K. R. Allakhverdiev

Effective nonlinear GaSe crystal. Optical properties and applications

We present an overview of the current state of the literature and research performed by the authors of the present paper on the experimental and theoretical results on the structural-, optical-, nonlinear optical (NLO)-properties (including two-photon absorption (TPA) and the terahertz (THz) range of spectra) and practical applications of a highly anisotropic Gallium Selenide (GaSe) semiconductor with emphasis on the ɛ-GaSe. Physical properties of ɛ-GaSe are important to researchers and designers developing different devices by using this material. This crystal possesses an outstanding NLO properties: high optical birefringence Δn ∼ 0.3 at 700 nm; high transparency range (0.7−18.0 μm) with low absorption coefficient (α ≤ 0.3 cm−1); very high nonlinear susceptibility χ(2) (d22 ≈ 86 ± 17 pm/V, corresponding to (2.0 ± 0.4) × 10−7 esu) that is used for phase matched second harmonic generation (SHG) in a wide transparency range; high power threshold for optical damage; possibility to perform optical frequency conversion under phase-matching conditions in the near- to mid-IR and THz range of spectra, etc. The domain structure of crystal in connection with the NLO properties is discussed as studied by confocal Raman microscopy experiments. Perspectives for future research of GaSe are considered in the present article, which does not pretend to be one reflecting all existing papers on GaSe crystal and discussed subjects.

Photoluminescence frequency up-conversion in GaSe single crystals as studied by confocal microscopy

The photoluminescence spectrum of melt-grown GaSe single crystals was investigated with a confocal Raman microscope equipped with a HeNe laser. Three luminescence bands of different intensity were observed, which are mainly located to the blue of the laser line. The luminescence signals show a quadratic dependence on excitation intensity. The effect is interpreted as second-harmonic generation in the strongly optically nonlinear material followed by the excitation of electrons into the conduction band and luminescence emission from direct-gap Wannier excitons. The relative intensities of the three luminescence peaks exhibit a spatial variation on the crystal surface, which was mapped with the confocal microscope. Possible explanations of this effect are discussed. In an external electric field the luminescence shows a strong increase and a quadratic redshift, whose magnitude is consistent with the Franz–Keldysh mechanism.

Dielectric properties and metastable states in ferroelectric TlInS2 crystals

Abstract A phase of antiferroelectric type of hysteresis preceding the transition to the ferroelectric state has been observed in Thallium Indium sulfide crystals. A possible coexistence of polar regions and domains of the ferroelectric and the T C1 - T C2 phases over a wide range of temperature is suggested from analysis of dielectric studies. A metastable behaviour of permittivity observed at thermocycling is explained by existence of a chaotic state below the ferroelectric phase transition temperature T C2 = 201 K. A model of two polar sublattices in the region of phase coexistence in TlInS2 is offered.

Succession of the low temperature phase transitions in TlInS2 crystals

Abstract Two additional incommensurate phase transitions (PTs) at T i = 206 K and T tr = 79 K were observed in TlInS 2 crystals. Peculiarities of the temperature behaviour of the dielectric susceptibility and the dielectric loops allowed to consider the PTs at T c1 = 204 K and T c2 = 201 K as incomplete lock-in transitions and the PT at T≈195 K as a temperature range of the final lock-in transition. The PT at T tr = 79 K was considered as an isomorphous transition. The experimental results of the second harmonic generation signal at low temperature are also in favour of such a conclusion. Peculiarities of the temperature behaviour of the dielectric properties at low temperature are qualitatively explained by means of the theory of two non-equivalent sublattices.

Phase‐matched second‐harmonic generation at 789.5 nm in a GaSe crystal

Phase‐matched second‐harmonic generation in the highly χ(2)‐active layered semiconductor GaSe is demonstrated in the near‐infrared frequency region. Due to the high indices of refraction, the internal phase‐matching angle of about 30° is beyond the critical angle of total reflection for a z‐cut crystal. This problem is overcome by sandwiching the crystal between two half‐cylindrical glass rods, which leads to a shift of the critical angle to larger values.

Memory effect in layered semiconductor TlInS2 with incommensurate phase

Abstract The temperature dependence of pyroelectric current was studied for the ferroelectric-semiconductor crystal TlInS2 in the temperature range of commensurate and incommensurate phases. A remarkable shift of the lock-in transition point to lower temperatures has been observed on heating runs after turning from a heating to a cooling from different temperatures within the incommensurate phase. The observed effect was interpreted by the pinning of modulation due to mobile defects in the crystal having layered structure and was attributed to a kind of memory effect.

Raman spectra of α-GaTe single crystals

Abstract Raman-active lattice vibrational modes of GaTe have been investigated at 300°K in the frequency range 13–300 cm -1 . The spectra of the Bridgman grown crystals were excited with the 1.06 μm line of the continuously operated YAG : Nd 3+ laser. Fourteen Raman bands were observed. All phonon modes have been assigned on the basis of their observed polarisation dependence to an irreducible representation of the appropriate point group. The results are consistent with the existence of GaTe in monoclinic α-polytype ( C 3 2 h ).

Vibrational properties of crystals with TlSe type structure

Abstract The lattice vibration spectra of TlS and TllnSe2 crystals were investigated by infrared reflectivity in the wavenumber range from 20 to 5000 cm−1. The frequencies of K = 0 phonons were determined by Kramers-Kronig analysis of the spectra. All five infrared active lattice vibrations predicted by the group theoretical analysis were identified. The normal coordinates of vibrations for five infrared active phonons were determined. In the approximation which does not take into consideration long range Coulomb interaction values of the force constant and effective changes are obtained for TlSe, TlS and TllnSe2 crystals.

Thermal hysteresis and memory effects in TIInS2

Abstract The dielectric susceptibility of layered TlInS2 was studied in the temperature range of successive phase transitions. Thermal hysteresis was observed in the incommensurate phase. It was shown that after annealing the crystal at a fixed temperature within the incommensurate phase, the existing temperature interval of this phase reveals noticeable broadening. The thermal memory effect is discussed using a defect density wave model.

The influence of hydrostatic pressure on Raman spectra of TIS and TlInSe2

Abstract First order Raman spectra of TIS and TlInSe2 single crystals excited with 1.064μm line of the continuously operated YAG: Nd3+ laser have been investigated in equilibrium conditions under various hydrostatic pressures up to 1.08 × 109 and 7.06 × 108 Pa, respectively. Mode parameters γj = (1/νj)(dνj/dP) were determined for all the Raman bands observed. Comparison of a set of these parameters in both crystals showed that the character of binding interatomic forces in these crystals appeared to be similar. For both crystals the intensity of Raman bands decreased with increasing the pressure.