Formation of Lead Halide Perovskite Based Plasmonic Nanolasers

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Formation of Lead Halide Perovskite Based Plasmonic Nanolaser and Nanolaser Array by Tailoring the Substrate can huang, Wenzhao Sun, Yubin Fan, Yujie Wang, Yisheng Gao, Nan Zhang, Kaiyang Wang, Shuai Liu, shuai wang, Shumin Xiao, and Qinghai Song ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01206 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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Formation of Lead Halide Perovskite Based Plasmonic Nanolaser and Nanolaser Array by Tailoring the Substrate Can Huang†, Wenzhao Sun†, Yubin Fan†, Yujie Wang†, Yisheng Gao†, Nan Zhang†, Kaiyang Wang†, Shuai Liu†, Shuai Wang†, Shumin Xiao†,‡ , #, Qinghai Song†, ‡,* †

State Key Laboratory on Tunable laser Technology, Ministry of Industry and

Information Technology Key Lab of Micro-Nano Optoelectronic Information System, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen, China, 518055. ‡

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan

030006, China.

KEYWORDS: hybrid plasmonic waveguide, spaser, lead halide perovskite, nanolaser array, wavelength control

ABSTRACT: Hybrid plasmonic nanolasers are intensively studied due to their nanoscale mode confinement and potentials in highly integrated photonic and quantum devices. Till now, the characteristics of plasmonic nanolasers are mostly determined by the crystal facets of top semiconductors such as ZnO nanowires or nanoplates. As a result, the spasers are isolated, and their lasing wavelengths are random and hard to be tuned. Herein we experimentally demonstrate the formation of lead halide perovskites (MAPbX3) based hybrid plasmonic nanolasers and nanolaser 1 ACS Paragon Plus Environment

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array with arbitrary cavity shapes and controllable lasing wavelengths. These spasers are composed of MAPbX3 perovskite nanosheets, which are separated from Au patterns with a 10 nm SiO2 spacer. In contrast to previous reports, here the spasers are determined by the boundary of Au patterns instead of the crystal facets of MAPbX3 nanosheets. As a result, whispering-gallery-mode based circular spaser and spaser array have been successfully realized by patterning the Au substrate into circles and gratings, respectively. The standard wavelength deviation of spaser array is as small as 0.3 nm. Meanwhile, owing to the anion-exchangeable property of MAPbX3 perovskite, the emission wavelengths of spasers have been tuned more than 100 nm back and forth by changing the stoichiometry of perovskite post-synthetically.

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Miniaturization of semiconductor lasers is a key step towards the faster, smaller, higher density, and lower energy consumption photon-based technologies, e.g. optical computing, highly sensitive detector, and on-chip quantum information et al.1-3 While many techniques have been developed to minimize the laser sizes, surface plasmon polaritons has been the most promising one to break the diffraction limit because it can allow the storage of energy in electron oscillations at the metal-dielectric interface.4-11 By coating the Au and Ag nanoparticles with gain medium, the light confinement and amplification in deep subwavelength scale have been facilitated with the localized surface plasmon modes.8 Meanwhile, the propagating hybrid surface plasmon within the gap between Ag film and a high index nanowire has also been used to produce nanolasers.12 Because the second technique is less affected by the Ohmic losses and has much longer amplification length along the axial direction, the hybrid plasmonic waveguide based spasers have been widely applied in a number of material systems, e.g. CdS,13 CdSe,14 ZnO,15,16 and GaN.17 In 2014, hybrid plasmonic nanolaser based explosive detection has also been reported.18 In spite of their rapid developments, the plasmonic nanolasers are facing severe challenges in practical applications. The performances of these spacers are mostly determined by the geometries of top semiconductors. As the above semiconductors are quite stable after synthesis, the lasing wavelength ranges are usually fixed and hard to be tuned.19,20 Meanwhile, owing to the randomness of synthesized nanostructures, the lasing wavelengths of the corresponding spasers are also random and hard to be repeated,


