Amir Capua

Static gain saturation in quantum dot semiconductor optical amplifiers.

Measurements of saturated amplified spontaneous emission-spectra of quantum dot semiconductor optical amplifiers demonstrate efficient replenishment of the quantum-dot ground state population from excited states. This saturation behavior is perfectly modeled by a rate equation model. We examined experimentally the dependence of saturation on the drive current and the saturating optical pump power as well as on the pump wavelength. A coherent noise spectral hole is observed with which we assess dynamical properties and propose optimization of the SOA operating parameters for high speed applications.

Direct correlation between a highly damped modulation response and ultra low relative intensity noise in an InAs/GaAs quantum dot laser

We describe modulation responses and relative intensity noise (RIN) spectra of an InAs/GaAs quantum dot laser operating near 1300 nm. A very large nonlinear gain compression coefficient yields a highly damped modulation response with a maximum 3 dB bandwidth of ~6.5 GHz and flat RIN spectra which reach as low a level as -158/-160 dB/Hz at frequencies up to 10 GHz.

Cross talk free multi channel processing of 10 Gbit/s data via four wave mixing in a 1550 nm InAs/InP quantum dash amplifier

We demonstrate multi wavelength processing in a broad band 1550 nm quantum dash optical amplifier. Two 10 Gbit/s signals, spectrally separated by 30 nm are individually wavelength converted via four wave mixing with no cross talk.

Determination of intrinsic damping of perpendicularly magnetized ultrathin films from time resolved precessional magnetization measurements

Magnetization dynamics are strongly influenced by damping. An effective damping constant {\alpha}eff is often determined experimentally from the spectral linewidth of the free induction decay of the magnetization after the system is excited to its non-equilibrium state. Such an {\alpha}eff, however, reflects both intrinsic damping as well as inhomogeneous broadening. In this paper we compare measurements of the magnetization dynamics in ultrathin non-epitaxial films having perpendicular magnetic anisotropy using two different techniques, time-resolved magneto optical Kerr effect (TRMOKE) and hybrid optical-electrical ferromagnetic resonance (OFMR). By using an external magnetic field that is applied at very small angles to the film plane in the TRMOKE studies, we develop an explicit closed-form analytical expression for the TRMOKE spectral linewidth and show how this can be used to reliably extract the intrinsic Gilbert damping constant. The damping constant determined in this way is in excellent agreement with that determined from the OFMR method on the same samples. Our studies indicate that the asymptotic high-field approach that is often used in the TRMOKE method to distinguish the intrinsic damping from the effective damping may result in significant error, because such high external magnetic fields are required to make this approach valid that they are out of reach. The error becomes larger the lower is the intrinsic damping constant, and thus may account for the anomalously high damping constants that are often reported in TRMOKE studies. In conventional ferromagnetic resonance (FMR) studies, inhomogeneous contributions can be readily distinguished from intrinsic damping contributions from the magnetic field dependence of the FMR linewidth. Using the analogous approach, we show how reliable values of the intrinsic damping can be extracted from TRMOKE.

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.

A nearly instantaneous gain response in quantum dash based optical amplifiers

We describe a unique phenomenon occurring in InAs/InP quantum dash amplifiers: a nearly instantaneous gain response taking place across the entire inhomogeneously broadened gain spectrum. The phenomenon is induced by a 100–150 fs pulse and is observable using multiwavelength pump probe measurements. Large bias levels and an intense pump pulse are imperative for enacting the ultrafast mechanism which is due to a combination of the gain inhomogeneity, nonlinear two photon absorption and the quantum wirelike density of state function.

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.

40 GHz small-signal cross-gain modulation in 1.3 μm quantum dot semiconductor optical amplifiers

Small-signal cross-gain modulation of quantum dot based semiconductor optical amplifiers (QD SOAs), having a dot-in-a-well structure, is presented, demonstrating superiority for ultrahigh bit rate wavelength conversion. Optimization of the QD SOA high speed characteristics via bias current and optical pump power is presented and a small-signal 3 dB bandwidth exceeding 40 GHz is demonstrated. The p-doped samples investigated here enable small-signal wavelength conversion within a range of 30 nm, limited mainly by the gain bandwidth.