K. Wundke

PbS quantum-dot-doped glasses for ultrashort-pulse generation

We investigate the use of PbS quantum-dot-doped glasses as saturable absorbers for ultrashort-pulse lasers by means of absorption bleaching experiments and numerical analysis of the pulse shaping process using the Haus’ master equation. We explain the mode-locking mechanism and the limitations of these absorbers. The generation of transform-limited fs pulses is predicted by soliton mode locking initiated by the absorption saturation of higher excited states of the quantum-dot saturable absorber.

Room-temperature gain at 1.3 μm in PbS-doped glasses

We report on room-temperature optical gain at the ground exciton transition of PbS quantum-dot-doped glasses while optical pumping into the next-higher exciton resonance. The material gain in the quantum dots is as large as 80 cm−1. The dot-size selective excitation provides tunability of the optical gain. This is demonstrated by tuning the gain from 1317 to 1352 nm by changing the pump wavelength from 900 to 980 nm.

Recovery dynamics of a PbS quantum dot doped glass passive mode locker

Summary form only given. Recently we demonstrated that PbS quantum dot doped glasses, fabricated by thermal treatment of an oxide molten glass, can be used as intracavity saturable absorbers to obtain self-starting mode locking in infrared solid-state lasers. In particular, using these absorbers in a Cr:forsterite laser we obtained pulses as short as 4.6 ps with a time-bandwidth product of 0.35 and pulse energies up to 1 nJ. Because of the large exciton Bohr radius (18 nm) and the small band gap energy (0.4 meV at RT) these saturable absorbers are suitable for operation over a wide spectral rang from visible to 3 /spl mu/m. However, it was unclear why shorter pulses then 4.6 ps could not be obtained. Here we perform degenerate pump-probe experiments to address this point and determine the recovery time of the optical nonlinearity.

Luminescence and gain around 1.3 /spl mu/m in PbS quantum dots

Summary form only given.Strong luminescence and gain is observed for PbS quantum dot (QD) doped glasses at the 1.3 /spl mu/m communications wavelength under CW and pulsed laser excitation, respectively, at 980 nm. This observation generates new interest in PbS QDs as promising candidates as an alternative to Er-doped amplifiers for communications applications. Thermal treatment of an oxide molten glass is an inexpensive method to manufacture semiconductor quantum dot doped glasses with well-defined exciton peaks and small. Inhomogeneous broadening. The PbS QDs studied here have radii much smaller than the bulk exciton Bohr radius providing strong quantum confinement effects. As a result, the exciton resonances can be tuned to the communication wavelengths of 1.3 and 1.55 /spl mu/m by varying the QD size.

Laser-induced Rabi oscillations in semiconductors

Summary form only given. We apply a two-color pump-probe scheme that enables the observation of several cycles of clearly resolved excitonic Rabi oscillations in a semiconductor quantum well. A 770 fs circularly polarized pump pulse with a narrow spectrum excites resonantly heavy hole (hh) excitons. The linear transmission spectrum of the InGaAs-GaAs sample and the pulse spectra are shown.

Experimental observation of multiple excitonic optical Rabi oscillations in a semiconductor

Data are presented which show eight excitonic density (Rabi) oscillations in an In0.1Ga0.9As/GaAs multiple quantum well at 5 K. Our time resolved, two-color pump-probe experimental technique for observing these oscillations is described. The quantum well sample geometry and linear spectrum are shown along with data characterizing the pump and probe pulses. Experimental data is shown to be in excellent agreement with our theoretical calculation of exciton density versus time, verifying the important affect of the Coulomb interaction between carriers in renormalizing the Rabi frequency. The theoretical calculations include the two-fold degenerate light-hole, heavy-hole, and valence bands in the Hartree-Fock form of the semiconductor Block equations, with all variables selected to conform to the experiment.

Many-body aspects of excitonic Rabi oscillations in semiconductors

A theoretical study of excitonic Rabi oscillations in semiconductor quantum wells is presented. Fundamental differences between excitonic Rabi oscillations and optical Rabi oscillations in ideal atomic or molecular 2-level systems are due to the existence of the continuous 1-particle spectrum in solids (energy bands) and the resulting strong effects of the Coulomb interaction, such as the attractive electron-hole interaction and the repulsive exchange interaction. In spite of these fundamental differences, Rabi-like oscillations of the excited electron-hole densities are computed for the case of resonant optical pumping of excitons. We compare our theoretical results with our recent experimental observation of excitonic Rabi oscillations.