Perovskite as a Platform for Active Flexible Metaphotonic Devices

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Perovskite as a platform for active flexible meta-photonic devices Longqing Cong, Yogesh Kumar Srivastava, Ankur Solanki, Tze Chien Sum, and Ranjan Singh ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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Perovskite as a platform for active flexible meta-photonic devices Longqing Cong,1,2 Yogesh Kumar Srivastava,1,2 Ankur Solanki,1 Tze Chien Sum,1 and Ranjan Singh1,2,* 1

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore 2 Centre for Disruptive Photonic Technologies, The Photonics Institute, 50 Nanyang Avenue, Nanyang Technological University, Singapore 639798 * Corresponding author: [email protected]

Abstract Solution-processed organic-inorganic hybrid halide perovskite has emerged as an excellent material for harnessing the solar energy. The ease of processing offers a fairly straightforward approach to integrate the methylammonium lead iodide perovskite with diverse technological platforms. Metamaterials, with an array of artificially designed meta-atoms and extraordinary electromagnetic properties, have enabled several functionalities in passive mode, especially for flexible photonic devices that have found application in the areas of flexible displays and wearable sensors. However, it is extremely challenging to induce a dynamic response in flexible photonic devices. Here, we experimentally demonstrate that the solution-processed perovskite with an intrinsic ultrafast response could be an ideal platform for developing active flexible photonic devices. A thin perovskite layer on a flexible metadevice enables an ultrafast all-optical switching of Fano resonance with a time constant of 500 picoseconds. The hybrid flexible meta-photonic device reveals an ultrasensitive dynamic switching with a deep-subwavelength thin film (λ/10000) of 1

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perovskite at extremely low fluence of photoexcitation. The low-cost, facile processing, ultrafast and highly photosensitive nature of the flexible meta-photonic device could find important applications in high-speed data encoding, flexible displays, wearable optoelectronics and ultrasensitive photodetectors. Our findings provide a new platform for ultrafast flexible photonic devices fueled by the remarkable photoconductivity of methylammonium lead halide perovskite.

Keywords: perovskite, ultrafast active switch, flexible devices, meta-photonic devices

2


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The

recent

photonics

based

technological

advancements

require

flexible

optoelectronic devices that could be integrated with display devices, flexible cell phones, portable solar cells and biosensors.1-7 With the discovery of metamaterials, many fascinating phenomena have been realized by artificially designed unit cells such as negative refractive index,8 anomalous wavefront deflection,9-10 invisibility cloaking11 and superlenses.12 The freedom of artificially designed structure provides us a platform to integrate these promising and custom-built functionalities with a flexible substrate, and thus enables the possibility of practical applications such as in wearable, energy efficient photonic devices. However, in most of the previous studies, the proposed phenomena or functionalities are usually based on rigid substrates such as silicon, sapphire, quartz and GaAs, especially for the actively controllable metadevices where there are no dynamic properties of a flexible substrate when it is subjected to the external stimuli.13-16 For the passive metadevices, a range of flexible substrates with excellent optical properties, easy processing and mechanical properties exist where the most common ones are polyethylene terephthalate (PET), polyethylene naphthalene (PEN), polymethylmethacrylate (PMMA), polystyrene, polyimide and polydimethylsiloxane (PDMS).17-18 Among them, polyimide and PDMS are the most widely used for applications in electronic and optoelectronic devices with better thermal stability (stable between -269 ºC to 400 ºC), mechanical strength (Young’s modulus of ~2.5 GPa), ease of fabrication and uniformity.19-20

There are stringent requirements for designing dynamic metadevices in choosing the 3


