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Single-Mode Lasing from Imprinted Halide-Perovskite Microdisks Alexey Zhizhchenko, Sergey Syubaev, Alexander Berestennikov, Alexey V. Yulin, Alexey Porfirev, Anatoly Pushkarev, Ivan Shishkin, Kirill Golokhvast, Andrey A. Bogdanov, Anvar A. Zakhidov, Aleksandr A. Kuchmizhak, Yuri S. Kivshar, and Sergey V. Makarov ACS Nano, Just Accepted Manuscript • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019
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Single-Mode Lasing from Imprinted Halide-Perovskite Microdisks Alexey Zhizhchenko,† Sergey Syubaev,† Alexander Berestennikov,¶ Alexey V. Yulin,¶ Alexey Porfirev,§ Anatoly Pushkarev,¶ Ivan Shishkin,¶ Kirill Golokhvast,† Andrey A. Bogdanov,¶ Anvar A. Zakhidov,¶ Aleksandr A. Kuchmizhak,∗,† Yuri S. Kivshar,¶ and Sergey V. Makarov∗,¶ †Far Eastern Federal University, Vladivostok 690091, Russia ‡Institute of Automation and Control Processes (IACP), Far Eastern Branch of the Russian Academy of Science, Vladivostok 690091, Russia ¶ITMO University, St. Petersburg 197101, Russia §Samara National Research University, Samara 443086, Russia kImage Processing Systems Institute of the RAS-Branch of FSRC “Crystallography & Photonics” of the Russian Academy of Science, Samara 443001, Russia ⊥University of Texas at Dallas, Richardson TX 75080, USA #Nonlinear Physics Centre, Australian National University, Canberra ACT 2601, Australia E-mail:
[email protected];
[email protected] 1
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Abstract Halide-perovskites microlasers have demonstrated fascinating performance owing to their low-threshold lasing at room temperatures and low-cost fabrication. However, being synthesized chemically, a controllable fabrication of such microlasers remains challenging, and it requires template-assisted growth or complicated nanolithography. Here, we suggest and implement an approach for the fabrication of microlasers by direct laser ablation of a thin film on glass with donut-shape femtosecond laser beams. The fabricated microlasers represent MAPbBrx Iy microdisks with 760 nm thickness and diameters ranging from 2 to 9 µm being controlled by a topological charge of the vortex beam. As a result, this method allows to fabricate single-mode perovskite microlasers operating at room temperatures in a broad spectral range (550–800 nm) with the Q-factors up to 5500. High speed of the fabrication and reproductivity of microdisks parameters, as well as a precise control of their location on a surface, make it possible to fabricate cm-size arrays of such microlasers. Our finding is important for direct writing of fully integrated coherent light sources for advanced photonic and optoelectronic circuitry.
Keywords halide perovskites, microlasers, laser fabrication, whispering gallery mode, microdisks. Microlasers are considered as active optical sources for photonic integrated circuits with ultrafast information processing. 1 Direct laser writing of wavelength-size microlasers can be employed for creating a platform for ultra-compact optical chips. However, the fabrication of semiconductor microlasers based on conventional materials is sensitive to overheating, melting, and laser ablation owing to rapid decay of the quantum efficiency with influence of heat-induced defects. Moreover, the fabrication of semiconductor microlasers requires epitaxial growth and matching the material lattice with that of a substrate. Therefore, low-index dielectrics (glass 2 or polymer 3 ) have been suggested for the fabrication of microdisk lasers,
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where relatively large mushroom-like designs are required owing to the low refractive index of the resonator material relative to a substrate. Thus, it is necessary to utilize materials with relatively high values of the refractive index for creating good optical resonators of small sizes, and with high defect tolerance to preserve its high gain properties after processing. Metal-halide perovskites (MAPbX3 , where X=[I, Br, Cl] and MA=CH3 NH3 ) represent a promising class of materials for advanced microlaser technology. They possess the refractive indices higher than two and they are also defect-tolerant materials, where defect sites are placed very close to the conduction or valence bands remaining the band gap. clean 4 It was shown that halide perovskites can be employed for compact nanophotonics designs 5–7 and near-wavelength laser applications. 8,9 Halide perovskites support excitons at room temperatures 10 yielding the luminescence quantum efficiency in the range of tens of percents. Perovskite microlasers of different shapes (nanowires, 11–13 microplates, 14–16 and various threedimensional shapes) generate laser emission in the range of 420−824 nm. 11,17–19 Individual perovskite microlasers can be fabricated by low-cost approaches such as CVD or chemical synthesis. However, with these methods the individual microlasers can not be placed in desired places on a substrate unless some additional multistep lithography is applied for the substrate. 20–22 or perovskite. 