No abstract

Nanostructured metasurfaces demonstrate extraordinary capabilities to control light at the subwavelength scale, emerging as key optical components to physical realization of multitasked devices. Progress in multitasked metasurfaces has been witnessed in making a single metasurface multitasked by mainly resorting to extra spatial freedom, for example, interleaved subarrays, different angles. However, it imposes a challenge of suppressing the cross-talk among multiwavelength without the help of extra spatial freedom. Here, we introduce an entirely novel strategy of multitasked metasurfaces with noninterleaved single-size Si nanobrick arrays and minimalist spatial freedom demonstrating massive information on 6-bit encoded color holograms. The interference between electric dipole and magnetic dipole in individual Si nanobricks with in-plane orientation enables manipulating six bases of incident photons simultaneously to reconstructed 6-bit wavelength-and spin-dependent multicolor images. Those massively reconstructed images can be distinguished by pattern recognition. It opens an alternative route for integrated optics, data encoding, security encryption, and information engineering.

Structural colors traditionally refer to colors arising from the interaction of light with structures with periodicities on the order of the wavelength. Recently, the definition has been broadened to include colors arising from individual resonators that can be subwavelength in dimension, e.g., plasmonic and dielectric nanoantennas. For instance, diverse metallic and dielectric nanostructure designs have been utilized to generate structural colors based on various physical phenomena, such as localized surface plasmon resonances (LSPRs), Mie resonances, thin-film Fabry-Pérot interference, and Rayleigh-Wood diffraction anomalies from 2D periodic lattices and photonic crystals. Here, we provide our perspective of the key application areas where structural colors really shine, and other areas where more work is needed. We review major classes of materials and structures employed to generate structural coloration and highlight the main physical resonances involved.

Unlike dye-based colorants, for which dilution results in a decrease in color saturation, a reduction of nanostructure density in plasmonic prints could increase color saturation instead. This interesting observation can be explained by the absorption cross-section of the nanostructure being larger than its physical cross-section. In this paper, we demonstrate the correlation between absorption cross-section and nanostructure density and use it to realize saturated colors by fabricating metal–insulator–metal aluminum nanostructures that support gap-surface plasmons (GSPs). We obtained structures with absorption cross-sections that exceed 10 times their physical cross-sections. The large absorption cross-sections of the GSP structures herald a color-mixing scheme where nanostructures of different hues are combined within subpixels at a constant pitch. The pitch is chosen such that the total absorption cross-section of individual constituents of the cell occupies the unit size area. Using a constant pitch of 320 nm, hence preserving the print resolution, our structures exhibit 45% coverage of the sRGB color space. By employing absorption cross-sections of the nanostructures, we produced black and saturated green pixels, which have been challenging to achieve in plasmonic color printing. The effects of square and hexagonal arrangements on color saturation are investigated, and point mixing effects are observed between individual nanostructures.

Dielectric optical nanoantennas are promising as fundamental building blocks in next generation color displays, metasurface holograms, and wavefront shaping optical devices. Due to the high refractive index of the nanoantenna material, they support geometry-dependent Mie resonances in the visible spectrum. Although phase change materials, such as the germanium–antimony–tellurium alloys, and post-transition metal oxides, such as ITO, have been used to tune antennas in the near-infrared spectrum, reversibly tuning the response of dielectric antennas in the visible spectrum remains challenging. In this paper, we designed and experimentally demonstrated dielectric nanodisc arrays exhibiting reversible tunability of Mie resonances in the visible spectrum. We achieved tunability by exploiting phase transitions in Sb2S3 nanodiscs. Mie resonances within the nanodisc give rise to structural colors in the reflection mode. Crystallization and laser-induced amorphization of these Sb2S3 resonators allow the colors to be switched back and forth. These tunable Sb2S3 nanoantenna arrays could enable the next generation of high-resolution color displays, holographic displays, and miniature LiDAR systems.

