Toroidal moment is an electromagnetic excitation that lies outside the familiar picture of electric and magnetic multipoles. It has recently been a topic of intense research in the fields of nanophotonics and metamaterials due to its weakly radiating nature and its ability to confine electromagnetic energy. Among extensive studies on toroidal moments and their applications, high quality factor (Q) toroidal resonances have been experimentally realized only in a very limited set of geometries and wavelengths. In this study, we demonstrate that a metasurface consisting of arrays of hollow dielectric cuboids supports a high Q-factor resonances at near-infrared and visible wavelengths due to the destructive interference between toroidal dipoles and magnetic quadrupoles. Using silicon as the high index dielectric, an experimental Q-factor of 728 is realized at a wavelength of 1505 nm, which is one of the highest values reported in the near-infrared using a dielectric metasurface. Importantly, our resonator geometry enables very efficient coupling of the toroidal resonance to the environment. This makes our metasurface design useful for refractometric sensing, where we measure a sensitivity of 161 nm per refractive index unit with a line width of 2.01 nm, efficiently distinguishing an index change of less than 0.02. We also find that a metasurface made of a relatively low-index dielectric, titanium dioxide (n < 2.4), is also capable of supporting the same toroidal mode with an observed Q-factor of 160 at visible wavelengths. With the versatility and robustness that dielectric metasurfaces provide, toroidal resonances are expected to be a powerful tool for investigating strong light–matter interaction and nonlinear phenomena at the nanoscale.

Ultrafast optical excitation of select materials gives rise to the generation of broadband terahertz (THz) pulses. This effect has enabled the field of THz time-domain spectroscopy and led to the discovery of many physical mechanisms behind THz generation. However, only a few materials possess the required properties to generate THz radiation efficiently. Optical metasurfaces can relax stringent material requirements by shifting the focus onto the engineering of local electromagnetic fields to boost THz generation. Here we demonstrate the generation of THz pulses in a 160 nm thick nanostructured GaAs metasurface. Despite the drastically reduced volume, the metasurface emits THz radiation with efficiency comparable to that of a thick GaAs crystal. We reveal that along with classical second-order volume nonlinearity, an additional mechanism contributes strongly to THz generation in the metasurface, which we attribute to surface nonlinearity. Our results lay the foundation for engineering of semiconductor metasurfaces for efficient and versatile THz radiation emitters.

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The effect of terahertz (THz) pulse generation has revolutionized broadband coherent spectroscopy and imaging at THz frequencies. However, THz pulses typically lack spatial structure, whereas structured beams are becoming essential for advanced spectroscopy applications. Nonlinear optical metasurfaces with nanoscale THz emitters can provide a solution by defining the beam structure at the generation stage. We develop a nonlinear InAs metasurface consisting of nanoscale optical resonators for simultaneous generation and structuring of THz beams. We find that THz pulse generation in the resonators is governed by optical rectification. It is more efficient than in ZnTe crystals, and it allows us to control the pulse polarity and amplitude, offering a platform for realizing binary-phase THz metasurfaces. To illustrate this capability, we demonstrate an InAs metalens, which simultaneously generates and focuses THz pulses. The control of spatiotemporal structure using nanoscale emitters opens doors for THz beam engineering and advanced spectroscopy and imaging applications.