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The ability to control the bandgap in two-dimensional graphene-like semiconductors is an essential task for the development of optoelectronic and nanoelectronic devices. Complex compositions alloys of transition metal dichalcogenides, such as MoxW1-x Se2yS2(1-y), are the most optimal candidates for this purpose. Here we present spectrally selective photodetectors based on such quaternary transition metal dichalcogenides monolayers. It is shown that the spectral selectivity of optical detectors can be changed in a wide range if the composition of these quaternary monolayer transition metal dichalcogenides varies. This effect is directly related to the significant influence of the composition of such two-dimensional semiconductors on their bandgap. A theoretical model for estimation of quaternary transition metal dichalcogenides optical coefficients and optical detectors on their basis is proposed. The optical absorption simulation in created detectors was made, the results of which qualitatively coincided with the detectors performance. Thus, the obtained results can contribute to the development of valleytronics for two-dimensional semiconductor structures.

Linearly polarized microphotoluminescence ($\ensuremath{\mu}$-PL) measurements of strained ${\mathrm{WSe}}_{2}$ monolayers in out-of-plane high magnetic fields are presented. At low temperature, a splitting of the bright exciton emission into two exciton components is observed, which is attributed to an in-plane uniaxial strain based on the full polarization dependence of the photoluminescence spectrum. High magnetic field measurements directly reveal a distinct evolution of the linear polarization and allows us to extract the valley coherence time constants (${T}_{s2}^{*}$) for both exciton components. For the high-energy transition of the exciton, the valley coherence time $\ensuremath{\simeq}0.45$ ps, closely matching ${T}_{s2}^{*}$ of an unstrained monolayer ($\ensuremath{\simeq}0.34$ ps). For the low-energy exciton, however, ${T}_{s2}^{*}$ is four times larger, $\ensuremath{\simeq}1.97$ ps. This valley coherence time observed here may be explored for future valleytronic applications.

Graphene-like semiconductor materials have attracted recently significant attention due to their unique properties which can be quite valuable for new generation of nano- and optielectronic devices. The use of various alloys of these materials is particularly promising for the application in highly efficient photovoltaic devices. In this paper, a simple method for the creation of monolayer flakes of transition metal dichalcogenide four-component alloys with substantially different properties is presented. It was shown that the band width of the flakes obtained by this method can vary over a wide range from 1.56 eV to 1.87 eV.

Second harmonic generation is studied in structures containing nanoscale layers of transition metal chalcogenides that are two-dimensional semiconductors and deposited on a SiO2/Si substrate. The second harmonic generation intensity is calculated with allowance for multibeam interference in layers of dichalcogenide and silicon oxide. The coefficient of reflection from the SiO2-layer-based Fabry–Perot cavity is subsequently calculated for pump wave fields initiating nonlinear polarization at every point of dichalcogenide, which is followed by integration of all second harmonic waves generated by this polarization. Calculated second harmonic intensities are presented as functions of dichalcogenide and silicon oxide layer thicknesses. The dependence of the second harmonic intensity on the MoS2 layer thickness is studied experimentally in the layer of 2–140 nm. A good coincidence of the experimental data and numerical simulation results has been obtained.

Photoluminescence and X-ray diffraction are used to investigate 2H and 3R polytypes of MoS2 layered crystals intercalated with Cl2 molecules. The spectral structure of the observed bound excitons emission is individual for each polytype, which can be used for unambiguous identification of the 2H and 3R polytypes. The DFT band structure calculations have been done for three molecular layers of 2H-MoS2 and 3R-MoS2 polytypes with and without intercalation of Cl2 molecules. The spectroscopic experimental results confirmed theoretical calculations performed for both cases and it has been demonstrated that halogen intercalation completely changes the local band structure of both 2H- and 3R-MoS2 polytypes.