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significantly limiting their applications in high throughput monitoring and pin-point detection.21 In searching an ideal material for spaser, we turn to the recently developed organo-lead halide perovskites. While this kind of materials are simply synthesized from solution,22 the optical gain of synthesized lead halide perovskites can still be as high as 3,200 ± 830 cm-1, which is comparable to single crystalline GaAs and more than an order of magnitude higher than CdS crystal.23 In past three years, the lasing actions have been thoroughly studied in perovskite based optical systems, especially in the single crystalline microrods and microplates. A number of interesting laser characteristics have been experimentally demonstrated,24-28 e.g. high quality (Q) factors,29 record low threshold,26 large external quantum efficiency,22 unidirectional laser emission,30 and integration with conventional optical systems.31,32 Very recently, the nanofabrication technologies have also been successfully developed to convert the lead halide perovskites from materials to nanodevices.33,34 Compared with conventional semiconductors such as CdS and ZnO, the emission wavelengths of lead halide perovskites can be widely tuned from ultraviolet to near infrared by controlling the halide mixture in initial solution or exchanging anions post-synthetically.35-39 Therefore, the ability of perovskites to encompass the full visible spectral range, associated with their single crystal nature and high refractive index, make the synthesized lead halide perovskites to be ideal materials for hybrid plasmonic nanolasers. In this research, by combing the anion-exchangeable property of MAPbX3 perovskite and the substrate-control mechanism, we experimentally demonstrate the 4 ACS Paragon Plus Environment

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perovskite-SiO2-Au hybrid plasmonic nanolaser and addressed the above challenges of typical spasers.

RESULTS AND DISCUSSIONS

Conventional MAPbX3-Au hybrid plasmonic nanolasers Figure 1(a) shows the schematic picture of a conventional MAPbX3 based hybrid plasmonic nanolaser. It is composed of a hexagonal MAPbX3 microplate and an Au film that are separated with a 10 nm SiO2 layer. The side length and thickness of MAPbX3 microplate are a and t, respectively. The light confinement within such kind of hybrid structure has been thoroughly studied in past few years.12,13 Due to the continuity of electric displacement (D = εE, smaller ε corresponds to larger E) in the vertical direction and the capacity effects,40 the intensity of transverse magnetic (TM, with E perpendicular to the plane) polarized light within the SiO2 nanogap shall be orders of magnitude larger than the surroundings. On the contrary, the transverse electric (TE, E is in-plane) polarized light has electric field parallel to the metal surface and cannot form the hybrid plasmonic mode. Since two polarizations can be simply separated in experiment, we mainly focus on the TM polarized electromagnetic waves below. Figure 1(b) shows the numerically calculated effective refractive index (neff) as a function of thickness t (see details in Methods). When the thickness of perovskite microplate is large, both of TM polarized photonic mode and hybrid plasmonic mode (see their field patterns and the differences in supplemental information) can propagate inside the hybrid waveguide. With the decrease of t, the 5 ACS Paragon Plus Environment

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TM-polarized photonic mode is cutoff at t = 90 nm and only the hybrid plasmonic mode always exists. Meanwhile, as the neff is more than 1.5 in a wide range of t, the total internal reflection can occur at the perovskite-air interface and thus in-plane whispering gallery modes can be generated.

Figure 1. (a) Schematic picture of the MAPbX3 based hybrid plasmonic nanolaser. (b) Numerically calculated effective refractive index (neff) as a function of thickness t. (c) Calculated quality (Q) factor around 780 nm. (d) Corresponding field pattern of high Q in figure 1(c). (e) Field confinement in vertical direction. Taking a perovskite microplate with a = 10 µm and t = 70 nm as an example, we have numerically studied the resonant modes within the hybrid system (see methods). As shown in Figure 1(c), some resonance can have relatively higher quality (Q) factor than the others. From the corresponding field patterns, it is easy to see that the long-lived modes are confined by total internal reflection along a 6-bounce hexagonal orbit (see Figure 1(d)). At the same time, the tight field confinement in vertical 6 ACS Paragon Plus Environment

