Brandon Scott Passmore

Doping tunable resonance: Toward electrically tunable mid-infrared metamaterials

We demonstrate metamaterials at the mid-infrared (mid-IR) wavelengths (8–12 μm) that can be widely tuned by doping in adjacent semiconductor epilayers. The metamaterials are based on metallic split ring resonators (SRRs) fabricated on doped indium antimonide (InSb). Finite integral time-domain simulation results and measured transmission data show that the resonance blueshifts when the semiconductor electron carrier concentration is increased while keeping the split ring geometry constant. A resonant wavelength shift of 1.15 μm is achieved by varying the carrier concentration of underlying InSb epilayer from 1×1016 to 2×1018 cm−3. This work represents the first step toward active tunable metamaterials in the mid-IR where the resonance can be tuned in real time by applying an electric bias voltage to control the effective carrier density.

Observation of Rabi Splitting from Surface Plasmon Coupled Conduction State Transitions in Electrically Excited InAs Quantum Dots

We demonstrate strong coupling between a surface plasmon and intersublevel transitions in self-assembled InAs quantum dots. The surface plasmon mode exists at the interface between the semiconductor emitter structure and a periodic array of holes perforating a metallic Pd/Ge/Au film that also serves as the top electrical contact for the emitters. Spectrally narrowed quantum-dot electroluminescence was observed for devices with varying subwavelength hole spacing. Devices designed for 9, 10, and 11 μm wavelength emission also exhibit a significant spectral splitting. The association of the splitting with quantum-dot Rabi oscillation is consistent with results from a calculation of spontaneous emission from an interacting plasmonic field and quantum-dot ensemble. The fact that this Rabi oscillation can be observed in an incoherently excited, highly inhomogeneously broadened system demonstrates the utility of intersublevel transitions in quantum dots for investigations of coherent transient and quantum coherence phenomena.

Mid-infrared doping tunable transmission through subwavelength metal hole arrays on InSb

Doping-tunable mid-infrared extraordinary transmission is demonstrated from a periodic metal hole array patterned on n-InSb. The polarization-dependent transmission was measured at room temperature and 77 K. In addition, the extraordinary transmission was measured for incident angles from 0 degrees to 35 degrees in 5 degrees steps. A fundamental resonance shift of approximately 123 cm-1 (1.4 microm) is observed by varying the doping from 1 x 10(16) to 2 x 10(18) cm(-3). The calculated transmission resonances were in good agreement with the experimental results. This suggests that InSb semiconductor-based plasmonic structures may be suitable for a variety of tunable mid-infrared device applications.

Spectral and spatial investigation of midinfrared surface waves on a plasmonic grating

A patterned metal film with a periodic array of subwavelength apertures, fabricated upon a semiconductor substrate and designed to possess transmission resonances in the midinfrared is interrogated with a wavelength-tunable external cavity quantum cascade laser. The interaction of the coherent light with this plasmonic structure is studied using a spatially resolved transmission experiment, allowing for the far-field imaging of propagating waves on the surface of the metal film. Spatial and spectral transmission is investigated for a range of near-normal incidence angles. For nonzero angles of incidence, coupling of laser light, at distinct frequencies, to surface waves propagating in opposite directions is demonstrated.

Metamaterial resonators on curved surfaces

Straightforward extension of canonical microwave metamaterial structures to optical and IR frequency dimensions is complicated by both the size scale of the resulting structures, requiring cutting edge lithography to achieve the requisite line-widths, as well as limitations on assembly/construction into final geometry. We present a scalable fabrication approach capable of generating metamaterial structures such as split ring resonators and split wire pairs on a micron/sub-micron size scale on concave surfaces with a radius of curvature ~ SRR diameter. This talk outlines the fabrication method and modeling/theory based interpretation of the implications of curved metamaterial resonators.

Loss mechanisms in mid-infrared extraordinary optical transmission gratings

The optical properties of periodic arrays of subwavelength apertures in metal films on GaAs substrates are studied. Specifically, geometric and material losses for these plasmonic structures are characterized using angular dependent transmission, normal incidence reflection, and angular dependent diffraction experiments, in addition to a crossed-polarizer transmission experiment. The optical properties of the samples as a function of engineered material losses are studied. Using this comprehensive approach to the characterization of the plasmonic structures, we are able to identify and isolate specific loss mechanisms, as well as identify the effect of free carriers on the optical properties of the structures.

Mid-Infrared Amplitude and Phase Measurement of Metamaterials Using Tandem Interferometry

A tandem interferometer system measuring the absolute phase and amplitude of planar split-ring resonators fabricated on a BaF2 substrate with a designed resonance at 10.5 ?m is presented.

Electrically tunable thermal-infrared metamaterials

We experimentally demonstrate that the resonance of a thermal-infrared metamaterial on a semiconductor substrate can be shifted by the substrate doping. We further study the electrical tuning of metamaterial resonance via finite integral time-domain simulation.

Mid-infrared emitters utilizing intersublevel transitions in self assembled InAs quantum dots

We demonstrate room temperature electroluminescence from intersublevel transitions in self-assembled InAs quantum dots in GaAs/AlGaAs heterostructures. The quantum dot devices are grown on GaAs substrates in a Varian Gen II molecular beam epitaxy system. The device structure is designed specifically to inject carriers into excited conduction band states in the dots and force an optical transition between the excited and ground states of the dots. A downstream filter is designed to selectively extract carriers from the dot ground states. Electroluminescence measurements were made by Fourier Transform Infrared Spectroscopy in amplitude modulation step scan mode. Current-Voltage measurements of the devices are also reported. In addition, both single period and multi-period devices are grown, fabricated, characterized, and compared to each other. Finally, we discuss the use of plasmonic output couplers for these devices, and discuss the unique emission observed when the quantum dot layer sits in the near field of the plasmonic top contacts.

Mid-infrared surface plasmon coupled emitters utilizing intersublevel transitions in InAs quantum dots.

We demonstrate mid-infrared electroluminescence from intersublevel transitions in self-assembled InAs quantum dots coupled to surface plasmon modes on metal hole arrays. Subwavelength metal hole arrays with different periodicity are patterned into the top contact of the broadband (9-15 μm) quantum dot material and the measured electroluminescence is compared to devices without a metal hole array. The resulting normally directed emission is narrowed and a splitting in the spectral structure is observed. By applying a coupled quantum electrodynamic model and using reasonable values for quantum dot distributions and plasmon linewidths we are able to reproduce the experimentally measured spectral characteristics of device emission when using strong coupling parameters.