Fabrication of SiN String Resonators and Their Applications in Probing Nanoplasmonic Heating

The number of applications of nanoplasmonic heating is increasing rapidly. Not too long ago, nanoplasmonic heating was considered to be a wasteful by-product that needed to be controlled, or better yet, reduced. The positive change in perception has brought about remarkable applications such as photothermal cancer therapy, photothermal imaging, and photothermal drug delivery.

This in turn has led to the emergence of the field of thermoplasmonics. The improvement of current applications of thermoplasmonics and the facilitation of new ones, demands the need for sensitive and reliable tools to probe nanoplasmonic heating at the nanoscale. However, most of the currently available techniques are either too invasive or suffer from drawbacks such as low resolution, lengthy calibration or the inability to probe structures with nanogaps.

The rapid advancement in nanofabrication techniques has led to development of micromechanical string resonators that have been shown to possess extraordinarily high quality factors, high temperature sensitivity and responsivity. Moreover, these microstructures can be fabricated using standard cleanroom manufacturing techniques.

Silicon nitride string resonators have already been used to probe nanoplasmonic heating in a single gold nanoslits and a single gold nanorod. The aim of this Ph.D. project was to explore the use of silicon nitride strings to probe more complex plasmonic systems. For the first time, silicon nitride string resonators have been used to probe nanoplasmonic heating in widely used metal nanoparticles such as nanospheres, nanostars, nanocubes and shell-isolated nanoparticles.

The nanoparticles were deposited on the strings followed by their subsequent probing using a 633 nm laser. A single nanoparticle and a dimer was probed for each of the four types. We compare the nanoplasmonic heating in the four sets of probed nanoparticles and observe that gold nanospheres generate the highest heating while gold nanostars generate the lowest.

A closer look at the results reveal that surface morphology and nanogap sizes influence nanoplasmonic heating strongly. Through the experiments, the strings exhibit an excellent sensitivity of ~2.4 Hz/μW.