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Abstract
Incorporating photosensitive material into structured metamaterials explores opportunities for dynamical operation across the terahertz functional devices, enabled by the efficient interaction between light and matter. In this work, the CsPbBr3 quantum dots are incorporated into the metasurfaces, realizing the active control of the plasmon-induced transparency. In the experiment, the normalized modulation depth of transparency effect is up to 74%. Rigorous numerical and theoretical simulations verify that the variation of dynamic physical process is associated with the charge storage capacity in the capacitive metasurface. An observed phase advance and group delay indicate the hybrid metasurface is useful for slow light application. In addition, the simple process provides a convenient way for the development of terahertz functional devices.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the development of wireless communication technology, increasingly requirements are presented for terahertz (THz) wave manipulation. In past decade, metamaterials are widely used to manipulate electromagnetic wave due to their functional rich and spectral scalable properties. Metasurfaces, as 2D planar metamaterials, are artificially designed periodic resonators, which can imitate effects in quantum mechanics [1–4]. Plasmon-induced transparency (PIT), which mimics the classical quantum electromagnetically induced transparency (EIT) phenomenon, is an effective strategy to control the transmission properties of the incident THz wave with designed metasurface structures [5–8]. The PIT phenomenon results from the destructive interference between two resonances, which are coupled to the radiation field on one chip. Transmission of the phenomenon is modulated passively by adjusting the geometric parameters of the metamaterial in the early stages. In 2008, Zhang et al. fabricated a π-shaped PIT structure as a pioneering work and control the slow light effect by changing the distance of the metallic nanorods [9]. After that, Yang et al. constructed an all-dielectric PIT-like system using silicon nanobars (bright mode) which coupled with ring nanocavity (dark mode) [10]. Amin et al. designed a PIT metasurface using four gold nanodisks as a unit cell, and achieved the modulation by tuning the radius of each nanodisk [11]. In recent years, THz metadevices, which are integrated novel active materials into metasurfaces, become a booming field of research [12–18]. In order to resemble the true atomic PIT system in optical control, Gu et al. innovatively embedded the photosensitive silicon islands into the split-ring gaps, realizing an active control of PIT structures with pump laser excitations [19]. Inspired by this work, many novel optical materials, such as germanium and 2D molybdenum disulfide monolayers have also been widely reported [20,21]. Recently, photosensitive perovskite has been emerged as an excellent optoelectronic material in the application of THz wave modulation [22,23]. More recently, enabled by excellent properties of the photoexcited carriers, high light conversion efficiency and low production cost compared with other quantum dots (QDs), perovskite QDs are promising candidates as the photoactive medium to endow the THz metadevices.
Perovskite has been exploited as a competitive optical material in photoelectric field since 2009. Especially, the inorganic halide perovskite QDs (e.g. CsPbBr3 QDs) cause a lot of attention on account of their excellent optical properties and quantum confinement effect. Kovalenko et al. firstly reported the optical properties of CsPbBr3 perovskite QDs, which showed high color purity, adjustable absorption, and striking quantum yield. The QDs have been extensively used in the field of light capture [24], light-emitting diode (LED) [25], and solar cell [26]. With the improvement of the time-resolved THz spectroscopy, the essential characteristics of CsPbBr3 QDs have been further analyzed in THz region. Yettapu et al. revealed that the remarkably transport properties of CsPbBr3 QDs were achieved by the absence of surface defects in the middle band gap of the QDs via the optical pump THz probe spectroscopy [27]. Sarkar et al. reported the efficient hot electron/hole transfer in CsPbBr3 [28]. Recently, CsPbBr3 QDs have drawn more attention in THz functional device, such as the high-speed THz modulator [29,30] and the ultrafast information encoder [31], enabled by the higher carrier mobility and larger diffusion length within the QDs. Compared with the traditional QDs, CsPbBr3 QDs have more advantages such as adjustable bandgap, wide excitation, narrow-band emission, and negligible self-absorption effects, which are a kind of promising photoactive materials in THz field [32]. However, although the CsPbBr3 QDs have excellent properties in contemporary optics, their applications in manipulating the resembling quantum mechanics are not fully demonstrated.
In this work, we design and fabricate a sensitive perovskite hybrid metasurface, and measure the active controllable THz PIT properties. The CsPbBr3 QDs solution is spin-coated on the surface of the closed-split ring resonators (CSRRs), wherein a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) transport layer is sandwiched to improve the control amplitude. An optical continuous wave (CW) laser is incident on the hybrid metasurface to photoexcite carriers in CsPbBr3 QDs and modulate the capacitive gap in split-ring, thereby changing the THz transparency window. The hybrid metasurface structure exhibits a fast and deep PIT phenomenon modulation. Meanwhile, the obvious phase advance and group delay indicate the perovskite QDs are good candidates for THz functional devices in wireless communications.
