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Robust Subwavelength Single-Mode Perovskite Nanocuboid Laser Zhengzheng Liu,†,§,∥,⊥ Jie Yang,‡,⊥ Juan Du,*,† Zhiping Hu,‡ Tongchao Shi,†,∥ Zeyu Zhang,† Yanqi Liu,† Xiaosheng Tang,*,‡ Yuxin Leng,*,†,§ and Ruxin Li†,§ †
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China ‡ Key Laboratory of Optoelectronic Technology and Systems (Ministry of Education) College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China § School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, China ∥ University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: On-chip photonic information processing systems require great research efforts toward miniaturization of the optical components. However, when approaching the classical diffraction limit, conventional dielectric lasers with all dimensions in nanoscale are difficult to realize due to the ultimate miniaturization limit of the cavity length and the extremely high requirement of optical gain to overcome the cavity loss. Herein, we have succeeded in reducing the laser size to subwavelength scale in three dimensions using an individual CsPbBr3 perovskite nanocuboid. Even though the side length of the nanocuboid laser is only ∼400 nm, single-mode Fabry−Pérot lasing at room temperature with laser thresholds of 40.2 and 374 μJ/cm2 for one- and two-photon excitation has been achieved, respectively, with the corresponding quality factors of 2075 and 1859. In addition, temperature-insensitive properties from 180 to 380 K have been demonstrated. The physical volume of a CsPbBr3 nanocuboid laser is only ∼0.49λ3 (where λ is the lasing wavelength in air). Its three-dimensional subwavelength size, excellent stable lasing performance at room temperature, frequency up-conversion ability, and temperature-insensitive properties may lead to a miniaturized platform for nanolasers and integrated on-chip photonic devices in nanoscale. KEYWORDS: cesium lead halide perovskites, subwavelength, single-mode laser, nanocuboid, Fabry−Pérot cavity
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to overcome the cavity losses for the surviving of laser oscillation modes. Perovskites have emerged as a class of cutting-edge photovoltaic and light-emitting materials due to their low cost, long-range charge transport, large absorption coefficient, and high photoluminescence quantum yield (PLQY).9−13 In particular, with controlled morphology, the halide perovskites with a general crystal structure of ABX3 could be synthesized as different nanostructures, such as quantum dots (QDs),14,15 submicron spheres,16 nanowires,17−19 nanoplates,20 and micro/ nanorods.21−23 Recently, the versatility of these perovskite nanostructures has been highlighted by demonstrating lowthreshold amplified spontaneous emission (ASE) as well as
iniaturized coherent light sources possessing excellent lasing performance are highly desired for many potential commercial applications such as onchip optical communication,1 sensing,2 computing,3 imaging,4 etc. In recent years, nanoscale metallic,5 metallo-dielectric,6 and plasmonic7,8 resonators have succeeded in confining light to volumes with dimensions considerably smaller than the wavelength. However, conventional all-dielectric structured lasers smaller than the wavelength in all dimensions have rarely been reported. Because differences in the refractive index of dielectrics are used to confine light in the cavity, the ultimate miniaturization of the possible cavity length for the conventional lasers is limited by L = mλ0/2n (where L is cavity length, λ0 is resonant wavelength, and m is an integer). Thus, the shortest possible cavity length is related to half a wavelength (m = 1). Another challenge for the size shrinking of the conventional dielectric small lasers is the optical gain. In general, for the small lasers, the higher optical gain is required © XXXX American Chemical Society
Received: March 22, 2018 Accepted: May 10, 2018 Published: May 10, 2018 A
DOI: 10.1021/acsnano.8b02143 ACS Nano XXXX, XXX, XXX−XXX
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quality F−P cavities for nanolaser. And then, we demonstrate the single-mode picosecond lasing at room temperature from individual CsPbBr3 nanocuboid with all three dimensions smaller than the lasing wavelength with low threshold, highquality factor (Q), and excellent photostability. Although the physical volume of the CsPbBr3 nanocuboid laser is only ∼0.49λ3, the up-conversion single-mode lasing threshold is as low as ∼374 μJ/cm2 and the Q is estimated to be as high as 1859. Meanwhile, the one-photon excited single mode lasing is also observed with a low lasing threshold of ∼40.2 μJ/cm2 with a Q of 2075. In addition, temperature-insensitive properties of nanocuboids from 180 to 380 K have been revealed by the temperature dependent emission study. Most importantly, the pulse duration of the nanolaser without employing any pulsecompression components is measured to be ∼22 ps, which can be attributed to the short lifetime of electron−hole plasma (EHP) of the CsPbBr3 nanocuboids. Our findings suggest that the subwavelength all-inorganic perovskite CsPbBr3 nanocuboid lasers are expected to provide miniaturized stable platforms for future nanophotonic exploration and development.
