Photonic nanojet mediated Raman enhancement: Vertical Raman mapping and simple ray matrix analysis

Abstract

A new method for enhancing the Raman scattering signal has emerged recently, based on dielectric enhancement. Especially promising is the dielectric method based on microspheres and photonic nanojet. In this paper, geometrical aspects and the influence of the incident beam parameters on Raman enhancement by silica microspheres were systematically investigated in three steps: by characterizing the incident beam using knife-edge method, performing horizontal and vertical Raman mapping imaging, and analyzing the results using ray transfer matrix analysis. Maps show a distinct enhancement (hotspot) area caused by the microsphere photonic nanojet and lens effect compared to a plain silicon substrate. Enhancement value on maps was the highest (5.7×) for 0.50 numerical aperture objective, when the incident beam size matched the microsphere diameter, and the focus of the incident beam was below the top of the sphere, so that the output beam focus was at the microsphere-substrate contact area. This geometrical configuration was confirmed as ideal by performing simple ray transfer matrix analysis. The ideal ranges of incident and output beam parameters match with the measured hotspot area. This three-step process and the usage of vertical Raman mapping have been, for the best of our knowledge, performed for the first time in such configuration. This research introduces a new way of investigating microsphere-assisted Raman enhancement, offers different approach to microsphere optics research, and improves current knowledge of the influence of the incident beam on the enhancement.

Microsphere‐Aided Super‐Resolution Scanning Spectral and Photocurrent Microscopy for Optoelectronic Devices

“…We also demonstrated that the contact mode (i.e., Δ d = 0) achieves the best Raman signal enhancement (Figure S7, Supporting Information). These results are in accordance with previous calculation results, [ 27 ] suggesting that the dielectric microsphere size, NA of the collection lens, and microsphere‐sample distance are all important parameters for PNJ‐based Raman enhancement.…”

“…An innovative method that enables imaging of specific regions of the sample surface is to trap polystyrene microspheres in water and achieve high-resolution Raman imaging through the optical tweezers mechanism. [24] In atmospheric environments, examples have also been demonstrated to attach dielectric microspheres as a scanning probe to specific locations and scan the sample surface, [25][26][27][28][29][30] among which an atomic force microscope (AFM) cantilever was introduced as one of the elegant mechanical holders for a microsphere. [28] However, the majority of these works applied the scanning microsphere to conventional optical microscopy.…”

Microlens‐assisted microscopy for biology and medicine

“…In 2007, the authors [74] for the first time achieved the enhancement of Raman scattering from a silicon wafer using close-packed micron and submicron spherical silica particles. So a new method, microsphere-enhanced Raman spectroscopy (MERS), appeared [75,76].…”

Multispectral Photonic Jet Shaping and Steering by Control of Tangential Electric Field Component on Cuboid Particle

“…However, there are very limited and clearly insufficient investigations of the role of tangential electric field components in the spatial and electromagnetic properties of the PJ. It could be noted that, in recent years, the studies in the field of the PJ effect based on the wavelength-scaled dielectric particles have gained more and more popularity and have found numerous applications, including optical trapping [23], nano-fabrications, and surface nano-patterning [24], quantum dot writing [25], microsphere-assisted microscopy and interferometry [26,27], auxiliary optomechanical manipulation [28], surface-enhanced Raman scattering [29], optical waveguides and switching [30,31], and biosensing [32], to name a few.…”