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The fabrication and study of the characteristics of a lattice-matched quantum cascade laser structure on an indium-phosphide substrate, designed for a wavelength of ~4.8 μm corresponding to one of the atmospheric windows are described. The heterostructure grown by molecular-beam epitaxy consisted of thirty cascades. Lasing was experimentally observed at temperatures up to 200 K at a wavelength coinciding with the calculated one, which confirms the high heterointerface quality and high precision of the layer thicknesses and active-region doping levels.

The process of obtaining a modified structure for quantum cascade lasers is studied; this process includes growth using molecular-beam epitaxy, plasma etching, photolithography with the use of liquid etching, and the formation of special contacts for decreasing losses in the waveguide. The use of a special type of structure makes it possible, even without postgrowth overgrowth with a high-resistivity material, to attain parameters satisfying requirements to heterostructures in high-quality quantum cascade lasers at maximal simplification of the entire preparation process.

Previously, GaAs/AlAs superlattices with a small active area (~1 μm2) were used by us to design mixer diodes. It was established that these superlattices can efficiently be used in the terahertz (THz) range. It was theoretically and experimentally shown that short-period (i.e., containing few periods) superlattices in the composition of harmonic mixers have significant advantages in comparison with multi-period (with 50–100 or more periods) superlattices at frequencies of up to 5.3 THz. In this study, the superlattice design is optimized and the operation efficiency of short-period superlattices is shown to be determined to a large extent by the transition regions located at the superlattice edges.

The driving current, chip area of high-power AlGaInN light emitting diodes (LEDs) and the level of integration of LED arrays are continuously increased to provide ever higher output light flux. The new developments require more attention to pay to the thermal management of LEDs, commonly assessed in terms of the thermal resistance. Temperature distribution in a LED array and its effect on the chip thermal resistance has been studied both theoretically and experimentally. The thermal measurements were performed with the help of a temperature-sensitive parameter - forward voltage drop on the p-n junction under the action of heating current. Two methods of heat exciting were used: step-like or harmonically pulse-width modulated heating current. Analysis of forward voltage relaxation at transient thermal processes allows determination of thermal impedance components corresponding to the structural elements of the LEDs and arrays. The temperature distribution in the LED array predicted by coupled simulations of the heat transfer agrees well with the experimental measured temperature mapping by the IR-microscopy.

A turn on of a quantum dot (QD) semiconductor laser simultaneously operating at the ground state (GS) and excited state (ES) is investigated both experimentally and theoretically. We find experimentally that the slow passage through the two successive laser thresholds may lead to significant delays in the GS and ES turn ons. The difference between the turn-on times is measured as a function of the pump rate of change ɛ and reveals no clear power law. This has motivated a detailed analysis of rate equations appropriate for two-state lasing QD lasers. We find that the effective time of the GS turn on follows an ɛ^{-1/2} power law provided that the rate of change is not too small. The effective time of the ES transition follows an ɛ^{-1} power law, but its first order correction in ln(ɛ) is numerically significant. The two turn ons result from different physical mechanisms. The delay of the GS transition strongly depends on the slow growth of the dot population, whereas the ES transition only depends on the time needed to leave a repellent steady state.

Vertical-external-cavity surface-emitting lasers (VECSEL), or semiconductor disk lasers (SDL), are attractive laser source for a wide range of applications owing to unique possibility to combine high output power with an excellent beam quality [1]. The intrinsic features of InAs quantum dots (QD) can offer low threshold, broad wavelength tunability, fast carrier dynamics and low temperature sensitivity. Recently, continuous wave (CW) operation of QD-based VECSEL emitting at 1.25 μm with output powers reaching multi-watt levels were achieved at room temperature [2]. However, extending the emission wavelength to 1.3 μm and beyond becomes more challenging. To date, QD-based VECSEL with optical power greater than 0.5 mW at 1305 nm has been demonstrated [3]. Here, we present a record-high power InAs/InGaAs QD-based VECSEL operating at the wavelength of 1.3 μm.