high-quality Fabry−Pérot (F−P) or whispering gallery mode (WGM) lasing.10,15−17,22−34 For example, followed by the first ASE and lasing demonstrated in MAPbX3 perovskite thin film in 2014 by Xing et al.,24 Zhang et al. obtained the perovskite lasing with high performance in hybrid organic−inorganic MAPbI3−x(Cl/Br)x nanoplates in 2014 and all-inorganic CsPbX3 nanoplates in 2016, respectively.25,26 In 2015, Yakunin et al. synthesized the CsPbX3 nanocrystals (NCs) via a simple one-step reaction between PbX2 and Cs-oleate in nonpolar solvent media and achieved ASE and room temperature WGM lasing by coating NCs onto single microspheres.27 In the same year, Wang et al. presented the CsPbBr3 QDs with lowthreshold ASE and realized the WGM lasing in a capillary tube.10 Zhu et al. reported the MAPbX3 nanowires lasing via low temperature solution processing with ultralow threshold in 2015.17 Eaton et al. and Fu et al. reported all-inorganic CsPbBr3 nanowires lasing with robust stability in 2016, respectively.28,29 Even though the physical size of current reported perovskite nanolasers has not been scaled down to subwavelength scale in all three dimensions, in particular, the cavity length along which the lasing modes oscillate is on the order of micron scale, the exceptional optoelectronic properties of perovskite together with their proper refractive index value provide an excellent opportunity for reducing the size of the laser to subwavelength scale. In addition to the size limitation, another scientific challenge for nanolasers based on lead halide perovskites is the intrinsic instability due to high sensitivity to the moisture.35,36 In contrast, the all-inorganic lead halide perovskites (CsPbX3, X = Cl, Br, and I) fabricated by replacing the organic cation, such as Cs+, exhibit better chemical and physical stability.15,16,22,23,26−32 Even though CsPbX3 so far has not yet demonstrated comparable properties in solar cells as the hybrid perovskites have done, their excellent photoluminescent characteristics have been well established.15,16,18,19,22−32 Even more recently, an improved low-temperature, solution-processed synthesis method capable of removing the organic ion completely has been developed for the fabrication of all-inorganic perovskites to improve their stability.28,29,31 For instance, it has been reported that the perovskite CsPbBr3 nanowires fabricated by this method can be maintained for over 109 excitation cycles exposed in N2 environment under a continuous intense pump laser.28,29 In our previous work, CsPbBrx/Cl3−x (0 ≤ x ≤ 3) perovskite nanocubes have been generated by this method, which demonstrated good photostability even after stored for several months under ambient conditions.31 Besides moisture and oxygen, temperature is another key environmental issue involved to the practical application of perovskites. It is wellknown that the optical gap of semiconductors changes with temperature.37 A linear blue shift (0.32 meV/K) from 80 to 220 K, a blue shift (0.27 meV/K) from 80 to 300 K, and a small shift (0.035 meV/K) from 5.8 to 295 K of the PL peak energy have been observed by PL measurements in CsPbBr3 QDs, nanocrystal films, and nanowires, respectively.18,38,39 Above room temperature, the bandgaps of hybrid lead halide perovskites at elevated temperature show a non-negligible blue shift.40 In contrast, the PL peaks of all-inorganic CsPbBr3 QDs have shown a temperature-insensitive behavior at relatively high temperature above 300 K.38,41 In the present work, we first fabricate CsPbBr3 nanocuboids with improved morphology by optimizing the low-temperature solution-processed synthesis parameters. These nanocuboids possess well-defined rectangular facets, which are natural high-
