We introduce Plasmene- in analogy to graphene-as free-standing, one-particle-thick, superlattice sheets of nanoparticles ("meta-atoms") from the "plasmonic periodic table", which has implications in many important research disciplines. Here, we report on a general bottom-up self-assembly approach to fabricate giant plasmene nanosheets (i.e., plasmene with nanoscale thickness but with macroscopic lateral dimensions) as thin as ∼40 nm and as wide as ∼3 mm, corresponding to an aspect ratio of ∼75,000. In conjunction with top-down lithography, such robust giant nanosheets could be milled into one-dimensional nanoribbons and folded into three-dimensional origami. Both experimental and theoretical studies reveal that our giant plasmene nanosheets are analogues of graphene from the plasmonic nanoparticle family, simultaneously possessing unique structural features and plasmon propagation functionalities.

The self-assembly of monodisperse inorganic nanoparticles into highly ordered arrays (superlattices) represents an exciting route to materials and devices with new functions. It allows programming their properties by varying the size, shape, and composition of the nanoparticles, as well as the packing order of the assemblies. While substantial progress has been achieved in the fabrication of superlattice materials made of nanospheres, limited advances have been made in growing similar materials with anisotropic building blocks, which is particularly true for free-standing two-dimensional superlattices. In this paper, we report the controlled growth of free-standing, large-area, monolayered gold-nanorod superlattice sheets by polymer ligands in an entropy-driven interfacial self-assembly process. Furthermore, we experimentally characterize the plasmonic properties of horizontally aligned sheets (H-sheets) and vertically aligned sheets (V-sheets) and show that observed features can be well described using a theoretical model based on the discrete-dipole approximation. Our polymer-ligand-based strategy may be extended to other anisotropic plasmonic building blocks, offering a robust and inexpensive avenue to plasmonic nanosheets for various applications in nanophotonic devices and sensors.

We present an improved analytical model describing transmittance of a metal-dielectric-metal (MDM) waveguide coupled to an arbitrary number of stubs. The model is built on the well-known analogy between MDM waveguides and microwave transmission lines. This analogy allows one to establish equivalent networks for different MDM-waveguide geometries and to calculate their optical transmission spectra using standard analytical tools of transmission-line theory. A substantial advantage of our model compared to earlier works is that it precisely incorporates the dissipation of surface plasmon polaritons resulting from ohmic losses inside any metal at optical frequencies. We derive analytical expressions for transmittance of MDM waveguides coupled to single and double stubs as well as to N identical stubs with a periodic arrangement. We show that certain phase-matching conditions must be satisfied to provide opt al filtering characteristics for such waveguides. To check the accuracy of our model, its results are compared with numerical data obtained from the full-blown finite-difference time-domain simulations. Close agreement between the two suggests that our analytical model is suitable for rapid design optimization of MDM-waveguide-based compact photonic devices.

Anisotropic plasmonic nanoparticles have been successfully used as constituent elements for growing ordered nanoparticle arrays. However, orientational control over their spatial ordering remains challenging. Here, we report on a self-assembled two-dimensional (2D) nanoparticle liquid crystalline superstructure (NLCS) from bipyramid gold nanoparticles (BNPs), which showed four distinct orientational packing orders, corresponding to horizontal alignment (H-NLCS), circular arrangement (C-NLCS), slanted alignment (S-NLCS), and vertical alignment (V-NLCS) of constituent particle building elements. These packing orders are characteristic of the unique shape of BNPs because all four packing modes were observed for particles with various sizes. Nevertheless, only H-NLCS and V-NLCS packing orders were observed for the free-standing ordered array nanosheets formed from a drying-mediated self-assembly at the air/water interface of a sessile droplet. This is due to strong surface tension and the absence of particle-substrate interaction. In addition, we found the collective plasmonic coupling properties mainly depend on the packing type, and characteristic coupling peak locations depend on particle sizes. Interestingly, surface-enhanced Raman scattering (SERS) enhancements were heavily dependent on the orientational packing ordering. In particular, V-NLCS showed the highest Raman enhancement factor, which was about 77-fold greater than the H-NLCS and about 19-fold greater than C-NLCS. The results presented here reveal the nature and significance of orientational ordering in controlling plasmonic coupling and SERS enhancements of ordered plasmonic nanoparticle arrays.

