Detecting Radio Waves with a Laser by means of a Nanomechanical Transducer
Albert Schliesser and Eugene S. Polzik
Affiliation: Niels Bohr Institute, Copenhagen University, Denmark.
Radio- and microwave frequency (RF) electric signals are the workhorse for countless applications in Science and Technology: cell phones transmit voice and data through them, medical doctors use them in magnetic resonance imagers to learn about the composition of biological tissue, and radio astronomers draw conclusions on the origin and structure of our universe by scrutinizing the microwave radiation that permeates it. Often, it would be desirable to recover extremely small signals, from objects with a weaker contrast, or further away, than what electronic RF signal receivers permit today: if the signals are too small, they get drowned in the noise added by the receiver.
Recently, we have taken a completely new approach at detecting such signals (Figure 1, and Reference ), following the proposal . Instead of using an electronic amplifier, we have applied the RF signal of interest to a simple LC-resonance circuit. The latter contains a special capacitor: one of its electrodes is a 100nm-thin, metallized nanomechanical membrane that hovers only a few microns above the other electrode. It is consequently rather susceptible to the electrostatic forces that arise from an RF voltage dropped at the capacitor. Due to the quadratic dependence of this force on voltage, the mechanical response can be further enhanced by a dc voltages bias. In that manner, membrane deflections on the order of a micron can be achieved already with signal voltages below a millivolt at the input of the LC circuit, provided the signal frequency lies close to the ~1 MHz resonance frequency of the membrane, and the LC circuit tuned to it.
Figure 1: Artist’s impression of the electro-optomechanical transducer. Radio wave signals (green, on the left) are guided to an on-chip capacitor. Above the capacitor’s bottom gold electrodes, a nanomechanical membrane, some 100 nm thick, and coated with a thin layer of aluminum is suspended, at a distance of only a few micron. Together they form a capacitor, and the resulting electrostatic forces deflect the membrane if an RF signal is applied. This deflection is read out with very high sensitivity by a laser beam (red) reflected from the membrane.
Now we combine this electromechanical device with a laser interferometer: by reflecting a laser beam from the capacitor’s membrane, and measuring the phase shift imparted on the laser beam if the membrane is displaced, we can most accurately determine the position of the membrane. Indeed, this measurement is only limited by the quantum noise of the light, which allows us to determine the displacement of the membrane at the level of 1 femtometer after one second of averaging time—that is the size of an atomic nucleus. As a result, by looking at the laser interferometer’s signal, we measure the voltage applied to circuit, and the precision of the optical measurement allows us to do so with very high sensitivity.
However, the optical readout noise is not the only mechanism that can degrade the quality of signal reception. One of the attractive features of our “electrooptomechanical transducer” is that it operates at room temperature. We have carefully studied the dependence of the noise of the transducer on the frequency and impedance of the signal source. One particularly interesting result we have found is that the thermal noise of the membrane at room temperature — its random agitation caused essentially by the Brownian motion of the atoms that constitute it — can be made negligible in most cases. The effective temperature of the membrane is the room temperature reduced by a cooperativity parameter C , which quantifies the strength of the electromechanical coupling, and reached values of up to 6800 in our work. Furthermore, any noise intrinsic to the LC resonance circuit can limit the sensitivity. After careful shielding of the inductor (L) against magnetic pickup, its Johnson noise was the dominant source of noise. It left us a total transducer noise at the level of (800 pV)2/Hz. All other sources of noise — from the light and the membrane — added up to about a percent of that value. By using electronic component of LC with lower loss and/or temperature, this number could be further improved. This allows us to dramatically surpass the performance of conventional electronic amplifiers, which, for now, is at a similar level.
At present our transducer operates around the LC frequency of about 1 MHz equal to the mechanical resonance frequency of the membrane. Switching from the dc bias voltage to an ac bias voltage  should allow for tuning the device to any desirable frequency in the MHz to GHz range.
Looking at this work from a more general perspective, it combines, for the first time, the functionalities of electro- and optomechanical systems, both of which have separately experienced extremely rapid progress over the last decade . One of the most remarkable feats accomplished in these still young research communities is the ability to control the mechanical dynamics at the quantum level by means of electromagnetic fields, either microwave [4,5] or optical [6,7]. Merging these platforms into a single device may thus enable a quantum-coherent interface between microwave and optical photons — a tool urgently needed if superconducting quantum computers are to be connected through an optical fiber network. Efforts along these lines are now undertaken in many groups worldwide [8,9].
This work was done in collaboration with T. Bagci, A. Simonsen, E. Zeuthen, J. Appel, A. Sorensen and K. Usami at the Niels Bohr Institute, Copenhagen, Denmark; S. Schmid, and L. G. Villanueva at the Danish Technical University, Kongens Lyngby, Denmark; and J. M. Taylor at the Joint Quantum Institute/NIST, College Park, USA.
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