J. Urayama

Temperature-dependent carrier dynamics in self-assembled InGaAs quantum dots

We measured the transient temperature-dependent carrier population in the confined states of self-assembled In0.4Ga0.6As quantum dots as well as those of the surrounding wetting layer and barrier region using differential transmission spectroscopy. Results show directly that thermal reemission and nonradiative recombination contribute significantly to the dynamics above 100 K. We offer results of an ensemble Monte Carlo simulation to explain the contribution of these thermally activated processes.

Gain dynamics and ultrafast spectral hole burning in In(Ga)As self-organized quantum dots

Using a femtosecond three-pulse pump-probe technique, we investigated spectral hole-burning and gain recovery dynamics in self-organized In(Ga)As quantum dots. The spectral hole dynamics are qualitatively different from those observed in quantum wells, and allow us to distinguish unambiguously the gain recovery due to intradot relaxation and that due to carrier capture. The gain recovery due to carrier–carrier scattering-dominated intradot relaxation is very fast (∼130 fs), indicating that this is not the factor limiting the bandwidth of directly modulated quantum dot lasers.

Density and temperature dependence of carrier dynamics in self-organized InGaAs quantum dots

We have used two- and three-pulse femtosecond differential transmission spectroscopy to study the dependence of quantum dot carrier dynamics on temperature. At low temperatures and densities, the rates for relaxation between the quantum dot confined states and for capture from the barrier region into the various dot levels could be directly determined. For electron–hole pairs generated directly in the quantum dot excited state, relaxation is dominated by electron–hole scattering, and occurs on a 5 ps time scale. Capture times from the barrier into the quantum dot are of the order of 2 ps (into the excited state) and 10 ps (into the ground state). The phonon bottleneck was clearly observed in low-density capture experiments, and the conditions for its observation (namely, the suppression of electron–hole scattering for nongeminately captured electrons) were determined. As temperature increases beyond about 100 K, the dynamics become dominated by the re-emission of carriers from the lower dot levels, due to the large density of states in the wetting layer and barrier region. Measurements of the gain dynamics show fast (130 fs) gain recovery due to intradot carrier–carrier scattering, and picosecond-scale capture. Direct measurement of the transparency density versus temperature shows the dramatic effect of carrier re-emission for the quantum dots on thermally activated scattering. The carrier dynamics at elevated temperature are thus strongly dominated by the high density of the high energy continuum states relative to the dot confined levels. Deleterious hot carrier effects can be suppressed in quantum dot lasers by resonant tunnelling injection.

Nonlinear propagation of negatively chirped pulses: Maximizing the peak intensity at the output of a fiber probe

A simple phenomenological scaling behavior is found for the power dependence of the pulse width for negatively pre-chirped pulses propagating in a normally dispersive fiber; the consequences for maximizing nonlinear signals such as two-photon fluorescence excited at the fiber output are considered.

Evidence of interdot electronic tunneling in vertically coupled In0.4Ga0.6As self-organized quantum dots

Ultrafast differential transmission spectroscopy with a resonant pump reveals evidence of electronic tunneling among the excited levels of vertically aligned In0.4Ga0.6As self-organized quantum dots. This evidence of tunneling is observed as a rapid spectral redistribution of electrons within a few hundred femtoseconds of optical excitation. Measurements show that this spectral spread is independent of carrier density and, therefore, indicate that carrier–carrier scattering is not the main mechanism for carrier redistribution. Instead, electronic tunneling is responsible for the interdot coupling; tunneling rate calculations agree reasonably with the experiment, supporting this conclusion.

Differential transmission measurement of phonon bottleneck in self-assembled quantum dot intersubband relaxation

Abstract Time-resolved measurements of self-assembled In 0.4 Ga 0.6 As quantum dots indicate a phonon bottleneck between the electronic levels. The bottleneck becomes observable in a capture experiment, where fast relaxation processes are removed at low carrier densities and nongeminate capture of carriers is allowed. Carrier-density-dependent measurements confirm nongeminate capture. We perform a Monte Carlo simulation based on a random carrier capture model to explain the bottleneck signal.

Carrier dynamics in In(Ga)As/Ga(Al)As self-organized quantum dots

Self-organized quantum dots are being incorporated in the active regions of interband lasers, modulators, and infrared detectors and sources. The unique hot-carrier relaxation rates in quantum dots play an important role in defining the device characteristics. We have conducted extensive theoretical and experimental studies of carrier dynamics in In(Ga)As/Ga(Al)As self-organized quantum dots grown by molecular beam epitaxy. Experimental techniques used include small and large-signal modulation of lasers and femtosecond pump-probe spectroscopy. It is found that the intersubband electron relaxation rates, which are strongly temperature dependent, are determined by electron-hole scattering in the dots. Theoretical calculations also show that electron-hole scattering is the dominant mechanism for the relaxation of hot carriers. It is also found that a phonon bottleneck exists in the dots for very weak excitations. The implications of these results on device performance will be discussed.

Dynamics of spectral hole burning in self organized quantum dot amplifiers

Femtosecond differential transmission spectroscopy on quantum dots in the gain regime enables direct observation of the spectral hole dynamics; we can independently determine the gain recovery due to intradot carrier relaxation and capture from the barrier region.

Nonlinear propagation of negatively chirped pulses: maximizing the intensity at the output of a fiber probe

A simple phenomenological scaling behavior is found for the power dependence of the pulse width for negatively pre-chirped pulses propagating in a normally dispersive fiber; the consequences for maximizing nonlinear signals such as two-photon fluorescence excited at the fiber output are considered.

Ultrafast carrier capture and relaxation in InGaAs and InAs self-organized quantum dots

We have used femtosecond differential transmission spectroscopy to directly time-resolve the carrier capture ad intersubband relaxation in In0.4Ga0.6As/GaAs ad InAs/GaAs self-organized quantum dots.