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Di Xing, Mu‐Hsin Chen, Zhiyu Wang, Chih‐Zong Deng, [Ya‐Lun Ho](https://orcid.org/0000-0001-8274-5978), Bo‐Wei Lin, Cheng‐Chieh Lin, Chun‐Wei Chen, [Jean‐Jacques Delaunay](https://orcid.org/0000-0003-2175-0620)

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[Solution‐Processed Perovskite Quantum Dot Quasi‐BIC Laser from Miniaturized Low‐Lateral‐Loss Cavity](https://mdr.nims.go.jp/datasets/f33f4cf4-255a-44d0-bf1b-d26c628cf7bb)

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Solution‐Processed Perovskite Quantum Dot Quasi‐BIC Laser from Miniaturized Low‐Lateral‐Loss CavityRESEARCH ARTICLEwww.afm-journal.deSolution-Processed Perovskite Quantum Dot Quasi-BICLaser from Miniaturized Low-Lateral-Loss CavityDi Xing, Mu-Hsin Chen, Zhiyu Wang, Chih-Zong Deng, Ya-Lun Ho, Bo-Wei Lin,Cheng-Chieh Lin, Chun-Wei Chen, and Jean-Jacques Delaunay*Laser devices produced via solution-processed perovskite quantum dots(QDs) offer broad spectral tunability as well as ease of fabrication. Utilizingquasi-bound states in the continuum (quasi-BIC) modes, solution-processedQD laser devices have been demonstrated with a nanostructure coated in athin-film gain media configuration. However, light leakage through thin-filmguiding from the cavity side edges becomes more pronounced whenshrinking the cavity size, posing challenges for the miniaturization ofquasi-BIC-based lasers. Here, the fabrication of well-defined patterns of QDsvia a solution process allows them to take advantage of the pattern edges toreduce losses through the cavity edges. A single-mode BIC laser is reported byusing CsPbBr3 QDs with a narrow linewidth of ≈0.1 nm. Importantly, aminiaturized quasi-BIC laser is realized with a device size as small as 10 × 10μm2, making it the smallest among existing solution-processed BIC lasers.This work provides a strategy for developing ultra-compact BIC lasers viasolution-processed gain media.D. Xing, M.-H. Chen, Z. Wang, B.-W. Lin, J.-J. DelaunaySchool of EngineeringThe University of Tokyo7-3-1 Hongo, Bunkyo-ku, Tokyo 113–8656, JapanE-mail: jean@mech.t.u-tokyo.ac.jpC.-Z. DengAdvanced Research Laboratory, Technology Infrastructure Center, Technol-ogy PlatformSony Group Corporation4-14-1 Asahi-cho, Atsugi-shi 243-0014, JapanY.-L. HoResearch Center for Electronic and Optical MaterialsNational Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanC.-C. Lin, C.-W. ChenDepartment of Materials Science and EngineeringNational Taiwan UniversityNo. 1, Sec. 4, Roosevelt Rd., Taipei 10617, TaiwanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adfm.202314953© 2024 The Authors. Advanced Functional Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivs License,which permits use and distribution in any medium, provided the originalwork is properly cited, the use is non-commercial and no modificationsor adaptations are made.DOI: 10.1002/adfm.2023149531. IntroductionSolution-processed laser devices utiliz-ing gain medium films obtained fromdye-doped resists,[1–3] perovskites,[4–8] col-loidal quantum wells,[9–11] and colloidalquantum dots (QDs)[12–18] have achievedsignificant progress. The ease of integra-tion with other photonic components,low cost, and the potential for large-areafabrication technologies make them highlyappealing for numerous applications, in-cluding integrated photonic circuitry,[19,20]displays,[21–23] communications,[24] sens-ing, and biomedical fields.[25,26] In therealm of solution-processed colloidal QDs,CsPbX3 QDs have attracted great interestdue to their remarkable optoelectronicproperties, such as a high photolumines-cence quantum yield (PLQY), narrow emis-sion line widths, and the wide spectrum oftunable emission wavelengths.[27–31] Unlikeother colloidal QDs, CsPbX3 QDs can be easily synthesized ona large scale at a relatively low temperature. Furthermore, theirhigh modal gain enables the realization of multiphoton pumplasers[32] and achieves ultralow thresholds for amplified sponta-neous emission (ASE) and lasing.