Laser-Induced Periodic Surface Structures on Layered GaSe Crystals: Structural Coloring and Infrared Antireflection.

We study structural and morphological transformations caused by multipulse femtosecond-laser exposure of Bridgman-grown ϵ-phase GaSe crystals, a van der Waals semiconductor promising for nonlinear optics and optoelectronics. We unveil, for the first time, the laser-driven self-organization regimes in GaSe allowing the formation of regular laser-induced periodic surface structures (LIPSSs) that originate from interference of the incident radiation and interface surface plasmon waves. LIPSSs formation causes transformation of the near-surface layer to amorphous Ga2Se3 at negligible oxidation levels, evidenced from comprehensive structural characterization. LIPSSs imprinted on both output crystal facets provide a 1.2-fold increase of the near-IR transmittance, while the ability to control local periodicity by processing parameters enables multilevel structural color marking of the crystal surface. Our studies highlight direct fs-laser patterning as a multipurpose application-ready technology for precise nanostructuring of promising van der Waals semiconductors, whose layered structure restricts application of common nanofabrication approaches.

Theoretical and experimental study on the electronic and optical properties of K0.5Rb0.5Pb2Br5: a promising laser host material.

The data on the electronic structure and optical properties of bromide K0.5Rb0.5Pb2Br5 achieved by first-principle calculations and verified by X-ray spectroscopy measurements are reported. The kinetic energy, the Coulomb potential induced by the exchange hole, spin-orbital effects, and Coulomb repulsion were taken into account by applying the Tran and Blaha modified Becke-Johnson function (TB-mBJ), Hubbard U parameter, and spin-orbital coupling effect (SOC) in the TB-mBJ + U + SOC technique. The band gap was for the first time defined to be 3.23 eV. The partial density of state (PDOS) curves of K0.5Rb0.5Pb2Br5 agree well with XES K Ll and Br Kß2, and XPS spectra. The valence band (VB) is characterized by the Pb-5d3/2 and Pb-5d5/2 sub-states locating in the vicinities of -20 eV and -18 eV, respectively. The VB middle part is mainly formed by K-3p, Rb-4p and Br-4s states, in which the separation of Rb-4p3/2 and Rb-4p1/2 was also observed. The strong hybridization of Br-p and Pb-s/p states near -6.5 eV reveals a major covalent part in the Br-Pb bonding. With a large band gap of 3.23 eV, and the remarkably high possibility of inter-band transition in energy ranges of 4-7 eV, and 10-12 eV, the bromide K0.5Rb0.5Pb2Br5 is expected to be a very promising active host material for core valence luminescence and mid-infrared rare-earth doped laser materials. The anisotropy of optical properties in K0.5Rb0.5Pb2Br5 is not significant, and it occurs at the extrema in the optical spectra. The absorption coefficient α(ω) is in the order of magnitude of 106 cm-1 for an energy range of 5-25 eV.

Optical and electronic properties of lithium thiogallate (LiGaS2): experiment and theory.

We report the relation between the optical properties and electronic structure of lithium thiogallate (LiGaS2) by performing XPS and XES measurements and theoretical calculations. According to the XPS measurements, the LiGaS2 crystals grown by the Bridgman-Stockbarger method possess promising optical qualities, low hygroscopicity and high stability upon middle-energy Ar+-ion irradiation. The difference in the LiGaS2 band gaps obtained by theoretical calculations and experimental measurements was, for the first time, reduced down to 0.27 eV by applying the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential where the Coulomb repulsion was considered by introducing Hubbard parameter, U. The TB-mBJ+U method also reproduces the XPS spectrum well. The TB-mBJ+U band-structure calculations of LiGaS2 are found to be in good agreement with the XPS and XES experimental data. The accurate electronic structure of LiGaS2 allows further investigation of the optical properties. The relation between the photoluminescence of LiGaS2 and its electronic structure was revealed. Moreover, the theoretical results show the possibility of emissions at higher energy levels in LiGaS2, that has not been measured in experiments yet. Good phase-matching of LiGaS2 was expected to occur at energy levels of 5, 6, 6.2, 7, 7.2, and 8 eV.

