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We report terahertz quantum cascade laser (QCL) sources based on intra-cavity difference-frequency generation processed into double-metal waveguides with surface-grating outcouplers. This configuration enables high confinement of the terahertz mode in the device active region and efficient surface extraction of terahertz radiation along the entire length of the waveguide. The devices operate at room temperature at 1.9 THz and produce over 110 μW of peak power output with the mid-infrared-to-terahertz conversion of 150 μW/W2. The results represent at least a factor of 2 improvement in the performance compared to the best Cherenkov difference-frequency generation QCL devices operating below 2 THz. [ABSTRACT FROM AUTHOR]

Contemporary research into diagonal active region terahertz quantum cascade lasers for high temperature operation has yielded little success. We present evidence that the failure of high diagonality alone as a design strategy is due to a fundamental trade-off between large optical oscillator strength and long upper-level lifetime. We hypothesize that diagonality needs to be paired with increased doping in order to succeed, and present evidence that highly diagonal designs can benefit from much higher doping than normally found in terahertz quantum cascade lasers. In assuming the benefits of high diagonality paired with high doping, we also highlight important challenges that need to be overcome, specifically the increased importance of carrier induced bandbending and impurity scattering. [ABSTRACT FROM AUTHOR]

Here, we present results of quantum cascade laser lifetime tests under various aging conditions including an accelerated life test. The total accumulated life time exceeds 1.5 x 106 device hours. The longest single device aging time was 46500 hours without failure in the room temperature aging test. Four failures were found in a group of 19 devices subjected to the accelerated life test with a heat-sink temperature of 60 °C and a continuous-wave current of 1 A. Failure mode analyses revealed that thermally induced oxidation of InP in the semi-insulating layer is the cause of failure. An activation energy of 1.2 eV is derived from the dependence of the failure rate on laser core temperature. The mean time to failure of the quantum cascade lasers operating at a typical condition with the current density of 5 kA/cm² and heat-sink temperature of 25 °C is expected to be 809 000 hours. [ABSTRACT FROM AUTHOR]

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 highpower, 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 2.55mm- and 1.28mm-long, respectively, for a total grating length of 5.1mm. The lasers are designed to operate in a symmetric (longitudinal) grating mode by causing resonant coupling of the guided optical mode to the antisymmetric surfaceplasmon 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 device's cleaved ends. Peak pulsed output powers of ~0.4W 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 2m. [ABSTRACT FROM AUTHOR]

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 lm-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 cm1) 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 prevent uncontrolled reflections from cleaved facets. Equations for the threshold-current density and the differential quantum efficiency of SE DFB/DBR QCLs are derived. For 7 mm-long, 8.0 lm-wide, 4.6 lm-emitting devices, with an Ag/InP grating of s39% duty cycle, and 0.22 lm tooth height, threshold currents as low as 0.45A are projected. Based on experimentally obtained internal efficiency values from high-performance QCLs, slope efficiencies as high as 3.4W/A are projected; thus, offering a solution for watt-range, single-lobe CW operation from SE, mid-infrared QCLs. [ABSTRACT FROM AUTHOR]

We present the design and realization of active photonic crystal (PhC) terahertz (THz) lasers operating in higher photonic bands. The structure consists of an array of isolated pillars fabricated from a THz quantum-cascade laser and embedded in a double-metal waveguide. The PhC geometry is adopted to achieve lasing in the first and second photonic bands. Thereby, the optical mode is pushed from the active pillars into the surrounding. The sensitivity of local sensors can be increased by almost one order of magnitude compared to designs operating in the lowest photonic band. [ABSTRACT FROM AUTHOR]

We demonstrated switching operation of lasing wavelength in GaAs-based mid-infrared coupled-cavity quantum cascade lasers. Our cavity structures consist of a ridge-waveguide Fabry–Perot cavity coupled with a microcylindrical cavity. Individual control of currents injected into both cavities enabled single-mode operation and facilitated intentional mode switching between coupled-cavity modes with spacing over 120 nm. The integrated microcylindrical cavity expanded the separation of coupled-cavity modes due to the small size of the cavity and enabled switching the lasing wavelength over a wide spectral range. [ABSTRACT FROM AUTHOR]