Vertical-cavity surface-emitting lasers (VCSELs) are low-cost high-performance light sources for high-speed data communication systems, optical interconnects and different sensors [1]. New VCSEL applications (spectroscopy, compact atomic clock) require single-mode operation with stable linear polarization combined with high temperature stability. While conventional GaAs-based VCSELs with small selectively-oxidized current apertures demonstrate stable fundamental transverse mode operation they have unstable polarization due to cylindrical symmetry and isotropic gain. Currently the most popular method for VCSEL polarization control is based on precise etching of sub-wavelength grating in output distributed Bragg reflector [2]. Drawbacks of this approach are relatively complicated fabrication technology and limited output power. In this work we discuss alternative approach for single-mode polarization-stable VCSELs based on rhomboidal selectively-oxidized current aperture combined with intracavity contacts.

We present a semiconductor disk laser (SDL) emitting at the wavelength of 1.3 μm. The active region of the SDL comprises InAs quantum dots (QDs) that are embedded into InGaAs quantum wells (QWs). An output power over 200 mW is obtained at 15°C, which represents the highest output power reported from QD-based SDLs in this wavelength range. The results demonstrate the feasibility of QD-based gain media for fabricating SDLs emitting at 1.3 μm.

We have investigated the optical orientation dynamics of positively charged excitons in p-doped InAs/GaAs quantum dots. We observe a hole spin memory effect in photoluminescence experiments. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical characteristics of quantum-cascade laser structures were studied in this work. Current-voltage characteristics and dependencies of photoresponse signal on current through the structure were measured together with the spontaneous and stimulated emission spectra. The stimulated emission with wavelength close to 9.56 μm with optical power of tens of milliwatt was observed in a pulsed mode.

The vertical transport in short-period InAs/AlSb and GaAs/AlAs superlattices was studied. The negative differential conductivity was observed in a miniband transport regime as a result of the overlapping of confined states in a periodic quantum well structure (Esaki-Tsu mechanism). Several maxima appeared on current–voltage characteristics of the superlattices found in the non-resonant tunnelling regime. They are shown to be due to the influence of the optical cavity on optical transitions within quantum wells (Purcell effect).

We have studied the spin dynamics in self-organized InAs/GaAs quantum dots (QD) by time-resolved photoluminescence (PL). After optical orientation of carriers photogenerated in the GaAs barrier by 1.2 ps light pulses, an unexpected increase of the QD excited-state luminescence polarization is observed. This effect is interpreted in terms of Pauli blocking which prevents the relaxation of electrons in the QD ground state already occupied by another electron with the same spin orientation. A theoretical model qualitatively describing the experimental results is developed.

A new possibility to obtain the intersubband population inversion under current injection (or optical pumping) of the electrons into the structure with GaAs/AlGaAs quantum wells (QWs) is discussed. The QWs are embedded into the i -layer of a p + – i –n + heterostructure. We consider QWs of funnel shape with three electron levels and find the conditions to realize mid-infrared (MIR)-stimulated emission due to intersubband electron transitions under simultaneous stimulated emission of near-infrared (NIR) radiation due to interband transitions. Experimental data on spontaneous MIR emission from InGaAs/GaAs QWs under simultaneous NIR lasing are presented.

We report on millimeter wave generation with a superlattice electronic device (SLED) operated at room temperature. The SLED, containing a wide-miniband GaAs/AlAs superlattice, had a quasi planar structure with two terminals lying in one plane. The device showed a negative differential conductance, due to Bloch oscillations of the miniband electrons. The SLED, mounted into a waveguide, delivered radiation in the 50 to 60 GHz range, with a maximum power (400 μW) corresponding to an efficiency of 1%. Additionally, harmonic radiation up to frequencies above 200 GHz was observed. We associate the generation of radiation with current oscillation caused by traveling dipole domains. We also present an analysis, taking elastic and inelastic scattering into account, of the miniband electrons, indicating that our SLED should, in principle, be suitable for generation of radiation up to 1 THz.

