![](https://csdnimg.cn/release/download_crawler_static/15173054/bg5.jpg)
optical pumping power and electrical conductivity. Finally, the device can offer
about 80% modulation depth.
In addition, employing the SOS can achieve ultra-broadband modulation. Jane E.
Heyes’s group (43) proposed a terahertz modulator com posed of a planar array of metal
cut-wires on an SOS substrate, as shown in Figure 1b, where the silicon layer serves as a
photoconductive switch. Under optical excitation, the device exhibits a controlled transi-
tion between the Lorentzian and Drude responses. Such a hybrid metasurface modulator
can offer ¡10 dB modulation depth, spanning 0.5–1.5 THz.
For the sensing application, the red shift and blue shift play prominent and significant
roles. Inductive–capacitive (LC) resonance frequency can usually be defined as a simple
expression, v D 1=
ffiffiffiffiffiffiffi
LC
p
. The effective inductance ( L) or capacitance (C) would be aug-
mented or abated when the hybrid metamaterial SOS is illuminated by a laser; thus, the
spectrum would exhibit a red shift or blue shift of fundamental LC resonance . In 2011,
Shen et al. also took advantage of SOS to make a device that is frequency agile and can
achieve broadband blue shift tunability (44). Analogically, using the same metamaterial
could also achieve red shift tunability (Figure 1c) (45).
Gallium Arsenide (GaAs)
Many theories and experimental results elucidate that the modulation speed is limited by the
lifetime of photocarriers. Silicon has a carrier lifetime only in nanosecond level, because of
its indirect bandgap. Some direct bandgap materials such as gallium arsenide (GaAs), which
is possessed of a lifetime of photocarrier at the (sub) picosecond level, are new and proper
candidates for high-speed modulation. For an efficient manipulation example, the incident
photon energy has to exceed the bandgap energy. However, the concentration of the free
charge carriers is determined by the intensity of pump light (46). The forbidden gap
(1.42 eV) of GaAs is ideally suited for photoexcitation by a high-power 800-nm fs laser; for
example, a Ti:sapphire laser. The electronic mobility (8,500 cm
2
V
¡1
s
¡1
)ofGaAsisalso
higher than silicon’s (1,400 cm
2
V
¡1
s
¡1
), which leads to a faster modulation reaction
speed, especially for some photoelectric cooperative manipulation approaches.
Chen et al. used GaAs material to study terahertz waves modulation (47). They tried
to use an ErAs/GaAs superlattice structure grown on an intrinsic GaAs wafer by MBE
(molecular beam epitaxy) to make a modulator, whose structure is illustrated in the exper-
iment. Compared to Fekete et al. (48), it experimentally unravels a switching recovery
time around 20 ps because of the fast carriers recombination in ErAs/GaAs. Figure 2a
also exhibits transmission variat ions in the hybrid substrate metamaterial with various
time delays between arrival of terahertz pulse and optical excitation. Then, the experi-
ment GaAs could offer faster modulation speed.
Being similar to the device made of silicon, the modulators composed of GaAs also
could realize phase modulation and resonance frequency shift. A device composed of
copper SRRs coated upon high resistivity GaAs subst rate was made by Manceau et al.
(49), and the experiment illustrates that the device could realize blue shift tunability of
the electric dipole metamaterial’s resonance, as well as broad band (»0.25 THz) phase
tunability up to ~p/4, depicted in Figure 2b. Analogically, the nonpolarization structure
also could be emp loyed in modulation. In 2012, Chatzakis et al. devised a polarization-
insensitive device, which consists of an array of double SRRs on GaAs (50). Under the
incident light, the density of photocarries could be change d, then it realized modulation
terahertz waves. Hence, such a device offers a new solution for designing nonlinear and
frequency-sensitive terahertz modulators.
710 Y. Bai et al.
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