Mode selection and high-quality upconversion lasing from perovskite CsPb2Br5 microplates Download: 630次
1. INTRODUCTION
The miniaturization and integration of photonic components in nano/micrometer-scale proximity on chip have boosted integrated information processing systems [1]. As an important component, great advances have been demonstrated in miniaturized optical sources for optoelectronic integration, sensing, and optical communications [13" target="_self" style="display: inline;">–
Compared with single-photon excitation, two-photon pumped lasers without phase-matching requirements have many attractive merits, such as large penetration depth, high spatial resolution, and reduced photodamage, rendering in optical storage, infrared detection, and medical imaging [1719" target="_self" style="display: inline;">–
More recently, a novel member of the perovskite family, all-inorganic , has a unique two-dimensional (2D) electrical structure composed of a cation between layers, which has been recognized as promising for applications in solar cells, photodetectors, and light-emitting devices [32–
Herein, we explicate the mechanism of optical gain from perovskite microplates, which establishes rapidly in less than 1 ps and lasts more than 30 ps. Subsequently, temperature-dependent amplified spontaneous emission (ASE) is observed with low threshold and high characteristic temperature of 403 K. Remarkably, the frequency-upconverted lasing with low thresholds and high quality factors operating in F-P mode or WGM is demonstrated from a pristine microplate at room temperature. Most importantly, the obtained quality factor () is the highest value obtained in upconversion perovskite micro/nanoplate lasings to our knowledge. All these findings suggest that perovskite microplates will provide versatile opportunities and stable platforms for the exploration of nonlinear nanophotonic integrated devices.
2. EXPERIMENT
2.1 A. Preparation and Synthesis of Microplates
Lead (II) bromide (, 99%, Xi’an Polymer Light Technology Corp.), cesium carbonate (, 99.9%, Adamas), oleic acid (OA, 90%, Sigma Aldrich), oleylamine (OLAM, 70%, Sigma Aldrich), 1-octadecene (ODE, 90%, Sigma Aldrich) were used. All chemicals were used without further purification in preparation of microplates. Cesium stock solution was obtained by mixing 100 mg , 0.4 mL OA, and 3.75 mL ODE in a 100 mL three-necked flask and degassing under nitrogen () flow at 120°C for 1 h until mixed in a 100 mL three-necked flask placed in a heating jacket with , and then heated to 120°C under . Temperature was set up to 135°C. 0.3 mL OLAM and 0.4 mL OA were injected into the crude solution quickly and the temperature of 135°C was maintained for 1 h until the was solubilized completely. 0.4 mL Cs-OA solution was swiftly injected and stirred for 2 h, and then was cooled by the ice bath. Finally, microplates were obtained after several purification processes and dispersed in toluene for testing.
2.2 B. Characterization of Microplates
The scanning emission microscopy (SEM) image was recorded using a ZEISS LIBRA 200FE microscope. The crystal phase of the sample was characterized by X-ray diffraction (XRD) with CuKa radiation (XRD-6100, SHIMADZU, Japan). The linear absorption spectrum was obtained by a scan UV–Vis spectrophotometer (UV–Vis: UV-2100, Shimadzu, Japan).
2.3 C. Ultrafast Transient Absorption Measurements
A femtosecond amplified Ti:sapphire laser source was used as the laser source with pulse duration of , repetition rate of 1 kHz, and central wavelength of 800 nm. The laser output is divided into two parts, and the major part is used to generate pump pulses at 355 nm () in an optical parametric amplifier. The pump beam is modulated at 500 Hz by a mechanical chopper. Another part of the laser is used to generate white light through a crystal plate. The white light can be focused onto the sample by an off-axis parabolic mirror and detected by a silicon detector. The time delay between pump and probe beams is controlled by a motorized stage. All experimental data are corrected for the chirp induced by the nonlinear process in white-light generation.
2.4 D. Temperature-Dependent ASE Experiments
For the two-photon-pumped ASE experiments, a commercial Ti:sapphire laser system with repetition rate of 1 kHz and pulse width of 35 fs was employed as the excitation source. The sample was put in a cryogenic sample chamber (MicrostatN2, Oxford Instruments), and the laser beam was focused by a lens (focus length: 100 mm). The photoluminescence (PL) spectra are measured at temperatures from 100 K to 300 K in a vacuum. A variable neutral density filter was employed to adjust the pump intensity on the sample, and the varied power is measured with a power meter. The adjustable slit is used to adjust the width of the excitation beam. Then the pump-intensity-dependent PL signal is collected from the sample by a backscattering geometry and the residual excitation light is blocked by a band-pass filter.
2.5 E. Lasing Characterization
The laser beam was focused by a microscope objective (MPLFLN , ) to excite the single microplate, and the emission signal was collected from the top of a micro-PL system by the same objective and detected by a spectrograph (Acton SpectraPro SP-2358, Princeton Instruments) and a silicon charge coupled device with a resolution of 0.04 nm.
3. RESULTS AND DISCUSSION
The perovskite microplates are synthesized by the improved solution process [32]. As investigated by the SEM image shown in Fig.
Fig. 1. Optical characterizations of perovskite microplates. (a) SEM image. The scale bar is 10 μm. (b) Schematic crystal structure. (c) Experimental and standard powder XRD patterns of tetragonal . (d) Linear absorption (green) and PL spectrum (red).
