Tunable terahertz wave difference frequency
generation in a graphene/AlGaAs surface
plasmon waveguide
TAO CHEN,
1,
*LIANGLING WANG,
1
LIJUAN CHEN,
1
JING WANG,
1
HAIKUN ZHANG,
1
AND WEI XIA
1,2
1
School of Physics and Technology, University of Jinan, Jinan 250022, China
2
e-mail: sps_xiaw@ujn.edu.cn
*Corresponding author: taochen426@hust.edu.cn
Received 14 November 2017; revised 18 January 2018; accepted 18 January 2018; posted 19 January 2018 (Doc. ID 313519);
published 27 February 2018
Graphene-based surface plasmon waveguides (SPWs) show high confinement well beyond the diffraction limit at
terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/
AlGaAs SPW structure for terahertz wave difference frequency generation (DFG) under near-infrared pumps.
The composite waveguide, which supports single-mode operation at terahertz frequencies and guides two pumps
by a high-index-contrast AlGaAs∕AlO
x
structure, can confine terahertz waves tightly and realize good mode field
overlap of three waves. The phase-matching condition is satisfied via artificial birefringence in an AlGaAs∕AlO
x
waveguide together with the tunability of graphene, and the phase-matching terahertz wave frequency varies from
4 to 7 THz when the Fermi energy level of graphene changes from 0.848 to 2.456 eV. Based on the coupled-mode
theory, we investigate the power-normalized conversion efficiency for the tunable terahertz wave DFG process by
using the finite difference method under continuous wave pumps, where the tunable bandwidth can reach 2 THz
with considerable conversion efficiency. To exploit the high peak powers of pulses, we also discuss optical pulse
evolutions for pulse-pumped terahertz wave DFG processes.
© 2018 Chinese Laser Press
OCIS codes: (190.4223) Nonlinear wave mixing; (240.6680) Surface plasmons; (230.7370) Waveguides; (160.6000) Semiconductor
materials.
https://doi.org/10.1364/PRJ.6.000186
1. INTRODUCTION
Graphene has recently attracted huge interest in the optoelec-
tronics field [1], such as passively mode-locked lasers [2,3],
broadband polarizers [4], chemical gas sensing [5], optical
modulators or switches [6,7], photodetectors [8], enhanced
four-wave mixing [9], terahertz (THz) and infrared spectros-
copy [ 10], and plasmonics [11,12]. Graphene-based plasmons
enable strong confinement of the optical field at subwavelength
scales and hold great potential for applications [13], which can
be tuned via gate bias voltage, chemical doping, the electric
field, or the magnetic field [14]. To guide graphene surface
plasmons with deep subwavelengths, many types of graphene
surface plasmon waveguides (GSPWs) have been proposed
[4,6,9,15,16]. At THz frequencies, the tunability, excitation,
and generation of graphene surface plasmons have been dem-
onstrated [17–19]. Liu et al. investigated the effective refractive
index, loss, and effective mode area of a 100 μm width
Si–SiO
2
–graphene–dielectrics–graphene–SiO
2
–Si waveguide
for different Fermi energy levels (chemical potentials) [20],
where the loss and effective index can be reduced with
increasing Fermi energy level when the resultant optical mode
area increases. Zhou et al. proposed a hybrid few-layer GSPW
to balance the loss and mode area through the strong coupling
between the dielectric waveguide and plasmon waveguide [21].
Furthermore, Xu et al. analyzed the single-mode operation re-
gion of a dielectric-loaded GSPW with different Fermi energy
levels [22]; the optical properties of the fundamental mode
can be tuned by changing the Fermi energy level, and the
waveguide is simple and easy for fabrication.
THz sources are important for many applications of THz
waves [23]. The four-wave mixing (FWM) and differen ce fre-
quency generation (DFG) in waveguide devices are important
techniques to generate coherent THz waves through nonlinear
wavelength conversion. Barh et al. reported an all-fiber FWM
THz source in a microstructured-core double-clad plastic fiber
pumped by a high-power CO
2
laser [24]. Sun et al. proposed a
hybrid graphene sheets waveguide for THz FWM generation
[25], where the waveguide supports graphene surface plasmon
modes at THz frequencies. Compared with the FWM process,
the DFG technique is more attractive because of its high
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Vol. 6, No. 3 / March 2018 / Photonics Research
Research Article
2327-9125/18/030186-07 Journal © 2018 Chinese Laser Press