23–25 On the other hand, successful ultrafast material removal from a perovskite film during femtosecond laser ablation would combine simplicity of the technological process and an ability to create various patterns with high precision. In this paper, we report on the demonstration of direct femtosecond (fs) laser printing perovskite whispering gallery mode (WGM) microlasers operating in the single-mode regime. To achieve high quality and good performance of the microdisks, we utilize advantages of material removal by fs laser pulses 26 with optimized beam structuring. 27,28 As a result, the developed simple and fast method allows for the fabrication of microdisks with single-mode lasing at room temperature in a broad spectral range (550–800 nm) and with excellent positioning precision. Centimeter-size fabrication of high-Q perovskite microlasers makes the developed approach suitable for on-chip integration and sensing applications.
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Fabrica on
Results
10 μm
MAPbI₃
Glass
Lasing
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WGM sion s i Em
1 cm Excita on Figure 1: Fabrication of large-scale arrays of perovskite microlasers and their photoexcitation. Right panel: a close-up false-color SEM image (top) as well as a photograph of 1×1 cm2 array of perovskite microlasers.
Fabrication of perovskite microlasers. To fabricate the microdisk perovskite lasers we use direct fs-laser multi-pulse imprinting on a 760-nm thick perovskite film covering a silica glass substrate with a specially designed laser beam having a donut-shaped lateral intensity profile, optical vortex (OV) beams. The proposed technology is convenient for rapid and high throughput fabrication of microlasers arrays as well as more complicated designs. Moreover, 4
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the technology is promising for up-scaling and application in optoelectronic devices, allowing to produce 1×1 cm2 lattice of closely packed perovskite lasers (Figs. 1) for 15 minutes only at 100 mm/s scanning speed and 200 kHz pulse repetition rate. The closest distance between the microdisks is around 0.77 µm, which is prospective for creation of coupled microlasers (see some realizations in Fig.S1). Our results show the advantage of direct laser printing as compared to time-consuming and costly focused-ion-beam milling, 29 whereas the fabrication design is more flexible as compared to the nanoimprinting technique. 30 To generate the donut-shape intensity patterns of variable sizes, we design appropriate phase masks implemented as a series of common diffractive optical elements, spiral phase plates, 31 in a transparent and robust fused silica, via combination of a photolithography and plasma etching (for fabrication details, see Supporting information). The output field of the OV beam generated by the spiral phase plates can be expressed by the following well-known expression 32
U (ρ) = πx
where x =
1 2
kωρ 2f
2
k 2 ω 4 −2x e I(`−1)/2 (x) − I(`+1)/2 (x) , 2 8f
(1)
, ρ is the radial coordinate in the focal plane of a spherical lens with
the focal length f , k = 2π/λ is the wavenumber of laser radiation, Il (x) is a modified Bessel function of order l. Using this basic expression and Fourier transform, one can calculate the output far-field intensity distributions and corresponding parameters defining the beam geometry (diameter D and thickness t of the donut) for the certain numerical aperture (NA) of the focusing lens and the OV beams having variable topological charge `. The results of these calculations at NA=0.32 are summarized in Fig.2a (solid curves), indicating almost linear increase of the beam diameter D versus the topological charge of the OV as well as weak increase of the beam thickness t versus `. The detailed measurements of the focalplane intensity distributions of the OV beams with variable topological charges (up to `=10) generated by the fabricated spiral phase plates reveal their central-symmetric shape as well
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Topological charge ℓ 16 12 10
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0.27
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155
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N=10
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2 µm
Figure 2: Direct laser printing of individual halide perovskite microdisk lasers. (a) Calculated (solid curves) and measured (markers) diameter D and thickness t of the donut-shaped vortex beams versus their topological charge `. (b) Intensity profiles of the vortex beams with `=2,4,8 and 10 (from top to bottom, respectively) captured in the focal plane of the dry microscope objective with NA=0.32. (c) Color-coded top-view SEM image of the microdisks printed in the 700-nm thick MAPbI3 film by gradually varying incident fluence F (vertical axis) as well as number of applied pulses N (horizontal axis). Close-up SEM images (insets I-IV) demonstrate several representative surface modifications produced at variable N and incident fluence fixed at F =50 mJ/cm2 . Image color indicates the lasing threshold (or absence of lasing characteristics) for each perovskite microdisk laser shown in the right color bar. (d) Side-view (view angle of 30o ) SEM images of several representative microdisks with diameters ranging from 2.1 to 9 µm printed at optimized conditions with the OV beams having variable topological charge `. 6
