Free Exciton Level Research Articles

Impact of excitation energy on the excitonic luminescence of cesium lead bromide perovskite nanosheets.

An excitation-energy-dependent luminescence phenomenon is reported in cesium lead bromide (CsPbBr3) perovskite nanosheets. At 10 K, the relative integrated luminescence intensity between the trapped exciton (TX) emission and the free exciton (FX) emission shows an interesting tendency with increasing the optical excitation energy from 2.431 eV (510 nm) to 3.758 eV (330 nm). To interpret such phenomenon, we develop a quantitative model on the basis of the biological population growth theory. A good agreement between experiment and theory is obtained. It is thus revealed that the lower capture coefficient of the TX level to the excited excitons via multiphonon emission relative to the FX level shall be the major cause of the observed phenomenon. These findings may help to deepen the current understanding of the complex luminescence mechanisms of these emerging light-emitting materials.

Influence of nonstimulated polariton relaxation on parametric scattering of microcavity polaritons

Stimulated parametric scattering of polaritons has been investigated in a GaAs-based microcavity for varied depths of the lower polariton (LP) branch energy minimum with respect to the free exciton level. We find that despite overall growing losses in the polariton system, for the deep LP branch (with the band minimum $>3.3\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ below the exciton level) the threshold for stimulation ${W}_{\text{th}}$ strongly decreases with temperature $T$. We show that for a deep LP branch the LP population arising from incoherent polariton relaxation at elevated $T$ plays the role of an efficient ``seed'' for the highly coherent process of the polariton parametric scattering. The influence of efficient nonstimulated relaxation on ${W}_{\text{th}}$ at low $T$ is also confirmed in the experiments when additional laser excitation above the band gap of GaAs is used to enhance the polariton relaxation. Our observations indicate that in addition to the four-wave-mixing formalism describing only four waves participating in the parametric process, the full physical picture including other relaxation channels should be taken into account to reproduce the observed nonlinear behavior accurately.

One-dimensional excitonic states and lasing in highly uniform quantum wires formed bycleaved-edge overgrowth with growth-interrupt annealing

High quality T-shaped quantum wire lasers are fabricated by cleaved-edge overgrowth withmolecular beam epitaxy on an interface improved by a growth-interrupt high temperatureannealing. Microphotoluminescence and photoluminescence (PL) excitation spectroscopy atlow temperatures reveals the formation of quantum wires with unprecedentedly highquality, and intrinsic structures of one-dimensional (1D) free excitons, exciton excitedstates, and 1D continuum states. At high pumping levels, the PL evolves from showing asharp free exciton peak via exhibiting a biexciton peak to a Coulomb correlatedelectron–hole plasma PL band. Lasing has been achieved with a low lasing threshold, andits emission patterns are measured in imaging experiments. The lasing energy is in theplasma PL band and is about 5 meV below the free exciton level. The observed shiftexcludes the possibility of free excitons in the lasing, and suggests a contribution from theelectron–hole plasma. Single T-wire samples such as a single-quantum-wire laser and afield-effect-transistor-type doped single quantum wire are fabricated and studied optically.

Third-order nonlinear optical properties of polydiacetylene single crystal.

We report the sub-picosecond time-resolved nonlinear optical properties of poly-DCH single crystal from near resonance to on resonance. The magnitude of the third-order nonlinear susceptibilities of poly-DCH for polarization along the main chain direction were found to be of the order 10-6esu on resonance and 10-7esu near resonance. A photo- induced absorptive nonlinearity was found near resonance and an absorption saturation was found on resonance. The sign change in the imaginary susceptibility is due to the mixing of two phenomena, the free exciton level satutation and the reabsorption of the self-trapped exciton. The self-trapped exciton lifetime was observed to be 1.5-2.3ps near resonance.

Resonant Raman Scattering in ZnS

Three Raman lines were observed in addition to excitonic emission lines in cubic ZnS using a He-Cd laser 325 nm line. The lines were assigned to the Raman lines of the laser line scattered by one or two phonons. The energies of the LO phonon and the TO phonon were estimated to be 44 meV and 35 meV from the energy of the Raman lines. These values agreed well with those measured by right-angle Raman scattering with an Ar laser 514.5 nm line. With the rise in temperature from 4.2 K, the intensity of the Raman lines declined until 80 K and was then enhanced to about 130 K. The temperatures where the intensity was enhanced corresponded to those where the energy of the free exciton level approached the energies of the Raman lines. The temperature dependence of the Raman intensity was interpreted in terms of resonance between the incident or scattered light with the temperature-dependent free exciton level.

Direct coupling of heavy-hole free excitons in InxGa1-xAs/GaAs quantum wells with free excitons in the GaAs barrier.

