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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.

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.

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.

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 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.

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.

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).

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 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.

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.

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.

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.