First observation of a plasmon-mediated tunable photoresponse in a grating-gated InGaAs/InP HEMT for millimeter-wave detection Nima Nader Esfahani* a,b,c , Robert. E. Peale a , Walter R. Buchwald b , Joshua R. Hendrickson c and Justin W. Cleary c a Department of Physics, University of Central Florida, Orlando FL, USA 32816 b Solid State Scientific Corporation, Nashua, NH 03060 c Air Force Research Laboratory, Sensors Directorate, Wright Patterson AFB OH 45433 ABSTRACT A tunable resonant photoresponse to millimeter-waves is demonstrated in a grating-gated high electron mobility transistor (HEMT) formed by an InGaAs/InP heterostructure. The gate consists of a metal grating with 9 μm period, which was designed to couple mm-radiation to plasmons in the two-dimensional electron gas (2DEG) of the HEMT. The resonant excitation of plasmons, which shifts with gate-bias, changes the channel conductance. These devices have potential as chip-scale frequency-agile mm-wave detectors, which may be scaled to THz frequencies. Keywords: HEMT, Plasmon, terahertz, 2DEG 1. INTRODUCTION Recent investigations of chip-scale, mm-wave and terahertz radiation detectors [1-4] have emphasized high-speed tunable resonant excitations of plasmons in a two-dimensional electron gas (2DEG). Such 2DEGs occur at the interface of two semiconductors with a conduction band discontinuity. Coupling structures such as gratings [2-6] or antennae [1] are required to compensate for the inherent momentum mismatch occurring between the incident radiation and excited plasmon mode. The excitation of plasmons results in significant absorption of the incident electromagnetic wave as has been observed by transmission spectroscopy in Si [5], InP [6], and GaN [7] materials systems. In some cases, this excitation gives rise to a change in channel conductance [1-4]. In the device investigated here, the grating also doubles as a gate, which allows for the tuning of the plasmon absorption frequency because of the dependence of the plasmon dispersion relation on sheet charge density. A tunable electrical photoresponse provides a basis for potential on-chip THz and mm-wave spectral detection. In general, the momentum matching grating period is much smaller than the incident wavelength, which results in the grating becoming polarized at the photon frequency. This polarization induces localized alternating fields with the grating spatial periodicity. These local fields penetrate the underlying semiconductor layers and perturb the 2DEG. The characteristic length scale of the grating structure defines the optical frequencies of the excited plasmons via their dispersion relation, and in consequence defines possible resonant changes in channel conductance. The plasmon dispersion relation is a function of both material and structure. The InGaAs/InP materials system has been selected in part because of the possibility of sharp THz plasmon resonances at high frequencies due to high sheet charge density, large mobility and low electron effective mass, which in principle allow narrow resonances at THz frequencies. This work presents our first observation of an electrical photo-response due to plasmon excitation in the 2DEG of an InGaAs/InP HEMT. For experimental convenience, due to the availability of stable and highly functional mm-wave sources, this first demonstration has been performed at millimeter wavelengths. 2. THEORETICAL CONSIDERATIONS The dispersion relation for two-dimensional plasmons is [5]  ௡ ଶ ൌ ௘ మ ௡ ೞ ௤ ೙ ௠ כ ఢ బ ൫ ߝ ௕ ൅ ߝ ௧ ݋ݐሺ ݍ ௡ ሻ൯ ଵ , (1) Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/app/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns. 8512 - 33 V. 2 (p.1 of 7) / Color: No / Format: Letter / Date: 7/19/2012 9:10:52 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes: