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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

High-power, low-lateral divergence broad area quantum cascade lasers with a tilted front facet

We introduce a simple technique to improve the beam quality of broad area quantum cascade lasers. Moderately tilted front facets of the laser provide suppression of higher order lateral waveguide modes. A device with a width of 60 μm and a front facet angle of 17° shows a nearly diffraction limited beam profile. In addition, the peak output power and the slope efficiency of the device are increased since most of the light inside the cavity is emitted through the tilted front facet by an asymmetric light intensity distribution along the cavity.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Mode switching in a multi-wavelength distributed feedback quantum cascade laser using an external micro-cavity

We demonstrate a multi-wavelength distributed feedback (DFB) quantum cascade laser (QCL) operating in a lensless external micro-cavity and achieve switchable single-mode emission at three distinct wavelengths selected by the DFB grating, each with a side-mode suppression ratio larger than 30 dB. Discrete wavelength tuning is achieved by modulating the feedback experienced by each mode of the multi-wavelength DFB QCL, resulting from a variation of the external cavity length. This method also provides a post-fabrication control of the lasing modes to correct for fabrication inhomogeneities, in particular, related to the cleaved facets position.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Imaging of free carriers in semiconductors via optical feedback in terahertz quantum cascade lasers

To monitor the density of photo-generated charge carriers on a semiconductor surface, we demonstrate a detectorless imaging system based on the analysis of the optical feedback in terahertz quantum cascade lasers. Photo-excited free electron carriers are created in high resistivity n-type silicon wafers via low power (≅40 mW/cm{sup 2}) continuous wave pump laser in the near infrared spectral range. A spatial light modulator allows to directly reconfigure and control the photo-patterned intensity and the associated free-carrier density distribution. The experimental results are in good agreement with the numerical simulations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Hydrogen peroxide detection with quartz-enhanced photoacoustic spectroscopy using a distributed-feedback quantum cascade laser

A quartz-enhanced photoacoustic spectroscopy sensor system was developed for the sensitive detection of hydrogen peroxide (H{sub 2}O{sub 2}) using its absorption transitions in the v{sub 6} fundamental band at ∼7.73 μm. The recent availability of distributed-feedback quantum cascade lasers provides convenient access to a strong H{sub 2}O{sub 2} absorption line located at 1295.55 cm{sup −1}. Sensor calibration was performed by means of a water bubbler that generated titrated average H{sub 2}O{sub 2} vapor concentrations. A minimum detection limit of 12 parts per billion (ppb) corresponding to a normalized noise equivalent absorption coefficient of 4.6 × 10{sup −9} cm{sup −1}W/Hz{sup 1/2} was achieved with an averaging time of 100 s.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Pump and probe spectroscopy with continuous wave quantum cascade lasers

This paper details infra-red pump and probe studies on nitric oxide conducted with two continuous wave quantum cascade lasers both operating around 5 μm. The pump laser prepares a velocity selected population in a chosen rotational quantum state of the v = 1 level which is subsequently probed using a second laser tuned to a rotational transition within the v = 2 ← v = 1 hot band. The rapid frequency scan of the probe (with respect to the molecular collision rate) in combination with the velocity selective pumping allows observation of marked rapid passage signatures in the transient absorption profiles from the polarized vibrationally excited sample. These coherent transient signals are influenced by the underlying hyperfine structure of the pump and probe transitions, the sample pressure, and the coherent properties of the lasers. Pulsed pump and probe studies show that the transient absorption signals decay within 1 μs at 50 mTorr total pressure, reflecting both the polarization and population dephasing times of the vibrationally excited sample. The experimental observations are supported by simulation based upon solving the optical Bloch equations for a two level system.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Genetic algorithm applied to the optimization of quantum cascade lasers with second harmonic generation

