Abstract
Nanoplasmonics is a potential game-changer in the development of next-generation on-chip photonic devices and computers, owing to the geometrically controlled and amplified linear and nonlinear optical processes. For instance, it resolves the limited light-matter interaction of the unique two-dimensional (2D) crystalline materials like semiconducting monolayer molybdenum disulfide (1L-MoS 2 ). Metal grating (MG) substrates excel at this because their surface plasmons (SPs) can lead to stark field confinement near the surface. This work studies optical amplification of 1L-MoS 2 on the gold (Au) MG substrate, which was designed to operate in a glycerol environment with SP resonance (SPR) at 850 nm excitation wavelength. Its design was verified by simulated and experimental reflectances, and topographically inspected by atomic force microscopy (AFM). Two advanced imaging modalities, second harmonic generation (SHG) and confocal Raman microscopy (CRM) were used to evaluate its 170-fold SHG on- and 3-fold CRM off-resonance optical amplifications, respectively. Some MoS 2 -to-grating adhesion issues due to trapped liquid showed as image nonuniformities. Possible improvements to limitations like surface roughness were also discussed. These Au MG substrates can boost conventional linear and nonlinear backscattering microscopies because they are tunable in the visible and near-infrared range by selecting geometry, metal, and environment. • Plasmonic optical amplification study of monolayer MoS 2 on Au/Si-grating substrate. • 170× on- and 3× off-resonance gains in second harmonic generation and Raman signals. • These nanogratings can boost both linear and nonlinear backscattering microscopies. • Tune by geometry, metal, and environment within the visible and near-infrared range. • Improve light-matter interaction (with less power and less light-induced damage).
Highlights
This work studies optical amplification of monolayer molybdenum disulfide (1L-MoS2) on the gold (Au) Metal grating (MG) substrate, which was designed to operate in a glycerol envi ronment with surface plasmon (SP) resonance (SPR) at 850 nm excitation wavelength
The MoO3 edge height slightly sloped down by 0–1 nm inwards, which were found to be 4 nm lower than the surrounding grating, indicating that the malleable gold grating has deformed by a few nano meters upon transfer. These atomic force micro scopy (AFM) images are helpful in the later discussion of the second harmonic generation (SHG) nonuniformity in Fig. 3c–f, which point to imperfections in the MoS2-to-grating adhesion
This work studied the optical amplification of the VLS-grown 1L-MoS2 on the Au MG substrate in on- and off-resonance cases by two advanced imaging techniques, SHG and confocal Raman microscopy (CRM), respectively
Summary
Nanoscale manipulation and boost of light-matter interaction via plasmonic metal nanostructures have enabled many attractive applications such as miniaturization of photonic devices [1,2,3,4,5], amplification of optical processes (nonlinear [4,6,7], refractive index [8,9], absorbance [6,10]), reduction of light-induced damage [11], and improvement of existing implementations Inter estingly, in the SPR-active MGs, both have been shown to contribute to the strong field confinement in the vicinity of the metal surface [7,17] Such plasmonic MGs can be used with conventional microscopes [6,13] to obtain the same optical signal but with much lower laser power, reducing the laser-induced effects like sample damage. The design flexibility and fabrication issues like surface roughness were discussed and improvements were proposed These MG substrates are useful in biological imaging applications due to the reduced photobleaching and the selective optical amplification, and are already being sold for this purpose [28]. They may even have uses as label-free biosensors based on the recent developments of plasmonic-based viral diagnostics [29,30]
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