[30,33–35] These characteristicsmake them hold great promise for advancements in the field ofsolution-processed nanolasers.In the field of nanophotonics, there has been a long-standingpursuit of achieving ultra-compact single-mode lasers. Besidesan efficient gain medium, strong light confinement in the cavityis the most important step toward achieving miniaturized lasers.Optical bound states in the continuum (BIC) have demonstratedtheir versatility as a powerful tool for achieving light confine-ment and a substantial enhancement of the quality factor (Q fac-tor) in periodic structures.[36] BIC states provide a complete lo-calized mode even though these states fall in the continuum ofstates above the light line, thus ensuring the complete suppres-sion of radiative losses.[36–38] In reality, an actual periodic struc-ture supporting the BIC mode has a limited Q factor due to fab-rication imperfections, material absorption, and the structure’sfinite size and is referred to as a quasi-BIC.[36,39,40] Particularly,shrinking the size of the periodic structure will cause a dramaticdrop in the Q factor due to the loss of light confinement fromthe quasi-BIC in the lateral direction.[39,41] To achieve miniatur-ized BIC lasers, overcoming the large lateral losses in the finite-size cavities is highly desired.[41,42] For now, some miniaturizedAdv. Funct. Mater. 2024, 2314953 2314953 (1 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbHhttp://www.afm-journal.demailto:jean@mech.t.u-tokyo.ac.jphttps://doi.org/10.1002/adfm.202314953http://creativecommons.org/licenses/by-nc-nd/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadfm.202314953&domain=pdf&date_stamp=2024-02-22www.advancedsciencenews.com www.afm-journal.deFigure 1. Schematic of a) CsPbBr3 QD slab waveguide-BIC laser and b) CsPbBr3 QD cavity-supported BIC laser. Simulated electric field norm distributionof c) QD slab waveguide-BIC laser and d) QD cavity-supported BIC laser with 19 × 19 unit cells. The top figures are first-order modes in the x-y plane.Middle figures are first-order modes in the x-z plane. The bottom figures are second-order modes in the x-z plane. e) and f) Micro-PL image of a CsPbBr3QD slab waveguide-BIC laser and a CsPbBr3 QD cavity-supported BIC laser. The CsPbBr3 QD cavity-supported BIC laser shows a strong lasing emissionunder a lower pump energy density.BIC lasers have been realized from some non-solution-processedgain media, such as by fabricating a suspended cavity with a min-imum cavity size of ≈10 μm,[40] introducing a surrounded pho-tonic crystal as a bandgap to decrease the lateral optical lossesand shrink the cavity size to ≈2.5 μm,[41,42] and designing Fano-BIC cavity based on photonic crystals with a cavity size down to≈2 μm.[43] These miniaturized BIC lasers showed impressive re-sults while the use of the surrounded boundary photonic crystalrequires a larger device size and careful design of the photoniccrystal. For conventional solution-processed BIC lasers, exceptfor some BIC lasers based on direct pattern metasurface,[5] thegain medium is usually spin-coated on top of the nanostructureto form a thin film and acts as a slab waveguide.[1,2,12,44,45] TheBIC modes arise from slab waveguide modes in the thin film andare coupled to the periodic nanocylinder array, which is referredto as slab waveguide BIC laser.[12] In this case, the leakage oflight through the sides becomes more pronounced, especially asthe entire cavity’s size decreases, thus presenting a challenge forachieving miniaturized BIC lasers based on solution-processedgain media.Here, we integrate an isolated CsPbBr3 QD cavity instead ofa QD film on top of the TiO2 nanocylinder array to achieve aQD cavity-supported BIC laser. Different from those miniatur-ized BIC lasers that utilize surrounded boundary photonic crys-tals, the direct fabrication of well-defined QD patterns of QDsvia a solution process allows us to take advantage of the pat-tern edges to reduce losses through the cavity edges withoutthe need for carefully designed photonic crystals. We report asingle-mode BIC laser by using CsPbBr3 QDs with a narrowlinewidth of around 0.1 nm. Compared with the conventionalQD slab-based BIC laser, the proposed QD slab waveguide-BIClaser consisting of the