Emitting Quantum Cascade Lasers Research Articles

Low temperature near-field scanning optical microscopy on infrared and terahertz photonic-crystal quantum cascade lasers

We report the development of a scattering-type near-field scanning optical microscope (sNSOM) which operates at temperatures down to 100 K with a scanning range of up to 400 μm. We have used this sNSOM to map the electromagnetic near-field on mid-IR and terahertz (THz) surface emitting quantum cascade lasers with photonic-crystal resonators. Mid-IR devices operate at λ=7.5 μm (40 THz) while THz devices operate at λ≈110 μm (2.7 THz). The near-field images—in agreement with numerical calculations—demonstrate an instrument resolution of 100's nm.

Photovoltaic detector based on type II heterostructure with deep AlSb/InAsSb/AlSb quantum well in the active region for the midinfrared spectral range

Photodetectors for the spectral range 2-4 μm, based on an asymmetric typeII heterostructure pInAs/AlSb/InAsSb/AlSb/( p, n)GaSb with a single deep quantum well (QW) or three deep QWs at the het� erointerface, have been grown by metal-organic vapor phase epitaxy and analyzed. The transport, lumines� cent, photoelectric, current-voltage, and capacitance-voltage characteristics of these structures have been examined. A highintensity positive and negative luminescence was observed in the spectral range 3-4 μm at high temperatures (300-400 K). The photosensitivity spectra were in the range 1.2-3.6 μm (T = 77 K). Large values of the quantum yield (η = 0.6-0.7), responsivity (Sλ = 0.9-1.4 A W -1 ), and detectivity ( = 3.5 × 10 11 to 10 10 cm Hz 1/2 W -1 ) were obtained at T = 77-200 K. The small capacitance of the structures (C = 7.5 pF at V = -1 V and T = 300 K) enabled an estimate of the response time of the photodetector at τ = 75 ps, which corresponds to a bandwidth of about 6 GHz. Photodetectors of this kind are promising for heterodyne detec� tion of the emission of quantumcascade lasers and IR spectroscopy.

Directional single mode quantum cascade laser emission using second-order metal grating coupler

We report on the design and experimental demonstration of a substrate emitting quantum cascade laser (QCL) with low beam divergence in the two directions. A low-loss, index-coupled, distributed feedback laser is coupled to a monolithic extraction area. Both functions are performed with a top metal grating spatially differentiated for improving the divergence of the QCL in the two directions. Spectrally single-mode InGaAs/AlInAs QCL emitting at a wavelength of 5.65 μm with a low beam divergence, represented by a full width at half maximum of 2.3° and 4°, is demonstrated at room temperature with a threshold current of 2.1 kA/cm2.

Design and fabrication technology for high performance electrical pumped terahertz photonic crystal band edge lasers with complete photonic band gap

We detail the design and fabrication technology of two-dimensional photonic crystal (PhC) band edge emitting quantum cascade lasers (QCLs) operating at terahertz frequencies (∼3.3 THz). The entire QCL active layer has been deeply etched to form a triangular lattice PhC, in which a complete in-plane photonic band gap for TM polarized light exists. Strong vertical optical confinement is provided by metal-metal plasmon waveguide fabricated with thermocompression bonding and planarization. Benzocyclobutene is employed as a low loss medium to planarize the PhC structure. By tailoring the top metal contact on the planarized PhC structures, electrical current injection pad can also be defined. The lasing properties can be controlled by engineering the photonic band structure. Broad band continuous single mode tuning over 30 GHz was observed; while lower current threshold density and higher operation temperature compared to Fabry–Perot (FP) asers were obtained.

Coherent coupling of ring cavity surface emitting quantum cascade lasers

We present investigations on coherent coupling of surface emitting quantum cascade lasers (QCLs). For this purpose, the evanescent field coupling method is exploited under incorporation of a ring cavity surface emitting QCL pair with an emission wavelength of ≈8 μm. A series of these pairs with intercavity separations of 1 and 3 μm was fabricated. The coupling gap of 1 μm yields robust phase-locked light source pairs with coherent emission at all injection currents and a corresponding threshold current density of 2.65 kA/cm2 at room-temperature.

Temperature dependence of the key electro-optical characteristics for midinfrared emitting quantum cascade lasers

The equations for threshold-current density Jth, differential quantum efficiency ηd, and maximum wallplug efficiency ηwp,max for quantum-cascade lasers (QCLs) are modified for electron leakage and backfilling. A thermal-excitation model of “hot” injected electrons from the upper laser state to upper active-region states is used to calculate leakage currents. The calculated characteristic temperature T0 for Jth is found to agree well with experiment for both conventional and deep-well (DW) QCLs. For conventional QCLs ηwp,max is found to be strongly temperature dependent; explaining experimental data. At 300 K for optimized DW-QCLs, front-facet, continuous-wave ηwp,max values >20% are projected.

Ring cavity induced threshold reduction in single-mode surface emitting quantum cascade lasers

We present ring cavity surface emitting (RCSE) quantum cascade lasers operating at temperatures as high as 380 K and above. A reduction in threshold current density and enhanced radiation efficiency are observed as compared to Fabry–Perot (FP) lasers. In continuous wave, the maximum operation temperature of RCSE lasers is 50 K higher than in FP emitters. The devices exhibit single-mode emission at a wavelength around 8 μm with a side mode suppression ratio of 30 dB at room temperature. A lithographic tuning of the resonance is achieved by a variation in the grating period. The emitters exhibit a low divergent ring-shaped beam pattern with a lobe separation of ∼1.5°.

