In an optomechanical cavity the optical and mechanical degree of freedom are strongly coupled by the radiation pressure of the light. This field of research has been gathering a lot of momentum during the last couple of years, driven by the technological advances in microfabrication and the first observation of quantum phenomena. These results open new perspectives in a wide range of applications, including high sensitivity measurements of position, acceleration, force, mass, and for fundamental research. We are working on low frequency pondero-motive light squeezing as a tool for improving the sensitivity of audio frequency measuring devices such as magnetic resonance force microscopes and gravitational-wave detectors. It is well known that experiments aiming to produce and manipulate non-classical (squeezed) light by effect of optomechanical interaction need a mechanical oscillator with low optical and mechanical losses. These technological requirements permit to maximize the force per incoming photon exerted by the cavity field on the mechanical element and to improve the element’s response to the radiation pressure force and, at the same time, to decrease the influence of the thermal bath. In this contribution we describe a class of mechanical devices for which we measured a mechanical quality factor up to 1.2 × 106 and with which it was possible to build a Fabry-Perot cavity with optical finesse up to 9 × 104. From our estimations, these characteristics meet the requirements for the generation of radiation squeezing and quantum correlations in the ∼ 100kHz region. Moreover our devices are characterized by high reproducibility to allow inclusion in integrated systems. We show the results of the characterization realized with a Michelson interferometer down to 4.2K and measurements in optical cavities performed at cryogenic temperature with input optical powers up to a few mW. We also report on the dynamical stability and the thermal response of the system.
The interaction of the radiation pressure with micro-mechanical oscillators is earning a growing interest for its
wide-range applications (including high sensitivity measurements of force and position) and for fundamental
research (entanglement, ponderomotive squeezing, quantum non-demolition measurements). In this contribution
we describe the fabrication of a family of opto-mechanical devices specifically designed to ease the detection of
ponderomotive squeezing and of entanglement between macroscopic objects and light. These phenomena are not
easily observed, due to the overwhelming effects of classical noise sources of thermal origin with respect to the
weak quantum fluctuations of the radiation pressure. Therefore, a low thermal noise background is required,
together with a weak interaction between the micro-mirror and this background (i.e. high mechanical quality
factors). The device should also be capable to manage a relatively large amount of dissipated power at cryogenic
temperatures, as the use of a laser with power up to a ten of mW can be useful to enhance radiation pressure
effects. In the development of our opto-mechanical devices, we are exploring an approach focused on relatively
thick silicon oscillators with high reflectivity coating. The relatively high mass is compensated by the capability
to manage high power at low temperatures, owing to a favourable geometric factor (thicker connectors) and
the excellent thermal conductivity of silicon crystals at cryogenic temperature. We have measured at cryogenic
temperatures mechanical quality factors up to 105 in a micro-oscillator designed to reduce as much as possible
the strain in the coating layer and the consequent energy dissipation. This design improves an approach applied
in micro-mirror and micro-cantilevers, where the coated surface is reduced as much as possible to improve the
quality factor. The deposition of the highly reflective coating layer has been carefully integrated in the micromachining
process to preserve its low optical losses: an optical finesse of F = 6×104 has been measured in a
Fabry-Perot cavity with the micro-resonator used as end mirror.
In a "Dual" gravitational wave (GW) detector a wide band sensitivity is obtained by measuring the differential displacement, driven by the GW, of the facing surfaces of two nested massive bodies mechanically
resonating at different frequencies. A "selective readout" scheme,
capable of specifically selecting the signal contributed by the vibrational modes sensitive to the gravitational waves, could then reduce the thermal noise contribution from the not sensitive modes. In a dual detector the sensitivity improvement in the displacement transduction could be pursued by means of mechanical amplification systems. This solution is innovative for the resonant GW detectors and we report about preliminary theoretical and experimental study.
A network of five cryogenic 'bar' gw detectors has been in operation in the years 1997-2000. A generic coincidence search has been performed with such a network, under the International Gravitational Events Collaboration, IGEC, for millisecond burst gw signals. A triple coincidence within the network has a false alarm rate below 10-2 y-1. No triple coincidence was found over the 173.2 days during which at least three detectors where on the air and improved upper limits have been established for the gw flux on earth. The typical search thresholds of the detectors correspond to a neutron star - neutron star coalescence at 10 kpc distance. The network is currently under upgrade and it is expected to usefully complement, in searches for ms gw signals, the interferometric detectors under completion.