Metasurfaces provide opportunities for wavefront control, flat optics, and subwavelength light focusing. We developed an imaging-based nanophotonic method for detecting mid-infrared molecular fingerprints and implemented it for the chemical identification and compositional analysis of surface-bound analytes. Our technique features a two-dimensional pixelated dielectric metasurface with a range of ultrasharp resonances, each tuned to a discrete frequency; this enables molecular absorption signatures to be read out at multiple spectral points, and the resulting information is then translated into a barcode-like spatial absorption map for imaging. The signatures of biological, polymer, and pesticide molecules can be detected with high sensitivity, covering applications such as biosensing and environmental monitoring. Our chemically specific technique can resolve absorption fingerprints without the need for spectrometry, frequency scanning, or moving mechanical parts, thereby paving the way toward sensitive and versatile miniaturized mid-infrared spectroscopy devices.

It has been a long-standing challenge to produce air-stable few- or monolayer samples of phosphorene because thin phosphorene films degrade rapidly in ambient conditions. Here we demonstrate a new highly controllable method for fabricating high quality, air-stable phosphorene films with a designated number of layers ranging from a few down to monolayer. Our approach involves the use of oxygen plasma dry etching to thin down thick-exfoliated phosphorene flakes, layer by layer with atomic precision. Moreover, in a stabilized phosphorene monolayer, we were able to precisely engineer defects for the first time, which led to efficient emission of photons at new frequencies in the near infrared at room temperature. In addition, we demonstrate the use of an electrostatic gate to tune the photon emission from the defects in a monolayer phosphorene. This could lead to new electronic and optoelectronic devices, such as electrically tunable, broadband near infrared lighting devices operating at room temperature.

We demonstrate experimentally ultrafast all-optical switching in subwavelength nonlinear dielectric nanostructures exhibiting localized magnetic Mie resonances. We employ amorphous silicon nanodisks to achieve strong self-modulation of femtosecond pulses with a depth of 60% at picojoule-per-disk pump energies. In the pump-probe measurements, we reveal that switching in the nanodisks can be governed by pulse-limited 65 fs-long two-photon absorption being enhanced by a factor of 80 with respect to the unstructured silicon film. We also show that undesirable free-carrier effects can be suppressed by a proper spectral positioning of the magnetic resonance, making such a structure the fastest all-optical switch operating at the nanoscale.

Highly selective and reconfigurable microwave filters are of great importance in radiofrequency signal processing. Microwave photonic (MWP) filters are of particular interest, as they offer flexible reconfiguration and an order of magnitude higher frequency tuning range than electronic filters. However, all MWP filters to date have been limited by trade-offs between key parameters such as tuning range, resolution, and suppression. This problem is exacerbated in the case of integrated MWP filters, blocking the path to compact, high performance filters. Here we show the first chip-based MWP band-stop filter with ultra-high suppression, high resolution in the MHz range, and 0-30 GHz frequency tuning. This record performance was achieved using an ultra-low Brillouin gain from a compact photonic chip and a novel approach of optical resonance-assisted RF signal cancellation. The results point to new ways of creating energy-efficient and reconfigurable integrated MWP signal processors for wireless communications and defence applications.The explosive growth in mobile communications demands radio-frequency (RF) technologies with exceptional spectral efficiency such as cognitive radios, which can adapt their frequencies to exploit the available spectrum in real-time [1,2]. Such frequency-agile systems will benefit hugely from RF filters that can be tuned over many gigahertz whilst keeping high MHz-scale resolution and high selectivity to prevent severe interference due to spectrumsharing. While this is difficult to achieve with all-electronic filters [3][4][5][6][7], integrated microwave photonic (IMWP) filters [8] can readily achieve multi-gigahertz tuning range without significant degradation in their frequency response. However, these filters typically exhibit limited resolution (GHz instead of MHz linewidths) and are plagued by trade-offs between key parameters, such as between the frequency tuning range and the resolution for multi-tap filters [9][10][11][12][13]; or between the peak rejection and the resolution for resonator-based filters [14][15][16][17][18].Stimulated Brillouin scattering (SBS) [19][20][21][22] offers a route to MHz-resolution IMWP filters. Although SBS has been widely studied in optical fibers, recently there has been a growing interest in harnessing SBS in nanophotonic waveguides [22][23][24][25][26][27]. The ability to control the coherent interaction of photons and acoustic phonons in chip-sized devices (as opposed to in optical fibers many kilometres long) promises not only fascinating new physical insights, but also opens the path to realising key technologies on-chip including slow light [28,29]; narrow linewidth lasers [30]; optical frequency combs [31,32]; RF signal processing [33][34][35] and filtering [36][37][38][39][40]. In particular, SBS filters can exhibit linewidths of the order of 10-100 MHz. Such a high resolution is unmatched by most on-chip devices because it requires extremely low material losses and impractically-large devices [41].Although IMWP filters exploiting SBS on ch...