M. Bonaldi

Feedback Cooling of the Normal Modes of a Massive Electromechanical System to Submillikelvin Temperature

We apply a feedback cooling technique to simultaneously cool the three electromechanical normal modes of the ton-scale resonant-bar gravitational wave detector AURIGA. The measuring system is based on a dc superconducting quantum interference device (SQUID) amplifier, and the feedback cooling is applied electronically to the input circuit of the SQUID. Starting from a bath temperature of 4.2 K, we achieve a minimum temperature of 0.17 mK for the coolest normal mode. The same technique, implemented in a dedicated experiment at subkelvin bath temperature and with a quantum limited SQUID, could allow to approach the quantum ground state of a kilogram-scale mechanical resonator.

3-Mode Detection for Widening the Bandwidth of Resonant Gravitational Wave Detectors

Along with peak sensitivity, an important parameter of a resonant gravitational wave detector is its bandwidth. In addition to the obvious advantage of making the detector more sensitive to short bursts, a wider bandwidth would allow, for instance, details of the signal emitted during a supernova gravitational collapse or the merger of compact binaries to be resolved [1]. Moreover, a wider bandwidth reduces the uncertainty in the burst arrival time [2] and consequently, with a detector network, permits a more precise source location and a higher efficiency of spurious events rejection [3]. The introduction of a mechanically resonant transducer, a standard practice in actual resonant detectors, has greatly improved the coupling between the bar and the amplifier, but the bandwidth is intrinsically limited [4], and in practice, according to the full width at half maximum (FWHM) definition applied to the two minima of the Shh strain noise spectra, values of a few Hz have been achieved [5]. The use of multimode resonant transducers should permit further improvements of the detector bandwidth [6]. This approach has been studied [7] in depth and a few 2-mode transducer prototypes have been realized [8] or are under development [9] to obtain 3mode operation of the resonant mass detectors. This Letter describes how a wider detection bandwidth can be obtained with an alternative 2-mode transduction system in which the resonant amplification is realized by means of a resonant mechanical mode plus a resonant electrical matching network. It also describes the key tests performed on the components of the transduction system in order to verify the achievement of the requirements set by analysis of the detector model. Figure 1 shows the electromechanical scheme of a cryogenic detector with a resonant capacitive transducer read by a SQUID amplifier. The matching transformer couples the output impedance of the transducer (a capacitance of a few nF) to the input impedance of the SQUID (a small

Random telegraph signals and low-frequency voltage noise in Y-Ba-Cu-O thin films

Excess low‐frequency noise extending to MHz frequencies was observed in dc current biased granular high‐Tc thin films. At particular bias conditions random telegraph signal produced by a single, fast two‐level fluctuator dominated the noise properties of the sample. Lifetimes of the low‐ and high‐voltage states of the fluctuating system were found to be exponentially distributed. Power spectra of the excess noise signal could be well fitted with a single Lorentzian contribution. Duty cycle dependence of the random telegraph signal on bias conditions was used to get an insight into physical mechanism causing the fluctuations. Charge trapping events in the intergranular intrinsic Josephson junctions and trapped flux hopping were identified as possible alternative sources of the observed noise.

Probing deformed commutators with macroscopic harmonic oscillators.

A minimal observable length is a common feature of theories that aim to merge quantum physics and gravity. Quantum mechanically, this concept is associated with a nonzero minimal uncertainty in position measurements, which is encoded in deformed commutation relations. In spite of increasing theoretical interest, the subject suffers from the complete lack of dedicated experiments and bounds to the deformation parameters have just been extrapolated from indirect measurements. As recently proposed, low-energy mechanical oscillators could allow to reveal the effect of a modified commutator. Here we analyze the free evolution of high-quality factor micro- and nano-oscillators, spanning a wide range of masses around the Planck mass mP (≈22 μg). The direct check against a model of deformed dynamics substantially lowers the previous limits on the parameters quantifying the commutator deformation.

