A. E. Zhukov

InAs/InGaAs quantum dot structures on GaAs substrates emitting at 1.3 μm

InAs self-organized quantum dots inserted in InGaAs quantum well have been grown on GaAs substrates by molecular beam epitaxy. The lateral size of the InAs islands has been found to be approximately 1.5 times larger as compared to the InAs/GaAs case, whereas the island heights and surface densities were close in both cases. The quantum dot emission wavelength can be controllably changed from 1.1 to 1.3 μm by varying the composition of the InGaAs quantum well matrix. Photoluminescence at 1.33 μm from vertical optical microcavities containing the InAs/InGaAs quantum dot array was demonstrated.

Gain and differential gain of single layer InAs/GaAs quantum dot injection lasers

We present gain measurements and calculations for InAs/GaAs quantum dot injection lasers. Measurements of the modal gain and estimation of the confinement factor by transmission electron microscopy yield an exceptionally large material gain of 6.8(±1)×104 cm−1 at 80 A cm−2. Calculations including realistic quantum dot energy levels, dot size fluctuation, nonthermal coupling of carriers in different dots, and band filling effects corroborate this result. A large maximum differential gain of 2×10−12 cm2 at 20 A cm−2 is found. The width of the gain spectrum is determined by participation of excited quantum dot states. We record a low transparency current density of 20 A cm−2. All experiments are carried out at liquid nitrogen temperature.

Multiphonon‐relaxation processes in self‐organized InAs/GaAs quantum dots

We report on optical studies of relaxation processes in self‐organized InAs/GaAs quantum dots (QDs). Near resonant photoluminescence excitation spectra reveal a series of sharp lines. Their energy with respect to the detection energy does not depend on QD size and their energy separations are close to the InAs LO phonon energy of 32.1 meV estimated for strained pyramidal InAs QDs. The shape of the PLE spectra is explained by multiphonon relaxation processes involving LO phonons of the QD as well as of the wetting layer, an interface mode, and low frequency acoustical phonons.

QUANTUM DOT LASERS: BREAKTHROUGH IN OPTOELECTRONICS

Abstract Semiconductor heterostructures with self-organized quantum dots (QDs) have experimentally exhibited properties expected for zero-dimensional systems. When used as active layer in the injection lasers, these advantages help to strongly increase material gain and differential gain, to improve temperature stability of the threshold current, and to provide improved dynamic properties. Molecular beam epitaxy (MBE) represents a developed technology well suited for fabrication of self-organized QDs. Optimization of deposition parameters can ensure that the self-organized islands are small (∼10 nm), have a similar size and shape and form dense arrays. Saturation material gain is as high as 150000 cm −1 compared with QW values of about 3000 cm −1 . Maximum differential gain reported for QD lasers approaches 10 −12 cm 2 and exceeds the QW laser values by about three orders of magnitude. Direct observation of relaxation oscillations reveals present cut-off frequencies close to 10 GHz. High internal (>96%) and differential (70%) efficiencies at 300 K are realized. Using the novel concept of electronically-coupled QDs and oxide-defined 10 μm apertures, CW lasing with J th =180 A/cm 2 , is realized in surface-emitting QD lasers (300 K). Wall-plug efficiencies are up to 16%. Total currents as low as 68 μA are measured for 1μm apertures. GaAs-based lasers for the 1.3 μm range with low J th (65 A/cm 2 ) at room temperature (RT) are realized using InAs/InGaAs/GaAs QDs obtained by activated spinodal decomposition. In stripes the lasing occurs via the QD ground state ( J th =90 A/cm 2 ) for cavity lengths L >1 mm (uncoated). Differential efficiency is 55% and internal losses are 1.5 cm −1 . A characteristic temperature near RT is 160 K. 3W CW operation at RT is achieved. The recent progress in lasers based on self-organized MBE QDs already made it possible to fabricate devices with dramatically improved characteristics as compared to recent QW devices for the most important commercial applications.