CsPbBr3 QD cavity-supported BIC lasermaintained a low lasing threshold for small cavity sizes. Impor-tantly, a miniaturized BIC laser with a device size down to 10× 10 μm2 is achieved using the proposed CsPbBr3 QD cavity-supported BIC laser design, which is the smallest among the ex-isting solution-processed BIC laser. This work provides a strategyto develop ultra-compact BIC laser via solution-processed gainmedia.2. Results and DiscussionFigures 1a and b give the schematic of the conventional CsPbBr3QD slab waveguide-BIC laser and CsPbBr3 QD cavity-supportedBIC laser. For the QD slab waveguide-BIC laser, a large areaof QD film is coated on the TiO2 nanocylinder array. Unlikethe CsPbBr3 QD slab waveguide-BIC laser, the CsPbBr3 QDcavity-supported BIC laser possesses boundaries along its foursides (see Figure S1, Supporting Information for the fabricationprocess). Figures 1c and d give the simulated near field distri-bution of first- and second-order BIC modes of a cavity with19 × 19 unit cells in the x-y plane (top) and x-z plane (bottom).The second-order BIC mode in the x-y plane is shown in FigureS2 (Supporting Information). The simulation results show thatthe QD slab waveguide-BIC laser shows higher optical losses inthe lateral direction because the QD layer can serve as a waveg-uide and guide the light away from the cavity. The optical lossesare more pronounced in the second-order BIC mode as higher-order BIC modes typically propagate within the slab waveguideand are more susceptible to size constraints and edge effectsthan first-order BIC modes. Due to the existence of side edgesin the CsPbBr3 QD cavity-supported BIC laser, the lasing modeis better confined within the cavity and shows lower scatteringlosses in the far field. It is worth noting that different from theconventional microdisk laser with in-plane WGM mode or thereported nanowire F-P laser with direction emission diffractedby top nanograting,[46,47] the optical feedback still mainly comesfrom the nanocylinder array. Figures S3a and b (Supporting In-formation) present the top-view and 30° tilted SEM images of thefabricated TiO2 nanocylinder array, respectively, where the geom-etry and the height of the nanocylinder array can be observed.Figures S3c and d (Supporting Information) show the SEMimages CsPbBr3 QD cavity fabricated on the TiO2 nanocylin-der array. From the enlarged SEM image shown in Figure S3dAdv. Funct. Mater. 2024, 2314953 2314953 (2 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202314953 by Cochrane Japan, Wiley Online Library on [20/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 2. a) Simulated transmission band diagram of first-order mode for the CsPbBr3 QD-TiO2 nanocylinder array under TM polarized incident light.The emission band of CsPbBr3 QD film is indicated by a semi-transparent white color. Lattice parameter, a= 305 nm, dTiO2 = 220 nm, hTiO2 = 120 nm, tQD= 240 nm. b) Simulated Q factor as a function of the incident angle of the first-order BIC mode, derived from Figure a. c) Simulated magnetic field normdistributions in the x-z plane of the leaky mode and the MQ BIC mode. The scale bar is in the log scale. d) Simulated magnetic field norm distributionsin the x-y plane of the leaky mode and the MQ BIC mode. e) Simulated BIC mode resonance wavelength of the CsPbBr3 QD-TiO2 nanocylinder array asa function of the QD film thickness.(Supporting Information), the CsPbBr3 QD cavity is well alignedon the TiO2 nanocylinder array. Finally, micro-PL images of theCsPbBr3 QD slab waveguide-BIC laser and CsPbBr3 QD cavity-supported BIC laser with a cavity size of 20 × 20 μm2 are shownin Figures 1e and f. Note that the pump energy density is around40 μJ cm−2. Due to the lateral loss in the QD layer, only las-ing from the CsPbBr3 QD cavity-supported BIC laser is observedhere.Figure 2a shows the simulated transmission band diagramof the CsPbBr3 QD coated TiO2 nanocylinder array under TM-polarized incident light with a cylinder diameter dTiO2 = 220 nm,cylinder height hTiO2 = 120 nm, period a = 305 nm, and QDthickness tQD = 240 nm. The emission band of CsPbBr3 QDs isindicated with a semi-transparent