Semiconductor heterostructures with quantum dots (QDs) are experimentally proved to exhibit properties expected for zero-dimensional systems, e.g. ultrasharp luminescence lines up to high temperatures, massively increased exciton oscillator strength per unit volume and temperature insensitivity of the radiative lifetime. When applied to the injection lasers these advantages help to increase strongly material gain, differential gain, to improve temperature stability of the threshold current and to suppress chirp. Threshold current densities as low as 60 A/cm 2 at 300 K are obtained. Formation of QDs with properties satisfying device requirements on QD size, shape, uniformity and density became possible by utilizing self-ordering phenomena on crystal surfaces.

Semiconductor quantum dot (QD) heterostructures created using self-ordering phenomena on crystal surfaces exhibit luminescence properties predicted for zero-dimensional systems, e.g. ultrasharp luminescence lines up to high temperatures, giantly increased material gain and practically complete temperature insensitivity of the laser threshold current. Faster than expected exciton capture and energy relaxation processes manifest minor role of the so-called phonon bottleneck effect. Formation of QDs with properties satisfying device requirements became possible.

We demonstrate stable mode-locking in a Yb : KYW laser by using a quantum-dot (QD) saturable absorber structure that incorporates a p-n junction. A reduction in the output pulse duration was measured when a reverse bias was applied to the QD saturable absorber mirror.

35GHz passive mode-locking of 1.3μm (InGa)As∕GaAs quantum dot lasers is reported. Hybrid mode-locking was achieved at frequencies up to 20GHz. The minimum pulse width of the Fourier-limited pulses was 7ps with a peak power of 6mW. Low uncorrelated timing jitter below 1ps was found in cross correlation experiments. High-frequency operation of the lasers was eased by a ridge waveguide design that includes etching through the active layer.

A 5-Gb/s data modulation and transmission is investigated using Fabry-Pe/spl acute/rot InGaAs quantum-dot lasers emitting at approximately 1.3 /spl mu/m. Error-free transmission of 5-Gb/s data at room temperature over 4 km of single-mode fiber (SMF) and over 500 m of installed grade multimode fiber are demonstrated for the first time. The temperature dependence of the data modulation performance is also studied. We report error-free 2.5-Gb/s data modulation up to 50/spl deg/C and transmission over 4 km of SMF with a Q-factor penalty of 0.5 dB. Error-free 5-Gb/s data modulation is observed up to 30/spl deg/C and 5-Gb/s data transmission over 4 km of SMF with a Q-factor penalty of 0.8 dB is obtained at 40/spl deg/C. The lack of overshoot and ringing in the eye diagrams is attributed to the large damping factor observed under small-signal modulation.

The results of development of InAlN/AlN/GaN heterostructures, grown on sapphire substrates by metal-organic chemical vapour deposition, and high electron mobility transistors (HEMTs) based on them are presented. The dependencies of the InAlN/AlN/GaN heterostructure properties on epitaxial growth conditions were investigated. The optimal indium content and InAlN barrier layer thicknesses of the heterostructures for HEMT s were determined. The possibility to improve the characteristics of HEMTs by in-situ passivation by Si3N4 thin protective layer deposited in the same epitaxial process was demonstrated. The InAlN/AlN/GaN heterostructure grown on sapphire substrate with diameter of 100 mm were obtained with sufficiently uniform distribution of sheet resistance. The HEMTs with saturation current of 1600 mA/mm and transconductance of 230 mS/mm are demonstrated.

