[Show abstract][Hide abstract] ABSTRACT: We investigate nuclear spin pumping with five-electron quadruplet spin states in a spin-blockaded weakly coupled vertical double quantum dot device. Two types of hysteretic steps in the leakage current are observed on sweeping the magnetic field and are associated with bidirectional polarization of nuclear spin. Properties of the steps are understood in terms of bias-voltage-dependent conditions for the mixing of quadruplet and doublet spin states by the hyperfine interaction. The hysteretic steps vanish when up- and down-nuclear spin pumping processes are in close competition.
[Show abstract][Hide abstract] ABSTRACT: Interband cascade (IC) lasers have been demonstrated based on type-I InGaAsSb/AlAsSb quantum
well (QW) active regions. These type-I IC lasers are composed of 6-cascade stages and InAs/AlSb
superlattice cladding layers. In contrast to the use of quinary AlGaInAsSb barriers for active region
in previous type-I QW lasers, the type-I QW active region in each stage is sandwiched by digitally
graded multiple InAs/AlSb QW electron injector and GaSb/AlSb QW hole injector. The fabricated
type-I IC lasers were able to operate in continuous wave and pulsed modes at temperatures up to
306 and 365 K, respectively. The threshold current densities of broad-area lasers were around
300 A/cm2 at 300 K with a lasing wavelength near 3.2 um. The implications and prospects of these
initial results are discussed.
Full-text · Article · Jan 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We outline power spectra and auto correlation analysis performed on temporal oscillations in the tunneling current of coupled vertical quantum dots. The current is monitored for similar to 2325 s blocks as the magnetic field is stepped through a high bias feature displaying hysteresis and switching: hallmarks of the hyperfine interaction. Quasi-periodic oscillations of similar to 2 pA amplitude and of similar to 100 s period are observed in the current inside the hysteretic feature. Compared to the baseline current outside the hysteretic feature the power spectral density is enhanced by up to three orders of magnitude and the auto correlation displays clear long lived oscillations about zero.
[Show abstract][Hide abstract] ABSTRACT: We outline the properties of the hyperfine-induced funnel structure observed in the two-electron spin blockade region of a weakly coupled vertical double quantum dot device. Hysteretic steps in the leakage current occur due to dynamic nuclear polarization when either the bias voltage or the magnetic field is swept up and down. When the bias voltage is swept, an intriguing similar to 3 mT wide cusp near 0 T appears in the down-sweep position, and when the magnetic field is swept, the current at 0 T can be switched from "low" to "high" as the bias is increased.
[Show abstract][Hide abstract] ABSTRACT: Single-mode operation of fibre-pigtailed distributed feedback semiconductor lasers in the 2.05 μm range has been demonstrated. The lasers are packaged inside standard butterfly modules with output powers in excess of 10 mW at the end of polarisation maintaining optical fibre. The fibre-pigtailed lasers show excellent sidemode suppression ratios (> 50 dB) and have a mode-hop free tunability larger than 1 nm. The output of the optical fibre has linear polarisation with better than 20 dB extinction over the operating current and temperatures.
Preview · Article · Nov 2013 · Electronics Letters
[Show abstract][Hide abstract] ABSTRACT: We demonstrate index-coupled distributed-feedback diode lasers at 2.65 µm that are capable of tuning across strong absorption lines of HDO and other isotopologues of H<sub>2</sub>O. The lasers employ InGaAsSb/AlInGaAsSb multi-quantum-well structures grown by molecular beam epitaxy on GaSb, and single-mode emission is generated using laterally coupled second-order Bragg gratings etched alongside narrow ridge waveguides. We verify near-critical coupling of the gratings by analyzing the modal characteristics of lasers of different length. With an emission facet anti-reflection coating, 2-mm-long lasers exhibit a typical current threshold of 150 mA at 20 °C and are capable of emitting more than 25 mW in a single longitudinal mode, which is significantly higher than the output power reported for loss-coupled distributed-feedback lasers operating at similar wavelengths.
[Show abstract][Hide abstract] ABSTRACT: We investigate two- and three-electron spin blockade in three vertical quantum dots (QDs) coupled in series. Two-electron spin blockade is found in a region where sequential tunneling through all QDs is forbidden but tunneling involving virtual hopping through an empty QD is allowed. It is observed only for the hole cycle with a distinct bias threshold for access to the triplet state. Three-electron spin blockade involving the quadruplet state is observed for nonequibilium conditions where sequential tunneling is allowed and the triplet state is accessible. Our results shine light on the importance of the nonequibilium conditions to obtain sufficient population of triplet and quadruplet states necessary for spin blockade.
