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Ultrafast frequency-agile terahertz devices using methylammonium lead halide perovskites

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Science Advances  04 May 2018:
Vol. 4, no. 5, eaar7353
DOI: 10.1126/sciadv.aar7353

Abstract

The ability to control the response of metamaterial structures can facilitate the development of new terahertz devices, with applications in spectroscopy and communications. We demonstrate ultrafast frequency-agile terahertz metamaterial devices that enable such a capability, in which multiple perovskites can be patterned in each unit cell with micrometer-scale precision. To accomplish this, we developed a fabrication technique that shields already deposited perovskites from organic solvents, allowing for multiple perovskites to be patterned in close proximity. By doing so, we demonstrate tuning of the terahertz resonant response that is based not only on the optical pump fluence but also on the optical wavelength. Because polycrystalline perovskites have subnanosecond photocarrier recombination lifetimes, switching between resonances can occur on an ultrafast time scale. The use of multiple perovskites allows for new functionalities that are not possible using a single semiconducting material. For example, by patterning one perovskite in the gaps of split-ring resonators and bringing a uniform thin film of a second perovskite in close proximity, we demonstrate tuning of the resonant response using one optical wavelength and suppression of the resonance using a different optical wavelength. This general approach offers new capabilities for creating tunable terahertz devices.

INTRODUCTION

Resonant devices allow for spectral manipulation of the incident radiation, which is useful for a broad range of applications that include spectroscopic studies, optoelectronic devices, and communication systems (1, 2). Because passive structures allow for only a fixed spectral response, devices that are frequency-agile offer better utilization of the available bandwidth and enable new capabilities. Examples of this include the potential for increased capacity in communication systems and ultrafast switching devices. At low frequencies, frequency-agile devices can be created using discrete components that are electrically switched. At higher frequencies, including the terahertz spectral range and beyond, discrete electrical components are impractical, and resonances based on the device geometry are more convenient. In the terahertz spectral range, these devices have been designed using metal structures that incorporate a variety of materials as the active medium, where tuning has been achieved using optical, thermal, magnetic, and electrical means (312). In the case of semiconductors, variation in the intensity of an above-bandgap optical pump can be used to alter the free-carrier density that can be used to tune the metamaterial response. In particular, if the optical excitation is pulsed, then ultrafast switching is possible on time scales that are limited only by the carrier recombination lifetime of the medium (1, 9). Existing implementations rely on uniform photoexcitation of a single active medium incorporated into metamaterial devices, which allows for switching between two states. Additional functionality and operational control can be achieved by incorporating two or more semiconductors with different bandgap properties into a single metamaterial unit cell; however, fabrication of these devices is technically challenging using conventional semiconductors. Here, we circumvent this issue by using organic-inorganic hybrid photoconductive materials that can be deposited by facile solution-based processes.

Organic-inorganic hybrid lead halide perovskites have recently gained significant attention in photovoltaic applications demonstrating power conversion efficiencies in solar cells that exceed 22% with moderate charge carrier mobilities (~20 cm2/V · s)(13, 14). The most commonly used material in this series, methylammonium lead iodine (MAPbI3), has a bandgap of 1.5 eV. Replacing the iodine atom with smaller, more electronegative halides, such as bromine or chlorine, yields larger bandgaps of 2.3 and 3.1 eV, respectively (15). In practice, any desired bandgap between these extreme values may be obtained by combining the halide groups in the desired stoichiometric ratio (16). Thin polycrystalline films of these materials have been shown to exhibit high optical absorption and carrier recombination lifetimes on the nanosecond time scale when excited using high optical pump fluences (17). The combination of these advantageous properties has stimulated interest in developing devices based on the hybrid perovskites for a variety of optoelectronic and photonic applications (1820).

