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  • 1
    Electronic Resource
    Electronic Resource
    Optimized energy separation for phonon scattering in three-level terahertz intersubband lasers (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 5504-5511 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We examine the impact of the complex phonon spectrum on the design rules for three-level THz intersubband emitters that utilize resonant optical phonon scattering to obtain intersubband population inversion. In GaAs/AlxGa1−xAs multiple quantum wells, electron–optical phonon scattering occurs due to interaction with "GaAs-like" modes at (h-dash-bar)ωLO∼36 meV and "AlAs-like" modes at (h-dash-bar)ωLO∼47 meV. Scattering rates are calculated for interface and confined phonon modes to determine the optimal subband energy separations for enhancement of fast depopulation scattering and reduction of parasitic nonradiative scattering. While the results depend sensitively on details of the structure, we find that, for electrically pumped THz lasers, there is not an overwhelming advantage in increasing the subband separation to allow scattering by the higher energy modes. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1413951
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  • 2
    Electronic Resource
    Electronic Resource
    Numerical study of a GaAs-based heterojunction bipolar transistor with stepwise alloy-graded base (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5400-5410 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We consider a GaAs-based heterojunction bipolar transistor with stepwise alloy-graded base. The step height is slightly greater than the longitudinal optical (LO) phonon energy (h-dash-bar)ωLO in order to facilitate LO-phonon-enhanced forward diffusion. Impurity and LO phonon scattering are considered as the dominant scattering mechanisms. We solve the Boltzmann transport equation (BTE) numerically in one dimension across the base and at arbitrary frequencies, unifying and extending previous theoretical treatments. The base transport factor is analyzed in the frequency domain by varying device parameters. The solution of the BTE is incorporated into the small-signal parameters of the intrinsic part of the device. Using a simple circuit approach for the extrinsic elements, high-frequency figures of merit are presented, casting light on the potential of the device as a high-gain amplifier and more importantly as a transit-time oscillator for millimeter-wave applications. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1462846
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  • 3
    Electronic Resource
    Electronic Resource
    Intrawell and interwell intersubband transitions in multiple quantum wells for far-infrared sources (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 9305-9320 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A theoretical study of electrically pumped unipolar lasers exploiting intrawell or interwell intersubband radiative transitions in multiple quantum-well heterostructures for the generation of IR and FIR radiation is presented. The feasibility of these coherent sources critically depends on the non-radiative intersubband transition rates. Numerical simulations of acoustical phonon, optical-phonon, and electron-electron scattering were implemented, including their temperature dependence. For far-infrared coherent sources, electron-electron scattering emerges as the dominating non-radiative relaxation mechanism. Interwell schemes offer distinctive advantages such as simplicity in design, greater tolerance in design and fabrication errors, field tunability of the emission frequency, improved internal quantum efficiency and aid in establishing population inversion. Design rules are put forward for such long wavelength sources. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362607
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  • 4
    Electronic Resource
    Electronic Resource
    Electrically pumped tunable terahertz emitter based on intersubband transition (1997)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 440-442 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An electrically pumped three-level system was designed and fabricated using an AlGaAs/GaAs multiple quantum well structure. Under appropriate biases, the structure emits terahertz (THz) radiation, as a result of diagonal (or interwell) intersubband transition. The emission spectra were resolved using an external Fourier transform infrared spectrometer. The center frequency of the emission was voltage tunable. The emission spectra and the power-voltage relation showed clear evidence that the middle level was depopulated by fast longitudinal optical-phonon scattering, thus, a population inversion between the two upper levels is feasible. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.119572
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  • 5
    Electronic Resource
    Electronic Resource
    Cryogenic picosecond sampling using fiber-coupled photoconductive switches (1997)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 70 (1997), S. 2644-2646 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A photoconductive sampling technique was developed that is well suited to probing cryogenic devices such as mesoscopic devices. Low-temperature-grown GaAs photoconductive switches were embedded in a coplanar waveguide and illuminated with pigtailed optical fibers. The chip was conduction cooled in a liquid helium cryostat. The fiber-coupled switches were designed to minimize external infrared-radiation loading and to obtain efficient optical to electrical conversion. Measurements at 300, 77, and 4.2 K demonstrate very low thermal loading and a temporal resolution of 4.7 ps—arising from dispersion of the femtosecond pulses (λ=810nm) in the optical fiber. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.119011
