This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrysta...

Low-dimensional systems: quantum size effects and electronic properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems

This is a review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity. There are also introductory discussions on physical size regimes, crystallite synthesis, quantum confinement theory, and linear optical properties. The experiments describe CdSe crystallites, exhibiting strong confinement of electrons and holes, and CuCl crystallites, exhibiting weak confinement of the exciton center of mass. In the CdSe system, neither the Stark effect nor the third order nonlinearity is well understood. The Stark shifts appear to be smaller than calculated, and field inducted broadening also occurs. The third order nonlinearity is only modestly stronger than in bulk material, despite theoretical prediction. Unexpectedly large homogeneous widths, due to surface carrier trapping, in the nominally discrete crystallite excited states appear to be involved. The CuCl system shows far narrower spectroscopic homogeneous widths, and corresponds more closely to an ideal quantum dot in the weak confinement limit. CuCl also exhibits exciton superradiance at low temperature. Surface chemistry and crystallite encapsulation are critical in achieving the predicted large Stark and third order optical effects in II-VI and III-V crystallites.

Quantum crystallites and nonlinear optics

Over the past 2 decades there has been tremendous advancements in the field of ultrafast carrier dynamics in semiconductors. The driving force behind this movement other than the basic fundamental interest is the direct application of semiconductor devices and the endless need for faster response and faster processing of information. To improve and develop microelectronics devices and address these needs, there must be a basic understanding of the various dynamical processes in the semiconductors which have to be studied in detail. Therefore, the excitation of semiconductors out of their equilibrium and the subsequent relaxation processes with various rates has become a key area of semiconductor research. With the development of lasers that can generate pulses as short as a few femtoseconds the excitation and subsequent probing of semiconductors on an ultrashort timescale have become routine. Processes such as carrier momentum randomization, carrier thermalization, and energy relaxation have been studied in detail using excite-and-probe novel techniques. This article reviews the status of ultrafast carrier and phonon dynamics in semiconductors. Experimental techniques such as excite-and-probe transmission, time-resolved up-conversion luminescence, and pump-probe Raman scattering along with some of the significant experimental findings from probing semiconductors are discussed. Finally, a selfconsistent theoretical model, which correlates the carrier and phonon dynamics in germanium on an ultrashort time scale, is described in detail.

Probing ultrafast carrier and phonon dynamics in semiconductors

The main mechanisms for the observed large optical nonlinearities in semiconductors are briefly discussed and illustrated for the example of the exciton ionization by plasma screening. These nonlinearities cause various types of optical bistability, either of intrinsic nature or evoked by an additional resonator feedback. Under certain operation conditions also higher instabilities such as oscillations and chaos can be obtained. As an example, the self-pulsing of an induced absorber in a ring cavity is treated. The resulting locked oscillations are shown to follow a Farey-tree scenarium.

Optical nonlinearities and instabilities in semiconductors

Quantum confined nanocrystals of PbS in glass were used as intracavity saturable absorbers to obtain passive continuous-wave mode locking in a Cr:forsterite laser. We obtained near transformed-limited 4.6 ps laser pulses at 110 MHz repetition rate, and a wide tunability range of 1207–1307 nm. The absorption saturation intensity of the quantum-dot PbS doped glasses was measured to be 0.2 MW/cm2.

PbS quantum-dot doped glasses as saturable absorbers for mode locking of a Cr:forsterite laser

A new type of optical bistability is presented. Instead of a resonator the intrinsic feedback between the optical excitation of carriers and their recombination is used. Optical bistability appears, if the absorption coefficient increases with growing carrier density. Both propagation effects and carrier diffusion are considered. Analytical model calculations are carried out for an abrupt behaviour of the absorption coefficient. The results explain the hysteresis in the transmittivity of CdS, recently observed.



Ein neuer Typ optischer Bistabilitat wird vorgestellt. Anstelle des Resonators wird die innere Ruckkopplung zwischen der optischen Anregung von Ladungstragern und ihrer Rekombination benutzt. Optische Bistabilitat entsteht, wenn der Absorptionskoeffizient mit zunehmender Tragerdichte wachst. Sowohl Ausbreitungseffekte als auch Tragerdiffusion werden betrachtet. Analytische Modellrechnungen werden fur einen abrupten Verlauf des Absorptionskoeffizienten durchgefuhrt. Die Ergebnisse erklaren die kurzlich experimentell beobachtete Hysterese im Transmissionsvermogen von CdS.

Resonatorless Optical Bistability Based on Increasing Nonlinear Absorption

Memory effect in the excitonic transmission of CdS

Visible-wavelength increasing absorption bistability at room temperature is shown due to lattice heating of CdS in a fully cw-regime. Cross talk of two beams and the switching behaviour of the bistable system are investigated. For the first time critical slowing down is demonstrated for this type of optical bistability. Quantitative theoretical investigations based on the heat flow equations yield an excellent agreement with the experimental data.



Optische Bistabilitat infolge wachsender Absorption im sichtbaren Wellenlangenbereich bei Raumtemperatur durch eine Gittererwarmung von CdS in einem vollig stationaren Regime wird gezeigt. Das Ubersprechen zwischen zwei Lichtstrahlen und das Schaltverhalten des bistabilen Systems werden untersucht. Erstmalig wird „critical slowing down” fur diesen Typ optischer Bistabilitat demonstriert. Quantitative theoretische Untersuchungen, die von der Warmeleitungsgleichung ausgehen, ergeben eine ausgezeichnete Ubereinstimmung mit den experimentellen Ergebnissen.

H. Rossmann

Papers

Resonatorless Optical Bistability Based on Increasing Nonlinear Absorption

Memory effect in the excitonic transmission of CdS

Increasing Absorption Bistability of CdS at Room Temperature

Molecular beam epitaxial growth and characterization of ZnSe on GaAs

Nonlinear optical properties of wide-gap II–VI bulk semiconductors and microcrystallites