The excitation spectra of the ZnAl2S4 spinel crystals codoped with chromium and vanadium are investigated in order to explain some features of the Cr3+ ions optical spectra and the inconsistency of the experimental absorption/emission bands position with the Tanabe–Sugano diagram, as well as the “missing” band in the absorption spectrum of V3+ ion although this band should be present as it is due to a spin allowed transition. The unusual high Racah parameters of the C/B ratio for Cr3+ ions have been induced to recalculate the Tanabe–Sugano diagram for the d3 electron configuration and the ratio C/B=8 in order to use it for the experimental results interpretation. The presence of Cr2+ ions in the low spin electron configurations in the octahedral coordination was confirmed; the temperature dependence of the Cr2+ emission at about 1.9 eV was studied and the Huang–Rhys factors were estimated for different temperatures. Despite the fact that the V3+ C/B ratio has a different value compared to the case of Cr3+ ions, the electron–phonon interaction is similar for both ions, with the Huang–Rhys parameter equal to 10 in both cases. Further research into optimizing the ZnAl2S4:Cr, V system to get an efficient enhancement of the vanadium emission on account on re-absorbing the chromium emission is suggested.

A systematic optical characterization of transition metal dichalcogenide layered crystals grown by chemical vapour transport method as well as of natural molybdenite were carried out by using piezoreflectance (PzR) measurements. From a detailed lineshape fit of the room-temperature PzR spectra over an energy range from 1.6 to 5.0 eV, the energies of the band-edge excitonic and higher lying interband direct transitions were determined accurately. The possible assignments of the different origins of excitonic transitions are discussed. The near direct band edge A and B excitonic transitions detected in PzR spectra show a linear red-shift with the temperature increasing up to 525 K. The values of temperature-dependent energies of the excitonic transitions A and B are evaluated and discussed.

The Z-scan technique was applied to investigate the nonlinear absorption and refraction in few layer sheets of 2D transition metal dichalcogenide: WS 2 , MoS 2 , WSe 2 and Mo 0.5 W 0.5 S 2 , which showed nonlinear saturable absorption and two photon absorption.

Photoluminescence and X-ray diffraction are used to investigate 2H and 3R polytypes of MoS2 layered crystals intercalated with Cl2 molecules. The spectral structure of the observed bound excitons emission is individual for each polytype, that can be used for unambiguous identification of the 2H and 3R polytypes. The DFT band structure calculations have been done for three molecular layers of 2H-MoS2 and 3R-MoS2 polytypes without and with intercalation of Cl2 molecules. The spectroscopic experimental results confirmed theoretical calculations performed for both cases and it has been demonstrated that halogen intercalation completely changes the local band structure of 2H- and 3R-MoS2 polytypes.

The experimental and theoretical studies of the optical properties of pure α-ZnAl 2 S 4 and α-ZnAl 2 S 4 :V 3+ crystals were carried out. The ab initio and crystal field calculations of the structural and optical properties of α-ZnAl 2 S 4 :V 3+ were compared with the corresponding experimental data. It was shown that the lowest vanadium 3d states are located at about 1.36 eV above the valence band's top. The complete energy level scheme of the α-ZnAl 2 S 4 :V 3+ system, which includes the host's electronic band structure and impurity ion's energy levels, was suggested on the basis of the performed calculations.

The absorption and luminescent properties of α-ZnAI2S4:V spinel type crystals in the temperature range 10–300 K are investigated. The spectra are assigned to the electronic transitions of trivalent vanadium ions located in octahedral sites. It is shown that at low temperatures the three main components of the revealed IR luminescence spectra are caused by the 1A1g(1G)→1Eg(1D), 1T2g(1D), 3T2g(3F)→3T1g(3F), and 1Eg(1D)→3T1g(3F) transitions. The observed dependencies of the emission components intensities on temperature are explained assuming that there is a phonon assisted tunnelling between 3T2g(3F) and 1Eg(1D) states. On temperature rise, the 3T2g(3F)→3T1g(3F) vibronic transitions suppress other emission channels, which leads to the enhancement of the integral luminescence intensity and to the broadening of the spectrum centred at λ=1.4 μm.

