PhD Defence by Tolga Bagci – Niels Bohr Institute - University of Copenhagen

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PhD Defence by Tolga Bagci

Opto-Electromechanical Devices for Low-Noise Detection of Radio Waves

There is currently an increasing interest in developing hybrid devices that unite the desirable features of different systems. Opto-electromechanics has emerged as one of these promising hybrid fields, where the functionality of conventional electrical circuits can be combined with the salient features of optical systems for various technological and sensing applications. Nanomechanical resonators stand as promising candidates in terms of linking the two systems, primarily thanks to their versatility in coupling to various physical systems, together with their excellent mechanical quality factors. For example, a hybrid system like this, would enable the use of well-established shot-noise limited optical sensing technologies for detecting weak radio-frequency (rf) signals, rf-to-optical photon conversion and transmission of information in low-loss fiber-optic links over long distances.

Driven mainly by potential sensing applications, we started an experimental project with the goal of realizing a hybrid opto-electromechanical device operating at room temperature. The device consists of an LC electrical circuit coupled to a metal-coated high-quality nanomechanical membrane, the vibrations of which are monitored as phase fluctuations via optical interferometry. At the first stage of the experiment, we have tested several bare, metal (aluminum)-coated and graphene-coated SiN (silicon nitride) membranes in terms of their capacitive interaction strength. Our findings support the expectation that metal and graphene coated membranes show similar performance that is significantly better than bare SiN membranes and  single layer graphene does not alter the mechanical quality and mass. Later on, we have incorporated an inductor to the system in order to achieve coupling between an aluminum coated membrane and an LC circuit (at ≈ 0.7 MHz). We have characterized the electromechanical coupling by both optical and electrical means, along with the observation of mechanically induced transparency and normal mode splitting due to strong coupling. Finally, we have analyzed the noise performance of our device for optical detection of radio waves. We demonstrate an actual Johnson noise-limited voltage sensitivity of ≈ 800 pV/√Hz and beyond that, we infer a sensitivity of 60 pV/√Hz both for the thermal noise of the membrane and shot noise (quantum) of the optical readout, at  the optimal electromechanical cooperativity  Cem=150 . Our findings are supplemented by additional Y-factor noise temperature measurements. This performance competes with the current state of the art operational amplifiers at room temperature and our device's performance can be improved with further advances. For a specific set of  parameters, we have achieved Cem =6800 meaning that the membrane noise can be suppressed down to Tm /Cem=40 mK. We believe our device will be of interest in sensing applications (NMR, radio astronomy etc.) where it is coupled to a cold signal input and the Johnson noise is strongly suppressed