Here, we demonstrate an ultrafast frequency-agile terahertz metamaterial device in which two different three-dimensional (3D) methylammonium lead halide perovskites (MAPbI3 and MAPbBr3) are incorporated into each metamaterial unit cell. The device uses metallic split-ring resonators (SRRs) fabricated on quartz substrates in which the perovskites are selectively patterned into the gaps with micrometer-scale precision. In contrast to 2D perovskites, which allow for enhanced photoexcitation-dependent terahertz absorption only when deposited directly on silicon (21), these materials exhibit photoexcitation-dependent conductivity when deposited on a broad range of substrates. To fabricate this device, we developed a process based on a polymer mask delamination technique. This approach ensures that no solvents come into contact with the deposited perovskites on the devices, thereby protecting the underlying organic component of the material while providing for high fidelity–patterned structures. When each perovskite is excited with above-bandgap optical pulses, the photoinduced terahertz conductivity alters the resonant response of the hybrid (that is, metal perovskite) structure. Using time-resolved optical pump–terahertz probe (TR-OPTP) spectroscopy (22, 23), we find that the transient response relaxes on nanosecond time scales, characteristic of the photocarrier recombination lifetime in these materials, enabling high-frequency operation (17). On the basis of this finding, we use a combination of excitation pulses at 800 and 400 nm to demonstrate ultrafast switching of the metamaterial response between three separate resonances. The switching time between these resonances can be controlled by varying the time delay between the two optical pump pulses, whereas the extent of the switching can be controlled by varying the optical fluence, thereby offering wide control over device operation. Finally, we demonstrate a device architecture using a layered arrangement of the two perovskites, where we can switch between a frequency-agile response and an amplitude modulation response, when excited using 800 and 400 nm pulses, respectively.

RESULTS

Three-dimensional perovskite synthesis

The MAPbI3 solution was prepared by mixing PbI2 (461 mg; Sigma-Aldrich) with methylammonium iodide (MAI; 159 mg; Solarmer) in dimethylformamide (2 ml). The MAPbBr3 solution was prepared by mixing PbBr2 (367 mg; Sigma-Aldrich) with MAI (112 mg; Solarmer) in dimethyl sulfoxide (2 ml). Both solutions were stirred for 3 hours at 50°C on a hotplate before use. We then spin-coated the perovskite solutions onto quartz substrates and annealed them at 110°C. Chloroform solvent was dripped on the substrates during the spin-coating process to improve the film morphology.

Carrier dynamics in perovskite films

In Fig. 1, we summarize the optical response dynamics of MAPbI3 and MAPbBr3 using above-bandgap pulse excitation. For these measurements, we used pristine perovskite films that were solution-cast onto quartz substrates. Figure 1 (A and B) shows the measured ultraviolet-visible (UV-vis) absorption spectra for the two perovskites that indicate a bandgap of ~1.5 eV (800 nm) for MAPbI3 and ~2.3 eV (540 nm) for MAPbBr3. We used TR-OPTP spectroscopy to examine the photocarrier dynamics in both perovskites. In Fig. 1C, we show the time-resolved terahertz transmission of MAPbI3 excited by an optical pump at ~800 nm as a function of the optical fluence. For the maximum optical fluence of 270 μJ/cm2 used here, the initial photocarrier density [n(t = 0)] was calculated to be ~ 5 × 1019 cm− 3, where we used the absorption properties of the medium and assumed an external quantum efficiency of ~0.5 for the generation of free carriers. The photocarrier recombination kinetics have been shown to be well modeled by a rate equation given by Embedded Image, where k1, k2, and k3 are recombination rate constants and n is the photocarrier density (16). For low optical fluences, the photocarrier dynamics are dominated by a monomolecular decay (−k1n), whereas at high pump fluences, the recombination dynamics are dominated by electron-hole bimolecular recombination (−k2n2) and Auger recombination (−k3n3), respectively (16). We estimated the monomolecular recombination lifetime [τrec (=k1− 1)] for MAPbI3 ≅ 7.8 ns at low optical fluence.

Fig. 1 UV-vis and terahertz properties of MAPbI3 and MAPbBr3 perovskites.

