Enhanced four-wave mixing process near the
excitonic resonances of bulk MoS
2
BRIAN A. KO,
1,2
ALEXEI V. S OKOLOV,
1,2
MARLAN O. SCULLY,
1,2,3,4
ZHENRONG ZHANG,
1,5
AND
HO WAI HOWARD LEE
1,2,
*
1
Department of Physics, Baylor University, Waco, Texas 76798, USA
2
The Institute for Quantum Science and Engineering, Texas A&M University, College Station, Texas 77843, USA
3
Princeton University, Princeton, New Jersey 08544, USA
4
e-mail: Scully@tamu.edu
5
e-mail: Zhenrong_Zhang@baylor.edu
*Corresponding author: Howard_Lee@Baylor.edu
Received 31 October 2018; revised 11 December 2018; accepted 11 December 2018; posted 11 December 2018 (Doc. ID 349643);
published 5 February 2019
Two-dimensional materials are generating great interest due to their unique electrical and optical properties.
In particular, transition metal dichalcogenides such as molybdenum disulfide (MoS
2
) are attractive materials
due to the existence of a direct band gap in the monolayer limit that can be used to enhance nonlinear optical
phenomena, such as Raman spectroscopy. Here, we have investigated four-wave mixing processes in bulk MoS
2
using a multiplex coherent anti-Stokes Raman spectroscopy setup. The observed four-wave mixing signal has a
resonance at approximately 680 nm, corresponding to the energy of the A excitonic transition of MoS
2
. This
resonance can be attributed to the increased third-order nonlinear susceptibility at wavelengths near the excitonic
transition. This phenomenon could open the path to using MoS
2
as a substrate for enhancing four-wave mixing
processes such as coherent anti-Stokes Raman spectroscopy.
© 2019 Chinese Laser Press
https://doi.org/10.1364/PRJ.7.000251
1. INTRODUCTION
Molybdenum disulfide (MoS
2
) is a transition metal dichalco-
genide that is generating great interest due to its optical [1–4],
electrical [5,6], and catalytic [7,8] properties. Bulk MoS
2
con-
sists of layers of S–Mo–S sheets bonded through van der Waals
forces. In bulk form, MoS
2
is a semiconductor with an indirect
bandgap of approxim ately 1.29 eV in the Γ valley. However, in
the monolayer limit, the indirect bandgap energy increases
and MoS
2
becomes a direct bandgap semiconductor with a
bandgap of 1.88 eV at the K point [9]. This indirect-to-direct
bandgap transition has been studied for its optical effects such
as photoluminescence in the monolayer limit [10] and Raman
enhancement [11,12].
The advance of MoS
2
monolayer production methods such
as micromechanical exfoliation and chemical vapor deposition
has led to an increase in the use of MoS
2
thin films to explore
various nonlinear optical phenomena [13–15], such as Raman
spectroscopy, where charge transfer interactions have been ob-
served to enhance the Raman signal [12]. However, while
monolayer MoS
2
has been studied intensively for its ability
to enhance nonlinear optical processes, it does have shortcom-
ings. In the Raman enhancement case, the photoluminescence
signal of MoS
2
disturbs the Raman signal, resulting in low
enhancement of Raman shifts at or near the direct bandgap
energy (659 nm) [12]. Monolayer MoS
2
fabrication methods
also suffer from low yield, making it difficult to implement
commercially. By increasing the layer count and turning
MoS
2
into an indirect bandgap semiconductor, the photolumi-
nescence signal becomes quenched while still maintaining the A
and B excitonic resonances [10]. Raman spectroscopy measure-
ments capitalizing on the electronic or excitonic resonances of
various substrates and samples have been extensively per-
formed, from gases [16] to nanoparticles [17]. In this paper,
we present our findings on the observation of the excitonic
photoluminescence in bulk MoS
2
via four-wave mixing
(FWM). We observe an enhancement of degenerate four-wave
mixing near the direct excitonic resonances of MoS
2
using a
broadband supercontinuum (SC) coherent anti-Stokes Raman
spectroscopy setup (Fig. 1). Previous studies on MoS
2
, espe-
cially that of Li et al., have demonstrated nonlinear optical
phenomena such as sum frequency generation (SFG) and
four-wave mixing [18]. However, our results are performed
on bulk MoS
2
substrates, much thicker than the few-layer sub-
strates typically studied. Our results also highlight the enhance-
ment of degenerate four-wave mixing that has not been
observed previously. These results show the potential of the
Research Article
Vol. 7, No. 3 / March 2019 / Photonics Research 251
2327-9125/19/030251-09 Journal © 2019 Chinese Laser Press