Mie resonance wavelengths of a dielectric structure are strongly dependent on the inherent material property and structural geometry. In particular, a high-index nanostructure enables light confinement within itself over the range of visible wavelengths, which allows the Mie resonator to be applied to a pixel in color printing of subwavelength-scale resolution. However, if the Mie resonator is packed into a smaller area in order to achieve better resolution, the interaction between adjacent resonators occurs depending on these spatial distances, leading to unexpected color changes. Here, we demonstrate metal-masked Mie-resonant color printing for suppressing undesirable color changes. We observed that the interaction between monocrystalline Si resonators can be suppressed by the addition of a Cr mask. The pixels with this functionality can produce individual colors even if operating as a single element or in other periodic arrays, resulting in the realization of higher resolution encoding. The coincidence of resonance peak positions derived from excited electric/magnetic dipoles enables the demonstration of brilliant full-color printing with higher color purity. Furthermore, a vivid printing image with a resolution of more than 100 000 dpi was achieved using the designed subwavelength pixels. This study can contribute not only to the improvement of the resolution of color printing but also to the suppression of unwanted interactions of Mie resonance in optical devices.

All-dielectric color printing by means of high-index Mie resonators has enabled wider control of reflection colors depending on structural geometry. However, modifying the geometry, including the height, by using conventional fabrication processes remains challenging, and drastic color modification approaches via the addition of a new tuning axis are required to extend color varieties and applications. Here, we demonstrate all-dielectric pixel color control through Si oxidation. Oxidized monocrystalline Si nanostructures exhibit wider tunability of brilliant reflection colors depending on the oxidation reaction. The different color change properties of each nanostructure enable the construction of an "invisible ink" that can hide color information. This approach for controlling printing color could be utilized to further extend color variation and reactive applications.

Silicon photonics have attracted significant interest because of their potential in integrated photonics components and all-dielectric meta-optics elements. One major challenge is to achieve active control via strong photon–photon interactions, i.e. optical nonlinearity, which is intrinsically weak in silicon. To boost the nonlinear response, practical applications rely on resonant structures such as microring resonators or photonic crystals. Nevertheless, their typical footprints are larger than 10 μm. Here, we show that 100 nm silicon nano-resonators exhibit a giant photothermal nonlinearity, yielding 90% reversible and repeatable modulation from linear scattering response at low excitation intensities. The equivalent nonlinear index is five-orders larger compared with bulk, based on Mie resonance enhanced absorption and high-efficiency heating in thermally isolated nanostructures. Furthermore, the nanoscale thermal relaxation time reaches nanosecond. This large and fast nonlinearity leads to potential applications for GHz all-optical control at the nanoscale and super-resolution imaging of silicon.

By virtue of its unique advantages such as natural abundance and mature fabrication engineering, silicon (Si) is widely utilized in the electronic industries. However, in the field of photonics, the indirect bandgap nature of Si prohibits emissionrelated applications. Despite this limitation, Si exhibits a relatively high refractive index that allows efficient light confinement, especially in nanostructures. [1][2][3] The strong confinement and accompanying field localization offer substantial enhancement of the intrinsically weak optical nonlinearity in Si, including twophoton absorption, [4,5] harmonic generation, [6] and photothermal effects, [7,8] leading to the emerging field of nonlinear Si nano-photonics [3] with applications covering all-optical switching, wavelength conversion, and superresolution imaging.Conventionally, optical nonlinearity is characterized via intensity-scan methods such as z-scan, [9,10] which is suitable to investigate thin-film samples. In the z-scan method, a thin sample moves along the propagation direction (z-axis) of a focused laser beam, and z-position dependent transmittance or divergence Nonlinear silicon nano-photonics has recently attracted significant attention due to the plethora of electric and magnetic Mie resonances that offer substantial enhancement of optical nonlinearities. Conventionally, the characterization of nonlinearity and its transient nature rely on intensity-scan methods (z-scan) in the spatial domain and pump-probe techniques in the temporal domain. However, most studied ultrafast nonlinear effects are instantaneous, that is, strongest at zero pump-probe delay, and have a solitary nonlinear power dependency (square, cubic, etc.). Here the authors found that when relaxation lifetime is dependent on pump fluence, transient nonlinearity appears. The effect is exemplified via Auger-based nonlinear carrier dynamics of a nano-silicon Mie-resonator. The Auger-induced transient nonlinearity not only locates at the time delay of several tens of picoseconds, but also displays diverse nonlinearities, including sub-linear, super-linear, and surprisingly full saturation, which features a "crossing point" where the probe scattering is pump-fluence independent. The crossing point exists when the relaxation lifetime is inversely dependent to second power of carrier density. Combining confocal intensity-scan (x-scan) and pump-probe temporal scan, the authors demonstrate that sub-linearity and super-linearity lead to swelled and reduced full-width-at-half-maximum (FWHM) of single-nanostructure images, further confirming the nonlinearity as well as the potential of sub-diffraction microscopy. The results open up a new avenue in nonlinear silicon nano-photonics by adding new degrees of freedom in temporally tuning the types of transient nonlinearities, which are valuable in all-optical signal processing and nano-imaging.

No abstract