Surface photovoltage in exciton absorption range in CdS

The high resolution, intrinsic spectra of surface photovoltage are reported for semiconducting n-type CdS single crystals. At reduced temperatures (120–160 K) the spectra exhibit three sharp maxima due to A, B and C free exciton transitions. Energy positions of these lines and valence band parameters (spin-orbit and crystal field splittings) estimated from surface photovoltage are in good agreement with values obtained by other methods. The excitonic transitions are very sensitive to surface treatment, i.e. polishing, etching, background illumination and surface doping. The mechanism of direct interaction of free excitons with surface states is proposed to explain exciton lines in surface photovoltage.     

Two-photon absorption in CdS by low power CW laser

Our aim is to study the two-photon induced luminescence in CdS crystals by using a low power CW laser (5 mW). We discuss briefly the possibility given by the electron-pulse-counting technique to detect weak optical signals. This technique enables us to investigate the luminescence spectra (which show two bands at 2.4 and 2.0 eV) and the dependence of the luminescence intensity on the laser power. The discussion of the experimental results makes it clear that the level responsible for the red emission is partially saturated at a laser intensity of 10²³ photon/cm² s.     

溶胶CdS毫微晶的双光子吸收及其量子尺寸效应

本文利用单光束透过率法研究了溶胶CdS毫微晶的双光子吸收。对于毫微晶尺寸为5.5～11.0nm的样品,测得其在波长510.6nm时的双光子吸收系数为1.31～5.24/GW,并由此得到相应的x~(3)为2.8×10~(-12)～1.1×10~(-11)esu。文中还从理论上分析了CdS溶胶的双光子吸收与毫微晶尺寸的关系,理论计算与实验结果基本符合。     

Two-photon transitions to exciton states in semiconductors

We consider the gauge invariance of the two-photon transition rate from the crystal ground state to exciton states in semiconductors in the framework of the Wannier model. We show that the non-locality correction in the velocity gauge is essential to preserve the gauge invariance. As a numerical example, we calculate in the length and in the velocity gauge the transition rates for the two-photon direct creation of 2p excitons in a two-band semiconductor.     

Two-photon absorption in hexagonal-CdS

The “composite sample technique” has been used to investigate the two-photon absorption coefficient (β) of CdS at 3.91 eV. The absolute value of β and its optical anisotropy have been studied for samples with different carrier densities. The present results may provide a new interpretation of experiments on CdS and other semiconductors reported in the literature.     

Two-photon absorption in CuCl

The two-photon absorption spectrum of CuCl at 4.2°K is studied in the energy range of the excitonic lines. The observed lines are suggested to be due to the creation of excitons in the 1S, 2P and 3P states and also to the simultaneous creation of 2P and 3P excitons and one or more longitudinal optical phonons.     

Resonant Brillouin scattering near the a exciton in CdS

We have studied resonant Brillouin scattering in CdS by tuning a narrow-band cw dye laser through the A-exciton transition at helium temperatures. We observed besides the resonance enhancement a pronounced asymmetry of the Stokes and anti-Stokes shift and a dramatic increase of the Brillouin shifts as a function of laser frequency. These observations are consistent with a model of exciton-polariton scattering by longitudinal acoustic phonons. For incident frequencies higher than the longitudinal exciton frequency additional features are found which cannot be explained by the two-branch model for exciton-polaritons.     

Two-photon interference in the presence of absorption

Experiments on two-photon interference are discussed in the case when there is absorption of all the modes participating in the process of spontaneous parametric down-conversion (SPDC) of light. The objects of investigation are 10-to 80-Å-thick ultrathin gold films deposited on fused-silica substrates. Conditions are determined under which the effect of absorption of the signal and pump waves on the interference pattern is small. It is shown that, under these conditions, the visibility of the interference pattern and the shape of the frequency-angular spectrum at the signal frequency are determined by the optical parameters of the medium at the idler frequency, which belongs to the near-infrared region.     

Two-photon fluorescence measurements in a hexagonal CdS crystal

Measurements of two-photon fluorescence (TPF) in a CdS crystal using a dye laser pumped by a Q-switched ruby laser are reported. The TPF is measured as a function of source intensity and wavelength. The total fluorescence shows linear and non-linear dependence on the source intensity; from these measurements the TPF part is determined. The measured transition of the crystal's fundamental gap is 2.5 ± 0.02 eV and another forbidden one-photon but allowed two-photon transition is found at 3.24 ± 0.02 eV.     

Reflectivity kinetics in the vicinity of exciton transitions in semiconductor nanostructures

The kinetics of the reflectivity of semiconductor samples with multiple GaAs/AlGaAs quantum wells has been investigated by the pump-probe method with a resolution of 150 fs. Excitons of the E1HH1 and E1LH1 types manifest themselves in the kinetics of transient reflectivity spectra despite the high density of the photogenerated two-dimensional electron gas. Differences are revealed in the shape and kinetics of the transient reflectivity spectra of samples with different detuning of exciton transitions from the pump frequency.     

Free exciton emission in forward-biased CdS MIS diodes

The radiative decay of free excitons with the emission of 0, 1, or 2LO phonons has been observed in electroluminescence in Cds MIS diodes operated in forward bias at temperatures in the range 40 – 77K. The line shapes and temperature dependence of the ILO and 2LO phonon assisted replicas are discussed in terms of the distribution of kinetic energy of the excitons with an effective temperature equal to that of the lattice.     

Optical activity of CdS crystals in exciton spectral region

Natural optical activity of wurtzite CdS crystals has been studied for the first time both theoretically and experimentally in the spectral region of the B_n=1 exciton resonance. It has been shown that the observed optical activity is due to mixing the longitudinal Γ_5L exciton state with the transverse Γ₁ state by the energy terms linear in wave vector.