Jeremy Kirch

Highly temperature insensitive, deep-well 4.8 μm emitting quantum cascade semiconductor lasers

4.8 μm emitting, quantum cascade (QC) lasers that suppress carrier leakage out of their active regions to the continuum have been realized by using deep (in energy) quantum wells in the active regions, tall barriers in and around the active regions, and tapered conduction-band-edge relaxation regions. The characteristic temperature coefficients T0 and T1 for the threshold current density Jth and slope efficiency, respectively, reach values of 238 K over the 20–60 °C temperature range, which means that Jth and the slope efficiency vary with temperature half as fast as those of conventional QC lasers. In turn, significantly improved continuous wave performance is expected.

Multidimensional Conduction-Band Engineering for Maximizing the Continuous-Wave (CW) Wallplug Efficiencies of Mid-Infrared Quantum Cascade Lasers

By tailoring the active-region quantum wells and barriers of 4.5-5.0-μm-emitting quantum cascade lasers (QCLs), the device performances dramatically improve. Deep-well QCLs significantly suppress carrier leakage, as evidenced by high values for the threshold-current characteristic temperature <i>T</i>₀ (253 K) and the slope-efficiency characteristic temperature <i>T</i>₁ (285 K), but, due to stronger quantum confinement, the global upper-laser-level lifetime τ_4g decreases, resulting in basically the same room-temperature (RT) threshold-current density <i>J</i>_th as conventional QCLs. Tapered active-region (TA) QCLs, devices for which the active-region barrier heights increase in energy from the injection to the exit barriers, lead to recovery of the τ_4g value while further suppressing carrier leakage. As a result, experimental RT <i>J</i>_th values from moderate-taper TA 4.8-μm emitting QCLs are ~14% less than for conventional QCLs and <i>T</i>₁ reaches values as high as 797 K. A step-taper TA (STA) QCL design provides both complete carrier-leakage suppression and an increase in the τ_4g value, due to Stark-effect reduction and strong asymmetry. Then, the RT <i>J</i>_th value decreases by at least 25% compared to conventional QCLs of same geometry. In turn, single-facet, RT pulsed and continuous-wave maximum wallplug-efficiency values of 29% and 27% are projected for 4.6-4.8-μm-emitting QCLs.

5.5 W near-diffraction-limited power from resonant leaky-wave coupled phase-locked arrays of quantum cascade lasers

Five, 8.36 μm-emitting quantum-cascade lasers (QCLs) have been monolithically phase-locked in the in-phase array mode via resonant leaky-wave coupling. The structure is fabricated by etch and regrowth which provides large index steps (Δn = 0.10) between antiguided-array elements and interelement regions. Such high index contrast photonic-crystal (PC) lasers have more than an order of magnitude higher index contrast than PC-distributed feedback lasers previously used for coherent beam combining in QCLs. Absorption loss to metal layers inserted in the interelement regions provides a wide (∼1.0 μm) range in interelement width over which the resonant in-phase mode is strongly favored to lase. Room-temperature, in-phase-mode operation with ∼2.2 kA/cm2 threshold-current density is obtained from 105 μm-wide aperture devices. The far-field beam pattern has lobewidths 1.65× diffraction limit (D.L.) and 82% of the light in the main lobe, up to 1.8× threshold. Peak pulsed near-D.L. power of 5.5 W is obtained, with 4...

Design for high-power, single-lobe, grating-surface-emitting quantum cascade lasers enabled by plasmon-enhanced absorption of antisymmetric modes

Resonant coupling of the transverse-magnetic polarized (guided) optical mode of a quantum-cascade laser (QCL) to the antisymmetric surface-plasmon modes of 2nd-order distributed-feedback (DFB) metal/semiconductor gratings results in strong antisymmetric-mode absorption. In turn, lasing in the symmetric mode, that is, surface emission in a single-lobe far-field beam pattern, is strongly favored over controllable ranges in grating duty cycle and tooth height. By using core-region characteristics of a published 4.6 μm-emitting QCL, grating-coupled surface-emitting (SE) QCLs are analyzed and optimized for highly efficient single-lobe operation. For infinite-length devices, it is found that when the antisymmetric mode is resonantly absorbed, the symmetric mode has negligible absorption loss (∼0.1 cm−1) while still being efficiently outcoupled, through the substrate, by the DFB grating. For finite-length devices, 2nd-order distributed Bragg reflector (DBR) gratings are used on both sides of the DFB grating to p...