Squeezing a Thermal Mechanical Oscillator by Stabilized Parametric Effect on the Optical Spring

We report the confinement of an optomechanical micro-oscillator in a squeezed thermal state, obtained by parametric modulation of the optical spring. We propose and implement an experimental scheme based on parametric feedback control of the oscillator, which stabilizes the amplified quadrature while leaving the orthogonal one unaffected. This technique allows us to surpass the -3  dB limit in the noise reduction, associated with parametric resonance, with a best experimental result of -7.4  dB. While the present experiment is in the classical regime, in a moderately cooled system our technique may allow squeezing of a macroscopic mechanical oscillator below the zero-point motion.

Multi-modal vibration based MEMS energy harvesters for ultra-low power wireless functional nodes

The aim of this contribution is to report and discuss a preliminary study and rough optimization of a novel concept of MEMS device for vibration energy harvesting, based on a multi-modal dynamic behavior. The circular-shaped device features Four-Leaf Clover-like (FLC) double spring-mass cascaded systems, kept constrained to the surrounding frame by means of four straight beams. The combination of flexural bending behavior of the slender beams plus deformable parts of the petals enable to populate the desired vibration frequency range with a number of resonant modes, and improve the energy conversion capability of the micro-transducer. The harvester device, conceived for piezoelectric mechanical into electric energy conversion, is intended to sense environmental vibrations and, thereby, its geometry is optimized to have a large concentration of resonant modes in a frequency range below 5-10 kHz. The results of FEM (Finite Element Method) based analysis performed in ANSYSTM Workbench are reported, both concerning modal and harmonic response, providing important indications related to the device geometry optimization. The analysis reported in this work is limited to the sole mechanical modeling of the proposed MEMS harvester device concept. Future developments of the study will encompass the inclusion of piezoelectric conversion in the FEM simulations, in order to have indications of the actual power levels achievable with the proposed harvester concept. Furthermore, the results of the FEM studies here discussed, will be validated against experimental data, as soon as the MEMS resonator specimens, currently under fabrication, are ready for testing.

Wideband mechanical response of a high-Q silicon double-paddle oscillator

We present experimental results of operation of a silicon double-paddle oscillator, namely the mechanical transfer function of the system and the quality factor of its resonant modes. We also describe the fabrication process, where efforts have been devoted to ensure strict dimensional tolerances for a proper functioning of the oscillator, and the setup used to drive the oscillation of the device and to detect its displacement with a sensitivity better that 10−12 m Hz−1/2 in the range 0.1–5 kHz. We observed mechanical quality factors higher than 105 in vacuum at room temperature for two specific resonant modes, while for all other modes the quality factor remains in the range 103–104. These values are in good agreement with the limit set by the thermoelastic dissipation in the device, evaluated by a finite element procedure, and demonstrate that our setup allows us to control losses coming from clamping and residual gas effects.

Status report and near future prospects for the gravitational wave detector AURIGA

We describe the experimental efforts to set up the second AURIGA run. Thanks to the upgraded capacitive readout, fully characterized and optimized in a dedicated facility, we predict an improvement in the detector sensitivity and bandwidth by at least one order of magnitude. In the second run, AURIGA will also benefit from newly designed cryogenic mechanical suspensions and the upgraded data acquisition and data analysis.

Selective readout and back-action reduction for wideband acoustic gravitational wave detectors

We present the concept of the ‘‘selective readout’’ for the recently proposed dual detector @M. Cerdonio et al., Phys. Rev. Lett. 87, 031101 ~2001!# of gravitational waves which is a sensitive and broadband resonantmass detector. The advantage of the proposed detection scheme is that it is made sensitive only to the acoustic modes of the masses forming the detector that have quadrupolar symmetry, and thus carry the signal, while it rejects efficiently the noise contribution from the other modes, either of thermal or back-action origin. The total effect is that the sensitivity of a dual detector equipped with the selective readout is flat within a wide frequency range and can be as good as ;8310 224 /AHz between 1.3 and 4 kHz for a silicon carbide detector, 3 m in diameter.

Ultralow-dissipation micro-oscillator for quantum optomechanics

Generating nonclassical states of light by optomechanical coupling depends critically on the mechanical and optical properties of micro-oscillators and on the minimization of thermal noise. We present an oscillating micromirror with a mechanical quality factor