Thermally Activated Delayed Fluorescence in Optically Accessed Softmatter Environment

Organic material compositions, able to demonstrate bright delayed fluorescence either by electrical excitation, or by optical excitation, are being applied in various fields of research, ranging from sunlight-powered photonic devices...

Локализация носителей заряда в квантовых ямах InGaN/GaN, ограниченная объемным зарядом

The mechanism of carrier tunneling through the potential walls of InGaN/GaN quantum well in the p-n structures is studied by means of the deep center tunneling spectroscopy. A number of humps on the current and photocurrent tunneling spectra, as well as on the forward bias dependences of the intensity and the peak energy of photoluminescence band from the quantum well are detected. These findings allow us to propose a model of carrier localization in the quantum well that permit to relate the tunneling transparency of the potential walls of the QW to the space-charge of deep-level centers in the quantum well barriers and its changes under optical excitation and forward biasing of p-n structure.

Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds

The efficiency of the generation of terahertz radiation from nitrogen-doped (∼0.1–100 ppm) diamonds was investigated. The synthetic polycrystalline and monocrystalline diamond substrates were pumped by a 400 nm femtosecond laser and tested for the photoconductive emitter operation. The dependency of the emitted THz power on the intensity of the optical excitation was measured. The nitrogen concentrations of the diamonds involved were measured from the optical absorbance, which was found to crucially depend on the synthesis technique. The observed correlation between the doping level and the level of the performance of diamond-based antennas demonstrates the prospects of doped diamond as a material for highly efficient large-aperture photoconductive antennas.

Fluorescence Spectroscopy of Enantiomeric Amide Compounds Enforced by Chiral Light

Chirality, the absence of mirror symmetry, governs behavior in most biologically important molecules, thus making the chiral recognition of great importance in the pharmaceutical and agrochemical industries, as well as medicine. Chiral molecules can be characterized by means of optical experiments based on chiro-optical excitation of molecules. Specifically, chiral absorptive materials differently absorb left- and right-circular polarized light, i.e., they possess circular dichroism (CD). Unfortunately, the natural CD of most molecules is very low and lies in the ultraviolet range. Fluorescence-detected CD is a fast and sensitive tool for investigation of chiral molecules which emit light; ultralow CD in absorption can be detected as the difference in emission. In this work, we perform fluorescence-detected CD on novel chiral amide compounds, designed specifically for visible green emission; we synthesize two enantiomeric fluorescent compounds using low-cost starting compounds and easy purification. We investigate different solutions of the enantiomers at different concentrations, and we show that the fluorescence of the intrinsically chiral compounds depends on the polarization state of the penetrating light, which is absorbed at 400 nm and emits across the green wavelength range. We believe that these compounds can be coupled with plasmonic nanostructures, which further shows promise in applications regarding chiral sensing or chiral emission.

EFFECT OF THE DENSITY OF SURFACE V-DEFECTS ON LASER PROPERTIES OF InGaN/GaN HETEROSCTRUCTURES WITH MULTIPLE QUANTUM WELLS GROWN ON SILICON SUBSTRATES

We investigated the radiative properties of InGaN/GaN heterostructures with multiple quantum wells (MQWs) grown on silicon substrates with different thicknesses of quantum wells at optical excitation. The correlation of laser and photoluminescent properties with the surface morphology of the gallium nitride coating layers and the density of V-defects has been established. It is shown that, with a growth in the density of V-defects, the threshold power density of the excitation of the generation of InGaN/GaN heterostructures with MQWs increases.

Molecule-molecule and atom-molecule collisions with ultracold RbCs molecules

Understanding ultracold collisions involving molecules is of fundamental importance for current experiments, where inelastic collisions typically limit the lifetime of molecular ensembles in optical traps. Here we present a broad study of optically trapped ultracold RbCs molecules in collisions with one another, in reactive collisions with Rb atoms, and in nonreactive collisions with Cs atoms. For experiments with RbCs alone, we show that by modulating the intensity of the optical trap, such that the molecules spend 75\% of each modulation cycle in the dark, we partially suppress collisional loss of the molecules. This is evidence for optical excitation of molecule pairs mediated via sticky collisions. We find that the suppression is less effective for molecules not prepared in the spin-stretched hyperfine ground state. This may be due either to longer lifetimes for complexes or to laser-free decay pathways. For atom-molecule mixtures, RbCs+Rb and RbCs+Cs, we demonstrate that the rate of collisional loss of molecules scales linearly with the density of atoms. This indicates that, in both cases, the loss of molecules is rate-limited by two-body atom-molecule processes. For both mixtures, we measure loss rates that are below the thermally averaged universal limit.

Ultrasonic and Spectroscopic Techniques for the Measurement of the Elastic Properties of Nanoscale Materials

Materials at the nanoscale often have properties which differ from those they have in the bulk form. These properties significantly depend on the production process, and their measurement is not trivial. The elastic properties characterize the ability of materials to deform in a reversible way; they are of interest by themselves, and as indicators of the type of nanostructure. As for larger scale samples, the measurement of the elastic properties is more straightforward, and generally more precise, when it is performed by a deformation process which involves exclusively reversible strains. Vibrational and ultrasonic processes fulfill this requirement. Several measurement techniques have been developed, based on these processes. Some of them are suitable for an extension towards nanometric scales. Until truly supramolecular scales are reached, the elastic continuum paradigm remains appropriate for the description and the analysis of ultrasonic regimes. Some techniques are based on the oscillations of purpose-built testing structures, mechanically actuated. Other techniques are based on optical excitation and/or detection of ultrasonic waves, and operate either in the time domain or in the frequency domain. A comparative overview is given of these various techniques.

Threshold increase and lasing inhibition due to hexagonal-pyramid-shaped hillocks in AlGaN-based DUV laser diodes on single-crystal AlN substrate

The presence of hexagonal-pyramid-shaped hillocks (HPHs) in AlGaN epitaxial films affects device character- istics; this effect is significant in DUV laser diodes (LDs) on AlN substrates, where the presence of HPHs under the p-electrode increases the threshold current density and inhibits the lasing. In this study, we investigated the difference between the lasing characteristics of LDs with and without HPHs. It was found that in the presence of HPHs, the threshold excitation power density increased and the slope efficiency decreased by optical excitation. To investigate the cause of these phenomena, we performed structural, optical, and electrical analyses of the HPHs. Various imaging techniques were used to directly capture the characteristics of the HPHs. As a result, we concluded that HPHs cause the degradation of LD characteristics due to a combination of structural, optical, and electrical factors.