RESULTS AND DISCUSSION The perovskite CsPbBr3 nanocuboids are synthesized by a lowtemperature solution process.28,29 During fabrication, the PEDOT:PSS solution is spin coated onto the glass substrate before the PbBr2 solution to make the PbBr2 solution more uniform, and the concentration of the CsPb solution is reduced to enhance the dispersion of synthesized CsPbBr3 nanocuboids. The morphology of the as-prepared CsPbBr3 nanocuboids is characterized by the scanning electron microscopy (SEM) in Figure 1a. Nearly all the products demonstrate a perfect cubic shape with well-defined rectangular facets and averaged side lengths around 400 nm. The flat and smooth end facets and subwavelength dimensions are ideal F−P cavities for nanolasers
Figure 1. Characterizations of perovskite CsPbBr3 nanocuboids. (a) SEM image of the CsPbBr3 nanocuboids. The scale bar is 500 nm. (b) Schematics of the crystalline structure (below) and stand-wave in F−P cavity (upper). (c) XRD pattern of the CsPbBr 3 nanocuboids. (d) Optical absorption (red line) and PL emission (blue line) spectra of the CsPbBr3 nanocuboids. B
DOI: 10.1021/acsnano.8b02143 ACS Nano XXXX, XXX, XXX−XXX
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comparable to the one obtained from CsPbBr3 nanocrystal film and is among the largest values of optical gain achieved under two-photon pumping.27,43 Furthermore, the two-photon absorption (TPA) coefficient is measured by the open aperture (OA) Z-scan technique, which is the most widely used method to display the nonlinear absorption.44 The absorption coefficient α can be express as α = α0 + αNLI, where α0 is the linear absorption coefficient and αNL is nonlinear absorption coefficient. The normalized transmittance for OA Z-scan measurement is given by43,45
with nanoscale for all dimensions, as shown in Figure 1b. In addition, the background is very clean, and the individual nanocuboid separates from others in space clearly, which can avoid the crosstalk between neighbor particles during the pump laser excitation. Therefore, even though the diffraction limit cannot be achieved by a pump laser focused by an objective (100X), the excitation of multiple particles can be easily avoided due to the distance between the neighbor particles. The X-ray diffraction (XRD) patterns of the perovskite nanocuboids is well consistent with that of CsPbBr 3 orthorhombic phase in Figure 1c. As shown in Figure 1d, the linear absorption spectrum (red line) reveals a narrow exciton absorption peak around 520 nm. The normalized PL peak (blue line) is centered at 530 nm with fwhm of 18 nm under twophoton excitation. In addition to the good quality of the nanocavity, the net modal gain is another key figure-of-merit for achieving efficient lasing. The optical gain can be evaluated using the variable stripe length (VSL) method42 by focusing the 800 nm excitation beam via a cylindrical lens into a striped line and adjusting the focus area on the sample via a slit, as shown in the inset of Figure 2a. The narrow ASE peak is observed distinctly
Topen = 1 −
1
αNLI0Leff
2 2 1 + (Z /Z R )2
(1)
where I0 is the on-axis peak intensity at the focus, Leff = (1 − exp(−α0Ls))/α0 is the sample effective length, Ls is the sample thickness, Z is the longitudinal displacement of the sample from the focus, and ZR is the Rayleigh diffraction length. For a sample with thickness much smaller than ZR, the Leff can be considerable as Ls. Fitting to the experimental data by eq 1 in Figure 2c, the TPA coefficient is estimated to be ∼5.53 cm/GW at a peak intensity of 759 GW/cm2. Another generally used method to determine the TPA coefficient is the static intensitydependent transmission measurement46,47 in which the sample is kept at the laser beam waist (Z = 0) and the transmission is obtained by monitoring the incident and transmitted intensity simultaneously. Figure 2d shows that the inverse transmission increases gradually as the incident intensity increases, which presents a typical feature for TPA. By fitting the model T(I) = exp(−α0Ls)/(1 + βILeff), where I is incident intensity and β is the TPA coefficient, the value of β is estimated to be 4.57 cm/ GW. In spite of the TPA coefficients measured by the above two methods show a slight difference due to the factors of reflection and scattering, the large TPA coefficients determined by them indicate the large