white color in the figure. Un-der this condition, the first-order BIC mode is excited within theemission band of CsPbBr3 QD. Note that by engineering the lat-tice parameters and QD thickness, the second-order BIC modescan also be shifted to match the emission band of the QD, asshown in the Supporting Information. The mode at normal inci-dence on the upper band is referred to as the diffraction-coupledband-edge mode, referred to as the leaky mode in the following,in which light can couple with the outgoing field and radiate. Thedark modes at the middle and bottom band are referred to as elec-tric dipole (ED) BIC mode and magnetic quadrupole (MQ) BICmode. The theoretical Q factors of the leaky mode and BIC modespresented in Figure 2b are derived from the simulated disper-sion band diagram of Figure 2a. Compared with the leaky modewhich possesses a low Q factor, the BIC modes show extremelyhigh Q factors which increase toward infinity as the angle of in-cidence approaches zero. The simulated near-field distributionsof the leaky and MQ BIC modes in the x-z plane and x-y planeare shown in Figures 2c and d, respectively. The mode distribu-tions of ED, magnetic dipole (MD), and electric quadrupole (EQ)BIC modes are also demonstrated in Figure S6 of the Support-ing Information. The leaky mode shows strong far-field radiationlosses, while the far-field radiation losses in the BIC mode arehighly suppressed, which also explains the extremely high Q fac-tor of this BIC mode. This simulation assumes no absorption inall materials of the investigated system, no scattering losses, andan infinitely extended lattice period. However, in the real case, theabsorption of material, fabrication defects, and the finite size ofthe cavity need to be considered. Consequently, the Q factor forthe quasi-BIC mode becomes finite. Figure 2e shows the simu-lated BIC mode resonance wavelengths of the first- and second-order of the CsPbBr3 QD-TiO2 nanocylinder array as a functionof the QD film thickness. For each mode, the lattice parame-ters are engineered to match the mode with the emission bandof CsPbBr3 QDs (see details in the Supporting Information).Adv. Funct. Mater. 2024, 2314953 2314953 (3 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202314953 by Cochrane Japan, Wiley Online Library on [20/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 3. Lasing behavior of the CsPbBr3 QD cavity-supported BIC laser, as defined in Figure 2. Lcavity = 40 μm. a) The emission spectra and b) theemission intensity as a function of the pump energy density. Inset of Figure a is the micro-PL image of the laser above the threshold. The scale bar is20 μm. c) Spontaneous emission spectra below lasing threshold (P < Pth) and lasing spectra above lasing threshold (P > 1.8 Pth) of the CsPbBr3 QDcavity-supported BIC laser shown over a large wavelength range. d) The high-resolution spectrum of the single-mode lasing (P >1.1 Pth) showing anFWHM of ≈0.1 nm. e) The far-field emission pattern collected at the back focal plane of a lens with a numerical aperture (NA) of 0.45. f) Polarizationdependence of the far-field radiation pattern for lasing. The direction of polarization of the output beam is marked by the white arrows.Figure 4. Optical characterizations of the CsPbBr3 QD slab waveguide-BIC laser and CsPbBr3 QD cavity-supported BIC laser. a, b, c) Lasing behavior ofthe CsPbBr3 QD slab waveguide-BIC laser. d, e, f) Lasing behavior of the CsPbBr3 QD cavity-supported BIC laser. For each figure, the left figure showsthe spontaneous and lasing emission spectra. The middle figure shows the emission intensity as a function of the pump energy density. The right figureshows the micro-PL image of the lasers above the lasing threshold. The parameters of the nanocylinder array and QD thickness have been optimizedto match the emission band to the second-order MD BIC mode: cylinder diameter dTiO2 = 240 nm, cylinder height hTiO2 = 120 nm, period a = 335 nm,and QD thickness tQD = 310 nm.Adv. Funct. Mater. 