Anisotropic 2D materials with unique thermoelectric, electrical, and optical characteristics offer prospects for various angle-dependent devices. Yet, few anisotropic 2D materials are exploited. Black phosphorus (BP) is a newly rediscovered 2D material with striking in-plane anisotropy. However, experimental illustrations of the optical anisotropy of 2D BP are very limited presumably due to the ultrathin thickness of the few-layered BP which weakens the lightmatter interaction. To solve this problem, herein a hybrid nanostructure is fabricated using a one-step self-assembly deposition of gold (Au) nanoparticle (NP) clusters onto the few-layered BP residing on a silica (SiO 2) substrate. Such a hybrid nanostructure can squeeze the light into AuNPs-SiO 2 gap through the gap plasmon resonance, enabling the confined light to interact efficiently with the ultrathin BP film. Angle-resolved polarized Raman spectra measurement is carried out to study the influence of the AuNPs on the in-plane optical anisotropy of the BP. About 20 times enhancement of Raman intensity anisotropy is experimentally achieved using AuNPs. The AuNPs-BP system is not only useful for high-performance polarization-dependent photonic devices but also a universal methodology for exploring the in-plane anisotropy of low-symmetry 2D materials. Furthermore, the high-throughput self-assembly fabrication shows its potential for industrial-scale manufacturability.
of metallic subwavelength antennas can generate optical elements with varied abilities. [6,7] Particularly, the PMMs advance interesting design methods to make light display and projection devices, [8,9] revolutionising state-of-the-art display technology. For example, a color display was obtained using the interference effect in pixelized metal-dielectric-metal multilayered structure. [10][11][12][13][14][15] As the spectral resonance of PMM color display depends on the meta-atom composition and geometry, most PMM color filters only produce static colors since the material and geometry of the PMMs are not varied once the devices are fabricated. Nevertheless, the fast development of color display technology demands tunable color filters/reflectors. [16] Recently, tunable color displaying technique has been studied using electrically tunable PMMs based on liquid crystal (LC). [17][18][19] Whereas, the tunability is rather restrained and the integration of the electrodes as well as the required layer structures for the LC tuning can be complicated in nanostructure. Meanwhile, a vanadium dioxide (VO 2 )-based PMM has been used to obtain switchable plasmonic colors, [20,21] where radical temperature-induced refractive index change in VO 2 leads to a variation in the reflected color. However, the VO 2 needs constant heating above the insulator-to-metal transition temperature to maintain the VO 2 in its high-temperature state. Chemical reaction induced metal-insulator transition can also change the plasmonic color. Very recently, a magnesium (Mg)-based plasmonic MM has been proposed to generate colors with high purity over a broad gamut under a hydrogen (H 2 ) environment. [22,23] However, the chemistry tuning process is slow; for example, it takes about 10 min of hydrogenation to remove the image and 37 min to restore it fully. Another method to produce tunable plasmonic colors is to reversibly deform the plasmonic MM residing on stretchable substrate. [24][25][26][27][28] However, a few crucial areas stay for further study such as integrated tuning schemes, cycle stability, and how to improve the optical performance while maintaining the tuning range. The most straightforward method to dynamically tune colors is to map the incident polarization states of polarization-dependent PMMs to various colors. [9,[29][30][31][32][33][34][35] Yet, the optical losses of PMMs and complexity of current nanofabrication techniques limit their further developments. [36,37] Anisotropic optical material provides a platform that paves up new avenues for polarization-dependent devices. Recently, Next-generation color display entails miniaturization, reconfigurablity, flexibility, integration, and excellent workability. Recently, emerging 2D van der Waals materials offer a new opportunity to satisfy these requirements and attract intense attention, owing to their intrinsic in-plane anisotropy for polarization dependent photosensitivity, straightforward integration with complicated nanostructures, and efficient quantum confinement for good...
2D black phosphorus (BP) possesses an in‐plane anisotropy, offering another degree of freedom to create new optoelectronic and photonic devices, such as waveplates with atomic thickness. Yet, due to its very low energy bandgap, the anisotropy of BP crystal is mainly studied in the near‐infrared region and remains elusive in the visible region. Furthermore, the strength of optical anisotropy of few‐layer BP, which is crucial for practical applications, is still limited. Herein, a BP‐based Fabry–Pérot (FP) cavity that can increase the strength of BP anisotropy twice compared with the bare BP flake is proposed. Notably, the FP cavity composed of BP/Au dual‐layer structure sitting on the Si substrate could realize a reconfigurable color switching over the whole visible region by varying the polarization of the incident light, utilizing polarization‐dependent complex permittivity of the anisotropic BP crystal. The finding not only offers a simple approach to improve the optical anisotropy of 2D layered materials but also advises an efficient method to develop BP optical devices, including a phase plate, a linear polarizer, and a color display.
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