optical gain for the high-performance laser devices.33 The above-mentioned well-defined end facets, high optical gain, and large absorption coefficient make our as-prepared nanocuboid promising for efficient lasing. In detail, the smooth end facet pair of the nanocuboid defines the F−P laser cavity (Figure 1b) and the efficient end facet reflectivity could be realized by the refractive index contrast at the end facets between the nanocuboid and its outside environment (atmosphere, substrate, etc.). Thus, the nanocuboid itself could act as both F−P laser cavity and gain medium, just like in the case of perovskite nanowire and nanorod.22,23,28,29 However, the subwavelength laser cavity length (equal to the gain medium length here) is much shorter than the micro ones in the case of nanowire and nanorod. Usually, only when the length of the gain medium is long enough to provide sufficient optical gain over the resonator losses can the lasing be achieved.48 Therefore, it will be a great challenge to realize efficient lasing action from a subwavelength-sized nanocuboid. Here, the frequency up-converted laser from one single CsPbBr3 nanocuboid has been successfully demonstrated. When the pump intensity of the femtosecond (fs) laser at 800 nm is low, the spontaneous mission dominates and the spectrum from an individual nanocuboid is centered at around 531 nm with a fwhm of 18 nm, as shown in Figure 3a. With the excitation fluence increased above 374 μJ/cm2 (Pth), lasing occurs with a sharp single-peak emission at around 539 nm, which first satisfies the resonance conditions of nanocavity. The output-integrated intensities with respect to pump intensity
Figure 2. Optical gain and nonlinear absorption coefficient of the CsPbBr3 nanocuboids. (a) Emission spectra with stripe lengths from 0.15 to 0.36 mm. The inset shows the scheme of VSL measurement. (b) ASE intensity under two-photon excitation as a function of stripe length. (c) Measured open aperture Z-scan result for CsPbBr3 nanocuboids. The red line shows the fit using eq 1. (d) Inverse transmission as a function of incident peak intensity for CsPbBr3 nanocuboid measured by static intensity-dependent transmission method. The red line is fitted by a TPA model.
from the sample when the stripe length is longer than 0.15 mm, indicating that the optical amplification dominates and offsets the propagation losses well. The two-photon pumped ASE spectra are measured with different stripe lengths and shown in a range from 0.15 to 0.36 mm in Figure 2a under the excitation fluence of ∼370 μJ/cm2. Figure 2b shows the PL spectra intensity corresponding the stripe length and fitted by using the model of net modal gain as I = A(egLg − 1)g,27,43 where I is the ASE intensity, g is the modal gain, and Lg is the length of the stripe. From the PL intensity data, the two-photon pumped modal gain coefficient is estimated to be ∼502 cm−1, which is C
DOI: 10.1021/acsnano.8b02143 ACS Nano XXXX, XXX, XXX−XXX
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length of ∼400 nm for the nanocuboid laser, the mode spacing is larger than the gain bandwidth; therefore, only one mode lases.56,57 Furthermore, even though the nanocuboids shown in Figure 1a exhibit similar side lengths around 400 nm, their sizes are not exactly the same. Considering that there is only a several nanometer difference between the side lengths of neighbor nanocuboids, the lasing peak wavelength output from them will also have a spectral difference of several nanometers. For example, we obtain the single-mode lasing peaked at 540.19 nm from another single nanocuboid as depicted in Figure S2. Therefore, if more than one nanocuboid are excited, the multimode lasing due to the delicate difference of nanocuboid size would be detected. In other words, the single mode lasing presented here can only be achieved from a single nanocuboid. The temporal shape and pulse period of output pulse from the nanocuboid laser are measured by oscilloscope, as shown in Figure S3. However, limited by the time resolution (rise time