UV-vis absorption spectrum for (A) MAPbI3 and (D) MAPbBr3 that show an optical gap of 1.5 eV (810 nm) and 2.3 eV (540 nm), respectively. The insets show the molecular arrangement of the perovskite structure. The photocarrier recombination dynamics in (B) MAPbI3 and (E) MAPbBr3 measured by OPTP spectroscopy for various optical pump intensities. The photocarriers are generated using 800-nm pulses in MAPbI3 and 400-nm pulses in MAPbBr3, respectively. The calculated photoinduced Drude conductivity values as a function of the pump intensity, measured at pump-probe delay τpp = 15 ps for (C) MAPbI3 and (F) MAPbBr3. OD, optical density.

In Fig. 1D, we show the time-resolved transmission properties of MAPbBr3 excited using an optical pump at ~400 nm. For an optical fluence of 110 μJ/cm2 and a film thickness of 60 nm,we find that n(t = 0) in MAPbBr3 was calculated to be ~ 1020 cm− 3 (see section S1). In contrast to MAPbI3, we estimate the recombination lifetime [τrec (=k1− 1)] for MAPbBr3 to be 500 ps. Because the absorption is small in these films, the accuracy of the fit parameters was not satisfactory. We also studied the photocarrier dynamics of MAPbI3 using a 400-nm pump excitation and MAPbBr3 using a 800-nm pump excitation (see section S1). In the former case, MAPbI3 exhibited somewhat higher conductivity along with a longer τrec, whereas no absorption was observed in MAPbBr3 when excited at 800 nm because the corresponding photon energy was less than the bandgap energy, and the pump intensity was too weak to generate photocarriers via two-photon absorption. Because the monomolecular recombination rate in the perovskite films could extend to tens of nanoseconds, time-resolved photoluminescence measurements at longer time scales might be warranted to provide more appropriate extraction of the recombination lifetimes (24).

We found that for all pump-probe delay times (τ), the photocarriers exhibit a photoinduced absorption spectrum having a Drude-like response (see section S2). This is in agreement with photogeneration of free carriers in these compounds because the binding energy of excitons is on the order of kBT at room temperature. (25). We calculated the photoinduced terahertz conductivity (Δσ) from the measured photoinduced transient terahertz transmission using the relation Embedded Image. Here, nsub is the refractive index of the quartz substrate (2.14), Z0(=377 ohms) is the impedance of free space, and tfilm is the film thickness (26). Δσ(t = 10 ps) for the two films obtained at different pump fluences is plotted in Fig. 1 (E and F, respectively), showing a linear increase with the pump fluence. The maximum terahertz conductivity was found to be ~13,000 S/m for MAPbI3 and ~6200 S/m in MAPbBr3. The lower conductivity for MAPbBr3 was limited by the available optical fluence at 400 nm because of nonlinear optical conversion in a 500-μm-thick β-barium borate (BBO) crystal.

Patterning of perovskite films

Hybrid lead halide perovskites have been shown to allow for solution-based synthesis while maintaining high-power conversion efficiencies and carrier mobilities, making them promising candidates for low-cost optoelectronic devices. Many of these applications necessitate patterning of the perovskite films. However, lead trihalide perovskites are known to be easily soluble in common solvents, rendering conventional lithographic patterning techniques unusable. Consequently, a number of alternate approaches for patterning perovskites have been developed in recent years, including imprint lithography (27), fluorinated solvent–based lithography (28), and wettability-assisted lithography (29). However, existing approaches have been applied to thin films of only a single perovskite. Because our goal is to pattern multiple perovskites into a single device (or a single unit cell), we needed to develop a new fabrication technique. The process uses Parylene C (hereafter referred to as Parylene) as a micropatterned mask for patterning perovskite thin films. The Parylene mask can be easily delaminated from the substrate, leaving behind clean, high fidelity–patterned structures. In this approach, the perovskite is spin-cast at the end of the process, thereby retaining the quality of the patterned organic-inorganic materials.

