Measurements of Radiation Pressure on Diffractive Films

One of the few ways to reach distant stars is by radiation pressure, in which photon momentum is harnessed from free sunlight or extraordinarily powerful laser systems. Large but low mass light-driven sails reflect photons and transfer momentum to the sailcraft, providing large velocity from continuous acceleration. Over the past decade, demonstrative reflective light sail missions were enabled by cost-efficient small satellites and the emerging private space economy. The maneuver of these metal-coated polyimide films is mechanically cumbersome because the sail must be rapidly tilted towards and away from the sun line during navigation. Modern diffractive films such as high-efficiency single-order gratings, liquid crystal cycloidal diffractive wave-plates, and meta-material gratings may provide enhanced control schemes with radiation pressure tangential to the sail surface. The potential to replace motorized control components with all-optical components also offers a reduction in mass and the risk of mission failure. Before spending considerable resources sending a rocket to deploy a solar sail, it must be verified that the sail will behave as expected in a lab on Earth. This is challenging since Earth’s gravity, electro-static forces, thermal effects, and environment vibrations exceed the relatively weak effects of radiation pressure. In this dissertation, we designed and constructed an opto-mechanical torsional pendulum in a vacuum environment that measures radiation pressure on diffraction films with sub-nano-Newton precision. With the system, we observed a large component of force parallel to the surface of a diffraction grating owing to “grating momentum”. Furthermore, we proposed, designed, and validated Diffractive Beam-Rider structures that enable spatially varying forces to pull and align the sailcraft to the beam. We parametrically “cooled” the turbulence on the Beam-Rider, which demonstrates its potential for implementation on a laser sail. This experimental stability verification was performed on a centimeter-sized bi-grating and a diffractive axicon with one and two-dimensional restoring force, respectively

Measurements of Radiation Pressure Owing to the Grating Momentum

The radiation pressure force on a nearly single-order diffraction grating was measured for a transmission grating near the Littrow angles at wavelengths of 808 and 447 nm. The component of force parallel to the grating agreed well with our prediction, being proportional to the product of the grating order and the ratio of the wavelength and grating period. The normal component of force varied with the incident angle, vanishing near the Littrow angle as expected. The measurements verify a correspondence between the Fourier grating momentum and the mechanical momentum. This Letter provides opportunities for in-space fly-by-light sailcraft as well as terrestrial applications

Search for intermediate-mass black hole binaries in the third observing run of Advanced LIGO and Advanced Virgo

International audienceIntermediate-mass black holes (IMBHs) span the approximate mass range 100−105 M⊙, between black holes (BHs) that formed by stellar collapse and the supermassive BHs at the centers of galaxies. Mergers of IMBH binaries are the most energetic gravitational-wave sources accessible by the terrestrial detector network. Searches of the first two observing runs of Advanced LIGO and Advanced Virgo did not yield any significant IMBH binary signals. In the third observing run (O3), the increased network sensitivity enabled the detection of GW190521, a signal consistent with a binary merger of mass ∼150 M⊙ providing direct evidence of IMBH formation. Here, we report on a dedicated search of O3 data for further IMBH binary mergers, combining both modeled (matched filter) and model-independent search methods. We find some marginal candidates, but none are sufficiently significant to indicate detection of further IMBH mergers. We quantify the sensitivity of the individual search methods and of the combined search using a suite of IMBH binary signals obtained via numerical relativity, including the effects of spins misaligned with the binary orbital axis, and present the resulting upper limits on astrophysical merger rates. Our most stringent limit is for equal mass and aligned spin BH binary of total mass 200 M⊙ and effective aligned spin 0.8 at 0.056 Gpc−3 yr−1 (90% confidence), a factor of 3.5 more constraining than previous LIGO-Virgo limits. We also update the estimated rate of mergers similar to GW190521 to 0.08 Gpc−3 yr−1.Key words: gravitational waves / stars: black holes / black hole physicsCorresponding author: W. Del Pozzo, e-mail: [email protected]† Deceased, August 2020