Y. Z. Hu

Nonlinear optical properties of quantum-confined CdSe microcrystallites

Quantum-confinement effects in CdSe microcrystallites in a glass matrix are investigated in the nanosecond time domain. The results of pump–probe and single-beam absorption saturation measurements show strong evidence for electron–hole quantization in the semiconductor microstructures, a different bleaching behavior from those of bulk semiconductors and multiple quantum wells, and a trend of increased saturation intensity with decreased microcrystallite size. A theory for the one- and two-pair states in quantum dots is developed that fully includes the relevant Coulomb interactions. The theory is evaluated numerically, and results are presented for intrinsic quantum dots and for quantum dots with impurities. The experimentally observed nonlinearities are attributed to saturation of one-electron–hole-pair resonances and induced absorption caused by two-pair resonances in the presence or absence of impurities (traps).

Quasicontinuous gain in sol-gel derived CdS quantum dots

We report evidence for quasicontinuous optical gain in CdS quantum dots fabricated by the sol‐gel process and embedded in glass. The gain spectra are obtained using the pump and probe technique and nanosecond (quasiresonant) excitation at 11 K. The dots are in the intermediate quantum confinement regime and the concentration of CdS is relatively high. The gain, which is spectrally broad, develops on the low energy side of the absorption band edge. The reason why the gain region is broad is not only the size distribution of the dots, but also the nature of the gain, which originates from the recombination of several excited levels between two and one electron‐hole pairs states (i.e., biexciton to exciton). The maximum measured gain reaches 200 cm−1 at 11 K and 17 cm−1 at 170 K.

ROOM TEMPERATURE SPECTRAL HOLE BURNING AND ELIMINATION OF PHOTODARKENING IN SOL-GEL DERIVED CDS QUANTUM DOTS

Spectral hole burning at room temperature is used as a technique to compare optical nonlinearities of semiconductor quantum dot materials prepared by the sol‐gel and glass fusion techniques. In an effort to understand the mechanism of photodarkening in quantum dots and assess its effect on optical nonlinearities, we prepared and characterized three representative samples with similar dot sizes. We found that photodarkening can be reduced substantially by controlling the media surrounding the dots. The sol‐gel derived samples with 80 wt % SiO2 do not exhibit appreciable photodarkening while the melt‐quenched glasses with 56 wt % SiO2 exhibit strong photodarkening effects at room temperature which results in reduction of their optical nonlinearity by a factor of ≂20 compared with the sol‐gel derived samples.

Femtosecond optical gain in strongly confined quantum dots.

Optical gain was found in strongly confined CdSe quantum dots. As a result of a multitude of one- and two-electron-hole pair transitions, the gain region is broad and quasi-continuous and stretches below the absorption edge. We present a model for gain in a quasi-zero-dimensional quantum confined semiconductor system that agrees well with the femtosecond experiments.

Coulomb effects and optical properties of semiconductor quantum dots

Abstract Earlier results of Coulomb effects and optical properties of semiconductor quantum dots are extended to include the confinement induced valence band mixing. The electron-hole-pari energies and wavefunctions are computed combining Luttinger theory with numerical matrix diagonalization techniques. Examples of optical absorption spectra are presented showing significant modification caused by the band mixing.

Linewidth broadening factor of a microcavity semiconductor laser

The emission linewidth above threshold is measured in a GaAs/AlGaAs microcavity surface‐emitting laser with a single cavity mode. The measured linewidth broadening factor is in good agreement with theoretical calculations that include the most important many‐body Coulomb effects of the electron‐hole plasma.

OPTICAL PROPERTIES OF CdTe AND CdS QUANTUM DOTS IN GLASS

We summarize the linear and nonlinear optical properties of a variety of CdTe and CdS quantum dots in glass. The measured linear absorption of the CdTe sample is compared with calculations involving valence-band mixing due to the quantum confinement. The temperature dependence of the lowest quantum-confined transition and its linewidth for samples with various crystallite sizes are measured and compared with a simple model. It is found that the shift of the energetically lowest quantum-confined transition as a function of temperature is the same as the temperature-dependent band-gap reduction in bulk materials. Excitation of the sample with pulses ranging from femtoseconds to microseconds allows distinguishing between various mechanisms responsible for the observed optical nonlinearities. At very early times, phase-space filling and Coulomb interaction between the excited charged carriers are responsible for the absorption changes. At later times, Coulomb effects due to “trapped” carriers remain and last for nanoseconds or microseconds.

QUANTUM CONFINEMENT AND COULOMB EFFECTS IN SEMICONDUCTOR QUANTUM DOTS

Coulomb and quantum confinement effects in small semiconductor microcrystallites are analyzed. Energies and wavefunctions for one- and two-electron-hole-pair states are computed and optical absorption spectra are evaluated.

Strongly confined semiconductor quantum dots: Pair excitations and optical properties

Abstract This paper reviews a microscopic model of basic electron-hole pair excitation processes in strongly confined semiconductor quantum dots (QD) and their influence on the optical QD properties. The effects of valence band mixing, Coulomb interaction, and surface polarization are taken into account. The exciton and biexciton wave functions and energies are obtained using a numerical diagonalization method. The computed optical spectra, such as absorption, gain, pump-probe, and two-photon absorption, agree well with experiments.

Instabilities of a microcavity laser with a weak injected signal

An external cw laser signal in injected into a microcavity laser, and the dynamics of the resulting coupled oscillator system are studied. By variation of the injection detuning and intensity, interesting nonlinear behavior and injection locking are experimentally observed. A theoretical model of this system based on coupled rate equations and including many-body gain effects is presented and yields good agreement with experiment.