January 19, 2017

Chip-sized, high-speed terahertz modulator raises possibility of faster data transmission

Tufts University engineers have invented a chip-sized, high-speed modulator that operates at terahertz (THz) frequencies and at room temperature at low voltages without consuming DC power. The discovery could help fill the “THz gap” that is limiting development of new and more powerful wireless devices that could transmit data at significantly higher speeds than currently possible.

Measurements show the modulation cutoff frequency of the new device exceeded 14 gigahertz and has the potential to work above 1 THz, according to a paper published online today in Scientific Reports. By contrast, cellular networks occupy bands that are much lower on the spectrum where the amount of data that can be transmitted is limited.

The device works through the interaction of confined THz waves in a novel slot waveguide with tunable, two-dimensional electron gas. The prototype device operated within the frequency band of 0.22-0.325 THz, which was chosen because it corresponded to available experimental facilities. The researchers say the device would work within other bands as well.

Confined terahertz waves interact with tunable, two-dimensional electron gas in a novel slot waveguide. Credit: Nano Lab, Tufts University School of Engineering.

Nature Scientific Reports - High Speed Terahertz Modulator on the Chip Based on Tunable Terahertz Slot Waveguide

Although there is significant interest in using the THz band of the electromagnetic spectrum, which would enable the wireless transmission of data at speeds significantly faster than conventional technology, the band has been underutilized in part because of a lack of compact, on-chip components, such as modulators, transmitters, and receivers.

“This is a very promising device that can operate at terahertz frequencies, is miniaturized using mainstream semiconductor foundry, and is in the same form factor as current communication devices. It’s only one building block, but it could help to start filling the THz gap,” said Sameer Sonkusale, Ph.D., of Nano Lab, Department of Electrical and Computer Engineering, Tufts University, and the paper’s corresponding author.

The proposed on-chip THz modulator device is based on the concept of enhanced interaction of THz wave with an electrically tunable two-dimensional electron gas (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. This frequency band was chosen based on the available measurement facilities and is by no means a limitation of the device itself. Based on the 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 increase in the charging/discharging time of the 2DEG. The drive voltage required for modulation is really low around 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, which posits practical applications. 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.

This paper presents an on-chip device that can perform gigahertz-rate amplitude modulation and switching of broadband terahertz 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. A prototype device is fabricated which shows THz intensity modulation of 96% at 0.25 THz carrier frequency with low insertion loss and device length as small as 100 microns. The demonstrated modulation cutoff frequency exceeds 14 GHz indicating potential for the high-speed modulation of terahertz waves. The entire device operates at room temperature with low drive voltage (less than 2 V) and zero DC power consumption. The device architecture has potential for realization of the next generation of on-chip modulators and switches at THz frequencies.

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