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Ultrastrong optomechanical coupling for quantum optomechanics experiments and novel radiation-pressure devices

Final Report Summary - USOM (Ultrastrong optomechanical coupling for quantum optomechanics experiments and novel radiation-pressure devices)

This project aimed to develop novel systems that exhibit ultrastrong coupling between light and mechanical motion through radiation pressure, and use these to gain control over the motion of macroscopic mechanical oscillators at the quantum level. In suitably engineered optical microcavities, optical cavity modes can be coupled strongly to the motion of acoustic modes of the system. Such optomechanical coupling allows sideband cooling of the mechanical oscillator mode using radiation pressure, and potentially enables control over the quantum state of motion of the mechanical oscillator with light. Manipulating and reading out the quantum motion of macroscopic objects is one of the most prominent goals in modern quantum physics. In the context of this project, miniaturised silica whispering gallery mode cavities were developed that allow microsecond photon storage times in combination with efficient optomechanical coupling to approximately 100 MHz acoustic modes. By employing a spokes-supported design, the oscillator mass could be minimised and the radiation pressure force per photon maximised, while effectively decoupling the oscillator from its environment. The systems were deployed in a He-3 cryogenic environment at approximately 0.6 K. Due to the large optomechanical coupling strength and small damping, resolved sideband laser cooling of the mechanical oscillator modes could allow cooling to mode temperatures of several mK, reducing the average number of thermal phonons in the oscillators below 2. As described by the fellow in a paper published in February 2012 (Verhagen et al., Nature 482, 63, 2012), the rate at which the optical and mechanical degrees of freedom are coupled exceeds both the optical and the mechanical decoherence rate, thus enabling control of the mechanical oscillator's quantum state of motion.

The optical and mechanical properties of optomechanical systems are crucial to their potential application as a realistic quantum technology. In particular, reduction of system size can optimise the optomechanical coupling strength. From detailed finite-element calculations, design rules for spokes-supported whispering gallery mode resonators were established that allowed the development of optimally small silica microtoroidal optomechanical systems demonstration of silica microtoroid optomechanical systems with a record-high vacuum optomechanical coupling rate of 6.5 kHz. These coupling rates were obtained in about 20 µm diameter microtoroids, which are close to the lower size limit given by the onset of bending losses in smaller cavities. A novel 'z-shaped' spokes design was used to allow sufficient isolation in these small samples, while respecting simple symmetry rules to avoid coupling to lossy torsional modes. Clamping losses were found to be indeed negligible, while an optical cavity decay rate of 2 MHz could still be realised. In combination with the mechanical resonance frequency of 100 MHz, this system therefore possesses a sideband factor (mechanical frequency / optical decay rate) - crucial for ground state cooling - that is unsurpassed at optical wavelengths.

After demonstrating near-ground-state cooling and quantum-coherent coupling, the improved optomechanical systems were combined with crucial experimental upgrades to enable the next generation of state-of-the-art quantum optomechanical experiments. The experimental advances included first the real-time spectral analysis of radio frequency (RF) signals for fast readout of mechanical motion. Second, a novel method was developed to achieve frequency locking of the laser to arbitrarily large detunings. This method, which is based on the subtraction of two PDH signals obtained with different modulation frequencies, does not require a modulation signal at frequency equal to the laser detuning, and can as such facilitate locking of the laser to the lower mechanical sideband of the cavity without driving the mechanical mode. Third, a new low-loss homodyne detection setup was implemented to allow readout of the mechanical motion with ultimate sensitivity. This setup includes advanced possibilities to modulate the cooling / control laser, either to generate microsecond pulses or a second beam at a frequency difference of twice the mechanical frequency, as well as the ability to vary the detected quadrature to enable state tomography. These techniques allow on the one hand faithful and ready cooling of optomechanical systems to the quantum regime and on the other hand, new ways to dynamically control and readout the quantum state of motion of the oscillators. This new generation of optomechanics experiments, making extensive use of the methods and systems developed in this project, are currently carried out at EPFL.

Additionally, in the context of this project an entirely novel class of optomechanical systems was explored based on plasmonic resonances. Surface plasmon polaritons can be used to confine light waves to deeply subwavelength volumes by using structured metallic surfaces. As such, a giant optomechanical coupling strength of several THz/nm can be realised if a plasmonic resonator is integrated with a nanomechanical oscillator. This can be used to realise efficient broadband plasmonic transducers of nanomechanical motion, even though the surface plasmon lifetime is small. In collaboration with the FOM Institute AMOLF, systems were developed that confine a resonant plasmonic mode in the nanoscale slit between two gold-coated silicon nitride nanobeams. In a conceptually simple light transmission experiment, this system facilitated the readout of the oscillator's thermal motion, with a spectral density of 10E-26 m2/Hz. This pioneering experiment paves the way for a new class of nano-optomechanical systems that could allow broadband, multiplexed readout and actuation of arrays of nanomechanical oscillators, of particular interest for sensing and signal processing.