Enhanced emission due to heavy-hole free-exciton (HHFE) collapse in narrow (${\mathit{L}}_{\mathit{z}}$\ensuremath{\le}50 \AA{} single ${\mathrm{In}}_{0.1}$${\mathrm{Ga}}_{0.9}$As/GaAs quantum wells was observed when the excitation energy was resonant with the GaAs-barrier free-exciton energy. The observed resonant excitation behavior along with the marked difference in the effect of an applied magnetic field on the HHFE emission strength for the cases of nonresonant and resonant excitation is consistent with a model of direct coupling between the GaAs-barrier free-exciton level and the ${\mathrm{In}}_{0.1}$${\mathrm{Ga}}_{0.9}$As-quantum-well heavy-hole free-exciton level. The direct coupling between the GaAs-barrier free-exciton level and the quantum-well HHFE level is a result of wave-function overlap accomplished by extension of the HHFE wave function into the barrier region. As a result of this direct coupling, when several ${\mathrm{In}}_{0.1}$${\mathrm{Ga}}_{0.9}$As quantum wells of different sizes are present in the same sample, excitation at the GaAs-barrier free-exciton formation energy simultaneously resonantly excites HHFE emission from all of the ${\mathrm{In}}_{0.1}$${\mathrm{Ga}}_{0.9}$As wells for the well sizes studied (${\mathit{L}}_{\mathit{z}}$\ensuremath{\le}50 \AA{}).

Resonant Raman scattering on low energy excited states of Fe 2+ in Cd1− xFexSe

The spectrum of low energy excited states of Fe 2+ in Cd 1− xFex Se has been studied through Raman scattering. (1) The energy position of the first two excited states of symmetry A 2 and E have been determined as a function of the applied magnetic field ⊥ c and ∥| c up to 6 T; they are in very good agreement with a crystal field model. (2) Strong resonant effects on free and bound exciton states have been observed and explained. (3) Above the free exciton level, a cascade process involving mainly n(A 1→A 2) excitations is studied. Backscattering on a face ⊥ c strongly favours crossed linear polarizations at each step of the cascade; a longitudinal magnetic field causes an analog of Hanle effect. Analysis of the data gives access to the polariton lifetime and its probability of inelastic scattering on Fe 2+ ions.

Absorption and Luminescence in Ultra-Thin Films of PbI2

We have measured the absorption and luminescence spectra of the ultra-thin PbI 2 film evaporated on a cleaved KCl crystal at 100 K. The energy of the exciton absorption band is analyzed by the one-dimensional quantum well, but the exciton energies in the platelet crystals consisting of the two and three layers deviate from the simple theoretical values because of the too thin crystals. From the appearance of the broad luminescence band in the ultra-thin film, it is concluded that the free exciton level may become higher than that of the self-trapped exciton by the quantum well effect.

Stacking fault density and splitting of exciton states in ϵ-GaSe

Abstract The distribution of stacking fault in single crystals of ϵ-GaSe grown by the Bridgman technique has been studied by combined measurements of X-ray diffraction and photoluminescence. The results show that the stacking fault is distributed randomly over the crystal with the fault layer normal to the c -axis. One stacking fault takes place approximately in every 15 layers for a typical crystal which shows multiple splittings of the free exciton level in the luminescence spectrum. For Bridgman-grown crystals of ϵ-GaSe, the average distance between the nearest neighbor fault planes is determined to be much longer than the Bohr diameter (60 A) of the direct 1 s exciton.

Resonant Electronic Raman Scattering and Electronic Structures of Bound Exciton Complex in ZnSe

Electronic Raman scattering of shallow donors in cubic ZnSe has been studied for the incident photon energy in the vicinity of the n =2 free exciton level. Raman scattering intensities for 1s→2s and 1s→2p excitations of donor electrons are resonantly enhanced as the photon energy coincides with the excited states of exciton-neutral donor complex. Since these excited states are located within the continuous dispersion of free exciton state, the large enhancement in the Raman scattering intensity has been attributed to the mixing of these excited states with the free exciton state. The resonant behavior of the electronic Raman scattering shows that these excited states correspond to the lowest lying electronic excited states of the exciton neutral donor complex.

Electron-hole liquid in GaP: N

The radiative recombination of the nitrogen-induced electron-hole liquid in GaP: N is observed. The liquid is formed by delocalization of excitons bound to nitrogen in crystals in which the nitrogen concentration exceeds ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, and under intense band-gap or selective excitation into the lowest $J=1$ state of the bound exciton ($A$ line). The observed binding energy is 17 meV relative to the free-exciton level. The liquid density is in the range of (1-4) \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ for excitation intensity varying in the range of ${10}^{14}$-${10}^{18}$ photons/${\mathrm{cm}}^{2}$/pulse.