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New Optomechanical Nanosensor Offers Fresh Possibilities for Signal Processing

The demand for nanosensors is large and growing, with not much sign of slowing, even though the world of nanosensors may be physically small. With electronic devices becoming smaller, their potential to provide accurate, chip-based sensing of dynamic physical properties such as motion becomes difficult to develop.

Schematic of torsional optomechanical resonator for sensing and frequency mixing. Credit Jianguo Huang

A literal twist on this challenge has been brought about by an international group of Researchers by demonstrating a new nanoscale optomechanical resonator capable of detecting torsional motion at near state-of-the-art sensitivity. Light is coupled into their resonator, which demonstrates torsional frequency mixing, an innovative ability to impact optical energies using mechanical motions. The Researchers have recently reported their work in the journal Applied Physics Letters, from AIP Publishing.

“With developments of nanotechnology, the ability to measure and control torsional motion at the nanoscale can provide a powerful tool to explore nature,” said Jianguo Huang from Xi’an Jiaotong University in China, one of the work’s authors. Jianguo Huang is also affiliated with the Nanyang Technological University and with the Institute of Microelectronics, A*STAR in Singapore.

We present a novel ‘beam-in-cavity’ design in which a torsional mechanical resonator is embedded into a racetrack optical cavity, to demonstrate nanoscale torsional motion sensing.

Jianguo Huang, One of the Authors, Xi’an Jiaotong University, China

Light is already being used in somewhat similar ways in order to detect the “breathing” or mechanical flexing of nanomaterials, usually requiring sensitive and complex coupling to the light source. This latest approach is novel not only in its detection of nanoscale torques, but also in its incorporated light-coupling design.

Huang and his team, used a silicon-based nanofabrication method, and designed the device in order to allow light to couple directly through an etched grating to a waveguide configuration, known as a racetrack cavity, in which the nanoresonator is made to sit.

“As light is coupled into the racetrack cavity through a grating coupler, mechanical torsional motion in the cavity alters the propagation of light and changes [the] power of output light,” said Huang. “By detecting the small variation of output light, the torsional motions can be measured.”

Besides detecting torques on their micron-length lever arms, the resonators are also capable of affecting the resulting optical properties of the incident signal. The torsional frequency of the mechanical system blends with the modulated optical signals.

The most surprising part is that when we modulate the input light, we can observe the frequency mixing. It is exciting for frequency mixing since it has only been demonstrated by flexural or breathing modes before. This is the first demonstration of torsional frequency mixing, which may have implications for on-chip RF signal modulation, such as super-heterodyne receivers using optical mechanical resonators.

Jianguo Huang, One of the Authors, Xi’an Jiaotong University, China

This is just the beginning for possible uses of torque-based nanosensors. Theoretically, these devices are capable of playing many frequency tricks for signal processing and sensing applications.  

We will continue to explore unique characters of this torsional optomechanical sensorand try to demonstrate novel phenomena, such as inference of dispersive and dissipative optomechanical coupling hidden behind the sensing. For engineering, magnetic or electrically-sensitive materials can be coated on the surface of torsional beams to sense small variations of physical fields, such as magnetic or electric fields to serve as multifunctional sensors.

Jianguo Huang, One of the Authors, Xi’an Jiaotong University, China

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