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as good agreement of both geometrical parameters of the OV beam with those predicted by the calculations (Fig.2a,b). Noteworthy, variation of the only parameter, the topological charge, provides convenient predictive way to gradually tune the diameter of the laser-printed structures without changing anything else in the optical system, which is crucial for robust and reproducible laser printing procedure. For the fixed diameter of the OV beam, one can optimize the fabrication process in terms of minimization of the laser-induced thermal effects via multi-pulse ablation at fluence F significantly below the single-pulse ablation threshold for the 760-nm thick MAPbI3 film used (Fab ≈ 0.37 J/cm2 , 33 ). The details of the ablation threshold measurements are presented in Fig.S2. The reason behind such an optimization is that the irradiated perovskite film is rather thick and it can be completely removed from the glass substrate via a single-pulse ablation to form isolated surface structure only at rather high incident fluence F > 0.4 J/cm2 (this fluence range is not shown in Fig.2c). At such fluences, the low-intense outer and inner shoulders of the generated donut-shape beam as well as the secondary donuts appeared for the OVs with higher topological charges ` ≥4 (see Fig. 2b) provide some irreversible destructive modification of the central microdisk structure as well as the surrounding film areas. Such modification appears at much lower threshold fluence of Fm ≈ 0.01 J/cm2 (for details, see Supporting information). The SEM image of the microdisk array fabricated using the OV beam with `=4 at variable number of pulses N (ranging from 1 to 500) and variable fluence F clearly demonstrates the advantages of the suggested multipulse printing approach (Fig. 2c as well as inset images therein). The sequence of laser pulses (N≈10), having the total incident fluence F in the range from 50 to 70 mJ/cm2 , which is well below the single-pulse ablation threshold and slightly higher than the modification threshold (see Supporting information), provides the delicate gradual material removal from the donut-shaped area without any visible damage of the remaining isolated microdisk as well as surrounding film (see inset I in Fig. 2c). As the perovskite represents hybrid semiconductor containing both organic and inorganic
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components, its ablation process is expected to involve multiple laser-induced phenomena: from boiling to photo-induced chemical reactions. In this respect, the observed “clean” removal of the material from the laser-irradiated areas can proceed either via evaporation of the organic products followed by melting and resolidification of inorganic counterparts or via spallation of the perovskite grains via acoustic relaxation of thermal-generated stress. Under the optimal laser irradiation conditions, the sidewalls of the perovskite microdisk appear to be free of melting as the laser-induced removal proceeds within a narrow ringshaped volume predominantly along the grain boundaries of the nanocrystalline perovskite film. Using laser confocal microscopy to map the PL signal from the produced perovskite microdisks, we confirm the minimized thermal impact on the microdisk material showing the averaged PL yield comparable to those for non-irradiated areas of the film (see Fig.S5). In a sharp contrast, neither the single-pulse ablation at higher fluences nor the combination of multi-pulse irradiation (N >50) and weak incident fluence do not provide evident improvement of the microdisk surface morphology (see insets II-IV in Fig. 2c). Noteworthy, similar efficient combination of the incident fluence and the number of applied pulses can be also found for the OVs with variable topological charge `, allowing to tune the diameter of the produced microdisks. More details related to the optimization of the laser printing conditions can be found in the Supporting information. For the fixed focusing conditions (NA=0.32) and our range of generated OV beams with ` varying from 2 to 10, we fabricate the high-quality perovskite microdisks with their diameters ranging from 2 to 9 µm. Noteworthy, diffraction limited downscaling of the microdisk diameter down to 0.5 µm as well as the spacing between the microdisks down to 0.2 µm are possible using tightly focused vortex beams. In Fig. 1 we show an example of an array of arranged microdisks fabricated by the donut-beam with a topological charge `=4. As shown in Fig.S3-S4 and in previous works, 34 the employing of OV beam allows for the best microdisk quality among direct laser-assisted perovskite microstructuring approaches. Perovskite lasers characterization
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We test basic lasing characteristics of the fabricated perovskite microdisks via irradiating them by focused 0.56-ns laser pulses at 532 nm wavelength and 2 orders of magnitude lower fluence than the measured threshold fluence of laser-induced modification of the perovskite film surface (for experimental details and schemes, see Methods and Supporting information).
