Strong optical force of a molecule enabled by the plasmonic nanogap hot spot in a tip-enhanced Raman spectroscopy system

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Abstract

Tip-enhanced Raman spectroscopy (TERS) offers a powerful means to enhance the Raman scattering signal of a molecule as the localized surface plasmonic resonance will induce a significant local electric field enhancement in the nanoscale hot spot located within the nanogap of the TERS system. In this work, we theoretically show that this nanoscale hot spot can also serve as powerful optical tweezers to tightly trap a molecule. We calculate and analyze the local electric field and field gradient distribution of this nanogap plasmon hot spot. Due to the highly localized electric field, a three-dimensional optical trap can form at the hot spot. Moreover, the optical energy density and optical force acting on a molecule can be greatly enhanced to a level far exceeding the conventional single laser beam optical tweezers. Calculations show that for a single $H2TBPP$ organic molecule, which is modeled as a spherical molecule with a radius of $rm=1 nm$, a dielectric coefficient $ε=3$, and a polarizability $α=4.5×10-38 C·m2/V$, the stiffness of the hot-spot trap can reach a high value of about $2 pN/[(W/cm2)·m]$ and $40 pN/[(W/cm2)·m]$ in the direction perpendicular and parallel to the TERS tip axis, which is far larger than the stiffness of single-beam tweezers, $～0.4 pN/[(W/cm2)·m]$. This hard-stiffness will enable the molecules to be stably captured in the plasmon hot spot. Our results indicate that TERS can become a promising tool of optical tweezers for trapping a microscopic object like molecules while implementing Raman spectroscopic imaging and analysis at the same time.

Li Long：School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510641, China
Jianfeng Chen：School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510641, China
Huakang Yu：School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510641, China
Zhi-Yuan Li：School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510641, China

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Li Long, Jianfeng Chen, Huakang Yu, and Zhi-Yuan Li, "Strong optical force of a molecule enabled by the plasmonic nanogap hot spot in a tip-enhanced Raman spectroscopy system," Photonics Research 8(10), 1573-1579 (2020)