Low-threshold thin-film III-V lasers bonded to silicon with front and back side defined features.

A III-V thin-film single-quantum-well edge-emitting laser is patterned on both sides of the epitaxial layer and bonded to silicon. Injected threshold current densities of 420 A/cm(2) for gain-guided lasers with bottom p-stripes and top n-stripes and 244 A/cm(2) for index-guided bottom p-ridge and top n-stripe lasers are measured with a lasing wavelength of approximately 995 nm. These threshold current densities, among the lowest for thin-film edge-emitting lasers on silicon reported to date (to our knowledge), enable the implementation of integrated applications such as power-efficient portable chip-scale photonic sensing systems.

Controlled growth of InGaAs/InGaAsP quantum dots on InP substrates employing diblock copolymer lithography

Selective metalorganic chemical vapor deposition growth with diblock copolymer nanopatterning is utilized to produce InGaAsP(Q1.15 μm)/In0.53Ga0.47As/InGaAsP(Q1.15 μm) and InP/In0.53Ga0.47As/InP quantum dots (QDs) on InP substrates. The QD patterning is prepared by dense nanoscale diblock copolymer lithography followed by pattern-transfer onto a dielectric template mask and reactive ion etching is utilized to form nanosized openings exposing the underlying InGaAsP layer. By varying the In0.53Ga0.47As layer thickness within the QDs, the emission wavelength can be selected within the 1.4–1.6 μm region. Strongest photoluminescence (PL) intensity is observed from QDs employing InP rather than InGaAsP barriers, demonstrating room temperature PL near 1.6 μm.

High-power, surface-emitting quantum cascade laser operating in a symmetric grating mode

Grating-coupled surface-emitting (GCSE) lasers generally operate with a double-lobed far-field beam pattern along the cavity-length direction, which is a result of lasing being favored in the antisymmetric grating mode. We experimentally demonstrate a GCSE quantum-cascade laser design allowing high-power, nearly single-lobed surface emission parallel to the longitudinal cavity. A 2nd-order Au-semiconductor distributed-feedback (DFB)/distributed-Bragg-reflector (DBR) grating is used for feedback and out-coupling. The DFB and DBR grating regions are 2.55 mm- and 1.28 mm-long, respectively, for a total grating length of 5.1 mm. The lasers are designed to operate in a symmetric (longitudinal) grating mode by causing resonant coupling of the guided optical mode to the antisymmetric surface-plasmon modes of the 2nd-order metal/semiconductor grating. Then, the antisymmetric modes are strongly absorbed by the metal in the grating, causing the symmetric mode to be favored to lase, which, in turn, produces a single-lobed beam over a range of grating duty-cycle values of 36%–41%. Simulations indicate that the symmetric mode is always favored to lase, independent of the random phase of reflections from the devices cleaved ends. Peak pulsed output powers of ∼0.4 W were measured with nearly single-lobe beam-pattern (in the longitudinal direction), single-spatial-mode operation near 4.75 μm wavelength. Far-field measurements confirm a diffraction-limited beam pattern, in agreement with simulations, for a source-to-detector separation of 2 m.