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material properties of substrate and ensuring the compatibility of fabrication processes. For example, one of the approaches for ultrafast switching in the metadevice is by integrating the patterned silicon islands into the metamaterial unit cells, which requires fabrication based on the epitaxially grown silicon on sapphire (SOS) substrate. The complex process flow and rigorous alignment required for two-step photolithography would be a severe limitation for large-scale production.13-14, 21 The excellent optical and electronic response of graphene provides an easier access to the dynamic metadevice by transferring the monolayer onto the metamaterials. However, the electronic doping of graphene reduces the modulation speed due to the RC time, especially

in

a

large-area

metadevice.22-23

The

recent

advances

in

the

solution-processed hybrid halide perovskite have enabled diverse opportunities in photonics and solar cells.24-25 In comparison to the silicon-based photovoltaic devices, perovskites26 have emerged as promising materials with higher charge-carrier mobility (>10 cm2 V-1 s-1) and longer diffusion lengths (up to 1 µm or beyond)27 that results in the better conversion efficiency of solar energy.28-30 The most remarkable property of solution-processed perovskite is the ease of process control and compatibility to be integrated with any platform using a simple one-step spin-coating method. Herein, we report an ultrafast all-optical switch for flexible photonics platform based on the low-cost, compatible nature, facile processing and high-efficiency halide perovskite. We demonstrate the excellent dynamic performance of the meta-photonic array based on terahertz asymmetric spit ring resonator (TASR) that exhibits an ultrafast modulation of the Fano resonance as a consequence of 4


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photoexcitation of extremely subwavelength thickness methylammonium lead iodide perovskite thin film.

The schematic architecture of the hybrid flexible meta-photonic device is presented in Figure 1a, where it comprises of a typical metallic resonator array on the flexible polyimide substrate and an ultrathin layer of spin-coated solution-processed methylammonium lead iodide perovskite on the top. The metamaterial array was fabricated using conventional photolithography followed by thermal evaporation of 200 nm aluminum (Al, σ = 3.72×107 S/m) on a 25 µm thick polyimide substrate (ε = 2.96+0.27i). Aluminum has the excellent stability and high conductivity in terahertz regime, and the oxidized layer in the thermal evaporation process of Al will not affect the overall conductivity which is much thinner than the penetration depth (See Supporting Information Note 1). With the metamaterial sample ready, an additional step of spin-coating 30±5 nm thin layer of methylammonium lead iodide perovskite was carried out to obtain the hybrid metadevice. The overall quality of the perovskite film distribution along with the TASR array is presented in Figure 1b, where we could observe a good quality overlayer film distribution on the device surface with all the split gaps of the TASR covered (morphology and profilometer of perovskite film can be found in Supporting Information Note 2). In order to test and verify the performance of the ultrafast all-optical switch in the hybrid meta-photonic device system using a simple processing procedure on a flexible substrate, we employ a TASR as the unit cell of metamaterial with the detailed geometrical parameters as 5


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shown in the inset image of Figure 1b. The key parameter of TASR is the displacement of asymmetric gap (δx = 20 µm) that determines the nature of the Fano resonance.31 In the hybrid meta-photonic device, the properties of the dynamic medium determine the overall switching performance. Here, we prepared the perovskite with unit cell of the pseudocubic lattice as shown in Figure 1c where the crystal structure consists of the organic cation (A), the metal cation (B) and the anion (X) in the perovskite (CH3NH3PbI3). A 10 wt.% perovskite solution was prepared by dissolving 78.3 mg/ml lead (II) iodide (PbI2 from Acros Organics) and 27.0 mg/ml methylammonium iodide (CH3NH3I from DyeSol) in N, N-Dimethylformamide (DMF from Sigma Aldrich) in a N2-filled glove box. Further stirring of the solution was performed at 70 ºC for 2 hours to obtain a clear yellow methylammonium lead iodide (CH3NH3PbI3) solution. Uniform thin film of perovskite on top of the device was obtained using a one-step spin-coating process of heated perovskite solution at 4500 RPM for 20 seconds. The thin perovskite film was modified during the spin-coating process, where toluene (3 parts of toluene for 1 part of CH3NH3PbI3 solution) was dripped at the 4th second after the start of the spin-coating process. This film was then annealed at 100 ºC for 30 minutes. The thickness of the perovskite film was measured to be around 30 nm by surface profilometer prepared on polyimide substrate, and X-ray diffraction (XRD) measurements show the dominant diffraction peaks in the Supporting Information Note 2. The Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) images were also taken to show the morphology of the perovskite film in Supporting Information Note 2. 6


