Mid-infrared w quantum-well lasers for noncryogenic continuous-wave operation

We review the recent progress of electrically injected and optically pumped mid-IR lasers based on antimonide quantum wells with the type II W configuration. W quantum-well diodes have achieved cw operation up to 195 K at lambda = 3.25 mum. Optically pumped devices that employ the diamond pressure bond heat sink have reached 290 K at 3 mum and 210 K at 6 mum. Pulsed power conversion efficiencies of up to 7% at 220 K have been attained by use of an optical pumping injection cavity approach, in which an etalon cavity for the pump beam significantly enhances its absorptance. The angled-grating distributed-feedback configuration has been used to obtain near-diffraction-limited output for an optical pumping stripe width of 50 mum.     

High-Power Single-Mode Distributed-Feedback Interband Cascade Lasers for the Midwave-Infrared

A single-mode output power of 41 mW has been obtained at T=120 K and lambdaap3.44 mum from a narrow-ridge interband cascade laser patterned with a Ge distributed-feedback grating. The sidemode suppression ratio at the maximum power is 23 dB, and the linewidth of 0.1 nm is instrument-limited. An alternate contacting geometry yielded robust single-mode output over a broad range of currents and temperatures, and current tuning of the wavelength by up to 17 nm     

Shallow-Etch Mesa Isolation of Graded-Bandgap “W”-Structured Type II Superlattice Photodiodes

Shallow-etch mesa isolation (SEMI) of graded-bandgap “W”-structured type II superlattice (GGW) infrared photodiodes provides a powerful means for reducing excess dark currents due to surface and bulk junction related processes, and it is particularly well suited for focal-plane array fabrication. In the n-on-p GGW photodiode structure the energy gap is increased in a series of steps from that of the lightly p-type infrared-absorbing region to a value typically two to three times larger. The wider gap levels off about 10 nm short of the doping-defined junction, and continues for another 0.25 μm into the heavily n-doped cathode before the structure is terminated by an n ⁺-doped InAs top cap layer. The increased bandgap in the high-field region near the junction helps to strongly suppress both bulk tunneling and generation–recombination (G–R) current by imposing a much larger tunneling barrier and exponentially lowering the intrinsic carrier concentration. The SEMI approach takes further advantage of the graded structure by exposing only the widest-gap layers on etched surfaces. This lowers surface recombination and trap-assisted tunneling in much the same way as the GGW suppresses these processes in the bulk. Using SEMI, individual photodiodes are defined using a shallow etch that typically terminates only 10 nm to 20 nm past the junction, which is sufficient to isolate neighboring pixels while leaving the narrow-gap absorber layer buried 100 nm to 200 nm below the surface. This provides for separate optimization of the photodiode’s electrical and optical area. The area of the junction can be reduced to a fraction of that of the pixel, lowering bulk junction current, while maintaining 100% optical fill factor with the undisturbed absorber layer. Finally, with the elimination of deep, high-aspect-ratio trenches, SEMI simplifies array fabrication. We report herein results from SEMI-processed GGW devices, including large-area discrete photodiodes, mini-arrays, and a focal-plane array. Current–voltage data show strong suppression of side-wall leakage relative to that for more deeply etched devices, as well as scaling of dark current with junction area without loss of quantum efficiency.     

Design optimization for high-brightness surface-emitting photonic-crystal distributed-feedback lasers

A new time-domain Fourier-Galerkin (TDFG) theory is developed to simulate the near-field, far-field and spectral characteristics of surface-emitting photonic-crystal distributed-feedback (SE PCDFB) lasers. It is found that a properly-designed two-dimensional hexagonal or square-lattice grating should efficiently couple the output into a single SE mode that retains coherence for aperture diameters of up to /spl ap/1 mm. We identify lattice structures and precise conditions under which all components of the transverse electric or transverse magnetic polarized optical fields constructively interfere to produce a single-lobed, near-diffraction-limited circular output beam. The TDFG simulations predict that quantum efficiencies as high as 30% (60% if reflectors are built into the waveguide structure) should be attainable. A surprising conclusion is that diffractive coupling into the surface-emitting direction must be relatively weak, in order to assure selection of the desired symmetric in-phase mode. Furthermore, gain media with a moderate linewidth enhancement factor should produce the best SE PCDFB performance, whereas edge emitters nearly always benefit from a very small value.     

High-power/low-threshold type-II interband cascade mid-IR laser-design and modeling

We present an optimized design and detailed numerical simulations for a mid-IR type-II interband InGaSb QW cascade laser (T2ICL) with InAs-In/sub 0.3/Ga/sub 0.7/Sb active quantum wells. It is shown that a 15-period T2ICL operating at 300 K and emitting at 3.15 /spl mu/m should achieve a much higher differential quantum efficiency (maximum of 0.9 W/A per facet at 300 K) than conventional mid-IR bipolar injection lasers, and a threshold current density much lower than for the intersubband quantum cascade laser.     