The absorption and luminescent properties of α-ZnAl 2 S 4 :Co spinel type crystals grown by chemical transport reactions method are investigated. The spectra are assigned to the electronic transitions of Co 2+ ions located in tetrahedral sites. It is shown, that the revealed four radiative spectral components are caused by the 2 A( 2 G)→ 4 T 1 ( 4 F), 4 T 1 ( 4 P)→ 4 T 2 ( 4 F), 2 E( 2 G)→ 4 T 2 ( 4 F) and 2 E( 2 G)→ 4 T 1 ( 4 F) transitions.

The photoexcitation and relaxation of carriers is studied in WSe2 nanolayers on Si/SiO2 substrate by optical pump–probe technique. The excitation wavelength falls in the vicinity of the band edge. The thickness variation of WSe2 layers acts as tuning of band gap of the layers. In this way, a band edge spectroscopy is carried out using the constant excitation wavelength. It is shown that the relaxation time constants increase with the thickness increase, while the amplitude of transient reflectivity changes nonmonotonically. A model is suggested describing the amplitude behavior.

The luminescence properties of the ZnAl 2(1− х ) Cr 2 х S 4 single crystals with different chromium content are investigated. The emission spectra of the samples with a small amount of chromium ( х ⩽ 0.005) consist of the R-line ( E R = 1.61 eV) and its phonon side band. At increasing of chromium concentration two new U-bands centered at Е U(a) = 1.45 eV and Е U(b) = 1.3 eV appear in the emission spectra. It is assumed, that the increase of the new broad bands intensity with increasing of chromium concentration takes place due to the formation of nonequivalent Cr +3 -ions occupying octahedral sites, in the first cationic coordination sphere of which one or two aluminum ions are replaced by chromium ions.

In this article, we report the theoretical evaluation of the vibronic constants for the spinel-type crystals ZnAl2S4, ZnAl2Se4, CdAl2Se4, and CdIn2S4 doped with the transition metal ions Cr3+, Cr4+, Ti2+, and Ti3+. In these crystals the ions Cr3+, Ti3+ and Cr4+, Ti2+ occupy positions with octahedral and tetrahedral surrounding correspondingly. In the case of a tetrahedral complex the interaction with full symmetric A1, tetragonal E and two trigonal vibrations T 2 ( 1 ) and T 2 ( 2 ) is considered, while for the octahedral cluster along with the A1 and E vibrations only one trigonal vibrational mode is taken into account. The examined semiconductor systems are mainly covalent and application of the crystal field point charge model loses proper exactity. For this reason, we employ the exchange charge model for the crystal field that accounts for the covalence effects and provides relatively simple expressions for the crystal field and vibronic parameters keeping at the same time a reasonable level of accuracy. The vibronic coupling constants are numerically calculated and for each case an appropriate multi-mode Jahn–Teller vibronic problem is found out.

Optical spectroscopy in high magnetic fields $B\leq65$ T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe$_{2}$, the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe$_{2}$ where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model., just accepted in Nano Letters http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b00626 (equations corrected)

The results of studying the optical properties of nanoscale single crystals of MoS2:Cl2 and WS2:Br2 semiconductor compounds are presented. In microscopic images obtained at the wavelength of the second (400 nm), edge effects are detected, which consist in enhancement or reduction in the second-harmonic signal intensity. Unlike previously proposed interference mechanisms of edge effects, non-interference mechanisms are considered. The occurrence of edge effects is associated with either an increased Cl2 and Br2 halogen molecule concentration or with an electrically induced second harmonic caused by band bending at the edges of individual crystal layers.

The radiative recombination in the layered transition metal dichalcogenide compounds 2H–WS2:(Br2 ,I 2), 2H–WSe2:I2 and 2H–MoS2:Cl2 has been investigated. It is shown that the strong photoluminescence (PL) of these indirect band gap semiconductors is caused by recombination of excitons bound to the neutral centres formed by halogen molecules intercalated in the well-defined sites of the van der Waals gap. These centres, located at energy ET0.1 eV below the conduction band, display similar properties as the isoelectronic traps in GaP, providing the efficient radiative recombination. The observed broad band IR emission is attributed to the deep centres, caused by the intrinsic defects of the host lattices. D 2005 Elsevier B.V. All rights reserved.

