Anthony J. Hoffman

University of Notre Dame, South Bend, Indiana, United States

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Publications (80)222.69 Total impact

  • W. Streyer · K. Feng · Y. Zhong · A.J. Hoffman · D. Wasserman
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    ABSTRACT: There has been increasing interest in so-called phononic materials, which can support surface modes known as surface phonon polaritons, consisting of electromagnetic waves coupled to lattice vibrations at the surface of a polar material. While such excitations have a variety of desirable features, they are limited to the spectral range between a material's longitudinal and transverse optical phonon frequencies. In this work, we demonstrate that for materials whose free-carrier concentrations can be controlled, hybrid plasmonic/phononic modes can be supported across a range of frequencies including those generally forbidden by purely phononic materials.
    No preview · Article · Dec 2015 · MRS Communications
  • K. Feng · W. Streyer · Y. Zhong · A.J. Hoffman · D. Wasserman

    No preview · Article · Nov 2015 · Optics Express
  • W. Streyer · K. Feng · Y. Zhong · A.J. Hoffman · D. Wasserman
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    ABSTRACT: We demonstrate engineered selective absorption and subsequent selective thermal emission from sub-wavelength thickness optical structures at far-infrared (30-40 μm) wavelengths. Control over absorption/emission wavelength is demonstrated, with both polarization-dependent and -independent structures fabricated. Samples are characterized experimentally by Fourier transform infrared reflection and emission spectroscopy, and modeled using three-dimensional rigorous coupled wave analysis. The ability to design and demonstrate strong selective absorption and thermal emission from optical structures in the far-infrared offers a potential route towards low-cost sources for the exploration of Reststrahlen band frequencies.
    No preview · Article · Aug 2015 · Applied Physics Letters
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    ABSTRACT: We demonstrate the excitation of localized surface phonon polaritons in an array of sub-diffraction pucks fabricated in an epitaxial layer of gallium nitride (GaN) on a silicon carbide (SiC) substrate. The array is characterized via polarization- and angle-dependent reflection spectroscopy in the mid-infrared, and coupling to several localized modes is observed in the GaN Reststrahlen band (13.4–18.0 μm). The same structure is simulated using finite element methods and the charge density of the modes are studied; transverse dipole modes are identified for the transverse electric and magnetic polarizations and a quadrupole mode is identified for the transverse magnetic polarization. The measured mid-infrared spectrum agrees well with numerically simulated spectra. This work could enable optoelectronic structures and devices that support surface modes at mid- and far-infrared wavelengths.
    No preview · Article · Aug 2015 · Applied Physics Letters
  • K. Feng · W. Streyer · S.M. Islam · J. Verma · D. Jena · D. Wasserman · A.J. Hoffman
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    ABSTRACT: GaN micro-disk resonator arrays were fabricated and measured within the Restrahalen band of GaN. Far-IR spectroscopy shows evidence for localized surface phonon polariton resonances, results which are confirmed by finite-element models of the fabricated structures.
    No preview · Article · May 2015
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    ABSTRACT: A wide-bandwidth, highly efficient method of inter-chip waveguide coupling suitable for on-chip, mid-infrared sensing is discussed. Simulations and preliminary fabrication work on laser-to-waveguide coupling are presented, with losses predicted to be better than 6 dB.
    No preview · Conference Paper · Jun 2014
  • Michael Harter · Yamac Dikmelik · Anthony J. Hoffman
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    ABSTRACT: The localization of electron wavefunctions due to interface roughness in a quantum cascade heterostructures is investigated by observing the electroluminescence spectra. Localization is more prominent in heterostructures with designed extended states.
    No preview · Conference Paper · Jun 2014
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    ABSTRACT: We demonstrate excitation of surface phonon polaritons on patterned gallium phosphide surfaces. Control over the light-polariton coupling frequencies is demonstrated by changing the pattern periodicity and used to experimentally determine the gallium phosphide surface phonon polariton dispersion curve. Selective emission via out-coupling of thermally excited surface phonon polaritons is experimentally demonstrated. Samples are characterized experimentally by Fourier transform infrared reflection and emission spectroscopy, and modeled using finite element techniques and rigorous coupled wave analysis. The use of phonon resonances for control of emissivity and excitation of bound surface waves offers a potential tool for the exploration of long-wavelength Reststrahlen band frequencies.
