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We analyze temperature-dependent photoluminescence (PL) behavior of patterned InAs/GaAs quantum dots (PQDs) formed by selective area epitaxy using metalorganic chemical vapor deposition. The processing scheme, described here, yields an ensemble of electronically isolated PQDs with PL characteristics that significantly differ from self-assembled (SA) QDs since neither a wetting layer nor neighboring QDs are available for coupling. The isolated nature of the PQDs supports a non-Fermi (nonequilibrium) carrier distribution which yields very different PL characteristics compared to the Fermi (equilibrium) distribution of the SAQDs especially at temperatures >100 K. Thus, the PQDs demonstrate a constant PL linewidth within the temperature range of 80–300 K along with improved temperature stability of PL intensity in comparison to SAQDs. The increased temperature stability allowed by electronic isolation may prove very important for high speed, temperature-insensitive QD laser development. [ABSTRACT FROM AUTHOR]

A method that reduces the lattice mismatch between the InAs quantum dots (QDs) and their surrounding material to extend the GaAs based QD emission wavelength is proposed. The density and the ground state transition of the resulting QD ensemble are lined out.

In this paper, a technology comparison between monolithic passively mode-locked lasers (MLLs) fabricated from 1.24 μm InAs dots-in-a-Well (DWELL) and 1.25 μm InGaAs single quantum well (SQW) structures grown using elemental source molecular beam epitaxy (MBE) is presented. 5 GHz optical pulses with sub-picosecond RMS jitter, high pulse peak power (1W) and narrow pulse width (< 10 ps) are typical of these monolithic two-section InAs DWELL passive MLLs. An InGaAs single quantum well MLL with the 42% indium is shown to exhibit a superior high-temperature performance. Compared with the typical operating range of the InAs DWELL devices (<60°C), the operation is in excess of 100 °C and is particularly attractive for clocking applications in next generation microprocessors. Based on an 8-band k.p analysis the reduction in the band edge density of states for such a quantum well is discussed. [ABSTRACT FROM AUTHOR]

An alternative segmented-contact method for accurate measurement of the optical gain and absorption of quantum-dot and quantum-dash active materials with small optical gain is reported. The usual error from unguided spontaneous emission is reduced by subtracting signals acquired from three independently controlled sections as opposed to just two found in the conventional technique. The quantum-dot gain spectra are measured to a precision of less than 0.2 cm-1 at nominal gain values below 2 cm-1, and gain spectrum of quantum-dash sample is calculated with an error less than 0.3 cm-1 at a gain less than 1 cm-1. These accuracies are checked with a self-calibrating method. The internal optical mode loss measurement is also described. [ABSTRACT FROM AUTHOR]

We have measured the passive modal absorption, modal gain, and spontaneous emission spectra of a quantum-dot system where the inhomogeneous broadening is sufficiently small so that the ground- and excited-state transitions can be spectrally resolved. Absorption by ground- and excited-state transitions is in the ratio 1:1.88 which is close to the ratio of 2 expected for dots with similar dimensions in two directions. The absorption cross section per dot is measured to be 1.1 × 10-14 cm². Optical gain from the ground-state saturates with current at a maximum value of one third of that predicted from the measured absorption if the system is fully inverted. The measured population inversion factor spectrum shows that the carrier distributions cannot be described only by a single global Fermi distribution and that the system is not in overall equilibrium. However, using parameters obtained by fitting the absorption spectrum, we find that for these particular samples the inversion factor spectrum can be described by a possible model where the ground- and excited-state occupancies are each described by a Fermi distribution but with different quasi-Fermi energy separations We speculate that photon mediation within the homogeneous linewidth could be one possible process which establishes quasi-equilibrium within each of the ground- and excited-state inhomogeneous distributions. [ABSTRACT FROM AUTHOR]

A unique, ultra-sensitive frequency-resolved optical gating system was constructed, and the pulse shape and chirp of a quantum-dot modelocked laser was unambiguously measured using this technique for the first time. A clear pulse asymmetry was detected, and evidence that the pulses are recompressible to sub-picosecond lengths is presented. [ABSTRACT FROM AUTHOR]

A novel ridge-waveguide quantum-dot superluminescent diode is reported. The multisection configuration enables the simultaneous realization of ultrawide 3-dB bandwidth (>150 nm) and an output power greater than 1 mW [ABSTRACT FROM PUBLISHER]

We report the device characteristics of stacked InAs/GaAs quantum dots (QDs) with GaP strain-compensation (SC) layers grown by metal organic chemical vapor deposition. By inserting GaP SC layers within the stacked structures, decrease in the density of QDs by stacking QDs can be suppressed due to reduction of overall compressive strain within the stacked QDs. We demonstrate ground-state lasing at 1.265 μm of six layers of InAs/GaAs QDs with GaP SC layers. The threshold current density is as low as 108 A/cm2. We also assess the internal loss and maximum modal gain of fabricated QD lasers by using a segmented contact method. The internal loss is as low as 5 cm-1, and the maximum modal gain of the ground state of the stacked QDs is approximately 10 cm-1. [ABSTRACT FROM AUTHOR]

We demonstrate an InAs/GaAs quantum dot (QD) laser based on a strain-compensated, three-stack active region. Each layer of the stacked QD active region contains a thin GaP (Δao=-3.8%) tensile layer embedded in a GaAs matrix to partially compensate the compressive strain of the InAs (Δao=7%) QD layer. The optimized GaP thickness is ∼4 MLs and results in a 36% reduction of compressive strain in our device structure. Atomic force microscope images, room-temperature photoluminescence, and x-ray diffraction confirm that strain compensation improves both structural and optical device properties. Room-temperature ground state lasing at λ=1.249 μm, Jth=550 A/cm2 has been demonstrated. [ABSTRACT FROM AUTHOR]

