Plasmonic nanodevices for terahertz sensing and spectroscopy
Tunable and coherent sources operating at terahertz (THz) frequencies currently represent one of the hottest topics in modern electronics. Historically, THz radiation was the province of astronomy and analytical science. Now, the technology is finding use in an increasingly wide variety of applications, such as in information and communications, biology and medical sciences, nondestructive evaluation, homeland security, quality control of food and agricultural products, global environmental monitoring, and ultrafast computing.1 Located between the microwave and IR regions of the electromagnetic spectrum, the THz band is unexplored in the sense that no commercially available microelectronic devices operate over its entire range of frequencies. Conventional electronics such as field-effect transistors (FETs) increase their operating frequencies by scaling down feature size at a substantial cost of output power. Photonic devices like semiconductor lasers, on the other hand, reduce their operating frequencies by scaling down the interband to intersubband transition energy. Decreasing radiation frequency to the THz range reduces the photon energy to the thermal-energy level or even lower, which severely disturbs the coherence of the radiation. This unfortunate state of affairs is known as the terahertz gap.
The channel of a FET can act as a resonator for plasma waves (collective electron-charge/density waves). When the energy of the plasma-wave frequencies is quantized, the quanta are called plasmons. The plasmon frequency depends on its dimension and easily exceeds THz by setting the gate length at hundreds of nanometers. Study of plasmon FET applications for THz imaging and spectroscopy began in the early 1990s with the pioneering theoretical work of Michael Dyakonov and Michael Shur.2,3 However, efficiently coupling nonradiative plasmons to radiative electromagnetic waves proved a major challenge.
To circumvent this problem, we introduced an interdigitated dual-grating gate (DGG) structure into a standard high-electron-mobility transistor (HEMT)4 (see Figure 1). The DGG periodically localizes the 2D plasmon in 100nm regions with an interval of a micrometer. When the DC drain-to-source bias is applied, 2D electrons are accelerated to produce a constant drain-to-source current. This distribution of plasmonic-cavity systems in periodic 2D electron-density modulation enables the DC current to excite the plasma waves in each cavity, leading to self-oscillation. The DGG also acts as a broadband THz antenna that converts nonradiative longitudinal plasmon modes to radiative transverse electromagnetic modes. Additionally, a vertical-cavity structure is introduced between the upper DGG plane and a THz mirror at the back side, working as a plasmon-resonant enhancer.
We have designed and fabricated our original plasmon-resonant emitters (PREs) using indium gallium phosphide/indium gallium arsenide/gallium arsenide (InGaP/InGaAs/GaAs) material systems in a double-deck HEMT.5 The intrinsic device area has a geometry of 30×75μm, where the DGG pattern is replicated on the upper-deck HEMT layer, while the periodic plasmon cavities are configured on the lower-deck HEMT. The DGG consists of 1500 and 1850nm lines aligned alternately with a spacing of 100nm. The number of DGG fingers is 37 (38) for the 1500nm (1850nm) grating. In experiments, the PREs showed 1μW broadband emission ranging from 0.5 to 6.5THz at room temperature (see Figure 2).
Compared with a commercially available high-pressure mercury (HP-Hg) lamp, the PRE exhibits a superior (two orders of magnitude higher) energy-conversion efficiency of 10−4 to 10−5, although the output power is one order of magnitude lower than that of the HP-Hg lamp.6 We confirmed that dual- and triple-chip operation of PREs successfully doubles and triples the emission intensity (see Figure 2). The results encouraged us to increase the output power of the HP-Hg lamp by integrating the PREs in a 10×10 array.
We incorporated the device as a light source into a Fourier-transform THz spectrometer. The water-vapor absorption lines can be clearly observed and coincide very well with results provided by NASA.6 In addition, we were able to identify fingerprints of several sugar-group materials in liquid. As shown in Figure 3, the poor signal-to-noise ratio for single-chip PRE operation is greatly improved by employing a triple chip, with results nearly approaching those for the HP-Hg lamp. The PRE also works as a broadband, highly sensitive THz detector.7 Recent efforts in collaboration with the CNRS (Montpellier, France) demonstrated excellent THz imaging of materials concealed inside an envelope.7
In conclusion, DGG-HEMT-based plasmon-resonant microchip emitters/detectors are well suited to spectroscopic measurement in the low-to-mid THz range, roughly 0.5 to 6.5THz. As a next step, we plan to develop a 10×10-arrayed integrated PRE as a milliwatt-power THz microchip light source.
The authors thank T. Nishimura of the Research Institute of Electrical Communication (RIEC), Tohoku University (Japan), E. Sano of Hokkaido University (Japan), T. Asano of Kyushu University (Japan), Y. M. Meziani of Salamanca University (Spain), W. Knap of CNRS (Montpellier, France), and V. Popov of Saratov University (Russia) for their contributions. We are also grateful to V. Ryzhii of the University of Aizu (Japan), M. Dyakonov of the University of Montpellier (France) and M. Shur of Rensselaer Polytechnic Institute for their valuable discussion and encouragement. This work was financially supported in part by the SCOPE Programme from the Ministry of Internal Affairs and Communications of Japan and by a Grant-in-Aid for Basic Research (S) from the Japan Society for the Promotion of Science.
Taiichi Otsuji is a professor. He received his DrEng in electronics in 1994 from the Tokyo Institute of Technology (Japan). His research interests include THz electron devices, circuits, systems, and plasmonic metamaterials. He is active in SPIE both as a program-committee member and as an invited and contributing speaker in the field of THz plasmonic nanodevices and systems.
Tsuneyoshi Komori and Takayuki Watanabe are presently working toward their ME degrees in the Graduate School of Engineering.
Tetsuya Suemitsu has been an associate professor at RIEC since 2006. He received his DrEng in electronics in 2000 from Waseda University (Japan). His research interests include ultrahigh-frequency electronic devices.