No preview · Article · Jan 2013 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: Time resolved photoluminescence (PL) spectroscopy has been applied to study dynamics of localized excitons in a single Ga0.7In0.3N0.015As0.985/GaAs quantum well (QW). The decay time constant, τPL, has been determined for different PL peak energies at various temperatures. An increase in temperature produced two effects: (i) a reduction of τPL and (ii) changes in the τPL dispersion. These two experimental observations as well as the shape of PL decay curves were very well reproduced by Monte-Carlo simulations of hopping excitons with parameters derived from PL and photoreflectance measurements for this QW.
Full-text · Article · May 2012 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Time-resolved photoluminescence (PL) characteristics of type-II GaAsSb/GaAs quantum wells are presented. The PL kinetics are determined by the dynamic band bending effect and the distribution of localized centers below the quantum well band gap. The dynamic band bending results from the spatially separated electron and hole distribution functions evolving in time. It strongly depends on the optical pump power density and causes temporal renormalization of the quantum well ground-state energy occurring a few nanoseconds after the optical pulse excitation. Moreover, it alters the optical transition oscillator strength. The measured PL lifetime is 4.5 ns. We point out the critical role of the charge transfer processes between the quantum well and localized centers, which accelerate the quantum well photoluminescence decay at low temperature. However, at elevated temperatures the thermally activated back transfer process slows down the quantum well photoluminescence kinetics. A three-level rate equation model is proposed to explain these observations.
[Show abstract][Hide abstract] ABSTRACT: A lateral etched-grating process was used to produce singlemode distributed feedback laser diodes at 3.23 μm. The devices are based on InGaAsSb/AlInGaAsSb type-I quantum well active regions grown on GaSb substrates by molecular beam epitaxy. The lasers were used in high-resolution spectroscopy of methane gas near the v3, R7 vibrational absorption transitions.
No preview · Article · Mar 2012 · Electronics Letters
[Show abstract][Hide abstract] ABSTRACT: We present the electronic properties of a triple quantum dot molecule
embedded inside a sub-micron mesa, made from a quadruple-barrier
triple-quantum-well structure, and surrounded by a single gate
electrode. We outline the design principles of the quadruple-barrier
triple-quantum-well structure and calculate the energy of the three
lowest states as a function of center well thickness. We observe regular
and irregular shaped Coulomb diamond regions similar to those for double
quantum dot devices. Variation in the Coulomb blockade region shape is
introduced by fluctuation in the offset energies between the quantum
dots likely associated with device processing and random impurity
potential in the material. We also present Coulomb blockade patterns
calculated with a constant interaction model for sequential tunneling
through the three series-coupled quantum dots.
No preview · Article · Feb 2012 · Japanese Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: The HF gas detection sensitivity of GaSb-based distributed feedback lasers was studied in order to evaluate the potential use of operating temperature to control the laser output power in field applications. Measurements were made in the 2f wavelength modulation spectroscopy mode to monitor a prominent HF absorption feature at 2396 nm. An Allan variance analysis indicates that the best sensitivity is obtained when a larger thermoelectric current is applied to maintain temperature control by actively heating or cooling the laser. The reduced detection sensitivity for a setpoint near the ambient laser operating temperature is similar to that obtained without active temperature control.
No preview · Article · Jan 2012 · Electronics Letters
[Show abstract][Hide abstract] ABSTRACT: Narrow ridge waveguide (5um) laser diodes were fabricated using type-I InGaAsSb/AlInGaAsSb quantum well active regions on GaSb. The devices operate in continuous-wave mode near 3254nm with a total light output of 7.4mW at 20°C (uncoated facets).
[Show abstract][Hide abstract] ABSTRACT: We outline the properties of fine structure in the high bias (~10-100
mV) current of a weakly coupled vertical double-dot device close to
``pinch-off''. Two features of particular interest at 0 T are
modulations in the strength along the length of the practically
degenerate 1s-2p+ and 1s-2p- resonance lines, and
a current step bisecting these two resonance lines whose height is very
different either side of the two resonance lines.
[Show abstract][Hide abstract] ABSTRACT: We characterize and model the single-particle energy level position and
resonant current strength at a three-level crossing in a coherent mixer
composed of two weakly coupled vertical quantum dots. In addition to
clear anticrossing behavior, an otherwise strong resonance is completely
extinguished at the center of the crossing. Despite the strong variation
in energy level position and resonant current strength throughout the
crossing region, the resonance widths and the sum of the branch currents
are found to be approximately constant.
