S. Yu. Sarkisov

GaSe1−xSx and GaSe1−xTex thick crystals for broadband terahertz pulses generation

We demonstrate the possibility of broadband THz pulse generation in mixed GaSe1−xSx and GaSe1−xTex crystals. The ordinary and extraordinary refractive indices of the crystals have been measured by the terahertz time-domain spectroscopy method, those values strongly influence the efficiency of THz generation process. The high birefringence and transparency of pure GaSe and mixed crystals allow optical rectification of femtosecond laser pulses in the several millimeters thick crystal using the еее interaction process (with two pumping waves and generated THz wave all having extraordinary polarization in the crystal).

Charge neutrality level and electronic properties of GaSe under pressure

Calculations of lattice parameters and electron band spectra of GaSe have been carried out from the first principles. The dependence of these parameters on hydrostatic compression as high as 5 GPa and on homogeneous biaxial tensile and compressive stresses (from −3 to 3 GPa) in the basal plane of the unit cell is considered. The calculations adequately reproduce the experimental features of the major interband transitions in GaSe under a hydrostatic pressure and, in the absence of experimental data, predict the dependence of structural and electronic properties of GaSe under the effect of a biaxial stress. On the basis of calculated band spectra, the energy position of the local charge neutrality level CNL, Ev + 0.8 eV, has been determined and electronic properties of the as-grown material and energy diagrams for interphase boundaries in GaSe have been analyzed.

Electronic properties of GaSe, InSe, GaS and GaTe layered semiconductors: charge neutrality level and interface barrier heights

Density functional theory calculations have been applied to study the structural and electronic properties of layered -GaSe, γ-InSe, β-GaS and GaTe compounds. The optimized lattice parameters have been obtained using vdW-DF2-C09 exchange-correlation functional, which is able to describe dispersion forces and produces interlayer distances in close agreement with experiments. Based on the calculated electronic band structures, the energy position of the charge neutrality level (CNL) in the III–VI semiconductors has been estimated for the first time. The room-temperature values of CNL are found to be 0.80 eV, 1.02 eV, 0.72 eV and 0.77 eV for -GaSe, β-GaS, GaTe and γ-InSe, respectively. The persistent p-type conductivity of the intentionally undoped -GaSe, β-GaS and GaTe and n-type conductivity of γ-InSe crystals are discussed and explained using the concept of CNL. We also estimated the barrier heights for a number of metal/semiconductor and semiconductor/semiconductor interfaces assuming partial Fermi level pinning at the CNL. A reasonable agreement between our calculations and the available experimental data has been obtained.

Compact 1.64 THz source based on a dual-wavelength diode end-pumped Nd:YLF laser with a nearly semiconfocal cavity

We describe a compact dual-wavelength (1.047 and 1.053 μm) diode end-pumped Q-switched Nd:YLE laser source which has a number of applications in demand. In order to achieve its dual-wavelength operation it is suggested for the first time to use essentially nonmonotonous dependences of the threshold pump powers at these wavelengths on the cavity length in the region of the cavity semiconfocal configuration under a radius of the pump beam smaller than the radius of the zero Gaussian mode. Here we demonstrate one of the most interesting applications for this laser: difference frequency generation in a GaSe crystal at a frequency of 1.64 THz. A superconducting hot-electron bolometer is used to detect the THz power generated and to measure its pulse characteristics.

Electronic properties and influence of doping on GaSe crystal nonlinear optical parameters for the applications in terahertz range

In the present paper the results of the study of semiconductor and optical properties of GaSe crystals important for their applications in nonlinear optics of terahertz and mid-IR ranges are reported as well as influence of doping with isovalent chemical elements on them. The performed first-principles calculations of charge neutrality level (CNL) in GaSe have shown its position at 0.8 eV above the top of the valence band. The location of CNL in the lower part of the band gap can explain GaSe intrinsic p-type of conductivity. In this case it is necessary to radically improve the growth technology to obtain GaSe crystals with low free carrier concentrations. Extraordinarily large birefringence of GaSe B~0.8 in terahertz range has been measured directly. In order to study the potential efficiency of application of doped GaSe crystals for terahertz generation and detection their study using terahertz time domain spectroscopy (THz-TDS) setup has been performed. According to the obtained results doped GaSe crystals are slightly less efficient for terahertz detection and generation in the frequency range 0.2-3.2 THz via optical rectification of laser pulses with λ=790 nm and τ=80 fs. On the other hand doping of GaSe with In, Al, S, Te leads to 2-3 times increase of the microhardness and the doped crystals become suitable for the mechanical treatment.

Ab initio calculations of optical constants of GaSe and InSe layered crystals

The dielectric functions, refractive indices, and extinction coefficients of GaSe and InSe layered crystals have been calculated within the density functional theory. The calculations have been performed for the values of theoretical structural parameters optimized using the exchange-correlation functional, which allows one to take into account the dispersion interactions. It has been found that optical functions are characterized by the most pronounced polarization anisotropy in the range of photon energies of ∼4–7 eV. The frequency dependences for InSe compound in the range up to 4 eV demonstrate the more pronounced anisotropy as compared to GaSe. The results obtained for GaSe crystal agree better with the experimental data as compared to the previous calculations.

Response to “Comment on ‘GaSe1−xSx and GaSe1−xTex thick crystals for broadband terahertz pulses generation’” [Appl. Phys. Lett. 100, 136103 (2012)]

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The optical properties of 9 MeV electron irradiated GaSe crystals

Two absorption bands in transparency region in high-energy electron irradiated GaSe crystals have been experimentally found. The optical transmission spectra of the crystals are studied in order to elucidate the origin and energy positions of related defect states. The measurements of optical transmission spectra temperature dependencies, radiation defect annealing and analysis of the optical spectra within existing theoretical models were performed.

Formation of intrinsic oxide nanocrystals on the surface of GaSe under laser irradiation

The process of GaSe surface oxidation is investigated. The work function is revealed to vary by 0.5 eV within a few hours after the formation of a cleaved facet of the semiconductor surface. It is demonstrated that low-energy laser irradiation with a wavelength of 650 nm leads to the generation of an intrinsic oxide on the crystallite surfaces. On account of continuous exposure to laser radiation for 6 h, the work function of the GaSe surface increases by 1 eV, i.e., becomes twice as large as that obtained without irradiation.

A comparison of terahertz electro-optic sampling in ZnTe, ZnSe, GaP and GaSe 1−x S x crystals

Efficiency of terahertz electro-optic sampling in a number of II-VI, III-V and III-VI crystals have been experimentally compared and analyzed. The terahertz pulses generated in femtosecond laser plasma filament were electro-optically detected in the crystals under investigation and recorded using conventional THz-TDS setup. Influence of crystal natural properties and chemical compound on the recorded spectra is discussed. In particular, influence of sulfur concentration in GaSe1-xSx mixed crystals on efficiency of terahertz detection was tested.