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Optical characterization of the thin-film perovskite was performed using the optical pump terahertz probe (OPTP) method in a nonlinear crystal (ZnTe) based terahertz time-domain spectroscopy system (THz-TDS) and the solution-processed perovskite film was first spin-coated on a blank polyimide substrate as the inset in Figure 2a illustrated. The dynamics of the perovskite was recorded in transient amplitude change when the femtosecond pump pulse (@ 400 nm, 1 kHz repetition rate, beam spot ~ Φ6 mm) strikes the perovskite film at 0 ps time delay, which leads to a steep increase in the change of amplitude (∆T) at the peak terahertz transient signal. The dynamic variation of terahertz electric amplitude was normalized to the maximal transmission electric amplitude of the relaxed perovskite film (-∆E/E) which indicates the real-time dynamics of photoconductivity in the perovskite thin film. This method obtains the frequency-integrated THz transient data at the peak amplitude of the transmitted THz electric field at varied time delays of the pump beam, which is proportional to the frequency-integrated photoconductivity. As observed in Figure 2a, the carriers in perovskite reach the excited state within ~20 ps after the photoexcitation, and then relax back to steady state within hundreds of picoseconds at varied pump fluences. We observe the strong pump fluence dependence of the peak THz transmission with the photoexcitation at 400 nm wavelength in terms of both the maximum of THz transmission amplitude change and temporal evolution nature. The evolution of THz transmission amplitude is simply interpreted from the photoconductivity of the perovskite film which depends on the fluence of the pump 7


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light below the threshold of the material (See Supporting Information Note 4). Regarding the temporal evolution nature, the decay essentially reflects the temporal evolution of charge carrier density by assuming the unchanged carrier mobilities since the photoconductivity is proportional to the product of carrier density and mobilities.26 At the lowest fluence (5 mW, 17.5 µJ/cm2), we attribute the slow decay to trap-assisted recombination (monomolecular recombination in nature) that is not fluence dependent, and the transient curve is fit by a single exponential expression (

∆E (t ) = Aslow e − t tslow + A E

). The intrinsic slow decay time constant of perovskite is

therefore obtained to be around 500 ps by retrieving the fitting parameters. At relatively higher fluences, fast decay process including bimolecular and trimolecular process (Auger recombination) should be taken into account while discussing the dynamics in order to fit the transients.32-35 Therefore, a double-exponential expression with the fast and slow decay contributions to the relaxation process is used with the intrinsic slow decay time constant (tslow=500 ps) which fits well with the transients. The double-exponential expression to fit the recombination process is:

∆E −t t (t ) = A fast e fast + Aslow e − t tslow + A E

(1)

where Afast and Aslow are the weights of the contribution from the fast and slow decay components, respectively, to the overall decay process, and tfast and tslow indicate the decay time constants.36-37 The retrieved fitting parameters are summarized in Supporting Information Note 3. At a relatively lower pump fluence of 35 µJ/cm2, the slow decay process still dominates the recombination process with a larger value of Aslow, however, with the stronger pump fluence (175 µJ/cm2), the fast decay process 8


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dominates with a larger value of Afast. In the OPTP measurement, the variation ratio of peak transmission amplitude (-∆E/E) versus time reflects the dynamics of photoconductivity that determines the dynamical performance of the hybrid meta-photonic device. Based on the fitting, we estimate the decay time as ~ 500 ps that is mainly determined by the slow decay of the monomolecular recombination process. The flexible sample reveals a relative short recombination time compared to the previous reports in terahertz regime that mainly originates from the different kinetics of interfacial electron transfer processes, film quality and thickness.35, 38

Another essential parameter to display the performance of perovskite film is the frequency-resolved photoconductivity at different pump fluences. The peak photoconductivity (at the peak point tp of the transient relaxation) was retrieved by: ∆σ ( t p ) =

ε 0c d

( na + nb )

−∆E ( t p )

(2)

E0 ( t p )

where ε 0 is the free space permittivity, c is the speed of light in free space, d is the perovskite film thickness, and na and nb are the refractive indices of the media on either side of the sample.26 The fluence dependent real-part of the photoconductivity is presented in Figure 2b (also see Supporting Information Note 4). According to the discussion, the photo-induced free carriers in the spin-coated solution-processed perovskite film on flexible polyimide substrate are generated within 20 ps and recombines within 500 ps, which thus enables the ultrafast switching functionality by integrating the perovskite thin film with a metadevice. The fluence dependent photoconductivity provides the dynamic platform to modulate the functionalities of 9


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the metadevices. As a proof of concept, we propose a perovskite-metal hybrid device by integrating a TASR subwavelength array based flexible metamaterial with the solution-processed perovskite thin film, and demonstrate the ultrafast modulation of Fano resonance in this flexible hybrid terahertz metadevice.

