Room-temperature long-wavelength (λ=13.3 μm) unipolarquantum dot intersubband laser

Long-wavelength (λ=13.3 μm) unipolar lasing at 283 K from self-organised In_0.4Ga_0.6As/GaAs quantum dots, due to intersubband transitions in the conduction band, is demonstrated for the first time. The threshold current density under continuous wave operation is 1.1 kA/cm² for a 60 μm×1.2 mm broad-area plasmon-enhanced waveguide device and the maximum power output is ≈ μW. The long intersubband relaxation time in quantum dots, together with the short lifetime in the ground state, due to interband stimulated emission, help to achieve the necessary population inversion and gain     

Interband cascade laser emitting >1 photon per injected electron

A 22-stage interband cascade laser with a “W” active region for enhanced gain has exhibited lasing at λ=3.0 μm. Threshold current densities are lower than the best reported intersubband quantum cascade laser results at all T up to the maximum lasing temperature of 225 K. At 100 K, output powers up to 430 mW are observed, and the slope of 274 mW/A per facet for high injection levels corresponds to a quantum efficiency of 1.3 photons emitted for every injected electron     

Optically pumped intersubband lasers based on quantum wells

A far infrared (FIR) laser based on intersubband transitions in quantum wells is proposed where a pumping laser is used to create population inversion in the structure. The goal is to develop a structure which operates essentially as a 4-level laser, to minimize bottlenecking of the lower laser state. Multiple quantum wells can be used in the active laser of these structures to enhance the laser gain and the minimum required reflectivity in the cavity structure. The possibility of using both conduction and valence band quantum-well structures are investigated. Our study shows that, due to high intersubband scattering rates in the valence band structure, the creation of population inversion is more difficult and requires a high pumping power density while in the conduction band structure, population inversion can be achieved by a moderate pumping power density. The maximum population inversion in the conduction band structure is estimated to be 2.1×10¹¹ cm², which requires a pumping power density 2 kW cm^-2 for a single quantum well. The threshold power as well as the minimum required reflectivity of the cavity structure for the conduction band scheme are estimated for different well numbers     

First demonstration of room temperature intersubband-interbanddouble-resonance spectroscopy of GaAs/AlGaAs quantum wells

Intersubband-interband double-resonance experiments in undoped GaAs/Al_0.33Ga_0.67As multiple quantum well (MQW) structures at room temperature are discussed. The well width is 78 Å. A Ti:sapphire laser is used to pump the interband transitions, while the first intersubband transition is probed with a CO₂ laser. The intersubband absorption is found to peak at 10.6 μm, and a 10-meV linewidth is measured. The absorption signal is also recorded at a fixed CO₂ tuning while varying the pump laser wavelength from 700 to 850 nm. A high-resolution spectrum is obtained, reflecting the steplike density of states with sharp peaks at the exciton resonances     

Single-mode tunable quantum cascade lasers in the spectral range ofthe CO2 laser at λ=9.5-10.5 μm

Widely tunable, single-mode quantum cascade distributed feedback (QC-DFB) lasers based on a complex coupling scheme and operating in the wavelength range of the CO₂ laser (λ≈9.5-10.5 μm) are reported. Dynamic single-mode emission up to high current levels is obtained. The continuous single-mode tuning range is 150 nm, while the tuning range of the equivalent Fabry-Perot laser is ~400 nm. By homogeneously reducing all layer thicknesses by 10%, the wavelength coverage of a single QC-laser design can be extended to cover one entire regular band of the fundamental CO₂ laser spectrum     

Analysis of reduced interband absorption mechanisms insemiconductor quantum wells

Mechanisms for reduced interband absorption using intersubband coherence or intersubband coupling in semiconductor quantum wells are analyzed by exactly solving the density matrix equation. By placing some mild limitations on the diagonal density matrix elements, general analytic solutions are obtained without weak-field restrictions. Our solutions demonstrate reduced absorption, various nonlinear processes, and coupling-field-modulation of interband transitions. Our results agree well with previous results for three-level atomic systems, give insights into nonlinear mechanisms, and point out the importance of nonperturbative approaches to this class of problems     

Interband nonlinear optical generation in presence of intersubbandlight in asymmetric quantum wells

The interband sum frequency generation process due to three-wave interaction of interband and intersubband coupling lights has been investigated in a semiconductor quantum well using the perturbational density matrix approach. The origin of the nonlinear process lies in the second-order susceptibility χ_c2-h1⁽²⁾ arising due to the optical transition between the second conduction subband and the first heavy hole state. Both the sign and the magnitude of the second-order susceptibility of the well may be controlled by the carrier density level and the frequency of the intersubband field     

Intersubband gain and stimulated emission in long-wavelength(λ=13 μm) intersubband In(Ga)As-GaAs quantum-dotelectroluminescent devices