A schematic diagram of the basic fabrication steps is shown in Fig. 2. First, the metal structures are patterned on a quartz substrate using standard lithography. A thin layer of Parylene C is then deposited by vapor deposition, such that the Parylene film completely encapsulates the sample. Using standard lithography, we then define the complementary pattern in the photoresist (that is, leaving photoresist where the Parylene film should not be etched away). The Parylene is subsequently etched using oxygen plasma, leaving through-holes where a perovskite can be deposited. After the desired perovskite film is solution-cast and annealed, the Parylene film is delaminated once the films begin to crystallize, typically after 60 s. The sample is then thoroughly annealed to obtain high-quality, polycrystalline-patterned structures. For the deposition and delineation of multiple perovskite layers, the entire process can be repeated multiple times. Finally, the completed sample is encapsulated with a thin layer of polymethyl-methacrylate (PMMA). We note that the Parylene encapsulation allows for use of regular solvents common to lithography so that different perovskites can be patterned in close proximity to one another. This noninvasive and facile process allows for high-fidelity, pinhole-free patterning of the perovskite layers. In general, the technique can also be used to fabricate stand-alone perovskite structures, allowing for an arbitrary number of materials to be patterned onto the same substrate.

Fig. 2 The fabrication process for the metal perovskite hybrid metamaterial device.

(Clockwise) A Parylene-based sacrificial mask is selectively patterned on a prefabricated metamaterial structure; subsequently, the perovskite solution is spin-coated. The Parylene film is delaminated to reveal the hybrid metamaterial structure. The process is sequentially repeated to pattern MAPbBr3., the second type of perovskite in alternate gaps of the metallic ring. The metal structure is composed of gold SRRs with six equally spaced gaps of 3 μm. The inner and outer diameters of the ring are 80 and 90 μm, respectively, and the periodicity is 220 μm.

Metal hybrid terahertz frequency-agile devices

In Fig. 3, we summarize the terahertz transmission properties of three separate metamaterial devices. Figure 3A shows the TR-OPTP transmission properties of a metamaterial device that contains a uniform thin film of MAPbI3. The inset shows an optical image of the sample taken from the quartz side (that is, back side). As the optical fluence was increased, the resonance depth progressively decreased and ultimately disappeared at a fluence of 270 μJ/cm2. However, there was no observed shift in the resonance frequency. These experimental observations are supported by numerical simulations, shown in Fig. 3B, using the extracted conductivity values from Fig. 1 (E and F). We note that the resonant response in the sample is slightly red-shifted because of a change in the environment surrounding the metal surface because perovskites have a higher refractive index than PMMA. This is consistent with recent work that uses only a uniform layer of MAPbI3 deposited on a metamaterial array (30).

Fig. 3 Terahertz frequency-agile response in metal-hybrid metamaterial device.

(A) Experimentally measured and (B) numerically simulated terahertz transmission through the metamaterial device covered with a thin uniform (unpatterned) film of MAPbI3 as a function of the photoexcitation fluence at 800 nm that was deposited over the entire array. (C) Experimentally measured and (D) numerically simulated terahertz transmission through the metamaterial device, where the MAPbI3 perovskite was patterned into the gaps of the metal rings, as a function of the photoexcitation fluence at 800 nm. (E) Experimentally measured and (F) numerically simulated terahertz transmission through the metamaterial device, where the MAPbBr3 perovskite was patterned into the gaps of the metal rings, as a function of the photoexcitation fluence at 400 nm. In all cases, the time-domain measurements were taken after a fixed pump delay of 30 ps after excitation.

Figure 3C shows the terahertz transmission properties for metal hybrid structures in which MAPbI3 perovskites were patterned in the gaps of the metal ring structure, as depicted by the inset to the figure. As the optical fluence was increased, the device terahertz response shifted progressively from the characteristic spectrum of the SRR to that of a closed-ring resonator. The photoinduced spectral modulation in the terahertz range is similar to that described in the study of Withayachumnankul and Abbott (1); however, the results here show the importance of patterning the perovskite film. Once again, numerical simulations shown in Fig. 3D agree well with the experimental results. Similar photoinduced terahertz modulation was observed when the sample was excited using a pump wavelength of 400 nm (see section S3). Finally, Fig. 3E shows terahertz transmission properties of a metal hybrid structure in which a MAPbBr3 perovskite film was patterned in the gaps of the SRR. Here, we observed frequency-agile operation only when the device was excited with 400-nm optical pulses. As we observed with the patterned MAPbI3 film, the terahertz resonance of the device red-shifted as the optical fluence was increased. However, the extent of the frequency shift was limited by the lower available optical fluence at 400 nm. Numerical simulations for this device, shown in Fig. 3F, agree well with the experimental results. In each case, the time-domain measurements were done after a fixed delay of 30 ps after optical excitation.