In order to understand the mechanism of the hybrid metadevice, we first explain the underlying Physics of the Fano resonance in the TASR resonator. We performed the numerical simulations using the commercially available software package CST Microwave Studio to investigate the properties of the sharp Fano resonance compared with the dipolar mode, and present the electric field and surface current distributions in Figure 3a. For the symmetric split ring resonator (δx = 0), it is well-known that a typical radiative dipolar resonance mode is excited for the x-polarized incidence inducing the parallel in-phase oscillated surface currents in identical branches of the metallic strips. As one of the capacitive gaps is displaced (δx ≠ 0) in TASR that breaks the structural symmetry, a sharp subradiant resonance mode is excited. This low frequency narrow linewidth mode is known as a Fano resonance which arises due to interference between a sharp discrete mode and a broad dipolar continuum.31, 39 The Fano mode enables the antiparallel currents that oscillate anti-phase in the two branches of the resonator forming a closed current loop. In the Fano mode, the two split gaps act as equivalent capacitors in the TASR equivalent circuit model. As we could observe in Figure 3a, most of the electric fields are concentrated in the capacitive gap areas at the Fano resonance frequency, and the anti-phase surface 10


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currents form a closed loop. The capacitive gaps would be gradually shorted if the conductivity of the medium in the gap increases, which then results in the larger damping of the Fano resonance. At large values of conductivity, we would observe a complete switch-off of the Fano resonance phenomena as the capacitive gaps become conductive and restores the symmetry of the split ring resonator.

The performance is experimentally verified using an ultrathin flexible hybrid meta-photonic device as shown in Figure 3b through the OPTP measurements. The dynamic response of the device is recorded by the transmitted terahertz time-domain signals at different pump powers and then Fourier transformed to frequency-domain spectra. We first measured the Fano and dipolar resonance modes of the TASR array without perovskite film as shown in Figure 3c where the two resonance modes are clearly visible. The subradiant Fano mode reveals a sharper resonance than the dipolar mode due to the weaker radiation to free space. After spin-coating the perovskite film on the array, we performed another quasi-steady state measurement of the hybrid meta-photonic device, and the spectrum is shown by the pink curve where we observe an obvious decrease in the amplitude of the Fano resonance due to the intrinsic conductivity of perovskite film that damps the resonance. However, the coupling coefficient of Fano radiation to free space is still larger than the damping rate so that the Fano feature remains observable which provides a nice platform for the dynamic modulation of Fano resonance by introducing the photo-induced ultrafast dynamic free carriers in the hybrid meta-photonic device. 11


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The damping rate of the Fano resonance is actively controlled by modulating the pump power that determines the photoconductivity of perovskite film in the capacitive gaps and the Fano resonance would be finally switched off when the damping rate is larger than the coupling coefficient. The modulation of Fano resonance at different pump powers (1 mW [3.5 µJ/cm2], 5 mW [17.5 µJ/cm2], 10 mW [35.0 µJ/cm2] and 50 mW [175 µJ/cm2]) could be clearly visualized in Figure 3c where the Fano dip gradually disappears along with the broadening of the dipolar mode. The overall amplitude of the transmission spectra decreases due to the larger reflection caused by the increased photoconductivity (See Supporting Information Note 4) of the perovskite thin film at higher pump powers, however, this screening effect is negligible due to the ultrathin film thickness. The modulation effect is more dominant at the Fano resonance profile due to the tightly confined electric field concentrated in the capacitive gaps which enables extremely sensitive features to external perturbance and thus strong modulation of Fano amplitude. Fano mode is finally switched off and restored to a dipolar mode at a low pump power (50 mW, fluence 175 µJ/cm2).

