Nanophotonic resonators can confine light to deep-subwavelength volumes with highly enhanced near-field intensity and therefore are widely used for surface-enhanced infrared absorption spectroscopy in various molecular sensing applications. The enhanced signal is mainly contributed by molecules in photonic hot spots, which are regions of a nanophotonic structure with high-field intensity. Therefore, delivery of the majority of, if not all, analyte molecules to hot spots is crucial for fully utilizing the sensing capability of an optical sensor. However, for most optical sensors, simple and straightforward methods of introducing an aqueous analyte to the device, such as applying droplets or spin-coating, cannot achieve targeted delivery of analyte molecules to hot spots. Instead, analyte molecules are usually distributed across the entire device surface, so the majority of the molecules do not experience enhanced field intensity. Here, we present a nanophotonic sensor design with passive molecule trapping functionality. When an analyte solution droplet is introduced to the sensor surface and gradually evaporates, the device structure can effectively trap most precipitated analyte molecules in its hot spots, significantly enhancing the sensor spectral response and sensitivity performance. Specifically, our sensors produce a reflection change of a few percentage points in response to trace amounts of the amino-acid proline or glucose precipitate with a picogram-level mass, which is significantly less than the mass of a molecular monolayer covering the same measurement area. The demonstrated strategy for designing optical sensor structures may also be applied to sensing nano-particles such as exosomes, viruses, and quantum dots.

The photonic spin Hall effect (PSHE) is a promising candidate for controlling the spin states of photons and exploiting next-generation photonic devices based on spinoptics. Herein, the influences of a perpendicular magnetic field on the PSHE appearing on the surface of monolayer black phosphorus (BP) are investigated. Results reveal that both the in-plane and transverse spin-dependent shifts are quantised and show an oscillating pattern due to the splitting of Landau levels (LLs) induced by the external magnetic field B. And the oscillation period of spin Hall shifts gradually increases with strengthening B because of the increase of LL spacings. By contrast, for a fixed magnetic field, as the LL spacings become smaller and smaller with increasing the LL index, the oscillation period of spin Hall shifts gradually decreases as the photonic energy increases. Moreover, it is possibly due to the synergistic role of intrinsic anisotropy, high crystallinity, and quantisation-incurred localised decreases in beating-like complex conductivities of the BP film, giant spin Hall shifts, hundreds of times of the incident wavelength, are obtained in both transverse and in-plane directions. These unambiguously confirm the strong impact of the external magnetic field on the PSHE and shed important insights into understanding the rich magneto-optical transport properties in anisotropic two-dimensional atomic crystals.

applications. As molecular vibrational absorption is generally proportional to the intensity of electric field experienced by the molecules, [2] surface-enhanced infrared absorption (SEIRA) sensors based on a variety of resonant nanophotonic structures have been demonstrated to significantly enhance molecular vibrational absorption and consequently achieve high sensitivity performance. [3] Resonant nanophotonic structures excited by incident light can confine highly enhanced electric field in deepsubwavelength regions, known as "hot spots," in which the interactions between light and analyte molecules can be drastically enhanced, leading to significant improvement of the sensing performance. Designing nanophotonic structures with smaller hot spots is a widely utilized strategy to increase field enhancement and improve sensor performance. A variety of nanophotonic structures with nanometric gaps have been demonstrated for sensing applications, such as dimer antennas, [4,5] split-ring resonators, [6] coaxial disk resonators, [7,8] and nanopatch antennas. [9][10][11] However, with the gap size decreasing down to the nanometric scale, it becomes increasingly difficult to deliver analyte molecules into these gaps (i.e., the hot spots), especially when the gap size is comparable to typical sizes of molecules. This issue fundamentally limits the further performance improvement of nanophotonic sensors. An effective approach to addressing this issue is to deliver the analytes before forming the nanometric gaps (or other types of hot spot structures). For example, metallic nanoparticles coated with analyte thin films can form supercrystals with nanometric separation gaps. [12,13] In a recent study, such supercrystals of gold nanoparticles coated with thiolated poly styrene molecules were demonstrated to function as SEIRA sensors for sensing the polystyrene molecules with high performance. [13] Another previous demonstration of SEIRA sensors based on graphene acoustic plasmon resonators realized effective delivery of analytes into nanometric gaps by first spin-coating a thin analyte film on gold nanoribbons, and subsequently transferring graphene onto the analyte film, which led to the successful detection of SEIRA signals from sub-nanometerthick analyte films. [14] Nevertheless, assembling supercrystals of metal nanoparticles or transferring graphene to form the complete sensor structures is not a simple and straightforward process, and hence may not be suitable for point-of-care applications.Surface-enhanced infrared absorption (SEIRA) spectroscopy can provide label-free, nondestructive detection and identification of analytes with high sensitivity and specificity, and therefore has been widely used for various sensing applications. SEIRA sensors usually employ resonant nanophotonic structures, which can substantially enhance the electric field and hence light-matter interactions by orders of magnitude in certain nanoscale hot spots of the devices. However, as ever, smaller hot spots are employed to further enhance the fi...

Surface plasmons in graphene have great potential for molecular sensing applications thanks to their exceedingly high sensitivity to environmental changes. Here, we demonstrate a type of hybrid graphene–metal metasurface that supports strong graphene plasmonic resonances in the terahertz range. Each unit cell of such a hybrid metasurface consists of a graphene antidot enclosing a metal disk realized using a self-aligned photolithography process. This hybrid design combines the advantages of both graphene- and metal-based photonic structures, leading to ∼3 times stronger tunable plasmonic resonances and an order of magnitude larger near-field intensity enhancement with respect to those of bare graphene antidot metasurfaces.

Plasmonic dark modes are not easy to be observed in the far field due to their weak photon emission. By contrast, it has been shown that a dark mode can be excited effectively by a near-field source such as an electron beam. In this Letter, we show theoretically that the photon emission from the monopole-like dark mode supported on a plasmonic nano-disc could be unexpectedly strong when excited by an electron beam through its hole. Even though this monopole mode is considered to be dark, it is found that the emission can be even "brighter" than the dipolar bright modes when the electron speed is higher than 0.6c. Due to the high conversion efficiency from electron energy loss to photon energy, the results could also suggest an optical method for the detection of high-energy electrons passing through the hole with negligible changes in electron speed.