[Show abstract][Hide abstract] ABSTRACT: Ultrashort-pulse lasers operating around the 2-μm spectral region are of great interest for applications in time-resolved spectroscopy, nonlinear frequency up-conversion to the mid/far-infrared spectral regions, mid-IR supercontinuum generation, optical communications and photomedicine. Tm 3+ doped crystalline gain media represent an attractive option for development of high-power, broadly tunable diode-pumped lasers around 2 μm that can be readily configured for ultrashort pulse generation through appropriate mode locking schemes. The best results in terms of pulse duration have to date been obtained with a Tm:KLu(WO4)2 laser  that produced 10-ps pulses at 1944 nm and a Tm:GdLiF4 laser that generated ∼17-44 ps pulses tuned from 1868 nm to 1926 nm.  Here we report, for the first time to our knowledge, a femtosecond Tm 3+:KY(WO4)2 (Tm:KYW) laser that operates in the 1985-2065 nm spectral range by incorporating a semiconductor saturable absorber mirror (SESAM)
[Show abstract][Hide abstract] ABSTRACT: Type-I interband laser diodes were developed for trace gas sensing applications in the 2-4um wavelength range. The devices were grown by molecular beam epitaxy on GaSb substrates using InGaAsSb/Al(In)GaAsSb active regions. Tunable, single-mode lasers were produced using distributed feedback grating processing or by incorporating Fabry-Perot lasers in an external cavity configuration. Sensitive gas detection was demonstrated using these lasers in tunable-diode laser absorption spectroscopy. I. INTRODUCTION There are many applications for gas sensing systems based on mid-infrared laser diodes. Trace gas sensing with a single- mode laser can be accomplished using tunable diode laser absorption spectroscopy (TDLAS), in which the wavelength of the laser is modulated through a strong absorption feature of the gas of interest, in a wavelength range free of interferences from other species. Laser diodes in the 2-3um range can be readily produced on GaSb substrates using compressively- strained InGaAsSb type-I quantum wells surrounded by AlGaAsSb waveguide and cladding layers which are lattice- matched to the substrate. Molecular beam epitaxy (MBE) is a preferred technique for the growth of these structures because of its excellent thickness and compositional control. These properties are even more important for the development of devices with wavelengths beyond 3um. In this case, it has been demonstrated that laser performance can be improved through the use of quinary AlInGaAsSb barrier layers, which simultaneously increase the hole confinement and decrease the conduction band offset (1). This more favourable band offset prevents hole leakage and improves the homogeneity of electron injection into multiple quantum well active regions. In this work we have developed single-mode lasers using quantum well active regions designed for specific operating wavelengths of 2476nm and 3240m. The shorter wavelength, single-mode devices were fabricated using a regrowth-free distributed feedback (DFB) process(2) involving laterally- coupled etched gratings to provide continuous single-mode tuning with current and temperature through absorption features of HF gas. Single-mode operation at longer wavelengths was achieved by controlled adjustment of a diffraction grating in an external cavity configuration. The resulting single-mode lasers operate at wavelengths around 3240nm, which is extremely important for the detection of methane and other hydrocarbons. II. EXPERIMENT The laser structures were grown on (100) GaSb:Te substrates in a V90 MBE system using conventional group-III effusion cells and valved cracker cells for As2 and Sb2. The structures consist of Te- and Be-doped Al0.6Ga0.4As0.052Sb0.948 cladding layers lattice-matched to the GaSb substrate (thicknesses in the range 1.5-3.0um). The composition and thickness of the quantum-well active region for each laser was carefully designed for the intended application. For the laser structure with target wavelength of 2476nm, the active region contains three 10.9nm In0.43Ga0.57As0.15Sb0.86 quantum wells separated by 30nm, with Al0.24Ga0.76As0.02Sb0.98 barrier and waveguide layers. Single-mode DFBs at 2476nm were produced using a two step inductively-coupled plasma reactive ion etching (ICP-RIE) procedure. In the first step, a narrow ridge waveguide structure was etched in the semiconductor. In the second step, first- order lateral gratings were etched on either side of the ridge to provide evanescent coupling to the optical mode (grating pitch λ=349.12nm, 50% duty cycle, thickness 330nm). E-beam lithography with fine-pitch control was used to write the lateral gratings using ZEP resist. Standard TiPtAu and NiGeAu metallization was used for the p- and n-contacts, respectively. After cleaving, the front output facet of the lasers was coated with a single layer of Al2O3 using ion-beam sputter deposition to provide a reflectivity of approximately 7%. The back facet of the laser was coated with a similar Al2O3 layer, followed by a multilayer stack of SiO2/TiO2 to provide a reflectivity of 90%. With these coatings, approximately 97% of the laser light should be emitted from the front facet.