994 Vol. 7, No.

9 / September 2019 / Photonics Research Research Article

Ultrasensitive polarization-dependent terahertz

modulation in hybrid perovskites plasmon-
induced transparency devices
JUNHU ZHOU,1,† YUZE HU,1,† TIAN JIANG,1,*,† HAO OUYANG,1 HAN LI,1 YIZHEN SUI,1 HAO HAO,2 JIE YOU,3
XIN ZHENG,3 ZHONGJIE XU,1 AND XIANG’AI CHENG1
1
College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China
2
State Key Laboratory of High Performance Computing, College of Computer, National University of Defense Technology,
Changsha 410073, China
3
National Innovation Institute of Defense Technology, Academy of Military Sciences PLA China, Beijing 100071, China
*Corresponding author: tjiang@nudt.edu.cn

Received 26 April 2019; revised 12 June 2019; accepted 3 July 2019; posted 3 July 2019 (Doc. ID 366071); published 9 August 2019

Active control of metamaterial properties with high tunability of both resonant intensity and frequency is es-
sential for advanced terahertz (THz) applications, ranging from spectroscopy and sensing to communications.
Among varied metamaterials, plasmon-induced transparency (PIT) has enabled active control with giant sensi-
tivity by embedding semiconducting materials. However, there is still a stringent challenge to achieve dynamic
responses in both intensity and frequency modulation. Here, an anisotropic THz active metamaterial device with
an ultrasensitive modulation feature is proposed and experimentally studied. A radiative-radiative-coupled PIT
system is established, with a frequency shift of 0.26 THz in its sharp transparent windows by polarization ro-
tation. Enabled by high charge-carrier mobility and longer diffusion lengths, we utilize a straightforwardly spin-
coated MAPbI3 film acting as a photoactive medium to endow the device with high sensitivity and ultrafast speed.
When the device is pumped by an ultralow laser fluence, the PIT transmission windows at 0.86 and 1.12 THz
demonstrate a significant reduction for two polarizations, respectively, with a full recovery time of 561 ps. In
addition, we numerically prove the validity that the investigated resonator structure is sensitive to the optically
induced conductivity. The hybrid system not only achieves resonant intensity and frequency modulations simul-
taneously, but also preserves the all-optical-induced switching merits with high sensitivity and speed, which en-
riches multifunctional subwavelength metamaterial devices at THz frequencies. © 2019 Chinese Laser Press
https://doi.org/10.1364/PRJ.7.000994

1. INTRODUCTION To date, the integration of metamaterials with actively

Anisotropic optoelectronic devices have emerged as an attrac- driven platforms by means of thermal [12,13], mechanical
tive and appealing research subject due to the recent progress [14,15], optical [16–22], or electrical [23–25] control has trig-
in photonics-based technologies, including communication, gered considerable interest, aiming at controlling and manipu-
sensing, and molecular spectroscopy [1–4]. However, it is lating THz waves. More precisely, these approaches have
extremely hard to attain the intrinsic linear dichroism that is successfully validated that the custom-built functionalities in
directly associated with the intrinsic nature of materials in the transmission of THz waves can be realized by either
the terahertz (THz) regime, which hinders the development strength modulation or resonance frequency shifting. In the
of abundant anisotropic THz applications. Benefiting from case of strength modulation, the near-field coupled metamate-
the emergence of metamaterials, a variety of fascinating phe- rials whose components are electromagnetically interfering with
nomena can be observed in their designed metal patterns, such each other are more likely to be actively manipulated than the
as negative refraction [5], invisibility cloaking [6], hyperbolic single metallic dipole resonators, mainly due to the enlarged
media [7], perfect absorbers [8,9], chiral metamaterials [10], influence induced by the disturbances of the interface environ-
and superlenses [11], which are barely seen in natural materials. ment. Recently, the Fano resonance-coupled optical-active
In this sense, it is possible to tailor the artificial metamaterials THz metadevices for ultrasensitive and ultrafast photoactive
for accurate manipulation of THz-wave polarization and to switching have been realized, but with the drawback of
exploit these materials with great flexibility. single-frequency modulation [26]. When it comes to the

2327-9125/19/090994-09 Journal © 2019 Chinese Laser Press

 Research Article Vol. 7, No. 9 / September 2019 / Photonics Research 995

tuning of resonance frequency, active control of electromagneti-

cally induced transparency has allowed a slight shifting of a
Fano dip [27] frequency. A new device that is an electrically
controlled connection between meta-atoms with graphene
bridges has exhibited a dipole resonance shift up to 0.65 THz
[28]. However, one significant disadvantage of such devices is
their relatively low modulation speed driven by electricity,
which may limit their application in ultrafast switching devices
and modulating. From an alternative perspective, the polariza-
tion control may be viewed as a promising approach to adjust
the resonance frequency without sacrificing the all-optical
switching merits [29], such as sensitivity and high speed. The
asymmetric Fano spectral profile and plasmon-induced trans-
parency (PIT) phenomena are two prime examples of Fano res-
onance manifestation by mimicking quantum phenomena in
classical metamaterial systems. In the case of PIT phenomenon, Fig. 1. (a) Schematic of the polarization-dependent metamaterial-
a sharp transparent window in the broad transmission dip is perovskite THz device. A periodic array of CRRs and SRRs tailors
mainly contributed to by the destructive interference between the PIT resonance at different frequencies determined by incident po-
two different modes. Following this definition, the PIT operat- larizations. A thin perovskite film is deposited on the quartz substrate
acting as a photoactive layer illuminated by optical pump pulses
ing in the destructive interference between closed-ring resona-
(400 nm). (b) Schematic view of the functional unit cell. The thickness
tors (CRRs) and split-ring resonators (SRRs) [30] could be of the quartz substrate is H
2 mm, the height of the Au metama-
engineered to generate an anisotropic Fano-type resonance. terial is h
127 nm, and the period is P x
150 μm, P y
110 μm.
More precisely, this radiative-radiative coupling with a twofold Geometric parameters of the structure are L1
120 μm, L2

