Session S34: Plasmonics

Molecular transport induced by plasmonic heating of periodic metal structure

Different molecules in mixed solutions (or gases) can be separated from each other by a large gradient of temperature. This phenomenon, so-called Soret effect, is quite important for molecular separation and condensation. Plasmonic heating induced by photo-excited periodic metal structures would be useful for creating such a large gradient of temperature. In this work, we fabricated the periotic gold (Au) triangle structure to achieve the plasmonic heating and used thermotropic liquid crystal (TLC) to detect the temperature. The temperature gradient of \textasciitilde 5K/5$\mu $m was created under the optical excitation (400 -- 440 nm). Then, molecular transport of bromothymol blue (BTB) from a dibutyl phthalate (DBP) solution was examined at the same condition as TLC. Details of the results will be presented on site.   

Optically-controlled plasmonic gate to modify the dielectric environment of semiconductor quantum dots

The detailed coupling between single quantum emitters and nearby plasmonic modes is an open area of research in the field of plasmonics. A lot of research is being done to study direct coupling effects between these two systems. In this talk I will instead focus on experimental aspects which indirectly affect their interaction. In a system of semiconductor InGaAs quantum dots very weakly coupled to a silver plasmonic structure, we observe effects similar to the quantum-confined Stark effect. This suggests the presence of a local dc electrical field generated by the launching of surface plasmon polaritons (SPPs) along the silver/GaAs interface. To understand this effect, we consider the electric field interaction between the SPPs and free carriers in the dielectric from the above band-gap optical excitation, as well as free charge carrier effects in the silver including phenomena such as an enhanced photon drag effect or plasmonic drag.   

Broadband Hot Electron Creation in Gap Plasmon Nanostructures

Plasmonic metallic nanostructures typically exhibit narrow resonances that can be tuned from the visible to infrared through geometry changes. Upon photoexcitation, plasmons rapidly decay to produce hot electrons near the metal's surface through either intra- or interband transitions. These charge carriers initially form a nonthermal energy distribution. Here we demonstrate the spectral and temporal response of both nonthermal and thermal hot electrons excited in a metasurface comprising gold or silver nanoparticles separated from a plasmonic thin film with a dielectric spacer of nanometric thickness. The ultrafast spectral response from hot electrons is shown to extend over 1000 nm from the ultraviolet to near-infrared wavelengths. Through experiment and modeling, we describe how such populations couple to the various optical frequencies and metasurface modes, and the response is shown to occur efficiently down to very low fluence. The extreme broadband and ultrafast response offers improved understanding of nonthermal systems far from equilibrium.   

Room Temperature Ultralow Threshold Plasmonic Nanolasers with Unusual Scaling Laws

Plasmonic nanolasers are a new class of quantum amplifiers that deliver coherent surface plasmons well below the diffraction barrier which brings fundamentally new capabilities to biochemical sensing, super-resolution imaging and on-chip optical communication. However, there is always a trade-off between field confinement and metallic absorption loss which has led to a long standing debate about whether metals could eventually enhance the performance of a laser. Particularly at room temperature, plasmonic nanolasers still face extremely high thresholds from MW cm$^{\mathrm{-2}}$ up to GW cm$^{\mathrm{-2}}$ preventing their practical usage. Here, we report a room temperature plasmonic nanolaser with record low threshold on the order of 10 KW cm$^{\mathrm{-2}}$ corresponding to a pump density in the range of modern laser diodes. We find unusual scaling laws that allow plasmonic lasers to be more compact and faster with lower threshold and power consumption than photonic nanolasers when the cavity size approaches or surpasses the diffraction limit. This provides unambiguous evidence that plasmonic lasers have superior performance over photonic laser at the nanoscale. In addition to clarifying this long standing debate, ultralow threshold nanolasers may lead to low-power-consumption nanophotonic circuitry and ultrasensitive biochemical sensors.   