The operation of these devices can be explained using a simple transmission line model for the resonator. When the metamaterial is covered with a uniform perovskite layer, the thin film can be modeled as impedance that is in parallel to the LC (inductance capacitance) resonator circuit. As photocarriers are generated in the perovskite film, the value of this impedance increases, thereby reducing the overall contribution of the LC resonance. At high photocarrier densities, the induced impedance dominates and suppresses the resonant response of the metamaterial device. In contrast, when the perovskites are selectively patterned into the gaps of the metal ring, the contribution of the perovskites can be modeled as an impedance and an inductor that are connected in parallel to the capacitance of the SRR gaps. Now, as photocarriers are generated, the impedance dominates for low conductivities, weakening the resonance. However, as the photoexcitation fluence increases, the increased conductivity in the perovskite causes an increase in the inductance, which, in turn, leads to a red shift in the resonance (that is, frequency-agile operation). A detailed model is presented in section S4.

Time-resolved resonance properties of the metal hybrid structure

To understand the temporal behavior of these devices, we consider an array of metal hybrid SRRs that are patterned with MAPbI3 in the gaps (see inset of Fig. 3C). In Fig. 4A, we show the measured resonance response of the device as a function of time after the arrival of a photoexcitation pulse having a fluence of 270 μJ/cm2 at 800 nm. Once the optical pump excites the SRRs, the high photocarrier density in the gaps at t = 0 causes the resonance minimum to shift from 0.71 to 0.54 terahertz (THz). Subsequently, at longer t, as the photocarriers begin to recombine, the frequency associated with the resonance minimum relaxes exponentially back to 0.71 THz, as shown in Fig. 4B. Equivalently, this can be viewed as a means for modulating the amplitude at a given frequency (for example, at the resonant frequency of 0.71 THz).

Fig. 4 Ultrafast frequency-agile response of terahertz resonances in MAPBI3-metal hybrid structure.

(A) Ultrafast tunability of terahertz resonances in a MAPbI3-based hybrid structure using photoexcitation pulses at 800 nm and at a fluence of 270 μJ/cm2. (B) Time-resolved transmission of the terahertz resonance minimum, approaching the resonant frequency in the dark at 0.715 THz. (C) Time-resolved change in the frequency of the terahertz resonance minimum.

Ultrafast switching with delayed pump excitation

Our results suggest the possibility of switching between multiple terahertz resonances on an ultrafast time scale when multiple perovskites are incorporated into the same unit cell of the device. A schematic diagram that shows the pump-probe arrangement is presented in Fig. 5A, along with an image of a device in which the two different perovskites are patterned into the six gaps of the unit cell in alternating fashion. In the top panel of Fig. 5B, we demonstrate ultrafast switching between multiple terahertz resonances with two excitation pulses at 800 and 400 nm, respectively, separated in time by t = 150 ps. The last two panels in Fig. 5B show the transient change in terahertz transmission upon optical excitation: Photocarriers are generated in MAPbI3 with both 800- and 400-nm pulses, whereas photocarriers are generated in MAPbBr3 with only 400-nm pulses. Figure 5C shows temporal switching of the terahertz resonances. Before either of the pulse trains arrives, the metamaterial metal-hybrid device exhibits a terahertz resonance having a minimum at 0.71 THz. Following photoexcitation at 800 nm, the resonance minimum shifts to 0.66 THz. When the sample is excited with a second optical pulse at 400 nm, the resonance minimum is further shifted to 0.61 THz. We note that larger frequency shifts are possible with larger photoexcitation fluences and, potentially, with alternate metamaterial geometries.

Fig. 5 Ultrafast terahertz frequency-agile device operation using two-wavelength time-delayed pump excitations.