symmetric structure can be endowed with sharp transparent 50 μm, L11
26 μm, L12
25 μm, L21
50 μm, L22
18 μm,
windows at separate frequencies via polarization rotation. respectively. Inset presentation shows the crystal structure of
As a dynamic medium, solution-processed MAPbI3 films T−CH3 NH3 PbI3 phases. Optical microscopic images of fabricated
usually present an astonishing photon-photoconductivity Au structures (c) before and (d) after covering a 55 nm perovskite thin
(Δσ) conversion efficiency that determines the high sensitivity film, where the scale bar represents 100 μm, and the inset picture
switching performance [31]. The use of organic-inorganic shows the thickness of the perovskite film.
metal halide perovskite (CH3 NH3 PbI3 ) has seen phenom-
enally rapid progress in the field of photovoltaic cells and pho-
tonic devices [32–36]. By virtue of high-charge carrier mobility
(∼500 cm2 ·V −1 · s−1 ) [37–39], long diffusion length (beyond resonance at different frequencies determined by incident
1 μm), and high photoluminescence (PL) quantum yields (up THz polarizations. The detailed geometrical parameters are re-
to 90%) [40–45], perovskites have prompted intense research vealed in Fig. 1(b), which describes the metamaterial structure
in power conversion efficiencies [46,47]. It is worth noting that along with the unit cell dimensions. In this figure, two pairs of
the rapid recovery of charge carriers in perovskites has greatly SRRs surround a CRR, with a bar located in the middle. Two
facilitated high-speed optoelectronic devices beyond traditional aspectant SRRs aligned with CRR in the horizontal (x) direc-
silicon-based devices [48–50]. These outstanding properties tion (L1 , L21 , and L22 ) have a different design compared to the
laid the foundation for the design of photoactive media on res- other pair of SRRs in the vertical (y) direction (L2 , L12 , and
onant structures, with an extremely simple and economical L11 ), which is the key point of the anisotropic PIT resonances.
spin-coating process. Herein, we demonstrate an ultrasensitive The metamaterial array is fabricated using the conventional
and high speed all-optical modulation of an anisotropic PIT photolithography technique, where the 127 nm thick gold
resonator on the basis of an inexpensive, uncomplicated (Au) structures is thermal-evaporated on a 2 mm thick z-cut
processing, and astonishing photoconductivity spin-coated quartz substrate as shown by the optical microscope image
MAPbI3 perovskite film. The well optimized PIT resonance in Fig. 1(c). Au has excellent chemical stability and ultrahigh
metaphotonic array demonstrates an outstanding dynamic re- conductivity (over 4.56 × 107 S∕m), which is often used as a
sponse at the transmission windows of 0.86 and 1.12 THz for perfect electrical conductor in THz structure arrays. The unit
two perpendicular polarizations, respectively. The present work cell in the size of P x
150 μm, P y
110 μm is repeated in
is an experimental demonstration of an anisotropic active THz the x and y direction, respectively, eventually forming a rectan-
metadevice fabricated in a simplified and cost-effective labora- gular array of 4 mm × 4 mm scale, as shown in Fig. 1(c).
tory method. With the metasurface ready, a 55 nm thin layer of solution
process MAPbI3 perovskite is spin-coated to obtain the hybrid
metadevice [51–53]. In the hybrid metaphotonic device sys-
2. RESULTS AND DISCUSSION tem, the properties of the photoactive medium are the domi-
A. Sample Preparation and Characterization nant factors for the overall dynamical performance. A thin,
The schematic architecture of the polarization-dependent dense, flat pinhole-free MAPbI3 film is fabricated by spin-coat-
metamaterial-perovskite THz device is presented in Fig. 1(a). ing a perovskite precursor poly(ethylene oxide) (PEO) blend
A periodic array of SRRs and CRRs tailors the PIT solution, followed by a thermal annealing process. Perovskite
 996 Vol. 7, No. 9 / September 2019 / Photonics Research Research Article

precursor solution (10 wt%) is prepared by dissolving methyl-

ammonium iodide (MAI) and lead (II) iodide (PbI2 ) powder
with a 1:1 molar ratio in dimethyl sulfoxide (DMSO) at 70°C
with vigorous stirring. PEO (3 wt% with respect to the
MAPbI3 precursor) is mixed with the precursor solution at
70°C. Before spin-coating, the prepared solution is cooled to
room temperature and the metamaterial is cleaned by acetone,
2-propanol and UV−O3 for 5 min each. Then the solution is
spin-coated on the top of the metamaterial at 4000 r/min for
30 s in an N2 -filled glovebox. The as-spun device is immedi-
ately annealed by a hotplate at 85°C for 10 min, and the trans- Fig. 2. (a) Simulated and (b) measured amplitude transmissions of
parent film turned brown upon the evaporation of the DMSO. the designed polarization-related metamaterial under illuminations of
x-polarized (blue) and y-polarized (red) THz electric fields without
The metasurface is wholly covered and has a uniform surface
perovskite coating. Dashed lines represent Fano-resonant frequencies
morphology along with the structure array in Fig. 1(d). The for two polarized THz electric fields.
high quality of perovskite film is verified by all the covered gaps
between Au structures, which is especially important for near-
field coupled resonances. An atomic force microscope (AFM)
attributed to their geometrical dimensions via the THz illumi-
image inserted in Fig. 1(d) shows a clear stage of 55 nm thick
nation with a polarization along the gap. The strongly coupled
film. In addition, we perform linear absorption and PL spectra
electric dipole oscillation of CRR and the weakly coupled fun-
tests on the prepared perovskite film. The film we used has a
damental eigenmode of SRR excited by THz electric field are
bandgap of 760 nm and an absorption peak at 740 nm
coupled back and forth through the magnetic field. Notably,
(see Appendix A, Fig. 7).
the resonance frequency of SRR is slightly higher than the de-
B. Experimental Section sign compared to the dipole mode of CRR. In order to
Optical characterization of the hybrid metamaterial-perovskite quantitatively determine the anisotropic functionality, our
device is performed by the optical-pump-THz-probe (OPTP) meta-atom sample consists of a CRR surrounded by two pairs
method. Two 1 mm thick nonlinear crystals (ZnTe) are used in of SRRs.
the THz generation and detection. The dynamic modulation The CRR and its corresponding SRRs are designed with dif-
process is recorded in the form of transient transmission am- ferent sizes along the x and y axis, respectively. The displacement
plitude change when the time delay of optical excitation pulse is length in the middle of the CRRs and SRRs is set to be 5 μm to
step-changed from 0 to 600 ps. A Spectra Physics regenerative ensure enough coupling strength between each other. The
amplifier system, which can provide a 1 kHz pulse train of strong coupling effect plays a key role in the giant enhancement
800 nm (center wavelength) pulses of 100 fs duration (full of the THz electric field along the gap. Such coupling effects are
width at half-maximum), is used as the light source of the demonstrated by the simulations of the intrinsic THz spectrum
whole OPTP setup. A part of 800 nm optical laser beam is of CRR and SRR resonators [54] (see Appendix A, Fig. 8). In
frequency-doubled (400 nm) by 1 mm thick β-BaB2 O4 crystal view of this, the enhanced mutual coupling between the SRR
for optical pump of MAPbI3 film, whose spot size is 5 mm in and CRR resonances exhibits unprecedented sensitivity of the
diameter, which is large enough to cover the THz beam gap environment, opening up new possibilities for ultrasensitive
spot (∼2.2 mm in diameter) for a uniform excitation over THz modulation beyond natural materials.
the surface of the device. A wire-grid THz polarizer is used The strong resonance feature is still maintained in the ex-
to project the polarization of THz pulses. The transmission perimentally measured transmission spectrum of the MAPbI3
spectra of the active device under different delay times and spin-coated sample. As presented in Figs. 3(a) and 3(c), the
pump fluence E S ω are normalized to the transmission hybrid metaphotonic device holds a typical PIT resonance
spectra of reference substrate E R ω following the relation transmission spectrum that is similar to the result in Fig. 2(b)
of jT ωj
jE S ω∕E R ωj. (x and y directions) when the pump beam is blocked, despite
the influence imposed by the intrinsic conductivity of the per-
C. Simulation and Experiment Results ovskite film. In order to characterize such an influence on the
To investigate the polarization-dependent PIT properties of the PIT window, we take into account a series of the pump fluence
designed structure, we numerically simulate and experimentally of a 400 nm pulse, ranging from 5 to 240 μJ∕cm2 . When the
study the amplitude transmissions of the designed polarization- pump fluence is as low as 5 μJ∕cm2 , as displayed in Figs. 3(a)
related metamaterial without perovskite coating, as shown in and 3(c), the obvious modulation effect in the x direction and
Fig. 2. To begin with, the numerical simulation results of y direction can be clearly visualized in the transmission spectra.
the typical PIT windows at different frequencies for x-polarized Specifically, the sharp PIT window is assigned to the destructive
and y-polarized incident waves are displayed in Fig. 2(a). In interference between the inductor-capacitor (LC) resonance
addition, the experimental data in Fig. 2(b) present that the and dipole resonance, which originate from the SRRs and
PIT window is located at 0.81 THz for the x-polarized wave, CRRs, respectively. Once the 55 nm MAPbI3 layer is excited
while at 1.11 THz for the y-polarized wave, in good agreement by an increasing pump fluence, the screening effect of the fring-
with the simulated results of Fig. 2(a). As demonstrated in ing field by the MAPbI3 layer is enhanced, while the PIT
Fig. 1(a), the resonances of CRR and SRR are primarily strength is gradually suppressed.
 Research Article Vol. 7, No. 9 / September 2019 / Photonics Research 997