The initial coupling coefficient and damping rate of the Fano resonance are readily engineered by tailoring the asymmetry degree, therefore, the threshold to switch off the Fano resonance can be lowered by increasing the quality factor which in turn enhances the sensitivity of the meta-photonic device. As we have discussed, the Fano 12


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resonance strongly depends on the degree of asymmetry given by δx. Smaller δx would give rise to a weaker coupling of radiation to free space in the array with the coherent anti-phase oscillating currents at Fano resonance, which thus enables an excitation of higher quality factor Fano resonance for a more sensitive hybrid meta-photonic device (See Supporting Information Note 5).31 We experimentally verify the enhanced sensitivity by changing the displacement of TASR to δx = 10 µm with all the remaining parameters being unchanged. We spin-coated the solution-processed perovskite thin film with identical processing procedures and then measured the quasi-steady state as well as the ultrafast switching of the Fano resonance. As shown in Figure 4a, we present an overall good quality of the perovskite overlayer distribution on the TASR array, which exhibits the excellent reproducibility of the spin-coating process on the metamaterial array. The measured transmission spectra are displayed in Figure 4b. We observe a similar spectrum with Fano and dipolar modes excited in the metamaterial array without perovskite thin film, whilst the Q factor of Fano resonance is enhanced to 10.7. Similarly, the Fano resonance is damped for the quasi-steady condition of the hybrid meta-photonic device due to the screening effect from the intrinsic conductivity of the perovskite film. However, with the higher Q factor of Fano resonance, it is readily modulated and switched off with a much lower threshold at extremely low pump power (10 mW, fluence 35 µJ/cm2) as shown in Figure 4b. Therefore, a more sensitive hybrid meta-photonic device is demonstrated by simply tailoring the resonance property of the functional metamaterial array. 13


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To provide a clearer comparison, we summarize the recently reported performance of dynamic metadevices with different control mechanisms in Table 1. Most of the dynamic metadevices are based on a rigid substrate with extremely complex fabrication procedures and require large thickness of the dynamic materials for efficient modulation depth. Although graphene based metadevice is flexible with ultrathin device thickness, the modulation speed and efficiency are limited. Based on this table, the perovskite based metadevice reveals an extraordinary performance beyond different dynamic materials/methods with the high-efficiency, facile processing, ultrafast and low-threshold features as a platform for active flexible meta-photonic devices.

In summary, we have demonstrated an excellent performance of solution-processed methylammonium lead iodide perovskite thin film as a platform for ultrafast all-optical switch integrated into a flexible metadevice. A single step spin-coating process of the perovskite film enables a passive metadevice to inherit the dynamically tunable photoconductivity from the perovskite layer, and thus enables the flexible hybrid meta-photonic device that behaves as an active, ultrafast, low threshold optical switch. This strategy could open up huge avenues for all-optical active control of low-cost flexible displays, wearable optoelectronic sensors, ultrafast, high-capacity data encoding and decoding using perovskite based flexible photonic platform.

ASSOCIATED CONTENT 14


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Supporting Information Available: Metamaterial sample fabrication; XRD spectra, profilometer, SEM and AFM images of perovskite thin film; double exponential fitting parameters; photoconductivity at different pump fluences; and Fano resonance at different asymmetric degrees in TASR. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.xxxxxxx.

AUTHOR INFORMATION Corresponding Author *

Email: [email protected].

ORCID Longqing Cong: 0000-0003-2839-5940 Tze Chien Sum: 0000-0003-4049-2719 Ranjan Singh: 0000-0001-8068-7428 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS: The authors thank Wen Xiang Lim of Nanyang Technological University for the help on taking SEM images and Abhishek Kumar of Nanyang Technological University for assistance in retrieving photoconductivity. Financial support from Nanyang Technological University start-up grants M4081282 and M4080514; the Ministry of Education Academic Research Fund Tier 1 grants M4011362, M4011534, RG184/14 15


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and RG101/15, and Tier 2 grants MOE2011-T3-1-005, MOE2014-T2-1-044, MOE2015-T2-2-015, MOE2015-T2-2-103 and MOE2016-T2-1-034 is gratefully acknowledged.

