General Relativity: Gravity of Photons

research highlights

OPTOMECHANICS reflectivity requires sufficiently thick OPTICAL ANTENNAS Reaching room temperature dielectric media, limiting the mechanical Reconfigurable resonance Phys. Rev. Lett. 116, 147202 (2016) characteristics. The team solved this problem Nano Lett. 16, 2680–2685 (2016) by incorporating hole-array photonic crystal ‘mirrors’ with a reflectivity of >99%, which Optical nanoantennas have potential are tethered by stressed SiN beams. The team applications in optomechanics, sensing and hopes that the concept will be applied to active plasmonics. However, a limitation silicon-based quantum networks operating at is that their resonance frequency of room temperature. DP operation is usually fixed by their size, shape and composition. Now, Kai Chen and SPECTROSCOPY co-workers in Germany have demonstrated Temporal flexibility reconfigurable nanoantennas operating Opt. Lett. 41, 1498–1501 (2016) in the visible spectral range. Plasmonic nanoantennas were made from a pair of Transient absorption spectroscopy is widely closely spaced, parallel single-crystal gold used to study short-lived substances. The wires by focused-ion beam milling. The usual pump–probe and continuous-wave antennas were suspended 300 nm above the approaches for studying the absorption bottom of a trench in a glass substrate. As

APS response of a sample, however, suffer from a voltage was applied, equal charges were limited temporal regimes of operation induced on both antenna wires. The resulting Many goals of the optomechanics and resolution. Tatsuo Nakagawa and equilibrium between the repulsive Coulomb community involve quantum mechanics, collaborators from Unisoku and Nihon force and the restoring elastic bending force for example, squeezing of optical and University in Japan have now devised a changed the gap between the antenna wires mechanical modes, or ground-state method based on a randomly interleaved from 40 nm at 0 V to 70 nm at 20 V, tuning cooling. These types of experiment are pulse train (RIPT), thus combining the antenna’s spectral response. White- technically non-trivial and are typically continuous-wave and pulsed strategies. The light scattering spectra of the nanoantenna conducted at cryogenic temperatures due scheme requires two asynchronous radiation without an applied voltage showed a to limitations of optomechanical resonators sources, in this case a picosecond laser resonance peak at 730 nm that shifted to a currently available. Now, Richard Norte, (producing the pump beam) with repetition shorter wavelength of 713 nm at 40 V. NH

João Moura and Simon Gröblacher, frequency Rrep = 1 kHz and a supercontinuum working at Delft University of Technology light source (generating probe pulses) GENERAL RELATIVITY in The Netherlands, have proposed and with Rrep = 20 MHz. Pump–probe delays Gravity of photons demonstrated an on-chip mechanical are evaluated passively through repeated New J. Phys. 18, 023009 (2016) resonator with suitable characteristics pumping cycles. The researchers determine for room-temperature optomechanical pump–probe delays with better than 10-ps Almost 90 years ago, Richard Tolman, experiments to reach the quantum regime. accuracy, and demonstrate measurement Paul Ehrenfest and Boris Poldolsky Mechanical quality factors as high as 108 time windows ranging from less than a published a paper titled ‘On the gravitation were reached by using ultrathin (down to nanosecond up to the microsecond scale. The field produced by light’ (Phys. Rev. 37,

15 nm thickness) Si3N4 membrane ‘tethers’ experimental transient absorption spectra for 602–615; 1931), which suggested that, under high tensile stress, close to breaking C60 shows that the RIPT method gives access via weak-field general relativity theory, a point (6.4 GPa). Previous works have noted to previously unobserved excited states for this cylindrical pulse, or ‘pencil’, of light causes difficulty in simultaneously achieving substance; the researchers also acquire spectra no acceleration to a co-propagating test ray. high mechanical quality and strong optical over visible to near-infrared wavelengths. One consequence of the analysis is that a reflectivity required for room-temperature Furthermore, this technique makes it possible light pulse does not self-interact with its own optomechanical quantum regime to correct transient absorption spectra for gravitational field. However, they noted that experiments. Typically, high optical fluorescence contamination. GD if the test ray is instead counter-propagating it can bend the trajectory of light, an effect similar to the bending of light by gravitational MICROCAVITIES fields of celestial bodies. Now, Dennis Rätzel, Slow-light benefits Phys. Rev. Lett. 116, 133902 (2016) Martin Wilkens and Ralf Menzel from the University of Potsdam, Germany, have theoretically proposed that the gravitational The development of optical microcavities with high Q-factors and thus long photon storage field of a linearly polarized light pulse is times would be useful for realizing high-performance optical filters, delay lines and sensors. ‘modulated’ by the electric field strength. A French team of researchers have now demonstrated a microcavity with a photon lifetime However, in the case of circularly polarized of 2.5 ms, corresponding to a Q factor of 3 × 1012, at a wavelength of 1,530 nm by harnessing light, no modulation is expected. The team slow-light effects. Vincent Huet and co-workers introduced a slow-light effect by exploiting theoretically investigated the gravitational coherent population oscillations in an erbium-doped fluoride glass whispering-gallery- effect on different test particles showing that mode microresonator. The result is a very strong refractive index dispersion and group delay both attraction and repulsion effects from the that thus slows down the signal light, increasing the photon storage time of the cavity from pulse trajectory may occur. DP 210 ps to 2.5 ms. The team says that the principle could be extended to suit devices intended for on-chip integration to create miniature ultrapure optical or microwave generators. OG Written by Gaia Donati, Oliver Graydon, Noriaki Horiuchi and David Pile.