A. M. Sergent

Stanford University, Stanford, CA, United States

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Publications (208)426.72 Total impact

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    ABSTRACT: We demonstrate a dual wavelength mid-infrared Quantum Cascade Laser (QCL) utilizing a single active region to emit at 5-μm and 9-μm. The novelty lies in the large energy difference between the two lasing energies, achieved through simultaneous injection into the top 2 levels of a 4-level cascade employing InGaAs/InAlAs heterostructures latticematched to InP. The gain and losses at both wavelengths were measured by two different methods, Hakki-Paoli and cutback method, and were compared with theoretical predictions. The results for the gain of the 9-μm laser from the two techniques are consistently lower than theoretical predictions. Moreover, the mid-infrared losses are larger than expected at both wavelengths. We are investigating these devices for their potential application of quantum coherence to achieve lasing without inversion. The intense fields generated by the 9-μm laser are expected to partially eliminate the resonant absorption on the transition of interest at an energy corresponding to the difference between the energies of the two lasers. Our results on the dual wavelength QCL provide insights in the detail charge transport and optical properties of this design concept and open up the possibility for future optimization of inversionless lasers.
    Proc SPIE 02/2012;
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    ABSTRACT: We present a study of the temperature and density dependence of the resistivity of an extremely high quality two-dimensional hole system grown on the (100) surface of GaAs. For high densities in the metallic regime (p > or approximately4x10;{9} cm;{-2}), the nonmonotonic temperature dependence ( approximately 50-300 mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of T=50 mK, the conductivity versus density data indicate an inhomogeneity driven percolation-type transition to an insulating state at a critical density of 3.8x10;{9} cm;{-2}.
    Physical Review Letters 01/2008; 99(23):236402. · 7.73 Impact Factor
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    ABSTRACT: Anisotropic charge transport is observed in a two-dimensional (2D) hole system in a perpendicular magnetic field at filling factors nu=7/2, nu=11/2, and nu=13/2 at low temperature. In stark contrast, the transport at nu=9/2 is isotropic for all temperatures. Isotropic hole transport at nu=7/2 is restored for sufficiently low 2D densities or an asymmetric confining potential. The density and symmetry dependences of the observed anisotropies suggest that strong spin-orbit coupling in the hole system contributes to the unusual transport behavior.
    Physical Review Letters 06/2007; 98(20):206804. · 7.73 Impact Factor
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    ABSTRACT: The authors report the growth and fabrication of 4.5 μ m quantum cascade lasers based on strain balanced In <sub>0.68</sub> Ga <sub>0.32</sub> As / Al <sub>0.64</sub> In <sub>0.36</sub> As heterostructures by molecular beam epitaxy. A device with 12.5 μ m waveguide width and 2.25 mm cavity length has a threshold current density of 5 kA / cm <sup>2</sup> and can deliver power from one facet of about 90 mW at room temperature under pulse mode. The characteristic temperature is 124 K . The measured waveguide losses at room temperature are 11.5 and 13.2 cm <sup>-1</sup> with ridge width of 14 and 11 μ m , respectively. The temperature dependence of the waveguide loss is correlated with the change of the carrier mobility.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/2007; · 1.36 Impact Factor
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    ABSTRACT: Using a combination of semiconductor overgrowth, epi-side down mounting, and high-reflectance coating of the back facet, continuous-wave operation up to 320 K of a quantum cascade laser emitting at 8 mum was achieved. In pulsed mode at 300 K the devices exhibit threshold current densities of 1.8 kA/cm2, peak output powers of 260 mW, and maximum slope efficiencies of 346 mW/A. In cw mode at 300 K the threshold current density was measured to be 2.5-kA/cm2, the slope efficiency was 52 mW/A, and the output power was 20 mW.
    04/2007;
  • Michael Manfra, L. Pfeiffer, A. M. Sergent
