Owing to the fundamental laws of diffraction, any dielectric system cannot focus light to a spot less than about half a wavelength of light (~λ/2). This diffraction limit imposes a lower limit of ~(/2)3 for the mode volume of a dielectric cavity, where is the wavelength of light inside the dielectric medium. Surface plasmons (SPs) can surpass [overcome，break]this diffraction limit by storing the electromagnetic energy partly in the kinetic energy of free electrons in conductors. This enables metallic nanostructures to concentrate light into deep-subwavelength volumes and to enhance the local density of states, which have propelled their use in a vast array of nanophotonics technologies and research endeavours[it has propelled use in a vast array of nanophotonics technologies and research endeavours because of the unprecedented abilities of nanometallic (that is, plasmonic) structures to concentrate light into deep-subwavelength volumes and to largely enhance the local density of states.]. After a rapid development in the past decades, plasmonics is now covering a broad range of branches, including information technology, biological/chemical sensing, medical therapy, renewable energy, and super-resolution imaging, etc.
Since 2012, our group has been working on the generation and manipulation of light at nanometer scale, and dedicating to explore its potential applications such as on-chip nanophotonic devices, plasmon rulers, biological/chemical sensing, etc. Our research issues[interests] can be subdivided into the following aspects:
1. Integrated NaInnophotonic Devices
Field confinement beyond the diffraction limit enables scaling down of photonic devices and facilitates the integration of nanophotonic devices with nanoelectronic devices, and researches on active and passive opto-electrical devices based on SPs is of epoch-making significance. We have launched experimental researches on electrically driven ultrafast photon sources base on MIM hetero-junctions, as shown in fig. 1a, aims to achieve on-chip integrated light sources with high efficiency. Coherent light source at single particle level based on silver nanorod-insulating layer-CdSe nanobelt (MIS) hetero-structure has been realized as well[1], as shown in fig. 1b. Meanwhile, series of works have been carried out to reduce power dissipation and optimize the performances of plasmonic-based nanophotonic devices[2] (fig. 1c).
Figure 1. (a) Schematics of electrically driven ultrafast photon sources base on MIM hetero-junctions. (b) Mimicking plasmonic nanolaser by selective extraction of electromagnetic near-field from photonic microcavity. (c) Theoretical study of the properties of SPPs in a metallic nanowire over substrate (NWOS) configuration.
2. Nanoscale Spectroscopy and Sensing
Localized surface plasmon resonance (LSPR) spectroscopy of metallic nanoparticles is a powerful technique for chemical and biological sensing experiments. On the one hand, LSPR wavelength is very sensitive to the changes in environmental refractive and gap distance, in this case, ultra-sensitive displacement and refractometirc sensing can be available by utilizing LSPR spectroscopy . In our recent work, cavity plamons in a metal nanowire-on-mirror setup was used to probe vertical dimensional changes with sub-picometer differential resolution[3], as shown in fig. 2a. On the other hand, LSPR is responsible for the electromagnetic field enhancement that leads to surface-enhanced Raman scattering (SERS) and other surface-enhanced spectroscopy, which makes it powerful in probing the near-field (fig. 2b) and detecting extremely trace of molecules (fig. 2c).
Figure 2. (a) Based on the MIM structure composited of metallic nanowire-insulate layer-ultrasmooth metallic film (NWOM), we experimentally evaluated its hypersensitivity functioning as a plasmonic ruler as well as a temperature and humidity sensor. (b) We use layered MoS2 as a two-dimensional lattice probe in nanoparticle-on-mirror nanoantennas to measure the limits of plasmonic enhancement in the gap by quantitative surface-enhanced Raman scattering. (c) The low consumption, multiplex, highly sensitive immunoassay is achieved by combination of surface chemistry, plasmon enhanced fluorescence and microdispersion techniques. Further study on integration of automatic arms and microfluidics will reduce the detection time from hours to minutes.
3. Physics of Light-Matter Interaction
The interaction between light and matter is a central scientific issue in the field of nanophotonics. In its own right, a deeper understanding of light-matter coupling is important in the design of optoelectronic devices such as lasers, switches, sensors, modulators and detectors. We are interested in plasmon-exciton coupling, especially in the field of strong coupling, and have launched several experimental researches on strong coupling regime based on single-particle/monolayer TMDCs[4]. What’s more, several theoretical works on the influence of plasmons on the elementary excitation (exciton, trion, Cooper pair) in 2D electron gas (2DEG) have been developed as well.
Figure 3. (a) Room-temperature formation of plexcitons with Rabi splittings as large as 49.5 meV is observed in a single silver nanorod on monolayer WSe2. The realization of strong plasmon-exciton coupling by in-situ tuning of the plasmon provides a novel route for manipulation of excitons in semiconductors. (b) We study the intermediate coupling between excitons in monolayer WSe2 and individual nanocavity plasmons, which inherits both of the advantages from weak and strong coupling. Meanwhile, high confinement of the plexcitons is achieved, which boosts the plexcitonic nonlinearity. (c) Theoretical analysis on the interaction between lattice plasmons and elementary excitations in 2DEG. (d) Theoretical works on the influence of localized electromagnetic field on the intermediate quantum structures within a particular nanogap region.
References:
[1] Deng Q, Kang M, Zheng D, et al. Mimicking plasmonic nanolaser emission by selective extraction of electromagnetic near-field from photonic microcavity[J]. Nanoscale, 2018, 10(16): 7431-7439.
[2] Zhang S, Xu H. Optimizing Substrate-Mediated Plasmon Coupling toward High-Performance Plasmonic Nanowire Waveguides[J]. ACS Nano, 2012, 6(9): 8128-8135.
[3] Chen W, Zhang S, Deng Q, et al. Probing of sub-picometer vertical differential resolutions using cavity plasmons[J]. Nature Communications, 2018, 9(1): 801.
[4] Zheng D, Zhang S, Deng Q, et al. Manipulating Coherent Plasmon–Exciton Interaction in a Single Silver Nanorod on Monolayer WSe2[J]. Nano Lett, 2017, 17(6): 3809-3814.