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direction (see Figure 1(e)) significantly reduces the effective mode volume (Veff) to 0.406 µm3, which is orders of magnitude smaller than conventional semiconductor microdisk. The large Q factor and small Veff can thus ensure large Purcell factors (~ Q/Veff) of the perovskite-SiO2-Au hybrid nanostructure. Therefore, considering the high gain coefficient of perovskites, the hybrid system can be a nice candidate to realize nanoscale coherent light sources. Based on above analysis and numerical calculations, we have experimentally fabricated the hybrid plasmonic nanostructures and studied their optical performances. In our experiment, the Au and SiO2 films were deposited with electron-beam evaporation (see methods). The thicknesses of Au film and SiO2 spacer were 70 nm and 10 nm, respectively. Then the CH3NH3PbI3 microplates were synthesized onto the SiO2 spaser through the combination of solution process and vapor-phase conversion method (see methods and details in figure S (1)). 41,42 Figure 2(a) shows the top-view scanning electron microscope (SEM) image of the synthesized MAPbI3 microplates, where many hexagonal microplates can be observed. The side lengths of these microplates vary between 10 µm to 100 µm. X-ray diffraction measurements and transmission electron microscope (TEM) have also been performed to investigate the crystal structures of the microplates. As shown in Figure 2(b), two dominant peaks are seen at 14.1o and 28.5o, which can be indexed to (110) and (220) facets of tetragonal structure.43,44 The other peaks in Figure 2(b) are generated by the thick PbI2 microplates that have not been fully converted to MAPbI3 perovskites (see figure S(3) ). The crystal structural of the as-grown perovskite microplates have been 7 ACS Paragon Plus Environment

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confirmed with high resolution transmission electron microscope (HRTEM) image and the selected area electron diffraction (SEAD) pattern. As shown in Figure 2(c), fringes that correspond to (220) and (200) planes of tetragonal single crustal can be clearly seen. The thickness and surface roughness of the synthesized perovskites microplate have also been measured by atomic force microscope (AFM). The CH3NH3PbI3 microplates were found to have quite smooth surface with route mean square (RMS) around 4.4 nm (see inset on Figure 2(a)). Figure 2(d) summarizes the thicknesses of synthesized perovskite microplates, which mostly vary between 40 nm – 70 nm. The single crystalline nature, the smooth surface, and the thicknesses make the performances of CH3NH3PbI3 perovskite microplates based hybrid plasmonic nanolasers very promising.


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Figure 2. (a) Top-view scanning electron microscope (SEM) image of the synthesized CH3NH3PbI3 microplates. Scale bar: 10 µm. Inset: Atomic force microscope (AFM) image of the CH3NH3PbI3 microplates with thickness around 50 nm. (b) X-ray diffraction measurements of the synthesized microplates. (c) High-resolution transmission electron microscope (HRTEM) of the synthesized microplates. Inset: the selected area electron diffraction(SAED) pattern confirms the crystal structure of CH3NH3PbI3 perovskite. (d) The thicknesses of synthesized perovskite microplates. Then the laser characteristics of hybrid plasmonic nanostructure were characterized with optical excitation at room temperature. The sample was mounted onto an optical microscope stage and excited with a frequency doubled Ti: Sapphire laser (wavelength at 400 nm, 100 fs pulse duration and 1000 Hz repetition rate). The pumping laser was focused by a 60x objective lens onto the top surface of the perovskite microplate. The emitted lights were collected and collimated by the same objective lens, and finally coupled to a ¾ m spectrometer and CCD camera (Princeton Instrument) via a multimode fiber. Figure 3(a) summarizes the emission spectrum of a CH3NH3PbI3 perovskite microplate based hybrid plasmonic nanostructure as a function of pumping density. The side length and thickness of the microplate are 10 µm and 50 nm, respectively. When the pumping density was low, a broad photoluminescence peak can be observed at 770 nm with a full width at half maximum ~ 40 nm, consistent with previous reports on CH3NH3PbI3 perovskites very well.24 With the increase of pumping power, the intensity of photoluminescence also increased slowly and narrow peaks emerged at P = 59.2 µJ/cm2. Further increasing the pumping power, the emission intensity increased dramatically and the emission 9 ACS Paragon Plus Environment

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spectrum was dominated by the sharp peaks. Figure 3(b) summaries the output intensity as a function of pumping density. The “S” curve clearly shows the transition from spontaneous emission to amplification and gain saturation and thus the lasing actions in perovskite-SiO2-Au hybrid nanostructure can be confirmed. According to Figure 1(b), the TM polarized photonic modes are cutoff in perovskite microplate with t = 50 nm and only hybrid plasmonic mode and TE polarized photonic mode can exist. Then the polarization can be an effective way to separate photonic laser and plasmonic nanolaser. To identify the fundamental mechanism, we have experimentally measured the polarization of laser emissions (see supplemental information). As depicted in inset of Figure 3(b), the emissions are clearly TM polarized with electric field perpendicular to the surface of microplate. Thus the TE polarized photonic mode can be excluded and the lasers shall be formed along the hybrid plasmonic mode. In semiconductor lasers, the proportion of light that couples into the laser mode (e g. spontaneous emission coupling factor β) is inversely proportional to the effective mode volume and corresponds to a higher x0 value.12, 45,46 Consequently, the ultrasmall Veff of hybrid plasmonic nanostructure shall correspond to near unity x0 value. This is exactly what we have observed. By fitting the threshold curve in Figure 3(b) (see methods), the calculated x0 value was as large as 0.38, which was much larger than the previous reports on microdisk lasers.47 As a control experiment, the laser characteristics of similar perovskite microplates on a glass substrate have also been experimentally studied. As shown in the figure S7, only TE