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    ABSTRACT: We report on the density and temperature dependence of the resistivity of an extremely high mobility, carbon-doped, two-dimensional hole system (2DHS) in the vicinity of the putative metal-to-insulator (MIT) transition. The high mobility of our structures allows us to probe the conduction properties at very low 2D densities, ˜10^9cm-2, a regime in which interactions are expected to play an important role. Using a back-gated structure, a mobility of 2.2x10^6cm^2/Vs is achieved at a density of 2.9x10^10cm-2 at T=50mK. Backgating allows us to monitor the evolution of the resistivity as the density is continuously tuned from 2.9x10^10cm-2 to 2.9x10^9cm-2. From analysis of the temperature dependence of the resistivity, the sample becomes insulating at 3.5x10^9cm-2. We compare our data to existing models of the MIT in high mobility, low density, structures.
    03/2007;
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    ABSTRACT: The authors present a systematic study on the growth of the ternary compound In <sub>x</sub> Al <sub>1-x</sub> N by molecular beam epitaxy. This work concentrates on In mole fractions x around 0.17, as this composition is in-plane lattice matched to GaN. At a growth temperature of 540 ° C , high quality material was obtained using a total metal to nitrogen flux ratio of ∼1 . Using these growth parameters, high quality Ga N / In Al N superlattices were obtained without growth interruptions.
    Applied Physics Letters 02/2007; · 3.52 Impact Factor
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    ABSTRACT: We demonstrate high-reflectivity crack-free Al0.18Ga0.82N/Al0.8Ga0.2N distributed Bragg reflectors (DBR) and monolithic microcavities grown by molecular beam epitaxy on thick c-axis GaN templates. The elastic strain energy in the epilayer is minimized by compensating the compressive and tensile stress in every period of the DBR structure. A 25 period DBR mirror provides a 26nm-wide stop band centered at 347 nm with the maximum reflectivity higher than 99%. The high-reflectivity DBRs can be used to form high Q-factor monolithic AlGaN/AlGaN microcavities.
    Proc SPIE 02/2007;
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    ABSTRACT: We report transport properties of two single-electron transistors (SETs) on a Ga N / Al Ga N heterostructure. The first SET formed accidentally in a quantum point contact near pinchoff. Its small size produces large energy scales (a charging energy of 7.5 meV and well-resolved excited states). The second, intentionally fabricated SET is much larger. More than 100 uniformly spaced Coulomb oscillations yield a charging energy of 0.85 meV . Excited states are not resolvable in Coulomb diamonds, and Coulomb blockade peak height remains constant with increasing temperature, indicating that transport is through multiple quantum levels even at the 450 mK base electron temperature of our measurements. Coulomb oscillations of both SETs are highly stable, comparable to the best GaAs SETs.
    Applied Physics Letters 08/2006; · 3.52 Impact Factor
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    ABSTRACT: We present low-temperature magnetoconductivity measurements of a density-tunable and high mobility two-dimensional electron gas confined in the wide band-gap GaN∕AlGaN system. We observed pronounced antilocalization minima in the low-field conductivity, indicating the presence of strong spin-orbit coupling. Density dependent measurements of magnetoconductivity indicate that the coupling is mainly due to the Bychkov-Rashba mechanism. In addition, we have derived a closed-form expression for the magnetoconductivity, allowing us to extract reliable transport parameters for our devices. The Rashba spin-orbit coupling constant is αso∼6×10−13 eV m, while the conduction-band spin-orbit splitting energy amounts to Δso∼0.3 meV at ne=1×1016 m−2.
    Physical review. B, Condensed matter 07/2006; 74(3). · 3.66 Impact Factor
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    ABSTRACT: High mobility two-dimensional electron systems in GaN/AlGaN heterostructures have been realized by plasma assisted molecular beam epitaxy on GaN templates. In the density range of 1011 cm–2 to 1012 cm–2, mobility values exceeding 160000 cm2/Vs have been achieved. Scattering mechanisms that presently limit the production of higher mobility samples are discussed. We present results of a systematic study of the weak localization and antilocalization corrections to the classical conductivity at very low magnetic fields. The unambiguous observation of a conductivity maximum at B = 0 suggests that spin–orbit scattering is not negligible in GaN heterostructures as one might expect for a wide-bandgap system. We have recently realized electron transport through GaN nanostructures. We report on the transport properties of the first quantum point contacts (QPCs) in GaN. These devices are used to study one-dimensional transport in the Nitride system. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (b) 05/2006; 243(7):1706 - 1712. · 1.49 Impact Factor
