Direct observation of ultrafast plasmonic hot electron transfer in the strong coupling regime

Hangyong Shan,Zheng Liu,Tianyang Han,Xingli Wang,Yang Luo,Jiarui Wu,Xing Zhu,Ying Yu,Shuai Zu,Beng Kang Tay,Bowen Du,Yu Li,Zheyu Fang,Kebin Shi,Feng Lin,Bowen Li

doi:10.1038/s41377-019-0121-6

Abstract

Achieving strong coupling between plasmonic oscillators can significantly modulate their intrinsic optical properties. Here, we report the direct observation of ultrafast plasmonic hot electron transfer from an Au grating array to an MoS2 monolayer in the strong coupling regime between localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs). By means of femtosecond pump-probe spectroscopy, the measured hot electron transfer time is approximately 40 fs with a maximum external quantum yield of 1.65%. Our results suggest that strong coupling between LSPs and SPPs has synergetic effects on the generation of plasmonic hot carriers, where SPPs with a unique nonradiative feature can act as an ‘energy recycle bin’ to reuse the radiative energy of LSPs and contribute to hot carrier generation. Coherent energy exchange between plasmonic modes in the strong coupling regime can further enhance the vertical electric field and promote the transfer of hot electrons between the Au grating and the MoS2 monolayer. Our proposed plasmonic strong coupling configuration overcomes the challenge associated with utilizing hot carriers and is instructive in terms of improving the performance of plasmonic opto-electronic devices.

Highlights

Surface plasmons (SPs), as the collective oscillation of free electrons at the interface between dielectric and metal layers[1], have aroused tremendous interest in diverse fields, such as solar energy conversion, superresolution, high harmonic generation, near-field imaging, and nonlinear phenomena[2–9]
The 20 nm Al2O3 layer can prevent hot carriers that decay from surface plasmon polaritons (SPPs) from tunneling into the MoS2 monolayer, most carriers are distributed in the low-energy region and can hardly cross the Schottky barrier
In summary, by virtue of femtosecond pump-probe spectroscopy, direct plasmonic hot electron transfer from an Au grating to an MoS2 monolayer was successfully observed in an MIM structure in the strong coupling regime

Summary

Introduction

Surface plasmons (SPs), as the collective oscillation of free electrons at the interface between dielectric and metal layers[1], have aroused tremendous interest in diverse fields, such as solar energy conversion, superresolution, high harmonic generation, near-field imaging, and nonlinear phenomena[2–9]. As nonpropagating SPs, localized surface plasmons (LSPs) can either dephase radiatively by reemitting photons or decay by Landau damping to form energetic electron−hole pairs[10,11] These pairs are nonthermal, and their intense collisions can redistribute accumulated energy in hundreds of femtoseconds, developing into hot carriers that obey a Fermi−Dirac-like distribution with an increased effective temperature[2,3]. If these hot carriers are exported at a rate faster than energy dissipation by electron−phonon scattering, they can be collected and utilized in external circuits for optoelectronic devices such as photodetectors[12–26].

Methods

Results

Conclusion