regarded as formed by a part-soft (from EuTiO3) part-rigid (from CaTiO3) sublattice. The anomalous κ(T) behavior of Eu1–xCaxTiO3 results from the combined effect of phonon transport between the normal phononic heat transport in the rigid sublattice and the strong damping of heat conduction in the soft sublattice

Anomalous thermal conductivity of alkaline-earth-metal-substituted EuTiO3 induced by resonant scattering

We have investigated thermal conductivities (κ) of polycrystalline Eu1–xAxTiO3 (A = Ca, Sr, Ba, 0 ≤ x ≤ 0.8) bulk materials in the temperature range of ∼ 2 K < T < 1173 K. The EuTiO3 demonstrates anomalous glass-like κ(T) behavior at low temperatures. Partial substitutions with Sr2+ and Ba2+ do not cause a significant change in the κ(T) behavior, while a κ(T) peak, which looks like a manifestation of a typical crystalline solid, appears only in the Ca-substituted samples with an orthorhombic structure when x ≥ 0.4. After excluding the magnetic effects on κ and discussing the possible phonon scattering mechanisms in depth, together with heat capacity Cp measurements and high-resolution X-Ray diffraction characterization, we find that the unusual low κ at low temperatures is attributed to resonant scattering induced by the intrinsic disordered local structure and lattice instability in EuTiO3. Due to the different lattice dynamics of ATiO3, the lattice structure of Eu1–xCaxTiO3 can be regarded as formed by a part-soft (from EuTiO3) part-rigid (from CaTiO3) sublattice. The anomalous κ(T) behavior of Eu1–xCaxTiO3 results from the combined effect of phonon transport between the normal phononic heat transport in the rigid sublattice and the strong damping of heat conduction in the soft sublattice.publishe