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    ABSTRACT: We demonstrate high-reflectivity crack-free Al0.18Ga0.82N/Al0.8Ga0.2N distributed Bragg reflectors (DBR) for the spectral region around 350 nm grown by molecular-beam epitaxy on thick GaN templates. The structural quality of the DBR layers is maintained by compensating the compressive and tensile stress in each λ/4 pair. This approach results in the lowest elastic strain energy and allows the growth of thick coherently strained DBRs. A 25 period mirror provides a 26 nm wide stop band centered at 347 nm with the maximum reflectivity higher than 99%.
    Applied Physics Letters 04/2006; 88(17):171101-171101-3. · 3.52 Impact Factor
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    ABSTRACT: Anisotropic charge transport is observed in a two-dimensional (2D) hole system in a perpendicular magnetic field at filling factors nu=7/2 and nu=11/2 for temperatures below 150mK. In stark contrast, the transport at nu=9/2 is isotropic for all temperatures. Our results for a two-dimensional hole system differ substantially from 2D electron transport where no anisotropy has been observed at nu=7/2, the strongest anisotropy occurs at nu=9/2, and reentrant behavior is not evident. We attribute this difference to strong spin-orbit coupling in the hole system.
    04/2006;
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    ABSTRACT: Mid-infrared electroluminescence from carbon-doped GaAs/AlGaAs quantum cascade structures was observed and studied to determine the prospects of hole intersubband transitions for possible use in light emitting devices. The luminescence spectra exhibit a complex three-peak structure consistent with two heavy-to-heavy hole transitions and thermal emission. The hole transition energies in emission are in agreement with corresponding absorption and photocurrent energies, but typically 17% smaller than expected from calculations of a six-band k∙p model. The upper-level lifetime was estimated to be 0.4 ps, in agreement with simple calculations considering approximations of the heavy-hole effective masses.
    Applied Physics Letters 02/2006; 88(8):081117-081117-3. · 3.52 Impact Factor
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    ABSTRACT: We report the demonstration of surface-plasmon microcylinder quantum cascade lasers with circular and deformed resonators. An improved self-alignment fabrication technique was developed that allows the use of wet etching, necessary to achieve smooth and clean surfaces, in combination with the deposition of the surface-plasmon-carrying metal layer up to the very edge of the resonator, where the optical mode is mostly located. The diameter of the microcylinders ranges from 75 to 180 μm while their deformation coefficient ε ranges from ε=0 to ε=0.32. Circular microcylinder lasers show a reduction of ∼50% of the threshold current density with respect to devices with standard ridge-waveguide resonators. On the other hand, highly deformed microcylinder lasers exhibit a complex mode structure, suggesting the onset of chaotic behavior.
    IEEE Journal of Selected Topics in Quantum Electronics 02/2006; · 4.08 Impact Factor
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    ABSTRACT: Anisotropic transport is observed at low temperatures in the N=1 and N=2 Landau levels of a very clean two-dimensional (2D) hole system. At nu = 7/2 and nu =11/2, the longitudinal magnetoresistance develops strong anisotropies which depend on the direction of current flow and temperature. Interestingly, the transport at nu=9/2 is isotropic for all temperatures. Our results for a two-dimensional hole system contrast sharply with 2D electron transport where no anisotropy has been observed in the N=1 Landau level, the strongest anisotropy occurs at nu=9/2, and no reentrant behavior is evident.
    01/2006;
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    ABSTRACT: We discuss polariton formation in high-quality stress compensated crack-free A10.13Ga0.87N/A10.56Ga0.44 N microcavities grown by molecular beam epitaxy on thick GaN templates. Microcavity design for studies of the strong coupling regime in GaN will be discussed.
    Lasers and Electro-Optics, 2006 and 2006 Quantum Electronics and Laser Science Conference. CLEO/QELS 2006. Conference on; 01/2006