Zinc oxide thin nanostructured film and single crystal were studied by photoluminescence (PL) and optical second harmonic generation (SHG). The single crystal reveals a strong blue excitonic luminescence, while for thin films “yellow”-defect PL is dominant, pointing to a strong contribution from the surface of film nanograins. In SHG the bulk contribution is dominant for both film and single crystal, although SHG enhancement was found in the film. Nonlinear parameters were calculated from the model and preferable orientation of the film grains was found.

The comparative study of the excitonic radiative recombination in 2H-MoS2:Cl2 and 2H-WS2:Br2 layered crystals is presented. The steady-state emission spectra of both the investigated materials include two distinct regions. The short-wavelength region consisting of sharp lines is produced by bound excitons, related to the halogen molecules intercalated in the layered semiconductors during the growth process. The second spectral region—a vibronic broad band identical for both types of crystals, is attributed to the deep radiative centres, caused by the intrinsic defects of the host lattices. The atypical temperature increase of the excitonic radiative decay time observed for 2H-WS2:Br2 in the range T = 2 – 7 K is related to the presence of a long lifetime sublevel, placed 0.3 meV above the lowest excitonic state.

Photoluminescence spectroscopy studies have been performed on self-assembled monolayers (SAMs) on Au(111) of thiophene oligomers with the number of thiophen rings N=3 and N=4. Photoluminescence spectra of SAMs reveal excitonic behavior with different band resolution and temperature dependence. These differences are attributed to different SAMs structure (degree of ordering).

is rather weak for applications. In order to enhance the luminescence and extend the functionality of the peptide tubes for optical applications, doping with dyes, photosensitizers and lanthanide ions can be done. [ 5,6 ] This approach increases the intensity of the generated light and adds new emission lines to the optical spectra of the peptide tubes but requires extra steps in the fabrication process. Also, such process does not easily allow to induce modifi cations of the spectra locally (e.g., an individual tube or part of it), which can be relevant for photonic integrated circuits. In this paper we report the appearance of two-photonluminescence (TPL) in the green spectral range as well as a strong enhancement of single-photon-luminescence (SPL) in tubular microstructures formed from FF molecules by laser- and thermo annealing without any doping process. The optical properties of the FFMT’s were probed by means of optical, confocal, and multiphoton microscopies combined with spectroscopy characterization. Moreover, morphology changes of the samples were confi rmed by scanning electron microscopy (SEM).

Resonant Raman spectra of single layer WS$_{2}$ flakes are presented. A second order Raman peak (2LA) appears under resonant excitation with a separation from the E$^{1}_{2g}$ mode of only $4$cm$^{-1}$. Depending on the intensity ratio and the respective line widths of these two peaks, any analysis which neglects the presence of the 2LA mode can lead to an inaccurate estimation of the position of the E$^{1}_{2g}$ mode, leading to a potentially incorrect assignment for the number of layers. Our results show that the intensity of the 2LA mode strongly depends on the angle between the linear polarization of the excitation and detection, a parameter which is neglected in many Raman studies., Comment: 6 pages, 4 figures

This publisher’s note amends the Funding section of a recent publication [Opt. Express24, 20685 (2016].

We studied the photoluminescence (PL) spectra of thiophene oligomers in a solution, and in a solid state: aggregated film and self-assembled monolayer (SAM). We showed a strong influence of packing conditions on the PL spectra. In the solid state a new electronic state was observed, which does not reveal itself in the solution.

Due to their layered structure, 2H layered transition metal dichalcogenide indirect bandgap compounds MoS2, WS2 can be intercalated with foreign atoms and molecules. The 2H-MoS2, 2H-WS2 and 2H-WSe2 crystals intercalated with halogen molecules at low temperatures exhibit a strong luminescence due to the bound excitons recombination. In this work, the results of comparative study of the excitonic luminescence kinetics for 2H-WS2:Br2 and 2H-MoS2:Cl2 single crystals are presented. A simple kinetic model describing the temperature dependencies of the steady-state luminescence intensity and of the radiative decay time is proposed. The lifetimes of the excitonic levels and the activation energy of the excitonic luminescence quenching caused by an extrinsic self-trapping process for both studied compounds were determined.

