Tufts University engineers have invented a chip-sized, high-speed modulator that operates at terahertz (THz) frequencies at room temperature and at low voltages without consuming DC power. The discovery could transmit data at significantly higher speeds than currently possible.
Great interest has emerged in the research and development of THz devices and related technology. Applications include material identification, imaging, and chemical and biological sensing. One of the greatest potential uses for THz frequencies could be high-data-rate wireless communication where an inherently high carrier frequency of THz wave will support much wider signal bandwidth compared to the radio frequency (RF) bands used today. Wireless communication is achieved by modulating a high-frequency carrier wave either in amplitude or phase, or both, by a modulating signal. Modulators are therefore a key building block in the realization of any communication platform.
Existing solutions for terahertz modulation are limited to free space propagation of quasi-optical signals that may be ideal for THz spectroscopy and imaging, but not suitable for THz communication, which requires that modulators be integrated with other components such as amplifiers and detectors on a single chip to enable realization of integrated THz transmitter and receiver systems.
The on-chip device invented in this work can perform gigahertz-rate amplitude modulation and switching of broadband THz electromagnetic waves. The operation of the device is based on the interaction of confined THz waves in a novel slot waveguide with an electronically tunable two-dimensional electron gas (2DEG) that controls the loss of the THz wave propagating through this waveguide.
The fabricated device is shown in the figure. The device is based on the concept of enhanced interaction of THz waves with the 2DEG serving as a loss medium. A novel low-loss metal slot waveguide is used to confine and couple the THz wave to the 2DEG.
Broadband operation for THz modulation is possible using this device. The modulator has been realized and verified experimentally for modulation of electromagnetic waves in the frequency band of 0.22–0.325 THz. Based on simulation, one can predict that this modulator can operate over signals up to 1 THz.
The modulation frequency exceeds 14 GHz, indicating that high-speed modulation of THz can be achieved. The achieved modulation depth is high, and can be increased further by increasing the length of the 2DEG in the device at the expense of modulation speed from increases in the charging/discharging time of the 2DEG. The drive voltage required for modulation is low — about 2 V — and there is no static DC power consumption. Also, given the fact that this device works at room temperature, it provides a unique advantage. All of these properties make the proposed modulator suitable for realization of THz transmitters/receivers on a single chip. Moreover, the high modulation index demonstrated with a short device length implies that one can utilize this device as a high-performance on/off switch at THz frequencies.