Population inversion is a fundamental prerequisite for lasing. When electrons are pumped into high excited states by absorbing photons, the optical gain could be possible if the population inversion is established when the accumulation of the emissive states is faster than the escaping processes. To track the gain dynamics, the ultrafast transient absorption (TA) spectroscopy is implemented. Figures
Fig. 2. TA spectra of perovskite . (a) 2D pseudo-color TA spectra. (b) TA spectra at different delay times. (c) The kinetics of prominent PB signal. (d) The kinetics of SE. Inset: a magnified view of the spectroscopic signature of SE before 11 ps.
Due to the excess energy, hot carrier cooling will play an essential role in determining the performance of optoelectronic devices. As shown in Fig.
To further demonstrate the gain properties, we explore the temperature-dependent ASE under two-photon excitation of perovskite microplates. As shown in Fig.
Fig. 3. Temperature-dependent ASE actions from microplates. (a) The PL spectra of two-photon pumped ASE at 300 K. Inset: photograph of the microplate excited above ASE threshold by a cylindrical lens. (b) The integrated intensity and FWHM with respect to pump intensity. (c) Normalized PL intensity as a function of excitation intensity at different temperatures. (d) Pump threshold intensity versus temperature. The solid red line is the fit according to Eq. (1 ).
Two-photon pumped lasers have been considered a promising strategy to achieve frequency upconversion, and they show promising nonlinear optical performances. In the following, we study the frequency upconversion application for lasers from microplates at room temperature. For a pristine perovskite nanostructure, F-P lasers and WGM lasers have been demonstrated in micro/nanowires and micro/nanoplates by virtue of their superior optical gain, respectively [9,14]. Owing to the smooth and regular morphology of microplates and the difference of refractive index between perovskite and its surroundings, the cavity modes inside could resonate following the unidirectional F-P mode between two facets and WGM among the four corners by total internal reflection.
First, the excitation laser beam is focused on the edge of a single microplate. The total spectra with increasing pump fluence are shown in Fig.
Fig. 4. F-P mode lasing characterization of a single microplate. (a) Pump-intensity-dependent emission spectra under two-photon excitation. Inset: the bright-field optical image and emission photograph from the single microplate above the lasing threshold, indicating the F-P mode. The scale bar is 5 μm. (b) Output intensity (blue) and FWHM (red) of laser with increasing pump fluence. (c) Fitting of the lasing oscillation mode. The FWHM is 0.15 nm with a quality factor of .
To better match the purpose of the miniaturized device, it is more desirable to pursue a single-mode laser. Then we turn to investigating the WGM lasing of a microplate. When the excitation beam is focused on the corner of the microplate, the significantly strong emission at the corners confirms the WGM lasing, which is clearly observed in the inset of Fig.
Fig. 5. WGM lasing characterization of a single microplate. (a) Pump-intensity-dependent emission spectra under two-photon excitation. Inset: the bright-field optical image and the emission photograph from the single microplate above the lasing threshold, indicating the WGM lasing. The scale bar is 5 μm. (b) Output intensity (red) and FWHM (blue) of laser with increasing pump fluence. (c) Fitting of the lasing oscillation mode. The FWHM is 0.16 nm with a quality factor of .
Fig. 6. Log–log plot of the integrated PL intensity as a function of the pump fluence under two-photon excitation.
Fig. 9. Two-photon pumped intensity-dependent emission spectra from a single microplate in the F-P cavity.
Fig. 11. Two-photon pumped intensity-dependent emission spectra from a single microplate in the WGM cavity.
Tailoring the lasing mode by the cavity size is a promising strategy for single-mode or multimode lasing. For a microplate, both the F-P mode oscillation between end facets and WGM oscillation among the four facets are possible, indicating that the optical mode could be modified as needed. Except for the microcavity effects due to the natural cavity structures, the high effective refractive index would result in excellent light confinement. Consequently, various resonance modes can be achieved. The distinct microcavity effects with mode selection would help us to fabricate functional miniaturized lasers by cavity structures, which will offer a versatile platform for the novel all-inorganic perovskites to be a flexible optical element into miniaturized photonic and optoelectronic devices.
4. CONCLUSION
In summary, we demonstrate the effective optical gain () with fast buildup in less than 1 ps from microplates for the first time to the best of our knowledge. Thanks to this, temperature-insensitive low-threshold ASE actions under two-photon excitation are observed from 100 K to 300 K with a high characteristic temperature of 403 K. Notably, two kinds of upconversion lasing, i.e., F-P mode with multimode lasing and WGM with single-mode lasing, are successfully realized at room temperature. The weak color shift of the lasing peak also confirms the strong temperature tolerance for heat induced by a pump laser with more stable chromaticity than the all-inorganic ones. The obtained ultrahigh quality factor () is among the best reported upconversion perovskite microplate lasers. Our findings suggest that the facile solution process ability and high performance lasing from a microplate will hold new opportunities in upconversion integrated photonic and mode selection optoelectronic devices for future exploration.
Article Outline
Zhengzheng Liu, Chunwei Wang, Zhiping Hu, Juan Du, Jie Yang, Zeyu Zhang, Tongchao Shi, Weimin Liu, Xiaosheng Tang, Yuxin Leng. Mode selection and high-quality upconversion lasing from perovskite CsPb2Br5 microplates[J]. Photonics Research, 2020, 8(9): 09000A31.