Monolayer transition metal dichalcogenides (TMDCs) have large second‐order optical nonlinearity owing to broken inversion symmetry in two‐dimensional (2D) crystals. However, despite the strong light–matter coupling in monolayer TMDCs, their nonlinear responses are ultimately limited by subnanometer thickness. Here, a dramatic enhancement of the second‐harmonic generation (SHG) is achieved from monolayer tungsten disulfide (WS2) incorporated onto a 2D silver (Ag) nanogroove grating with subwavelength pitch. By tuning surface plasmon mode and second‐harmonic frequency in resonance with the C exciton in WS2, a large SHG enhancement factor (≈400) and a large conversion efficiency (≈2.0 × 10–5) can be obtained. Furthermore, the azimuthal angle dependence of polarized SHG from monolayer WS2 can be manipulated by the nanogroove plasmonic mode. Based on this property, a polarization‐modulated optical encoding technique is demonstrated. The results suggest that 2D TMDC–plasmonic hybrid metasurface structures can provide an ideal integration platform for on‐chip nonlinear photonics and plasmonics.

In comparison to noble metals (gold and silver), aluminum is a sustainable and widely applicable plasmonic material owing to its abundance in the Earth’s crust and compatibility with the complementary metal–oxide–semiconductor (CMOS) technology for integrated devices. Aluminum (Al) has a superior performance in the ultraviolet (UV) regime with the lowest material loss and good performance in the full visible regime. Furthermore, aluminum films can remain very stable in ambient environment due to the formation of surface native oxide (alumina) acting as a passivation layer. In this work, we develop an epitaxial growth technique for forming atomically smooth aluminum films on transparent c-plane (0001) sapphire (Al-on-Sapphire, ALOSA) by molecular-beam epitaxy (MBE). The MBE-grown ALOSA films have small plasmonic losses and enable us to fabricate and utilize high-quality plasmonic nanostructures in a variety of optical configurations (reflection, transmission, and scattering). Here, the surface roughness and crystal orientation of ALOSA films are characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD). Moreover, the formation of smooth native oxide layer and abrupt heterointerfaces are investigated by transmission electron microscopy (TEM). We have also measured the optical dielectric function of epitaxial aluminum films by using spectroscopic ellipsometry (SE). These results show that the structural and optical properties of epitaxial aluminum films grown by MBE are excellent compared to polycrystalline aluminum films grown by other deposition methods. To illustrate the capability of device applications for the full visible spectrum, we demonstrate clear surface plasmon polarition (SPP) interference patterns using a series of double-groove surface interferometer structures with varied groove–groove separations under white-light illumination. Finally, we show the device performance of zinc oxide (ZnO) nanowire (UV) and indium gallium nitride (InGaN) nanorod (blue and green) plasmonic lasers prepared by using the epitaxial Al films. The measured lasing thresholds are comparable with the best available data obtained on the Ag films. According to these result, we suggest that epitaxial ALOSA films are a versatile plasmonic material platform in the UV and full visible spectral regions.

Surface-enhanced Raman spectroscopy (SERS) is an ultrasensitive technique to identify vibrational fingerprints of trace analytes. However, present SERS techniques suffer from the lack of uniform, reproducible, and stable substrates to control the plasmonic hotspots in a wide spectral range. Here, we report the promising application of epitaxial aluminum films as a scalable plasmonic platform for SERS applications. To assess the uniformity of aluminum substrates, atomically thin transition metal dichalcogenide monolayers are used as the benchmark analyte due to their inherent two-dimensional homogeneity. Besides the distinctive spectral capability of aluminum in the ultraviolet (325 nm), we demonstrate that the aluminum substrates can even perform comparably with the silver counterparts made from single-crystalline colloidal silver crystals using the same SERS substrate design in the visible range (532 nm). This is unexpected from the prediction solely based on optical dielectric functions and illustrate the superior surface and interface properties of epitaxial aluminum SERS substrates.

beyond the diffraction limit, provide a promising platform for the development of all-optical logic circuits. [12][13][14][15][16][17][18] Recently, several groups have reported that plasmonic microstructures, either nanowires or nanogrooves, could be used to build microscale all-optical logic gates. [19][20][21][22][23] Using different microfabrication techniques, devices with varying scales and output intensity contrast ratios have been demonstrated. In spite of the progress so far, these devices either have a small contrast ratio which affects the signal extraction, or have a big loss which is hard for integration. More importantly, broadband response and multifunctionality have not been demonstrated, which are crucial for high-density data transmission and processing.In this work, we propose and demonstrate broadband (over 100 nm in free-space wavelength) Boolean logic devices, including seven fundamental logic gates, while maintaining their high-contrast-ratio properties. Each device contains a curved grating with a rectangular groove profile for the input port(s) and a subwavelength hole as the output port. Because of its unique geometric property, the curved grating can transduce the input optical signals to propagating surface plasmon polaritons (SPPs), and focus them to the focal point without using any additional guiding structure. Consequently, the SPPs from different input ports can interfere at the focal point, and are coupled out to the far-field by using the hole as the output signal port. Essentially, this configuration is a zero-order interferometer, which naturally has a wide operating bandwidth. Besides, when propagating freely, the SPPs experience minimal losses in comparison with the case propagating in waveguides. [24] The loss is further decreased by using single-crystalline silver, instead of the polycrystalline silver films usually prepared by thermal evaporation. Meanwhile, these logic gates can maintain the high intensity contrast ratio even using front illumination. Here, we obtain a maximum contrast ratio of 29 dB. In addition, we used only one geometric configuration to realize all the fundamental logic gates, making this method easy for device fabrication and system integration.The designed device configuration is illustrated in Figure 1a. The input optical signal is incident on the curved grating and transduced into SPPs, which are focused to the grating focal point. To build a broadband device, two conditions need to be met: the first one is that the grating has a broadband spectral response to the incident signals, and the second one is that the focus of the input signals from different input ports is achromatic, i.e., independent of the operating wavelength.Plasmonic logic gates provide a promising platform to realize diffractionunlimited all-optical logic circuits. However, previous plasmonic logic gates suffer from narrow bandwidth and high loss. Here, broadband plasmonic logic gates are proposed and demonstrated using a unique design based on curved silver gratings. These log...

Non-Hermitian photonic systems with gains and/or losses have recently emerged as a powerful approach for topology-protected optical transport and novel device applications. To date, most of these systems employ coupled optical systems of diffraction-limited dielectric waveguides or microcavities, which exchange energy spatially or temporally. Here, we introduce a diffraction-unlimited approach using a plasmon–exciton coupling (polariton) system with tunable plasmonic resonance (energy and line width) and coupling strength. By designing a chirped silver nanogroove cavity array and coupling a single tungsten disulfide monolayer with a large contrast in resonance line width, we show the tuning capability through energy level anticrossing and plasmon–exciton hybridization (line width crossover), as well as spontaneous symmetry breaking across the exceptional point at zero detuning. This two-dimensional hybrid material system can be applied as a scalable and integratable platform for non-Hermitian photonics, featuring seamless integration of two-dimensional materials, broadband tuning, and operation at room temperature.