Stefan Schmitt-Rink

Far-infrared light generation at semiconductor surfaces and its spectroscopic applications

Femtosecond pulses of far-infrared (FIR) radiation can be generated in the depletion field near semiconductor surfaces under optical excitation via the inverse Franz-Keldysh effect or electric-field-induced optical rectification. This coherent mechanism differs from previously proposed incoherent mechanisms and is expected to dominate for nonresonant excitation as well as for resonant excitation with optical fields. Two femtosecond time-resolved spectroscopic applications using a visible excitation pulse and a FIR probe pulse are described. >

Dissipative dynamics of an electronic wavepacket in a semiconductor double well potential

The authors present a detailed study of coherent tunneling oscillations of an optically excited electronic wavepacket in a semiconductor double quantum-well structure. The oscillation of the wavepacket between the two wells is traced by time-resolved pump-probe spectroscopy as well as time-resolved degenerate four-wave mixing. The experimental observations are compared with a simple theoretical model. The motion of the wave packet is studied as a function of both an external electric field and the lattice temperature. While the oscillation frequency shows the expected dependence on electric field, it increases with temperature at fixed electric field when perfectly delocalized electronic states exist at low temperature. >

SINGLE SPIN FLIP IN THE INFINITE U HUBBARD MODEL: HUBBARD OPERATORS, THREE-BODY FADEEV EQUATIONS AND GUTZWILLER WAVE FUNCTIONS

We investigate a recently proposed many-body theory for composite (Hubbard) operators (A.E. Ruckenstein and S. Schmitt-Rink, Phys. Rev. B38, 7188 (1988)) in the context of the problem of a single spin flip in the saturated ferromagnetic state of the infinite U Hubbard model. We prove that the suitably defined strong coupling Hartree-Fock mean field theory leads to results identical to those obtained from the Gutzwiller wave function through exact evaluation of the kinetic energy. Most interestingly, we also show how exactly the same results can be obtained starting from the weak coupling limit by solving analytically the three-body t-matrix (Fadeev) equations in the infinite U limit. This work also sheds light on the physical content of slave boson approximations to which our approach was previously shown to be equivalent in the limit of large spin or orbital degeneracy. For the single spin flip problem we compare our results with those obtained by Bethe-Goldstone perturbation theory, Bethe Ansatz in one dimension, and exact diagonalization studies.

INTERFERENCE OF THE FERMI EDGE SINGULARITY WITH AN EXCITONIC RESONANCE IN DOPED SEMICONDUCTORS

Low-temperature emission spectra are calculated for an n-doped semiconductor in which the Fermi level is almost degenerate with an exciton of a higher conduction band. It is shown that in such a sample the virtual transitions of electrons from the Fermi surface into the excitonic state can lead to a strong enhancement of the optical oscillator strength. In particular, due to this effect, the Fermi edge singularity, which is usually only seen in absorption, can become visible also in the emission spectrum. This explains the optical spectra recently obtained in experiments on modulation-doped heterostructures.

QUANTUM BEATS OF EXCITONS IN QUANTUM WELLS

We briefly review our recent observations of quantum beats of excitons in quantum wells. The quantum beats are observed as an oscillatory structure in the polarization decay of energetically closely spaced excitons which are coherently excited by ultrashort laser pulses.

Temperature dependence of semiconductor lasers (Invited Paper)

For many years it has been assumed that nonradiative recombination plays a dominant role in determining the high temperature performance of long wavelength laser diodes. We show that this view is inconsistent with the measured temperature dependence of spontaneous emission from light emitting diodes. We conclude that net gain primarily determines the temperature sensitivity of threshold in long wavelength semiconductor lasers.

THEORY OF MULTIPLE EXCITONIC PEAKS IN THE LUMINESCENCE SPECTRA OF DOPED QUANTUM WELLS IN A MAGNETIC FIELD

We report calculations of the luminescence spectra of doped quantum wells in a perpendicular magnetic field. We use the variational Hartree-Fock approximation and include Coulomb coupling of different Landau levels to all orders. In the case of localized holes we obtain multiple excitonic peaks with an oscillator strength which increases towards the chemical potential, in good agreement with recent experiments.

Brownian oscillator analysis of femtosecond pump-probe spectroscopy of polydiacetylene☆

Abstract Pump-probe difference absorption experiments on polydiacetylene- para -toluene sulfonate (PTS) are analyzed using a three-mode Brownian oscillator model for the intramolecular and intermolecular dynamics. Excellent agreement between theory and experiment is obtained.

Prospects for THz quantum well optoelectronics

We discuss the possibility of generating, propagating and detecting THz electrical pulses in semiconductor quantum well structures and present preliminary experimental results.

Nonlinear optical response of two-dimensional magneto-excitons

We study the coherent nonlinear optical response of two-dimensional excitons in a perpevthcular magnetic field. The transition from two-dimensional behaviour in the low-field limit to quasi-zerodimensional behaviour in high fields is investigated and pronounced manybody effects which are highly field dependent are found. Differential pump-probe spectra are calculated for various excitation conditions. It is shown that cyclotron motion can be time resolved with ultrashort optical pulces.