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(12)Lu, D.; Liu, Z., Hyperlenses and metalenses for far-field super-resolution imaging. Nature Communications 2012, 3, 1205. (13)Zhang, S.; Zhou, J.; Park, Y. S.; Rho, J.; Singh, R.; Nam, S.; Azad, A. K.; Chen, H. T.; Yin, X.; Taylor, A. J.; Zhang, X., Photoinduced handedness switching in terahertz chiral metamolecules. Nat Commun 2012, 3, 942. (14) Gu, J.; Singh, R.; Liu, X.; Zhang, X.; Ma, Y.; Zhang, S.; Maier, S. A.; Tian, Z.; Azad, A. K.; Chen, H.-T., Active control of electromagnetically induced transparency analogue in terahertz metamaterials. Nature communications 2012, 3, 1151. (15) Padilla, W. J.; Taylor, A. J.; Highstrete, C.; Lee, M.; Averitt, R. D., Dynamical electric and magnetic metamaterial response at terahertz frequencies. Phys. Rev. Lett. 2006, 96, 107401. (16) Singh, R.; Azad, A. K.; Jia, Q. X.; Taylor, A. J.; Chen, H.-T., Thermal tunability in terahertz metamaterials fabricated on strontium titanate single-crystal substrates. Opt. Lett. 2011, 36, 1230-1232. (17) Cong, L.; Xu, N.; Han, J.; Zhang, W.; Singh, R., A Tunable Dispersion-Free Terahertz Metadevice with Pancharatnam–Berry-Phase-Enabled Modulation and Polarization Control. Adv. Mater. 2015, 27, 6630-6636. (18) Pryce, I. M.; Aydin, K.; Kelaita, Y. A.; Briggs, R. M.; Atwater, H. A., Highly Strained Compliant Optical Metamaterials with Large Frequency Tunability. Nano Lett. 2010, 10, 4222-4227. (19) MacDonald, W. A., Engineered films for display technologies. J. Mater. Chem. 2004, 14, 4-10. (20) MacDonald, W. A.; Looney, M. K.; MacKerron, D.; Eveson, R.; Adam, R.; Hashimoto, K.; Rakos, K., Latest advances in substrates for flexible electronics. Journal of the Society for Information Display 2007, 15, 1075-1083. (21) Manjappa, M.; Srivastava, Y. K.; Cong, L.; Al-Naib, I.; Singh, R., Active Photoswitching of Sharp Fano Resonances in THz Metadevices. Adv. Mater. 2017, 29, 1603355. (22) Yan, R.; Arezoomandan, S.; Sensale-Rodriguez, B.; Xing, H. G., Exceptional Terahertz Wave Modulation in Graphene Enhanced by Frequency Selective Surfaces. ACS Photonics 2016, 3, 315-323. (23) Li, Q.; Tian, Z.; Zhang, X.; Xu, N.; Singh, R.; Gu, J.; Lv, P.; Luo, L.-B.; Zhang, S.; Han, J.; Zhang, W., Dual control of active graphene–silicon hybrid metamaterial devices. Carbon 2015, 90, 146-153. (24) Veldhuis, S. A.; Boix, P. P.; Yantara, N.; Li, M.; Sum, T. C.; Mathews, N.; Mhaisalkar, S. G., Perovskite Materials for Light-Emitting Diodes and Lasers. Adv. Mater. 2016, 28, 6804-6834. (25) Sutherland, B. R.; Sargent, E. H., Perovskite photonic sources. Nat Photon 2016, 10, 295-302. (26)Yettapu, G. R.; Talukdar, D.; Sarkar, S.; Swarnkar, A.; Nag, A.; Ghosh, P.; Mandal, P., Terahertz Conductivity within Colloidal CsPbBr3 Perovskite Nanocrystals: Remarkably High Carrier Mobilities and Large Diffusion Lengths. Nano Lett. 2016, 16, 4838-48. (27) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; 18