A computational model for the optimization of the second order optical nonlinearities in GaInAs/AlInAs quantum cascade laser structures is presented. The set of structure parameters that lead to improved device performance was obtained through the implementation of the Genetic Algorithm. In the following step, the linear and second harmonic generation power were calculated by self-consistently solving the system of rate equations for carriers and photons. This rate equation system included both stimulated and simultaneous double photon absorption processes that occur between the levels relevant for second harmonic generation, and material-dependent effective mass, as well as band nonparabolicity, were taken into account. The developed method is general, in the sense that it can be applied to any higher order effect, which requires the photon density equation to be included. Specifically, we have addressed the optimization of the active region of a double quantum well In{sub 0.53}Ga{sub 0.47}As/Al{sub 0.48}In{sub 0.52}As structure and presented its output characteristics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Directional collimation of substrate emitting quantum cascade laser by nanopores arrays

We report a beam reshaping phenomena via nanopore array defined on the emission window of a substrate emitting quantum cascade lasers. A pronounced beam divergence reduction by a factor of 6 in the elongated beam direction is achieved. A collimated laser beam with small divergence (2.9° × 0.12°) is observed. These single-mode devices preserve good power and spectral performances at room temperature continuous wave operation. This beam reshaping design is simple to fabricate and able to be adapted for other wavelengths ranging from the visible to mid-infrared regimes, which would open up major opportunities for the control of beam divergence.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Nano-optical observation of cascade switching in a parallel superconducting nanowire single photon detector

The device physics of parallel-wire superconducting nanowire single photon detectors is based on a cascade process. Using nano-optical techniques and a parallel wire device with spatially separate pixels, we explicitly demonstrate the single- and multi-photon triggering regimes. We develop a model for describing efficiency of a detector operating in the arm-trigger regime. We investigate the timing response of the detector when illuminating a single pixel and two pixels. We see a change in the active area of the detector between the two regimes and find the two-pixel trigger regime to have a faster timing response than the one-pixel regime.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Cascade phonon-assisted trapping of positrons by divacancies in n-FZ-Si(P) single crystals irradiated with 15 MeV protons

The trapping of positrons by the radiation defects in moderately doped oxygen-lean n-FZ-Si(P) single crystal irradiated with 15 MeV protons has been investigated in a comparative way using the positron lifetime spectroscopy and Hall effect measurements. The experiments were carried out within a wide temperature interval ranging from 25 K – 29 K to 300 K. The positron trapping rate for divacancies was reconstructed in the course of many-stage isochronal annealing. The concentration and the charged states of divacancies (V{sub 2}{sup −} and V{sub 2}{sup −−}) were estimated. The temperature dependency of the trapping cross section of positrons by the negatively charged divacancies is in a good agreement with the data of calculations based on the assumptions of the cascade phonon-assisted mechanism of exchange of the energy between the positron and acoustic long-wave phonons. Obeying ∼ T{sup −3} law, the cross-section of the trapping of positrons by divacancies changes considerably ranging from ∼1.7×10{sup −12} cm{sup 2} (66 – 100 K) to ∼2×10{sup −14} cm{sup 2} (≈ 250 K). The characteristic length of trapping of the positron by V{sub 2}{sup −−} divacancy was estimated to be l{sub 0}(V{sub 2}{sup −−})≈(3.4±0.2)×10{sup −8} cm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Coherent detection of metal-metal terahertz quantum cascade lasers with improved emission characteristics

Coherent detection of emission from quantum cascade lasers with metal-metal waveguides is demonstrated through free-space coupling of a THz pulse to the sub-wavelength waveguide. We implement a simple, monolithic planar horn antenna design on the metal-metal waveguide that reduces the impedance mis-match to the waveguide. The resulting devices show up to 10 times more directed output power than conventional metal-metal waveguides. This enhanced coupling to free-space allows a more efficient injection of broad-band THz pulses into the waveguide. Through this, we are able to seed the laser emission and coherently detect the laser emission by electro-optic sampling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Surface-emitting terahertz quantum cascade lasers with continuous-wave power in the tens of milliwatt range

We demonstrate efficient surface-emitting terahertz frequency quantum cascade lasers with continuous wave output powers of 20–25 mW at 15 K and maximum operating temperatures of 80–85 K. The devices employ a resonant-phonon depopulation active region design with injector, and surface emission is realized using resonators based on graded photonic heterostructures (GPHs). GPHs can be regarded as energy wells for photons and have recently been implemented through grading the period of the photonic structure. In this paper, we show that it is possible to keep the period constant and grade instead the lateral metal coverage across the GPH. This strategy ensures spectrally single-mode operation across the whole laser dynamic range and represents an additional degree of freedom in the design of confining potentials for photons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Coherent frequency combs produced by self frequency modulation in quantum cascade lasers

