M. V. Maximov

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.

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.

Nanoimprint lithography: an alternative nanofabrication approach

A status report of nanoimprint lithography is given in the context of alternative nanofabrication methods. Since the ultimate resolution of nanoimprint appears to be determined by the stamp, this is discussed in detail, particularly the recent developments on polymer stamps. The scope of the technique is illustrated with applications in passive optical structures and organic devices. Throughout the report, critical dimensions are discussed, as well as other challenges facing nanoimprint lithography.

High power temperature-insensitive 1.3 µm InAs/InGaAs/GaAs quantum dot lasers

We report on GaAs-based broad area (100 µm) 1.3 µm quantum dot (QD) lasers with high CW output power (5 W) and wall-plug efficiency (56%). The reliability of the devices has been demonstrated beyond 3000 h of CW operation at 0.9 W and 40 °C heat sink temperature with 2% degradation in performance. P-doped QD lasers with a temperature-insensitive threshold current (T0 > 650 K) and differential efficiency (T1 = infinity) up to 80 °C have been realized.

Gain studies of (Cd, Zn)Se quantum islands in a ZnSe matrix

By inserting stacked sheets of nominally 0.7 monolayer CdSe into a ZnSe matrix we create a region with strong resonant excitonic absorption. This leads to an enhancement of the refractive index on the low-energy side of the absorption peak. Efficient waveguiding can thus be achieved without increasing the average refractive index of the active layer with respect to the cladding. Processed high-resolution transmission electron microscopy images show that the CdSe insertions form Cd-rich two-dimensional (Cd, Zn)Se islands with lateral sizes of about 5 nm. The islands act as quantum dots with a three-dimensional confinement for excitons. Zero-phonon gain is observed in the spectral range of excitonic and biexcitonic waveguiding. At high excitation densities excitonic gain is suppressed due to the population of the quantum dots with biexcitons.

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.

AlGaAs/GaAs photovoltaic cells with an array of InGaAs QDs

Specific features of the fabrication of AlGaAs/GaAs single-junction photovoltaic cells with an array of quantum dots (QDs) by molecular beam epitaxy have been studied. It was shown for the first time that, in principle, vertically coupled QDs can be incorporated, with no dislocations formed, into the structure of photovoltaic cells without any noticeable deterioration of the structural quality of the p-n junction. Owing to the additional absorption of the long-wavelength part of the solar spectrum in the QD medium and to the subsequent effective separation of photogenerated carriers, a ∼1% increase in the short-circuit current density Jsc was demonstrated for the first time in the world for photovoltaic cells with QDs. The maximum efficiency of the photovoltaic cells was 18.3% in conversion of the unconcentrated ground level solar spectrum AM1.5G.

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%).

Complete suppression of filamentation and superior beam quality in quantum-dot lasers

Comparative near-field and beam-quality (M2) measurements on narrow stripe quantum-dot (QD) and quantum-well (QW) lasers of identical structure, both emitting at 1100 nm, are presented. Intrinsic suppression of filamentation in the QD lasers is observed. QD lasers emitting at 1300 nm again show no filamentation. For a 6-μm-stripe, QW laser, M2 increases from 2.6 to 6.1 with output power increasing from 5 to 60 mW and with increasing stripe width (20 mW, 3→10 μm, M2=2.6→4.7). In the QD lasers, filamentation is suppressed up to 8 μm (1100 nm) and 9 μm (1300 nm) stripe width and no dependence on output power is observed.