Surface-Emitting Perovskite Random Lasers for Speckle-Free Imaging

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Surface-Emitting Perovskite Random Lasers for Speckle-Free Imaging Yilin Liu, Wenhong Yang, Shumin Xiao, Nan Zhang, Yubin Fan, Geyang Qu, and Qinghai Song ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04925 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Surface-Emitting Perovskite Random Lasers for Speckle-Free Imaging Yilin Liu,† Wenhong Yang,† Shumin Xiao,†,

‡, #

Nan Zhang,† Yubin Fan,† Geyang Qu,† and

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, 518055, China. ‡Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006 China Email: # [email protected]; * [email protected];

ABSTRACT. Random lasers have been ideal illumination sources for speckle-free and highspeed imaging. Despite of their successes, the real applications of random lasers are facing a long-standing challenge, i.e. the cumbersome size of the illuminating system. Herein we demonstrate a perovskite-based surface emitting random lasers (SERLs) and explore their applications in speckle-free imaging. The random lasers are generated by the multiple scattering in a perovskite polycrystalline film sandwiched by two distributed Bragg reflectors (DBRs). Owing to the tight confinement in vertical direction and large number of random resonances, the wavevectors of random lasers are dominated by their vertical components and thus multimode SERLs with divergence angle ~3-5o and low spatial coherence are produced. By directly

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illuminating the patterns with the SERLs, the notable speckle noises of conventional optical images have been dramatically suppressed. This research shall provide a strategy towards the integrated spectral-free imaging systems.

KEYWORDS. random laser, perovskite, vertical cavity surface-emitting laser, speckle-free imaging, spatial coherence


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Lead halide perovskites (MAPbX3, X=Cl, Br, I) have been intensively studied as cost-effective photovoltaic materials.1,2 Owing to their long carrier lifetime and carrier diffusion length, the power conversion efficiency (PCE) of perovskite solar cells has been dramatically increased from 3.9% to 24.2%.3 The continuous successes in photovoltaics have also driven the rapid developments of other perovskite devices, e.g. photodetectors,4 light-emitting diodes,5-7 metasurfaces,8-12 and the microlasers.13-32 Soon after the discovery of their exceptional gain, MAPbX3 perovskite lasers have been experimentally obtained from various perovskite nanostructures including nanowires, microrods, microplates, photonic crystals, and films. Among all of these lasers, perovskite random laser is interesting but relatively less-explored one.19-25 Unlike the conventional lasers, random lasers are independent to the well-designed cavities and can be simply generated by the multiple scattering in gain medium.33, 34 As a result, perovskite random lasers have been widely observed in numerous of perovskite films and powders. However, due to the strong scattering and the modal interaction,35 the quality (Q) factors of modes from different random cavities can be very close and multiple laser peaks are thus excited simultaneously. The mode numbers, wavelengths, directionalities, and Q factors of perovskite random lasers are relatively random and hard to be controlled.34,35 Consequently, such kind of generic lasing actions have been widely accepted as useless light sources and typically ignored in practical applications. In contrast to the conventional understanding, we note that the perovskite random lasers also have their unique properties. One prominent example is the spatial coherence.36,37 In random lasers, multiple laser modes from different random cavities are excited simultaneously, significantly reducing the spatial coherence without spoiling the laser intensities. On one hand, the preserved high intensity of random lasers ensures their applications in extreme conditions