Negative thermal expansion and electronic structure variation of chalcopyrite type LiGaTe2.

The LiGaTe2 crystals up to 5 mm in size were grown by the modified Bridgman-Stockbarger technique and the cell parameter dependence on temperature in the range of 303-563 K was evaluated by the X-ray diffraction analysis. The thermal behavior of LiGaTe2 is evidently anisotropic and a negative thermal expansion is found along crystallographic direction c with coefficient -8.6 × 10-6. However, the normal thermal expansion in two a directions with coefficient 19.1 × 10-6 is dominant providing unit cell volume increase on heating. The atomic mechanism is proposed to describe this pronounced anisotropic expansion effect. The electronic structure of LiGaTe2 is measured by X-ray photoelectron spectroscopy and the band structure is obtained by DFT calculations. The pressure response from 0 to 5 GPa was calculated and a normal crystal compression is found. This work indicates that LiGaTe2 is promising as an IR NLO or window material for many practical applications because the thermal expansion coefficients of this telluride are not big. We believe that these results would be beneficial for the discovery and exploration of new IR optoelectronic polyfunctional metal tellurides.

A luminescence spectroscopy and theoretical study of 4f-5d transitions of Ce3+ ions in SrAlF5 crystals.

This research is focused on the 4f-5d transitions in Ce(3+) centers doped into tetragonal ß-SrAlF(5) single crystals belonging to the I4(1)/a space group. The presence of four non-equivalent Sr(2+) sites in this compound leads to the appearance of three spectroscopically non-equivalent Ce(3+) luminescence centers, which can be well distinguished using a time-resolved laser spectroscopy technique. All 4f-5d transitions have slightly varying excitation and emission energies with characteristic probabilities resulting in several decay times that can be determined experimentally. One of these centers experiences strong perturbation due to a defect nearby, probably the O(2-) impurity ion substituting for the F(-) ion and acting as a charge compensator as well. Identification of these photoluminescence centers is performed using crystal field calculations. The crystal field parameters are calculated for two identified centers using the structural data for SrAlF(5); diagonalization of the crystal field Hamiltonian results in obtaining the splitting of the Ce(3+) 5d states. This method allows 'regular' unperturbed Ce(3+) centers with selected Sr(2+) sites to be assigned.

Luminescence properties of undoped LiBaAlF6 single crystals.

This paper presents the results of the study of electronic excitations in undoped LiBaAlF(6) single crystals by means of luminescence spectroscopy and complimentary optical methods. The intrinsic emission at 4.2 eV due to self-trapped excitons was identified. The fast nanosecond defect-related luminescence was revealed at 3.0 eV. Both emissions degrade under electron beam irradiation, the most probable reason of which is defect creation introducing an additional non-radiative relaxation channel prohibiting energy transfer to luminescence centers. These defects can be recovered and luminescence intensity restored at higher temperatures (>200 K). The permanent damage by electron beam irradiation results only in overall growth of the absorption coefficient in the whole 1.5-6.5 eV spectral region studied. The analysis of thermally stimulated luminescence glow curves in the temperature range of 5-410 K revealed two shallow charge carrier traps with the activation energies of 0.22 and 0.33 eV, respectively. The luminescence of an impurity peaked at 2.5 eV was found and tentatively assigned to an oxygen-related emission center.

Optical properties of LiGaS(2): an ab initio study and spectroscopic ellipsometry measurement.

Electronic and optical properties of lithium thiogallate crystal, LiGaS(2), have been investigated by both experimental and theoretical methods. The plane-wave pseudopotential method based on DFT theory has been used for band structure calculations. The electronic parameters of Ga 3d orbitals have been corrected by the DFT+U methods to be consistent with those measured with x-ray photoemission spectroscopy. Evolution of optical constants of LiGaS(2) over a wide spectral range was determined by developed first-principles theory and dispersion curves were compared with optical parameters defined by spectroscopic ellipsometry in the photon energy range 1.2-5.0 eV. Good agreement has been achieved between theoretical and experimental results.