CuGaSe2 single crystals were doped with B and Ge by ion implantation and analyzed by Hall effect, photoluminescence, and reflected second harmonic generation measurements. After ion implantation the crystals exhibited a destroyed surface structure. Measurements of the band edge photoluminescence and the second harmonic generation before and after annealing clearly reflected the thermal healing of the implantation-induced crystalline defects. As expected the analysis of the electrical measurements showed the incorporation of donor levels due to implantation with B and Ge, which led to electrically compensated samples. However no n-type conductivity could be obtained. This is ascribed to self-compensation of the defects for implantation ion doses higher than 1015 cm−2.

The optical properties of the chromium doped spinel type semiconductor α-ZnAl2S4 have been examined over the temperature range 2–540 K. The intrinsic photoluminescence (PL) showed an intense ultraviolet peak at 381 nm at 296 K. From extrinsic absorption and emission spectra the transitions between the ground state 4A2g and the excited levels 2Eg, 2T1g, 2T2g, 4T2g, and 4T1g of Cr3+ ions were observed. A fit of the measured temperature dependence of the PL decay, using a four level model, yielded the lifetimes of the 2Eg, 2T1g, and 4T2g levels and the energy gaps between the 2Eg–2T1g and 2Eg–4T2g states which were found to be very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr3+ ions in a α-ZnAl2S4:Cr spinel host has been constructed. Optical gain measurements at 198, 298, and 380 K displayed two separate spectral regions associated to the Cr3+ ions and possibly Cr4+ impurity centers. Gain values of up to 25 cm−1 at 198 K were obtained.

The experimental and theoretical studies of the optical properties of pure α-ZnAl2S4 and α-ZnAl2S4:Co2+ crystals were carried out. The ab initio and crystal field calculations of the structural and optical properties of α-ZnAl2S4:Co2+ were compared with the corresponding experimental data. It was shown that the lowest cobalt 3d states are located at about 0.5 eV above the valence band’s top. The complete energy level scheme of the α-ZnAl2S4:Co2+ system, which includes the host’s electronic band structure and impurity ion’s energy levels, was suggested on the basis of the performed calculations.

Photoluminescence measurements have been performed on chromium doped α-ZnAl 2 S 4 spinel-type crystals in the temperature range 2–540 K. a single configuration coordinate diagram for the Cr 3+ ions in the α-ZnAl 2 S 4 host is constructed to interpret the absorption and emission spectra and to allow a determination of the energy gaps between the 2 E g − 2 T 1g and 2 E g − 4 T 2g states. A fit of the measured temperature dependence of the luminescence decay, using a simplified four level model gave the lifetimes of the 2 E g , 2 T 1 g and 4 T 2 g excited levels and confirmed the gap values obtained from steady state measurements.

The increase of the resistance up to two orders of magnitude under laser illumination (λ=760 nm) was observed in La0.7Ca0.3MnO3 (LCMO) epitaxial thin films in ferromagnetic state. Optical absorption also increases by 10–15 % and the magnetic second-harmonic generation signal decreases down to zero under the irradiation. The light induced changes are reversible with characteristic relaxation times τ∼1–30 s. Magnetic field, B=4 T, suppresses the photoconductivity and decreases its relaxation time. Photoinduced effects are caused by the injection of a large number of extra carriers, which change the (antiferromagnetic) AFM/FM phase balance in LCMO, favoring the insulating AFM state.