1.1 Fth D = 2.10 µm
1.5 Fth δλ = 38 nm
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5
3 MAPbBr₃
MAPbBrI₂
MAPbI₃
2 1 0 510
570
630 690 750 Wavelength (nm)
810
Figure 3: Lasing properties of a single microdisk. (a) Lasing of D=3.8 µm disk of MAPbI3 at different intensities below and after the threshold. (b) Intensity and full-width-halfmaximum (FWHM) of emission spectrum of the microdisk at the lasing wavelength. (c) Emission spectra of perovskite microdisks with different diameters from 2 to 9 µm. (d) Statistical data on modes number in each perovskite microlaser and the wavelength of the most intensive mode (inset). (e) Lasing spectra from perovskite microlasers of different compositions: MAPbI3 (brown curve), MAPbBrI2 (red curve), and MAPbBr3 (green curve). Insets: optical images of the emission from microdisks of various compositions. Figure 3a shows the spectral evolution of photoluminescence with the increase of the laser pump fluence around threshold 0.3 mJ/cm2 for a microdisk with diameter 3.8 µm. The observed behaviour demonstrates all basic lasing features: clear threshold for emission at a lasing mode at around ≈ 0.2–0.3 mJ/cm2 and its sharp spectral narrowing by two orders of magnitude (from 38 nm down to less than 0.2 nm) at the red-wing (760–800 nm) of PL spectra owing to lower losses in this spectral range as compared with the blue-wing (710– 760 nm). Noteworthy, the lasing characteristics of the perovskite microdisks correlate with
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their geometrical features, which are defined by the laser fabrication conditions discussed in the previous section. Specifically, the lasing threshold fluence depends strongly on the quality of the microdisks increasing from 0.2 mJ/cm2 for structures printed at optimized conditions (highlighted by a green lines in Fig. 2c) up to 3 mJ/cm2 – for microdisks produced at higher fluences and the increased number of applied pulses. The lowest observed lasing threshold is around 0.15 mJ/cm2 , as shown in Fig. S10. Corresponding Q-factors (Q = λ/∆λ) for microdisks printed at optimized conditions are higher than 5000. As shown in Fig. 3b, the experimentally measured Pin -Pout plot can be fitted by a theoretical model based on coupled equations for electron-hole photoexcitation, their radiative recombination via spontaneous and stimulated emission, as well as non-radiative recombination on defects and via Auger process (for details, see Section 11 in Supporting information). Some discrepancy between the experimental and theoretical curves is caused by non-linear slopes below and after the lasing threshold and originates from state-filling near the conduction band edge. 35 According to our modeling, the lasing condition is fulfilled at density of electron-hole pairs around 1.5×1018 cm−3 , corresponding to the gain around 100 cm−1 . Further increase of the calculated gain is observed to be up to 200 cm−1 , which is in a good agreement with previous works on MAPbI3 thin films. 35,36 According to our measurements (for details, see Methods and Supporting information), the lasing regime in the microdisks can also be achieved at one order of magnitude lower fluences (≈7 µJ/cm2 ) under fs-laser photoexcitation, which is comparable with the thresholds for much larger resonators made of MAPbI3 films. 30,37 We observed the lowest thresholds for D
