Self-phase modulation of a single-cycle terahertz pulse by nonlinear free-carrier response in a semiconductor

We demonstrate the self-phase modulation (SPM) of a single-cycle THz pulse in a semiconductor, using bulk n-GaAs as a model system. The SPM arises from the heating of free electrons in the electric field of the THz pulse, leading to an ultrafast reduction of the plasma frequency, and hence to a strong modification of the THz-range dielectric function of the material. THz SPM is observed directly in the time domain. In the frequency domain it corresponds to a strong frequency-dependent refractive index nonlinearity of n-GaAs, found to be both positive and negative within the broad THz pulse spectrum, with the zero-crossing point defined by the electron momentum relaxation rate. We also observed the nonlinear spectral broadening and compression of the THz pulse.Comment: 5 pages, 6 figure

Exciton spin-flip rate in quantum dots determined by a modified local density of optical states

The spin-flip rate that couples dark and bright excitons in self-assembled quantum dots is obtained from time-resolved spontaneous emission measurements in a modified local density of optical states. Employing this technique, we can separate effects due to non-radiative recombination and unambiguously record the spin-flip rate. The dependence of the spin-flip rate on emission energy is compared in detail to a recent model from the literature, where the spin flip is due to the combined action of short-range exchange interaction and acoustic phonons. We furthermore observe a surprising enhancement of the spin-flip rate close to a semiconductor-air interface, which illustrates the important role of interfaces for quantum dot based nanophotonic structures. Our work is an important step towards a full understanding of the complex dynamics of quantum dots in nanophotonic structures, such as photonic crystals, and dark excitons are potentially useful for long-lived coherent storage applications.Comment: 5 pages, 4 figure

Carrier Dynamics in Submonolayer InGaAs/GaAs Quantum Dots

Carrier dynamics of submonolayer (SML) InGaAs/GaAs quantum dots (QDs) were studied by micro-photoluminecence (MPL), selectively excited photoluminescence (SEPL), and time-resolved photoluminescence (TRPL). MPL and SEPL show the coexistence of localized and delocalized states, and different local phonon modes. TRPL reveal shorter recombination lifetimes and longer capture times for the QDs with higher emission energy. This suggests that the smallest SML QDs are formed by perfectly vertically correlated 2D InAs islands, having the highest In content and the lowest emission energy, while a slight deviation from the perfectly vertical correlation produces larger QDs with lower In content and higher emission energy.Comment: 12 pages, 5 figure

Ultrafast gain dynamics in InAs/InGaAs quantum dot amplifiers

The ultrafast dynamics of gain and refractive index in an electrically pumped InAs-InGaAs quantum-dot (QD) optical amplifier are measured at room temperature using differential transmission with femtosecond time resolution. Both absorption and gain regions are investigated. While the absorption bleaching recovery occurs on a picosecond time scale, the gain compression recovers with /spl sim/100-fs time constant, making devices based on such dots promising for high-speed optical communications

Spectral hole-burning and carrier-heating dynamics in InGaAs quantum-dot amplifiers

The ultrafast gain and index dynamics in a set of InAs-InGaAs-GaAs quantum-dot (QD) amplifiers are measured at room temperature with femtosecond resolution. The role of spectral hole-burning (SHB) and carrier heating (CH) in the recovery of gain compression is investigated in detail. An ultrafast recovery of the spectral hole within /spl sim/100 fs is measured, comparable to bulk and quantum-well amplifiers, which is contradicting a carrier relaxation bottleneck in electrically pumped QD devices. The CH dynamics in the QD is quantitatively compared with results on an InGaAsP bulk amplifier. Reduced CH for both gain and refractive index dynamics of the QD devices is found, which is a promising prerequisite for high-speed applications. This reduction is attributed to reduced free-carrier absorption-induced heating caused by the small carrier density necessary to provide amplification in these low-dimensional systems