Complex influence of photon lifetime (controlled by the mirror loss) and aperture size on the performance of 850 nm InGaAlAs oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with fully doped AlGaAs-based distributed Bragg reflectors (DBR) was investigated. We find a tradeoff between photon lifetime and gain nonlinearity for maximizing the optical bandwidth, leading to the optimum aperture size close to 4-6 μm. In spite of the reduced photon lifetime (from 4 ps to 1 ps), the excess damping caused by the current-induced self-heating limits the ultimate modulation bandwidth for the given VCSELs design at 24-25 GHz. Further improvement in high frequency characteristics can be facilitated by decrease of the heat generation and improvement of the heat removal from the active region as well as by proper engineering of the scattering loss at the oxide aperture while keeping the low capacitance optimizing design of the oxide aperture.

A type-II GaAs/GaAlAs short-period superlattice (SPSL) used as an electro-optic medium for the spectral range 820–850 nm is studied in a vertical microcavity geometry. SPSL is sandwiched between two GaAlAs distributed Bragg reflectors. Optical power reflectance (OR) spectra are measured as a function of applied reverse bias at different tilt angles and temperatures. All spectra reveal a blue shift of the reflectivity dip upon applied voltage which evidences a negative electrorefraction of the electro-optic medium. The shift enhances up to ~0.6 nm once the exciton resonance is brought close to the wavelength of the reflectivity dip. As opposed to those modulators based on quantum–confined Stark effect, no increased absorption is observed at an applied bias, because the integrated intensity of the reflectivity dip in the OR spectra is virtually constant. This indicates a low absorption loss with applied bias and consequently a high potential for the increased dynamic range of the related modulator.

Self-organized nano-objects fabricated in different semiconductor systems are currently at the focus of scientific interest because of their unique electronic properties. Transmission electron microscopy and high-resolution electron microscopy have been used to study the InAs quantum dots grown by molecular-beam epitaxy (MBE) on GaAs and InP substrates. Optimal imaging conditions for visualization of quantum dots were established. Size, shape, and stability of the equilibrium island arrays were analyzed with respect to the growth conditions. Both decrease and increase of the As pressure compared to the optimal value were shown to destroy the regular arrangement of the islands. Energy benefit due to the strain relaxation in the InAs islands is likely to be the driving force for their formation.

Recent achievements in self-organized quantum dots (QDs) have demonstrated their potential for long-wavelength laser applications. However, the wavelength of QD structures pseudomorphically grown on GaAs substrate is typically not longer than 1.3 μm. In this work we study a novel approach for extension of the spectral range of GaAs-based diode lasers up to 1.5 μm. We use a sensitivity of QD emission to the band gap energy of surrounding matrix. The method is based on formation of a QD array inside a metamorphic InGaAs epilayer. Growth regimes of metamorphic buffer that enable mirror-like surface morphology in combination with effective dislocation trapping are discussed. Structural and optical properties of metamorphic InAs/InGaAs QDs are presented. It is shown that the wavelength of QD emission can be controllably tuned in the 1.37-1.58 μm range by varying the composition of metamorphic InGaAs matrix. Details of formation, fabrication, and characterization of metamorphic-based diode lasers are also presented. We demonstrate a lasing wavelength as long as 1.48 μm in the 20-80 °C temperature interval. The minimum threshold current density is 800 A/cm2 at RT. The external differential efficiency and pulsed power maximum exceed 50% and 7 W, respectively.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We observe FIR emission and magnetotransport phenomena of modulation doped p- GaAs AlGaAs multi-quantum wells structures of different well width in a streaming regime. This FIR emission originates from radiative transitions between the upper quantum subbands, where hot holes are trapped by the emission of LO-phonons and the ground quantum state. The importance of the light-heavy states mixing with an increase of confinement effect is shown experimentally. The magnetoresistance in a strong in-plane electric field applied along the [110] crystallographic direction is found to be negative, which is explainedDby strong warping of the constant hole energy surfaces in two-dimensional quantum wells.

By forming twin contacts on a 1300 nm quantum dot laser, a small signal modulation bandwidth of 4.6 GHz is achieved. This bandwidth value is 2.3 times greater than that for the equivalent single contact device. The quantum dot lasers studied are 800 /spl mu/m long with 8 /spl mu/m wide ridge guides and 5 InAs quantum dot stacks in the active layer.