2. Design and characterization
The designed PIT sample consists of 200 nm-thick gold resonator array deposited on 500 nm-thick Si wafer (N-type, resistivity is 5000 Ω cm), with CsPbBr3 QDs and PEDOT:PSS hybrid layer spin-coated on their surface. THz pulse is normally incident and transmits through the sample with a 450 nm CW laser applied in an angle of 40 degrees in the measurement, as revealed in Fig. 1(a). The unit cell of the designed metasurface consists of a split ring inside a closed ring, as shown in Fig. 1(b). The detailed geometric parameters of the structure are shown in Fig. 1(c), and the dimensions are as follows: a=50 μm, b=40 μm, c=20 μm, d=4 μm, and w=3 μm. The fabrication of the photoactive medium started with spin-coating PEDOT:PSS on the metasurface at 5000 rpm and annealed in 130℃ for 20 min, followed by depositing CsPbBr3 QDs on the top at 3000 rpm. The fabrication of the QDs is in high-temperature synthesis method with an eventual concentration of 20 mg/mL [33]. CsPbBr3 QDs exhibit a spectrally narrow photoluminescence (PL) spectrum around 510 nm wavelength. And the full width at half maximum (FWHM) of the emission peak is just 23 nm as shown in Fig. 1(d). The typical optical absorption spectrum in the range of 400-510 nm wavelength denotes that the QDs can be excited by the violet and blue laser. The TEM photograph shows that the QDs are in the typical rectangle shape and the particle size is about 12 nm, as shown in the inset image of Fig. 1(d). The transmission spectra with and without spin-coating PEDOT: PSS/ CsPbBr3 QDs hybrid layer are shown in Fig. 1(e). Without the operation of spin-coating, the transparency window is at 0.98 THz with the transmittance of 23%. The transparency window shifts to 0.91THz and the transmittance decreases to 15% with spin-coating. This red-shift is caused by the increase of the relative permittivity at metasurface interface from air to CsPbBr3 hybrid layer.
Fig. 1. (a) Schematic illustration of the CsPbBr3 QDs based PIT structure. (b) Optical microscopy of the designed metasurface. (c) Design dimensions of a unit cell. (d) PL intensity and absorption spectrum of CsPbBr3 QDs. The inset shows the TEM image of the synthesized QDs. (e) Measured transmission spectra of the PIT structure with and without spin-coating PEDOT: PSS/ CsPbBr3 QDs.
To determine primarily geometrical dimensions and explore the underlying mechanism of the PIT phenomenon, the structures are characterized in the simulation. The simulation is investigated using the numerical computer simulation technology (CST) software with perfect magnetic field boundary in x-direction and perfect electric field boundary in y-direction. The permittivity of silicon is 11.9 and the conductivity of gold is 4.56 × 107 S/m. The incident THz wave is propagated in kz direction and the electric field is parallel with the gap along y direction. Three sets of structures are simulated in this work. The first structure is a split-ring array with an inductive-capacitive (LC) resonance and narrow linewidth at 0.98 THz, as shown in Fig. 2(a). The second structure is a closed-ring array with a wide linewidth at 0.91 THz. The third structure is the closed-spilt rings array with a transparency window at 0.91 THz. To understand the PIT formation mechanism better, the field distribution at the resonance center is analyzed, as shown in Figs. 2(d)–2(f). The resonance in the single split ring is strongly concentrated in the gap of the split ring while the resonance in the single closed ring is symmetrically distributed in two side arms. It is evident that both resonances of these two rings are decreased once coupled in PIT structure, which indicates that it is the destructive interference of the two rings with deviating linewidth that results the transparency window.
Fig. 2. Simulated transmission spectra of (a) single split ring, (b) single closed ring, and (c) CSRRs resonator. (d-f) Corresponding field distribution of (a-c). All simulation is under Ey polarization.
