Vitalii Ivanov

High gain 1.55 μm diode lasers based on InAs quantum dot like active regions

InP diode lasers with InAs quantum dot (QD) like active regions emitting at 1.55 μm have been fabricated. The QDs were grown in an As2 mode, which reduces the degree of elongation of the nanospecies yielding nearly circular shapes. Lasers with four to six dot layers show low absorption αi<10 cm−1 and high modal gain Γg0 of 10 cm−1 per QD layer (QDL) and above. The high gain values are compatible with an inhomogeneous linewidth that is much narrower than in quantum dash material, which is the common nanoscale gain material in the InP system.

High Speed 1.55 μm InAs/InGaAlAs/InP Quantum Dot Lasers

We report static and dynamic characteristics of InAs/InP quantum dot (QD) lasers emitting near 1.55 μm. The gain section was optimized for a high speed operation using a unique spatially resolved model. The measured modulation capability dependence on structural parameters (barrier width and the number of QD layers) is consistent with the model predictions. Short cavity lasers with a modal gain of more than 10 cm-1 per dot layer exhibit a small signal modulation bandwidth above 9 GHz and large signal modulation at up to 22 Gb/s with an on/off ratio of 3 dB.

High-Speed Low-Noise InAs/InAlGaAs/InP 1.55-

We present the static and dynamic properties of InAs quantum-dot (QD) lasers emitting near 1.55 μm. The used laser material comprises four QD layers and exhibits a high modal gain of about 40 cm-1. The 340-μ.m-long lasers show a room temperature threshold current of 38 mA and a maximum output power of 16 mW. The small signal modulation response is highly damped and carrier transport limited with a moderate 3-dB bandwidth of 5 GHz. This is accompanied by a flat relative intensity noise spectrum at a low level of -150 dBc/Hz. Neverthe- less, the laser exhibits record large signal modulation capabilities for a 1.5-μ.m QD laser: 15 Gb/s with a 4-dB on/off ratio.

\mu{\rm m}

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.

Quantum-Dot Lasers

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.

Rabi oscillations and self-induced transparency in InAs/InP quantum dot 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.

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

We demonstrate the ability to control quantum coherent Rabi-oscillations in a room-temperature quantum dot semiconductor optical amplifier (SOA) by shaping the light pulses that trigger them. The experiments described here show that when the excitation is resonant with the short wavelength slope of the SOA gain spectrum, a linear frequency chirp affects its ability to trigger Rabi-oscillations within the SOA: A negative chirp inhibits Rabi-oscillations whereas a positive chirp can enhance them, relative to the interaction of a transform limited pulse. The experiments are confirmed by a numerical calculation that models the propagation of the experimentally shaped pulses through the SOA.

Coherent control in a semiconductor optical amplifier operating at room temperature

Based on a novel quantum dot (QD) growth technique, high density dot-like QDs were grown on (100)InAlGaAs/InP surfaces, which resulted in a strongly improved modal gain in 1.55 μm QD lasers. The influence of the number of QD layers on static properties, e.g., modal gain, threshold current density and spectral properties, are presented and discussed. For a large number of QD layers, e.g., 6 QD layers, a high modal gain of > 70cm-1 could be obtained. By reducing the number of QD layers, i.e., lowering the modal gain, the wavelength shift with temperature can be reduced to < 0.2 nm/K. Systematic dependence of laser properties on structural parameters is observed.

Coherent control in room-temperature quantum dot semiconductor optical amplifiers using shaped pulses

With high modal gain InAs/AlGaInAs/InP quantum dot laser material short cavity ridge waveguide lasers were fabricated with cavity lengths down to 275 μm. These devices show new record values in direct digital signal modulation at 22 GBit/s. In addition also strong improvement are expected from this laser material in the static properties, in particular on the linewidth due to the suppression of the linewidth enhancement factor. First distributed feedback lasers on similar quantum dot laser material were processed and preliminary linewidth measurements indicate a significant linewidth reduction. Quantitative investigations are under way and will be presented at the conference.

High modal gain 1.5 μm InP based quantum dot lasers: dependence of static properties on the active layer design

The dynamical properties of InP-based quantum dot (QD) gain media are surveyed and analyzed for three time scales ranging from tens of picoseconds to less than 200 fs, where quantum coherent phenomena are observable. Each of these time scales determines the important properties of QD devices, i.e., modulation capabilities, nonlinear gain, and coherent light-matter interactions. Experimental investigations and modeling results are described, highlighting the state of the art in QD devices for the important 1550-nm telecom wavelength range.