Alexander V. Uskov

The linewidth enhancement factor α of quantum dot semiconductor lasers

We show that the various techniques commonly used to measure the linewidth enhancement factor can lead to different values when applied to quantum dot semiconductor lasers. Such behaviour is a direct consequence of the intrinsic capture/escape dynamics of quantum dot materials and of the free carrier plasma effects. This provides an explanation for the wide range of values experimentally measured and the linewidth re-broadening recently measured.

Auger carrier capture kinetics in self-assembled quantum dot structures

We establish rate equations to describe Auger carrier capture kinetics in quantum dot structures, calculate Auger capture coefficients for self-assembled quantum dots, and analyze Auger capture kinetics using these equations. We show that Auger capture times can be of the order of 1–100 ps depending on barrier carrier and dot densities. Auger capture rates depend strongly on dot diameters and are greatest at dot diameters of about 10–20 nm.

Sensitivity of quantum-dot semiconductor lasers to optical feedback

The sensitivity of quantum-dot semiconductor lasers to optical feedback is analyzed with a Lang-Kobayashi approach applied to a standard quantum-dot laser model. The carriers are injected into a quantum well and are captured by, or escape from, the quantum dots through either carrier-carrier or phonon-carrier interaction. Because of Pauli blocking, the capture rate into the dots depends on the carrier occupancy level in the dots. Here we show that different carrier capture dynamics lead to a strong modification of the damping of the relaxation oscillations. Regions of increased damping display reduced sensitivity to optical feedback even for a relatively large alpha factor.

Carrier-induced refractive index in quantum dot structures due to transitions from discrete quantum dot levels to continuum states

The carrier-induced refractive index in quantum dot (QD) structures due to optical transitions from QD levels to continuum states is considered. It is shown that, for large photon energies, the refractive index change is given asymptotically by the Drude formula. Calculations of the linewidth enhancement factor, α, show that α∼1 due to this contribution to the total refractive index. Furthermore, for highly localized QD states, the absorption coefficient at the photon energies ∼0.8–1.0 eV due to these transitions can be on the order of 103 m−1.

On ultrafast optical switching based on quantum-dot semiconductor optical amplifiers in nonlinear interferometers

It is shown that interferometers containing quantum-dot semiconductor optical amplifiers can be effective for ultrafast cross-phase modulation and digital signal processing with low dependence on the specific random data pattern.

Carrier capture dynamics of InAs/GaAs quantum dots

Carrier dynamics of a 1.3μm InAs∕GaAs quantum dot amplifier is studied using heterodyne pump-probe spectroscopy. Measurements of the recovery times versus injection current reveal a power law behavior predicted by a quantum dot rate equation model. These results indicate that Auger processes dominate the carrier dynamics.

Auger carrier relaxation in self-assembled quantum dots by collisions with two-dimensional carriers

Carrier relaxation in self-assembled quantum dots due to Coulomb interaction with two dimensional (2D) carriers is studied theoretically. Auger coefficients for carrier relaxation rates are calculated in the dipole approximation for Coulomb interaction. The dipole approximation allows one to derive selection rules for Auger relaxation in a cylindrical quantum dot, and to describe a general picture of Auger relaxation via energy levels in self-assembled quantum dots. A numerical example for InAs/GaAs self-assembled quantum dots demonstrates that the Auger effect may lead to relaxation times in the order of 1–10 ps at 2D carrier densities of 1011–1012 cm−2. This result demonstrates the possibility of fast carrier relaxation in quantum dots if the carrier density in the surrounding barrier is sufficiently high. Analytical formulas for Auger coefficients are derived for moderate temperatures of the 2D carriers.

Experimental demonstration of slow and superluminal light in semiconductor optical amplifiers

Tunable delays in semiconductor optical amplifiers are achieved via four wave mixing between a strong pump beam and a modulated probe beam. The delay of the probe beam can be controlled both electrically, by changing the SOA bias, and optically, by varying the pump power or the pump-probe detuning. For sinusoidal modulated signal at 0.5 GHz, a tunable delay of 1.6 ns is achieved. This corresponds to a RF phase change of 1.6 pi. For 1.3 ns optical pulses propagating through the SOA a delay of 0.59 ns is achieved corresponding to a delay-bandwidth product exceeding 0.45. For both the cases, slow light and superluminal light are observed as the pump-probe detuning is varied.

Delay limit of slow light in semiconductor optical amplifiers

In contrast to absorbers, where the delay time via wave mixing saturates to its maximum with increasing device length due to decay of pump power along the device, in semiconductor optical amplifiers (SOAs) the delay time increases with the device length and is limited by the corresponding growth of the SOA gain, which can lead to unacceptable amplified stimulated emission in the SOA. The number of pulses which can be stored in an SOA, defined by the gain and group velocity dispersion, also is given ultimately by this SOA gain limitation, and is estimated between one and two.

Dynamics of light propagation in spatiotemporal dielectric structures

Propagation, transmission and reflection properties of linearly polarized plane waves and arbitrarily short electromagnetic pulses in one-dimensional dispersionless dielectric media possessing an arbitrary space-time dependence of the refractive index are studied by using a two-component, highly symmetric version of Maxwells equations. The use of any slow varying amplitude approximation is avoided. Transfer matrices of sharp nonstationary interfaces are calculated explicitly, together with the amplitudes of all secondary waves produced in the scattering. Time-varying multilayer structures and spatiotemporal lenses in various configurations are investigated analytically and numerically in a unified approach. Several effects are reported, such as pulse compression, broadening and spectral manipulation of pulses by a spatiotemporal lens, and the closure of the forbidden frequency gaps with the subsequent opening of wave number band gaps in a generalized Bragg reflector.