(A) Schematic showing the time-delayed pump excitations at 800 and 400 nm, which photogenerate free carriers in MAPbI3 and MAPbBr3, respectively. (B) Top to bottom panels: Transient photoinduced terahertz transmission response of MAPbI3 and MAPbBr3 films excited using 800- and 400-nm pulse excitations that are time-delayed by 150 ps. (C) From left to right: The frequency-agile terahertz response of dual perovskite-metal hybrid metamaterial device plotted on the time scale axis. The three transient spectra are measured at time delays of t = −30, 15, and 165 ps, when a 150-ps delay was introduced between the 800- and 400-nm excitation pulses. (D) Top and middle panels: Same as in (B) except for a delay of 50 ps between the 800- and 400-nm pulses. (E) Same as in (C) but measured at time delays of t = −10, 15, and 65 ps, respectively.

If we reduce the delay time t between the two optical pump pulses, then the resonance can be switched between the three states, that is, the resonant state in the dark and at 800- and 400-nm excitations, on different time scales, as shown in Fig. 5D. Here, the time delay (t) between the two photoexcitation pulses was reduced to 50 ps. The higher residual conductivity from the first excitation adds to the conductivity induced by the second pulse. Further manipulation of the resonances can be achieved by varying the time delay between the two pulses and the photoexcitation fluences at each wavelength. An additional example is given in section S5.

Selective operation using a tandem structure

The different bandgap properties of the hybrid perovskite SRRs also allow for the design of terahertz devices that exhibit various terahertz response properties. Using a hybrid SRR metamaterial patterned with a MAPbI3 perovskite in contact with a MAPbBr3 layer spin-coated on thin Parylene film, the device response can be switched from one that shifts in frequency upon photoexcitation to another where the resonance in the dark is simply suppressed upon photoexcitation. Figure 6A illustrates such a structure based on two different perovskite compounds. Here, the MAPbBr3 layer was spin-cast onto a Parylene-coated quartz substrate. The 6-μm Parylene film was then released from the quartz substrate, yielding a flexible perovskite/Parylene membrane. This free-standing film was then attached to a quartz substrate containing SRRs with MaPbI3 fabricated into the gaps (see Fig. 3C for this latter structure). Figure 6B demonstrates the dual operation of the device where photocarriers generated in MAPbI3-filled gaps using an 800-nm optical pump induce red shift of the device terahertz resonance, whereas excitation at 400 nm (from the MAbBr3 side) primarily suppresses the resonance. We note in passing that the obtained slight red shift in the device terahertz resonance frequency arises from photocarrier generation in the MAPbI3 in the gaps of the metal SRR with the shorter wavelength excitation.

Fig. 6 Selective control of terahertz resonance using MAPBI3- and MAPBBr3-tandem hybrid metamaterial structure.

(A) Schematic of metamaterial heterostructure structure formed by tandem arrangement of metal perovskite metamaterial structure, where MAPbI3 is selectively patterned into the gaps of the metal SRR, and a uniform film of MAPbBr3 is capped on a thin Parylene membrane. (B) Selective response of the tandem device upon photoexcitation with 800- and 400-nm pulses at fluences of 210 and 110 μJ/cm2, respectively.

DISCUSSION

We have demonstrated ultrafast frequency-agile terahertz devices in which multiple lead halide perovskites are incorporated into an array of metallic metamaterial unit cells. To accomplish this, we have developed a unique fabrication technique that relies on the use of polymer masks, which shield already deposited perovskites from organic solvents so that multiple perovskites with micrometer-scale dimensions can be patterned in close proximity to one another. When only a single perovskite is patterned in the SRR gaps, the resonance response can be tuned by varying the photoexcitation intensity. However, when two different perovskites having different optical gaps are patterned into every unit cell of the device, tuning the resonant terahertz frequencies is also dependent on the photoexcitation wavelength. By varying the time delay between the two optical pump pulses, the terahertz resonances can be switched on an ultrafast time scale. The use of multiple perovskites also allows for increased functionality (that is, tuning of the resonance with one optical wavelength and suppression of the resonance with another optical wavelength). Further variation in the response can be expected by using a greater number of different perovskites or other metamaterials.

