Anthony J. Hoffman

Room-temperature continuous-wave quantum cascade lasers grown by MOCVD without lateral regrowth

We report on room-temperature continuous-wave (CW) operation of lambda~8.2 mum quantum cascade lasers grown by metal-organic chemical vapor deposition without lateral regrowth. The lasers have been processed as double-channel ridge waveguides with thick electroplated gold. CW output power of 5.3 mW is measured at 300 K with a threshold current density of 2.63 kA/cm2. The measured gain at room temperature is close to the theoretical design, which enables the lasers to overcome the relatively high waveguide loss

Dispersive photon blockade in a superconducting circuit.

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.

Tunable coupling in circuit quantum electrodynamics using a superconducting charge qubit with a V-shaped energy level diagram.

Recent progress in superconducting qubits has demonstrated the potential of these devices for the future of quantum information processing. One desirable feature for quantum computing is independent control of qubit interactions as well as qubit energies. We demonstrate a new type of superconducting charge qubit that has a Vshaped energy spectrum and uses quantum interference to provide independent control over the qubit energy and dipole coupling to a superconducting cavity. We demonstrate dynamic access to the strong coupling regime by tuning the coupling strength from less than 200 kHz to more than 40 MHz. This tunable coupling can be used to protect the qubit from cavity-induced relaxation and avoid unwanted qubit-qubit interactions in a multi-qubit system.

High-Performance Quantum Cascade Lasers: Optimized Design Through Waveguide and Thermal Modeling

We present a comprehensive model to study the thermal effects in quantum cascade (QC) lasers for continuous-wave (CW) operation at and above room temperature. This model self-consistently solves the temperature-dependent threshold current density equation and heat equation to determine the CW threshold current density, maximum heat sink temperature, and core temperature at threshold for a given laser design. The model includes effects from temperature dependence on thermal backfilling, thermal conductivity, phonon lifetimes, gain bandwidth, thermionic emission, and resistive heating in waveguide layers. Studies on these effects yield results not simultaneously considered by previous models. By including these results in laser designs, lasers with lower core temperatures, with higher operating temperatures, and requiring lower electrical power than current high-performance lasers are predicted. Additionally, experimental results are presented, exploring various methods of improving CW laser performance for a lambda ~ 8 mum QC laser and are compared to the model.

Midinfrared semiconductor optical metamaterials

We report on a novel class of semiconductor metamaterials that employ a strongly anisotropic dielectric function to achieve negative refraction in the midinfrared region of the spectrum, ∼8.5–13 μm. We present two types of metamaterials, layered highly doped/undoped heterostructures and quantum well superlattices that are highly anisotropic. Contrary to other optical metamaterials these heterostructure systems are optically thick (up to 20 μm thick), planar, and require no additional fabrication steps beyond the initial growth. Using transmission and reflection measurements and modeling of the highly doped heterostructures, we demonstrate that these materials exhibit negative refraction. For the highly doped quantum well superlattices, we demonstrate anomalous reflection due to the strong anisotropy of the material but a determination of the sign of refraction is still difficult. This new class of semiconductor metamaterials has great potential for waveguiding and imaging applications in the long-wave inf...

ZnCdSe/ZnCdMgSe quantum cascade electroluminescence

This letter reports electroluminescence emission from a ZnCdSe∕ZnCdMgSe quantum cascade (QC) structure. With a two-well QC active region design, the II-VI heterostructure was grown lattice matched on an InP substrate by molecular beam epitaxy. Deep etched mesas were electrically pumped at current densities up to 10kA∕cm2, producing optical emission centered near 4.8μm, in good agreement with the structure design. The light is predominantly TM polarized, confirming its intersubband origin. Electroluminescence was observed from 78to300K.

Voltage Tunability of Quantum Cascade Lasers

The voltage tunability of three types of quantum cascade laser designs is investigated. The tuning coefficients and tuning ranges of electroluminescence and laser emission from all designs are measured and compared with the calculated results. A reduced tunability was observed in all lasers above threshold. This is attributed to the decrease of resistance across the laser active region (AR) as the photon density increases. A resumed tunability high above threshold occurs in all lasers with anticrossed injector ground and upper laser states. Lasers based on the anticrossed diagonal transition are tunable above threshold, with a tuning range of about 30 cm-1 (~3% of the laser emission wavenumber), i.e., a tuning rate of 750 cm-1 V-1middotperiod-1 of the AR and the injector.

Photonic materials, structures and devices for Reststrahlen optics

We present a review of existing and potential next-generation far-infrared (20-60 μm) optical materials and devices. The far-infrared is currently one of the few remaining frontiers on the optical spectrum, a space underdeveloped and lacking in many of the optical and optoelectronic materials and devices taken for granted in other, more technologically mature wavelength ranges. The challenges associated with developing optical materials, structures, and devices at these wavelengths are in part a result of the strong phonon absorption in the Reststrahlen bands of III-V semiconductors that collectively span the far-infrared. More than just an underexplored spectral band, the far-IR may also be of potential importance for a range of sensing applications in astrochemistry, biology, and industrial and geological processes. Additionally, with a suitable far-IR optical infrastructure, it is conceivable that even more applications could emerge. In this review, we will present recent progress on far-infrared materials and phenomena such as phononic surface modes, engineered composite materials, and optoelectronic devices that have the potential to serve as the next generation of components in a far-infrared optical tool-kit.

Quantum cascade lasers with voltage defect of less than one longitudinal optical phonon energy

Efficient use of applied voltage in quantum cascade (QC) lasers is a critical factor in achieving high wall-plug efficiency and low compliance voltage. We demonstrate a QC laser emitting at 4.2 μm featuring a low voltage defect and short injector with only four quantum wells. Devices with a voltage defect of 20 meV, well below the energy of the longitudinal optical phonons, and a voltage efficiency of 91%, a record value for QC lasers, are reported for pulsed operation at 180 K. Voltage efficiencies of greater than 80% are exhibited at room temperature. Overall performance showed wall-plug efficiencies ranging from 21% at cryogenic temperatures to 5.3% at room temperature.

Selective absorbers and thermal emitters for far-infrared wavelengths

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.