depth is already close to saturation under the pump fluence

of 240 μJ∕cm2 . From the simulation result [Fig. 3(d),
1440 S/m], the saturation of the modulation depth is actually
derived from the photoconductivity response of the surface
photoactive film. Considering that the photoactive film has
a thickness of only 55 nm, this problem may be solved by
appropriately increasing the thickness of the spin-coated perov-
skite. The anisotropic modulation is achieved under an ultra-
low pump fluence (5 μJ∕cm2 ), which is markedly different
from previous studies [20,26,31] (detailed comparison is pro-
vided in Table 1). In fact, to improve the sensitivity of this
device, we use perovskite as photoactive material and particu-
larly employ a short gap (5 μm) design for the purpose of low-
ering the threshold to switch off the near-field coupling of LC
resonance and dipole resonance. The effect of this design is cer-
tified by the simulation analysis shown in Figs. 3(b) and 3(d).
The Lorentzian mechanical oscillator is a commonly used theo-
retical model for explaining the phenomenon of PIT metama-
terial resonance [55,56]. We analyzed this PIT effect by
Fig. 3. Results of the optical modulation of anisotropic THz wave.
Measured transmission spectra of the designed perovskite-based device theoretical calculation, as shown in Appendix A (Fig. 9).
of the (a) x-polarized and (c) y-polarized incident THz electric field The modulation of the PIT peak can only be viewed as the
under different pump powers. Corresponding numerically simulated change of the damping rate of SRR arms. Therefore, it indicates
transmission spectra of the (b) x-polarized and (d) y-polarized incident that the suppression of the bright mode in SRRs is the major
THz electric field under different conductivities of the perovskite factor of our modulation. Intending to rule out the fake PIT
thin film. The dashed lines mark the frequencies corresponding to effect that may be hidden in the transmission amplitude [57],
the Fano resonance peaks. we extracted the group delay curves from the experimental
results as the derivative of phase with respect to circular fre-
quency. Significant slow-light effect is seen in Appendix A
In addition, the normalized ratio of transmission amplitude (Fig. 10), which could prove the authenticity of the PIT effect
at the resonance peak of 0.86 and 1.12 THz along the x and y exhibited by our device.
direction, respectively, is decreased from 0.44 and 0.60 to From these figures, one can see that the resonance peaks
0.37 and 0.45 when the pump fluence changes from 0 to almost disappear when the conductivity of the perovskite film
240 μJ∕cm2 . This observed amplitude modulation of the is just set to a relatively low level, namely, 960 and 1440 S/m in
PIT window is attributed to the reduced capacitance at the the x and y direction, respectively. Importantly, this ultrasensi-
gap of CRRs and SRRs throughout the unit cell, which is tive (5 μJ∕cm2 ) resonant modulation behavior could open up a
the result of the optical excitation and photoconductivity of new direction for the design and construction of low-threshold
MAPbI3 in the gap. In our study, the resonance peak (y) is perovskite-based plasmonic devices in the THz regime. Please
not completely switched off, even though the modulation note that the overall amplitude reduction in the excited
ultrathin 55 nm thick perovskite can be negligible when com-
pared to the modulation effect of the PIT resonance, since the
electric field is tightly confined in the 5 μm gaps and
enables an extremely sensitive response to the interface environ-
ment, resulting in the strong modulation of the PIT resonance.
In addition, the experimental and simulation results exhibit a
significant discrepancy in the modulation depth along the x
polarization. Theoretical calculations show that the x direction
is more sensitive than the y direction (the near-field simulation
below gives an explanation), but experimentally, the modula-
tion in the x direction is relatively small. This is due to the
coupling effect of PIT resonance and optically active phonon
mode of MAPbI3 film, which was studied by earlier OPTP
characterization at room temperature [26,58–60]. In the recent
study [26], MAPbI3 perovskite has shown an active phonon
mode when the pump fluence is greater than 7 μJ∕cm2 .
Fig. 4. Calculated z-component field distributions in the transverse The observed mode coupling of the phonon mode of the per-
plane of the Au metasurface varying the conductivity of the perovskite ovskite film with the dipole resonance (SRR resonator, Fig. 8 in
film under the x-polarized THz electric field from 0 S/m to 960 S/m. Appendix A) at 1.0 THz in the former study could provide
Incident fields are normalized as 1 V/m. preliminary evidence for our experimental result (x direction,
 998 Vol. 7, No. 9 / September 2019 / Photonics Research Research Article

