In Praise of Resonators

Resonators, as the name suggests, are devices that display the property of resonance. They oscillate at their natural frequencies at which the amplitudes are much larger. This means that they can be used to either generate waves that have a certain frequency or to select desired frequencies from a given signal. Therefore, they can act as actuators and/or as sensors!

If this definition sounds too abstract, it might be helpful to note that resonators are all around you and can be found in many familiar objects. For example, musical instruments are essentially resonators that are acoustic in nature. Wooden bars of a xylophone, strings, and pipes are all resonators that produce soothing tunes. Resonators are also present in exhaust pipes of automobiles. Here, they work with mufflers and their role is the exact opposite, i.e., to lower any sound or noise! In electrical circuits, mechanical resonators are often used to produce signals of a precise frequency.

Finally, these devices are also seen in practically every gadget these days. For example, gyroscopes and accelerometers contain resonators and are used to detect rotation in several smartphones and entertainment units. Yet, very few people are aware of their presence or the significant role played by them. Unfortunately, while iPads bask in their glory, and GPS or satnavs enjoy the limelight, these modest yet powerful devices are not given the same attention.

Even as resonators continue to improve your Wii and iPad experience, they are also used constantly in other Microelectromechanical systems (MEMS) devices and applications. For example, they can be found in atomic force microscopes (AFMs) in the form of cantilevers (which happens to be the most commonly used detection methodology). The cantilever is driven at a frequency (close to resonance), and the corresponding variations in the amplitude or phase of the cantilever vibration are detected that indicate the force gradients and hence the topology of the surface one is trying to probe.

Resonators are also found in evaporation sources (used to deposit metallic films) within clean rooms where they monitor the rate of deposition. Once the evaporated material deposits on the surface of the crystal oscillator, the resonant frequency is modified and this change is detected by the corresponding circuit. This principle also illustrates the use of a resonator in applications related to sensing.

A quick note on sensors.

Sensors are devices that are typically frequency-modulated, that is, they undergo a change in the output frequency which is related to the physical variable that one is trying to measure.

One obviously desires a precise measurement from these sensors. In order to obtain such a measurement, the frequency stability of the sensor’s output should be high. This depends on the damping in the resonator which is described by the quality factor. Therefore, frequency and quality factor are two important parameters when describing a resonator which is to be used in a sensor.

As sensors, resonators currently have various applications. For example, they are used for high sensitivity detection of bacteria, measurement of the stoichiometry of surface compounds, evaluation of hydrogen storage capacity, monitoring of air pollution, laser cooling, nanotechnology, and gravitational wave detection.

Impressive breakthroughs have been made in terms of the resolution achieved by these resonators. For example, microfabricated silicon resonators at 4.2 K have managed to demonstrate a mass resolution of ~7 zg (10^-21g)!

Design requirements

Regardless of the application in which beam or plate resonators are being used and which can range anywhere from sensing (physical, biological and chemical signals) and communications (timers, frequency references, filters) to energy harvesting (conversion of ambient mechanical movement into portable electrical power) and fundamental studies of quantum mechanical systems, three fundamental requirements dominate their design.

Firstly, high values of natural frequency of vibration (10kHz – 1GHz) are desired. This is because high frequencies can assist sensing at high rates as well as match the frequencies of the signals of interest. Secondly, the damping factor must be very low, between 10^-6 and 10^-4 (i.e., the quality factor, Q, should be high). This will ensure the sensitivity is improved and the frequency selectivity (and hence, the performance) is enhanced. Thirdly, the stability of vibrations must be ensured and there should be minimum drift in the frequency of operation due to nonlinear effects and environmental coupling.

If these three requirements are met, the possibilities are endless. Progress in this field will ensure that micro and nano resonators from silicon, metals, graphene, nanotubes, and other materials will serve several functions and open the door to new sets of devices and applications. Ultrasensitive resonators are already assisting the biological sciences and helping them achieve single particle resolution which is invaluable for detecting and/or counting biological nanoparticles, and individual viruses. Resonators are also important for environmental sciences because they can be used to monitor chemical changes within air or water. It is expected that sensitive nanomechanical resonators will continue to improve probe microscopy and metrology. ‘Smart’ clothes, shoes, cosmetics and other wearable devices will become common. Biosensors particularly will benefit immensely.

It is thus amazing to see the countless applications that employ these resonators. Their significant role in sensing, detection, and timekeeping truly makes them the unsung heroes in the world of MEMS.

References

• P. Meystre, “Cool Vibrations,” Science, vol. 333, pp. 832-833, 2011.
• B. Lassagne et al., “Ultrasensitive Mass Sensing with a Nanotube Electromechanical Resonator,” Nano Letters, vol. 8, pp. 3735-3738, 2008.
• L. He et al., “Detecting single viruses and nanoparticles using whispering gallery microlasers,” Nat Nano, vol. 6, pp. 428-432, 2011.
• K. Y. Yasumura et al., “Quality factors in micron- and submicron-thick cantilevers,” Microelectromechanical Systems, Journal of, vol. 9, pp. 117-125, 2000.