The transfer matrix method has been widely used to calculate wave propagation through the layered structures consisting entirely of either linear or nonlinear optical materials. In the present work, we develop the transfer matrix method for structures consisting of alternating layers of linear and nonlinear optical materials. The result is presented in a form that allows one to directly substitute the values of material constants, refractive index and absorption coefficient, into the expressions describing the second harmonic generation (SHG) field. The model is applied to the calculation of second harmonic (SH) field generated in nano-thin layers of transition metal dichalcogenides exfoliated on top of silicon oxide/silicon Fabry-Perot cavity. These structures are intensively studied both in view of their unique properties and perspective applications. A good agreement between experimental and numerical results can be achieved by small modification of optical constants, which may arise in an experiment due to a strong electric field of an incident focused pump laser beam. By considering the SHG effect, this paper completes the series of works describing the role of Fabry-Perot cavity in different optical effects (optical reflection, photoluminescence and Raman scattering) in 2D semiconductors that is extremely important for characterization of these unique materials.

Photoconductivity in \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$ single crystals was observed in the temperature range of 80--250 K. The energy gap, 0.89 eV at 85 K, and its temperature dependence were determined. The values of the average phonon energy, 55 meV, and the electron-phonon coupling parameter, S=2.75, were evaluated. The observed quenching of photoconductivity is explained in assuming the two-center model. The donor and acceptor activation energy of 70 and 120 meV, respectively, as well as the hole mobility due to lattice scattering were determined.

Synthetic two-dimensional transition metal dichalcogenides such as tungsten disulphide (WS2), tungsten diselenide (WSe2), molybdenum disulphide (MoS2) as well as mixed molybdenum tungsten disulphide (Mo0.5W0.5S2) single crystals were grown by the chemical vapor transport method using halogens (bromine or chlorine) as transport agents. Multi-layer samples were cleaved from the single crystals, and their nonlinear optical (NLO) properties were obtained from both open aperture and closed aperture Z-scan measurements using a picosecond mode-locked Nd:YAG laser operating at a wavelength of 1064 nm, with pulse duration of 25 ps and 20 Hz repetition rate. Both WS2 and MoS2 exhibited nonlinear saturable absorption (SA), whereas WSe2 and Mo0.5W0.5S2 showed nonlinear two-photon absorption (2PA). A large 2PA coefficient β as high as + 1.91x10−8 cm/W was obtained for the Mo0.5W0.5S2, and an index of refraction coefficient γ = −2.47x10−9 cm2/W was obtained for the WSe2 sample.

The mapping of two-photon excited luminescence in aggregations of free-standing zinc oxide microrods has been carried out at room temperature. Two-photon luminescence spectra in the excitonic region for individual microrods have been recorded. The luminescence intensity exhibits a power-law dependence on the optical pump power with the exponent n > 2. This fact, along with the existence of a threshold power above which the dependence in the exciton region deviates from a quadratic one, indicates the onset of light amplification in individual ZnO microrods and the conditions preceding laser oscillation.

ZnSe:Cr-based infrared (IR) laser operating at room temperature is of great interest shows the near-edge photoluminescence (PL) spectra of Cr-doped ZnSe observed under 337.1 nm excitation. A broad blue PL band at 427 nm is observed for lightly Cr-doped samples and the sample doped with Cr from the Bi + Cr melt. Blue PL band is excited by two-step complementary photoionization transitions of chromium ions. Free carriers photo-generated in the conduction and valence bands of ZnSe participate in the band-to-band transitions responsible for the blue emission. Free holes generated in the valence band under chromium ionization are efficiently trapped by shallow acceptor centres being active in the blue emission process. The second photon, which is absorbed by photo-generated Cr+ state, induces a complementary ionization transition to the conduction band. Thus, both types of free carriers are photo-generated and responsible for the blue PL.

Second harmonic generation (SHG) of a high intensity was found in MoS2 flakes of different thicknesses exfoliated on a silicon substrate. Reduction of the SHG intensity was observed only for a small portion of flakes, for both very thin and quite thick ones. This was attributed to the presence of polytypism, i.e., of 3R non-centrosymmetric and 2H centrosymmetric polytypes, in a source bulk crystal grown by the chemical vapor transport technique. The presence of two polytypes in the sample was confirmed by the spectral structure of the photoluminescence of bound excitons observed in flakes at low temperature. Absolute values of nonlinear susceptibility of MoS2 flakes of different thicknesses were estimated.

