Optical metasurfaces are regular quasi-planar nanopatterns that can apply diverse spatial and spectral transformations to light waves. However, metasurfaces are no longer adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. We experimentally realise an ultrafast tunable metasurface consisting of subwavelength gallium arsenide nanoparticles supporting Mie-type resonances in the near infrared. Using transient reflectance spectroscopy, we demonstrate a picosecond-scale absolute reflectance modulation of up to 0.35 at the magnetic dipole resonance of the metasurfaces and a spectral shift of the resonance by 30 nm, both achieved at unprecedentedly low pump fluences of less than 400 μJ cm–2. Our findings thereby enable a versatile tool for ultrafast and efficient control of light using light.

Transition metal dichalcogenide materials have recently been shown to exhibit a variety of intriguing optical and electronic phenomena. Focusing on the optical properties of semiconducting WS2 nanotubes, we show here that these nanostructures exhibit strong light-matter interaction and form exciton-polaritons. Namely, these nanotubes act as quasi 1-D polaritonic nano-systems and sustain both excitonic features and cavity modes in the visible-near infrared range. This ability to confine light to subwavelength dimensions under ambient conditions is induced by the high refractive index of tungsten disulfide. Using "finite-difference time-domain" (FDTD) simulations we investigate the interactions between the excitons and the cavity mode and their effect on the extinction spectrum of these nanostructures. The results of FDTD simulations agree well with the experimental findings as well as with a phenomenological coupled oscillator model which suggests a high Rabi splitting of ∼280 meV. These findings open up possibilities for developing new concepts in nanotube-based photonic devices.

Saturable optical elements lie at the cornerstone of many modern optical systems. Regularly patterned quasi-planar nanostructuresmetasurfacesare known to facilitate nonlinear optical processes. Such subwavelength semiconductor nanostructures can potentially serve as saturable components. Here we report on the intensity-dependent reflectance of femtosecond laser pulses from semiconductor metasurfaces with Mie-type modes, caused by the absorption saturation. Arrays of GaAs nanocylinders with magnetic dipole resonances in the spectral vicinity of the GaAs bandgap demonstrate a reduced saturation intensity and increased selfmodulation efficiency, an order of magnitude higher than bulk GaAs or unstructured GaAs films. By contrast, the reflection modulation is shown to be negligible in the CW regime for the same average intensities, indicating that the process is not the result of temperature effects. Our work provides a novel idea for low-power saturable elements based on nonthermal nature of saturation. We conclude by devising a high-quality metasurface that can be used, in theory, to further reduce the saturation fluence below 50 nJ/cm 2 .

Optical materials are undergoing revolutionary transformations driven by nanotechnology. Our ability to engineer structures at a scale smaller than the wavelength of light enables new properties and functionalities otherwise not available in natural bulk optical materials. A class of such components—dielectric metasurfaces—employs two-dimensional arrays of designer resonant nanoscale elements whose optical response is defined by their geometry. While linear regimes of interactions between dielectric metasurfaces and moderately intense light have already formed a mature field of applied research and engineering, new frontiers are being actively explored in the nonlinear optical regime describing interactions of metasurfaces with strong optical fields. In this Research Update, we cover the most recent progress along with several directions of research within the field of nonlinear optics of dielectric metasurfaces. Specifically, we review approaches to design and fabricate metasurfaces with high local field enhancements that facilitate nonlinear light–matter interactions, outline nonlinearity-enabled functionalities of dielectric metasurfaces, explore resonant metasurfaces in the strong-field non-perturbative regime, and discuss the implications of the time-variant refractive index in metasurfaces that interact with strong optical fields produced by laser pulses.

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