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polarized photonic lasers have been obtained when the thickness of microplate was smaller than 135 nm. And their fitted x0 values are only around 0.026. More than the TM polarization and the large x0 value, the double-pump excitation measurements can be an additional way to confirm the onset of the hybrid plasmonic nanolaser.16 Following the research in Ref. 16, we measured the total double-pump response against time delay above pumping threshold. The pumping density of two pulses were fixed at 1.2 times and 0.24 times (5:1) of laser threshold. In principle, the first pulse must create population conversion. If the second pulse reaches during the lasing process, it won’t be absorbed and contribute to the total output intensities. With the increase of delay time τ, the first laser pulse will be terminated and the absorption of the second pulse recovers. As a result, the total output shall be increased and reach a maximum at τmax. According to the experimental results in Figure 3(c), we can see that the τmax of the plasmonic mode (blue dots in figure 3(c)) is around 2 ps, which is an order of magnitude smaller than the corresponding photonic mode (yellow dots in figure 3(c)). This result is consistent with the previous report in Ref. 16 and confirms the onset of hybrid plasmonic nanolaser well.


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Figure 3. (a) The emission spectrum of a CH3NH3PbI3 perovskite microplate as a function of pumping density. (b) The output intensity as a function of pumping density. The fitted x0 value of the “S” curve is 0.3824.

Inset shown the collected

emission spectrum for TM (E field norm to the sample surface) and TE (E field parallel to the sample surface) mode. Here the 0° was defined as the direction which the axis of the polarizer norm to the plane (e g, TM mode, with E perpendicular to the plane). (c) Total double-pump response of the emission intensity versus time delay near the threshold. (d) Threshold intensity of plasmonic and photonic lasers versus perovskite thickness, the solid lines were used for guidance. To exclude the accident phenomena, we have repeated the experiments with the other 30 samples. All of the experimental details are shown in Figure 3(d). The thresholds of hybrid plasmonic lasers slightly increase with the decrease of device thickness from 150 nm to 20 nm. On the contrary, the TE polarized photonic lasers keep much lower thresholds at large thickness. When the thickness of CH3NH3PbI3 12 ACS Paragon Plus Environment

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perovskite microplate approaches 50 nm, the thresholds dramatically increase and no TE polarized lasers can be experimentally observed at t < 40 nm. All of these observations are consistent with the numerical calculations in Figure 1(b), where both TE and TM polarized photonic modes are cutoff at t ~ 40 nm and only hybrid plasmonic mode exists. Therefore, based on the TM polarization, the relatively large x0 values, the laser emissions at very small t, and the temporal response, we can conclude that hybrid

plasmonic nanolasers

have

been generated in the

perovskite-SiO2-Au hybrid nanostructure. Wavelength controllable plasmonic nanolaser The above experiments show that the single crystalline lead halide perovskite microplate, similar to CdS and GaN nanorod and nanoplate, can also be applied to generate plasmonic nanolasers. Compared with the conventional semiconductors, the bandedge and the emission wavelengths of lead halide perovskites can be widely tuned by changing the stoichiometry. This is an intrinsic advantage of lead halide perovskite, especially for the case that the devices have been fabricated. Below, by using the vapor assisted anion exchange method, we demonstrate the applications of lead halide perovskites in controlling the wavelengths of fabricated plasmonic nanolasers.