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such as scattering or absorptive media.37,38 On the other hand, the reduced spatial coherence can effectively eliminate the notable speckles of optical images when the patterns are illuminated with random lasers, simply expanding their applications to bio-medical imaging.38,39 In addition, the lead bromide perovskite (MAPbBr3) random lasers perfectly fill the well-known “green gap”, where the quantum efficiency of conventional III-V semiconductors drops drastically.13-21, 38-41 Therefore, perovskite random lasers can be ideal sources for wide-field and high-speed imaging, optical storage, and holography etc. Despite of the above advantages, the real applications of perovskite random lasers are still facing one severe challenge. The emissions of perovskite random lasers are omni-directional and their spectra vary with the detection angle.38 While external optical lens can collect and collimate partial laser emissions, there is still a remarkable waste of laser energy and laser modes, and the eventual system is also too cumbersome for pointof-care applications. Here, we demonstrate the perovskite based green SERLs with low spatial coherence and explore their applications in integrated speckle-free imaging. RESULTS AND DISCUSSION Random lasers in perovskite films. The lead halide perovskite films used in this study were prepared by directly spin-coating the perovskite precursor on substrates (see Methods). The thickness of perovskite film is controllable via the concentration of precursor and the spincoating speed (see Figure S1). The corresponding X-ray diffraction (XRD) spectrum, absorption spectrum, and the energy dispersive X-Ray spectroscopy (see Figure S1) are consistent with the literatures and confirm the formation of MAPbBr3 films. Figure 1a shows the top-view scanning electron microscope (SEM) image of perovskite film on glass substrate. Since no additional solvent and nitrogen blowing were applied, the films look more like the aggregation of irregular perovskite islands than a uniform film. The in-plane sizes of perovskite islands are around 20–80


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µm. The optical properties of the perovskite films were characterized by pumping them with a femtosecond Ti:Sapphire laser (see Methods and Figure S2). The emission spectra at different pumping densities are shown in Figure 1b. There is only a broad spontaneous emission peak at low pumping fluence. With the increase of pumping power, sharp peaks with linewidths below 1 nm emerge and quickly dominate the emission spectrum at higher pumping fluence. Figure 1c summarizes the integrated emission intensity as a function of the pumping density, where an “S” curve can be clearly seen in the log-log plot. Based on the threshold behavior and the rapid reduction of linewidth, we can conclude the onset of lasing actions in the perovskite films. Because no cavities have been pre-defined in the perovskite film, the lasing actions must relate to its internal structures. The fluorescent microscope image in inset-I of Figure 1c shows that the lasers are mostly confined within one island. The high-resolution SEM image of the island is depicted as inset in Figure 1a. It contains numerous perovskite flakes and has quite irregular rough boundaries. Then the generation of perovskite-based multimode lasers becomes clear. The light is strongly scattered by the rough boundaries of the island. The resonances with random wavelengths, random field distribution, and similar low Q factors are formed by the well-known dynamical localization (see Figure S3). Such kind of random resonances are similar to previous reports in other material system and consistent with our experimental observations.23,

38-41

As

depicted in Figure 1b, the laser spectrum from the perovskite film varies with the pumping density. At the same time, we also notice that the emission spectrum is also strongly dependent on the pumping position (see Figure 1d). The emission spectra at different positions are totally different, further confirming the formation of random lasers in perovskite film.


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Figure 1. The random lasers in MAPbBr3 perovskite film. (a) The top-view SEM image of the perovskite film. Scale bar 200 μm. The inset shows the high-resolution image of one perovskite domain. Scale bar 10 μ m. (b) The emission spectra at different pumping densities. (c) The dependence of output intensity on the pumping density. Inset-I and inset-II shows the fluorescent microscope image and the far field distribution. (d) The emission spectra recorded from different perovskite islands at the same pump power. The insets are their corresponding SEM images. As mentioned above, spatial coherence is one of the important characteristics of random lasers. Here the spatial coherence of perovskite random lasers is measured with Young ’ s double slits experiment.41,42 The random laser was focused on a chrome plated glass with two slits, whose width were 30 μm, and spacing 100 μm. A CCD is used to collect the interference fringe images behind a lens. The degree of spatial coherence γ is calculated use γ =(Imax-Imin)/(Imax+Imin). Imax and Imin are the intensities of light stripe and dark stripe in the centre. The γ of perovskite film


