A. Tierno

Saturation of absorption and gain in a quantum dot diode with continuous-wave driving

The nonlinear interaction of laser light with self-assembled quantum dots (QD) is an important subject of current research. Significant work has been done on gain saturation due to the relevance for semiconductor optical amplifiers (SOAs). In contrast, studies under absorptive conditions seem to be limited either to pulsed excitation for semiconductor saturable absorption mirror (SESAM) applications, or to cryogenic temperatures and single dots due to their relevance for quantum information devices. However, room temperature absorptive and refractive index nonlinearities of ensembles of QD without inversion might be useful for photonic devices displaying optical bistability, optical pattern formation, spatial solitons or slow light [1–3]. Here, we are reporting on the saturation of the cw absorption of InAs QD close to the telecommunication O-band.

High density InAlAs/GaAlAs quantum dots for non-linear optics in microcavities

Structural and optical properties of InAlAs/GaAlAs quantum dots grown by molecular beam epitaxy are studied using transmission electron microscopy and temperature- and time-resolved photoluminescence. The control of the recombination lifetime (50 ps-1.25 ns) and of the dot density (5.10–8-2.1011 cm–3) strongly suggest that these material systems can find wide applications in opto-electronic devices as focusing non-linear dispersive materials as well as fast saturable absorbers.

Bistability and opto-thermal-pulsations in a quantum-dot edge-emitting laser diode

Self-sustained pulsations in the output of an InAs quantum dot laser diode in the MHz range are reported for the first time. The characteristics (shape, range and frequency) are presented for the free running laser and when optical feedback in the Littrow configuration is applied. The time scale suggest that these are opto-thermal pulsation similar to those reported in quantum well amplifiers. Bistability in the light-current characteristics is observed for wavelengths smaller the gain peak (λ = 1225 nm), but it is not present for wavelength above the gain peak and for the free running lasers.

Time-resolved spectra of a self-pulsing quantum dot laser

Self-sustained pulsations in the output of an InAs quantum dot laser diode in the MHz range are reported for the first time. The characteristics (shape, range and frequency) are presented for the free running laser and when optical feedback in the Littrow configuration is applied. The frequency resolved optical spectra reveal different envelope shifts between the two cases. This might be related to a change of phase-amplitude coupling across the gain maximum in agreement with the expectation for a two level system. The time scale and bifurcation scenario suggest that these are opto-thermal pulsation like those reported in quantum well amplifiers.

Optical and electrical properties of stacked binary InAs-GaAs quantum dot structures prepared under Surfactant-mediated growth conditions

The structural, optical and electrical properties of a 10-layer InAs/GaAs quantum dots (QDs) system having InAs layers (2.9ML) grown under surfactant growth conditions, using only an impinging In beam, were investigated. This growth mode still resulted in the formation of quantum dots, but with dot sizes smaller and sample quality better than those for normal growth (NG) of ~3ML InAs-GaAs QD structures. Room temperature photoluminescence measurements showed PL emission from this sample at 1200-1300 nm, i.e. reaching the telecom O-band. At low substrate growth temperatures (LT), 250?C, and under the same Arsenic free growth condition an InAs/GaAs superlattice structure without the formation of QD was successfully grown with up to 2.9MLs of InAs, which was not achievable under NG conditions. Both samples showed a noticeable photocurrent when illuminated with 1.2 ? 1.3 ?m lasers. Thus they can be used as photoconductive materials that can be excited with wavelength longer than that used for the well-known LT-GaAs ultrafast material and close to the telecom wavelengths of 1.3 ?m for Terahertz imaging or other optoelectronic applications.

Low- noise adaptive optics for gravitational wave interferometers

We present the results of an adaptive optics prototype system used to test the feasibility of active control for corrections of geometrical fluctuations of a gravitational wave interferometers input laser beam. It is shown that the efficiency of the system in correcting fluctuations extends up to 80 Hz unity gain frequency and that the upper limit of reintroduced noise is within the Virgo interferometer requirements.

Nonlinear Optics and Saturation Behavior of Quantum Dot Samples Under Continuous Wave Driving

The nonlinear optical response of self-assembled quantum dots (QD) is relevant to the application of QD-based devices in nonlinear optics, all-optical switching, slow light, and self-organization. Theoretical investigations are based on numerical simulations of a spatially and spectrally resolved rate equation model, which takes into account the strong coupling of the quantum dots to the carrier reservoir created by the wetting layer (WL) states. The complex dielectric susceptibility of the ground state is obtained. The saturation is shown to follow a behavior in between the one for a dominantly homogeneously and inhomogeneously broadened medium. Approaches to extract the nonlinear refractive index change by fringe shifts in a cavity or self-lensing are discussed. Experimental work on saturation characteristic of InGa/GaAs quantum dots close to the telecommunication O-band (1240–1280 nm) and of InAlAs/GaAlAs QD at 780 nm is described and the first demonstration of the cw saturation of absorption in room temperature QD samples is discussed in detail.

Nonlinear phase-shifts in self-assembled quantum dot samples under CW driving

We analyze the strength of the nonlinear refractive index change associated with the saturation of the ground state absorption or gain of self-assembled quantum dot samples under cw optical injection. The model is based on a two-level framework but takes into account the strong coupling of quantum dots to the wetting layer by thermal excitations and Auger processes. Due to the inhomogeneous broadening and limited density the effect turns out to be weak though it should be experimentally detectable using self-lensing, the shift of Fabry-Perot resonances or interferometric schemes.

Nonlinear optics and self-lensing in self-assembled quantum dots with cw driving

Quantum dots (QD) offer new and promising possibilities for device applications in the optical communication 1.3 µm wavelength range. We are currently theoretically investigating the nonlinear optics of QD, in the continuous wave regime, of thin and thick quantum dot samples to have information on transmission properties, saturation parameter, self-lensing and nonlinear refractive index change. In particular we are exploiting whether self lensing can be used as indication for the nonlinear refractive index change. The long-term aim to use self-lensing in QD to have cavity solitons as predicted in [1]. The quantum dot material parameters used in the simulation are typical for the ones grown by Innolume and used e.g. in [2]. They are made of 10 layers of InAs quantum dots inside GaInAs quantum wells with size and composition of the dots such that the peak of the emission is around 1280 nm. The sheet density is of the order of 5×1010 cm2. We follow a model developed by [1] that describes QD as a collection of inhomogeneously broadened two-level systems characterized by a homogeneous linewidth γ peaked around a frequency ω0 with coupling to a wetting layer. For a starting point we focus on a short sample so we do not need to consider diffraction and diffusion inside the medium. We calculated the response of these dots to a Gaussian beam of certain beam radius (15 µm), real and imaginary part of susceptibility as function of space. From that we obtain the transmission through these samples in absorption and gain regimes (Fig. 1). Fitting a suitable saturable absorption model we obtain absorption coefficient and saturation intensity as function of the detuning of the beam. The saturation is shown to follow a behavior in between the one for a dominantly homogeneously and inhomogeneously broadened medium.

Fast and low noise adaptive optics system for the correction of micro-aberrations of laser beam

We present the results of an Adaptive Optics prototype system used to test the feasibility of active control for corrections of geometrical aberrations of gravitational wave interferometers input laser beam. It is shown that the efficiency of the system in correcting fluctuations extends up to 80 Hz unity gain frequency and that the upper limit of reintroduced noise is within gravitational wave interferometers requirements.