The dynamics of injected carriers and the conditions for intersubband gain and population inversion in In(Ga)As-GaAs self-organized quantum dots have been studied. Direct femtosecond pump-probe spectroscopy as a function of temperature and excitation density confirms earlier results and shows a long (>100 ps) electron relaxation time between the excited states and ground state in the dots. Intersubband gains as high as 170 cm^-1 are calculated in the dots. Far-infrared spontaneous emission centered around 13 μm is observed in edge-emitting light-emitting diodes. Stimulated emission, with a distinct threshold around 1.1 kA/cm² in the light-current characteristics, is observed in plasmon-enhanced waveguide devices. The intersubband threshold occurs after a threshold is observed for interband lasing (~1 μm) in the same device     

Doping effects on intersubband and interband optical transitions inGaSb-InAs superlattices

Normal incidence intersubband and interband absorptions of a novel type II GaSb-InAs superlattices can be obtained by utilizing the various doped-type cap and buffer layers. Moreover, the types and intensities of the absorptions could also be modulated by changing doping concentration. The intersubband transition can occur due to the strong mixing of the heavy-hole band and the light-hole band for InAs n-type cap and buffer layers. But the interband transition is a result of coupling between the wave-functions of the first conduction subband and the first heavy-hole subband for GaSb p-type cap and buffer layers. Both the intensities of intersubband can be modulated by changing doping concentration, and the corresponding wavelengths are in the ranges of 3-5 μm and 8-14 μm, respectively. Hence, it shows the potential application as an infrared photodetector     

Analysis of transient interband light modulation by ultrashortintersubband resonant light pulses in semiconductor quantum wells

We explore the transient characteristics of an interband resonant light modulation process by ultrashort intersubband resonant light pulses in semiconductor quantum wells (QW's). The modulation characteristics in a three-level semiconductor QW system, including the effects due to in-plane momentum, have been investigated by a numerical analysis of the coupled Bloch equations using a density-matrix approach. We have studied the effect of the carrier density and the nature of the intersubband coupling light on the transient absorption features of the interband light. The modulation process has been compared in doped and undoped QW's. The switching behavior of the strong interband light field in presence of a train of intersubband light pulses has also been discussed     

Intersubband χ3 in coupled InGaAs-AlGaAs multiplequantum wells

The third-order nonlinearity, χ³(ω,ω,-ω), is measured for a mid-infrared intersubband transition in strained InGaAs-AlGaAs multiple quantum wells (MQW's). The high conduction band offset of this system allows an intersubband transition at 3.1 μm. The level structure of the quantum well is designed to include a meta-stable trapping level, resulting in a peak saturation intensity of 6 MW/cm² at Brewster's angle, approximately 20 times lower than would be found in a square quantum well with similar linewidth. A near-resonant n₂ of 8.4×10^-7 cm²/W at 3.1 μm is calculated. The off-resonant n₂ is also calculated and shown to be attractive at wavelengths as short as 1.55 μm     

Pulsed and continuous-wave operation of long wavelength infrared(λ=9.3 μm) quantum cascade lasers

The operation of quantum cascade lasers at a wavelength (λ≃9.3 μm) well within the 8-13-μm atmospheric window is reported. A detailed study of intersubband luminescence in a vertical transition structure shows linewidths as narrow as ~10 meV at cryogenic temperatures, increasing to 20 meV at room temperature. Pulsed operation is demonstrated up to 220 K with a peak power ≈10 mW and ≈35 mW at 140 K. The temperature dependence of the threshold current density (J _th) is described by a high T₀ (128 K), J_th is also systematically studied as a function of cavity length to determine the peak gain and waveguide losses. Continuous-wave, single-mode operation is demonstrated up to 30 K with powers ≈2 mW     

Characterization and modeling of quantum cascade lasers based on aphoton-assisted tunneling transition

A detailed characterization and modeling of long-wavelength (λ~10 μm) quantum cascade (QC) lasers based on a photon-assisted tunneling transition are presented. In particular, the influence of the finite lifetime of the lower state of the laser transition on the current-voltage and threshold current versus temperature characteristics have been studied both theoretically and experimentally. It is shown that, for our structure, the value of the lower state lifetime can be extracted from the voltage-current curve; the value we found was 2.6 ps. In addition, this model allows to understand the abrupt degradation of the performance of the device for T>150 K. Low temperature (T=10 K) threshold current densities of 1.1 kA/cm² and a tuning range of 85 cm^-1 in pulsed mode are reported. In continuous-wave mode, the emission linewidth of a free-running laser was determined to be 3.9 MHz     