Quantum dot (QD) edge emitting and vertical cavity lasers are realized using a self-organized growth approach. Threshold current densities at room temperature (RT) of about 60 A/cm/sup 2/ for edge emitting and 170 A/cm/sup 2/ for vertical cavity lasers are measured. High internal (>96%) and differential (70%) efficiencies are obtained for InGaAs-AlGaAs lasers based on vertically coupled QDs and RT 1 W continuous wave operation is demonstrated. QD lasers exhibit much larger gain, differential gain and smaller linewidth enhancement factor as compared to conventional quantum well devices.

Pulsed lasing at 1.3 /spl mu/m via the exciton ground state is demonstrated for vertical cavity surface emitting lasers containing three uncoupled sheets of InAs quantum dot active layers. The dots lie within InGaAs quantum wells separated by GaAs barrier layers. The structures are grown on GaAs substrates and when fabricated include selectively oxidized AlO current apertures and AlO/GaAs distributed Bragg reflectors. Experimental devices operate pulsed at 20/spl deg/C with threshold currents below 2 mA and differential slope efficiencies of 40%.

Heterostructures with submonolayer insertions attract interest due to optical properties and to possibility of the studying of the self-organization effect. Besides, investigation of SML heterostructures is retarded as consequence of problems of the structural and composition characterization. The situation is changed on account of development of quantitative methods for HREM image analysis. There formation of superlattice heterostructures with InAs submonolayer insertions in GaAs matrix and them optical properties are investigated in the work.

A series of narrow emission lines (halfwidth 0.5 - 2 meV) corresponding to quantum-dot-like compositional fluctuations have been observed in low temperature near-field photoluminescence spectra of GaAsN and InGaAsN alloys. The estimation of the size, density, and nitrogen excess of individual compositional fluctuations (clusters) using scanning near-field magneto-spectroscopy reveals phase-separation effects in the distribution of nitrogen. We found a strong effect of In on the exciton g -factor in InGaAsN alloys.

Optical and structural properties of self organized InGaAs quantum dots (QD), deposited in Al0.3Ga0.7As matrix, were investigated. Samples were grown by molecular-beam epitaxy (MBE). It is shown, that deposition of 1.7 - 4 monolayer of InAs on Al0.3Ga0.7As surface results in formation of nanoscale QDs on 1 - 2 monolayer thick wetting layer (Stranski-Krastanov growth mode). Large exciton localization energy of the InAs QDs in Al0.3Ga0.7As in compare with QDs in GaAs is demonstrated. This is due to increase in size of these QDs and significant bandgap offset in the case of InAs/AlGaAs system in compare with InAs/GaAs one.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

For memory structures based on optically induced charge in self-organized quantum dots, the concept of wavelength-domain multiplexing in the quantum dot ensemble is an essential prerequisite. The electric properties of quantum dots in various material systems as studied by time-resolved capacitance spectroscopy are summarized, and candidates suitable for future memory applications are discussed. By combining optical excitation and capacitance spectroscopy, direct evidence is obtained for energy-selective hole charge generation and storage in InAs/GaAs quantum dots. A clear dependence of the activation energy of the emitted holes on the energy of the excitation is observed.

By selective doping (Be) of the well and barrier regions of p-GaAs/AlGaAs structures we have realized the situation where the upper Hubbard band (A+ centers) has been occupied by holes in the equilibrium. We studied temperature behavior of the Hall effect, variable range hopping conductivity and the photoluminescence spectra of the corresponding structures. The experimental data demonstrated that the binding energy of the A+ states significantly increases with respect to 3D case and strongly depends on well width (9nm,15nm). The localization radii of the A+ states are of the order of well widths., Comment: 6 pages, 5 figures