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Figure 4. (a) Schematic diagram of the stoichiometric ratio changing of synthesized CH3NH3PbI3 by thermal annealing in LPCVD system. (b) Photoluminescence(PL) of CH3NH3PbI3 perovskite microplates changed by the effects of thermal annealing. (c) The emission wavelengths of lead halide perovskite as a function of the annealing time. As depicted in Figure 4(a), the stoichiometry of synthesized CH3NH3PbI3 perovskite microplates can be changed by thermal annealing in a low pressure chemical vapor deposition (LPCVD)

with CH3NH3Br vapor environment (see

methods). During the annealing, the iodine ions are replaced by bromide ions. The degree of anion exchange in lead halide perovskites was controlled by the annealing time. Figure 4(b) illustrates the effects of thermal annealing on the photoluminescence of CH3NH3PbI3 perovskite microplates. With the increase of annealing time, more iodine ions were replaced by bromide ions and the emission wavelength shifted from ~ 770 nm (for CH3NH3PbI3) to ~ 550 nm (for CH3NH3PbBr3). Interestingly, we found that the wavelength shift of lead halide perovskite was reversible. When the 14 ACS Paragon Plus Environment

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microplates were further annealed in CH3NH3I vapor environment, the emission wavelength can gradually shift back to 770 nm. Figure 4(c) summarizes the emission wavelengths of lead halide perovskite as a function of the annealing time. We can see that the emission wavelength is almost linearly dependent on the reaction time. The decrease slope and increase slope are around -2 nm/min and 4 nm/min, respectively.

Figure 5. (a) Peak wavelengths of hybrid plasmonic nanolasers. Inset: the corresponding microscopy image (680 nm). (b) Dependence of lasing wavelength on the anion exchange time. (c) Laser threshold as a function of wavelength. Based on the wavelength shift of photoluminescence, we have experimentally explored the possibility of controlling the emissions of hybrid plasmonic nanolasers. All of the experimental results have been summarized in Figure 5. In a CH3NH3PbI3 microplate-SiO2-Au hybrid plasmonic nanostructure, the plasmonic nanolasers emission has been confirmed by recording the thickness of microplate, the laser polarization, and the fitted x0 value (see the experimental details in figure S9). When the hybrid plasmonic nanostructure was annealed in LPCVD, the wavelength shift of plasmonic nanolasers has been experimentally observed. Figure 5(a) shows that the peak wavelengths of hybrid plasmonic nanolasers gradually shifted from 779 nm (original wavelength) to 740 nm (15 min), 710 nm (30 min), 680 nm (50 min), and 15 ACS Paragon Plus Environment

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662 nm (60 min). Owning to the anion-exchangeable property of lead halide perovskite, the wavelength controllable hybrid plasmonic nanolasers have been experimentally demonstrated. The dots in Figure 5(b) summarize the dependence of lasing wavelength on the anion exchange time. The blue shift of plasmonic nanolasers was also found to be linearly dependent on the anion exchange time and the maximum wavelength shift was more than 100 nm. Further increasing the anion exchange time can shift the emission wavelength to green light spectrum. However, as shown in Figure 5(c), the laser threshold also increased dramatically with the reaction time. When lasing wavelengths are below 660 nm, the threshold is so high that the nanostructures have been rapidly damaged. This is because that CH3NH3PbI3 and CH3NH3PbBr3 have different crystal structures. Simply replacing iodine ions with bromide ions will partially degrade the crystal quality and reduce the optical gain. In the shorter wavelength ranges, similar wavelength controllable hybrid plasmonic nanolasers can be realized by utilizing CH3NH3PbBr3 and CH3NH3PbCl3 perovskites. Similar to the photoluminescence, we note that the wavelength shift of hybrid plasmonic lasers is also reversible (see Figure 5b). After thermal annealing in CH3NH3I vapor environment, the laser wavelengths and threshold can be simply recovered (see the square in Figure 5c). Therefore, the wavelength control of MAPbX3 perovskite based spaser is reversible and repeatable. Plasmonic nanolaser with arbitrary cavity shape


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Although the perovskite spasers have been successfully demonstrated, as shown in Figure 1(d), the main light confinements are still caused by the total internal reflection at the facets of MAPbBr3 nanosheet. According to the SEM and AFM images (e.g. Figure 2), we find that all the perovskite nanosheets have hexagon shapes. Meanwhile, the sizes and thicknesses of the perovskite nanosheets are quite random. Therefore, the perovskite spasers must follow the orbit families of polygon cavities. Their wavelengths are hard to be pre-defined and repeated in other devices. These limitations strongly hinder the practical applications of the perovskite spasers, especially for the integrated photonic systems that require spasers with various of shapes and sizes. While the semiconductor nanosheets can be tailored with top-down fabrication technique, their excellent gain coefficients are hard to be preserved and it is thus difficult to support high-performance lasing actions. To date, a hybrid plasmonic nanolaser with arbitrary cavity shape has not been explored. The flexibility of the thin perovskite microplates supplied us the convenience to achieve spaser with arbitrary cavity shape. In contrast to the modifications on the gain materials, we control the characteristics of hybrid plasmonic nanolasers by tailoring the metallic substrate. As illustrated in Figure 6(a), the configuration is also composed of a single crystalline perovskite nanosheets, a SiO2 insulating layer, and patterned gold substrates. The perovskite nanosheets also have hexagonal boundaries and their sizes are much larger than the gold patterns. As the thickness of nanosheets is smaller than the cutoff wavelengths of photonic modes, the light can only propagate as hybrid plasmonic waveguide mode at the perovskite-SiO2-Au hybrid interfaces. Therefore, 17 ACS Paragon Plus Environment