CuIn5Se8 bulk single crystals were characterized using ellipsometry, photoluminescence (PL), and optical second harmonic generation (SHG). The refractive n(E) and absorption k(E) indices as functions of photon energy E are determined. The structure of the PL spectrum suggests the presence of the Cu-rich nonstoichiometric phases at the natural surfaces of the CuIn5Se8 sample. The best fit of the experimental SHG data is achieved by assuming that CuIn5Se8 belongs to the 4¯2m symmetry group. This result is in contradiction with x-ray analysis (6¯2m group), which testifies to the difference in surface and bulk structure.

Photoluminescence (PL) of chromium-doped spinel-type semiconductor α-ZnAl 2 S 4 has been studied in the temperature range 2–540 K. The transitions between ground state 4 A 2g and 2 E g , 2 T 1g , 2 T 2g , 4 T 2g , 4 T 1g excited levels of Cr 3+ ions in absorption and emission spectra have been observed. A fit of the measured temperature dependence of the PL decay, using a four-level model, gave the lifetimes of the 2 E g , 2 T 1g and 4 T 2g levels. The energy gaps between 2 E g – 2 T 1g and 2 E g – 4 T 2g states have been determined and they are very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr 3+ ions in the α-ZnAl 2 S 4 :Cr spinel host has been constructed.

For the first time the photoluminescence of the tungsten disulphide 2H-polytype single crystals have been observed and the photoconductivity spectra in the region of indirect transition have been investigated. The analysis of the experimental data has been performed taking into account the band structure of 2H-WS/sub 2/ layered crystals.

Optical absorption were investigated in CdGa/sub 2/S/sub 4/ in the temperature range 85-300 K. The energy gap (3.17 eV at 85 K) and its temperature dependence were determined. The latter was studied by means of a three-parameter thermodynamic model and the Einstein model. Band tail states were found to give rise to the absorption below the fundamental edge.

Optical absorption and photoconductivity in ReS/sub 2/ single crystals were investigated in the temperature range 80-300 K. For the first time, the direct energy gap E/sub g//sup dir/=1.55 eV (BOK) and its temperature dependence were determined. From the absorption spectrum shape in the indirect optical transition region (E/sub g//sup ind/=1.27 eV, 300 K) an optical phonon energy value of 0.06 eV was estimated.

The spectroscopic characteristics of the α-ZnAl(2)S(4) wide bandgap semiconductor doped with Ti ions are investigated. It is shown, that the ZnAl(2)S(4):Ti spinel-type crystals exhibit luminescence in the IR spectral range 0.8-1.4 µm. The observed spectroscopic characteristics are assigned to the emission bands arising from the ligand -Ti(4+) charge transfer for octahedral sites of titanium that is in agreement with the experimental evidence for the absence of the EPR signal from Ti ions. A qualitative explanation of the experimental data is given.

Structural characterization of sample was carried out through the Laue showing a very perfect ternary symmetry and a high crystallinity. Optical absorption was investigated in CuIn3Se5 in the temperature range 85-300K. The energy gap (1.42 eV at 85K) and its temperature dependence were determined. The latter was studied by means of a three-parameter thermodynamic model and the Einstein model. The values of the band gap at zero temperature, average phonon energy, and a dimentionless constant related to the electron-phonon coupling were estimated. Band tail states were found to give rise to the absorption below the fundamental edge.

Steady state and time-resolved photoluminescence of Cr3+ ions in CdIn2S4:Cr single crystals has been investigated at resonant excitation. Fluorescence band associated with 4T2g→4A2g vibronic transitions has been observed. The nature of the nonequivalent Cr3+ octahedral sites in partially inversed spinel structure are discussed.