3. Results and discussion
3.1 Experiment and numerical analysis
The optical characterization of PIT metasurface is investigated under different pump powers, as shown in Fig. 3. The measurement is carried out by THz time-domain spectroscopy (THz-TDS) system. The measured transmission spectra with different pump powers of 450 nm CW laser are shown in Fig. 3(a). When there is no photoexcitation, a typical PIT peak is obtained with a transmission amplitude of 15.18% at 0.91 THz. As the CW pump power is varied from 0.22 W/cm2 to 0.88 W/cm2, the transparency window gradually decreases to 8.89%, while the corresponding resonance dips at 0.82 THz and 1.1 THz gradually increase in magnitude. Transparency window doesn't disappear completely because the phonon modes of the perovskite are coupled with the designed metallic structure in the lower frequency of THz band [34,35]. To investigate the relationship between photoexcitation of the hybrid layer and deterioration of the capacitive gap in split-ring, the optical control of the PIT hybrid metasurface is simulated. In the simulation, the CsPbBr3 QDs hybrid layer is modelled as a 100 nm thick normal material with the measured permittivity of 5. The conductivities are fitted from 1 S/m (corresponding to the pump power of 0 W/cm2) to 1200 S/m (corresponding to the pump power of 0.88 W/cm2) to model the carrier excitations in hybrid layer. With the conductivity of the hybrid layer increased, the transparency window gradually diminishes, as illustrated in Fig. 3(b). This effect further verifies that the suppression of the transparent phenomenon is due to the carrier excitations of the spin-coated layer. Theoretical fitting curves, as shown in Fig. 3(c), are calculated to explore the physical mechanism of the active modulation in PIT effect. The calculation uses the coupled differential equations as follows [19]:
Fig. 3. (a) Measured transmission spectra of the hybrid metasurface with different pump powers. (b) Simulated results with different conductivities of the QDs layer. (c) Theoretical fitting results with the variation of damping rates. (d) Fitting parameters in the theoretical calculation. (e) Normalized amplitude variation derived from the measured transmission spectrum.
Table 1. Some reported modulation depth and group delay of the optical controlled PIT phenomenon
E-field distributions are simulated to further clarify the dynamic mechanism. Figure 4 provides the field distributions of blank metasurface and hybrid metasurface with different pump powers. The distributions with the pump powers of 0, 0.44 and 0.88 W/cm2 correspond to the regression of the PIT effect due to the different carrier excitations. Without the CsPbBr3 QDs hybrid layer, the field distribution is mainly concentrated in the split-ring gap and the intensity is maximum as shown in Fig. 4(a). After spin-coating the hybrid layer, the resonance intensity shows a marginal decrease in Fig. 4(b). When the 450 nm CW laser is incident on the metasurface with the pump power of 0.44 W/cm2, the E-Field distribution decreases substantially. With the pump power increasing to 0.88 W/cm2, the intensity in the split-ring gap almost disappears. However, the field distribution in the side arms of the closed ring is inverse to strengthen gradually. As the split-ring resonance is suppressed, the destructive interference between the split ring and the closed ring is disrupted. Hence, the electric field redistributes to the side arms of the closed ring, just like the case shown in Fig. 2(e).
Fig. 4. E-Field distributions at the resonance frequency for (a) the blank metasurface without QDs (b) the hybrid metasurface without the photoexcitation (c) with the pump power of 0.44 W/cm2 (d) with the pump power of 0.88 W/cm2.
3.2 Discussion of slow light capability
The phase advance and the group delay behavior of the designed hybrid metasurface are analyzed to show the slow light capability. The phase advance spectra prove a strong normal dispersion in the transparency region, as shown in Fig. 5(a). When the pump power increases from 0 to 0.88 W/cm2, the phase slop of the transparency window decreases from 1.016 to 0.454. The group delay $\Delta {t_g}$ is extracted from the measurements, which reveals the slow light capability directly. It is defined as [12]:
where $\varphi $ denotes the phase advance in Fig. 5(a) and $\omega $ stands for the angular velocity. The results show that the $\Delta {t_g}$ decreases from 2.09 to 1.07 ps as the photoexcitation increases to 0.88 W/cm2. The wave packet is delayed by 1.02 ps at 0.91 THz, as shown in Fig. 5(b). This obvious group delay testifies the reliability of the PIT effect in this work.4. Conclusion
In summary, this work has demonstrated an optical controlled PIT hybrid metasurface by spin-coating CsPbBr3 QDs hybrid layer as the photoactive material. The measurement results show that the modulation depth of PIT phenomenon is up to 74% and the group delay is over 1 ps. The numerical and theoretical calculations reveal the inherent reasons. With the pump power increasing, the electric field confinement capacity is changed in the split gap, which modulates the PIT phenomenon in the designed metasurface. These proofs convincingly demonstrate that the inorganic halide perovskite QDs are the powerful candidates in modulating the PIT and the slow light effect. It also provides a convenient way for the development of THz functional devices.
Funding
National Natural Science Foundation of China (61705162, 61735010).
Acknowledgments
The authors acknowledge useful discussions with Dr. Xueqian Zhang and Mr. Zhiliang Chen.
Disclosures
The authors declare no conflicts of interest.
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