Ouri Karni

Electronic structure, morphology and emission polarization of enhanced symmetry InAs quantum-dot-like structures grown on InP substrates by molecular beam epitaxy

The optical and structural properties of a new kind of InAs/InGaAlAs/InP quantum dot (QD)-like objects grown by molecular beam epitaxy have been investigated. These nanostructures were found to have significantly more symmetrical shapes compared to the commonly obtained dash-like geometries typical of this material system. The enhanced symmetry has been achieved due to the use of an As2 source and the consequent shorter migration length of the indium atoms. Structural studies based on a combination of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) provided detailed information on both the structure and composition distribution within an individual nanostructure. However, it was not possible to determine the lateral aspect ratio from STEM or APT. To verify the in-plane geometry, electronic structure calculations, including the energy levels and transition oscillator strength for the QDs have been performed using an eight-band k·p model and realistic system parameters. The ...

Rabi oscillations in a room-temperature quantum dash semiconductor optical amplifier

Quantum coherent light-matter interactions have been at the forefront of scientific interest since the fundamental predictions of Einstein and the later work of Rabi. Direct observation of quantum coherent interactions entails probing the electronic wavefunction which requires that the electronic state of the matter does not de-phase during the measurement, a condition that can be satisfied by lengthening the coherence time or by shortening the observation time. The short de-phasing time in semiconductors has dictated that all coherent effects reported to date have been recorded directly only at cryogenic temperatures. Here we report on the first direct electronic wavefunction probing in a room-temperature semiconductor. Employing an ultrafast characterization scheme we have demonstrated Rabi oscillations and self-induced transparency in an electrically driven, room-temperature semiconductor laser amplifier, revealing the most intimate details of the light-matter interactions seen to date. The ability to employ quantum effects in solid-state media, which operate at elevated temperatures, will finally bring true quantum mechanical concepts into the realm of practical devices.

Rabi oscillations and self-induced transparency in InAs/InP quantum dot semiconductor optical amplifier operating at room temperature.

We report direct observations of Rabi oscillations and self-induced transparency in a quantum dot optical amplifier operating at room temperature. The experiments make use of pulses whose durations are shorter than the coherence time which are characterized using Cross-Frequency-Resolved Optical Gating. A numerical model which solves the Maxwell and Schrödinger equations and accounts for the inhomogeneously broadened nature of the quantum dot gain medium confirms the experimental results. The model is also used to explain the relationship between the observability of Rabi oscillations, the pulse duration and the homogeneous and inhomogeneous spectral widths of the semiconductor.

A Finite-Difference Time-Domain Model for Quantum-Dot Lasers and Amplifiers in the Maxwell–Schrödinger Framework

We describe a finite-difference time-domain (FDTD) model of a long (edge-emitting) gain medium based on a quantum-dot (QD) in-a-well structure under the framework of the Maxwell-Schrödinger equations. The model includes the dynamic behavior of a QD gain medium including an excited state incorporated within carrier rate equations and considers the carrier density dependence of the refractive index. The model enables us also to calculate carrier diffusion effects, which, unlike in quantum well based structures, play an important role in QD devices, since carrier capture and escape processes modify the effective carrier diffusion length. We present results of basic static and dynamic lasers properties as well as of the interaction of a QD amplifier with short, 150 fs pulses. We identify four regimes of operation for the pulse-QD interaction, two of which are important: the linear-saturated regime and the Rabi-oscillation dominated regime. The latter leads to Rabi floppings with a period shorter than the pulse itself. The model can be easily employed for any complicated process such as four-wave mixing, saturable absorption, semiconductor pulse laser sources, etc.

Nonlinear pulse propagation in InAs/InP quantum dot optical amplifiers: Rabi oscillations in the presence of nonresonant nonlinearities

We study the interplay between coherent light-matter interactions and non-resonant pulse propagation effects when ultra-short pulses propagate in room-temperature quantum-dot (QD) semiconductor optical amplifiers (SOAs). The signatures observed on a pulse envelope after propagating in a transparent SOA, when coherent Rabi-oscillations are absent, highlight the contribution of two-photon absorption (TPA), and its accompanying Kerr-like effect, as well as of linear dispersion, to the modification of the pulse complex electric field profile. These effects are incorporated into our previously developed finite-difference time-domain comprehensive model that describes the interaction between the pulses and the QD SOA. The present, generalized, model is used to investigate the combined effect of coherent and non-resonant phenomena in the gain and absorption regimes of the QD SOA. It confirms that in the QD SOA we examined, linear dispersion in the presence of the Kerr-like effect causes pulse compression, which counteracts the pulse peak suppression due to TPA, and also modifies the patterns which the coherent Rabi-oscillations imprint on the pulse envelope under both gain and absorption conditions. The inclusion of these effects leads to a better fit with experiments and to a better understanding of the interplay among the various mechanisms so as to be able to better analyze more complex future experiments of coherent light-matter interaction induced by short pulses propagating along an SOA.