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random laser is as low as 0.3 (see Figure S4). According to the experimental results in Figure 1d, the mode number of perovskite random laser is controllable with the selection of lasing position. So does the degree of spatial coherence γ (see Figure S4).36 Then the perovskite random lasers have been applied as illumination sources and the speckle patterns are greatly suppressed experimentally (see Figure S4). However, as depicted in inset-II of Figure 1c, the emissions of such random lasers are nearly isotropic in the far field. External optical lens is required to collect and collimate the random laser emissions. As a result, the total size of our final system for speckle-free imaging is around 40cm × 10cm × 5cm (see detail discussion below), which is too cumbersome for compact and integrated applications. Surface emitting random lasers (SERLs). In order to improve the integrability and compactness of the low spatial coherence light source, it is a key step to develop directional random laser emissions and release the dependence on external optical elements. Here we utilize the configuration of vertical cavity surface emitting laser (VCSEL) to improve the directionality of low-spatial-coherence lasers. As depicted in Figure 2a, the MAPbBr3 perovskite film is sandwiched by two DBR mirrors (see Methods). The DBR mirrors were designed using 7-pair TiO2/SiO2 bi-layers. After measured the gain spectrum and laser spectrum to confirm the generation of random laser and the operation wavelength.(Figure S5A), the thicknesses of TiO2 and SiO2 layers were calculated with either transfer matrix or a commercial software (COMSOL Multiphysics). The high reflection band of DBR mirrors can be tuned to the spectral range from 476 nm to 646 nm. The reflectance around the lasing wavelength (from 545 nm to 560 nm) is about 95% (Figure S5). Here, the thicknesses of TiO2 and SiO2 were 60 nm and 95 nm, respectively. The reflective index of TiO2 and perovskite were purchased from previous work of our group.23, 43 The reflective index of SiO2 is 1.47. The device was made of spin-coating the


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disorder perovskite film onto the DBR and covering it with another DBR mirror. The steps to realize the vertical planar microcavity has been shown in Figure S5. The orange and black lines in Figure 2b are the simulation and experimental reflection spectrum of perovskite based planar microcavity, respectively. The simulation model was consisted by a 2.3 μm perovskite layer and an air gap of 500 nm sandwiched between a pair of DBR mirrors mentioned in Figure 2a. Within the high reflection band, several resonant dips can be barely seen at 628 nm and 596 nm. Because the thickness of the perovskite film is not so uniform, the strong scattering from the rough perovskite films can make resonant peaks become shallow and hard to distinguish under decoherence (see Figure S6). There are also several features that are different from conventional planar microcavity. The reflectivity of microcavity is more than 10% lower than the value of a single DBR (see comparison in Figure S7). Such difference becomes more significant at shorter wavelength and the depth of the resonant dips can only be resolved in a logscale plot. These phenomena are caused by the strong in-plane scattering in the perovskite film and the increased absorption around the bandedge of MAPbBr3 perovskite. The laser properties of perovskite-DBR sandwich were then studied via optical excitation (see Methods). When the pumping density is low, the emission spectrum is a broad peak of spontaneous emission. Different from the smooth photoluminescence in Figure 1b, here the emission spectrum is associated with two small peaks at 543 nm and 554 nm in Figure 2c. According to the corresponding numerical simulation, these modes match the Fabry-Perot modes of planar microcavity in vertical direction well (see the orange line in Figure 2b), demonstrating the Purcell effect of planar microcavity.44-46 Because of the low signal to noise ratio at weak pump power and the presence of strong scattering in the perovskite microcavity, the modulation is not as strong as conventional planar microcavities (see Figure S6). With the increase of


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pumping power, one peak at 554 nm increases dramatically and discrete spikes start to appear. The linewidths of such modes are below 1 nm. Further increasing the pumping density, additional narrow peaks randomly emerge and the laser intensities are orders of magnitude narrower than the spontaneous emission.

Figure 2. The SERL in MAPbBr3 perovskite based planar microcavity. (a) The schematic picture of the SERL. (b) The experimentally recorded reflectance spectra (black line) and numerically fitted (orange line) reflection spectra of the perovskite planar microcavity. The inset is the SEM image of the DBR mirror with fake color. The scale bar is 1 μm. (c) The evolution of emission spectrum from the perovskite planar microcavity as a function of pumping density. (d) The dependence of integrated output intensity on the pumping power. The inset is the photograph of the highly directional angular emission in far field.