Fundamental materials studies of undoped, In-doped, and As-doped Hg1−xCdxTe

Variable magnetic-field Hall and transient photoconductance-lifetime measurements were performed on a series of undoped, In-doped, and As-doped HgCdTe samples grown by molecular beam epitaxy (MBE). Use of quantitative mobility-spectrum analysis (QMSA) combined with multiple carrier-fitting (MCF) techniques indicates that the majority of samples contain an interfacial n-type layer that significantly influences the interpretation of the electrical measurements. This n-type layer completely masks the high-quality electrical properties of undoped or low n-type In-doped HgCdTe, as well as complicating the interpretation of activation in As-doped p-type HgCdTe. Introduction of an intentional n-type background, typically created through doping with In to “recover” high mobility, is actually shown to increase the “bulk” layer conductivity to a level comparable to the interface layer conductivity. Photoconductance-lifetime measurements suggest that In-doping may introduce Shockley-Read-Hall (SRH) recombination centers. Variable-field Hall analysis is shown to be essential for characterizing p-type material. Photoconductance-lifetime measurements suggest that trapping states may be introduced during the incorporation and activation of As. Two distinctly different types of temperature dependencies were observed for the lifetimes of As-doped samples.     

A comparison of optoelectronic properties of lattice-matched andstrained quantum-well and quantum-wire structures

The k·p formalism is used to study the absorption spectra, material and differential gain in quantum wires as a function of orientation, built-in strain, and wire dimensions. The results for material and differential gain are compared with those for an optimized quantum-well structure. We find that for quantum wires at 300 K, the gain becomes positive at a carrier density of 1.27·10¹⁸ cm^-3, while in quantum wells this density is calculated to be 1.82·10¹⁸ cm^-3. Incorporating tensile strain in the wires reduces the transparency carrier concentration to 0.96·10¹⁸ cm^-3 while compressive strain allows one to obtain positive gain for densities greater than 1.08·10¹⁸ cm^-3. Orienting the wire along the [111] direction reduces the transparency carrier density to 0.60·10¹⁸ cm^-3. The differential gain in quantum-well structures for injections near the threshold is on the order of 10^-14 cm^-4, while for 50 Å·100-Å quantum wires the differential gain near the threshold is found to be on the order of 10^-13 cm^-4 . The differential gain in wires whose wire axis is parallel to the [111] direction has also been found to be on the order of 10^-13 cm^-4 for carrier injections close to the threshold     

Ultra-low temperature sensitive deep-well quantum cascade lasers (λ - 4.8 μm) via uptapering conduction band edge of injector region

A new design feature for deep-well quantum cascade (QC) lasers, in which the conduction band edge of the injector region is uptapered, results in virtual suppression of carrier leakage out of the active regions of 4.8 mm emitting devices. For heatsink temperatures in the 20?90°C range the characteristic temperature coefficients for threshold, T₀, and slope efficiency, T₁, reach values as high as 278 and 285 K, respectively, which are nearly twice the values for conventional QC lasers. At 20°C, the threshold current density for uncoated, 30 period, 3 mm-long devices is only ~1.8 kA/cm².     

Interband Cascade Lasers with Wavelengths Spanning 2.9 μm to 5.2 μm

We report an experimental investigation of four interband cascade lasers with wavelengths spanning the mid-infrared spectral range, i.e., 2.9 μm to 5.2 μm, near room temperature in pulsed mode. One broad-area device had a pulsed threshold current density of only 3.8 A/cm² at 78 K (λ = 3.6 μm) and 590  A/cm² at 300 K (λ = 4.1 μm). The room-temperature threshold for the shortest-wavelength device (λ = 2.6 μm to 2.9 μm) was even lower, 450 A/cm². A␣cavity-length study of the lasers emitting at 3.6 μm to 4.1 μm yielded an internal loss varying from 7.8 cm⁻¹ at 78 K to 24 cm⁻¹ at 300 K, accompanied by a decrease of the internal efficiency from 77% to 45%.     

Steady-state and transient characteristics of microcavitysurface-emitting lasers with compressively strained quantum-well activeregions

In conventional semiconductor lasers, the dimensions of the optical cavity greatly exceed the photon wavelength, and the photon density of states forms a continuum since it is essentially that of a bulk system. On the other hand, in an ideal laser, one would like to have a single optical mode coincident with the maximum in the gain spectrum of the active medium. We show that substantial density-of-states quantization and enhancement of the fraction of photons spontaneously emitted into the lasing mode can be obtained by reducing the lateral width of the surface-emitting laser. For emission at λ=0.954 μm, the threshold current density can be drastically reduced by increasing the coupling factor to a few percent. For a cavity structure width of 0.3 μm, the threshold current density is 50 A/cm², compared with 250 A/cm² for the 0.6-μm cavity. At lower still lateral widths, the cavity loses its vertical character, and confinement of the lateral optical mode rapidly deteriorates. The large-signal response of microcavity lasers is slightly improved primarily due to elimination of mode competition in intrinsically single-mode microcavities, with relaxation times close to 1 ns. The enhancement of the spontaneous emission coupling factor results in an increase of the relaxation oscillation frequency and improvement in the standard small-signal response of microcavity lasers. For J=10J_th, the -3 dB modulation frequency exceeds 40 GHz. Since low threshold current densities may be achieved in microcavity lasers, the gains in small-signal performance are primarily extrinsic, i.e., higher modulation bandwidths ace accessible for the same injection