Carrier dynamics in inhomogeneously broadened InAs/AlGaInAs/InP quantum-dot semiconductor optical amplifiers

We report on a characterization of fundamental gain dynamics in recently developed InAs/InP quantum-dot semiconductor optical amplifiers. Multi-wavelength pump-probe measurements were used to determine gain recovery rates, following a powerful optical pump pulse, at various wavelengths for different bias levels and pump excitation powers. The recovery was dominated by coupling between the electronic states in the quantum-dots and the high energy carrier reservoir via capture and escape mechanisms. These processes determine also the wavelength dependencies of gain saturation depth and the asymptotic gain recovery level. Unlike quantum-dash amplifiers, these quantum-dots exhibit no instantaneous gain response, confirming their quasi zero-dimensional nature.

Extreme nonlinearities in InAs/InP nanowire gain media: the two-photon induced laser

We demonstrate a novel laser oscillation scheme in an InAs / InP wire-like quantum dash gain medium. A short optical pulse excites carriers by two photon absorption which relax to the energy levels providing gain thereby enabling laser oscillations. The nonlinear dynamic interaction is analyzed and quantified using multi-color pump-probe measurements and shows a highly efficient nonlinear two photon excitation process which is larger by more than an order of magnitude compared to common quantum well and bulk gain media. The dynamic response of the nonlinearly induced laser line is characterized by spectrally resolved temporal response measurements, while changes incurring upon propagation in the stimulating short pulse itself are characterized by frequency resolved optical gating (FROG).

Coherent control in a semiconductor optical amplifier operating at room temperature

Quantum decoherence times in semiconductors are extremely short, particularly at room temperature where the quantum phase is completely erased in a fraction of a picosecond. However, they are still of finite duration during which the quantum phase is well defined and can be tailored. Recently, we demonstrated that quantum coherent phenomena can be easily accessed by examining the phase and amplitude of an optical pulse following propagation along a room temperature semiconductor optical amplifier. Taking the form of Rabi oscillations, these recent observations enabled to decipher the time evolution of the ensemble states. Here we demonstrate the Ramsey analogous experiment known as coherent control. Remarkably, coherent control occurs even under room temperature conditions and enables to directly resolve the dephasing times. These results may open a new way for the realization of room temperature semiconductor-based ultra-high speed quantum processors with all the advantages of upscaling and low-cost manufacturing.

Complex characterization of short-pulse propagation through InAs/InP quantum-dash optical amplifiers: from the quasi-linear to the two-photon-dominated regime

We describe direct measurements at a high temporal resolution of the changes experienced by the phase and amplitude of an ultra-short pulse upon propagation through an inhomogenously broadened semiconductor nanostructured optical gain medium. Using a cross frequency-resolved optical gating technique, we analyze 150 fs-wide pulses propagating along an InP based quantum dash optical amplifier in both the quasi-linear and saturated regimes. For very large electrical and optical excitations, a second, trailing peak is generated and enhanced by a unique two-photon-induced amplification process.

Nonlinear pulse propagation in a quantum dot laser

We investigate the nonlinear propagation of an ultra-short, 150 fs, optical pulse along the waveguide of a quantum dot (QD) laser operating above threshold. We demonstrate that among the various nonlinear processes experienced by the propagating pulse, four-wave mixing (FWM) between the pulse and the two oscillating counter-propagating cw fields of the laser is the dominant one. FWM has two important consequences. One is the creation of a spectral hole located in the vicinity of the cw oscillating frequency. The width of the spectral hole is determined by an effective carrier and gain relaxation time. The second is a modification of the shape of the trailing edge of the pulse. The wave mixing involves first and second order processes which result in a complicated interaction among several fields inside the cavity, some of which are cw while the others are time varying, all propagating in both directions. The nonlinear pulse propagation is analyzed using two complementary theoretical approaches. One is a semi-analytical model which considers only the wave mixing interaction between six field components, three of which propagate in each direction (two cw fields and four time-varying signals). This model predicts the deformation of the tail of the output signal by a secondary idler wave, produced in a cascaded FWM process, which co-propagates with the original injected pulse. The second approach is a finite-difference time-domain simulation, which considers also additional nonlinear effects, such as gain saturation and self-phase modulation. The theoretical results are confirmed by a series of experiments in which the time dependent amplitude and phase of the pulse after propagation are measured using the cross-frequency-resolved optical gating technique.