Carrier dynamics in MOVPE-grown bulk dilute nitride materials for multi-junction solar cells

Dilute nitride materials with a 1eV band-gap lattice matched to GaAs substrates are attractive for high-efficiency multi-junction solar cells. Carrier lifetime measurements are crucial in optimizing material growth and p-i-n field-aided carrier-extraction-device design. One research group has reported carrier lifetimes of MBE-grown bulk InGaNAsSb materials, but there has been no report of carrier lifetime measurements from bulk InGaNAsSb grown by MOVPE. In this study, we report the growth of bulk InGaNAsSb by MOVPE and the first carrier lifetime measurement from MOVPE-grown bulk InGaNAsSb materials with Eg= 1.0 - 1.2eV at 300K. We studied carrier dynamics in MOVPE-grown bulk dilute nitride materials nominally lattice matched to GaAs (100) substrates: 1μm thick In0.035GaN0.025As (Eg= 1.0eV at 300K) and ~0.2μm thick In(0.05-0.07)GaN(0.01-0.02)AsSb(0.02-0.06) layers (Eg= 1.2eV at 300K). Both structures are fully strained. The incorporation of N in InGaNAs leads to degradation in photoluminescence efficiency, but prior studies indicate the addition of Sb in MBE-grown InGaNAsSb improved the PL efficiency. Two-step post-growth thermal annealing processes were optimized to obtain maximum PL efficiencies that yielded a typical blue shift of 50 and 30meV for InGaNAs and InGaNAsSb, respectively. We employed a streak camera to measure carrier lifetimes from both as-grown and thermally annealed samples. Carrier lifetimes of <30psec were obtained from the InGaNAs samples, whereas carrier lifetimes of up to ~150psec were obtained from the InGaNAsSb samples. We discuss possible reasons for short carrier lifetimes measured from MOVPE-grown InGaNAs(Sb) materials.

Spontaneous Radiative Efficiency and Gain Characteristics of Strained-Layer InGaAs–GaAs Quantum-Well Lasers

The optical gain spectra, unamplified spontaneous emission spectra, and spontaneous radiative efficiency are extracted from the measurement of amplified spontaneous emission (ASE) on a single pass, segmented contact 0.98-mum-emitting aluminum-free InGaAs-InGaAsP-GaAs quantum-well (QW) laser diode. These measurements provide a baseline for which to compare higher strain InGaAs QW lasers emitting near 1.2 mum. The peak gain-current relationship is extracted from gain spectra and the peak gain parameter go is found to agree within 25% of the value extracted using conventional cavity length analysis for 0.98-mum-emitting devices. The spontaneous radiative current is extracted using the fundamental connection between gain and unamplified spontaneous emission, which in turn gives an estimate of the amount of nonradiative recombination in this material system. The spontaneous radiative efficiency, the ratio of spontaneous radiative current to total current, at room temperature of 0.98-mum-emitting InGaAs QW laser material is found to be in the range of 40%-54%, which is 2.5-3.5 times larger than that of highly strained InGaAs QW laser emitting near lambda = 1.2 mum. Whereas the gain parameter, g0 = dg/d(ln j), was measured to be 1130 and 1585 cm-1 for the 0.98-mum- and 1.2-mum-emitting materials, respectively. From the calculated below threshold current injection efficiency of 75%-85%, we deduce that the internal radiative efficiency of the QW material is ~ 20% higher than the ratio of internal radiative current to external injected current extracted directly from ASE measurements.

Highly temperature insensitive, low threshold-current density (λ = 8.7–8.8 μm) quantum cascade lasers

By stepwise tapering, both the barrier heights and quantum-well depths in the active regions of 8.7–8.8 μm-emitting quantum-cascade-laser (QCL) structures, virtually complete carrier-leakage suppression is achieved. Such step-taper active-region-type QCLs possess, for 3 mm-long devices with high-reflectivity-coated back facets, threshold-current characteristic temperature coefficients, T0, as high as 283 K and slope-efficiency characteristic temperature coefficients, T1, as high as 561 K, over the 20–60 °C heatsink-temperature range. These high T0 and T1 values reflect at least a factor of four reduction in carrier-leakage current compared to conventional 8–9 μm-emitting QCLs. Room temperature, pulsed, threshold-current densities are 1.58 kA/cm2; values comparable to those for 35-period conventional QCLs of similar injector-region doping level. Superlinear behavior of the light-current curves is shown to be the result of the onset of resonant extraction from the lower laser level at a drive level of ∼1.3×...