Optical Excitation Intensity Research Articles

Absorption saturation mechanism in short-period [formula omitted] superlattice self-electro-optic effect devices based on Wannier-Stark localization

We have studied absorption saturation characteristics in short-period GaAs AlAs superlattice self-electro-optic effect devices based on Wannier-Stark localization. The disappearance of negative differential resistances due to absorption saturation was observed under high optical excitation intensities. To understand the mechanism, we measured the time-resolved photocurrent and time-resolved photoluminescence. Results revealed that the absorption saturation is caused by electric field screening originating from the space-charge of remaining holes due to effective mass filtering.

Intersubband Raman scattering in [formula omitted] quantum wells

Raman spectroscopy has been used to study intersubband transitions in InAs AlSb single quantum wells grown by molecular-beam epitaxy on (100) GaAs substrates using strain-relaxed AlSb or GaSb buffer layers. From the measured energies of the coupled longitudinal optical phonon-intersubband plasmon modes the single particle transition energies between the first and second confined electron subbands were deduced as a function of the width of the pseudomorphically strained InAs well. Subband spacings calculated including the effects of strain and nonparabolicity were found to be in agreement with the experimental transition energies. For a given well width, the two-dimensional electron concentration deduced from the Raman measurements was found to be lower than the concentration measured in the dark by Hall effect, but showed a significant increase with increasing optical excitation intensity.

Free-exciton states in crystalline GaTe.

Polarized properties of both the singlet and triplet ground exciton states in the photoluminescence and transmission spectra of crystalline GaTe are explained based on the possible symmetry properties of the energy band edge of GaTe. Some experimental results about excited exciton states in GaTe are presented and discussed. The energy positions of exciton series in GaTe follow the three-dimensional direct allowed Wannier exciton formula just as in the the other III-VI layered compounds of GaSe and InSe. The nonthermalized, ``hot'' nature of excitons inside GaTe under higher optical excitation intensities is also discussed.

Intensity dependence of the photoreflectance amplitude in semiconductors.

A rigorous calculation of the intensity dependence of the photoreflectance (PR) amplitude is presented, and the derived relation is compared with published experimental results. The method utilizes a Taylor-series expansion to determine the change in reflectance in terms of the modulation of the surface electric field, and the Fourier-series technique is employed to explicitly develop the harmonic components of the photoreflectance amplitude for square-wave excitation. In particular, it is shown that if the photoreflectance amplitude depends upon the optical excitation intensity I as ln(\ensuremath{\gamma}I+1), which is normally the case experimentally, then the small-modulation PR signal should have a line shape proportional to the first derivative of the sample reflectance with respect to the surface electric field. In the high-field limit, the nature of the Franz-Keldysh oscillations is explained for both small- and large-modulation conditions, and the theoretical predictions are correlated with recent experimental data. Overall, this theoretical study of the photoreflectance effect clarifies certain issues regarding the connection between the observed intensity dependence of the PR amplitude and the PR line shape, and it also illustrates how the nature of the PR line shape changes as one moves from low- to high-field conditions, in both the small- and large-modulation limits.

Carrier recombination in a periodically δ-doped multiple quantum well structure

We have theoretically and experimentally investigated the optical-excitation-dependent carrier recombination lifetime in a periodically /spl delta/-doped InGaAs/GaAs multiple-quantum-well structure. The spatial separation of photogenerated electrons and holes results in an increased sensitivity to the optical excitation intensity in proportion to the increase in carrier lifetime. Experimentally, we find more than six orders of magnitude reduction in the carrier recombination rate over that for spatially direct transitions under low-excitation conditions. On the other hand, theory predicts intrinsic recombination rates for ideal structures far below those found experimentally. Various mechanisms such as electric-field-enhanced redistribution of the dopants during epitaxial growth, statistical variations in the separation of the dopants, and extrinsic recombination channels caused by misfit dislocations are discussed as possible origins for this discrepancy. >

Femtosecond optical response of Y-Ba-Cu-O thin films: The dependence on optical frequency, excitation intensity, and electric current.

We have performed a series of femtosecond reflectivity experiments on various Y-Ba-Cu-O thin films at temperatures ranging from 12 to 300 K. In particular, the dependence of the optical response on probing laser frequency, pumping laser intensity, and bias electric current has been measured. Results obtained at room temperature provide quantitative information on the position of the Fermi level in films with different oxygen content. Systematic analysis of the measurements performed in the superconducting state indicates that the optical response associated with nonequilibrium properties of Y-Ba-Cu-O depends strongly on excitation intensity, sample thickness, and bias current. The results cannot be satisfactorily interpreted as the relaxation dynamics of quasiparticles, and a simple two-fluid model is shown to fail to explain data obtained under low laser excitation. Several tentative explanations are proposed, which provide a more comprehensive understanding of the transient optical response of Y-Ba-Cu-O.

Room‐Temperature Optical Nonlinearity in Semiconductor‐Doped Glasses

AbstractThe bleaching effect in commercial samples of glasses is investigated at the fundamental absorption edge of semiconductor microcrystals responsible for the colour of the glass. At room temperature under an optical excitation intensity up to 107 W/cm2 an increase of the optical transmission by five orders and more is observed. The spectral band width in which nonlinear optical response occurs reaches 0.5 eV. The results are compared with the results obtained for nonlinear absorption of semiconductor monocrystals.

Red-shift of the excitonic level in two beam absorption spectra of GaSe

Optical absorption spectra of GaSe samples immersed in superfluid helium have been measured as a function of the optical excitation intensity, by using a two beam technique. As the electron-hole density increases a broadening and a red-shift of the fundamental excitonic absorption line occur. We compare our observation with the results of a simplified first order theory which accounts for electron-hole correlation effects in static screening approximation. The optical spectra, both calculated theoretically and observed experimentally, suggest that the observed Mott transition is, at finite temperatures, smooth and continuous. Our simple model explains surprisingly well the observed optical spectra.

Photoluminescence spectra of Li-intercalated indium selenide

The photoluminescence (PL) spectra of pure and Li intercalated InSe using the n-butyl-lithium method are presented in the temperature range from 5 to 150 K and the effect of Li intercalation on the structure of the emission spectrum is examined. The new characteristics demonstrate the presence of new PL centers that modify the long-wavelength region of the spectrum. The observed modifications, which are analysed in detail, taking into account the dependence of the spectra on temperature, optical excitation intensity and dose of intercalation, enable us to give an interpretation of their origin.

Changes of the excitonic spectra of GaSe induced by optical excitation

Abstract The optical spectra of GaSe at and below the excitonic levels were measured as a function of optical excitation intensity. The experiments were performed using a two beam technique. With increasing excitation, the absorptions due to the excitonic levels continuosly become weaker and broaden. Finally, they disappear almost completely, while a weak band of induced absorption persists. The well known stimulated luminescence appears at excitation intensities for which also the excitonic lines are perturbed. For light propagating parallel to the c-axis of the crystal, no gain could be observed, i.e. also the stimulated transitions are essentially allowed only if polarized E⌈c.

Intermolecular Triplet Energy Transfer in Naphthalene–Pentacene Mixed Crystals via Higher Excited Triplet States

AbstractRadiationless intermolecular host‐guest and guest‐host triplet energy transfer involving a hot triplet state of pentacene embedded in a crystalline naphthalene matrix has been observed. The latter process gives rise to delayed fluorescence which increases with the fourth power of the intensity of optical excitation.