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the hybrid nanocavities are defined by the boundaries of gold patterns instead of top semiconductor nanosheets. Due to rapid progresses in plasmonic nanostructures and metamaterials, 48 this concept makes the control of cavity shape to be simple. Here we take the hybrid plasmonic nanolaser with a circular cavity boundary shape as an example to illustrate this concept (see Figure 6(a)). Figure 6(b) shows the numerically calculated mode confinement within the hybrid plasmonic nanolaser (Calculation details see the methods). Similar to the above analysis, the resonant mode is well confined along the boundary of Au circle instead of the boundary of nanosheet. To verify the above numerical calculations, we have experimentally fabricated a circular shaped hybrid plasmonic nanolaser and examined its laser performances (fabrication process is described in the method section). According to the SEM image and the AFM image (see inset of figure 6(d)), we know that the thicknesses of Au, SiO2, and perovskite nanosheet are 100 nm, 10 nm, and 75 nm, respectively. The in-plane radius of Au disk is 10 micron. When the pumping density is above 30 µJ/cm2, a series of narrow peaks appear in the emission spectrum. The corresponding output intensity as a function of pumping density in Figure 6(e) clearly shows the threshold behaviors at around 30 µJ/cm2 and thus confirms the laser actions. The inset shows the fluorescent microscope image of the perovskite/Au/SiO2/ disk, in which the boundary is much brighter, corresponding to the WGM mode spaser well. Following the simulation results in Figure 1, hybrid waveguide with 75 nm perovskite nanosheet can either support TE0 waveguide mode or hybrid plasmonic waveguide mode. As two modes have reversed polarization, they can be easily distinguished from the 18 ACS Paragon Plus Environment

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polarization. As depicted in Figure 6(f), the emissions are TM polarized with E perpendicular to the plane and clearly demonstrate the onset of hybrid plasmonic nanolaser.

Figure 6. (a) The schematic picture of perovskite nanosheets covered on the Au/SiO2 disk. (b) The numerically calculated mode confinement within the hybrid plasmonic nanolaser in cross-section. Here the light is confined along the boundary of Au pattern. Inset shows the 3D filed pattern reconstructed with the Azimuthal number m. (c) The SEM image of perovskite plates covered on Au/SiO2 disks. Scale bar: 2 µm. Inset shows the SEM image (scale bar: 5 µm) and its high-resolution magnification image of Au/SiO2 disk. Scale bar: 500 nm. (d) Emission spectrum of the perovskite microplates on Au/SiO2 disk above threshold. Inset: AFM image of perovskite plates 19 ACS Paragon Plus Environment

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covered on Au/SiO2 disks. Scale bar: 5 µm. (e) Integrated output intensity as a function of pumping density. The fitted x0 value of the “S” curve is 0.282. Inset shows the fluorescent microscope image of perovskite hybrid plasmonic nanolaser. (d) The TM polarization of photoemission. Different from the results in Figures 3, the lasing modes in Figure R6 (d) are well separated. The mode spacing is around 2.8 nm, which is consistent with the free spectral range of WGMs in circular shaped hybrid plasmonic nanocavity very well. For a direct comparison, we have measured the emission spectrum out of the Au circle. Only photoluminescence has been observed no matter how high the pumping density is. Therefore, we can confirm that the hybrid plasmonic nanolasers have been formed within the circular shaped nanocavities. Plasmonic nanolaser array Another challenge of hybrid plasmonic laser is the repeatability. As mentioned above, the exact dimensions of chemically synthesized single crystal nanosheets are quite random. Thus it is quite challenging to realize a hybrid plasmonic nanolaser array with uniform laser emissions. In contrast to the synthesis of uniform single crystalline nanostructures, based on the same mechanism in Figure 6, here we show that a hybrid nanolaser array can be realized by tailoring the substrate. Figure 7(a) shows the top-view SEM and AFM images of the final sample. A periodic Au-SiO2 strips have been realized on silicon substrate and covered with a perovskite nanosheet (the fabrication process could be seen in the supplemental information). The period of Au grating is 10 µm and the width of Au-SiO2 strip is 5 µm. Due to its small 20 ACS Paragon Plus Environment