2024, 2314953 2314953 (4 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202314953 by Cochrane Japan, Wiley Online Library on [20/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deTable 1. Comparison between BIC lasers.Gain material Wavelength [nm] Qlasing Pump method Threshold peakpower [mW]Threshold average powerdensity [kW cm2]Smallest cavitysize [μm2]Smallest devicesize [μm2]Refs.Non-solution-processedInGaAsP MQWs 1551.4 4701 Pulsed pump 15.6 4 9.6 × 9.6 9.6 × 9.6 [40]InGaAsP MQWs ∼1550 – Pulsed pump 73 – – – [51]GaAs 825 2750 Pulsed pump 8.8 × 105 7.0 × 104 – – [52]InGaAsP MQWs ∼1620 7300 Pulsed pump 0.34 1.47 23 × 23 23 × 23 [53]InGaP MQWs 621 2070 Pulsed pump 1.5 × 107 7.5 × 105 50 × 50 50 × 50 [54]GaAs MQWs 940 150 Pulsed pump 1.1 × 105 – 35 × 10 35 × 10 [55]GaN 376.5 3765 Pulsed pump 5.2 × 105 2.7 × 104 8 × 8 8 × 8 [56]InAs/GaAs epitaxialQDs1260∼1275 32500 cw pump 0.041 0.08 7.5 × 7.5 21 × 21 [42]InAs/GaAs epitaxialQDs1303 2327 cw pump 0.012 0.052 2.5 × 2.5 13 × 13 [41]InGaAsP QWs 1560 78000 cw pump 3.5 12.38 ≈2.2 32 × 11 [43]Solution-processedMAPbBr3 552 – Pulsed pump 5.3 × 105 4.2 × 104 – – [5]CdSe/CdZnS CQDs 647.7 2590 Pulsed pump 5.1 × 106 1.8 × 105 – – [15]MAPbBr3 548.5 1120 Pulsed pump 6.2 × 106 4.9 × 105 – – [6]Rhodamine 101 614 614 Pulsed pump 7.9 × 103 1.0 × 102 – – [48]CdSe/CdS CQDs 626 – Pulsed pump 3.1 × 102 11 – – [12]IR-140 878∼912 ∼1820 Pulsed pump 8.3 × 103 20 – – [2]Rhodamine 6G ∼600 ∼1960 Pulsed pump 1.6 × 103 1.0 × 102 – – [44]MAPbI3 775 1370 Pulsed pump – – – – [4]MAPbI3 791∼797 797 Pulsed pump 5.7 × 105 8.0 × 104 – – [7]MaPbBr3 550 1100 Pulsed pump 2.2 × 104 1.1 × 103 – – [49]IR-792 ∼867 1734 Pulsed pump 2.2 × 106 2.8 × 104 20 × 20 20 × 20 [45]IR-140 ∼897 1382 Pulsed pump 1.6 × 108 2.2 × 106 85 × 85 85 × 85 [50]CsPbBr3 CQDs 532 5320 Pulsed pump 1.6 × 104 2.0 × 102 10 × 10 10 × 10 This workMQWs: multiple quantum wells CQDs: colloidal quantum dots.First-order BIC modes have a higher tolerance for thicknessvariation compared with the second-order BIC modes. Forthe first-order modes, a QD thickness variation of 150 nmcan still maintain the BIC mode within the gain emissionband. For the second-order modes, the QD thickness vari-ation needs to be controlled within ≈60 nm, which needsmore accurate control of the QD thickness in the fabricationprocess.The lasing properties of the CsPbBr3 QD cavity-supported BIClaser (structure parameters indicated in Figure 2) with a finitesize of 40 × 40 μm2 are characterized at room temperature (seeFigure S4, Supporting Information for details about the mea-surement setup). Figure 3a presents the lasing emission spec-trum of the CsPbBr3 cavity-supported BIC laser under differ-ent pump energy densities pumped by a 355-nm nanosecondlaser. As the pump energy density increases, the output intensityof the PL band linearly increases. Once the pump energy den-sity reaches the lasing threshold, a single sharp peak appears at532 nm. Figure 3b gives the emission intensity as a function ofthe pump energy density. A clear threshold behavior is observedat 123.0 μJ cm−2. Note that a lasing threshold of 29.6 μJ cm−2is observed by pumping the CsPbBr3 QD cavity-supported BIClaser using a 400-nm fs laser (Figure S5, Supporting Informa-tion). Figure 3c shows the lasing and spontaneous emission spec-trum of the CsPbBr3 QD cavity-supported BIC laser over a largewavelength range. The single-mode lasing peak shows up withinthe gain region of the CsPbBr3 QDs. For the CsPbBr3 QD cavity-supported BIC laser, a best-measured FWHM is ≈0.1 nm. Thefar-field emission is collected at the back focal plane of a lenswith a numerical aperture (NA) of 0.45. The lasing profile fromthe QD cavity-supported BIC laser shows a dark area in the mid-dle part of the image surrounded by a donut-shape bright ring.The dark area in the middle may be related to suppressed ra-diation in the strict normal direction for the BIC mode and atopological singularity at the beam axis.[5,12] The lasing radiatedin the vicinity of the BIC mode. The polarization dependence ofthe far-field lasing profile is also characterized by rotating the lin-ear polarizer in front of the CCD camera as shown in Figure 3f.These results agree well with simulated the electric field direc-tion shown in the near-field distribution in Figure 2d, indicat-ing a lasing is achieved via an MQ BIC mode. The period ofthe nanocylinder array is engineered to match the ED, MD, andEQ BIC modes to the gain emission band, and lasing emissionsfor these modes are observed. The far-field emission propertiesAdv. Funct. Mater. 