InSe as a case between 3D and 2D layered crystals for excitons.

InSe is a promising material in many aspects where the role of excitons is decisive. Here we report the sequential appearance in its luminescence of the exciton, the biexciton, and the P-band of the exciton-exciton scattering while the excitation power increases. The strict energy and momentum conservation rules of the P-band are used to reexamine the exciton binding energy. The new value ≥20 meV is markedly higher than the currently accepted one (14 meV), being however well consistent with the robustness of the excitons up to room temperature. A peak controlled by the Sommerfeld factor is found near the bandgap (~1.36 eV). Our findings supported by theoretical calculations taking into account the anisotropic material parameters question the pure three-dimensional character of the exciton in InSe, assumed up to now. The refined character and parameters of the exciton are of paramount importance for the successful application of InSe in nanophotonics.

Localization and transient emission properties in InGaN/GaN quantum wells of different polarities within core-shell nanorods.

Transient photoluminescence (PL) characteristics and localization phenomena in InGaN/GaN core-shell nanorods (NRs) were investigated from 6 K up to 285 K. The NRs exhibit three well-defined PL bands in the near-UV, blue, and green range ascribed to the emission of quantum well (QW) areas situated at the (1.00) sidewalls, (10.1) top facets, and (00.1) tip, respectively. At low temperature, time-resolved PL shows a fast decay time of about 0.5 ns for the semi- and non-polar QWs, while the polar QWs exhibit at least a twice-longer time. Rapid delocalization of carriers above 50 K indicates shallow potential fluctuations in the QWs. At room temperature, the characteristic fast PL decay time of the three QW bands stabilizes around 300 ps. The slow decaying PL components have different characteristic decay times that are explained by additional localization at basal stacking faults (BSFs), taking into account the quantum confined Stark effect. In addition, narrow excitonic luminescence lines are observed in the BSF-enriched polar QWs, providing direct evidence of the impact of the BSF/QW crossings on the optical properties of the NRs. A PL rise time of about 100 ps does not show any deviation between bands. These findings are suggestive of similar transport mechanisms in temperature equilibrium without inter-facet transport between different QWs. We believe that predictable transient characteristics can play a key role in creating uniform NR ensembles for device applications.

Towards the indium nitride laser: obtaining infrared stimulated emission from planar monocrystalline InN structures.

The observation of a stimulated emission at interband transitions in monocrystalline n-InN layers under optical pumping is reported. The spectral position of the stimulated emission changes over a range of 1.64 to 1.9 µm with variations of free electron concentration in InN layers from 2·1019 cm-3 to 3·1017 cm-3. The main necessary conditions for achieving the stimulated emission from epitaxial InN layers are defined. In the best quality samples, a threshold excitation power density is obtained to be as low as 400 W/cm2 at T = 8 K and the stimulated emission is observed up to 215 K. In this way, the feasibility of InN-based lasers as well as the potentials of crystalline indium nitride as a promising photonic material are demonstrated.

Insight into the performance of multi-color InGaN/GaN nanorod light emitting diodes.

We report on the thorough investigation of light emitting diodes (LEDs) made of core-shell nanorods (NRs) with InGaN/GaN quantum wells (QWs) in the outer shell, which are grown on patterned substrates by metal-organic vapor phase epitaxy. The multi-bands emission of the LEDs covers nearly the whole visible region, including UV, blue, green, and orange ranges. The intensity of each emission is strongly dependent on the current density, however the LEDs demonstrate a rather low color saturation. Based on transmission electron microscopy data and comparing them with electroluminescence and photoluminescence spectra measured at different excitation powers and temperatures, we could identify the spatial origination of each of the emission bands. We show that their wavelengths and intensities are governed by different thicknesses of the QWs grown on different crystal facets of the NRs as well as corresponding polarization-induced electric fields. Also the InGaN incorporation strongly varies along the NRs, increasing at their tips and corners, which provides the red shift of emission. With increasing the current, the different QW regions are activated successively from the NR tips to the side-walls, resulting in different LED colors. Our findings can be used as a guideline to design effectively emitting multi-color NR-LEDs.

Optically Addressable Silicon Vacancy-Related Spin Centers in Rhombic Silicon Carbide with High Breakdown Characteristics and ENDOR Evidence of Their Structure.

We discovered a family of uniaxially oriented silicon vacancy-related centers with S=3/2 in a rhombic 15R-SiC crystalline matrix. We demonstrate that these centers exhibit unique characteristics such as optical spin alignment up to the temperatures of 250°C. Thus, the range of robust optically addressable vacancy-related spin centers is extended to the wide class of rhombic SiC polytypes. To use these centers for quantum applications it is essential to know their structure. Using high frequency electron nuclear double resonance, we show that the centers are formed by negatively charged silicon vacancies V_{Si}^{-} in the paramagnetic state with S=3/2 that is noncovalently bonded to the neutral carbon vacancy V_{C}^{0} in the nonparamagnetic state, located on the adjacent site along the SiC symmetry c axis.

III-nitride tunable cup-cavities supporting quasi whispering gallery modes from ultraviolet to infrared.

Rapidly developing nanophotonics needs microresonators for different spectral ranges, formed by chip-compatible technologies. In addition, the tunable ones are much in demand. Here, we present site-controlled III-nitride monocrystal cup-cavities grown by molecular beam epitaxy. The cup-cavities can operate from ultraviolet to near-infrared, supporting quasi whispering gallery modes up to room temperature. Besides, their energies are identical in large 'ripened' crystals. In these cavities, the refractive index variation near an absorption edge causes the remarkable effect of mode switching, which is accompanied by the spatial redistribution of electric field intensity with concentration of light into a subwavelength volume. Our results shed light on the mode behavior in semiconductor cavities and open the way for single-growth-run manufacturing the devices comprising an active region and a cavity with tunable mode frequencies.