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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TTHZ.2018.2853991, IEEE
Transactions on Terahertz Science and Technology
Abstract—We report an active broadband terahertz modulator
based on an Fe
3
O
4
nanoparticle/Si structure, where the interface
effects were measured in a homemade THz time-domain
spectroscopy system. The approximately 100nm Fe
3
O
4
nanoparticle thin film on the high-resistance Si substrate was
easily attained by spin-coating ferrofluids. In our experiment, a
modulation depth as high as 92% was achieved at an external
laser irradiance of 3.6 W/cm
2
. This result can be explained by the
accumulation of carriers at the interface of the hybrid structure,
which induces intense absorption of the terahertz transmission. In
addition, the limit modulated frequency of the device is ~12 kHz.
The superior performance of this device for THz wave modulation
in comparison to other nano-material-based terahertz modulators,
as well as the ease of fabrication both illustrate that this is a
promising method in the modulation of THz transmission.
Furthermore, this modulator could also potentially provide an
essential component in a wide variety of technologies, such as
terahertz communications and terahertz imaging, etc.
Index Terms—Fe
3
O
4
, modulator, nanoparticle, terahertz
I. INTRODUCTION
HE terahertz (THz) frequency range, lying between 0.1
and 10 THz, is one of the most promising frequency ranges
in the electromagnetic spectrum, and has engrossed the
attention of many researchers in recent decades. Located in the
middle of the spectrum, THz radiation has numerous
similarities with its well-established neighboring bands [1].
This has enabled its use in various different technological areas
such as security [2], biological science [3], and
communications [4]. To effectively apply THz in these fields,
an effective THz modulator has emerged from the research as
an essential component [5-7].
The establishment of THz time-domain spectroscopy
(THz-TDS) has provided a technical maturity for the
This research was supported by the National Natural Science Foundation of
China (Grant No. 61505125) and the National Instrumentation Program of
China (Grant No. 2012YQ140005), Youth Innovative Research Team of
Capital Normal University and Support Project of High-level Teachers in
Beijing Municipal Universities in the Period of 13th Five–year Plan.
(Corresponding author: Bo Zhang and Jing-Ling Shen.)
Lu-Yao Xiong, Bo Zhang, Hong-Yu Ji, Wei Wang, Xin Liu, Shu-Li He,
Jing-Ling Shen are with the Key Laboratory of Terahertz Optoelectronics,
Ministry of Education, Advanced Innovation Center for Imaging Technology,
Beijing Key Laboratory for Terahertz Spectroscopy and Imaging, Beijing Key
Laboratory of Metamaterials and Devices, Department of Physics, Capital
Normal University, Beijing 100048, China. (e-mail: bzhang@cnu.edu.cn (B.
Zhang) and sjl-phy@cnu.edu.cn (J. Shen).)
development of the modulator. Owing to this important
contribution, multifarious modulators have emerged in recent
years based on different materials and their corresponding
various physical methods for modulator manufacture.
According to the reports, coatings of nanoparticles on silicon
substrates can be effectively used to improve the performance
of THz modulators under continuous wave (CW) laser
irradiation. Wen et al. have demonstrated a modulator based on
a gold nanoparticle monolayer that exhibited a 75%
transmission change with the bandwidth between 0–1.2 THz
and a pump power density of 10.47 W/cm
2
[8]. Further, Lai et al.
have fabricated MnFe
2
O
4
nanoparticles and coated them on a Si
substrate, and have indicated that the transmission change
through the modulator based on this structure could reach up to
91% at a pump power density of 7.07 W/cm
2
with the
bandwidth between 0–1.2 THz [9]. However, these
nanoparticle-based THz modulators suffer from several
significant drawbacks such as a complicated preparation
process, high cost, high pumped power and low bandwidth.
Therefore, to identify a new method for constructing
modulators, Fe
3
O
4
nano-ferrofluids have recently inspired
interest in the THz regime. These Fe
3
O
4
nano-ferrofluids are
known as one type of stable colloidal system comprising
nano-grade Fe
3
O
4
particles in a carrier liquid [10], exhibiting
some beneficial properties such as low external magnetic field
requirements[11, 12] and wide commercial availability. Given
these properties, Fe
3
O
4
nano-ferrofluids may be suitable for
THz modulator development. In 2013, Fan et al. realized a 40%
THz intensity modulation depth at 150 mT in a ferrofluid-filled
photonic crystal [13]. Further, in 2014, Shalaby et al.
investigated a polarization-sensitive magnetic modulator based
on Fe
3
O
4
nano-ferrofluids operating at THz wavelengths that
reached a modulation depth of 66% when using a field as low as
35 mT [14]. Nevertheless, the efficiency of these modulators
for THz intensity modulation does not reach a sufficient level in
these studies by focusing only on the magnetic property of the
Fe
3
O
4
nano-ferrofluids.
In this work, a high-efficiency broadband optical THz
modulator based on Fe
3
O
4
nanoparticles was demonstrated by
integrating Fe
3
O
4
nano-ferrofluid on a silicon substrate. This
device was optically controlled over a broad range of
frequencies from 0.2–2.6 THz to realize attenuation of the THz
transmission. The thin film of Fe
3
O
4
nanoparticles, formed by
spin-coating the Fe
3
O
4
nano-ferrofluid on the silicon substrate,
was compared with a bare Si substrate sample both in
Active optically-controlled broadband terahertz
modulator based on Fe
3
O
4
nanoparticles