2024, 2314953 2314953 (5 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202314953 by Cochrane Japan, Wiley Online Library on [20/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 5. a) Simulated electric field norm distribution of second-order mode for QD slab waveguide-BIC laser (top) and QD cavity-supported BIC laser(bottom) with 19 × 19 unit cells. The TiO2 nanocylinders surrounding the QD cavity-supported BIC laser are taken into account here. b) Simulatedpassive Q factor as a function of the cavity size. c) The lasing threshold of the QD slab waveguide-BIC laser and QD cavity-supported BIC laser fordifferent cavity sizes. d) The experimental far-field emission pattern and corresponding beam divergence angle of CsPbBr3 QD cavity supported-BIClaser for different cavity sizes.of the lasing generated from ED, MD, and EQ BIC modes arealso characterized in Figure S7 (Supporting Information). Thepolarization-dependant radiation patterns of these modes matchwell with the corresponding simulated results. Note that the ef-fect of pump light polarization on lasing emission is not consid-ered in this work.To investigate the effect of the isolated cavity on the BIC laser,the lasing behavior of the CsPbBr3 QD slab waveguide-BIC lasersand CsPbBr3 QD cavity-supported BIC lasers with different cav-ity sizes are characterized by using a fs laser as a pump (exci-tation at 400 nm), as shown in Figure 4. Note that here we fo-cus on the second-order MD BIC mode for obvious comparison,as the lateral losses of the high-order mode in infinite-size thinfilm are higher than that of the first-order mode as mentionedabove. The parameters of the nanocylinder array and QD thick-ness have been optimized to match the emission band to thesecond-order MD BIC mode: cylinder diameter dTiO2 = 240 nm,cylinder height hTiO2 = 120 nm, period a= 335 nm, and QD thick-ness tQD = 310 nm. The corresponding simulated transmissionband diagram and near-field distribution are shown in Figure S8(Supporting Information). For the CsPbBr3 QD slab waveguide-BIC lasers, a sharp single-mode lasing within the gain regionof CsPbBr3 QD is observed. The lasing emission intensity as afunction of pump energy density also shows a clear thresholdbehavior. The lasing threshold decreased slightly as the cavity sizeincreased, which could be attributed to better confinement fromthe lattice and a higher gain contribution from the QD. Note thatthe minimum size that exhibits a lasing is 20 μm for CsPbBr3 QDslab waveguide-BIC lasers as shown in Figure S9 (Supporting In-formation). When increasing the pump energy density, a broadpeak initially appears on the PL. Once the pump energy densityreaches 67.5 μJ cm−2, a relatively sharp peak which should likelycorrespond to the lasing onset of the BIC-supported mode is ob-served. For the CsPbBr3 QD cavity-supported BIC lasers, besidesthe single-mode lasing, a lower threshold behavior is observedcompared with the same-size CsPbBr3 QD slab waveguide-BIClasers. This may be due to the better lateral confinement from theisolated cavity. The optical microscope images showing the lasersabove their lasing thresholds are given in the right part of eachfigure. The CsPbBr3 QD cavity-supported BIC laser exhibits a lowlateral loss, however, from the optical image, scattering light isonly observed from the edge of the CsPbBr3 QD cavity-supportedBIC laser. For the CsPbBr3 QD slab waveguide-BIC laser, the QDlayer can act as a waveguide and guide light in the lateral direc-tion so that less scattered light can be observed from the objec-tive lens. Note that the minimum device size that exhibits las-ing is 10 × 10 μm2 for CsPbBr3 QD cavity-supported BIC lasers,which is the smallest BIC laser among the BIC lasers basedon solution-processed gain media and also comparable with thenon-solution-processed BIC lasers (Table 1).