The authors describe electrical beam steering of mid-infrared quantum cascade lasers with a monolithically integrated Y-coupled cavity and electrically separated branches. The deflection of the laser beam is achieved without any additional components such as optic or mechanic systems. This is done by injecting additional direct current into one of the two emitting branches, locally increasing the temperature. We estimate that the required temperature difference between left and right branch is approximately 12 K to achieve 2° of the beam steering. This value is in a good agreement with heat transfer simulations. [ABSTRACT FROM AUTHOR]

We report on the observation of midinfrared intersubband (ISB) absorption in InGaAs/GaAsSb multiple quantum wells grown lattice-matched to InP substrates by molecular beam epitaxy. ISB absorption in a broad wavelength region (5.8–11.6 μm) is observed in quantum wells with well widths ranging between 4.5 and 12 nm. The conduction band offset at the InGaAs/GaAsSb heterointerface is measured to be 360 meV, giving an excellent agreement between the theoretically calculated ISB transition energies and the Fourier-transform infrared spectroscopy measurements over the whole range of well widths under investigation. [ABSTRACT FROM AUTHOR]

Surface emitting photonic-crystal quantum cascade lasers operating at λ≈7.3 μm are demonstrated. The photonic crystal resonator is written solely on the top metallization layer. The mismatch between the modes supported by metallized and nonmetallized regions yields enough optical feedback to achieve laser action. The devices exhibit single-mode emission with a side mode suppression ratio of ≈20 dB, the wavelength is lithographically tunable across a range of almost 70 cm-1, and the radiation is emitted from the surface. The maximum operating temperature is 220 K. The divergence of the output beam, which is doughnut-shaped, is approximately 9°. [ABSTRACT FROM AUTHOR]

We have calculated the performance of a recently proposed four-level scheme terahertz quantum cascade laser (4L terahertz-QCL) with the nonequilibrium Green’s function method. The calculation result for 40 K showed that the 4L QCL has a larger terahertz gain than the conventional resonant phonon QCL. This is because a large number of electrons accumulate in the upper lasing level and contribute to lasing in the new scheme. When the temperature is increased, the advantage of gain decreases due to thermally activated phonon scattering. [ABSTRACT FROM AUTHOR]

A quantum cascade laser based on a three well active module and emitting at 3 THz is demonstrated. The optical transition is vertical in real space and localized in an Al0.03Ga0.97As quantum well. Maximum operating temperature of 123 K in pulsed mode is reported, with threshold current densities as low as 110 A/cm2 at 10 K and 175 A/cm2 at 100 K. High slope efficiency values testify the good internal quantum efficiency of the structure. [ABSTRACT FROM AUTHOR]

We describe the fabrication and operation of surface emitting second-order distributed feedback quantum cascade ring lasers. The devices exhibit single-mode emission at a wavelength of 3.95 μm with a side mode suppression ratio of 25 dB. A linear tuning coefficient of 0.13 cm-1/K is observed. A single longitudinal mode in the ring shaped resonator results in a highly symmetric far-field pattern and a low beam divergence, represented by a full width at half maximum of ∼3°. Based on these characteristics the presented compact coherent light source may find its way into today’s midinfrared spectroscopy applications. [ABSTRACT FROM AUTHOR]

We report a surface-emitting terahertz source based on intracavity difference-frequency generation in dual-wavelength midinfrared quantum cascade lasers with integrated giant second-order nonlinear susceptibility. The terahertz light is coupled out of the waveguide by a second-order grating etched into the laser ridges. In contrast to sources where the difference-frequency radiation is extracted from the facet, this approach enables extraction of the terahertz emission from the whole length of the device even when the coherence length is small. [ABSTRACT FROM AUTHOR]

We report on the fabrication and operation of quantum cascade ring lasers providing grating-coupled surface emission. The devices exhibit tunable far fields, ranging from spot- to ring-shaped symmetric beam cross sections, depending on the grating period. This—along with threshold current densities as low as for comparable Fabry–Pérot lasers—demonstrates the compatibility of reduced beam divergence and two-dimensional integrability, resulting in an attractive light source for applications in midinfrared spectroscopy and imaging. [ABSTRACT FROM AUTHOR]

The effect of extraction barrier width on the performance of a resonant-phonon terahertz quantum cascade laser is studied using the ensemble Monte Carlo method. The width of extraction barrier is varied from 48 to 33 Å with a stepsize of -3 Å. The calculated threshold current density increases monotonously from 436 to 1054 A/cm2 with decreasing extraction barrier width, while the peak gain shows nontrivial behavior. These findings agree well with an experimental study. The simulation results suggest an optimal extraction barrier width of 36 Å for peak gain and for the best temperature performance. [ABSTRACT FROM AUTHOR]