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thickness, the MAPbI3 perovskite microplate was tightly attached to the Au-SiO2 strip and the Si substrate. There is almost no air gap between microplate and the Si substrate (see figure S10) Then the perovskite nanosheet was divided by the Au-SiO2 strips into many segments. Because silicon has much higher refractive index, there is no guiding mode at perovskite-Si region and the in-plane whispering gallery modes are eliminated. Considering the relatively high neff of hybrid plasmonic mode, Fabry-Perot resonances can be formed at the SiO2-Au strips in the transverse horizontal direction. The corresponding numerical calculations are shown in figure 7(b). The light is confined along the hybrid plasmonic mode in vertical direction. The in-plane confinement is mainly caused by the reflection at two edges of each Au strip. We then experimentally measured the lasing actions in such structures via optical excitation. The results are summarized within the supplemental information. The spasers have been confirmed by measured the polarization, the emission spectrum, and the L-I curves. The lasing actions have been further confirmed by studying the linear dependence of mode spacing on 1/neffL (see supplemental information). More than the confirm of Fabry--Perot laser, the numerical calculations shows that the Fabry-Perot modes are confined within each Au-SiO2 strip. As a result, the Fabry-Perot modes on different SiO2-Au strips can be utilized to generate hybrid plasmonic nanolaser array. As the widths of SiO2-Au strips can be easily controlled, the emission from different spasers should be quite uniform. To confirm this information, we have selectively pumped different SiO2-Au strips on the same sample 21 ACS Paragon Plus Environment

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and recorded their optical characteristics. The experimental details are shown in the supplemental information and parts of results are plotted in Figure 7(c). We can see that the emissions from different strips are very close and the standard deviations are only as small as 0.3 nm. We note that the standard deviation is mostly caused by the fabrication deviation. This value can be simply improved by replacing standard photolithography (resolution > 500 nm) with high resolution nanofabrication technique. Similar to Figure 4 and Figure 5, the emission wavelengths of plasmonic nanolaser array can also be controlled. As shown in Figure 7(d), the emission wavelengths can be tuned from 775 nm to 690 nm by annealing the sample in MABr vapor environment. As a result, the laser emissions can be experimentally observed. The inset in Figure 7(d) shows the corresponding fluorescent microscope image of the


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hybrid

plasmonic

nanolaser.

Figure 7. (a) Top-view SEM and AFM images of the periodic Au/SiO2 strips covered with a perovskite nanosheet. The period of Au grating is 10 µm and the width of Au/SiO2 strip is 5 µm. (b) The numerically calculated F-P mode hybrid plasmonic nanolaser in cross-section. The light is confined along the hybrid plasmonic mode between two edges of each Au strip. The inset shows the magnified field pattern within the SiO2 gap. (c) Spectrum of selectively pumped different SiO2/Au strips on the same sample. Inset: the corresponding optical image.

(d) The blue-shifted

emission wavelength spectrums by annealing the sample in CH3NH3Br vapor environment. Inset: the corresponding fluorescent microscope image. CONCLUSION


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Overall, we have synthesized ultrathin (40 nm – 70 nm) lead halide perovskite microplates and experimentally studied their potential in forming the hybrid plasmonic nanolasers. Due to the cutoff of photonic mode, the lead halide perovskite based hybrid plasmonic nanolaser has been confirmed with the polarization of laser emission, the fitted large x0 value, and the time-domain behaviors. Most importantly, we have demonstrated the substrate-control concept to form hybrid plasmonic nanolaser. We show that the resonances within hybrid plasmonic nanolaser can be precisely controlled by the shape and size of bottom Au patterns instead of the top semiconductors. As a result, by patterning the substrate into Au disks and Au strips, we have experimentally realized the circular hybrid plasmonic nanolaser and the uniform plasmonic nanolaser array. We note that the substrate-control concept in this research is not limited in lead halide perovskite and hybrid plasmonic lasers. It can be extended to other gain materials and photonic lasers as well. We believe this research will be important for application of lead halide perovskites in quantum information technology and high throughput biological sensing. This research shall also accelerate the developments of plasmonic nanolasers and their applications.