We describe the growth mechanisms of highly mismatched (Δao/ao=13%) defect-free AlSb on Si(001) substrates. Nucleation occurs during the first few monolayers of AlSb deposition by crystalline quantum dot formation. With continued growth, the islands coalesce into a bulk material with no vertically propagating defects. Strain energy from the AlSb/Si interface is dissipated by crystallographic undulations in the zinc-blende lattice, as confirmed by high-resolution transmission electron microscopy (TEM) images. Reciprocal space analysis of the TEM images corroborates a crystallographic rotation associated with the undulations. The resulting AlSb material is >98% relaxed according to x-ray diffraction analysis. [ABSTRACT FROM AUTHOR]

We describe the growth and characterization of InAs quantum dots (QDs) on a patterned GaAs substrate using metalorganic chemical vapor deposition. The QDs nucleate on the s001d plane atop GaAs truncated pyramids formed by a thin patterned SiO2 mask. The base diameter of the resulting QDs varies from 30 to 40 nm depending on the size of the mask. With specific growth conditions, we are able to form highly crystalline surface QDs that emit at 1.6 μm under room-temperature photopumped conditions. The crystalline uniformity and residual strain is quantified in high-resolution transmission electron microscopy images and high-resolution x-ray reciprocal space mapping. These strained QDs may serve as a template for selective nucleation of a stacked QD active region. [ABSTRACT FROM AUTHOR]

The first monolithic GaInNAsSb quantum well 1.55 µm passively modelocked lasers grown on a GaAs substrate are reported. A repetition rate of up to 13.2 GHz and an optical pulse width as small as 26 ps have been realised. [ABSTRACT FROM AUTHOR]

This article discusses 740 nm InP/GaInP quantum-dot laser with 190 A cm-2 room temperature threshold current density. In recent years, self-assembled quantum dots have attracted much interest for laser applications due, in part, to predictions of improvements in terms of threshold current, a reduced threshold current temperature dependence and increased differential gain. The facets were uncoated and the devices were mounted p-side up and measured in pulsed mode. Measurements carried out in Cardiff were performed with a pulse length of 1000 ns and duty cycle of 1% while measurements at the University of New Mexico were performed with 500 ns pulses at 1% duty cycle.

Combining with electrochemical corrosion measurements, immersion and hydrogen evolution testing performed in 0.9 wt.% NaCl solution at 37 °C, the corrosion resistance of an as-rolled Mg-3%Al-1%Zn alloy before and after a 3% compressive strain along the rolling direction was investigated. Results revealed that the corrosion behavior of differently oriented surfaces of the as-rolled samples with a strong basal texture was obviously different. Among them, the corrosion rate of sample surface with the orientation parallel to the normal direction (ND) of the plate was the fastest, the corrosion rate of sample surface with the orientation parallel to the rolling direction (RD) of the plate took the second place and the corrosion rate of sample surface with the orientation parallel to the transverse direction (TD) was the slowest. After being pre-strained, the activation of a high density of {10-12} twins could remarkably reduce the corrosion rate of surrounding α-Mg matrix and simultaneously weaken the corrosion anisotropy between differently oriented samples. The main reason was that similar to grain boundaries, twin boundaries acted as physical barriers to the corrosion attack. Moreover, the activated twins increased the protectiveness of surface films and then suppressed the micro corrosion couples occurred in twinned grains.

To conduct grain refinement induced by plastic deformation, NiTi shape memory alloy is processed by surface mechanical attrition treatment. The process leads to surface nanocrystallization and consequently surface hardening. The cross sectional microhardness of the treated NiTi is measured and compared to those of annealed NiTi specimens with residual stress relaxation and recrystallization. Our results show that surface nanocrystallization induced by surface mechanical attrition treatment is an effective method to enhance the surface hardness and anti-wear properties of NiTi shape memory alloy for the biomedical application.

By extending the net-gain modulation phasor approach to account for the discrete distribution of the gain and saturable absorber sections in the cavity, a convenient model is derived and experimentally verified for the cavity design of two-section passively mode-locked quantum dash (QDash) lasers. The new set of equations can be used to predict functional device layouts using the measured modal gain and loss characteristics as input. It is shown to be a valuable tool for realizing the cavity design of monolithic long-wavelength InAs/InP QDash passively mode-locked lasers.

We investigate the dynamical response of a quantum dot photonic integrated circuit formed with a combination of eleven passive and active gain cells operating when these cells are appropriately biased as a multi-section quantum dot passively mode-locked laser. When the absorber section is judiciously positioned in the laser cavity then fundamental frequency and harmonic mode-locking at repetition rates from 7.2GHz to 51GHz are recorded. These carefully engineered multi-section configurations that include a passive wave-guide section significantly lower the pulse width up to 34% from 9.7 to 6.4 picoseconds, as well increase by 49% the peak pulsed power from 150 to 224 mW, in comparison to conventional two-section configurations that are formed on the identical device under the same average power. In addition an ultra broad operation range with pulse width below ten picoseconds is obtained with the 3rd-harmonic mode-locking configuration. A record peak power of 234 mW for quantum dot mode-locked lasers operating over 40 GHz is reported for the first time.