    No preview · Article · Mar 2014 · Applied Physics Letters
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    ABSTRACT: We present Finite-Difference Time-Domain (FDTD) simulations to explore feasibility of chip-to-chip waveguide coupling via Optical Quilt Packaging (OQP). OQP is a newly proposed scheme for wide-bandwidth, highly-efficient waveguide coupling and is suitable for direct optical interconnect between semiconductor optical sources, optical waveguides, and detectors via waveguides. This approach leverages advances in quilt packaging (QP), an electronic packaging technique wherein contacts formed along the vertical faces are joined to form electrically-conductive and mechanically-stable chip-to-chip contacts. In OQP, waveguides of separate substrates are aligned with sub-micron accuracy by protruding lithographically-defined copper nodules on the side of a chip. With OQP, high efficiency chip-to-chip optical coupling can be achieved by aligning waveguides of separate chips with sub-micron accuracy and reducing chip-to-chip distance. We used MEEP (MIT Electromagnetic Equation Propagation) to investigate the feasibility of OQP by calculating the optical coupling loss between butt coupled waveguides. Transmission between a typical QCL ridge waveguide and a single-mode Ge-on-Si waveguide was calculated to exceed 65% when an interchip gap of 0.5 μm and to be no worse than 20% for a gap of less than 4 μm. These results compare favorably to conventional off-chip coupling. To further increase the coupling efficiency and reduce sensitivity to alignment, we used a horn-shaped Ge-on-Si waveguide and found a 13% increase in coupling efficiency when the horn is 1.5 times wider than the wavelength and 2 times longer than the wavelength. Also when the horizontal misalignment increases, coupling loss of the horn-shaped waveguide increases at a slower rate than a ridge waveguide.
    No preview · Article · Sep 2013 · Proceedings of SPIE - The International Society for Optical Engineering
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    Yu Yao · Anthony J. Hoffman · Claire F. Gmachl
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    ABSTRACT: Mid-infrared quantum cascade lasers are semiconductor injection lasers whose active core implements a multiple-quantum-well structure. Relying on a designed staircase of intersubband transitions allows free choice of emission wavelength and, in contrast with diode lasers, a low transparency point that is similar to a classical, atomic four-level laser system. In recent years, this design flexibility has expanded the achievable wavelength range of quantum cascade lasers to similar to 3-25 mu m and the terahertz regime, and provided exemplary improvements in overall performance. Quantum cascade lasers are rapidly becoming practical mid-infrared sources for a variety of applications such as trace-chemical sensing, health monitoring and infrared countermeasures. In this Review we focus on the two major areas of recent improvement: power and power efficiency, and spectral performance.
    Full-text · Article · Jun 2012 · Nature Photonics
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    ABSTRACT: We present a tunable coupling qubit (TCQ), which has independent and fast control over the qubit energy and the coupling strength to a superconducting microwave cavity in a circuit quantum electrodynamics architecture.
    No preview · Article · May 2012
  • Yanbing Liu · Srikanth Srinivasan · Anthony Hoffman · Andrew Houck
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    ABSTRACT: We discuss various experimental approaches to improve the single shot measurement fidelity of a superconducting charge qubit. Dispersive readout is optimized on a transmon coupled to a superconducting coplanar waveguide resonator. Measurement parameters, such as microwave power and frequency are varied. Also control theory is adapted to construct a genetic algorithm which optimizes the shape of the drive pulse. Additionally, we attempt to reduce noise and increase SNR by employing a SLUG amplifier. Using these techniques, we discuss the feasibility of reaching the measurement fidelity needed for scalable quantum computation with superconducting circuits.
    No preview · Article · Feb 2012
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    ABSTRACT: We report measurements of the coupling between a superconducting microwave resonator and a transmon qubit fabricated on a separate chip and mounted to a three-dimensional cryogenic translation stage. The qubit-resonator system reached the strong coupling regime with a coupling strength in excess of 80 MHz. We use the translation stage to explore the position dependence of the coupling strength. With a scanning qubit stage, it is possible to measure many qubits in succession and study the statistics of the fabrication process. The system can also be used as a local probe of a large array of microwave cavities and superconducting qubits.
    No preview · Article · Feb 2012
  • Devin Underwood · Will Shanks · Anthony Hoffman · Jens Koch · Andrew Houck
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    ABSTRACT: It has been proposed that arrays of electromagnetic cavities, coupled to two level quantum systems can be used to realize quantum phase transitions of polaritons. One possible experimental realization is a circuit quantum electrodynamics architecture, in which transmon qubits are coupled to superconducting coplanar waveguide resonators (CPWRs); however, for this to be successful, arrays of resonators must be fabricated with low disorder. Results will be reported on characterization of an array of 12 niobium resonators on a sapphire substrate in a honeycomb pattern with the photonic lattice sites forming a Kagome star. These arrays were characterized by measuring many devices of the same design, and using statistical methods for analysis. Furthermore we investigate the origins of disorder, and its dependence on fluctuations in the CPWR geometry.