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Figure 2d shows the dependence of integrated output intensities on the pumping energy. Clear transitions from spontaneous emission to amplification and finally to gain saturation can be seen and the lasing actions within the perovskite microcavity are thus confirmed. While multiple peaks have also been achieved in Figure 2c, the corresponding far-field patterns are dramatically different from the one in Figure 1. When the pumping energy is below threshold, the emission is dominated by the spontaneous emission (see Figure S8), demonstrating the highly directional output of the random lasers in planar microcavities. Such kind of directional output can be clearly seen from the photography in the inset of Figure 2d. A laser spot with diameter < 3 mm is recorded on a screen 5 cm away from the sample, giving a divergent angle less than 3.5 degrees. The lasing photostability has been measured (see Figure S9). The device was measured at room temperature under atmospheric condition. The pump intensity was about P=1.2Pth using 400 nm femtosecond laser. The intensity of the SERL reduced to 1/e after 30 minutes, consistent with the previous reports well. Interestingly, the photostability can be extended to 3 hours by simply coating a 200 nm PMMA layer onto the perovskite film before it is clamped with another DBR mirror. The formation of highly directional output is understandable. Basically, the wavevectors of the emissions from the microcavity can be decomposed into kV and kT,47 which are the vertical components and the in-plane components, respectively. Since the vertical cavity size is usually a few wavelengths and the in-plane sizes of perovskite islands are tens of microns, the in-plane wavevector components kT shall be much smaller than their vertical counterparts kV.40 As a result, the emission wavevectors are dominated by their vertical components and highly directional laser emission can be generated along the direction normal to the cavity surfaces. Although similar vertical emitting perovskite lasers have also been observed, here the surface


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emitting laser is intrinsically different from the previously reported perovskite-based VCSELs.44,45 Due to the presence of multiple scattering, here the perovskite surface emitting lasers typically produce multi-modes instead of the single laser mode. The laser modes are strongly dependent on the pumping conditions. Besides the power-dependence of emission spectrum, Figure 3 summarizes the recorded emission spectra from different pump positions. It is easy to see that the peak positions, the mode number, and the threshold values vary significantly with the excitation positions. Therefore, we can thus confirm that surface emitting random lasers (SERLs) have been generated in the perovskite-based planar microcavities. As mentioned in the simulation results in Figure S3, the large numbers of modes with similar Q factors are generated by the surface scattering within the perovskite islands. Since the spatial distributions of such modes are totally different, they can be considered as different transverse modes instead of longitude modes in the cavity. From the first order of coherence function, it is easy to know that the spatial coherence is inversely proportional to the number of transverse modes. As a result, the spatial coherence of the SERLs shall also be low. According to the experimental results in Figure 3, the degree of spatial coherence γ is also controllable. The spatial coherence γ has also been measured. The corresponding spatial coherence γ are computed with the maximum and minimum intensities of the interference fringes. As depicted in Figure S10, their spatial coherence γ are 0.93, 0.8, 0.614, 0.538, respectively. These controllable values, associated with their well-preserved directionality in Figure 2, make the SERLs ideal for practical applications, i.e. speckle-free imaging. Additionally, the temporal coherence has been measured in Figure S11, the coherence distance of the SERLs is about 6.3992 mm, and the coherence time is 21.3307 ps.


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Figure 3. The SERLs from different positions of the perovskite-based planar microcavity. (a)-(d) show the SERLs from different pumping positions. Column-I are their corresponding emission spectra at P = 1.2Pth. Pth are the threshold values of different SERLs, which can be obtained from the L-I curve in column-II. Column-III are the corresponding far field patterns. The SERLs in (a)-(d) are named as SERL-I to SERL-IV. SERLs based speckle-free imaging. Speckle noises are considered as the most common manifestation of coherent artefacts. When the incident laser is scattered by a rough pattern, the


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scattered light from different locations on the pattern can reach the same position. Because the typical laser diodes have very high spatial coherence, the scattered light can thus interfere with each other and form the notable speckles. As mentioned above, the employments of random lasers as illuminating sources are an effective way to reduce the speckle noises. The top row of Figure 4a illustrates a typical configuration for speckle-free imaging. The isotropic random laser emissions can only be partially collected and collimated by an optical lens and an objective lens before it illuminates the pattern. Such system is cumbersome and induces a large waste of laser energy. By utilizing the SERLs, the highly directional laser beam can directly illuminate the sample. Then the conventional light source can be simplified to a thin film and is compatible with the conventional optical microscopes (see the bottom row of Figure 4a.) Figure 4b summarizes the optical images of a ground glass. Here the ground glass is used to provide the scattering, and transmitted signal is imaged onto a CCD camera. Figure 4b-i is the image when the ground glass is illuminated with a conventional laser diode. Because its degree of spatial coherence γ is close to 1, strong speckles with notable intensity variation can be clearly observed. Then we calculated the speckle contrast C=σ/, where σ is the standard deviation of the intensities and is the average intensity from the image.37,41 The speckle contrast of the laser diode is C = 0.4943. When the illumination source is replaced with SERL-I in Figure 4a, the speckles are still observable. But the speckle contrast is reduced to C = 0.1441 (see Figure 4b-ii). If the same ground glass is illuminated with SERL-II, the speckle noises are further reduced and the speckle contrast is only C = 0.1284 (see Figure 4b-iii). These improvements are mainly attributed to the increase of mode numbers (can be considered as transverse random modes) and the corresponding reduction of spatial coherence. Consequently, the image quality can be further improved by using SERL-III and SERL-IV with more lasing modes. As shown in