GaAs-AlGaAs quantum cascade lasers: physics, technology, andprospects

In recent years, the performance of GaAs-AlGaAs-based quantum cascade (QC) lasers has improved markedly. These devices are capable of pulsed room temperature operation and can deliver respectable average powers (11 mW at λ~9 μm) operating on a Peltier cooler. This performance has been achieved by the suppression of thermally activated carrier leakage through increases in the heterobarrier band offset. We demonstrate that QC lasers, with wavelengths λ⩾9 μm, can operate using heterostructures encompassing the entire composition range of Al_xGa_1-xAs, without encountering potential problems-of the satellite X-minima for x>45%. Furthermore, we present particular characteristics of these devices, such as a phonon-limited temperature dependence, electrical and optical self-oscillations, and novel design concepts that exploit this closely lattice matched material system. Finally, we discuss improvements in device fabrication to lower the operating current through a reduction of the area of current injection. Using this technology, devices can be designed to selectively pump the fundamental lateral mode. We, therefore, observe single spatial-mode operation over the entire current range of operation     

The role of intersubband optical transitions on the electrical properties of InGaAs/GaAs quantum dot solar cells

We report on an experimental study of the intersubband optical response of an In_0.5Ga_0.5As/GaAs quantum dot solar cell (QDSC). By calculating the quantum dot absorption cross section, the strength of the intersubband optical transitions is gauged, and their importance and influence on the electrical properties of the solar cell can be compared with those of other physical processes such as thermal intersubband and optical interband transitions. The temperature‐dependent photocurrent and dark current characteristics of the QDSC have also been analyzed in detail, leading to an understanding of the specific effects reducing the open‐circuit voltage. The deviation of QDSCs from idealized models is discussed, and some conditions required for an improved open‐circuit voltage are suggested. Copyright © 2012 John Wiley & Sons, Ltd.     

1.5 μm λ/4 shifted multiple quantum well distributedfeedback laser diodes

1.5 μm λ/4 shifted multiple quantum well distributed feedback laser diodes have been achieved for the first time. A characteristic temperature value for a threshold current at around room temperature was as high as 88 K. Spectra at 0.9 times the threshold current showed substantial TM mode suppression. The MQW active region consists of four GaInAs wells (75 Å thick) and GaInAsP barriers (λ_g=1.15 μm, 150 Å thick) grown by metalorganic vapour phase epitaxy (MOVPE). 1.3 μm GaInAsP was grown as an optical guide layer     

Proposal of an optical modulator based on resonant tunneling andintersubband transitions

We propose and analyze an optical modulator based on intersubband transitions. The absorption is modulated by modulating the carrier density in the ground state of a quantum well (QW). Electrons are injected resonantly into this subband from a QW reservoir subband through a single barrier. When the two states are tuned out of resonance, the electrons are rapidly evacuated by means of the optical field. A waveguide based on surface plasmons is assumed in order to have a high optical mode overlap. Calculations are performed for a cascaded structure with four periods, assuming InGaAs-InIAs QWs. The considered modulator structure operates at λ=6.0 μm and is RC limited to 27 GHz. An extinction ratio of 4 is obtained with a low applied voltage of 0.6 V. At larger applied voltages, the absorption is bistable. Absorption at shorter/longer wavelengths can be obtained by using materials with a larger/smaller conduction band offset. We also assess resonant tunneling from a 2-D electron gas reservoir into an array of quantum dots and compare it to the 2-D-2-D tunneling resonance     

Continuous wave operation of λ~19 μm surface-plasmonquantum cascade lasers

Continuous wave laser action has been achieved in a superlattice quantum cascade device operating on surface plasmon waveguide modes. The emission wavelength λ~19 μm is by far the longest ever reported for continuous wave III-V semiconductor lasers. The output power at cryogenic temperature is of the order of the mW     

Large Stark effects for transitions from local states to globalstates in quantum well structures

The electric-field dependence of the optical absorption for the type of quantum well structure in which one or more small wells are embedded in a big well is discussed. In such structures, local energy states confined by the small well(s) and global states confined by the big well have different electric-field dependences while their wave functions remain overlapped. Thus, a large Stark effect (large energy shift and oscillator strength) can be achieved for the optical transition from a local state to a global state. This concept of using the local and global states can be applied to both interband and intersubband transitions. For intersubband transitions, a typical 10-20 meV shift is predicted, compared to a reported 1.1 meV blue shift at the field 30 kV/cm. For interband transitions, the Stark shift is larger than that in the single quantum wells     

Pulse operation and threshold characteristics of 1.55-μmInAlGaAs-InAlAs VCSELs

All-monolithic air-post index-guided vertical-cavity surface-emitting lasers have been demonstrated under pulsed electrical injection at room temperature. The structure grown in single step by metal-organic chemical vapor deposition employs InP lattice matched InAlAs/InAlGaAs Bragg mirrors and a 2λ-thick periodic gain active region with 15 InGaAs quantum wells (QWs). We report threshold current characteristics of these devices grown on a 2-in wafer with wide emission wavelength range of 1.51~1.59 μm. For the devices larger than 30-μm in diameter, we found the minimum threshold current density of ~2.93 kA/cm² at the emission wavelength of 1.57 μm, corresponding to about 20 nm wavelength offset between photoluminescence peak of InGaAs QWs and resonant cavity wavelength