Table 1. Some Reported Active Modulation of the Optically Controlled THz Modulator
Maximum Modulation Pump Wavelength and
Year Active Material Depth Minimum Working Fluence Ref.
2018 310 nm Ge 29% 800 nm, 254 μJ∕cm2 [20]
2017 300 nm GaAs nanodisks 35% 800 nm, 310 μJ∕cm2 [61]
2017 400 nm, drop coated MoS2 film ∼20% 800 nm, 12.7 μJ∕cm2 [18]
2012 500 nm Si 49% 800 nm, 35 μJ∕cm2 [50]
2018 284 nm VO2 film 138 deg (phase shifting) 800 nm (cw laser), 3.5 W∕cm2 [13]
2018 50 nm high-Tc YBCO 42% 800 nm, 64 μJ∕cm2 [62]
This work 55 nm MAPbI3 film 25% 400 nm, 5 μJ∕cm2 This work

To clarify the underlying mechanism of the modulation ef-

fect in Fig. 3, a commonly used finite-element method is em-
ployed to analyze the electric field distributions, considering
various conductivities of the perovskite film. As described in
Fig. 1(b), a base unit cell is illuminated by the incident
THz radiation with an electric field intensity of 1 V/m.
Here, the Floquet boundary condition is utilized to describe
the continuous metamaterial arrays. In consideration that
the influence of resonance is best exemplified by the z compo-
nent of electric field E z , which is located 27.5 nm above the
upper surface of the substrate, it is shown to characterize res-
onant behaviors at the center of PIT windows. Since the afore-
mentioned resultant transmission through the perovskite-based
anisotropic THz device is a function of frequency, PIT peaks
exist at 0.86 and 1.12 THz for x- and y-polarized THz inci-
dences, respectively. Therefore, we restrict our attention to the
Fig. 5. Calculated z-component field distributions in the transverse
electric fields with various perovskite film conductivities at both
plane of the Au metasurface varying the conductivity of the perovskite
film under the y-polarized THz electric field from 0 S/m to 1440 S/m. of these frequencies, in order to investigate the change of elec-
Incident fields are normalized as 1 V/m. tric field concentrated at the gaps of the metamaterial structure.
One significant finding from Fig. 4(a) is that the electric
field confinement in the gap along the x-polarized THz wave
is around 54 times stronger than in the other region, allowing
at ∼0.86 THz), which is repeatedly tested. In order to more for a giant sensitivity of conductivity change within the gap
intuitively reflect the influence of the coupling between the along the x axis. Particularly, the electric field in the CRR is
phonon mode and the PIT resonance mode on the device, almost completely suppressed, which identifies and character-
we verified it by the simulation with the addition of the phonon izes the feature of a typical PIT effect. Without an optical
mode (see Appendix A, Fig. 11). pump, the perovskite thin film can be considered as a semicon-
ductor with small conductivity of 60 S/m, and the resonators
exhibit very strong electric confinement, which is comparable
to the case without perovskite, as presented in Fig. 4(b). On
photoexcitation, the controllably increased conductivity in-
duced by the pump beam allows for the active tuning under
variable coupling strengths with specified polarization direc-
tion. It is visible that the electric field enhancement shows a
moderate and even strong attenuation when the perovskite con-
ductivity goes up to 240 S/m and 960 S/m, respectively. This
confirms the weakening coupling between CRR and SRR,
as we predicted in previous sections. The underlying physics
can be intuitively understood as follows: the radiative LC mode
Fig. 6. Time-evolution dynamics of the metasurface-perovskite de- resonance in the SRR pair is hampered by the photoexcitation
vice. (a) Transient transmission spectra of the y-polarized THz electric enhancement, leading to a weaker circulating current in the
field at different pump-probe delay values for an average pump fluence
of 30 μJ∕cm2 . (b) Measured transient THz excitation dynamics for
SRR pair. Based on this mechanism, we reiterate the designed
perovskite (CH3 NH3 PbI3 ) thin film spin-coated on the Fano- structure as the image illustrated in Fig. 1 to provide a deeper
resonant metasurface implemented by using OPTP measurements insight into this process. Two gaps, namely, the gap between
for various pump fluences. Solid curves represent the fittings of recom- the CRR and SRR and the gap in the SRR, would be connected
bination processes utilizing rate equations, where the dotted lines are by the perovskite film with a considerably high conductivity.
measured by experiment. Hence, the circulating current of SRR is difficult to be
 Research Article Vol. 7, No. 9 / September 2019 / Photonics Research 999

