Research on two-dimensional designer optical structures, or metasurfaces, has mainly focused on controlling the wavefronts of light propagating in free space. Here, we show that gradient metasurface structures consisting of phased arrays of plasmonic or dielectric nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Based on this design principle, we experimentally demonstrate waveguide mode converters, polarization rotators and waveguide devices supporting asymmetric optical power transmission. We also demonstrate all-dielectric on-chip polarization rotators based on phased arrays of Mie resonators with negligible insertion losses. Our gradient metasurfaces can enable small-footprint, broadband and low-loss photonic integrated devices.

A functional hybrid of reduced graphene oxide (RGO)-Fe 3 O 4 nanoparticles (NPs) has been chemically synthesized with exceptionally high yield and tunable RGO/Fe 3 O 4 ratio. The adsorption behaviors of a series of dyes using this hybrid as the adsorbent are systematically investigated in aqueous solutions through real-time monitoring of the fingerprint spectral changes of the dyes. The results show that, benefiting both from the surface property of RGO and from the magnetic property of Fe 3 O 4 , the hybrid possesses quite a good (although unoptimized) and versatile adsorption capacity to the dyes under investigation, and can be easily and rapidly extracted from water by magnetic attraction. Most importantly, it is found that by simply annealing in moderate conditions, this hybrid adsorbent can be easily and efficiently regenerated for reuse with hardly any compromise of the adsorption capacity. Furthermore, the adsorbability of this hybrid shows satisfactory tolerance against the variations in both pH environment and dye concentration. Even when exposed to a multi dye cocktail, the hybrid can work well without suppressing the adsorption capacity for each of the dyes, as compared with that measured separately. The inherent advantages of this nanostructured adsorbent, such as noncompromised adsorption capacity, low cost, easy, rapid extraction and regeneration, good tolerance, multiplex adsorbability, and handy operation, may pave a new, efficient and sustainable way towards highly-efficient dye pollutant removal in Earth's water environments.

Virtual and augmented realities are rapidly developing technologies, but their large-scale penetration will require lightweight optical components with small aberrations. We demonstrate millimeter-scale diameter, high-NA, submicron-thin, metasurface-based lenses that achieve diffraction-limited achromatic focusing of the primary colors by exploiting constructive interference of light from multiple zones and dispersion engineering. To illustrate the potential of this approach, we demonstrate a virtual reality system based on a home-built fiber scanning near-eye display.

The phase-matching condition is a key aspect in nonlinear wavelength conversion processes, which requires the momenta of the photons involved in the processes to be conserved. Conventionally, nonlinear phase matching is achieved using either birefringent or periodically poled nonlinear crystals, which requires careful dispersion engineering and is usually narrowband. In recent years, metasurfaces consisting of densely packed arrays of optical antennas have been demonstrated to provide an effective optical momentum to bend light in arbitrary ways. Here, we demonstrate that gradient metasurface structures consisting of phased array antennas are able to circumvent the phase-matching requirement in on-chip nonlinear wavelength conversion. We experimentally demonstrate phase-matching-free second harmonic generation over many coherent lengths in thin film lithium niobate waveguides patterned with the gradient metasurfaces. Efficient second harmonic generation in the metasurface-based devices is observed over a wide range of pump wavelengths (λ = 1580–1650 nm).

Birefringence occurs when light with different polarizations sees different refractive indices during propagation. It plays an important role in optics and has enabled essential polarization elements such as wave plates. In bulk crystals, it is typically constrained to linear birefringence. In metamaterials with freeform meta-atoms, however, one can engineer the optical anisotropy such that light sees different indices for arbitrary—linear, circular, or elliptical—orthogonal eigen-polarization states. Using topology-optimized metasurfaces, we demonstrate this arbitrary birefringence. It has the unique feature that it can be continuously tuned from linear to elliptical birefringence, by changing the angle of incidence. In this way, a single metasurface can operate as many wave plates in parallel, implementing different polarization transformations. Angle-tunable arbitrary birefringence expands the scope of polarization optics, enables compact and versatile polarization operations that would otherwise require cascading multiple elements, and may find applications in polarization imaging, quantum optics, and other areas.

We show that large modulation of the amplitude and phase of mid-infrared light can be achieved by dynamically shifting the resonance of graphene-metal plasmonic antennas via electrical tuning of the optical conductivity of graphene. Intensity modulation with on-off extinction ratio exceeding 100 and phase modulation over a range of 240° are demonstrated by simulations of scattered light from arrays of such antennas. The modulation rate is estimated to be on the order of a few GHz. These properties are promising for creating reconfigurable flat optical components such as spatial light modulators in the mid-infrared spectral range.

Optical devices are highly attractive for biosensing as they can not only enable quantitative measurements of analytes but also provide information on molecular structures. Unfortunately, typical refractive index-based optical sensors do not have sufficient sensitivity to probe the binding of low-molecular-weight analytes. Non-optical devices such as field-effect transistors can be more sensitive but do not offer some of the significant features of optical devices, particularly molecular fingerprinting. We present optical conductivity-based mid-infrared (mid-IR) biosensors that allow for sensitive and quantitative measurements of low-molecular-weight analytes as well as the enhancement of spectral fingerprints. The sensors employ a hybrid metasurface consisting of monolayer graphene and metallic nano-antennas and combine individual advantages of plasmonic, electronic and spectroscopic approaches. First, the hybrid metasurface sensors can optically detect target molecule-induced carrier doping to graphene, allowing highly sensitive detection of low-molecular-weight analytes despite their small sizes. Second, the resonance shifts caused by changes in graphene optical conductivity is a well-defined function of graphene carrier density, thereby allowing for quantification of the binding of molecules. Third, the sensor performance is highly stable and consistent thanks to its insensitivity to graphene carrier mobility degradation. Finally, the sensors can also act as substrates for surface-enhanced infrared spectroscopy. We demonstrated the measurement of monolayers of sub-nanometer-sized molecules or particles and affinity binding-based quantitative detection of glucose down to 200 pM (36 pg/mL). We also demonstrated enhanced fingerprinting of minute quantities of glucose and polymer molecules.

The electron-doping-induced phase transition of a prototypical perovskite SmNiO induces a large and non-volatile optical refractive-index change and has great potential for active-photonic-device applications. Strong optical modulation from the visible to the mid-infrared is demonstrated using thin-film SmNiO . Modulation of a narrow band of light is demonstrated using plasmonic metasurfaces integrated with SmNiO .