Vitalii Sichkovskyi

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.

Temperature-Insensitive High-Speed Directly Modulated 1.55-

Modulation properties and temperature stability of short cavity ridge waveguide lasers based on high-quality InAs quantum dots exhibiting a total modal gain of ~90 cm-1 are reported. The 338-μm-long lasers show a threshold current of 20 mA at room temperature and an output powers of up to 36 mW. A maximum small signal modulation bandwidth of 15 GHz was obtained at 14 °C, which degrades to 13 GHz at 60 °C and 8 GHz at 80 °C. Digital modulation at 25 Gb/s between 15 °C and 50 °C was obtained with clear open eyes under constant drive conditions (dc and ac). The maximum data rates of 32 and 35 Gb/s were obtained for 338- and 230-μm-long lasers, respectively, at 14 °C.

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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.

Quantum Dot Lasers

The charge carrier dynamics of improved InP-based InAs/AlGaInAs quantum dot (QD) semiconductor optical amplifiers are examined employing the multi-wavelength ultrafast pump-probe measurement technique. The transient transmission response of the continuous wave probe shows interesting dynamical processes during the initial 2-3 ps after the pump pulse, when carriers originating from two photon absorption contribute the least to the recovery. The effects of optical excitations and electrical bias levels on the recovery dynamics of the gain in energetically different QDs are quantified and discussed. The experimental observations are validated qualitatively using a comprehensive finite-difference time-domain model by recording the time evolution of the charge carriers in the QDs ensemble following the pulse.

Coherent control in a semiconductor optical amplifier operating at room temperature

The ever-growing need for higher data rates is a driving force for the implementation of higher order coherent communication formats. A key element in coherent detection is the local oscillator (LO) of the decoding unit. This device has to provide coherent light with a narrow linewidth in order to distinguish between different phase and amplitude states of the incoming signal. As predicted by theory, a drastic linewidth reduction is expected from quantum dot (QD) laser materials by the quasi zero-dimensional nature of the gain function. The impact of different gain materials consisting of different numbers of QD layers on the linewidth of distributed feedback (DFB) lasers was investigated and shows an unambiguous dependence on the layer design. Intrinsic linewidths as low as 110 kHz could be determined.

Ultra-fast charge carrier dynamics across the spectrum of an optical gain media based on InAs/AlGaInAs/InP quantum dots

Self-organized InAs quantum dot (QD) lasers based on InP substrate were grown by means of solid source molecular beam epitaxy (SSMBE). Six InAs QD layers with high dot density and highly uniform dot sizes were used as active medium. Broad area (BA) and ridge waveguide (RWG) lasers with different cavity lengths were processed and characterized. Also the influence of a post-growth rapid thermal annealing (RTA) process on the laser characteristics was investigated. The lasers showed a high modal gain of 12 - 14.5 cm-1 per dot layer and a threshold current density for infinite cavity length of 120 A/cm2 per dot layer. In pulsed operation, as-cleaved BA lasers with a cavity length of 292 μm can be operated up to 120 °C. High characteristic temperature values were obtained with T0 = 125 K (20 °C to 45 °C) and T0 = 100 K up to 120 °C. The slope efficiency of about 0.28 W/A can be kept constant over a wide operating temperature range of up to 100 °C. Mounted RWG lasers with 388 μm cavity length and operated in pulsed mode showed a maximum output power of 120 mW a slope efficiency of 0.42 W/A at 15 °C. The lasers can be operated at 150 °C with 25 mW output power. These results demonstrate very well the temperature insensitive lasing performance expected in nearly ideal QD lasers due to the high density of states localized at the transition energy, which allow a very robust ground state lasing.

Narrow-linewidth 1.5μm quantum dot distributed feedback lasers

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.

1.5μm quantum dot laser material with high temperature stability of threshold current density and external differential efficiency

Carrier relaxation in self-assembled InAs/In0.53Ga0.23Al0.24As/InP(001) quantum dots emitting at 1.55 μm and quantum dots coupled to the In0.64Ga0.36As/In0.53Ga0.23Al0.24As quantum well through a thin In0.53Ga0.23Al0.24As barrier is investigated employing high-temporal-resolution (< 0.3 ps), time-resolved spectroscopic techniques at cryogenic temperatures, supported additionally with photoluminescence, photoluminescence excitation, and theoretical modelling. We focused on intra-band carrier relaxation pathways that solely determine the observed non-equilibrium carrier population kinetics. We ascertained relatively fast carrier capture and intra-band relaxation process in a reference structure with quantum dots only (∼8 ps time constant) and even faster initial relaxation in the coupled system (∼4 ps). An evident bottleneck effect is observed for the final relaxation stage in the coupled quantum dots-quantum well system slowing down the overall relaxation process by a factor of 5. The effect is attributed ...

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

A monolithically integrated widely tunable narrow-linewidth light source was realized on an InP-based quantum dot (QD) gain material. The quasi zero-dimensional nature of QDs and the resulting low linewidth enhancement factor enabled standalone distributed feedback (DFB) lasers with intrinsic linewidths as low as 110 kHz. An integrated device comprising four DFB lasers with on-chip micro-heaters, a 3 dB-coupler network, and a semiconductor optical amplifier (SOA), which covers the entire C+ telecom band, exhibits a linewidth of below 200 kHz independent of the SOA operation current.

Carrier relaxation bottleneck in type-II InAs/InGaAlAs/InP(001) coupled quantum dots-quantum well structure emitting at 1.55 μm

Due to the discrete density of states distribution and spatial localization of carriers in quantum dot (QD) material, the dynamics should be strongly enhanced in comparison to quantum well material. Based on improved 1.5 μm InAs/InGaAlAs/InP QD gain material short cavity ridge waveguide lasers were fabricated. Devices with cavity, lengths of 230 to 338 μm with high reflection coatings on the backside exhibit record value for any QD laser in small and large signal modulation performance with up to 15 GHz and 36 GBit/s, respectively, obtained at 14 °C. Due to the high temperature stability of threshold current and external differential efficiency, the lasers exhibit also nearly constant modulation bandwidth between 14-60 °C.