Double-quantum-well field-effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity. The voltage tuned resonance is determined by the plasma oscillations of the composite structure. The resonant photoconductivity requires a double-quantum well but the mechanism whereby plasma oscillations produce changes in device conductance is not understood. The phenomenon is potentially important for fast, tunable terahertz detectors. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1497433͔ Double-quantum-well ͑DQW͒ heterostructures are important in the scientific exploration of correlated electron states in two-dimensional electron systems 1 and potentially important for novel field-effect transistors that add functionality by controlling electron transfer between the quantum wells. 2 Interwell transfer can also be promoted by terahertz photon assisted tunneling, opening the possibility of fast, voltage tunable terahertz ͑THz͒ detectors. 3 This motivated our research on THz response of DQW field-effect transistors ͑FETs͒. We report the THz photoconductivity of DQW FETs in which the gate is a periodic metallic grating. Strong photoresponse occurs at the plasma resonance of the composite structure. Other detector proposals make use of plasmon modes in single two-dimensional electron gas ͑2DEG͒ systems. 4 But, the relatively strong resonant response that we report here appears to require the presence of a DQW. We model the resonant response with a transmission line model of the collective modes of the 2DEGs and correlate the observed resonances with standing plasmon resonances under the metallic part of the grating gate. While the work was motivated by the concept of interwell transfer, the actual mechanism that gives rise to this response is not understood.The FETs are fabricated from modulation doped GaAs/ AlGaAs DQW heterostructures grown by molecular beam epitaxy. Both wells are 200 Å wide and are separated by a 70 Å barrier. The nominal electron densities are n upper ϭ1.7 ϫ10 11 cm Ϫ2 and n lower ϭ2.57ϫ10 11 cm Ϫ2 : the 4.2 K mobility is ϳ1.7ϫ10 6 cm 2 /V s. A 2ϫ2 mm mesa is defined and ohmic contacts to both quantum wells form source and drain. A 700-Å-thick TiAu grating gate ͑with no metallization between the grating fingers͒ is evaporated with the lines of the grating perpendicular to the current flow. We explored 4 and 8 m periods; half the period is metal. The grating modulates the electron density when a voltage is applied, selects wave vectors of the excited plasmon and, coincidentally, produces both normal and transverse THz electric fields. See inset in Fig. 1 for a cross section of the sample.We apply a constant source-drain current of 100 A, focus the radiation onto the sample, and study the photoconductive response of the DQW as a function of gate voltage, THz frequency, and temperature. The radiation sources are the free-electron lasers at the University of California, Santa Barbara, which cover a frequency range between 120 GHz and 4.8 THz. The respons...

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Three-dimensional periodic microstructures of aluminum oxide, which are important for creating photonic band-gap structures (PBGs), were fabricated by laser rapid prototyping by means of laser-induced direct-write deposition from the gas phase. The structures consisted of layers of parallel rods forming a face-centered tetragonal lattice with lattice constants of 66 and 133 micrometers. These structures showed transmission minima centered around 4 terahertz (75 micrometers) and 2 terahertz (150 micrometers), respectively. PBGs will allow precise control of the optical properties of materials, including lasers without threshold.

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