Quantum versus optical interaction contribution
to giant spectral splitting in a strongly coupled
plasmon–molecules system
BO WANG,
1,†
XIAN-ZHE ZENG,
2,†
AND ZHI-YUAN LI
1,
*
1
School of Physics and Optoelectronic Technology, South China University of Technology, Guangzhou 510640, China
2
School of Physics, Peking University, Beijing 100871, China
*Corresponding author: phzyli@scut.edu.cn
Received 12 August 2019; revised 5 January 2020; accepted 6 January 2020; posted 6 January 2020 (Doc. ID 375135); published 27 February 2020
Vacuum Rabi splitting, which stems from a single photon interaction with a quantum emitter (a single atom,
molecule, or quantum dot), is a fundamental quantum phenomenon. Many reports have claimed that using
J aggregate coupling to highly localized plasmon can produce giant Rabi splitting (in scattering spectra) that
is proportional to
N
p
, where N is the number of excitons in J aggregates, and this splitting originates purely
from quantum interaction between excitons and plasmons. In this work, we show that the scattering spectra are
very sensitive to the surrounding matter, and the giant spectral splitting stems both from the quantum interaction
of a single molecule with plasmons (Rabi splitting) and from the classical optical interaction of multiple molecules
with plasmons. We develop a Lorentzian model to describe molecules and plasmon and find that the collective
optical interaction is dominant in generating the giant splitting (in scattering spectra), which is also proportional
to
N
p
, upon the quantum interaction of single-molecule Rabi splitting. Simply speaking, the observed giant
spectral splitting is not a pure quantum Rabi splitting effect, but rather a mixture contribution from the large
spectral modulation by the collective optical interaction of all molecules with plasmons and the modest quantum
Rabi splitting of a single molecule strongly coupled with plasmons.
© 2020 Chinese Laser Press
https://doi.org/10.1364/PRJ.375135
1. INTRODUCTION
The radiation of light from quantum emitters such as atoms,
molecules, and quantum dots (QDs) can reflect both the
intrinsic and extrinsic prop erties of quantum emitters, as it de-
pends not only on the intrinsic quantum energy states of
emitters themselves but also on their extrinsic background
electromagnetic properties. The background material and
structure can accelerate, decelerate, or even inhibit the sponta-
neous emission of quantum emitters [1]. They can also cause
strong coupling of emitting light with the quantum emitter and
lead to Rabi splitting in the radiation spectrum in some special
situations, such as micro/nanocavity with high-quality factor
(high-Q) and small modal volume [2]. The strong light–matter
interactions at the single-photon level, which are described by
the cavity quantum electrodynamics (QED), offer a promising
means to explore quantum optical application such as single-
photon switches, coherent control, and entanglement of distin-
guishable quantum systems [3–5]. The interactions are rooted
in the strong coupling between a quantum emitter and a radi-
ation field mode with the condition g ≫ κ, γ, where g, κ, and γ
are the coupling coefficient, cavity loss, and decay rate of the
quantum emitter, respectively [6,7]. When a two-level atom is
strongly coupled to a single-mode vacuum field, the excited
state will split into two states and electrons travel circularly be-
tween them via the process of releasing and capturing photons
[8]. This will induce two splitting peaks (so-called Rabi split-
ting) in photoluminescence spectra, and the interval between
two peaks is called vacuum Rabi frequency Ω
R
. Ideally, the best
way to observe Rabi splitting is to use an atomic system, since
their behaviors and characteristics are easily understood as
quantum level [9–11]. However, it is hard to fix an atom in
a macroscopic cavity QED system. Even if a relatively station-
ary trapped atom can be achieved, a decrease in the coupling
strength g is inevitable, since it needs a big room for trapping
[12,13]. Moreover, the dipole moment μ of atom is too small
so that it needs to further decrease the mode volume V and
increase the quality factor (Q) in order to observe single-atom
Rabi splitting to happen in a sufficiently large optical energy
density.
Recently, strong coupling has been reported for quantum
well (QW) [14], QD [15–18], and J aggregates [19–22]as
quantum emitters in two types of systems; one is a traditional
cavity system [15–18] such as photonic crystal cavity [16,17],
and the other is a plasmonic nanocavity system [19–31].
Research Article
Vol. 8, No. 3 / March 2020 / Photonics Research 343
2327-9125/20/030343-09 Journal © 2020 Chinese Laser Press