Terahertz electromagnetic (EM) waves (1 THz=1012 Hz), lying between the millimeter wave and mid-infrared, have many important applications such as imaging, spectroscopy, wireless communication, security screening and so forth. Great efforts are being made to bridge the so called “terahertz gap” by developing more efficient key terahertz devices such as sources, detectors and modulators for applications. Among the key terahertz devices, Schottky-barrier diodes and field-effect transistors have been developed as sensitive terahertz detectors for room-temperature applications. On the other hand, terahertz quantum cascade lasers and resonant tunneling diodes are being developed into powerful and compact terahertz sources by using the photonic and electronic approaches, respectively.
In bridging the terahertz gap, not only detectors and sources but also high-performance terahertz modulators are highly desired for terahertz communication and imaging systems. Most of the terahertz modulators reported so far rely on the absorption of terahertz EM wave by free electrons which is described by the Drude conductivity with a single-particle nature. Such a non-resonant absorption mechanism is usually not sufficient for achieving a large modulation depth even when a resonant coupling structure for terahertz EM wave is employed. Consequently, a compromise between modulation depth, bandwidth, insertion loss and polarization dependence has to be made. Nevertheless, a few different types of single-particle-based terahertz modulators have been demonstrated by using either bulk or two-dimensional semiconductors such as bulk GaAs, two-dimensional electron gas (2DEG) in AlGaAs/GaAs heterostructure and graphene.
Recently, the terahertz research group led by Prof. QIN Hua at Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), a research institute of Chinese Academy of Sciences (CAS), has proposed and demonstrated a terahertz modulator based on the collective electron excitation (plasmons) in grating-coupled 2DEG in AlGaN/GaN heterostructure. 2D plasmon as an elementary excitation mode of 2DEG, its frequency can be continuously tuned with the electron density by using a field-effect gate. In comparison with the terahertz absorption by free electrons of single-particle nature, resonant excitation of 2D plasmons allows for an extremely high modulation depth.
In the prototype modulator, a grating gate is utilized to tune the electron density in the AlGaN/GaN 2DEG channel underneath and to couple the 2DEG with the incident terahertz EM wave. The device exhibits excellent modulation performances. In the test, Dr. Yongdan Huang (now at Shanghai Institute of Technical Physics, Chinese Academy of Sciences) demonstrated a maximum intensity modulation depth of 93% (11.5 dB) at 0.501 THz. Over a broad spectrum of 82 GHz from 0.436 THz to 0.518 THz, the modulation depth is kept above 90%. Not only the resonant characteristic of 2D plasmons but also the strong light-matter coupling realized by the grating coupler (naturally broadband) in combination with the Fabry-Perot terahertz cavity (the sapphire substrate supporting the 2DEG) is beneficial to the high modulation depth.
The transmission of terahertz EM wave can be switched on and off at a high speed (~400 kHz) by the gate voltage alternatively set to the resonant and non-resonant conditions (2 V apart) for the 2D plasmon. The maximum switching speed is limited by the large gate capacitance and can be improved by making an impedance matching to the driving circuit.
However, the above superior modulation characteristics were achieved at a temperature of 8.7 K. Although the maximum operation temperature is about 170 K, modulation depth decreases with the elevating temperature. To enable practical and effective terahertz modulators at room temperature, both the quality factor of 2D plasmon and the coupling strength between the plasmon and the terahertz EM wave have to be improved. The former could be improved by using electronic materials with higher carrier mobility and the latter could be achieved by using a Fabry-Perot cavity with a higher Q factor.
The present device structure also provides a convenient platform for studying the light-matter interaction in terahertz frequency regime in many different types of low-dimensional electronic materials, such as graphene and black phosphorus. For more information on terahertz modulation based on resonant plasmon excitation, please refer to the new publication entitled "Plasmonic terahertz modulator based on a grating-coupled two-dimensional electron system" appeared in Applied Physics Letters [Appl.Phys.Lett. 109, 201110 (2016)]. This work was supported by the National Natural Science Foundation of China (61505242, 61401456, 11403084 and 61271157). The authors acknowledge support from the Nanofabrication Facility at SINANO-CAS.
Contact information: Prof. QIN Hua
Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences
Suzhou, Jiangsu 215125,China.