[2,4–7,12,15,40–45,48–56]To further investigate the isolated cavity effect, simulated near-field distributions for the CsPbBr3 QD slab waveguide-BIC laserand CsPbBr3 QD cavity-supported BIC laser are presented inFigure 5a. In this case, TiO2 nanocylinders surrounding the QDcavity are taken into account to match the real case. We also per-formed simulations to investigate the effect of different edge con-ditions and surface roughness on the passive Q factor of theCsPbBr3 QD cavity-supported BIC laser as shown in FiguresS10 and Figure S12 (Supporting Information). For the CsPbBr3QD slab waveguide-BIC laser, light being guided out of theAdv. Funct. Mater. 2024, 2314953 2314953 (6 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202314953 by Cochrane Japan, Wiley Online Library on [20/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.denanostructure is observed and thus larger optical losses are tobe expected. This can also be confirmed by the observation ofscattered light under lasing operation, as shown in Figure S11(Supporting Information). For the CsPbBr3 QD cavity-supportedBIC laser, light is better confined within the slab and only a littlelight is found to be scattered at the edge. The simulated Q fac-tor of the CsPbBr3 QD slab waveguide-BIC laser and CsPbBr3QD cavity-supported BIC laser with different cavity sizes are pre-sented in Figure 5b. The CsPbBr3 QD cavity-supported BIC lasershows a higher Q factor compared with that of the CsPbBr3 QDslab waveguide-BIC laser, especially for smaller cavity sizes (≈3times for a size of 10 × 10 μm2). Figure 5c gives the measuredlasing threshold power of the lasers under different cavity sizes.The threshold difference becomes large when the cavity size isbelow 20 μm. As the cavity size increases, the difference in thresh-old becomes small, which can be explained by the improvementin the confinement with the number of unit cells, and the ef-fect of lateral confinement with and without boundary becomesnegligible. The far-field emission patterns of the correspondingCsPbBr3 QD cavity-supported BIC laser are shown in Figure 5d.The beam divergence of the lasing beam becomes larger asthe cavity size decreases, which agrees well with the simulationresults.3. ConclusionIn conclusion, we demonstrate a CsPbBr3 QD cavity-supportedBIC laser by integrating an isolated CsPbBr3 QD cavity on a TiO2nanocylinder array. A small footprint single mode BIC laser witha narrow linewidth of ≈0.1 nm is realized. By engineering the lat-tice parameters, BIC lasing generated from ED, MD, EQ, and MQmodes are observed. Compared with the conventional CsPbBr3QD slab waveguide-BIC laser, the CsPbBr3 QD cavity-supportedBIC laser shows a lower lasing threshold, especially at a smallfootprint size. Finally, lasing with a miniaturized BIC laser hav-ing a device size down to 10 × 10 μm2 is achieved owing to thedecreased lateral optical losses.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was supported by JSPS KAKENHI (JP21H01383, JP23KF0107,JP23H01461). “Advanced Research Infrastructure for Materials and Nan-otechnology in Japan (ARIM)” of the Ministry of Education, Cul-ture, Sports, Science and Technology (MEXT) (Proposal Number JP-MXP1223UT1077). The authors would like to extend their grateful appre-ciation to Prof. Kuniaki Konishi and Prof. Junji Yumoto from the School ofScience, The University of Tokyo for technical support.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from thecorresponding author upon reasonable request.Keywordsbound states in the continuum, CsPbBr3 quantum dots, low lateral-losscavity, miniaturized lasers, solution-processed lasersReceived: November 25, 2023Revised: January 22, 2024Published online:[1] S. I. Azzam, K. Chaudhuri, A. Lagutchev, Z. Jacob, Y. L. Kim, V. M.Shalaev, A. Boltasseva, A. V. Kildishev, Laser Photonics Rev. 2021, 15,2000411.[2] Z. Zhai, Z. Li, Y. Du, X. Gan, L. He, X. Zhang, Y. Zhou, J. Guan, Y. Cai,X. Ao, ACS Photonics 2023, 10, 437.[3] J.-H. Yang, Z.-T. Huang, D. N. Maksimov, P. S. Pankin, I. V. Timofeev,K.-B. Hong, H. Li, J.-W. Chen, C.-Y. Hsu, Y.-Y. Liu, T.-C. Lu, T.-R. 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