Engineering spontaneous emission of fluorophores using metallic nanostructures is among the most prosperous and promising research fields in plasmonics [1-3]. The capability to confine light within a nanoscale volume is of major importance to enable efficient change in the local electromagnetic environment, thus providing enhancement  or suppression  of the fluorescent emission. Metallic nanostructures such as single nanoparticles [6, 7], metallic pairs [8-10], and periodic metallic arrays , have been fabricated to provide enhanced fluorescent emission using localized surface plasmon (LSP). Furthermore, coupling between nanoscale metallic elements or arrays also provides a variety of localized electromagnetic modes, such as gap mode [12-14], lattice mode [15, 16], and magnetic plasmon polaritons (MPPs) [17-20]. The control of fluorescence via multi-mode resonances in plasmonic metasurface has been widely studied. These plasmon-enhanced fluorescence and absorption processes for molecules have an extensive range of potential applications, including optical sensors [21, 22], biological imaging [23, 24], light-emitting devices [25, 26], and solar energy harvesters .
To provide prominent enhancement, the size or gap of a nanoantenna structure must be carefully adjusted to ensure that the plasmonic resonance wavelength matches the fluorescent radiation spectrum. Coupling with plasmonic mode can modify efficiency , polarization  and directivity  of the fluorescent emission. Typical works in this field emphasize only the control of the emission process with plasmonic resonance [31, 32]. However, for photoluminescence, coupling between the pump laser beam and the plasmonic resonance also contributes to enhancement of fluorescent radiation . One possible way to enhance both excitation and radiation of the emitter is to adopt a broad plasmonic resonance that overlaps the emitter's excitation and radiation spectra simultaneously . Another preferable solution is to design metallic nanostructures that have multiple resonance modes corresponding to the fluorescence absorption and emission spectra , which will then provide greater freedom for manipulation of the radiation of fluorescent emitters.
In this work, the photoluminescence of Nile Red molecules buried in metal-dielectric-metal (MDM) fishnet nanostructures is enhanced by matching both the excitation and radiation wavelengths with the LSP and MPP modes, respectively. We demonstrate that the spatial electromagnetic distributions of these two modes are of major importance to acquire enhanced fluorescent emission. Because the spatial distribution of LSP mode is dependent on the polarization of excitation laser, the fluorophores can be spatially-selectively excited by rotating the polarization of the pump laser beam; therefore, the wavelength and the polarization of the enhanced fluorescence can be manipulated using MPP mode. The results of finite-difference time-domain (FDTD) simulations of the electromagnetic characteristics of the MDM fishnet metasurface support all the experimentally measured results.Ⅱ. SAMPLE PREPARATION
FIG. 1(a) shows a schematic of the MDM fishnet metasurface used to manipulate the photoluminescence of Nile Red molecules sandwiched in the gap. The MDM fishnet metasurface was fabricated by perforation of rectangular holes in MDM films that were deposited on a glass substrate (the refractive index of glass substrate is 1.518). The top and bottom silver films, which have the same thickness
As previously demonstrated, multiple plasmonic resonant modes occur in MDM fishnet metasurfaces, including LSP, MPP, and surface plasmon polariton-Bloch waves (SPP-BWs). The resonant wavelengths of these modes can be tuned by modifying the geometric sizes of the fishnet lattice structure. To identify the resonant modes of the lattice sample (FIG. 1(b)), polarization-dependent transmittance spectra were measured in visible wavelength between 500 nm and 800 nm using a local spectra detection setup. Linearly-polarized white light was used to provide normal illumination of the sample. The transmitted light was then collected using an objective lens (60
The presence of these multiple resonant modes is further corroborated using three-dimensional finite-difference time-domain simulation (FDTD solutions, Lumerical Solutions Ltd.). In this case, the dielectric coefficient used for Ag was taken from Palik's data (0-2
A 532 nm linearly polarized laser was used to pump the device. A polarizer and a half-wavelength plate were inserted in front of the MDM fishnet device to control the polarization and maintain constant beam intensity for the incident light. The light was then focused normal on the plane of the sample using an objective lens (10
To clarify the physics behind these enhanced spectra, we conducted FDTD simulations to disclose the excitation and emission coupling behavior between the localized electromagnetic field and the fluorescent emitters. When the broad spectrum of LSP mode is taken into consideration, it is reasonable to expect that the incident 532 nm laser light will induce LSP mode in the MDM fishnet structure. Therefore, the absorption of fluorescent emitter is enhanced by the localized electromagnetic field that occurs in the gap. The electric field distributions at excitation wavelength of 532 nm are inspected at the
This coupling between plasmonic modes and fluorescent emitters in the MDM fishnet structure can be further corroborated through investigation of the polarization characteristics of the fluorescent emission. To excite all the fluorescent molecules homogeneously within the gap of the MDM fishnet lattice, the device was illuminated using 532 nm laser light with a polarization orientation angle
To understand these results, we simulated the emission process. Because the localized electric field within the gap at 532 nm is dominated by the
In conclusion, this work demonstrates that the absorption and emission properties of fluorescent emitters can be enhanced simultaneously using two independent plasmonic modes in an MDM fishnet structure. This double enhancement produces more intense photoluminescent radiation. More importantly, the coupling of the two independent plasmonic modes enables manipulation of the excitation and emission properties of the fluorescent emitters. Use of specific spatial distributions of LSP mode for different polarizations allows the fluorescent molecules dispersed within the gap of the MDM fishnet structure to be excited selectively, which thus enables shaping of the spectrum via tuning of the polarization of excitation laser beam. In addition, the resulting polarization-dependent multimode-mediated emission enables selectivity in terms of both radiation wavelength and polarization. This work opens up the possibility of generation of color light-emitting diodes or laser sources on the nanoscale for use in photonic integrated circuits at visible frequencies.Ⅶ. ACKNOWLEDGEMENTS
This work was supported by the National Nature Science Foundation of China (No.11674303 and No.11574293), and the USTC Center for Micro and Nanoscale Research and Fabrication.
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