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  • 6
    Electronic Resource
    Electronic Resource
    Micromachined room-temperature microbolometers for millimeter-wave detection (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 2020-2022 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have combined silicon micromachining technology with planar circuits to fabricate room-temperature niobium microbolometers for millimeter-wave detection. In this type of detector, a thin niobium film, with a dimension much smaller than the wavelength and fabricated on a 1 μm thick Si3N4 membrane, acts both as a radiation absorber and temperature sensor. Incident radiation is coupled into the microbolometer by a 0.37λ dipole antenna of center frequency 95 GHz with a 3 dB bandwidth of 15%, which is impedance matched with the Nb film. An electrical noise equivalent power (NEP) of 4.5×10−10 W/(square root of)Hz has been achieved. This is comparable to the best commercial room-temperature millimeter-wave detectors. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.115624
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  • 7
    Electronic Resource
    Electronic Resource
    A low-noise micromachined millimeter-wave heterodyne mixer using Nb superconducting tunnel junctions (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 1862-1864 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A heterodyne mixer with a micromachined horn antenna and a superconductor-insulator- superconductor (SIS) tunnel junction as mixing element is tested in the W-band (75–115 GHz) frequency range. Micromachined integrated horn antennas consist of a dipole antenna suspended on a thin Si3N4 dielectric membrane inside a pyramidal cavity etched in silicon. The mixer performance is optimized by using a backing plane behind the dipole antenna to tune out the capacitance of the tunnel junction. The lowest receiver noise temperature of 30±3 K (without any correction) is measured at 106 GHz with a 3-dB bandwidth of 8 GHz. This sensitivity is comparable to the state-of-the-art waveguide and quasi-optical SIS receivers, showing the potential use of micromachined horn antennas in imaging arrays. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.116038
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  • 8
    Electronic Resource
    Electronic Resource
    Design analysis of a high Tc superconducting microbolometer (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 2444-2446 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We propose an antenna-coupled microbolometer based on the resistive transition of a high Tc superconducting film as a detector for far infrared and millimeter waves. Such microbolometers can be mechanically stronger, more easily fabricated, and much faster than conventional bolometric infrared detectors. A design analysis shows that a noise equivalent power of 2.5×10−12 W Hz−1/2 is achievable for modulation frequencies up to 10 kHz. The superconducting film must be of high quality with narrow resistive transition and low 1/f noise.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.101998
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  • 9
    Electronic Resource
    Electronic Resource
    Quantum-limited heterodyne detection of millimeter waves using superconducting tantalum tunnel junctions (1990)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 57 (1990), S. 2487-2489 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have made accurate measurements of the noise and gain of heterodyne mixers employing small-area (1 μm2) Ta/Ta2O5/Pb0.9Bi0.1 superconductor-insulator-superconductor tunnel junctions. These junctions have very low subgap leakage current and an extremely sharp current rise at the sum gap. We have measured an added mixer noise of 0.61±0.31 quanta at 95.0 GHz, which is within 25% of the quantum limit of 0.5 quanta for a single-sideband mixer. Values of the imbedding admittances are deduced from the shapes of I-V curves pumped at the upper and lower sideband frequencies. Using these admittances, the mixer performance calculated from the quantum theory is in good agreement with the experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.104111
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  • 10
    Electronic Resource
    Electronic Resource
    Fabrication of high-quality superconductor-insulator-superconductor junctions on thin SiN membranes (1993)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 63 (1993), S. 1002-1004 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have successfully fabricated high-quality and high-current density (Jc≈4 000 A/cm2) superconductor-insulator-superconductor (SIS) junctions on freestanding thin (∼1 μm) silicon nitride (SiN) membranes. These devices can be used in a novel millimeter-wave and THz receiver system which is made using micromachining. The SIS junctions with planar antennas were fabricated first on a silicon wafer covered with a SiN membrane, the Si wafer underneath was then etched away using an anisotropic KOH etchant. The current-voltage characteristics of the SIS junctions remained unchanged after the whole process, and the junctions and the membrane survived thermal cycling.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.109877
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