generated by strong oscillation with the incident THz wave. ΔT t t

− t
A1 e −t 1  A2 e −t 2  A0 , (3)
On the other hand, the electric field confinement of T
1.12 THz in the gap along the y-axis direction corresponding is more suitable for the bimolecular recombination-dominated
to the y-polarized THz incidence shows similar patterns with process. Because the first-order recombination process (i.e., the
the change of optically controlled conductivity, as illustrated in trap-assisted monomolecular recombination) is reported to last
Fig. 5. These results contribute to the effective and active mod- longer than 10 ns, far beyond the delay line in the MAPbI3
ulations of meta-atoms via optical control of the perovskite sample, we fix the decay time constant t 1 at 10 ns for conven-
conductance. ience [41]. The best-fitting curves are shown as solid lines in
In order to explore the dynamic response of ultrafast modu- Fig. 6(b). Additionally, the trimolecular decay time constant t 3
lation aspect, we have experimentally characterized the temporary at 20 and 30 μJ∕cm2 is obtained to be 6.2 and 6.7 ps, respec-
resonant switching behavior in our proposed perovskite- tively. What follows is the bimolecular decay time constant t 2 ,
functionalized metamaterial hybrid device. For illustration, the which is 63, 96, and 141 ps for the 5, 20, and 30 μJ∕cm2
temporal evolution of the dynamic modulation response in y pump fluence, respectively. This indicates that the fast decay
polarization and the transient transmission amplitude change process (i.e., the second- and third-order recombination) of
of solution spin-coated perovskite film on quartz in the absence photoconductivity in perovskite film is mainly completed within
of Au structure are shown in Fig. 6. Notably, the relative time ∼200 ps, which is consistent with the result of Fig. 6(a), where
delay between the pump and the THz probe is controlled by a the response of PIT resonance is quickly repaired within 151 ps
step motor, and the 400 nm pump pulse is assumed to stimulate and then changed slowly. It is well known that the trap-assisted
the sample at a 0 ps time delay. A main conclusion from Fig. 6(a) monomolecular recombination rate of the MAPbI3 perovskite is
is that under a 30 μJ∕cm2 pump fluence, the PIT amplitude at exceptionally low, and the carriers’ relaxation curve can be high-
the frequency of 1.12 THz gradually restores from the state at quality fitted when just incorporating bimolecular and trimo-
0 ps time delay and becomes very close to the original transmis- lecular decay components [48]. Based on this point—that the
sion amplitude without photoexcitation at 561 ps. It can be seen first-order decay can be ignored—we can understand the weak
that our sample exhibits a faster response speed (<1 ns, GHz) effect of a slow recombination process on device response speed.
than the millisecond response of conventional silicon-based pho- Therefore, it can be concluded that the high-speed response
toactive devices. The heavily curtailed PIT amplitude at a 0 ps of our sample is caused by the fast recombination process
time delay implies that the maximum density of photoexcited (i.e., the second- and third-order recombination) of nonequili-
carriers exists in the perovskite film. brium carriers.
In Fig. 6(a), we use the transmission amplitude of the PIT
resonance 3. CONCLUSION
E t − E on In summary, we have experimentally demonstrated the ultra-
× 100%, (1)
E off − E on sensitive modulation (5 μJ∕cm2 ) and THz transmission modu-
lation characteristics of an active metamaterial device by
to quantify the recovery of device. E off is the PIT amplitude
constructing a perovskite-hybridized anisotropic PIT resonator
without pump, E on is the PIT amplitude at the time delay
array with a spin-coated 55 nm MAPbI3 film as the photoactive
of 0 ps, and E t is the PIT amplitude at the time delay of t.
medium. The high-speed dynamic behavior of all-optical
Among the time delays of 18, 151, and 561 ps, the calculation
modulation along the x and y polarizations with different PIT
results of recovery ratio are 43.3%, 66.7%, and 82.3%, respec-
frequencies is obtained with a giant sensitivity and a far faster
tively. And the recovery rate of the PIT response is progressively
speed compared to silicon-based devices, which implies the
decreased, as revealed in the recombination curve of the non-
excellent potential of easily solution-processed perovskite in
equilibrium carriers from Fig. 6(b). By step-collecting the
application of fast optical response devices. Moreover, we have
change of amplitude (ΔT ) throughout the delay line at the
performed accurate numerical simulations that prove that our
peak THz transient signal (T ), time-resolved relaxing of photo-
resonator structure is sensitive to the surface dielectric envi-
conductivity in the MAPbI3 film is obtained as −ΔT ∕T , which
ronment, a prerequisite for weak-light detection and sensing.
is proportional to Δσ. Unlike the PIT resonance, which is more Further, the polarization-dependent design has provided a
susceptible to interface environment, the light-induced change promising alternative to modulate the resonance frequency
of perovskite film is relatively small (where the maximum is while preserving the all-optical switching merits (e.g., high sen-
∼7% under a 30 μJ∕cm2 pump fluence) and displays a clear sitivity and ultrafast speed). Therefore, the anisotropic PIT
pump power dependence. Extracting the characteristic time structure can be regarded as a potential candidate for multi-
from this temporal evolution is essential for the study of other functional subwavelength metamaterial devices operating in
MAPbI3 -based devices. At the pump fluence of 20 and the THz range.
30 μJ∕cm2 , a triple-exponential decay equation,
ΔT t t −t
− t
A1 e −t 1  A2 e −t 2  A3 e t 3  A0 , (2) APPENDIX A
T The physical origin of photon-active modulation of the near-
is used to fit the dynamic curves for the presence of trimolecular field coupling effect between two bright models in the hybrid
processes (Auger recombination). Meanwhile, at the lowest flu- perovskites metasurface is explained by a typical coupled har-
ence, a double-exponential expression, monic oscillator model.
 1000 Vol. 7, No. 9 / September 2019 / Photonics Research Research Article

Fig. 11. Simulated results with a phonon mode at 1.0 THz in the
Fig. 7. Normalized linear absorption and PL spectra of the spin- perovskite thin film as a function of optical conductivity of the per-
coated perovskite (CH3 NH3 PbI3 ) film. The PL and absorption peaks ovskite thin film.
are located at 760 and 740 nm, respectively.

ω−2
1 p̈ t  Γ1 ω−1
1 p_ t  pt
f t − κqt,
ω−2
2 q̈ t  Γ2 ω−1
2 q_ t  qt
−κpt, (A1)
where κ is the linear coupling strength between the two reso-
nators. Γ1 , Γ2 , ω1 , and ω2 are damping factors and resonance
frequencies of the two bright (CRR and SRR) resonators. Γ1
and Γ2 are defined by the excitation pt and qt, respectively.
f t is the external motivating force that projects Γ2 . We could
express the susceptibility χ̃ e as a function of the above-
mentioned parameters, and then the transmission amplitude
Fig. 8. Intrinsic THz spectra of SRR and CRR resonators along can be calculated from the susceptibility [56].
(a) x and (b) y directions. The near-field coupling effect between In order to explain the relative lower modulation depth
CRR and SRR is the origin of PIT resonance. along the x polarization, a Lorentz–Drude model is used here
to determine the material properties as
σω
−iε0 ωε∞ − 1
X ε0 ω2p,m ω σ
 2 2 i , (A2)
m
1
iω 0,m − ω   ωΓ m ωε 0

where ωp,m , ω0,m , and Γm are plasma frequencies, phonon res-

onance frequencies, and the scattering rate, respectively. σ is the
optical conductivity generated by the optical pump. We uti-
lized this model to roughly describe the coupled phonon mode
and optical conductivity of perovskite film.
Fig. 9. Theoretical calculation results of the Lorentzian mechanical A wide array of photosensitive materials is explored for the
oscillator model along the (a) x-polarized and (b) y-polarized incident advancement of active THz metadevices, including semicon-
THz electric field. ductors (Ge, GaAs, Si, and perovskites), 2D transition metal
dichalcogenides (MoS2 ), phase-change materials (VO2 ), and
superconductors (YBCO). Traditional semiconductors such
as Ge, Si, and GaAs generally have a large thickness (∼300 nm)
when used as a photon doping medium, and their pump
fluence usually needs more than 100 μJ∕cm2 . There are few
reports on the use of chemical vapor deposition (CVD)-grown
large-area transition metal dichalcogenides (TMDCs) film for
THz metasurfaces, but the application of MoS2 nanosheets
on the surface of metal structures has achieved a good
all-optical modulation effect. The optical properties of super-
conductors and phase change materials are usually related to the
ambient temperature, and a high power is required when their
state is significantly changed by means of optical pumping. In
Fig. 10. Group delay data extracted from experiment results as a this work, our device can work at pump fluence as low as
function of pump fluence: (a) x-direction and (b) y-direction. 5 μJ∕cm2 , and this result is based on the premise that our
 Research Article Vol. 7, No. 9 / September 2019 / Photonics Research 1001