One salient characteristic of Quantum Cascade Laser (QCL) is its very short τ ∼ 1 ps gain recovery time that so far thwarted the attempts to achieve self-mode locking of the device into a train of single pulses. We show theoretically that four wave mixing, combined with the short gain recovery time causes QCL to operate in the self-frequency-modulated regime characterized by a constant power in time domain and stable coherent comb in the frequency domain. Coherent frequency comb may enable many potential applications of QCL's in sensing and measurement.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Detection of a 2.8 THz quantum cascade laser with a semiconductor nanowire field-effect transistor coupled to a bow-tie antenna

The use of a high-electron mobility semiconductor nanowire as transistor channel has recently allowed the extension of the spectral coverage of THz field-effect transistor detectors up to 1.5 THz. In this report, we demonstrate efficient operation of a field-effect transistor detector based on a semiconductor nanowire at a much higher frequency, 2.8 THz, with a responsivity ≈5 V/W in a bandwidth ≈100 kHz, thus proving the full potential of such approach for the detection of THz quantum cascade lasers. Finally, such a THz sensing system is exploited to perform raster scan transmission imaging, with high spatial resolution, signal-to-noise ratio, and acquisition rate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

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{sup −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 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 μm-wide, 4.6 μm-emitting devices, with an Ag/InP grating of ∼39% duty cycle, and ∼0.22 μm tooth height, threshold currents as low as 0.45 A are projected. Based on experimentally obtained internal efficiency values from high-performance QCLs, slope efficiencies as high as 3.4 W/A are projected; thus, offering a solution for watt-range, single-lobe CW operation from SE, mid-infrared QCLs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Integrated patch and slot array antenna for terahertz quantum cascade lasers at 4.7 THz

Our work presents a slot and a patch array antenna at the front facet of a 4.7 THz quantum cascade laser as extractor, decreasing the facet reflectivity down to 2.6%. The resulting output power increases by a factor 2 and the slope efficiency by a factor 4. The simulated and the measured far-fields are in good agreement.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

On-chip focusing in the mid-infrared: Demonstrated with ring quantum cascade lasers

We report on collimated emission beams from substrate emitting ring quantum cascade lasers with an on-chip focusing element fabricated into the bottom side of the device. It is formed by a gradient index metamaterial layer, realized by etching subwavelength holes into the substrate. The generated optical path length difference for rays emitted under different angles from the ring waveguide flattens the wavefront and focuses the light. Our far field measurements show an increased peak intensity corresponding to 617% of the initial value without the focusing element. Far field calculations, based on a Fourier transformation of the metamaterial area, are in good agreement with our experimental data.

42 ENGINEERING↗
DOE OSTI2014

Tunable single-mode slot waveguide quantum cascade lasers

We report experimental demonstration of tunable, monolithic, single-mode quantum cascade lasers (QCLs) at ∼10 μm with a two-section etched slot structure. A single-mode tuning range of 77 cm{sup −1} (785 nm), corresponding to ∼7.8% of the relative tuning range, was realized with a ∼20 dB side mode suppression ratio within the whole tuning range. Compared with integrated distributed feedback QCLs, our devices have the advantages of easy fabrication and a broader tuning range. Further theoretical analyses and numerical simulations show that it is possible to achieve a broad continuous tuning range by optimizing the slot structures. The proposed slot-waveguide design could provide an alternative but simple approach to the existing tuning schemes for realizing broadly continuous tunable single-mode QCLs.

42 ENGINEERING↗
DOE OSTI2014

Superconducting cascade electron refrigerator

The design and operation of an electronic cooler based on a combination of superconducting tunnel junctions is described. The cascade extraction of hot-quasiparticles, which stems from the energy gaps of two different superconductors, allows for a normal metal to be cooled down to about 100 mK starting from a bath temperature of 0.5 K. We discuss the practical implementation, potential performance, and limitations of such a device.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