Walter R. Buchwald

Ultrasensitive silicon photonic-crystal nanobeam electro-optical modulator: Design and simulation

Design and simulation results are presented for an ultralow switching energy, resonator based, silicon-on-insulator (SOI) electro-optical modulator. The nanowire waveguide and Q ~8500 resonator are seamlessly integrated via a high-transmission tapered 1D photonic crystal cavity waveguide structure. A lateral p-n junction of modulation length L(m) ~λ is used to alter the index of refraction and, therefore, shift the resonance wavelength via fast carrier depletion. Differential signaling of the device with ΔV ~0.6 Volts allows for a 6 dB extinction ratio at telecom wavelengths with an energy cost as low as 14 attojoules/bit.

Fano-Resonance Photonic Crystal Membrane Reflectors at Mid- and Far-Infrared

We report here single-layer ultracompact Fano-resonance photonic crystal membrane reflectors (MRs) at mid-infrared (IR) and far-IR (FIR) bands, based on single layer crystalline Si membranes. High-performance reflectors were designed for surface-normal incidence illumination with center operation wavelengths up to the 75-μm FIR spectral band. Large-area patterned MRs were also fabricated and transferred onto glass substrates based on membrane transfer processes. Close to 100% reflection was obtained at the ~ 76-μm spectral band, with a single-layer Si membrane thickness of 18 μm. Such Fano-resonance-based membranes reflectors offer great opportunities for high-performance ultracompact dielectric reflectors at IR and THz regions.

Millimeter-wave photoresponse due to excitation of two-dimensional plasmons in InGaAs/InP high-electron-mobility transistors

A polarized photoresponse to mm-wave radiation over the frequency range of 40 to 108 GHz is demonstrated in a grating-gated high electron mobility transistor (HEMT) formed by an InGaAs/InP heterostructure. The photoresponse is observed within the plasmon resonance absorption band of the HEMT, whose gate consists of a 9 μm period grating that couples incident radiation to plasmons in the 2D electron gas. Gate-bias changes the channel carrier concentration, causing a corresponding change in photoresponse in agreement with theoretical expectations for the shift in the plasmon resonance band. The noise equivalent power is estimated to be 235 pW/Hz1/2.

Planar integrated plasmonic mid-IR spectrometer

Mid-IR spectrometers with adequate resolution for chemical sensing and identification are typically large, heavy, and require sophisticated non-stationary optical components. Such spectrometers are limited to laboratory settings. We propose an alternative based on semiconductor micro-fabrication techniques. The device consists of several enabling parts: a compact broad-band IR source, photonic waveguides, a photon-to-surface-plasmon transformer, a surfaceplasmon sample-interaction region, and an array of silicon ring-resonators and detectors to analyze the spectrum. Design considerations and lessons learned from initial experiments are presented.

Millimeter and terahertz detectors based on plasmon excitation in InGaAs/InP HEMT devices

Recent progress in the investigation of millimeter-wave and THz detectors based on plasmon excitation in the twodimensional electron gas (2DEG) of a high electron mobility transistor (HEMT) is reported. A tunable resonant polarized photoresponse to mm-wave radiation in the frequency range of 40 to 110 GHz is demonstrated for a gratinggated InGaAs/InP based device. The gate consisted of a metal grating with period of 9 μm specifically designed for excitation of sub-THz plasmons. The resonant excitation of plasmons, which shifts with gate-bias, changes the channel conductance. This resonant change in channel conductance enables potential applications in chip-scale frequency-agile detectors, which can be scaled to mid-THz frequencies.

Tunable excitation of two-dimensional plasmon modes in InGaAs/InP HEMT devices for terahertz detection

THz electromagnetic waves resonantly excite plasmons in the two dimensional electron gas (2DEG) of high electron mobility transistors (HEMTs) via grating-gate couplers. These excitations can induce measureable photoresponse. Biasing the grating gate tunes the photoresponse via control of 2DEG carrier density. Plasmons are investigated here in an InGaAs/InP HEMT with a 9 μm period grating gate at 78 and 106 GHz free-space radiation and 4K sample temperature. The dependence of the photoresponse on applied Source-Drain bias is also investigated. The minimum noise equivalent power (NEP) is estimated to be 113 pW/Hz1/2 , with maximum responsivity of 200 V/W. Such plasmonic alterations in channel conductance provide a means for voltage-tunable THz and sub-THz detectors or filters.

Active frequency selective surfaces

Split ring resonator arrays are investigated for use as active elements for the realization of voltage controllable frequency selective surfaces. Finite difference time domain simulations suggest the absorptive and reflective properties of such surfaces can be externally controlled through modifications of the split ring resonator gap impedance. In this work, such voltage-controlled resonance tuning is obtained through the addition of an appropriately designed high electron mobility transistor positioned across the split ring resonator gap. It is shown that a 0.5μm gate length high electron mobility transistor allows voltage controllable switching between the two resonant conditions associated with a split ring resonator and that of a closed loop geometry when the surface is illuminated with THz radiation. Partial switching between these two resonant conditions is observed at larger gate lengths. Such active frequency selective surfaces are proposed, for example, for use as modulators in THz detection schemes and as RF filters in radar applications when scaled to operate at GHz frequencies.