perovskite thin film is only 55 nm thick. If the thickness of the 14. M. Manjappa, P. Pitchappa, N. Wang, C. Lee, and R. Singh, “Active
control of resonant cloaking in a terahertz MEMS metamaterial,”
film is appropriately increased, the modulation depth of our
Adv. Opt. Mater. 6, 1800141 (2018).
device may be better. 15. L. Cheng, Z. Jin, Z. Ma, F. Su, Y. Zhao, Y. Zhang, T. Su, Y. Sun, X. Xu,
Z. Meng, Y. Bian, and Z. Sheng, “Mechanical terahertz modulation
Funding. National Natural Science Foundation of China based on single-layered graphene,” Adv. Opt. Mater. 6, 1700877
(NSFC) (11802339, 11804387, 11805276, 61801498, (2018).
16. P. Pitchappa, A. Kumar, S. Prakash, H. Jani, T. Venkatesan, and R.
61805282); Scientific Researches Foundation of National
Singh, “Chalcogenide phase change material for active terahertz
University of Defense Technology (ZK16-03-59, ZK18-01- photonics,” Adv. Mater. 31, 1808157 (2019).
03, ZK18-03-22, ZK18-03-36); Natural Science Foundation 17. H. Cai, Q. Huang, X. Hu, Y. Liu, Z. Fu, Y. Zhao, H. He, and Y. Lu,
of Hunan Province (2016JJ1021); Open Director Fund of “All-optical and ultrafast tuning of terahertz plasmonic metasurfaces,”
State Key Laboratory of Pulsed Power Laser Technology Adv. Opt. Mater. 6, 1800143 (2018).
18. Y. K. Srivastava, A. Chaturvedi, M. Manjappa, A. Kumar, G. Dayal, C.
(SKL2018ZR05); Open Research Fund of Hunan Provincial Kloc, and R. Singh, “MoS2 for ultrafast all-optical switching and modu-
Key Laboratory of High Energy Technology (GNJGJS03); lation of THz Fano metaphotonic devices,” Adv. Opt. Mater. 5,
Opening Foundation of State Key Laboratory of Laser 1700762 (2017).
Interaction with Matter (SKLLIM1702); Youth Talent 19. A. Kumar, Y. K. Srivastava, M. Manjappa, and R. Singh, “Color-
Lifting Project (17-JCJQ-QT-004). sensitive ultrafast optical modulation and switching of terahertz plas-
monic devices,” Adv. Opt. Mater. 6, 1800030 (2018).
20. W. X. Lim, M. Manjappa, Y. K. Srivastava, L. Cong, A. Kumar,
†
These authors contributed equally to this work. K. F. MacDonald, and R. Singh, “Ultrafast all-optical switching of
germanium-based flexible metaphotonic devices,” Adv. Mater. 30,
1705331 (2018).
REFERENCES 21. J. Gu, R. Singh, X. Liu, X. Zhang, Y. Ma, S. Zhang, A. S. Maier, Z.
1. G. P. Neupane, K. Zhou, S. Chen, T. Yildirim, P. Zhang, and Y. Lu, Tian, K. Azad, A. Chen, H.-T. Chen, A. Taylor, J. Han, and W.
“In-plane isotropic/anisotropic 2D van der Waals heterostructures for Zhang, “Active control of electromagnetically induced transparency
future devices,” Small 15, 1804733 (2019). analogue in terahertz metamaterials,” Nat. Commun. 3, 1151
2. W. Zhou, J. Chen, H. Gao, T. Hu, S. Ruan, A. Stroppa, and (2012).
W. Ren, “Anomalous and polarization-sensitive photo-response of 22. L. Cong, Y. K. Srivastava, H. Zhang, X. Zhang, J. Han, and S. Ranjan,
Td-WTe2 from visible to infrared light,” Adv. Mater. 31, 1804629 “All-optical active THz metasurfaces for ultrafast polarization switch-
(2018). ing and dynamic beam splitting,” Light Sci. Appl. 7, 28 (2018).
3. J. Liu, Y. Zhou, Y. Lin, M. Li, H. Cai, Y. Liang, M. Liu, Z. Huang, F. Lai, 23. H. Jung, H. Jo, W. Lee, B. Kim, H. Choi, M. S. Kang, and H. Lee,
F. Huang, and W. Zheng, “Anisotropic photo-response of the ultrathin “Electrical control of electromagnetically induced transparency by
GeSe nanoplates grown by rapid physical vapor deposition,” ACS terahertz metamaterial funneling,” Adv. Opt. Mater. 7, 1801205
Appl. Mater. Interfaces 11, 4123–4130 (2019). (2018).
4. Z. Zhou, M. Long, L. Pan, X. Wang, M. Zhong, M. Blei, J. Wang, J. 24. Z. Chen, X. Chen, L. Tao, K. Chen, M. Long, X. Liu, K. Yan, R. I.
Fang, S. Tongay, W. Hu, J. Li, and Z. Wei, “Perpendicular optical re- Stantchev, E. Pickwell-MacPherson, and J.-B. Xu, “Graphene con-
