Search

The inherent crystal anisotropy of hexagonal boron nitride (hBN) provides the ability to support hyperbolic phonon polaritons, that is, polaritons that can propagate with very large wave vectors within the material volume, thereby enabling optical confinement to exceedingly small dimensions. Indeed, previous research has shown that nanometer-scale truncated nanocone hBN cavities, with deep subdiffractional dimensions, support three-dimensionally confined optical modes in the mid-infrared. Because of optical selection rules, only a few of the many theoretically predicted modes have been observed experimentally via far-field reflection and scattering-type scanning near-field optical microscopy (s-SNOM). The photothermal induced resonance (PTIR) technique probes optical and vibrational resonances overcoming weak far-field emission by leveraging an atomic force microscope (AFM) probe to transduce local sample expansion caused by light absorption. Here we show that PTIR enables the direct observation of previously unobserved, dark hyperbolic modes of hBN nanostructures. Leveraging these optical modes and their wide range of angular and radial momenta could provide a new degree of control over the electromagnetic near-field concentration, polarization in nanophotonic applications.

Nonradiative carrier recombinations at deep centers in semiconductors are of great importance for both fundamental physics and device engineering. In this article, we provide a revised analysis of Huang’s original nonradiative multi-phonon (NMP) theory with ab initio calculations. First, we confirmed at the first-principles level that Huang’s concise formula gives the same results as the matrix-based formula, and that Huang’s high-temperature formula provides an analytical expression for the coupling constant in Marcus theory. Secondly, we correct for anharmonic effects by taking into account local phonon-mode variations for different charge states of a defect. The corrected capture rates for defects in GaN and SiC agree well with experiments. [ABSTRACT FROM AUTHOR]

Abstract: Optical absorption spectra, emission spectra, and fluorescence lifetimes were measured for a number of visible and infrared transitions in Ho3+ doped KPb2Cl5. Judd–Ofelt parameters were obtained by using the conventional approach, as well as by a modified approach that minimizes the fractional difference, rather than the absolute difference, between calculated and measured oscillator strengths. Both procedures gave good overall agreement between measured and calculated lifetimes, with an average difference of ≈15%. This confirms the expectation from the multiphonon energy gap law that most observed transitions in Ho3+:KPb2Cl5 are highly radiative. Emission cross section spectra for the transitions 5F3 → 5I8, 5G5 → 5I7, 5F3 → 5I7, 5F4/5S2 → 5I7, and 5I7 → 5I8 were obtained from measured emission spectra, with the peak cross section values scaled by the Judd–Ofelt calculated oscillator strengths. [Copyright &y& Elsevier]

Abstract: Enhanced optical efficiency of nanosized semiconductors is due to reduction in the nonradiative recombination rate, as the size of the grain is decreased. A theoretical basis for such a reduction is obtained by a classical approach by calculating surface recombination velocity in a confined configuration. This explains the reduction of nonradiative recombination in small size grains. [Copyright &y& Elsevier]

An experimental investigation of the general characteristics of nonradiative and radiative recombination of charge carriers in strongly excited porous silicon is presented. It is shown that photoconductivity, photomagnetoelectric effect, quantum yield, and intensity of visible radiation of porous silicon demonstrates strong nonlinearities against laser excitation intensity. It is suggested that the band-to-band Auger recombination is dominant similar to that in crystalline silicon, whereas the visible luminescence is determined by the bimolecular process. The nonequilibrium density of charge carriers Δn ≈ 1019 cm−3, and the bimolecular radiative recombination coefficient Brad ≤ 9 × 10−14 cm3/s have been found. [ABSTRACT FROM AUTHOR]

Abstract: Through the time-resolved photoluminescence (TR-PL) measurement for excitons, we have studied the enhancement of spatial fluctuation (SF) and the generation of nonradiative-recombination-centers (NRC) due to high Si-doping into GaAs/AlAs short-period-superlattices (SPS''s). We have carried out the exciton transport analysis according to Krivorotov et al. [I.N. Krivorotov, T. Chang, G.D. Gilliland, L.P. Fu, K.K. Bajaj, Phys. Rev. B 58 (1998) 10687]. From this analysis, we have obtained the temperature dependence of the exciton diffusivity, the concentration of the NRC and the average distant between adjacent localized states of excitons. The temperature dependence of the exciton diffusivity is found to be given by the sum of the temperature-independent contribution and the activation-type contribution. For the exciton diffusivity in undoped GaAs/AlAs SPS''s, only the activation-type contribution has been observed. Therefore, we point out the possibility that the temperature-independent contribution comes from the tunneling through the impurities. In this experiment, the activation energy and the concentration of the NRC are found to be larger than those of undoped GaAs/AlAs SPS''s. We infer that high Si-doping into GaAs/AlAs SPS causes the enhancement of the SF and the generation of NRC. [Copyright &y& Elsevier]

Organic-inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley-Queisser limit efficiency (30.5%). Obviously, trap-assisted nonradiative (also called Shockley-Read-Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post-treatment are explored. The focus herein is on the recombination at perovskite/electron-transporting material and perovskite/hole-transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap-assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The nonradiative relaxation channels of gas-phase tryptophan excited along the S1-S4 excited states (287-217 nm) have been tracked by femtosecond time-resolved ionization. In the low-energy region, λ ≥ 240 nm, the measured transient signals reflect nonadiabatic interactions between the two bright La and Lb states of ππ* character and the dark dissociative πσ* state of the indole NH. The observed dynamical behavior is interpreted in terms of the ultrafast conversion of the prepared La state, which simultaneously populates the fluorescent Lb> and the dissociative πσ* states. At higher energies, after excitation of the S4 state, the tryptophan dynamics diverges from that observed in indole, pointing to the opening of a relaxation channel that could involve states of the amino acid part. The work provides a detailed picture of the processes and electronic states involved in the relaxation of the molecule, after photoexcitation in the near part of its UV absorption spectrum.