Remote heating in Au bowtie constrictions by propagating plasmons

Gold bowtie nanowires attached to larger electrodes are convenient devices for combining electronic and optical measurements, because they can be modified to form plasmonically-active junctions that serve as surface-enhanced Raman scattering (SERS) substrates with single-molecule sensitivity. Direct optical excitation of the molecular junction by focused incident laser can cause a dramatic temperature increase in the metal, resulting in molecular configuration instability and breakdown. Adding metallic gratings to the electrodes of these devices allows for remote excitation of the junction. By shining light on the gratings, propagating plasmon modes travel to and remotely excite the junction with far less heating than direct excitation. These plasmons have propagation lengths over 10 microns and decrease the overall heating of the junction by over 70\% as measured with a bolometric detection method. We will discuss how this simple addition to the electrode design allows for reliable remote heating of the constriction via plasmon propagation and its potential use in low-temperature, single-molecule SERS measurements.   

Dependence of plasmon-induced hot-carrier properties on nanoparticle size, material and environment.

Plasmon-induced hot electron processes in metallic nanostructures are currently intensely investigated because of their potential for technological applications in photocatalysis, photodetection and solar energy harvesting. An accurate theoretical description of these phenomena is required to guide experimental progress and device design, but developing such a theory is challenging because of the need to combine a description of their optical properties with a theory of their electronic structure. To address this challenge, we employ a two-step procedure proposed by Manjavacas et al.[1]: after solving Maxwell’s equations for the optical properties of the spherical nanoparticle, we determine hot-electron generation rates by evaluating Fermi’s golden rule within a jellium approximation. We use this approach to compute hot carrier properties in different materials (Li, Na, K, Al, Cu, Ag, Au) and study their dependence on nanoparticle size and the dielectric constant of the environment. [1] A. Manjavacas, J. G. Liu, V. Kulkarni, and P. Nordlander, ACS Nano 8, 7630 (2014), pMID: 24960573, http://dx.doi.org/10.1021/nn502445f   

Surface Plasmon Polariton-Assisted Long-Range Exciton Transport in Monolayer Semiconductor Lateral Heterostructure

Recently, two-dimensional (2D) semiconductor heterostructures, i.e., atomically thin lateral heterostructures (LHSs) based on transition metal dichalcogenides (TMDs) have been demonstrated. In an optically excited LHS, exciton transport is typically limited to a rather short spatial range (\textasciitilde 1 micron). Furthermore, additional losses may occur at the lateral interfacial regions. Here, to overcome these challenges, we experimentally implement a planar metal--oxide--semiconductor (MOS) structure by placing a monolayer of WS2/MoS2 LHS on top of an Al2O3 capped Ag single-crystalline plate. We found that the exciton transport range can be extended to tens of microns. The process of long-range exciton transport in the MOS structure is confirmed to be mediated by an exciton--surface plasmon polariton--exciton conversion mechanism, which allows a cascaded energy transfer process. Thus, the planar MOS structure provides a platform seamlessly combining 2D light-emitting materials with plasmonic planar waveguides, offering great potential for developing integrated photonic/plasmonic functionalities.   

Modeling non-locality of plasmonic excitations with a fictitious film

Non-local effects, requiring a wavevector ($q)$ dependent dielectric response are becoming increasingly important in studies of plasmonic and metamaterial structures. The phenomenological hydrodynamic approximation (HDA) is the simplest, and most often used model, but it often fails. We show that the $d$-function formalism [1], exact to first order in $q$, is a powerful and simple-to-use alternative. Recently, we developed a mapping of the $d$-function formalism into a purely local fictitious film [2]. This geometric mapping allows for non-local extensions of any local calculation scheme, including FDTD. We demonstrate here, that such mapped FDTD simulation of metallic nanoclusters agrees very well with various experiments. [1] P. J. Feibelman, \textit{Progress in Surface Science} 12, 287 (1982); A. Liebsch, \textit{Physical Review B} 48, 11317 (1993). [2] A. J. Shvonski, J. Kong and K. Kempa, \textit{Physical Review B} (\textit{in review}).   

Optimization of dual-width plasmonic nanogap gratings with substrate thickness for optical enhancement applications.