versal of the linear dichroism and polarized photodetection in 2D trolled Brewster angle device for ultra-broadband terahertz modula-
GeAs,” ACS Nano 12, 12416–12423 (2018). tion,” Nat. Commun. 9, 4909 (2018).
5. S. Yoo, S. Lee, and Q.-H. Park, “Loss-free negative-index metamate- 25. T.-T. Kim, H.-D. Kim, R. Zhao, S. S. Oh, T. Ha, D. S. Chung, Y. H. Lee,
rials using forward light scattering in dielectric meta-atoms,” ACS B. Min, and S. Zhang, “Electrically tunable slow light using graphene
Photon. 5, 1370–1374 (2018). metamaterials,” ACS Photon. 5, 1800–1807 (2018).
6. R. Schittny, M. Kadic, T. Buckmann, and M. Wegener, “Invisibility 26. M. Manjappa, Y. K. Srivastava, A. Solanki, A. Kumar, T. C. Sum, and
cloaking in a diffusive light scattering medium,” Science 345, 427– R. Singh, “Hybrid lead halide perovskites for ultrasensitive photoac-
429 (2014). tive switching in terahertz metamaterial devices,” Adv. Mater. 29,
7. R. Macedo, T. Dumelow, R. E. Camley, and R. L. Stamps, “Oriented 1605881 (2017).
asymmetric wave propagation and refraction bending in hyperbolic 27. S. J. Kindness, N. W. Almond, B. Wei, R. Wallis, W. Michailow, V. S.
media,” ACS Photon. 5, 5086–5094 (2018). Kamboj, P. Braeuniger-Weimer, S. Hofmann, H. E. Beere, D. A.
8. Y. S. Jonathan, F. Kebin, and J. P. A. Willie, “Zero-rank, maximum Ritchie, and R. Degl’Innocenti, “Active control of electromagnetically
nullity perfect electromagnetic wave absorber,” Adv. Opt. Mater. 7, induced transparency in a terahertz metamaterial array with graphene
1801632 (2019). for continuous resonance frequency tuning,” Adv. Opt. Mater. 6,
9. X. Zhao, Y. Wang, J. Schalch, G. Duan, K. Cremin, J. Zhang, C. Chen, 1800570 (2018).
D. R. Averitt, and X. Zhang, “Optically modulated ultra-broadband all- 28. H. Jung, J. Koo, E. Heo, B. Cho, C. In, W. Lee, H. Jo, J. H.
silicon metamaterial terahertz absorbers,” ACS Photon. 6, 830–837 Cho, H. Choi, M. S. Kang, and H. Lee, “Electrically controllable
(2019). molecularization of terahertz meta-atoms,” Adv. Mater. 30, 1802760
10. Z. Wang, B. Ai, Z. Zhou, Y. Guan, H. Möhwald, and G. Zhang, “Free- (2018).
standing plasmonic chiral metamaterials with 3D resonance cavities,” 29. X. Chen, S. Ghosh, Q. Xu, C. Ouyang, Y. Li, X. Zhang, Z. Tian, J. Gu,
ACS Nano 12, 10914–10923 (2018). L. Liu, A. K. Azad, J. Han, and W. Zhang, “Active control of
11. Q. Yang, J. Gu, Y. Xu, X. Zhang, Y. Li, C. Ouyang, Z. Tian, J. Han, and polarization-dependent near-field coupling in hybrid metasurfaces,”
W. Zhang, “Broadband and robust metalens with nonlinear phase pro- Appl. Phys. Lett. 113, 061111 (2018).
files for efficient terahertz wave control,” Adv. Opt. Mater. 5, 1601084 30. M. Liu, Z. Tian, X. Zhang, J. Gu, C. Ouyang, J. Han, and W. Zhang,
(2017). “Tailoring the plasmon-induced transparency resonances in terahertz
12. M. T. Nouman, J. Hwang, M. Faiyaz, G. Lee, D. Y. Noh, and J.-H. metamaterials,” Opt. Express 25, 19844–19855 (2017).
Jang, “Dynamic metasurface based cavity structures for enhanced 31. L. Q. Cong, K. S. Yogesh, S. Ankur, C. S. Tze, and S. Ranjan,
absorption and phase modulation,” ACS Photon. 6, 374–381 “Perovskite as a platform for active flexible metaphotonic devices,”
(2019). ACS Photon. 4, 1595–1601 (2017).
13. Y. Zhao, Y. Zhang, Q. Shi, S. Liang, W. Huang, W. Kou, and Z. Yang, 32. T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P.
“Dynamic photoinduced controlling of the large phase shift of terahertz Müller-Buschbaum, M. B. Johnston, and L. M. Herz, “Photon reab-
waves via vanadium dioxide coupling nanostructures,” ACS Photon. sorption masks intrinsic bimolecular charge-carrier recombination in
5, 3040–3050 (2018). CH3NH3PbI3 perovskite,” Nano Lett. 17, 5782–5789 (2017).
 1002 Vol. 7, No. 9 / September 2019 / Photonics Research Research Article