While structural colors are ubiquitous in nature, saturated reds are mysteriously absent. This long-standing problem of achieving Schrödinger’s red demands sharp transitions from “stopband” to a high-reflectance “passband” with total suppression of higher-order resonances at blue/green wavelengths. Current approaches based on nanoantennas are insufficient to satisfy all conditions simultaneously. Here, we designed Si nanoantennas to support two partially overlapping quasi–bound-states-in-the-continuum modes with a gradient descent algorithm to achieve sharp spectral edges at red wavelengths. Meanwhile, high-order modes at blue/green wavelengths are suppressed via engineering the substrate-induced diffraction channels and the absorption of amorphous Si. This design produces possibly the most saturated and brightest reds with ~80% reflectance, exceeding the red vertex in sRGB and even the cadmium red pigment. Its nature of being sensitive to polarization and illumination angle could be potentially used for information encryption, and this proposed paradigm could be generalized to other Schrödinger’s color pixels.

color displays, [4][5][6][7] optical anti-counterfeiting, [8][9][10][11] and camouflaging. [12] Various systems have been employed to realize structural colors, such as plasmonic resonators, [13][14][15] all-dielectric systems, [16][17][18][19][20] and diffraction gratings. [21][22][23][24] These systems take advantage of various physical resonances and mechanisms such as surface plasmons, gap plasmons, [25][26][27] Mie resonances, [28][29][30] and thin-film interference. [19,31,32] In particular, structural colors based on thin-film interference such as Fabry-Perot (FP) cavity are promising as they can be easily scaled up, patterned using different processes, and produce vibrant colors. [19,33] Structural color elements can be designed such that they visually respond to different external stimuli through the use of suitable materials in an optically resonant system. Various optical systems such as FP, [34][35][36] Bragg reflector, [37] plasmonic, [38][39][40] and photonic crystal [41] structure types have been employed to realize such stimuli-responsive systems. Various materials such as hydrogels, [34,35,38,39] 2D materials, [37] and phase changing materials (i.e., VO 2 , Sb 2 S 3 ) [36,42] have been exploited in structural color responsive systems. Depending on the material that is incorporated into the optical system, the system can respond to one or multiple stimuli such as temperature and humidity. [38,39] While multiple demonstrations have been made, the issue of cost and simplicity of device design has yet to be fully addressed.Of the various external stimuli, a chemical response is of great importance. In chemical sensing, changes in the environment are detected by transforming the change into an analytically useful output such as color. [43][44][45] To this end, materials that respond to certain chemicals are integrated into an optical resonator. For instance, the poly(2-hydroxyethyl methacrylate-coacrylic acid) hydrogel was covalently bonded to a silicon (Si) substrate to realize a sensor through the thin-film interference effect to detect volatile organic compounds. [34] In another work, copper ions and glycoprotein were detected using an FP cavity made by sandwiching poly(acrylamide-co-acrylic acid-co-N-allyl acrylamide) hydrogel between Si substrate and a gold (Au) film through spectrometric measurements. [35] Previous reports do not provide enough color variation, lack saturated colors that are easy to distinguish visually, need spectrometric measurements, or use costly materials and processes.The cost-effective colorimetric detection of chemicals can be a potential substitute for expensive spectrometers. Here, a structural color sensor is presented that can distinguish seven different organic solvents through a timed sharp color change. The color sensor is based on interference effects in a metalinsulator-metal Fabry-Perot (FP) cavity with polydimethylsiloxane (PDMS) serving as the dielectric layer. By tuning the cross-linker to monomer ratio of PDMS and employing a porous nickel (Ni) top layer, t...

Dual-functional aggregation-induced photosensitizers (AIE-PSs) with singlet oxygen generation (SOG) ability and bright fluorescence in aggregated state have received much attention in image-guided photodynamic therapy (PDT). However, designing an AIE-PS with both high SOG and intense fluorescence via molecular design is still challenging. In this work, we report a new nanohybrid consisting of gold nanostar (AuNS) and AIE-PS dots with enhanced fluorescence and photosensitization for theranostic applications. The spectral overlap between the extinction of AuNS and fluorescence emission of AIE-PS dots (665 nm) is carefully selected using five different AuNSs with distinct localized surface plasmon (LSPR) peaks. Results show that all the AuNSs can enhance the 1O2 production of AIE-PS dots, among which the AuNS with LSPR peak at 585 nm exhibited the highest 1O2 enhancement factor of 15-fold with increased fluorescence brightness. To the best of our knowledge, this is the highest enhancement factor reported for the metal-enhanced singlet oxygen generation systems. The Au585@AIE-PS nanodots were applied for simultaneous fluorescence imaging and photodynamic ablation of HeLa cancer cells with strongly enhanced PDT efficiency in vitro. This study provides a better understanding of the metal-enhanced AIE-PS nanohybrid systems, opening up new avenue towards advanced image-guided PDT with greatly improved efficacy.