Tuning of the emission wavelength in GaAs/AlxGa1-xAs bound-to-continuum quantum cascade lasers with different Al mole fractions (x) is reported. By varying x in the range of 0.37–0.52, a shift of the emission wavelengths of over 4 μm has been observed. Using this method, laser action in the range of 11.2–15.3 μm at temperatures T>=260 K has been demonstrated with a record value of ∼340 K for GaAs based QCLs operating at 13.5 μm. [ABSTRACT FROM AUTHOR]

Second-harmonic (SH) emission in the form of Cˇerenkov-type phase-matched radiation has been measured for quantum-cascade lasers (QCLs) with built-in nonlinearities. The QCLs operate at 10.9 μm wavelength and, due to material dispersion, show a high collinear phase mismatch, resulting in low SH external conversion efficiencies on the order of 2 μW/W2. Due to our waveguide design, we were able to couple out and measure the generated SH light in the form of Cˇerenkov-type phase-matched radiation from the substrate of the device. The SH power collected from the Cˇerenkov beam leads to an increased external conversion efficiency of ∼50 μW/W2. [ABSTRACT FROM AUTHOR]

Midinfrared quantum cascade lasers with monolithically integrated Y-junctions are investigated. Two different emission wavelengths of 10.5 and 4.2 μm were realized in two different material systems, lattice-matched GaAs/AlGaAs on GaAs and strain-compensated InGaAs/InAlAs/AlAs on InP. With identical Y-junction dimensions, phase locking is observed in both structures. In GaAs based devices, fundamental lateral modes are present in the coupled waveguides, which are coherently synchronized at the Y-junction. In InP based devices, modes of higher order are excited, which originate from coupling. The generation of multiple modes yields an out-of-phase fraction which reduces the level of coherence. [ABSTRACT FROM AUTHOR]

A hybrid GaAs quantum cascade laser system obtained by Au–Au thermocompression bonding epilayer down onto gold coated silicon substrates is presented in this paper. The performance of the hybrid laser in low-duty-cycle pulsed operation in comparison to an unbonded one was not deteriorated. The lasers run with a threshold of 4.6 kA/cm2, emit around 12 μm, and with a maximum optical output power of 550 mW at cryogenic temperatures. The key advantage of such hybrid chips is the possibility of integrating III-V cascade lasers with established silicon photonics technology, such as silicon-on-insulator waveguides, V-groove fiber coupling and microfluidics. [ABSTRACT FROM AUTHOR]

By using a semiconductor/metal grating formed on the episide of a quantum-cascade structure, distributed feedback lasing has been achieved with beam emission through the substrate. Using short-pulse excitation (100 ns, 16 kHz), single-longitudinal-mode operation near 5.1 μm is demonstrated over wide ranges in heatsink temperature and drive current. The beam divergence in the longitudinal direction at a distance 40 cm away from the 2.5 mm wide aperture is ∼0.5°. [ABSTRACT FROM AUTHOR]

A monolithic coupling scheme in which two active waveguides merge into a single waveguide is presented for a GaAs/AlGaAs quantum cascade laser. The evolving fields interfere and a constant phase is observed in the Y-shaped laser cavity, resulting in a far field pattern of a double slit. The mode distribution is comprehensively derived by matching the far field profiles to simulated values and shows a weak current dependence. The device demonstrates the feasibility of coherent laser resonators with prospective applications in interferometric sensing and high power laser arrays. [ABSTRACT FROM AUTHOR]

The authors present the effects of the doping concentration on the performance of a set of terahertz quantum-cascade lasers emitting around 2.75 THz. The chosen design is based on the longitudinal-optical-phonon depopulation of the lower laser state. An identical structure is regrown varying the sheet density from 5.4×109 to 1.9×1010 cm-2. A linear dependency of the threshold current density on the doping is observed. The applied field where lasing takes place is independent of the doping. The field is responsible for the alignment of the cascades and therefore the transport of the electrons through the structure. [ABSTRACT FROM AUTHOR]