33. S.-F. Leung, K.-T. Ho, P.-K. Kung, V. K. S. Hsiao, H. N. Alshareef, 48. W. Christian, E. E. Giles, B. J. Michael, J. S. Henry, and M. H. Laura,
Z. L. Wang, and J.-H. He, “A self-powered and flexible organometallic “High charge carrier mobilities and lifetimes in organolead trihalide
halide perovskite photodetector with very high detectivity,” Adv. Mater. perovskites,” Energy Environ. Sci. 7, 2269 (2014).
30, 1704611 (2018). 49. W. S. De, J. Holovsky, S. J. Moon, P. L. Per, B. Niesen, M. Ledinsky,
34. L. Najafi, B. Taheri, B. Martín-García, S. Bellani, D. D. Girolamo, A. F. J. Haug, J.-H. Yum, and C. Ballif, “Organometallic halide perov-
Agresti, R. Oropesa-Nunez, S. Pescetelli, L. Vesce, E. Calabro, M. skites: sharp optical absorption edge and its relation to photovoltaic
Prato, A. E. D. R. Castillo, A. D. Carlo, and F. Bonaccorso, “MoS2 performance,” J. Phys. Chem. Lett. 5, 1035–1039 (2014).
quantum dot/graphene hybrids for advanced interface engineering 50. J. Gu, R. Singh, X. Liu, X. Zhang, Y. Ma, S. Zhang, A. S. Maier,
of a CH3NH3PbI3 perovskite solar cell with an efficiency of over Z. Tian, K. Azad, A. Chen, H.-T. Chen, A. Taylor, J. Han, and
20%,” ACS Nano 12, 10736–10754 (2018). W. Zhang, “Active control of electromagnetically induced transpar-
35. J. Zhen, W. Zhou, M. Chen, B. Li, L. Jia, M. Wang, and S. Yang, ency analogue in terahertz metamaterials,” Nat. Commun. 3, 1151
“Pyridine-functionalized fullerene additive enabling coordination inter- (2012).
actions with CH3NH3PbI3 perovskite towards highly efficient bulk het- 51. H. Cho, S.-H. Jeong, M.-H. Park, Y.-H. Kim, C. Wolf, C.-L. Lee,
erojunction solar cells,” J. Mater. Chem. A 7, 2754–2763 (2019). J. H. Heo, A. Sadhanala, N. Myoung, S. Yoo, S. H. Im, R. H.
36. M. H. Laura, “Charge-carrier dynamics in organic-inorganic metal hal- Friend, and T.-W. Lee, “Overcoming the electroluminescence
ide perovskites,” Annu. Rev. Phys. Chem. 67, 65–89 (2016). efficiency limitations of perovskite light-emitting diodes,” Science
37. D. A. Valverde-Chávez, C. S. Ponseca, C. C. Stoumpos, A. Yartsev, 350, 1222–1225 (2015).
M. G. Kanatzidis, V. Sundström, and D. G. Cooke, “Intrinsic femtosec- 52. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, and H. J.
ond charge generation dynamics in single crystal CH3NH3PbI3,” Snaith, “Efficient hybrid solar cells based on meso-superstructured
Energy Environ. Sci. 8, 3700–3707 (2015). organometal halide perovskites,” Science 338, 643–647 (2012).
38. Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao, and J. 53. B. Jeong, H. Han, Y. J. Choi, S. H. Cho, E. H. Kim, S. W. Lee, J. S.
Huang, “Electron-hole diffusion lengths > 175 μm in solution-grown Kim, C. Park, D. Kim, and C. Park, “All-inorganic CsPbI3 perovskite
CH3NH3PbI3 single crystals,” Science 347, 967–970 (2015). phase-stabilized by poly(ethylene oxide) for red-light-emitting diodes,”
39. O. E. Semonin, G. A. Elbaz, D. B. Straus, T. D. Hull, D. W. Paley, A. M. Adv. Funct. Mater. 28, 1706401 (2018).
van der Zande, J. C. Hone, I. Kymissis, C. R. Kagan, X. Roy, and J. S. 54. C. W. Singh, R. Zhang, C. J. G. Han, M. Tonouchi, and W. L. Zhang,
Owen, “Limits of carrier diffusion in n-type and p-type CH3NH3PbI3 per- “Plasmon-induced transparency in metamaterials: active near field
ovskite single crystals,” J. Phys. Chem. Lett. 7, 3510–3518 (2016). coupling between bright superconducting and dark metallic mode
40. Y. Kim, E. Yassitepe, O. Voznyy, R. Comin, G. Walters, X. Gong, P. resonators,” Appl. Phys. Lett. 103, 101106 (2013).
Kanjanaboos, A. F. Nogueira, and E. H. Sargent, “Efficient lumines- 55. X. Zhang, N. Xu, K. Qu, Z. Tian, R. Singh, J. Han, and W. L. Zhang,
cence from perovskite quantum dot solids,” ACS Appl. Mater. “Electromagnetically induced absorption in a three-resonator meta-
Interfaces 7, 25007–25013 (2015). surface system,” Sci. Rep. 5, 10737 (2015).
41. F. Zhang, H. Zhong, C. Chen, X.-G. Wu, X. Hu, H. Huang, J. Han, B. 56. P. Tassin, L. Zhang, R. Zhao, A. Jain, T. Koschny, and C. M.
Zou, and Y. Dong, “Brightly luminescent and color-tunable colloidal Soukoulis, “Electromagnetically induced transparency and absorption
CH3NH3PbX3 (X = Br, I, Cl) quantum dots: potential alternatives for in metamaterials: the radiating two-oscillator model and its experimen-
display technology,” ACS Nano 9, 4533–4542 (2015). tal confirmation,” Phys. Rev. Lett. 109, 187401 (2012).
42. Q. A. Akkerman, V. D’Innocenzo, S. Accornero, A. Scarpellini, A. 57. A. S. Roberts, D. Wang, D. Fei, D. Wang, P. K. Kristensen, S. I.
Petrozza, M. Prato, and L. Manna, “Tuning the optical properties of Bozhevolnyi, and K. Pedersen, “Multilayer tungsten-alumina-based
cesium lead halide perovskite nanocrystals by anion exchange reac- broadband light absorbers for high-temperature applications,” Opt.
tions,” J. Am. Chem. Soc. 137, 10276–10281 (2015). Mater. Express 6, 2704–2714 (2016).
43. K. Wei, T. Jiang, Z. Xu, J. Zhou, J. You, Y. Tang, H. Li, R. Chen, X. 58. C. Laovorakiat, J. M. Kadro, T. Salim, D. Zhao, T. Ahmed, Y. M. Lam,
Zheng, S. Wang, K. Yin, Z. Wang, J. Wang, and X. Cheng, “Ultrafast J. X. Zhu, R. A. Marcus, M. E. Michel-Beyerle, and E. E. M. Chia,
carrier transfer promoted by interlayer Coulomb coupling in 2D/3D per- “Phonon mode transformation across the orthohombic-tetragonal
ovskite heterostructures,” Laser Photon. Rev. 12, 1800128 (2018). phase transition in a lead-iodide perovskite CH3NH3PbI3: a terahertz
44. H. Li, X. Zheng, Y. Liu, Z. P. Zhang, and T. Jiang, “Ultrafast interfacial time-domain spectroscopy approach,” J. Phys. Chem. Lett. 7, 1–6
energy transfer and interlayer excitons in the monolayer WS2/CsPbBr3 (2016).
quantum dot heterostructure,” Nanoscale 10, 1650–1659 (2018). 59. M. Bokdam, T. Sander, A. Stroppa, S. Picozzi, D. D. Sarma, C.
45. K. Wei, Z. J. Xu, R. Z. Chen, X. Zheng, X. A. Cheng, and T. Jiang, Franchini, and G. Kresse, “Role of polar phonons in the photo excited
“Temperature-dependent excitonic photoluminescence excited by state of metal halide perovskites,” Sci. Rep. 6, 28618 (2016).
two-photon absorption in perovskite CsPbBr3 quantum dots,” Opt. 60. C. La-o-vorakiat, L. Cheng, T. Salim, R. A. Marcus, M. E. M. Beyerle,
Lett. 41, 3821–3824 (2016). Y. M. Lam, and E. E. M. Chia, “Hybrid tandem quantum dot/organic
46. X. Guichuan, M. Nripan, S. L. Swee, Y. Natalia, L. Xinfeng, S. Dharani, photovoltaic cells with complementary near infrared absorption,” Appl.
G. Michael, M. Subodh, and C. S. Tze, “Low-temperature solution- Phys. Lett. 110, 123901 (2017).
processed wavelength-tunable perovskites for lasing,” Nat. Mater. 61. M. R. Shcherbakov, S. Liu, V. V. Zubyuk, A. Vaskin, P. P.
13, 476–480 (2014). Vabishchevich, G. Keeler, T. Pertsch, T. V. Dolgova, I. Staude, I.
47. J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C. S. Lim, J. A. Chang, Brener, and A. A. Fedyanin, “Ultrafast all-optical tuning of direct-
Y. H. Lee, H.-J. Kim, A. Sarkar, M. K. Nazeeruddin, M. Grätzel, and gap semiconductor metasurfaces,” Nat. Commun. 8, 17 (2017).
S. Seok, “Efficient inorganic-organic hybrid heterojunction solar cells 62. Y. K. Srivastava, M. Manjappa, L. Cong, H. N. S. Krishnamoorthy, V.
containing perovskite compound and polymeric hole conductors,” Savinov, P. Pitchappa, and R. Singh, “A superconducting dual-
Nat. Photonics 7, 486–491 (2013). channel photonic switch,” Adv. Mater. 30, 1801257 (2018).