introducing abrupt phase changes via an ultrathin sheet of subwavelength resonators (usually known as meta-atoms). The first such metasurface was proposed by Yu and Capasso [2] Afterward, significant number of novel metasurfaces have been proposed in microwave, [3] terahertz, [4] near-infrared, [5] and visible ranges. [6] These metasurfaces are utilized for realizing many applications such as focusing with subwavelength planar lenses, [7] optical cloaking, [8,9] generating nondiffracting beams, [10] manipulating phase profiles having orbital angular momentum (OAM), [11] photon spin Hall effect, [12] and performing computation with the coding metasurfaces. [13,14] Although metasurfaces have proved their unique freedom in wavefront manipulation, most metasurfaces generally suffer from low efficiencies. This is especially critical for transmissive metasurfaces where both the magnetic and electric resonances should be precisely controlled. For example, the efficiency of the first ever demonstrated metasurface was only 5%. [2] A few transmissive metasurfaces have since been introduced with higher efficiencies based on the Huygens' principle [15,16] or meta-transmit arrays. [17,18] However, the operation of these metasurfaces is severely restricted, e.g., may require specific polarization, added complexity in manufacturing, or demand complex, costly computations for each distinct phase.Recently, it has been demonstrated that metasurfaces can be realized based on the photon spin Hall effect. [19][20][21][22] Spin Hall effect is a phenomenon where moving electrons with opposite spins can be transversely separated. Spin Hall effect can be intrinsic based on spin-orbit coupling of electrons [23] or extrinsic due to the spin-dependent scatterings by impurities. [24] Both of these could be utilized to achieve extraordinary wave propagation. It is known that the experimentally observed intrinsic phenomena are usually very weak. [23] On the contrary, extrinsic spin Hall is observed in a special class of metasurfaces, [12] which utilizes subwavelength scatterers to achieve full phase control in the range of 0-2 π. The transmission/ reflection phase control of a cross-polarized wave is achievable through the rotation of the designed scatterers, which in fact make it more promising than the other types of meta-atoms that require phase dispersions to achieve phase control. The maximum achievable efficiency of such metasurfaces was theoretically predicted to be 25% in a single layer structure since only electric responses can be realized in such structures. [25,26] This limitation was overcome by designing reflective metasurfaces with ground planes, [27][28][29] where both the electric and the Metasurfaces offer unprecedented freedom to manipulate electromagnetic waves at deeply subwavelength scales. However, realizing a highly efficient metasurface, yet simple enough to conceptualize, design, and fabricate, is a challenging task. In this paper, a novel approach is proposed for designing meta-atoms which can achieve full phase cont...

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We systematically investigated the size- and shape-dependent SERS activities of plasmonic core-shell nanoparticles towards detection of the pesticide thiram. Monodisperse Au@Ag nanocubes (NCs) and Au@Ag nanocuboids (NBs) were synthesized and their Ag shell thickness was precisely adjusted from ∼1 nm to ∼16 nm. All these nanoparticles were used as SERS substrates for thiram detection, and the Raman intensities with three different lasers (514 nm, 633 nm and 782 nm) were recorded and compared. Our results clearly show that: (1) the excitation wavelength discriminated particle shapes regardless of particle sizes, and the maximized Raman enhancement was observed when the excitation wavelength approaches the SERS peak (provided there is significant local electric field confinement on the plasmonic nanostructures at that wavelength); (2) at the optimized laser wavelength, the maximum Raman enhancement was achieved at a certain threshold of particle size (or silver coating thickness). By exciting particles at their optimized sizes with the corresponding optimized laser wavelengths, we achieved a detection limit of roughly around 100 pM and 80 pM for NCs and NBs, respectively.

COMMUNICATIONstronger than those obtained from a commercial Klarite SERS substrate, which was rigid, opaque, and could not be used for direct chemical identifi cation on banknotes or coins.A combination of polymer-ligand-based strategy and dryingmediated air-water interfacial self-assembly was utilized for the fabrication of plasmene nanosheets with Au@Ag nanocubes (NCs) as model building blocks. [7][8][9] The physical steps towards obtaining a free-standing plasmene nanosheet are demonstrated in Figure 1 a. In brief, a concentrated drop of polystyrene (PS)-capped NCs is spread and allowed to solidify into a silvery refl ection on the surface of a water droplet. Subsequent slow evaporation resulted in plasmene nanosheets that covered almost the entire grid. One striking feature of our plasmene nanosheets is their high mechanical fl exibility, which allows them to be transferred using a high-fi delity, polydimethylsiloxane (PDMS) elastomer. This enables their use as powerful "SERS adhesive" for chemical identifi cation of trace amount of chemicals on solids of different materials with complex surface structures.To prove this exciting capability, 4-aminothiophenol (4-ATP) was used as the model target analyte because of its strong affi nity to silver and its apparent, large SERS signal. For this 40 µL of 4-ATP (1 µM, corresponding to around 4 ng cm −2 ) drops were deposited and allowed to dry onto the surface of respectively a Malaysian banknote (fi brous), an Australian banknote (polymer), and an Australian coin (rough metal) before PDMS-mediated stamping of the plasmene nanosheets. The transferred sheets had intimate contact with these complex surfaces because of their high fl exibility and robustness, and they enabled a substantial signal enhancement in the SERS detection of the trace amounts of 4-ATP molecules (Figure 1 b-d). The characteristic Raman peaks for 4-ATP could not be observed on the plain and ATP-covered solid surfaces (blue curves and black curves), but they became evident for the surfaces with attached plasmene nanosheets (red curves). This demonstrates a simple non-destructive manner for chemical identifi cation on non-planer surfaces and we believe that the observed signalenhancing capability can be potentially extended to any other complex solid surfaces.We further systematically investigated how the size and shape of the constituent nanoparticles could infl uence the SERS enhancement. We deliberately synthesized six plasmonic building blocks with different geometries (Supporting Information, Section 3.1) -small-(s-), medium-(m-), large-(l-) nanocubes (NCs) and nanobricks (NBs), which were used to produce six different sheets with the same capping ligands of identical molecular lengths. As opposed to NC building blocks, which tend to pack side by side with a regularity imposed by its highly ordered symmetrical nature, isotropic NBs are aligned horizontally with random packing directions to form unique, Plasmonic nanoparticles have become a prominent research subject in the fi eld of chemical ide...