The authors report on tunable, miniaturized, two-segment coupled cavity quantum cascade lasers fabricated in a process with a single dry etch step. They observed controllable vernier-effect-based mode switching, induced by the spectral shift of the short cavity’s Fabry-Pérot mode comb due to Joule heating. With the help of this discontinuous tuning mechanism, single-mode operation with routinely ∼20 dB side mode suppression ratio was achieved over a broad spectral range. The authors measured a quasicontinuous tuning range of 12 cm-1 (138 nm) and an entire single-mode tuning range of 16.5 cm-1 (190 nm), centered around 933 cm-1 (10.7 μm). [ABSTRACT FROM AUTHOR]

The authors have measured the output spectrum and the threshold current in 9.2 μm wavelength GaAs/Al0.45Ga0.55As quantum-cascade lasers at 115 K as a function of hydrostatic pressure up to 7.3 kbars. By extrapolation back to ambient pressure, thermally activated escape of electrons from the upper lasing state up to delocalized states of the Γ valley is shown to be an important contribution to the threshold current. On the other hand leakage into the X valley, although it has a very high density of states and is nearly degenerate with the Γ band edge in the barrier, is insignificant at ambient pressure. [ABSTRACT FROM AUTHOR]

The authors have fabricated GaAs/Al0.45Ga0.55As quantum cascade lasers (QCLs) with two-dimensional photonic crystal (PhC) mirrors. The monolithically integrated PhC mirrors consist of a triangular lattice of ∼14 μm deeply etched semiconductor pillars. The PhC is designed to exhibit a photonic band gap for both the ΓM and the ΓK crystal orientation and provides reflectivities of >80%. Compared to a reference laser with the same cavity length of 600 μm but as-cleaved facets, the QCLs with ΓM and ΓK oriented PhCs show reductions of the threshold current by 34% and 30%, respectively. [ABSTRACT FROM AUTHOR]

The reliability of high power strain-balanced Ga0.331In0.669As/Al0.659In0.341As/InP intersubband quantum cascade lasers (QCLs) emitting at a wavelength of 4.6 μm has been studied. Constant current aging is reported for two randomly selected high-reflectivity-coated QCLs with an output power over 100 mW. QCLs are tested under continuous-wave operation at a heat sink temperature of 298 K(25 °C) corresponding to an internal temperature of 378 K (105 °C). Over 4000 h of continuous testing is reported without any decrease in output power. [ABSTRACT FROM AUTHOR]

We report on a detailed experimental and theoretical analysis of the role of injector doping density on both the threshold current density (Jth) and the saturation current density (Jsat), determining the dynamic range of the quantum cascade lasers. The experimental results were obtained from two growth series of λ≈9 μm GaAs/Al0.45Ga0.55As quantum-cascade lasers based on single and double phonon resonance depopulation mechanisms. We derive a quasilinear dependence of Jth on the injector doping density of both designs for doping range as wide as (4–10)×1011 cm-2. Despite threshold current increase the faster rise of saturation current with doping results in an enhanced dynamic range for injector doping up to ∼8×1011 cm-2. For higher doping levels, Jsat itself saturates. Furthermore, our investigations yield that single phonon resonance devices exhibit clear current saturation simultaneously with a decrease of the optical power, whereas two phonon resonance devices show only power saturation, which we attribute to increased leakage currents. These deteriorate the laser performance of the latter design at higher operational temperatures. [ABSTRACT FROM AUTHOR]

We present two InP-based quantum cascade detectors (QCDs) in the mid-infrared wavelength range. Their narrow band detection spectra are centered at 5.3 and 9 μm. A vertical intersubband transition followed by a carefully designed extraction cascade, which is adapted to the LO-phonon energy, leads to 10 K responsivities R of 3.2 and 9.0 mA/W and background limited detectivities DBLIP* of 2×108 and 3×109 Jones, for the 5.3 and the 9 μm devices, respectively. Detection has been observed up to device temperatures of 300 K (RT), albeit reasonable performance is restricted to temperatures below 150 K (5.3 μm) and 70 K (9 μm). Designed for zero bias operation, QCDs do not produce any dark current and therefore do not suffer from dark current noise and capacitance saturation at long integration times, making them ideal devices for large focal plane arrays. [ABSTRACT FROM AUTHOR]

We present second-harmonic surface emission from distributed feedback GaAs/AlGaAs quantum-cascade lasers with integrated intersubband nonlinearities. The devices show single mode fundamental and second-harmonic emission at wavelengths of 10.7 and 5.35 μm, respectively. Only 5.35 μm light is emitted from the surface, as the fundamental light is not coupling to radiating modes for the grating of our choice. The second-harmonic peak optical power via the surface at 78 K is 150 μW for a fundamental peak power of 1.1 W. [ABSTRACT FROM AUTHOR]

