Alexey E. Zhukov

Quantum dot lasers

We presents both edge emitting and surface emitting quantum dot laser research. Growth is by both MOCVD and MBE.

GaAs-based long-wavelength lasers

The present paper reviews recent achievements in the fabrication of diode lasers for the near-infrared range on GaAs substrates. 1.3??m light emitters are currently widely used in fibre-optic communication systems. GaAs-based devices are potentially advantageous compared to their InGaAsP counterparts in several aspects, such as improvement of thermal stability, possibility to grow vertical-cavity surface-emitting lasers in a single growth run and the use of large-area high-quality inexpensive GaAs substrates. Three main approaches have been suggested so far to achieve the 1.3??m emission from structures grown on GaAs substrates. They are InGaAs and GaAsSb quantum wells, GaInAsN quantum wells and InAs/GaAs quantum dots. In the present paper we discuss all these approaches including material growth, optical properties and laser characteristics. The results obtained by these methods are compared and their potential advantages discussed.

Negative characteristic temperature of InGaAs quantum dot injection laser

The range of negative characteristic temperatures in temperature dependences of threshold current density of low-threshold (In, Ga)As/(Al, Ga)As quantum dot injection lasers has been observed. A model describing the decrease in threshold current density with temperature at low temperatures is proposed.

Direct modulation and mode locking of 1.3 μm quantum dot lasers

We report 7u2009GHz cut-off frequency, 2.5 and 5u2009Gbu2009s−1 eye pattern measurements upon direct modulation of 1.3u2009μm quantum dot lasers grown without incorporating phosphorus in the layers. Passive mode-locking is achieved from very low frequencies up to 50u2009GHz and hybrid mode-locking is achieved up to 20u2009GHz. The minimum pulse width of the Fourier-limited pulses at 50u2009GHz is 3u2009ps, with an uncorrelated timing jitter below 1u2009ps. The lasers are optimized for high frequency operation by a ridge waveguide design that includes etching through the active layer and ridge widths down to 1u2009μm. The far-field shape for 1u2009μm is close to circular with a remaining asymmetry of 1.2.

QD lasers: physics and applications

Quantum dot (QDs) heterostructures structurally represent tiny 3D insertions of a narrow bandgap material, coherently embedded in a wide-bandgap single-crystalline matrix. The QDs are produced by conventional epitaxial techniques applying self-organized growth and behave electronically as artificial atoms. Strain-induced attraction of QDs in different rows enables vertically-coupled structures for polarization, lifetime and wavelength control. Overgrowth with ternary or quaternary alloy materials allows controllable increase in the QD volume via the island-activated alloy phase separation. Repulsive forces during overgrowth of QDs by a matrix material enable selective capping of coherent QDs, keeping the defect regions uncapped for their subsequent selective evaporation. Low-threshold injection lasing is achieved up to 1350 nm wavelength at 300K using InAs-GaAs QDs. 8 mW VCSELs at 1.3 μm with doped DBRs are realized. Edge-emitters demonstrate 10 GHz bandwidth up to 70°C without current adjustment. VCSELs show ~4 GHz relaxation oscillation frequency. QD lasers demonstrate above 3000 h of CW operation at 1.5 W at 45°C heat sink temperature without degradation. The defect reduction technique (DRT) applied to thick layers enables realization of defect-free structures on top of dislocated templates. Using of DRT metamorphic buffer layers allowed 7W GaAs-based QD lasers at 1500 nm.

Long-wavelength (1.3-1.5 micron) quantum dot lasers based on GaAs

The molecular beam epitaxy of self-assembled quantum dots (QDs) has reached a level such that the principal advantages of QD lasers can now be fully realized. We overview the most important recent results achieved to date including excellent device performance of 1.3 μm broad area and ridge waveguide lasers (Jth<150A/cm2, Ith=1.4 mA, differential efficiency above 70%, CW 300 mW single lateral mode operation), suppression of non-linearity of QD lasers, which results to improved beam quality, reduced wavelength chirp and sensitivity to optical feedback. Effect of suppression of side wall recombination in QD lasers is also described. These effects give a possibility to further improve and simplify processing and fabrication of laser modules targeting their cost reduction. Recent realization of 2 mW single mode CW operation of QD VCSEL with all-semiconductor DBR is also presented. Long-wavelength QD lasers are promising candidate for mode-locking lasers for optical computer application. Very recently 1.7-ps-wide pulses at repetition rate of 20 GHz were obtained on mode-locked QD lasers with clear indication of possible shortening of pulse width upon processing optimization. First step of unification of laser technology for telecom range with QD-lasers grown on GaAs has been done. Lasing at 1.5 μm is achieved with threshold current density of 0.8 kA/cm2 and pulsed output power 7W.

Multi-Stacked InAs/InGaAs/InP Quantum Dot Laser (Jth=11 A/cm2, λ=1.9 µm (77 K))

Self-organized InAs quantum dots inserted in an (In, Ga)As matrix lattice matched to InP substrate were used as an active region of an injection laser. Low threshold (11 A/cm2) lasing at 1.9 nm (77 K) via the quantum dot states was realized. Temperature dependencies of the main laser parameters demonstrate the important role of the nonradiative recombination. An analysis of basic mechanisms of leakage shows that the Auger recombination share is negligible.

Anomalous dynamic characteristics of semiconductor quantum-dot lasers generating on two quantum states

Dynamics of the emission spectra of semiconductor quantum dot (QD) lasers generating on two quantum states has been experimentally studied. Being pumped with 30-ns current pulses, a QD laser ceased to generate 2–5 ns after switch-on and exhibited a pause up to 10 ns or longer, depending on the pumping pulse amplitude. After the subsequent switch-on, the laser generated short (200–300 ps) pulses of emission from the excited state of QDs followed by minima of comparable duration (dark pulses) corresponding to the ground-state emission. This behavior is explained in terms of the laser Q-switching due to the charge carrier density redistribution between the excited and ground states.

GaAsN-on-GaAs MBE using a DC plasma source

A new dc plasma source for MBE growth of GaAsN layers is suggested. The efficiency of nitrogen incorporation, crystal perfection, surface morphology, and luminescent properties of the epilayers vs. operation conditions of the source are studied.

Room Temperature Lasing in 1-μm Microdisk Quantum Dot Lasers

Lasing characteristics of InAs/InGaAs quantum dot microdisks with diameter varied from 1 to 6 μm were studied under optical pumping. The disks were fabricated with standard photolithography and two-step wet etching. We demonstrate room temperature lasing in the 1.29-1.32-μm wavelength range (ground-state transition) in microlasers as small as 1 μm in diameter. The microlasers demonstrate narrow linewidths (40-60 pm), low thermal impedance (85°C/mW), and low threshold powers (50-100 μW).