L. Bányai

Optical nonlinearities of glasses doped with semiconductor microcrystallites.

The origin of optical nonlinearities of glasses doped with CdS(x)Se(1-x) microcrystallites is studied both experimentally and theoretically. At room temperature a large shift ( congruent with50 meV) of the absorption spectrum to higher energies is observed in a CdS(x)Se(1-x) glass with increased light intensity. This blue shift is analyzed by using a semiconductor plasma theory. Experimental and theoretical results suggest that band filling is the dominant mechanism for the observed nonlinearity. An independent measurement for the intensity-dependent dispersive changes is presented.

Optical switching in low-dimensional systems

The workshop was a part of the NATO Special Program on Condensed Systems of Low Dimensionality (1983-1988). Contributors concentrate on the development and study of optical and electro-optical devices, as well as experimental and theoretical investigations of the underlying optical nonlinearities. T

Three types of electronic optical bistabilities in CdS

The authors investigate optical bistability in CdS, induced by photo-electronic non-linearities. Varying the photon energy and the temperature of the sample they find three different types of optical bistability. The switching times are always around 1 ns or below, notably 300 ps for dispersive optical bistability. Optical bistability by bleaching of absorption at room temperature shows a contrast ratio of about 50 between the two stable branches. The dependence on various parameters, including the microscopic foundations, is discussed experimentally as well as theoretically.

High-excitation photoluminescence studies of CdS1−xSex quantum dots

Linear absorption and excitation dependent luminescence measurements on CdSSe quantum dots in glass are reported. The new aspects of the spectra are: (i) Stokes shift of linear emission versus absorption and (ii) appearance of new structures in the high energy wing of the luminescence peak under high excitation, which are interpreted as consequences of Coloumb effects, important for dot radii around the exciton Bohr radius.

Two-photon absorption and third-order nonlinearities in GaAs quantum dots

Third-order optical nonlinearities of GaAs quantum dots are investigated theoretically. Two quantum confinement regimes are analyzed, and large optical nonlinearities are predicted for sufficiently narrow linewidths. The appearance of an induced (two-photon) absorption resonance is predicted energetically above the exciton resonance for quantum-dot radii, which are between the electron and hole Bohr radii.

Quantum kinetics for differential transmission spectra in GaAs with LO-phonon interaction

We calculate the differential transmission at the band edge in GaAs at low temperatures within a resonant femtosecond pulse pump-probe configuration for low pair densities, simulating recent experiments performed by Furst et al. The treatment is based on the semiconductor Bloch equations with quantum kinetic electron-phonon collision integrals, taking into account fully the time features of the pulse experiment. A thorough comparison with the Boltzmann version of the theory, as well as with experimental data is performed. Good agreement with the experiment is obtained only by a nonlinear analysis with quantum kinetic scattering.

Improved spectral functions for quantum kinetics

We improve the calculation of the two-time-dependent spectral functions which determine the memory kernel of the retarded quantum kinetics by using an interpolation between the ultrashort time limit of the dissipative part of the self-energy and its stationary limit. By these means we get a spectral function which decays in the form of a power of a hyperbolic secans. For short time intervals the decay of the retarded Green function starts with zero slope, while the long-time behaviour is exponential. The real part of the Fourier transform of a hyperbolic secans is again a hyperbolic secans which is localised enough in frequency space, so that run-away effects of the asymptotic kinetics are avoided.

Nonlinear optical properties of semiconductor quantum dots

Abstract The shift of the lowest quantized state ▿ E 1 in quantum dots (QDS) is connected with the crystallite radius R , determined, e.g. by X-ray techniques. Theory gives ▿ E 1 = CE B ex ( a B π / R ) 2 with C = 0.67. C varies from 0.3 to 10 when using it as a fit parameter for samples from different sources indicating some inconsistencies in the determination of R . The splitting ▿ E 1/2 between first and second absorption peak is independent of R but is a function of x for R / a B ⪆ 1 and agrees with the spin-orbit splitting and / or other valence band pecularities. ▿ E 1/2 agrees for R / a B ⪅ with the R dependent separation between first and second quantized conduction band states. From the Stokes shift of absorption and emission and their halfwidths, we determine an upper limit for the Huang-Rhys factor S . It increases with decreasing R up to S ⩽ 2.0. Pump and probe beam (DTS) measurements show bleaching of the absorption structures almost independent of ℏ pump and sample temperature. The width of the DTS signal confirms S values between 1 and 2. One-beam bleaching measurements agree with theory and give values for the saturation intensity around 6 MW / cm 2 depending on ℏ pump and x . Luminescence under high excitation shows an additional high energy peak, which is the result of recombination of an excited two electron-hole pair state in the dot, in agreement with theory. This state can also be seen in DTS measurements as induced absorption. Spectroscopy with self-diffraction from laser-induced gratings at room temperature shows rather broad (≈ 20 nm) efficiency spectra. The nonlinearity is gain the bleaching of absorption, which yields values of some 10 -9 esu if expressed in terms of an effective x (3) . This value is much lower than for bulk CdS due to the dilution of the semiconductor in the glass.

Linear and nonlinear optical studies of CdS1?x Se x quantum dots

In this contribution we present and discuss our measurements on CdS1−xSex quantum dots in a glass matrix. In linear absorption measurements we find the typical blue shift of the transitions with decreasing crystallite radius due to quantization. The luminescence shows a significant Stokes shift with respect to absorption, which is interpreted in terms of strong exciton-phonon coupling and allows to deduce the Huang-Rhys factor S. Under high excitation we find an additional emission band on the high energy side, which can be attributed to the recombination of an excited two electron-hole pair state to a one electron-hole pair state in agreement with theory. Pump and probe beam experiments give a bleaching but no hole burning. Finally we discuss some open questions especially concerning the high energy structures in the absorption spectrum.

Study of the Coulomb Boltzmann kinetics in a quasi-twodimensional electron gas by eigenfunction expansions and Monte Carlo simulations

For carrier-carrier scattering the relaxation kinetics of a quasi-twodimensional degenerate electron gas is studied in the framework of the Boltzmann equation using a static RPA screening. The time development of an initial nonequilibrium distribution is calculated both by ensemble Monte Carlo simulations and by expansions in terms of eigenfunctions of the linearized collision operator. A systematic comparison of the results of both methods yields a rather detailed understanding of the Coulomb relaxation kinetics and of the strengths and limitations of both methods. The dependence of the relaxation rate on the position and width of the initial population deviation is investigated and conditions for nonexponential decay are given.