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    ABSTRACT: Bound-to-bound intersubband absorption in the valence band of modulation-doped GaAs quantum wells with digitally alloyed AlGaAs barriers was studied in the midinfrared wavelength range. A high-purity solid carbon source was used for the p-type doping. Strong narrow absorption peaks due to heavy-to-heavy hole transitions are observed with out-of-plane polarized light, and weaker broader features with in-plane polarized light. The heavy-to-heavy hole transition energy spans the spectral range between 206 to 126 meV as the quantum well width is increased from 25 to 45 Å. The experimental results are found to be in agreement with calculations of a six-band k∙p model taking into account the full band structure of the digital alloy.
    Applied Physics Letters 08/2005; 87(9):091116-091116-3. · 3.52 Impact Factor
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    ABSTRACT: We report on the temperature dependence of the mobility μ of the two-dimensional (2D) electron gas in a variable density Al Ga N / Ga N field-effect transistor, with carrier densities ranging from 0.4×10<sup>12</sup> to 3.0×10<sup>12</sup> cm <sup>-2</sup> and a peak mobility of 80 000 cm <sup>2</sup>/ V s . Between 20 and 50 K we observe a linear dependence μ<sub> ac </sub><sup>-1</sup>=αT , indicating that acoustic phonon scattering dominates the temperature dependence of the mobility, with α being a monotonically increasing function of decreasing 2D electron density. This behavior is contrary to predictions of scattering in a degenerate electron gas, but consistent with calculations that account for thermal broadening and the temperature dependence of the electron screening. Our data imply a deformation potential D=12–15 eV .
    Applied Physics Letters 07/2005; · 3.52 Impact Factor
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    ABSTRACT: Nonlinear light generation in quantum-cascade lasers (QCLs) has the potential to extend the operating wavelength of these devices outside the limits imposed by the fundamental properties of the materials of choice. The giant nonlinear susceptibilities of resonant intersubband transitions have been studied intensively both theoretically and experimentally over the past twenty years. However, the practical applications have been limited by the lack of efficient laser pumping and of convenient phase matching techniques. The first obstacle was overcome by monolithically integrating the nonlinear intersubband transitions within the active region of a quantum cascade (QC) laser. Sum-frequency and second-harmonic (SH) generation were the first nonlinear processes observed in QCLs. The optimization of the second-harmonic generation in InGaAs/InAlAs QCLs will be discussed in detail. The second challenge for achieving high efficiency nonlinear power conversion is the phase-matching of the fundamental and nonlinear light. We have developed a technique for modal phase-matching that takes advantage of the flexibility in the design of the QCL waveguide. An additional degree of freedom for tuning into exact phase-matching conditions is provided by the dependence of the refractive indices on the laser ridge width. Record nonlinear power of 2 mW at 4.45 µm was achieved using an InP top-cladding waveguide and high-reflectance coating on the laser back facet. Reaching the milliwatt power range is significant as such power levels are sufficient for trace gas point sensors using mid-infrared light sources. The practical limitations of the phase-matching method will be evaluated and the experimental results will be compared with theoretical predictions.
    Proc SPIE 04/2005;

Publication Stats

3k Citations
426.72 Total Impact Points

Institutions

  • 2006
    • Stanford University
      • Department of Applied Physics
      Stanford, CA, United States
  • 2003–2006
    • Université Paris-Sud 11
      Orsay, Île-de-France, France
  • 2005
    • Princeton University
      • Department of Electrical Engineering
      Princeton, NJ, United States
  • 2001
    • Politecnico di Bari
      Bari, Apulia, Italy
  • 1990–1999
    • AT&T Labs
      Austin, Texas, United States
  • 1997
    • University of Illinois, Urbana-Champaign
      • Department of Electrical and Computer Engineering
      Urbana, IL, United States
  • 1996
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, CA, United States
  • 1995
    • University of Maryland, College Park
      • Department of Electrical & Computer Engineering
      College Park, MD, United States
  • 1994
    • University of Maryland, Baltimore
      Baltimore, Maryland, United States
  • 1993
    • Colorado State University
      • Electrical & Computer Engineering
      Fort Collins, CO, United States
  • 1983
    • University of Oregon
      Eugene, Oregon, United States