PbTe/PbS Multilayer Mirror for EuTe/PbTe Surface Emitting Quantum Cascade Lasers

A PbTe/PbS multilayer mirror was prepared by Hot Wall Epitaxy. A reflectivity as high as 99% in the mid infrared region was obtained using about seven periods of the multilayer structure, owing to the relatively large difference in the refractive index of PbTe and PbS. Such a mirror has potential applications for electrically pumped EuTe/PbTe vertical cavity surface emitting quantum cascade lasers.

Emission and Propagation Properties of Midinfrared Quantum Cascade Lasers

We report divergence, astigmatism, and beam propagation factor (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) measurements of quantum cascade lasers (QCLs) with emission wavelengths of 8.77 mum. Emission profiles from the facet showed full-width at half-maximum divergence angles of 62deg and 32degplusmn2deg for the fast and slow axes, respectively. Diffraction-limited Ge aspheric microlenses were designed and fabricated to efficiently collect, collimate, and focus QCL emission. A confocal system comprised of these lenses was used to measure M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> yielding 1.8 and 1.2 for the fast and slow axes, respectively. Astigmatism at the exit facet was calculated to be about 3.4 mum, or less than half a wave. To the best of our knowledge, this is the first experimental measurement of astigmatism and M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> reported for midinfrared QCLs.

HIGH-MAGNETIC-FIELD SPECTROSCOPY OF InGaAs-AlAs/InP QUANTUM-CASCADE LASER

The magnetic-field dependence of the mid-infrared quantum-cascade laser emission spectra is used to identify the particular Wannier-Stark states responsible for the laser action in two different laser designs. The active regions in both QCLs are based on the bound-to-continuum design, but have differing degrees of coupling between the injector miniband and the bound state. The effects of the magnetic field and the injection-barrier width on the emission wavelength indicates that the laser emission in the QCL with less coupling between the injector and the bound state originates from a transition between the injector and extractor minibands. The transition from injector miniband to extractor miniband has both a lower energy and a lower oscillator strength than the transition originating from the bound state, but dominates because of the low population of the upper bound state. This result has important implications for further miniband engineering of QCL active regions for laser action at the shortest possible wavelengths. New quantum-cascade laser sample, designed regarding the high-magnetic-field spectroscopy results mentioned above, indeed operates at shorter wavelength and demonstrates outstanding performance up to room temperature.

Probing the population inversion in intersubband laser by magnetic field spectroscopy

The magnetic field dependence of the midinfrared quantum-cascade laser emission spectra is used to identify the particular Wannier-Stark states responsible for the laser action in two different laser designs. The active regions in both quantum-cascade lasers are based on a modified bound-to-continuum design, but have differing degrees of coupling between the injector miniband and the bound state. The effects of the magnetic field and the injection-barrier width on the emission wavelength indicate that the laser emission in the quantum-cascade laser with less coupling between the injector and the bound state originates from a transition between the injector and extractor minibands. The transition from injector miniband to extractor miniband has both a lower energy and a lower oscillator strength than the transition originating from the bound state, but dominates because of the low population of the upper bound state. This result has important implications for further miniband engineering of quantum-cascade-laser active regions for laser action at the shortest possible wavelengths.

Room-temperature operation of λ≈7.5μm surface-plasmon quantum cascade lasers

We report the pulsed, room-temperature operation of λ≈7.5μm quantum cascade lasers (QCLs) in which the optical mode is a surface-plasmon polariton excitation. Previously reported devices based on this concept operate at cryogenic temperatures only. The use of a silver-based electrical contact with reduced optical losses at the QCL emission wavelength allows a reduction of the laser threshold current by a factor of 2 relative to samples with a gold-based contact layer. As a consequence, the devices exhibit room-temperature operation with threshold current densities ∼6.3kA∕cm2. These devices could be used as all-electrical surface-plasmon generators at midinfrared wavelengths.

Single-mode surface-emitting quantum-cascade lasers

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.

Strain-compensated AlAs/(In,Ga)As heterostructures for short-wavelength intersubband absorption and laser emission

We have investigated intersubband transitions in strain-compensated AlAs/(In,Ga)As heterostructures, demonstrating both absorption and quantum-cascade laser emission at short wavelengths. Short-wavelength optical transitions in such structures are associated with a number of challenges in both the growth and design, including managing the internal strain and designing around indirect valleys. We achieve absorption peaks at wavelengths as short as 1.7 μ m in fully strain-compensated AlAs/(In,Ga)As structures. Quantum cascade lasers based on similar heterojunctions exhibit laser emission as short as 3.7 μ m . These lasers exhibit low-temperature threshold current densities of 860 A / cm 2 in pulsed mode and output power as high as 6 W per facet (12 W total). At room temperature, the threshold current density is 4.5 kA / cm 2 and the maximum power 240 mW per facet.

Finite-Difference Time-Domain Simulation of Mid- and Far-Infrared Quantum Cascade Lasers

We present simulations of mid- and far-infrared quantum cascade lasersoperating with/without external magnetic fleld. Maxwell{Bloch solver basedon the flnite-difierence time-domain method was used in our investigation.Reduction of the far-infrared quantum cascade laser emission intensity isassociated with increased optical losses in highly doped layers when magneticfleld is changed from 4.2 T to 6.2 T. A simulated emission spectrum of mid--infrared disc-shaped quantum cascade laser with 60

Investigation of the spectral width of quantum cascade laser emission near 5.2 μm by a heterodyne experiment

We report line width measurements of a quantum cascade distributed feedback laser by a heterodyne experiment. At currents slightly above threshold and a laser output power higher than 1 mW, the full width at half maximum of the beat signal was narrower than 0.5 MHz, which gives us an upper limit of the laser line width. As reference laser we used a carbon monoxide laser. Both lasers were operating unstabilized in continuous wave mode emitting light at about 5.2 μm.