Optical detection of radio waves through a nanomechanical transducer

Low-loss transmission and sensitive recovery of weak radio-frequency (rf) and microwave signals is an ubiquitous technological challenge, crucial in fields as diverse as radio astronomy, medical imaging, navigation and communication, including those of quantum states. Efficient upconversion of rf-signals to an optical carrier would allow transmitting them via optical fibers instead of copper wires dramatically reducing losses, and give access to the mature toolbox of quantum optical techniques, routinely enabling quantum-limited signal detection. Research in the field of cavity optomechanics [1, 2] has shown that nanomechanical oscillators can couple very strongly to either microwave [3–5] or optical fields [6, 7]. An oscillator accommodating both these functionalities would bear great promise as the intermediate platform in a radio-to-optical transduction cascade. Here, we demonstrate such an opto-electro-mechanical transducer following a recent proposal [8] utilizing a high-Q nanomembrane. A moderate voltage bias (Vdc < 10V) is sufficient to induce strong coupling [4, 6, 7] between the voltage fluctuations in a radio-frequency resonance circuit and the membrane’s displacement,
which is simultaneously coupled to light reflected off its metallized surface. The circuit acts as an antenna; the voltage signals it induces are detected as an optical phase shift with quantum-limited sensitivity. The corresponding half-wave voltage is in the microvolt range, orders of magnitude below that of standard optical modulators. The noise added by the mechanical interface is suppressed by the electro-mechanical cooperativity Cem 6800 and has a temperature of TN = Tm/Cem 40mK, where Tm is the room temperature at which the entire device is operated. This corresponds to a sensitivity limit as low as 5 pV/pHz, or −210dBm/Hz in a narrow frequency band around 1MHz. Our work introduces an entirely new approach to all-optical, ultralow-noise detection of classical electronic signals, and sets the stage for coherent upconversion of low-frequency quantum signals to the optical domain [8–12].