Quantum-dot heterostructure lasers

Quantum-dot (QD) heterostructures are nanoscale coherent insertions of narrow-gap material in a single-crystalline matrix. These tiny structures provide unique opportunities to modify and extend all basic principles of heterostructure lasers and advance their applications. Despite early predictions, fabrication of QD heterostructure (QDHS) lasers appeared to be a much more challenging task, as compared to quantum well (QW) devices. The breakthrough occurred when techniques for self-organized growth of QDs allowed the fabrication of dense arrays of coherent islands, uniform in shape and size, and, simultaneously, free from undesirable defects. Recently, the figure of merit of QDHS lasers surpasses some of the key characteristics of QW devices in some of the most important applications.

InAs–GaAs Quantum Pyramid Lasers: In Situ Growth, Radiative Lifetimes and Polarization Properties

We have realized injection lasers based on InAs–GaAs and InGaAs–GaAs quantum pyramids (QPs) with a lateral size ranging from 80 to 140 A. The structures with relatively small dots (~80 A) exhibit properties predicted earlier for quantum dot (QD) lasers such as low threshold current densities (below 100 Acm-2) and ultrahigh characteristic temperatures (T0=350–425 K). For operation temperatures above 100–130 K, T0 decreases and the threshold current density increases (up to 0.95–3.3 kAcm-2 at room temperature) due to carrier evaporation from QPs. Larger InAs QPs (~140 A) providing better carrier localization exhibit saturation of the ground-state emission and enhanced nonradiative recombination rate at high excitation densities. The radiative lifetime shows a weak dependence on the dot size in the range 80–140 A being close to ~1.8–2 ns, respectively. A significant decrease in radiative lifetime is realized in vertically coupled quantum dots formed by a QP shape-transformation effect. The final arrangement corresponds to a three-dimensional tetragonal array of InAs islands inserted in a GaAs matrix each composed of several vertically merging InAs parts. We achieved injection lasing in such an array for the first time.

High-power picosecond and femtosecond pulse generation from a two-section mode-locked quantum-dot laser

We demonstrate mode locking in a two-section quantum-dot laser that produces output powers up to 45 mW at 1260 nm. The pulse duration could be varied from 2 ps to as short as 400 fs at the 21 GHz pulse repetition rate.

Structural and optical properties of InAs-GaAs quantum dots subjected to high temperature annealing

Annealing at higher temperature (700 °C) of structures with two‐dimensional and three‐dimensional arrays in InAs–GaAs quantum dots (QDs) results in an increase in the size and in a corresponding decrease in the indium composition of the QDs. The change in the In composition is monitored by the contrast pattern in the plan‐view transmission electron microscopy (TEM) images viewed under the strong beam imaging conditions. Increase in the size of the QDs is manifested by the plan‐view TEM images taken under [001] zone axis illumination as well as by the cross‐section TEM images. We show that the dots maintain their geometrical shape upon annealing. Luminescence spectra demonstrate a shift of the QD luminescence peak toward higher energies with an increase in the annealing time (10–60 min) in agreement with the decrease in indium composition revealed in TEM studies. The corresponding decrease in the QD localization energy results in an effective evaporation of carriers from QDs at room temperature, and the in...

Continuous-wave operation of long-wavelength quantum-dot diode laser on a GaAs substrate

Continuous-wave operation near 1.3 /spl mu/m or a diode laser based on self-organized quantum dots (QDs) on a GaAs substrate is demonstrated. Multiple stacking of InAs QD planes covered by thin InGaAs layers allows us to prevent gain saturation and achieve long-wavelength lasing with low threshold current density (90-105 A/cm/sup 2/) and high output power (2.7 W) at 17/spl deg/C heatsink temperature. It is thus confirmed that QD lasers of this kind are potential candidates to substitute InP-based lasers in optical fiber systems.

Long-wavelength lasing from multiply stacked InAs/InGaAs quantum dots on GaAs substrates

An InAs quantum dot (QD) array covered by a thin InGaAs layer was used as the active region of diode lasers grown by molecular beam epitaxy on GaAs substrates. The wavelength of the ground-state transition in such heterostructures is in the 1.3 μm range. In the laser based on the single layer of QDs, lasing proceeds via the excited states due to insufficient gain of the ground level. Stacking of three QD planes prevents gain saturation and results in a low threshold (85 A/cm2 in broad-area 1.9-mm-long stripe) long-wavelength (1.25 μm) lasing at room temperature via the QD ground state with relatively high differential efficiency (>50%).