Session Z2: Plasmonic Nanogaps: From Single Molecule Sensing to Light Manipulation and Beyond

Emission and propagation properties of surface plasmons on metal nanowires

Manipulating light on the nanometer scale is a challenging topic not only from a fundamental point of view, but also for applications aiming at the design of miniature optical devices. Nanoplasmonics is a rapidly emerging branch of photonics, which offers variable means to manipulate light using surface plasmon excitations on metal nanostructures. Here we report our recent studies about emission and propagation properties of surface plasmons on metal nanostructures. For the propagating properties, we found the propagating plasmons can remotely excite surface enhanced Raman scattering at a few molecules level, and excite the excitons of quantum dots directly. For the emission properties, we observed that light from the end of a silver nanowire, following excitation of plasmons at the other end of the wire, is emitted in a cone of angles peaking at nominally 45-60 degrees from the nanowire axis, with virtually no light emitted along the direction of the nanowire. This surprising characteristic can be explained in a simple picture invoking Fabry- P\'erot resonances of the forward and back-propagating plasmons on the nanowire.   

Quantum description of plasmons in strongly coupled metallic nanostructures

The plasmonic couplings between closely positioned metallic nanoparticles can induce extraordinary large electric field enhancements in the junctions between the particles of relevance for surface enhanced spectroscopies such as SERS.[1] Such plasmonic couplings can also lead to plasmonic interference and coherence effects that manifest themselves as narrow Fano resonances in the optical spectra with extraordinary sensitivities to their dielectric environment.[2] Until very recently, the modeling of the plasmonic response of closely coupled metallic nanoparticles has been made using classical approaches neglecting quantum mechanical effects such as electron tunneling between the particles and screening due to the finite electron density in the junction. In this talk we will present a fully quantum mechanical investigation of the plasmonic response of two coupled metallic nanoparticles as a function of interparticle separation.[3] We identify three distinct regimes of interaction. In the classical regime for separations larger than 1 nm, the nanoparticles remain neutral and the plasmonic response is well described using classical theory. In the cross-over regime for separations between 0.5 and 1nm, electrons begin to tunnel between the nanoparticles and a reduction of the plasmonic couplings and field enhancements result. In the conductive regime for separations smaller than 0.5nm, a large conductive overlap is established between the two particles and a blue-shifted Charge Transfer Plasmon (CTP) emerges.[4] The CTP is a collective plasmon mode which both includes a polarization of the electron distribution of each individual nanoparticle and a significant electron current between the two particles. [1] F. Le et al., ACS Nano 2(2008)707-718 [3] N.A. Mirin, K. Bao, and P. Nordlander, J. Phys. Chem. A 113 (2009)4028-4034 [3] J. Zuloaga, E. Prodan, and P. Nordlander, Nano Lett. 9(2009) 887-891 [4] J.B. Lassiter et al., Nano Lett. 8(2008)1812-1816   

The nano-gap and the emitting molecule: Control of polarization and spectral shape

The realization of single-molecule surface-enhanced Raman scattering (SERS) from molecules positioned within nano-gaps between metallic nanopraticles has opened up exciting opportunities for studying plasmonic fields and their effects on quantum emitters. We recently showed that constructs made of pairs of nanoparticles with an individual molecules bridging their gap can be systematically formed and studied [1]. By changing the size of the particles, we were able to tune the position of the plasmon resonance spectrum, so that the overlap with different parts of the molecular Raman spectrum changed, leading to significant modulation of its shape. More intricate control over molecular properties can be achieved if a third particle is added to the contstruct. It was found that by breaking the dimer symmetry, a third particle can couple strongly to the emitted Raman field and modulate its polarization in a wavelength-dependent fashion [2]. This surprising experimental result was backed up by a series of Generalized Mie calculations, showing the effect of the distance of the third particle, its size and position [3]. Interestingly, the refractive index of the surrounding medium serves as another control parameter that allows changing the coupling between the particles and modulating the polarization of emitted light. \\[4pt] [1] Dadosh T, et al. (2009) Plasmonic Control of the Shape of the Raman Spectrum of a Single Molecule in a Silver Nanoparticle Dimer. Acs Nano 3:1988-1994. \\[0pt] [2] Shegai T, et al. (2008) Managing light polarization via plasmon-molecule interactions within an asymmetric metal nanoparticle trimer. Proc Natl Acad Sci USA 105:16448-16453. \\[0pt] [3] Li ZP, Shegai T, Haran G, Xu HX (2009) Multiple-Particle Nanoantennas for Enormous Enhancement and Polarization Control of Light Emission. Acs Nano 3:637-642.   

Accurate tuning of the electronic coupling and emergent magnetic properties of metal nanoparticle dimers from the linear to nonlinear dielectric-response regime

Closely-packed nanoparticle aggregates have sparked great interest in recent years due to their potential applications in new functional materials and devices at the nanoscale. Experimentally, it has been demonstrated that the properties of such aggregates crucially depend on the electronic coupling between nanoparticles. This coupling originates from the overlap of electronic wavefunctions between neighboring particles, thus requiring a full quantum-mechanical treatment. In this talk, I will discuss the tuning of the electronic coupling via particle separation and external electric field, as well as its effect on the dielectric and magnetic properties of a nanoparticle dimer system [1,2]. Using atomistic real-space first-principles calculation, we find that there is an optimal separation at which the static polarizability reaches its maximal value. Such a peak structure is completely missing in the classical electromagnetic theory, and is associated with the ``bond''-breaking process between the two nanoparticles. In some systems, the electronic coupling can be strong enough to give rise to a net magnetic moment of the dimer, even though the isolated nanoparticles are nonmagnetic. Furthermore, we show that the electronic coupling can be tuned by a modest electric field, resulting in an electric-field tunable magnetic moment. We discuss these results in the context of spin-polarized molecular transport, nanoscale multiferroics, and nanoplasmonics. [1] K. Zhao, M. C. Troparevsky, D. Xiao, A. G. Eguiluz, Z. Y. Zhang, Phys. Rev. Lett. 102, 186804 (2009). [2] M. C. Troparevsky, K. Zhao, D. Xiao, Z. Y. Zhang, and A. G. Eguiluz, ``Tuning the Electronic Coupling and Magnetic Moment of a Metal Nanoparticle Dimer in the Nonlinear Dielectric-Response Regime'', to be published in Nano Lett.   

High harmonic generation by surface plasmon resonance: Design of plasmonic devices and their applications

Seung-Woo Kim has been researching femtosecond ultrafast optics for ultraprecision manufacturing technologies including EUV and X-ray generation. Recently, he and his colleagues achieved a novel method of high-harmonic generation by exploiting the local field enhancement in the nanogap induced by resonant plasmons within a metallic nanostructure consisting of bow-tie shaped gold elements on a sapphire substrate. Plasmonic gold elements enhance the pulse intensity enough to induce high harmonic generation with no extra cavities at all. By injection of argon and xenon gas jets onto bow-tie nanostructures, high harmonics up to 21st (38 nm) order were produced while the incident laser intensity remained only 10$^{11}$ Wcm$^{-2}$. Other nanostructures such as tapered cones are now being investigated to construct laptop-sized coherent EUV sources for advanced lithography and high resolution imaging applications.