Tip Enhanced Rayleigh Scattering via a Gap Mode in Transmission Geometry

Conventional far-field optical microscopy is diffraction limited and hence to surpass the diffraction limit and resolve nanostructures, near-field scattering optical microscopy (NSOM) is being studied¹. The resolution offered by NSOM too is 60-100nm. To improve the resolution further still, an alternate technique was demonstrated which involves local field enhancement via a metallic tip. The resolution in this case is determined by the curvature of the tip. Imaging techniques that follow this are a) tip-enhanced Raman scattering², b) tip-enhanced photoluminescence³, c) tip-enhanced Rayleigh scattering and d) tip-enhanced coherent anti-stokes Raman scattering⁴ to name a few.<br/>In this study, we report tip-enhanced Rayleigh scattering of Au nanoparticles grown on a glass substrate using thermal vapor deposition technique. When two plasmonic structures approach within a few nanometers there is a huge enhancement of the plasmon intensity in the gap between the two structures as energy is transferred from one structure to the other. The two plasmonic structures can be imagined as 1) an approaching gold coated quartz tip and 2) a gold thin film and energy transfer can be captured as Rayleigh scattering. We employ a gap mode in transmission geometry to observe a tip enhanced Rayleigh scattering. The detection frequency is locked-in and varied from f₀, f₀-1f’ and f₀-2f’ where f₀ is the frequency of the tip and f’ is dithering frequency of the stage. Two sets of experiments were performed to illuminate the tip via a) a top excitation geometry and b) side excitation. We compare the difference between the excitation geometry based on Force-distance curves measured for each of the above-mentioned experiments. Balanced detection scheme is also utilized to improve the signal to noise ratio.<br/><br/><br/><u>References: </u><br/>1) Y. De Wilde and P.A. Lemoine, Review of NSOM Microscopy for Materials, AIP Conference Proceedings <b>931</b>, 43 (2007); https://doi.org/10.1063/1.2799414<br/>2) Naresh Kumar, Sandro Mignuzzi, Weitao Su & Debdulal Roy(2015),Tip-enhanced Raman spectroscopy: principles and applications, EPJ Techniques and Instrumentation volume 2, Article number: 9<br/>3) Wang, C.-F., Zamkov, M., & El-Khoury, P. Z. (2021). Ambient Tip-Enhanced Photoluminescence with 5 nm Spatial Resolution. The Journal of Physical Chemistry C, 125(22), 12251–12255.<br/>4) SATOSHI KAWATA ,TARO ICHIMURA, NORIHIKO HAYAZAWA, YASUSHI INOUYE and MAMORU HASHIMOTO, TIP-ENHANCED NEAR-FIELD CARS MICROSCOPY, Journal of Nonlinear Optical Physics & MaterialsVol. 13, No. 03n04, pp. 593-599 (2004)Organic Electronics