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Sensors that produce a measurable response to the applied mechanical deformation are useful in many applications including personal and structural health monitoring. One of the ways to make these strain sensors by fabricating small string-like vibrating structures called nanoresonators.  These tiny (sub 10um) structures, made of materials such as graphene, vibrate at very high frequencies (10-100MHz) and are exquisitely sensitive to changes in the strain. However, introducing the strain stimuli into these sensors implies that the devices have to fabricated on flexible substrates such as PDMS and PET. The fabrication process on these substrates is complex and device yields are small.

Swapnil More has developed a simple method to fabricate these devices on a thin diaphragm on silicon substrates. He then utilized air pressure to deform the substrate on which NEMS are fabricated. This controlled deformation of the substrate induces strain in the nano resonator and changes the resonant frequency. The magnitude of this strain change can be deduced using the frequency tuning of the nano resonator. We estimate that strain changes as low as 10^-6 can be measured using these devices. This method can be used with a wide variety of nanomechanical systems. Besides their utility as sensors, strain tuneable nanoresonators are interesting tools to study some of the most intriguing dynamical phenomena such as synchronization, mode coupling, internal resonance.

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In a recently published research in Nano Letters, we study coupling mechanism between two modes of a nanoscale resonators. Strong coupling between nanoresonators have previously been used to demonstrate novel effects such as cooling and electromagnetically induced transparency. The nanoscale resonator in this work is drum shaped resonator made of few atomic layers thick membrane and the modes are accommodated in this single nano drum resonator. The two different mechanical vibration modes have frequencies in 100 MHz range, the frequency range where FM radio works. The modes interact with each other via tension in the membrane which can be controlled electrically. In our experiment, the coupling is manipulated to enhance or reduce the energy exchange between the vibrational modes. The coupling can be increased to such an extent that the energy between the modes is exchanged more than 500 times back and forth before the information is lost to the environment. This is more than an order of magnitude improvement of in coupling compared to previous demonstrations. This demonstration of strong tunable coupling between high-frequency vibrational modes at nanoscale could lead to improvements in sensitivity of nano-mechanical sensors and has major implications for mechanical logic circuits and quantum limited measurement.

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Congratulations Parmeshwar and Nishta!

Thermal tuning is an easily implementable, low-cost method often employed in electronic and mechanical devices to extract optimal performance or to match the operating points of multiple devices. Similarly, in photonic integrated circuits (PICs), thermal tuners are essential for matching the operating wavelengths of multiple photonic devices such as resonators, gratings, and filters. They are also used for low frequency optical modulation and optical memory applications. The working principle is thermally induced change in refractive index of the materials interacting with guided light.

Conventional thermo-optic tuners are implemented using metal heaters. However, metals absorb near-IR wavelengths, where most of the applications of PICs are, and thus have to be placed a few microns away from the devices to be tuned to avoid degradation in performance of the optical devices. The heating, therefore, is not localized and leads to thermal cross-talk among multiple optical devices. This prevents dense integration of integrated optics. Moreover, these heaters have low power efficiency and a large thermal transient. It is desirable to have a heater closer to the device, yet have it be non-absorbing.

Continue reading “Carbon-nanotube-on-waveguide thermo-optic tuners”

Chandan and Nishta’s work on tuning of nonlinearities using electrostatic gate voltage has now been accepted in Applied Physics Letters.

The work demonstrates the ability to control and manipulate the nonlinearities in ultra-thin resonators. This control over the nonlinearities is used to cancel out the strongest two nonlinearities in the device. This can not only be used to improve the linear dynamic range for NEMS based sensors but can also be used to probe higher order nonlinearities that are typically masked by the quadratic and cubic nonlinearities.

In this work, we observe higher order stiffening nonlinearities as well as some hints of nonlinear damping. Previous reports of nonlinear damping in 2D materials required cooling down to sub 100mK temperatures and quality factor of close to a million. The fact that we are able to see these weak effects at room temperature with modest quality factors of about 100demonstrates the usefulness of results shown in this manuscript.

More in this paper.