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Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. (28) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C., Long-range balanced electron-and hole-transport lengths in organic-inorganic CH3NH3PbI3. Science 2013, 342, 344-347. (29) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348, 1234-1237. (30) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A., High-efficiency solution-processed perovskite solar cells with millimeter-scale grains. Science 2015, 347, 522-525. (31) Cong, L.; Manjappa, M.; Xu, N.; Al-Naib, I.; Zhang, W.; Singh, R., Fano Resonances in Terahertz Metasurfaces: A Figure of Merit Optimization. Advanced Optical Materials 2015, 3, 1537-1543. (32) Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M., Charge-carrier dynamics in vapour-deposited films of the organolead halide perovskite CH3NH3PbI3−xClx. Energy & Environmental Science 2014, 7, 2269. (33) Ponseca, C. S., Jr.; Tian, Y.; Sundstrom, V.; Scheblykin, I. G., Excited state and charge-carrier dynamics in perovskite solar cell materials. Nanotechnology 2016, 27, 082001. (34) Johnston, M. B.; Herz, L. M., Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diffusion, and Radiative Efficiencies. Acc. Chem. Res. 2016, 49, 146-54. (35) Marchioro, A.; Teuscher, J.; Friedrich, D.; Kunst, M.; van de Krol, R.; Moehl, T.; Grätzel, M.; Moser, J.-E., Unravelling the mechanism of photoinduced charge transfer processes in lead iodide perovskite solar cells. Nature Photonics 2014, 8, 250-255. (36) Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M., High Charge Carrier Mobilities and Lifetimes in Organolead Trihalide Perovskites. Advanced Materials (Deerfield Beach, Fla.) 2014, 26, 1584-1589. (37) Herz, L. M., Charge-Carrier Dynamics in Organic-Inorganic Metal Halide Perovskites. Annu. Rev. Phys. Chem. 2016, 67, 65-89. (38) La, O. V. C.; Salim, T.; Kadro, J.; Khuc, M. T.; Haselsberger, R.; Cheng, L.; Xia, H.; Gurzadyan, G. G.; Su, H.; Lam, Y. M.; Marcus, R. A.; Michel-Beyerle, M. E.; Chia, E. E., Elucidating the role of disorder and free-carrier recombination kinetics in CH3NH3PbI3 perovskite films. Nat Commun 2015, 6, 7903. (39) Zheludev, N. I.; Prosvirnin, S. L.; Papasimakis, N.; Fedotov, V. A., Lasing spaser. Nature Photonics 2008, 2, 351-354. (40) Singh, R.; Xiong, J.; Azad, A. K.; Yang, H.; Trugman, S. A.; Jia, Q. X.; Taylor, A. J.; Chen, H.-T., Optical tuning and ultrafast dynamics of high-temperature superconducting terahertz metamaterials. Nanophotonics 2012, 1, 117-123. (41) Chen, H. T.; Padilla, W. J.; Zide, J. M.; Gossard, A. C.; Taylor, A. J.; Averitt, R. D., Active terahertz metamaterial devices. Nature 2006, 444, 597-600. 19


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Table 1. A comparison of different dynamic metadevices Materials /methods

Speed

Stimuli 2

(µJ/cm ) Superconductor

~ps

100

Mod.

Area

Amp.

2

Thickness

(mm )

(nm)

70%

10×10

100

Flex.

Mode

Comment

(Q) No

40

Dipole

@ 20 K

(~1)

(YBCO)

Silicon on sapphire

~ps

4725

100%

10×10

500

No

(SOS)14

Fano

Undoped

(~8)

GaAs (Gated)41

~kHz

16 V

50%

5×5

1000

No

LC

-

(~2) ~1 ns

GaAs (photoexcitation)

43

Liquid crystal

2

55%

~10×10

670000

No

42

LC

Substrate

(~2) ~1 kHz

15 V

~75%

3×3

5000

No

Dipole

Reflection

(~1) VO244

50 ms

40.5 K

~10%

-

90

No

LC

Thermal

(~1.5) MEMS45

Graphene46

30 kHz

100 kHz

16 V

350 V

56%

0.1×0.1

47%

15×15

-

~1

No

Yes

Dipole

Reflection

(~0.2)

@6.2 µm

Dipole

-

(~1.5) Perovskite

500 ps

35

100%

10×10

30

Yes

Fano

-

(10.7)

(Our work)

Speed: modulation speed of the metadevice response; Stimuli: the external pump by light (µJ/cm2, all are @ 800 nm except perovskite),

heat (K) or voltage (V), default stimuli is by light; Mod. Amp.: modulation amplitude of the respective resonances; Thickness: the

thickness of the dynamic materials; Flex.: whether the whole hybrid metadevice is flexible or not; Mode (Q): the resonant mode that is

discussed for the dynamic modulation with the estimated quality factors (Q).