    No preview · Article · Feb 2012
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    ABSTRACT: Strong photon-qubit coupling in the circuit quantum electrodynamics architecture may lead to quantum phase transitions of light. Recent theoretical and experimental efforts have been made toward examining such quantum phase transitions in large systems; however, interesting crossovers may also exist in significantly smaller and more controllable systems. A sharp nonequilibrium self-trapping transition of light has been predicted in a system comprising two coupled resonators each containing a single qubit. A delocalized regime, where photons coherently oscillate between the two cavities, transitions via dissipation into a localized regime, where photons cannot tunnel. We realized this system experimentally using two capacitively coupled superconducting microwave coplanar waveguides each containing a single transmon qubit. We present our experimental investigation of the system using time and frequency domain measurements to probe its dynamics.
    No preview · Article · Feb 2012
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    ABSTRACT: We present results of time domain measurements on a tunable coupling qubit (TCQ) coupled to a superconducting coplanar waveguide resonator. The TCQ has the benefit of independently tunable qubit frequency and cavity-qubit coupling. We show that the TCQ's frequency and coupling can be dynamically controlled in tens of nanoseconds by using two on-chip flux control lines. Using this dynamic control, Rabi oscillations were measured at various coupling strengths showing that the coupling can be reduced by a factor greater than 1500. To measure qubit coherence at low coupling, the TCQ was tuned to a high coupling region, excited by a synchronized pi-pulse and then returned to the zero coupling region where the qubit state was measured. Coherence times of several microseconds were measured and are comparable to other superconducting qubits
    No preview · Article · Feb 2012
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    ABSTRACT: We demonstrate coherent control and measurement of a superconducting qubit coupled to a superconducting coplanar waveguide resonator with a dynamically tunable qubit-cavity coupling strength. Rabi oscillations are measured for several coupling strengths showing that the qubit transition can be turned off by a factor of more than 1500. We show how the qubit can still be accessed in the off state via fast flux pulses. We perform pulse delay measurements with synchronized fast flux pulses on the device and observe $T_1$ and $T_2$ times of 1.6 and 1.9 $\mu$s, respectively. This work demonstrates how this qubit can be incorporated into quantum computing architectures.
    Preview · Article · Aug 2011 · Physical review. B, Condensed matter
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    ABSTRACT: All-semiconductor, highly anisotropic metamaterials provide a straightforward path to negative refraction in the mid-infrared. However, their usefulness in applications is restricted by strong frequency dispersion and limited spectral bandwidth. In this work, we show that by stacking multiple metamaterials of varying thickness and doping into one compound metamaterial, bandwidth is increased by 27% over a single-stack metamaterial, and dispersion is reduced.
    Preview · Article · Aug 2011 · Optics Express
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    ABSTRACT: Mediated photon-photon interactions are realized in a superconducting coplanar waveguide cavity coupled to a superconducting charge qubit. These nonresonant interactions blockade the transmission of photons through the cavity. This so-called dispersive photon blockade is characterized by measuring the total transmitted power while varying the energy spectrum of the photons incident on the cavity. A staircase with four distinct steps is observed and can be understood in an analogy with electron transport and the Coulomb blockade in quantum dots. This work differs from previous efforts in that the cavity-qubit excitations retain a photonic nature rather than a hybridization of qubit and photon and provides the needed tolerance to disorder for future condensed matter experiments.
    Preview · Article · Jul 2011 · Physical Review Letters
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    ABSTRACT: Recently, quantum phase transitions of light have been the focus of much theoretical attention. One possible experimental realization relies upon the circuit quantum electrodynamics architecture (cQED); however, in order for this to be successful, coupled arrays of superconducting resonators must first be realized with low disorder. Here we fabricate and characterize an array with low disorder consisting of 12 niobium resonators on a sapphire substrate in a honeycomb pattern with the photonic lattice sites forming a Kagome star. The structure is characterized by measuring transmission through different input-output port pairs and by varying the hopping rate between resonators. A family of resonant peaks corresponding to the various modes of the coupled array is identifiable and agrees well with both a tight-binding Hamiltonian and simulations from a commercial microwave software package. These experiments are an important step in realizing strongly correlated interactions in cQED.
    No preview · Article · Mar 2011