R. Flaminio

A Xylophone Configuration for a third Generation Gravitational Wave Detector

Achieving the demanding sensitivity and bandwidth, envisaged for third-generation gravitational wave (GW) observatories, is extremely challenging with a single broadband interferometer. Very high optical powers (megawatts) are required to reduce the quantum noise contribution at high frequencies, while the interferometer mirrors have to be cooled to cryogenic temperatures in order to reduce thermal noise sources at low frequencies. To resolve this potential conflict of cryogenic test masses with high thermal load, we present a conceptual design for a 2-band xylophone configuration for a third-generation GW observatory, composed of a high-power, high-frequency interferometer and a cryogenic low-power, low-frequency instrument. Featuring inspiral ranges of 3200 Mpc and 38 000 Mpc for binary neutron stars and binary black holes coalesences, respectively, we find that the potential sensitivity of xylophone configurations can be significantly wider and better than what is possible in a single broadband interferometer.

A study of coating mechanical and optical losses in view of reducing mirror thermal noise in gravitational wave detectors

Mirror coatings play a crucial role in the performance of laser interferometers devoted to gravitational wave detection such as Virgo and LIGO. Mechanical losses in the coating material limit the sensitivity of the detectors due to the associated mirror thermal noise. The absorption of light in the coating induces a thermal lens in the mirror substrate which reduces the quality of the optical interference and requires sophisticated thermal compensation systems. This paper describes the work ongoing at LMA in order to reduce mechanical losses and optical absorption in the coating. The results obtained by doping Ta2O5 layers and testing different high-index materials are described. Finally the performances of different potential coatings are compared and the results obtained with a 40 kg mirror are reported. Titania doped Ta2O5 shows mechanical losses of 2 × 10−4 and absorption below 0.5 ppm. Nb2O5 appears to be the best competitor from the thermal noise point of view but it has an optical absorption four to five times larger.

Non-planar four-mirror optical cavity for high intensity gamma ray flux production by pulsed laser beam Compton scattering off GeV-electrons

As part of the R&D toward the production of high flux of polarised Gamma-rays we have designed and built a non-planar four-mirror optical cavity with a high finesse and operated it at a particle accelerator. We report on the main challenges of such cavity, such as the design of a suitable laser based on fiber technology, the mechanical difficulties of having a high tunability and a high mechanical stability in an accelerator environment and the active stabilization of such cavity by implementing a double feedback loop in a FPGA.

The Virgo Project

The Virgo project is a Italian-French collaboration aiming at the construction of a long baseline interferometric antenna for the detection of gravitational radiation signals of cosmic origin. We describe the principles of the system, and high-light the technical challenges we need to overcome for reaching a sensitiity as low as 10−23Hz−1/2.The gravitational clustering of collisionless particles in an expanding universe is modelled using some simple physical ideas. I show that it is possible to understand the nonlinear clustering in terms of three well defined regimes: (1) linear regime; (2) quasilinear regime which is dominated by scale-invariant radial infall and (3) nonlinear regime dominated by nonradial motions and mergers. Modelling each of these regimes separately I show how the nonlinear two point correlation function can be related to the linear correlation function in hierarchical models. This analysis leads to results which are in good agreement with numerical simulations thereby providing an explanation for numerical results. Using this model and some simple extensions, it is possible to understand the transfer of power from large to small scales and the behaviour of higher order correlation functions. The ideas presented here will also serve as a powerful analytical tool to investigate nonlinear clustering in different models.

Observation of three-mode parametric instability

Three-mode parametric interactions occur in triply resonant optomechanical systems: Photons from an optical pump mode are coherently scattered to a high-order mode by mechanical motion of the cavity mirrors, and these modes resonantly interact via radiation pressure force when some conditions are met. They can either pump energy into acoustic modes, leading to parametric instability, or extract mechanical energy, leading to optomechanical cooling. Such effects are predicted to occur in long baseline advanced gravitational-wave detectors, possibly jeopardizing their stable operation. We have demonstrated both three-mode cooling and amplification in two different three-mode optomechanical systems. We report an observation of the three-mode parametric instability in a free-space Fabry-Perot cavity, with ring-up amplitude saturation.

Gravitational wave astronomy: the current status

In the centenary year of Einstein’s General Theory of Relativity, this paper reviews the current status of gravitational wave astronomy across a spectrum which stretches from attohertz to kilohertz frequencies. Sect. 1 of this paper reviews the historical development of gravitational wave astronomy from Einstein’s first prediction to our current understanding the spectrum. It is shown that detection of signals in the audio frequency spectrum can be expected very soon, and that a north-south pair of next generation detectors would provide large scientific benefits. Sect. 2 reviews the theory of gravitational waves and the principles of detection using laser interferometry. The state of the art Advanced LIGO detectors are then described. These detectors have a high chance of detecting the first events in the near future. Sect. 3 reviews the KAGRA detector currently under development in Japan, which will be the first laser interferometer detector to use cryogenic test masses. Sect. 4 of this paper reviews gravitational wave detection in the nanohertz frequency band using the technique of pulsar timing. Sect. 5 reviews the status of gravitational wave detection in the attohertz frequency band, detectable in the polarisation of the cosmic microwave background, and discusses the prospects for detection of primordial waves from the big bang. The techniques described in sects. 1–5 have already placed significant limits on the strength of gravitational wave sources. Sects. 6 and 7 review ambitious plans for future space based gravitational wave detectors in the millihertz frequency band. Sect. 6 presents a roadmap for development of space based gravitational wave detectors by China while sect. 7 discusses a key enabling technology for space interferometry known as time delay interferometry.

Cryogenic measurements of mechanical loss of high-reflectivity coating and estimation of thermal noise

We report on low-frequency measurements of the mechanical loss of a high-quality (transmissivity T<5 ppm at λ(0)=1064 nm, absorption loss <0.5 ppm) multilayer dielectric coating of ion-beam-sputtered fused silica and titanium-doped tantala in the 10-300 K temperature range. A useful parameter for the computation of coating thermal noise on different substrates is derived as a function of temperature and frequency.

A micropillar for cavity optomechanics

We have designed a micromechanical resonator suitable for cavity optomechanics. We have used a micropillar geometry to obtain a high-frequency mechanical resonance with a low effective mass and a very high quality factor. We have coated a 60-μm diameter low-loss dielectric mirror on top of the pillar and are planning to use this micromirror as part of a high-finesse Fabry-Perot cavity to laser cool the resonator down to its quantum ground state and to monitor its quantum position fluctuations by quantum-limited optical interferometry.

Mirrors used in the LIGO interferometers for first detection of gravitational waves

For the first time, direct detection of gravitational waves occurred in the Laser Interferometer Gravitational-wave Observatory (LIGO) interferometers. These advanced detectors require large fused silica mirrors with optical and mechanical properties and have never been reached until now. This paper details the main achievements of these ion beam sputtering coatings.

Toward a new generation of low-loss mirrors for the advanced gravitational waves interferometers

The new generation of advanced interferometer needs fused silica mirrors having better optical and mechanical properties. This Letter describes the way to reduce the ion beam sputtering coating absorption at 1064 nm and to improve the layer thickness uniformity in order to coat two large mirrors (diameter 35 cm) at the same time.