In this paper the photoconductivity and photoluminescence spectra as a function of temperature (90–300 K) of CuGaSe2 single crystals are discussed. The photoconductivity spectra peculiarities (90 K), have been explained by the interband transitions and by trasitions from splitting levels of valence band and excited 3d level of copper to the top of the valence band. A peculiarities at 300 K (1.28 eV and 1.55 eV) can be connected with a deep acceptor centers. A peculiarity at 1.38 eV can be ascribed to a deep donor center due to iodine impurity. It has been shown that the photoluminescence spectra are greatly influenced by the near-surface layer of the CuGaSe2 single crystals.

Recombination radiation in Cd3(AsxP1−x)2 solid solutions near cadmium phosphide (x ⪅ 0.1) is studied. A complex structure of the stimulated emission spectra in Cd3(AsxP1−x)2 single crystals is detected. It is shown that for Cd3P2 the width of emission spectrum and its position relative to the transmission edge may be adequately described in terms of a direct interband-transition model. The same model is used to calculate the luminescence Icv1(ω) and the optical transmission Tcv1(ω) in the presence of a small (i.e. up to 5% of the Brillouin zone size) K1 shift between the extrema of the heavy hole valence band V1 and conduction band C. It is shown that at k1 ≠ 0 the dependence Icv1(ω) may have two maxima. The structure of stimulated emission spectra and the reference data on luminescence in Cd3(AsxP1−x)2 single crystals are interpreted taking into account the calculations carried out in this work. Rekombinationsstrahlung in Cd3(AsxP1−x)2-Mischkristallen in der Nahe von Kadmiumphosphid (x ⪅ 0,1) wird untersucht und eine komplexe Struktur der stimulierten Emissionsspektren in Cd3(AsxP1-x)2-Einkristallen entdeckt. Es wird gezeigt, das fur Cd3P2 die Breite des Emissionsspektrums und seine Lage bezuglich der Transmissionskante adaquat mit einem Modell direkter Interbandubergange beschrieben werden kann. Dasselbe Modell wird fur die Berechnung der Lumineszenz Icv1(ω) und optischen Transmission Tcv1(ω) beim Vorliegen einer kleinen (d. h. bis zu 5% der Grose der Brillouinzone) K1-Shift zwischen den Extremwerten des Valenzbandes schwerer Locher V1 und des Leitungsbandes C benutzt. Es wird gezeigt, das bei k1 ≠ 0 die Icv1(ω)-Abhangigkeit zwei Maxima besitzen kann. Die Struktur der stimulierten Emissionsspektren und die Bezugswerte der Lumineszenz in Cd3(AsxP1−x)2-Einkristallen werden unter Berucksichtigung der durchgefuhrten Berechnungen interpretiert.

Optical absorption, photoacoustic spectroscopy and photoconductivity were investigated in ReS2 single crystals in the temperature range 50–300 K. The energy gap (1.55 eV at 80 K) and its temperature dependence, the value of the average phonon energy (17 meV) and the electron phonon coupling parameter (S=2.40) as well as the electron‐hole mobility due to lattice scattering were determined.

Plenary Lecture; International audience; Excitons in polar group-III nitride quantum wells (QWs) are naturally indirect, because electron and hole within such excitons are spatially separated. As a consequence, they have strong dipole moments and long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown, that a combination of these two features results in many interesting properties of IXs: they can propagate over large distances, can be controlled in-situ by light and external gate voltage, cool down to the lattice temperature before recombination, and form cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in nitride QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over two orders of magnitude higher density range. Using spatially- and time-resolved photoluminescence experiments we examined temperature, density, substrate material (GaN vs Sapphire) and excitation regime (cw vs pulsed excitation) dependence of the exciton transport in 7 nm wide GaN QW sandwiched between Al0.19Ga0.81N barriers, the structure is characterized by internal electric field of order of 1 MV/cm. Comprehensive analysis of the data combined with numerical modeling show that in GaN QWs efficient propagation of IX takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole - dipole interaction. Under these conditions, exciton mobility is almost temperature-independent from 10 K up to room temperature. This suggests that exciton-exciton interaction is by far dominant scattering mechanism, compared to scattering on the interface disorder. In this context, we argue that nitride-hosted IXs constitute a promising system for formation of collective bosonic states in semiconductors.