research highlights

NONLINEAR OPTICS fourth-harmonic generations, sum- PHOTONICS Metasurface mixer frequency generation, two-- Single-mode operation Nat. Commun. 9, 2507 (2018) absorption-induced photoluminescence, Sci. Adv. 4, eaar6782 (2018) four- mixing and six-wave mixing). Applications in communications, The concept of optical parity–time (PT) quantum optics, sensing and biology symmetry, whereby optical gain and loss are expected. RW are carefully controlled within a photonic https://doi.org/10.1038/s41566-018-0230-4 system, has enabled the generation of pure, low-noise single-mode microwave signals without the need for complex filtering. In an POLARITONS optoelectronic oscillator, an optical feedback Spin orientation switching loop is used to generate high-frequency Phys. Rev. B 97, 235303 (2018) microwave signals with low phase noise, but ensuring single-mode operation usually For the realization of integrated spintronic requires the use of an ultra-narrow-band 3 µm devices, a spin-switching device is desired. optical filter. Now, Jiejun Zhang and Now, Alexis Askitopoulos and co-workers Jianping Yao from the University of Ottawa, from the UK, Russia and Greece have Credit: Springer Nature Ltd Canada have shown that employing PT experimentally observed that the two spin symmetry can remove the need for such a components of a polariton condensate can Optical frequency mixers generally rely filter. In their approach, a polarizing beam switch their orientation via illumination splitter and controllers are on bulk nonlinear crystals and weak with . A polariton condensate was nonlinear optical processes. Sheng Liu used to create two feedback loops that have generated in a high-quality (Q =​ 16,000) a variable splitting ratio, thus allowing the and colleagues from Sandia National GaAs-based microcavity consisting of Laboratories, USA have now demonstrated level of gain and loss to be controlled and 10 nm GaAs quantum wells and a PT symmetry to be broken for just one an ultra-compact all- metamixer Rabi splitting of 9 meV. Light from a that enables many simultaneous nonlinear mode, enabling single-mode operation. The continuous-wave Ti:sapphire laser emitting approach has been tested with optical fibre optical processes across a broad spectral at 754 nm was sent to the microcavity range. The device is a gallium arsenide- loops of 20.31 m, 433.1 m and 9.166 km and the first excited state of the polariton in length and demonstrated to produce a based metasurface consisting of an condensate was optically trapped. 840-nm periodic square array of microwave signal at a frequency of 9.867 GHz The spin component of the condensate with a phase noise of –142.5 dBc Hz–1. OG nanocylinders with a diameter of was investigated by means of polarization- ~400 nm. Each nanocylinder is composed resolved spectroscopy. When the excitation https://doi.org/10.1038/s41566-018-0229-x of an ~300-nm-thick SiOx etch mask, density of the light was increased from an ~450-nm-thick GaAs nanodisk and 12.48 mW to 14.56 mW, the dominant an ~400-nm-thick (AlxGa1–x)O3 layer. spinor state with a p-orbital symmetry On pumping the metamixer with two Deep-learning boost was rotated by π​/2. The phenomenon is ACS Nano 12, 6326–6334 (2018) femtosecond pulses at ~1.24 μ​m and explained by the spatially inhomogeneous ~1.57 μ​m, near the magnetic and electric depletion of the exciton reservoir and the Wei Ma and colleagues from Northeastern dipole resonances of the metasurface, repulsive exciton–polariton interaction. University, USA have developed an accurate, the team observed the generation of 11 In principle, the approach could lead to a efficient deep-learning-based model new frequencies spanning from ~380 nm spin-demultiplexing device for polariton- for designing three-dimensional chiral to ~1,000 nm, generated by 7 different based spin circuits. NH metamaterials. The model consists of two concurrent nonlinear processes bidirectional neural networks, with a forward (second-harmonic, third-harmonic and https://doi.org/10.1038/s41566-018-0227-z combiner and an inverse combiner. It is trained heuristically with multiple functions for fast prototyping, optimization and inverse PHOTONIC CRYSTALS design. The unit cell of the chiral Cherenkov detector Nat. Phys. http://doi.org/crv7 (2018) under study consists of two stacked gold split- ring resonators twisted at a certain angle and Cherenkov — light emission that occurs when a propagates separated by two spacing dielectric layers with through a medium at a that is faster than the phase of light in the medium a continuous gold reflector at the bottom. — is of importance to applications such as particle detection. However, the relation The team demonstrated that the model can between the angle of Cherenkov emission and the particle velocity is inherently limited significantly shorten the prediction time for by the of the material. Now, an international team led by Xiao Lin generating chiral dichroism and can solve has removed this restriction by using a one-dimensional to control the design-on-demand inverse problem the Cherenkov angle at will. The photonic crystal was constructed by alternating two accurately and efficiently, retrieving geometric transparent dielectric materials, and the Cherenkov angle can be engineered to be suitable parameters of the metamaterial from specific for particle identification of high momenta near 500 GeV/c , which is hard to achieve with requirements of its optical response. RW a conventional Cherenkov detector. Another advantage of the one-dimensional photonic https://doi.org/10.1038/s41566-018-0231-3 crystal approach is that the crystal only needs to be a few millimetres thick. NH https://doi.org/10.1038/s41566-018-0228-y Oliver Graydon, Noriaki Horiuchi and Rachel Won

Nature Photonics | VOL 12 | AUGUST 2018 | 443 | www.nature.com/naturephotonics 443