Xuexin Ren's research while affiliated with University of California, Berkeley and other places

The demand for essential pixel components with ever-decreasing size and enhanced performance is central to current optoelectronic applications, including imaging, sensing, photovoltaics and communications. The size of the pixels, however, are severely limited by the fundamental constraints of lightwave diffraction. Current development using transmissive filters and planar absorbing layers can shrink the pixel size, yet there are two major issues, optical and electrical crosstalk, that need to be addressed when the pixel dimension approaches wavelength scale. All these fundamental constraints preclude the continual reduction of pixel dimensions and enhanced performance. Here we demonstrate subwavelength scale color pixels in a CMOS compatible platform based on anti-Hermitian metasurfaces. In stark contrast to conventional pixels, spectral filtering is achieved through structural color rather than transmissive filters leading to simultaneously high color purity and quantum efficiency. As a result, this subwavelength anti-Hermitian metasurface sensor, over 28,000 pixels, is able to sort three colors over a 100 nm bandwidth in the visible regime, independently of the polarization of normally-incident light. Furthermore, the quantum yield approaches that of commercial silicon photodiodes, with a responsivity exceeding 0.25 A/W for each channel. Our demonstration opens a new door to sub-wavelength pixelated CMOS sensors and promises future high-performance optoelectronic systems.

Elastic ceramics Aerogels hold promise as lightweight replacements for thermal insulation. However, poor mechanical stability has hampered progress in moving toward commercialization. Xu et al. designed a mechanical metamaterial that pinches in a small amount when you compress it (see the Perspective by Chhowalla and Jariwala). This is characteristic of materials with a negative Poisson's ratio and dramatically improves mechanical stability. The trick was using three-dimensional graphene structures to template the ceramic aerogels, thus producing a superinsulating material endowed with excellent mechanical properties. Science , this issue p. 723 ; see also p. 694

The control of the exciton intervalley coherence renders transition metal dichalcogenides monolayers promising candidates for quantum information science. So far, generating intervalley coherence has the need for an external coherent field. Here, we theoretically demonstrate spontaneous generation (i.e., without any external field) of exciton intervalley coherence. We achieve this by manipulating the vacuum field in the vicinity of the monolayer with a designed polarization-dependent metasurface, inducing an anisotropic decay rate for in-plane excitonic dipoles. Harnessing quantum coherence and interference effects in two-dimensional materials may provide the route for novel quantum valleytronic devices.

Metasurfaces are optically thin layers of subwavelength resonators that locally tailor the electromagnetic response at the nanoscale. Our metasurface research aims at developing novel designs and applications of metasurfaces that go beyond the classical regimes. In contrast to conventional phase gradient metasurfaces where each meta-atom responds individually, we are interested in developing metasurfaces where neighboring meta-atoms are strongly coupled. By engineering a non-Hermitian coupling between the meta-atoms, new degrees of freedom are introduced and novel functionalities can be achieved. We are also interested in combining classical metasurface with quantum emitters, which may offer opportunities for on-chip quantum technologies. Additionally, we have been designing metasurfaces to realize exciting phenomena and applications, such as ultrathin metasurface cloak and strong photonic spin-Hall effect. In this paper, we review our research efforts in optical metasurfaces in the past few years, which ranges from conventional to novel type of metasurface and from classical to quantum regime.

Entanglement-based quantum science exploits subtle properties of quantum mechanics into applications such as quantum computing, sensing and metrology. The emerging route for quantum computing applications, which calls for ultracompact, integrated and scalable architecture, aims at on-chip entangled qubits. In this context, quantum entanglement among atomic qubits was achieved via cold controlled collisions which are only significant at subwavelength separations. However, as other manifolds of quantum state engineering require single site addressability and controlled manipulation of the individual qubit using diffraction-limited optics, entanglement of qubits separated by macroscopic distances at the chip level is still an outstanding challenge. Here, we report a novel platform for on-chip quantum state engineering by harnessing the extraordinary light-molding capabilities of metasurfaces. We theoretically demonstrate quantum entanglement between two qubits trapped on a chip and separated by macroscopic distances, by engineering their coherent and dissipative interactions via the metasurface. Spatially scalable interaction channels offered by the metasurface enable robust generation of entanglement, with large values of concurrence, and remarkable revival from sudden death. The metasurface route to quantum state engineering opens a new paradigm for on-chip quantum science and technologies.

Quantum coherence and interference offer novel pathways to control light-matter interaction at the atomic scale, with enticing prospects in both fundamental and applied science. Here, we demonstrate coherent control over transient excitation of localized surface plasmon modes of a silver nanosphere adjacent to a quantum coherence medium composed of three-level quantum emitters, using a theoretical approach. We show that quantum interference enables more than two orders of magnitude enhancement in the surface plasmon field generated when the quantum emitters are driven coherently, in comparison to incoherent pumping. Furthermore, under optimal conditions, intense surface plasmon lasing can be induced without population inversion on the spasing transition. Our results open up the possibility of overcoming dissipative losses in plasmonic modes in the transient regime when steady-state population inversion is difficult to achieve due to fast relaxation rates and impractical pumping requirements.