Dual-width plasmonic gratings have been previously shown to demonstrate additional light enhancement over standard single-width gratings. This benefit is further increased by utilizing sub-10 nm gaps between the grating structures. Optimization of the grating geometry for specific combinations of incident light wavelength and polarization as well as different gratings materials is crucial to fabricating ideal grating devices for light enhancement applications. Geometric parametric studies have been conducted, but substrate effects have not yet been thoroughly investigated. This work calculates the effect of varying SiO2 thickness on a Si substrate containing a nanogap dual-width Au plasmonic grating with a Ti adhesion layer, as has been shown possible to fabricate in previous work. Computational results will potentially be confirmed with experimental measurements such as surface-enhanced Raman spectroscopy, dark field scattering spectroscopy, or cathodoluminescence.   

In-plane directivity of a plasmonic wireless communication system

On-chip communication is important for many future technologies. While present information transfer rates are high enough to perform some communication, there remain barriers to overcome. We recently reported the first nanoscale wireless communication system (nWCS) driven by plasmonic antennas in the visible spectrum that can perform communication faster than present on-chip technologies [1]. Toward optimizing this system to improve performance, antenna directivity is one of the most important parameters, since this influences transfer efficiency. Here, we report directivity measurements on an nWCS using visible light. Our findings are consistent with antenna theory, and suggest that manipulation of the polarization of incident light is important in order to obtain maximum directivity. Also, due to the plasmonic nature of the antennas, fabrication defects have important impact on the in-plane emitted signal, affecting the overall performance of the system. Finally, we suggest alternative designs for such plasmonic antennas [2]. [1] J.M. Merlo, \textit{et al.} Sci. Rep. 6, 31710; doi: 10.1038/srep31710 (2016). [2] J.M. Merlo, \textit{et al.} (manuscript in preparation).   

Spectrally-selective THz plasmonic sensor based on thin InSb layer with metallic gratings

We present a novel terahertz plasmonic sensor based on metallic grating on micrometer-thin InSb layer. Two strong absorption modes are found computationally in the transmission spectra and we interpret them as standing surface plasmon modes by investigating the dispersion relations and the electric field spatial distribution of these two modes. We show that the dispersion of the plasmon modes is well explained by the air/InSb/air trilayer theory of surface plasma waves. This structure is very sensitive to the dielectric environment and the analyte (eg. lactose) at the InSb interface, which shows the capability as a terahertz plasmonic sensor. The sensitivity is determined to exceed 0.2 THz per refractive index unit. We demonstrate the splitting of the plasmonic mode as we tune it in resonance with the lactose vibrational mode at 1.37THz, which indicates that the structure is a potential candidate for a new sensing modality that allows direct detection of terahertz vibrational fingerprints of an analyte.   

Quantum and Classical Plasmonic Phenomena in Nanoparticle Arrays

Using both classical and quantum approaches, we model plasmonic phenomena in nanoparticle (NP) dimers and trimers. Using a model of three nanoparticles, we propose a mechanism of non-dissipative and ultrafast plasmon passage assisted by hot spots. For this, the NP trimer should include two Au-NPs and one Ag-NP. In the Au-Ag-Au trimer, the two Au-plasmons become coupled via the virtual plasmon of the Ag-NP. The efficient and ultra-fast passage of the Au-plasmons assisted by the virtual Ag-plasmon only becomes possible when the inter-NP gaps in the trimer are small. In this coupling regime, the inter-NP gap regions become plasmonic hot spots that greatly enhance the plasmonic passage effect. At this moment, the plasmonic passage phenomenon was already observed experimentally using optical spectroscopy and the DNA-origami NP complexes. Other systems of our interest were a NP dimer and a nanostar with plasmonic hot spots. For those systems, we predict strong enhancement of the generation of energetic (hot) carriers [1,2]. [1] Besteiro, L.V.; Govorov, A.O. \textit{J. Phys. Chem. C} \textbf{120}, 19329 (2016). [2] Kong, X.-T.; Wang, Z.; Govorov, A.O. \textit{Adv. Optical Mater.}, doi: 10.1002/adom.201600594.