Supporting Information

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1 Supporting Information Longqing Cong, 1,2 Yogesh Kumar Srivastava, 1,2 Ankur Solanki, 1 Tze Chien Sum, 1 and Ranjan Singh 1,2,* 1 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore , Singapore 2 Centre for Disruptive Photonic Technologies, The Photonics Institute, 50 Nanyang Avenue, Nanyang Technological University, Singapore * Corresponding author: ranjans@ntu.edu.sg 1. Fabrication of metamaterial samples The metamaterial sample was fabricated using standard photolithography technique on a flexible polyimide substrate with thickness 25 µm. The substrate was first cleaned using acetone and IPA. After drying the substrate at 100 C for 10 mins on a hot plate, a positive photoresist of thickness 1.5 µm was spin-coated. The photoresist coated polyimide substrate was further prebaked at 105 C on a hot plate, and then exposed by UV light through a photomask with predesigned pattern. The complementary pattern in form of photoresist was developed by soaking the substrate in the developer solution which removes the exposed part of the photoresist. On the top of the patterned substrates, 200 nm thick aluminum was deposited using thermal evaporation. The ultrathin, flexible and robust metamaterial sample was finalized by soaking it in acetone to liftoff the undesired aluminum on top of the patterned photoresist with the left metallic TASR array on the polyimide substrate. The sample image is shown in Figure S1(a) and the flexibility is revealed in Figure S1(b). The dark color in the sample area of Figure S1(a) was induced by the intense visible light while taking the microscopic images of the sample after the OPTP measurements. 1

2 Figure S1. (a) The ultrathin, flexible hybrid meta-photonic device image with the spin-coated solution-processed perovskite film and the red square presents the measured area; (b) Image of the flexible device. We choose aluminum as the constituent metal in our experiments due to the excellent stability and its large conductivity in terahertz regime where the DC conductivity (σ = S/m) is approximately applied for discussion. With this conductivity, the skin depth of Al at around 0.5 THz is calculated to be 116 nm using the equation δ π µσ = 1 f 0 DC where f is the frequency, 0 µ is the vacuum permeability and σ DC is the DC conductivity of Al. 1 We deposited Al in the thermal evaporator at the pressure of Torr in order to minimize the probability of oxidization. 2 Compared to this metal thickness (200 nm, to enable a strong resonance of the metal resonator), the oxidized layer of Al 2 O 3 during the thermal evaporation is negligible which is estimated to be around several nanometers 2-3. The oxidized top layer would not affect the effective conductivity of Al with ~116 nm penetration depth in the terahertz regime. Therefore, we ignore the effect of oxidization of Al in our experiments and discussion. 2

3 2. XRD spectra, profilometer, SEM and AFM images We carried out X-ray diffraction (XRD) measurements on the spin-coated perovskite sample to confirm the growth of perovskite. The corresponding spectrum in comparison to the theoretical predictions (from the literature, ICSD ) is presented in Figure S2. The measured XRD spectrum shows most of the dominant CH 3 NH 3 PbI 3 diffraction peaks with the minute amount of PbI 2 phase detected in the spin-coated perovskite film at 2θ = 12.5º. A good agreement of the measured XRD data with the literature values confirms the purity of the prepared perovskite sample spin-coated on the TASR metamaterial structure. Figure S2. The measured XRD spectrum with the spectrum from literature for CH 3 NH 3 PbI 3. 3

4 The Scanning electron microscope (SEM) and Atomic Force Microscope (AFM) were also taken to show the morphology of the perovskite film as shown in Figure S3a, S3b and S3c. The profilometer data confirms the thickness of the film at 30±5 nm in Figure S3d. Figure S3. SEM and AFM images of the perovskite thin film on polyimide substrate and the profilometer data to show the film thickness. 3. Double exponential fitting parameters for photoconductivity dynamics Fluence t slow A slow t fast A fast 4

5 (µj/cm 2 ) (ps) (ps) 17.5 * ± ±5 0.6 * Single-exponential expression E t tslow ( t ) A slow e = + A was used to estimate the slow decay at low fluence. E 4. Photoconductivity at different pump fluences In order to numerically present the frequency-resolved photoconductivity of the perovskite thin film in situ, we performed the optical pump terahertz probe (OPTP) measurements of the film at different pump fluences and present the real-part of the photoconductivity as shown in Figure S3. The peak photoconductivity (at the peak point t p of the relaxation) is retrieved using the equation: 4 0 ( ( p) ε 0c E t σ( t p) = ( na + nb) (S1) d E t where ε 0 is the free space permittivity, c is the speed of light in free space, d is the perovskite film thickness, and n a and n b are the refractive indices of the media on either side of the sample. From Figure S4a, we observe the increasing photoconductivity with increased pump power below 50 mw (175 µj/cm 2 ). However, the saturation occurs at pump power larger than 50 mw and the sample deteriorates leading to decreased photoconductivity at further increase in pump power as shown in Figure S4b. 5

6 Figure S4. The measured frequency-resolved photo-conductivity at different pump powers. (a) At lower pump powers below 50 mw (175 µj/cm 2 ), the photoconductivity increases with larger pump powers. (b) At higher pump powers above 50 mw, the photoconductivity saturates and with further increase in fluence, the conductivity begins to fall due to the deterioration of the perovskite sample. 5. The Fano resonance in TASR With the smaller value of δx, the coherently oscillating currents on the surface of the resonators cancel strongly which results in a weakly radiating Fano resonance. In Figure S5, we show the simulated Fano resonance spectra at different values of δx and calculate the respective quality factors. We observe the enhancement in the quality factor of the Fano resonance (as large as 69.5 at δx = 2 µm) by reducing the asymmetric degree that is the signature of the reduced radiative loss. In addition to the engineered quality factor, the Fano footprint indicated by the peak to dip intensity also reveals the dependence on the structural asymmetry. With the smaller δx, the Fano 6

7 resonance intensity reveals a smaller amplitude that is difficult to be measured in the far field die to its weakly radiating nature. Therefore, there exists a tradeoff between the Fano resonance quality factor and its intensity for the experimental implementation. 5 Figure S5. The Fano resonances at different values of asymmetric degree (δx). References: (1) Singh, R.; Smirnova, E.; Taylor, A. J.; O Hara, J. F.; Zhang, W., Optically thin terahertz metamaterials. Opt. Express 2008, 16, (2) Hollars, C. W.; Dunn, R. C., Evaluation of thermal evaporation conditions used in coating aluminum on near-field fiber-optic probes. Rev. Sci. Instrum. 1998, 69, (3) Evertsson, J.; Bertram, F.; Zhang, F.; Rullik, L.; Merte, L. R.; Shipilin, M.; Soldemo, M.; Ahmadi, S.; Vinogradov, N.; Carlà, F.; Weissenrieder, J.; Göthelid, M.; Pan, J.; Mikkelsen, A.; Nilsson, J. O.; Lundgren, E., The thickness of native oxides on aluminum alloys and single crystals. Appl. Surf. Sci. 2015, 349, (4) 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, (5) Cong, L.; Manjappa, M.; Xu, N.; Al-Naib, I.; Zhang, W.; Singh, R., Fano Resonances in Terahertz Metasurfaces: A Figure of Merit Optimization. Advanced 7

8 Optical Materials 2015, 3,