METHODS

Numerical calculations: The numerical calculations were performed using finite-element method (COMSOL). The effective index as a function of perovskite 24 ACS Paragon Plus Environment

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thickness was calculated by setting the perovskite/SiO2/Au configuration with different thickness t.

Since the model is too huge for full 3D simulation, we solved

for the plasmonic mode in 1D and then utilized the corresponding TM wave effective mode indexes inside and outside the cavity in a 2D eigenfrequency calculation.

The

effective mode volume was calculated by using effective mode area calculated in 2D eigenfrequency times the gap thickness. During the simulation, the refractive indices of perovskite and SiO2 are set by 2.5 and 1.46 at λ=770 nm, respectively, whereas the Au film is described by a Drude model following Johnson’s experiment.49-50 The high spontaneous emission factor for the plasmonic devices was fitted to Casperson's model with a single fitting parameter, x0, in the integrated emission spectrum against the pump power. Here, for the photonic mode, usually x00.1.16,45 Device fabrication: The gold film and SiO2 film are deposited with electron-beam evaporation. The deposition rates are both 0.3 Å/s and the base vacuum pressure is 2

×10-7 Torr. The final thicknesses of Au and SiO2 are 70 nm and 10 nm, respectively. Synthesis of MAPbX3 microplate: The perovskite microplates were prepared using a combined solution process and vapor-phase conversion method proposed by Bao et al.41,42 Basically, the saturated PbI2 aqueous solution was prepared by dissolving 2 mg PbI2 powder in 5 mL of deionization (DI) water and heated at 110 °C for 1 h and cooled naturally. Then the solution was casted on the SiO2/Au substrate and PbI2 microplate started to appear. During the evaporation of DI water, the microplates were moved by a tapered fiber equipped with a 3D translation platform to the designed 25 ACS Paragon Plus Environment

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position before the PbI2 nanocrystals stick to the substrate. During the whole vapor conversion process, the CH3NH3I powder was placed at the center of a CVD furnace while the as-grown PbI2 microplates on the Au/SiO2 or silica substrate were mounted downstream of the apparatus. The central heating zone was increased to 125 °C (8 °C/min heating rate, 10 mins as buffer ) under low-pressure conditions (40−50 Torr) and maintained from 25 min to 2 h. Ar and H2 were used as carrier gases with flow rates of 35 and 15 sccm, respectively. The furnace was then naturally cooled to room temperature and MAPbI3 microplates have been obtained. Post-synthetic control of the emission wavelength: The wavelength of perovskite microplate was changed by controlling the stoichiometry after the fabrication of the samples. By controllably annealing CH3NH3PbI3 nanosheets in an excess CH3NH3Br vapor environment at 125 °C under low pressure (40−50 Torr), the iodine ions were replaced by the bromide ions. Then the CH3NH3PbI3 was gradually transferred to CH3NH3PbBrxI3−x. The number x was controlled by the annealing time (from 5 min to 2 h).

ASSOCIATED CONTENT

Supporting Information Available:. This material is available free of charge via the Internet at http://pubs.acs.org This supplemental information includes additional experimental details for 26 ACS Paragon Plus Environment

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sample synthesis and optical characterization, the optical characterization of photonic laser, the experimental details for wavelength-controllable plasmonic nanolaser, and the characterization of F-P type plasmonic nanolaser. AUTHOR INFORMATION Corresponding Author Email: #[email protected]; *[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The author would like to thank the financial support from Shenzhen Fundamental research projects (JCYJ20160427183259083), Public platform for fabrication and detection of micro- & nano-sized aerospace devices, and Shenzhen engineering laboratory on organic-inorganic perovskite devices.

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Formation of lead halide perovskite based plasmonic nanolaser and nanolaser array by tailoring the substrate In contrast to previous reports, we show that the resonances within hybrid plasmonic nanolaser can be precisely controlled by the shape and size of bottom Au 32 ACS Paragon Plus Environment

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patterns instead of the top semiconductors. As a result, by patterning the substrate into Au disks and Au strips, we have experimentally realized the circular hybrid plasmonic nanolaser and the uniform plasmonic nanolaser array.


Formation of Lead Halide Perovskite Based Plasmonic Nanolasers

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