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Figure 4b-iv and Figure 4b-v, no obvious speckles can be seen and the contrast is reduced to almost C = 0.0649. This value is already close to the one (C = 0.0241) under white light illumination (Figure 4b-vi), but the intensity SERL is much larger than the white light source. Figure 4c summarizes the experimentally recorded speckle contract C as a function of the degree of spatial coherence γ. The speckle contrast decreases monotonously with the decrease of spatial coherence, consistent with the theoretical model well.36,37

Figure 4. The speckle-free imaging under the illumination of SERLs with different spatial coherence. (a) The schematic pictures of the optical setup for imaging with SERLs. (b) The recorded images of a ground glass, which is illuminated with conventional laser diode (i), SERL-


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I (ii), SERL-II (iii), SERL-III (iv), SERL-IV (v), and white light (vi). (c) The calculated speckle contrast as a function of degree of spatial coherence. More than the complex ground glass, the advantages of SERLs as illumination sources are also apparent in illuminating relatively simple pattern, e.g. a figure of “ HIT ” . Figure 5a shows the experimental results with the illumination of conventional laser diodes. While the speckle noises are not as dramatic as Figure 4, the intensity variation of the optical image is still significant. For a high incident power, all parts of “ HIT ” are visible but some areas are saturated. When the incident power is low, parts of the “ HIT ” are too dark to be detected with a CCD camera. In contrary, the intensity of “HIT” is always uniform under the illumination of SERLs. As shown in Figure 5b, very uniform “HIT” with C ~ 0 can always be seen no matter the intensity is high (right column) or close to the noise level (left column). Till now, based on the results in Figure 4 and Figure 5, we can confirm that the SERLs can serve as more compact light sources for speckle-free imaging of micro- & nanoscale objects or patterns.

Figure 5. The speckle-free imaging of “HIT”. (a) Illuminated with conventional laser diode. (b) Illuminated with SERLs. The smallest feature size ~3 μm.


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Till now, we have experimentally realized the SERLs with controllable spatial coherence. As a large number of perovskite lasers have been demonstrated, it is essential to compare them in terms of the light confinement mechanism, the directionality, the numbers of transverse modes, and the potential in compact speckle-free imaging etc. All the results are summarized in table-1. The perovskite-based surface emitting lasers are based on conventional Fabry-Perot cavity. While highly directional laser emission has been experimentally achieved, they are mostly single-mode operation microlasers, which will produce significant speckles in imaging applications. In case of perovskite based random laser, the light is confined by multiple scattering in disorder gain medium and the directional output is not considered. As a result, additional lens groups are required for practical applications and the entire system is cumbersome. From the point view of practical applications, the SERLs with controllable spatial coherence lengths in this research provide an effective solution to realize a concise, easypreparation and controllable imaging system. Table 1. Comparison on the light confinement mechanism, the directionality, the numbers of transverse modes, and the potential in compact speckle-free imaging of this work with previous reports. Cavity

Laser modes

Directionality

Compactness for speckle free imaging

Ref.

Fabry-Perot

Single set of longitude mode

2.72°

Not applicable

48

Fabry-Perot

Single set of longitude mode

3.6°

Not applicable

49

Fabry-Perot

Single-mode

6°

Not applicable

31


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Fabry-Perot

Single-mode

Surface-Emitting Perovskite Random Lasers for Speckle-Free Imaging

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