MATERIALS AND METHODS

Terahertz measurements

The TR-OPTP system used an amplified Ti-sapphire laser (4 mJ at 85-fs pulses) as the optical source. The laser output at 800 nm was split into two beams: an optical pump, used to excite the sample, and a remainder that was used for conventional terahertz time-domain spectroscopy based on optical rectification and electro-optic sampling (31). TR-OPTP measurements were performed by varying the time delay between the optical pump and terahertz-probe pulses. A 500-μm-thick BBO crystal was used to obtain second harmonic optical pump pulses. Frequency-dependent terahertz measurements were made by fixing the optical pump at a desired position and sampling the terahertz probe using a second delay stage. The delayed pump setup was designed by modifying the standard TR-OPTP setup, where the optical pump beam was further split into two beams and an additional delay stage was used to vary the time delay between the two excitation pulses.

Sample fabrication

The metallic SRRs were fabricated on quartz substrates using standard lithography procedures by depositing 5-nm chromium and 60-nm gold films. The sample was then thoroughly cleaned with solvents and a piranha solution (3:1 volumetric solution of sulfuric acid and hydrogen perovskite). Then, a 10-μm-thick layer of Parylene C was deposited using vapor deposition (SCS Parylene Deposition System), such that the Parylene film completely encapsulated the sample. Using conventional lithography, we developed a complementary pattern in 14-μm-thick photoresist (AZ 9290; MicroChem). In the mask aligner, the sample was aligned to the gaps in the underlying metal pattern and exposed. The sample was then hard-baked at 95°C for ~30 min. The Parylene film was later etched at a rate of ~0.25 μm/min using oxygen plasma (Oxford Plasmalab 80) with a radio frequency power of 200 W. The remaining photoresist was cleaned using standard solvents. The Parylene layer provided an excellent seal, preventing solvent seepage. The samples were treated once again with a 50-W oxygen plasma for ~5 min to activate the surface for perovskite deposition. The Parylene etching was minimal during this step. The desired perovskite solution was then spin-cast onto the sample. After annealing at 110°C, the Parylene film was immediately delaminated once the films began to crystallize (after approximately 60 s). The films were then thoroughly annealed for 60 min to obtain high-quality, polycrystalline patterned structures. Finally, the sample was encapsulated with thin layer of PMMA. The process was repeated to pattern a second perovskite, if needed.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaar7353/DC1

section S1. Ultrafast photocarrier dynamics in the perovskite films

section S2. Photocarrier dynamics in MAPbI3 with 400-nm excitation

section S3. Frequency-agile terahertz metamaterial response at 400-nm and 800-nm excitation for MAPbI3- and MAPbBr3-based devices

section S4. Transmission line modeling of frequency-agile metamaterial devices

section S5. Example of variable pump intensity response in delayed pump excitation

fig. S1. Ultrafast photocarrier dynamics of MAPbI3 and MAPbBr3 perovskites.

fig. S2. Extracted Drude conductivity in MAPbI3 and MAPbBr3 perovskites.

fig. S3. Ultrafast photocarriers dynamics of MAPbI3 at 400-nm excitation.

fig. S4. Terahertz frequency-agile response in metal-hybrid metamaterial device.

fig. S5. Transmission line model for metal SRR and metal SRR perovskite hybrid structures.

fig. S6. A variation in ultrafast terahertz frequency-agile device operation using two-wavelength time-delayed pump excitations with different pump intensities.

table S1. Extracted rate constants from the curves presented in fig. S1.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

REFERENCES AND NOTES

Acknowledgments: We thank B. Sensale-Rodriguez for helpful discussions. Funding: This work was supported by the NSF Materials Research Science and Engineering Center program at the University of Utah under grant no. DMR 1121252. Support from NSF Electrical, Communications and Cyber Systems award no. 1607516 for sample preparation is also acknowledged. Author contributions: A.C., X.L., and C.Z. fabricated the devices and characterized the optical properties of the perovskites. A.C. performed the calculations, conducted all the terahertz experiments, and analyzed the data. A.N. and Z.V.V. supervised the project. All authors discussed the results and contributed to the writing of the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from A.N. (ajay.nahata{at}utah.edu).
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