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Figure Captions: Figure 1. A hybrid flexible active meta-photonic device. (a) The schematic illustration of the all-optical switch using the flexible perovskite-metal hybrid metadevice. (b) The optical microscope image of the hybrid metadevice with the spin-coated perovskite thin film as the dynamic medium; inset is the zoomed image of one square unit cell with the geometrical parameters where the period is 75 µm with l = 60, d = 6, and g = 3 (in µm). (c) Perovskite unit cell of the pseudocubic lattice with the organic cation (A), the metal cation (B) and the anion (X).

Figure 2. (a) The ultrafast relaxation of the solution-processed perovskite thin film performed by OPTP method at pump powers of 5 mW (fluence 17.5 µJ/cm2), 10 mW (35.0 µJ/cm2) and 50 mW (175 µJ/cm2). The dynamics is presented by the normalized transient amplitude change of terahertz electric field versus time delay of the 400 nm (3.1 eV, bandgap ~2.5 eV) pump beam. The recombination dynamics is fitted using a single-exponential expression for 5 mW pump to estimate the slow decay time constant and a double-exponential expression for 10 mW and 50 mW pump (see Supporting Information). (b) The retrieved frequency-resolved photoconductivity at the respective pump fluences.

Figure 3. The ultrafast modulation of the Fano resonance. (a) The analysis of electric field concentration and surface current distribution at Fano resonance of a TASR array. (b) The schematic illustration of the ultrathin flexible hybrid 22


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ACS Photonics

meta-photonic device and OPTP measurement. (c) The measured transmission spectra of the device with the ultrafast modulation of Fano resonance.

Figure 4. Ultrasensitive Fano switching with lower asymmetry of the resonators. (a) The optical microscope image of the hybrid meta-photonic device with smaller asymmetry (δx = 10 µm). (b) The measured transmission spectra of the device which shows larger sensitivity towards the switching effect of the Fano resonance.

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Perovskite as a platform for active flexible meta-photonic devices Longqing Cong,1,2,* Yogesh Kumar Srivastava,1,2 Ankur Solanki,1 Tze Chien Sum,1 and Ranjan Singh1,2,*

For Table of Contents Use Only

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Terahertz

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Pu

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

mp

@

ACS Photonics

40

0n

m

E(x)

Al

a b

c

l 30 μm

g

k(z)

20 d


A: Organic cation B: Metal cation X: Halide anion

H(y)

a

ACS Photonics ite Perovsk

30 nm e 25 μm Polyimid

2

1

Pump power (mW) 5 10 50

~20 ps

-ΔE/E (10-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 b14 15 16 17 18 19 20 21 22 23 24 25 26

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Fitting

0 tp

0

500

1000

Time delay (ps)

1500

Δσ1 (×103　S/m)

12 9 6 3 0 ACS Paragon Plus Environment @ 1 mW 0.3

0.6 0.9 1.2 Frequency (THz)

1.5

Electric field a Page 27 of 28 ACS Photonics

Surface current

Fano

Pump

x

Dipole

p

m @ 0

40

~25μm

Polyimide w/o perovskite w/ perovskite (0mW) 1mW 5mW 10mW 50mW

0.9

0.6

0.3

metasurface

nm

Perovskite

Terahertz

y

Pu

9 10 11 12 13 14 15 16 c 17 18 19 20 21 22 23 24 25 26 27 28

Fano

Amplitude Transmission

1 2 3 4 5 6 7 8b

Q=4.5

Dipole

Fano Plus Environment ACS Paragon

0.6

0.9 1.2 Frequency (THz)

1.5

a

ACS Photonics

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Amplitude Transmission

1 2 3 10 4 l d 5 g 6 7b 8 0.9 w/o perovskite 9 10 w/ perovskite (0 mW) 11 2 mW 12 10 mW 13 0.6 14 15 16 Q=10.7 17 18 0.3 19 ACS Paragon Plus Environment 20 0.6 0.9 1.2 1.5 21 Frequency (THz)

Perovskite as a Platform for Active Flexible Metaphotonic Devices

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