Spontaneous emission

About: Spontaneous emission is a research topic. Over the lifetime, 12855 publications have been published within this topic receiving 323684 citations.

Determination of the refractive index of InGaAsP epitaxial layers by mode line luminescence spectroscopy

Abstract: A new method of detecting optical waveguide modes of semiconductor epitaxial layers is presented. The modes are excited by photoluminescence and coupled out of the semiconductor wafer by a grating etched on the surface of the guiding layer. As a consequence of this coupling, the spectrum of optical emission from the sample at fixed angles has "mode lines" superimposed on the spontaneous emission spectrum. This method is used to measure the refractive indexes of eight InGaAsP layers of different compositions. These data together with published data for the refractive index of InP were fit with a two-oscillator model to yield an algebraic description of the refractive index of the InGaAsP/InP system. The method was also used to measure the change in mode index of a laser in going from low current up to threshold. The observed change was measured to be -0.023 ± 0.006.

Exciton-polariton spectra and limiting factors for the room-temperature photoluminescence efficiency in ZnO

Abstract: Static and dynamic responses of excitons in state-of-the-art bulk and epitaxial ZnO are reviewed to support the possible realization of polariton lasers, which are coherent and monochromatic light sources due to Bose condensation of exciton–polaritons in semiconductor microcavities (MCs) To grasp the current problems and to pave the way for obtaining ZnO epilayers of improved quality, the following four principal subjects are treated: (i) polarized optical reflectance (OR), photoreflectance (PR) and photoluminescence (PL) spectra of the bulk and epitaxial ZnO were recorded at 8 K Energies of PR resonances corresponded to those of upper and lower exciton–polariton branches, where A-, B- and C-excitons couple simultaneously to an electromagnetic wave PL peaks due to the corresponding polariton branches were observed Longitudinal–transverse splittings (ωLT) of the corresponding excitons were 15, 111 and 131 meV, respectively The latter two values are more than two orders of magnitude greater than that of GaAs being 008 meV (ii) Using these values and material parameters, corresponding vacuum-field Rabi splitting of exciton–polaritons coupled to a model MC mode was calculated to be 191 meV, which is the highest value ever reported for semiconductor MCs and satisfies the requirements to observe the strong exciton–light coupling regime necessary for polariton lasing above room temperature (iii) Polarized OR and PR spectra of an out-plane nonpolar ZnO epilayer grown by laser-assisted molecular beam epitaxy (L-MBE) were measured, since ZnO quantum wells (QWs) grown in nonpolar orientations are expected to show higher emission efficiencies due to the elimination of spontaneous and piezoelectric polarization fields normal to the QW plane They exhibited in-plane anisotropic exciton resonances according to the polarization selection rules for anisotropically-strained wurzite material (iv) Impacts of point defects on the nonradiative processes in L-MBE ZnO were studied using time-resolved PL making a connection with the results of positron annihilation measurement Free excitonic PL intensity at room temperature naturally increased with the increase in nonradiative lifetime (τnr) The value of τnr increased and density or size of Zn vacancies (VZn) decreased with increasing growth temperature (Tg) in heteroepitaxial films grown on a ScAlMgO4 substrate, and the use of homoepitaxial substrates further reduced VZn density The value of τnr was shown to increase with the decrease in gross density of positively and negatively charged and neutral point defects including complexes rather than with the decrease in VZn density The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VZn, such as VZn-defect complexes As a result of defect elimination by growing the films at high Tg followed by subsequent post-growth in situ annealing, combined with the use of high-temperature-annealed ZnO self-buffer layer, a record long τnr for spontaneous emission of 38 ns was obtained at room temperature By using progressively improving epitaxial growth methods, the polariton laser effect is expected to be observed at room temperature in the near future

Optical gain spectra for near UV to aquamarine (Al,In)GaN laser diodes.

Abstract: Optical gain spectra presented for (Al,In)GaN laser diodes with lasing wavelength ranging from UV (375 nm) to aquamarine (470 nm) show a strong increase in inhomogeneous broadening, caused by Indium composition and quantum well width fluctuations which increase with Indium mole fraction. These gain spectra provides a standard data set for the calibration of microscopic many–body simulations. We demonstrate by comparison with basic simulations that the different assumptions of a global constant carrier density or of global constant quasi–Fermi levels for electrons and holes lead to a strikingly different dependency of optical gain on carrier density. For constant quasi–Fermi levels the threshold carrier density becomes insensitive to inhomogeneous broadening for realistic parameters. This is in agreement with the observation that the threshold current is nearly independent over the wavelength range from near UV to aquamarine.

Spin-dependent donor-acceptor pair recombination in ZnS crystals showing the self-activated emission

Abstract: Donor-acceptor recombination has been investigated in ZnS by optically detected magnetic resonance (ODMR). Resonances due to both donors and A-centre acceptors have been observed in hexagonal and cubic ZnS containing group IIIB and group VIIB dopants. Spectral dependence measurements show that both resonances are associated with the blue self-activated emission, which lies at 2.70 eV in ZnS:Cl and 2.60 eV in ZnS:Al,I. The resonances are observed as increases of order 0.4% in the total emission intensity and are accounted for in terms of a donor-acceptor recombination process. It is found that the signals are predominantly from donor-acceptor pairs that are sufficiently close for recombination to occur before thermalisation is achieved but which are sufficiently well separated for the exchange interactions and associated line broadening to be small. The investigation confirms that the self-activated emission is due to radiative recombination of an electron from a shallow donor with a hole on an A-centre.

Spontaneous emission and collection efficiency enhancement of single emitters in diamond via plasmonic cavities and gratings

Abstract: We demonstrate an approach, based on plasmonic apertures and gratings, to enhance the radiative decay rate of single NV centers in diamond, while simultaneously improving their collection efficiency. Our structures are based on metallic resonators formed by surrounding sub-wavelength diamond nanoposts with a silver film, which can enhance the spontaneous emission rate of an embedded NV center. However, the collection efficiency of emitted photons remains low due to losses to surface plasmons and reflections at the diamond-air interface. In this work, we mitigate photon losses into these channels by incorporating grating structures into the plasmonic cavity system.