Summary form only given. Vertical cavity surface emitting lasers (VCSELs) emitting at 1.3 /spl mu/m and integrated with GaAs-based microelectronic circuits have many potential applications in optical communication systems. Several approaches exist for the realization of 1.3 /spl mu/m VCSELs. A promising approach for the realization of longer wavelength VCSELs is the use of self-organized InAs-InGaAs quantum dot (QD) active gain regions. In 1997 we demonstrated QD VCSELs on GaAs with peak emission wavelengths near 1.0 /spl mu/m and sub-milliampere threshold currents. We have now extended the QD peak emission wavelength to 1.3 /spl mu/m by selectively varying the physical size and composition of the InAs-InGaAs dots and herein report room temperature continuous wave (CW) 1.3 /spl mu/m QD VCSELs. While striving to maximize the uniformity of the QDs to increase device efficiency at a single emission wavelength, we have also experimented with sheets of QDs wherein the dot size is purposely nonuniform. Though less efficient, these nonuniform QD active regions have potential applications in tunable VCSELs where a broad gain bandwidth is required.

Transmission electron microscopy has been applied to study the ordering in size and shape of InAs quantum dots and in their lateral distribution. InAs islands were grown by MBE on GaAs substrates at different As-pressures and growth temperatures. Experiments with growth interupption support the theoretical predictions concerning equilibrium island size, shape and arrangement. The stability of the equilibrium dot arrays to changing of growth conditions was studied by varying the deposition temperature, arsenic pressure or growth interruption time. Significant deviation from the optimal As-pressure towards both the lower and higher values was shown to supress the formation of InAs dots resulting either in mesoscopic InAs clusters or 2D corrugated islands. Energy benefit due to the strain relaxation at island edges explains the experimental results better than kinetic consideration.

A quantum-dot-based saturable absorber has been demonstrated to initiate the generation of femtosecond pulses from a passively mode-locked solid-state laser. Control and tuning of the pulse duration from 58 ps to 158 fs was achieved. The 158 fs transform-limited pulses at 1280 nm are the shortest pulses that were produced from the Cr:forsterite laser passively mode locked by an InAs/InGaAs quantum-dot semiconductor saturable absorber mirror.

18 GHz passive modelocking with Fourier-transform-limited optical pulses in InGaAs/GaAs quantum dot lasers emitting at /spl sim/1.3 /spl mu/m has been achieved for the first time. An upper limit of 600 fs is estimated for the root-mean-square timing jitter (2.5-50 MHz).

We report on the observation of hot luminescence increase from the second electronic subband in undoped GaAs-AlGaAs multi-quantum-wells (MQWs) structure under application of a magnetic field perpendicular to the layers. We consider it to be due to suppression of acoustic phonon emission which has an oscillating character with magnetic field. This novel effect proves the influence of magnetic field on the occupation of a higher subband and provides a new possibility for creation of population inversion between quantum well subbands.

The room-temperature vertical transport in InAs/AlSb and GaAs/AlAs short-period superlattices was studied. The influence of optical cavity on current-voltage characteristics was found in the range of both resonant and nonresonant tunneling. Possible explanations are suggested.

Optical absorption and birefringence due to intersubband electron transitions were investigated in GaAs/AlAs MQW structures. These structures are intended for creation of a mid-infrared laser of a new type. Experimental results on electron redistribution between size-quantization levels under electron heating were obtained up to an electric field of 3500?V?cm-1.

Optical phenomena in the mid-infrared range connected with interlevel and intersubband charge-carrier transitions in quantum dot and quantum well (QW) heterostructures under optical and electrical pumping were investigated. Spectra of interband photoluminescence are also presented. The existence of a metastable level in funnel-shaped QWs is experimentally confirmed. The intersubband transition dynamics in asymmetrical pairs of tunnel-coupled QWs was studied by means of pump-and-probe time-resolved spectroscopy. This paper was presented at the 3rd Russian Workshop on Nanophotonics, Nizhnii Novgorod, Russia, 26-29 March 2001.