Operation of a quantum-cascade laser based on the In0.52Al0.48As–In0.53Ga0.47As/InP material is demonstrated at a frequency of 3.6 terahertz. The active region is based on a bound-to-continuum transition combined with an optical phonon extraction. The optical waveguide exploits a combination of metallic and dielectric confinement. Threshold current density of 460 A/cm2 at 10 K and a maximum operating temperature of 45 K are achieved. [ABSTRACT FROM AUTHOR]

We present high-power surface-emitting second-order distributed feedback quantum-cascade lasers in GaAs and InP material systems. The GaAs device, grown by molecular-beam epitaxy, showed single-mode peak output powers of 3 W at 78 K in pulsed operation. With the InP-based devices, which are grown by metalorganic vapor phase epitaxy, we obtained single-mode peak output powers of 1 W at room temperature. These are the highest output powers for surface emission of quantum-cascade lasers reported so far. The InP-based distributed feedback lasers also have very low threshold current densities and are working well above room temperature. [ABSTRACT FROM AUTHOR]

The influence of doping density on the performance of GaAs/AlGaAs quantum-cascade lasers is presented. A fully self-consistent Schrödinger–Poisson analysis, based on a scattering rate equation approach, was employed to simulate the above threshold electron transport in laser devices. V-shaped local field domain formation was observed, preventing resonant subband level alignment in the high pumping-current regime. The resulting saturation of the maximal current, together with an increase of the threshold current, limits the dynamic working range under higher doping. Experimental measurements are in good agreement with the theoretical predictions. [ABSTRACT FROM AUTHOR]

Midinfrared single mode emission, up to 20 °C, for a distributed feedback structure consisting of lateral index variation on both sides of a rib waveguide is achieved on a quantum cascade laser. Fabrication was made possible by a high density plasma process developed for high aspect ratio etching of GaAlAs/GaAs structures. A lateral grating considerably enhances design freedom by decoupling the vertical waveguide structure from the grating strength. The grating coupling, now being mainly determined by waveguide width, can then be exploited for device applications. [ABSTRACT FROM AUTHOR]

We report on postgrowth heat treatment of quantum-cascade lasers. Intermixing of the atoms at the barrier–well interfaces in the temperature range between 850 and 875 °C shifts the energy levels and thus the gain of the structure. The achieved emission wavelength shift is from 10.3 to 11.9 μm. X-ray rocking curves confirm Al–Ga interdiffusion at the interfaces. A fit to the data yields interface widths of 1–2 nm. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

GaAs-based quantum-cascade lasers based on a bound-to-continuum transition have been realized and characterized. This band structure design combines the advantages of the well known three-well and superlattice active regions. We observed lasing of Fabry–Pérot lasers in pulsed mode up to a temperature of 100 °C. Multimode emission with a pulsed peak power of 340 mW is observed at room temperature and 42 mW at 80 °C. Further, from aging tests we expect a lifetime of over 60 years for these devices. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Terahertz quantum cascade lasers incorporating lattice-matched quaternary AlInGaAs barriers grown by molecular beam epitaxy on InP substrate are reported. Four quantum well active region devices exhibited lasing at 3.8 THz with threshold current densities as low as 74 A/cm2 at 10 K. From optical characterization and a doping study of the active region, an upper state lifetime of 8 ps, as well as a long transport time across the active region of 68 ps and a ratio of free carrier loss to gain cross sections of 4.6%, is reported. A maximum operating temperature of 130 K was achieved for a device with a conduction band discontinuity of 0.14 eV. [ABSTRACT FROM AUTHOR]

We demonstrate that dopant migration and the resulting impurity scattering can strongly influence the performance of GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers. A nominally symmetric structure allows us to compare the negative and positive bias behavior of the very same device. Dopants, migrated towards the upper laser level during sample growth, degrade the lasing performance due to enhanced impurity scattering rates. The consequences are a higher threshold current (+30%) and reduced optical output power (-29%) in the affected operating direction. This polarity dependent performance is reversed in an asymmetrically doped sample, which imitates the migration of dopants against the growth direction. [ABSTRACT FROM AUTHOR]