Controlled Outcoupling of Whispering-Gallery-Mode Lasers Based on

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Controlled Outcoupling of Whispering-Gallery-Mode Lasers Based on Self-Assembled Organic Single-Crystalline Microrings Yuanchao Lv, Xiao Xiong, Yingying Liu, Jiannian Yao, Yong Jun Li, and Yong Sheng Zhao Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04402 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Controlled Outcoupling of Whispering-GalleryMode Lasers Based on Self-Assembled Organic Single-Crystalline Microrings Yuanchao Lv,†,§ Xiao Xiong,‡ Yingying Liu,†,§ Jiannian Yao,†,§ Yong Jun Li,*,† and Yong Sheng Zhao*,†,§ †

Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences,

Beijing 100190, China ‡

Key Laboratory of Quantum Information, Synergetic Innovation Center of Quantum

Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China §

University of Chinese Academy of Sciences, Beijing 100049, China

KEYWORDS: moleular self-assembly, organic laser, WGM laser, organic nanophotonics

ABSTRACT: The outcoupling of whispering-gallery-mode (WGM) lasers is crucial for the realization of various photonic functionalities, yet present material structures still suffered the unexpected surface damages or contaminations in the multistep micro/nanofabrications. Here, we propose a strategy to achieve controlled outcoupling of WGM lasers in self-assembled organic

1 ACS Paragon Plus Environment

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single-crystalline microrings. The microrings with molecular-smooth surfaces functioned as organic crystalline whispering-gallery-mode microlasers with a lasing threshold of ∼14.2 μJ cm-2 and a quality factor on the order of 103 to 104. The circular self-assembly allowed us to design different derived ring-based structures towards desired outcoupling of the WGM lasers, including unidirectional laser output from wire-ring coupled structures, and single-mode lasing in double-ring coupled systems. The results would provide an alternative avenue to construct versatile organic nanoscale lasers and related components with specific photonic applications.

Whispering-gallery-mode (WGM) lasers with high quality factors and small mode volumes have been intensively investigated because of their promising applications ranging from highthroughput sensing to on-chip optical communication.1-7 The ever-increasing demand for the outcoupling of WGM lasers,8,9 which is of great significance to light filtering,10 directional couplings,11 and modulations,12,13 calls for the integration of WGM resonators with other optical components.14,15 With the development of top-down processing techniques, various functional structures,16,17 including wire-disk,18 wire-ring,19 and multi-ring coupled systems,20 have been demonstrated to support the outcoupling of WGM modes. However, these structures still suffer from

the

unexpected

surface

defects

and

contaminations

in

the

multistep

micro/nanofabrications,19 which is a major obstacle limiting their applications in device integrations. Self-assembled organic crystals with loose crystal lattice offer high flexibility in producing highly-curved micro/nanostructures with molecular-smooth surfaces,21 like microrings,22-25 which can function as superb optical resonators with low scattering density and high quality factors.26,27 The various intermolecular interactions28,29 make it possible to modify the self-


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assembled structures and even to realize composite systems for desired optical functionalities.3032

The organic assemblies with abundant excited-state processes have shown great potential for

achieving low-loss optical waveguide and efficient lasing.33,34 More importantly, single crystal organic micro/nanostructures with effective light confinement permit the strong evanescent fields along the boundary,33 which would benefit the coupling with other optical components for specific modulation and outcoupling of WGM lasers.13 In this work, we realized controlled outcoupling of WGM lasers from organic singlecrystalline microrings synthesized through a surface tension-assisted molecular self-assembly. With well-defined circle boundaries and ultra-smooth surfaces, the single-crystalline microrings functioned as organic crystalline WGM microlasers with a lasing threshold of ∼14.2 μJ cm-2 and a quality factor on the order of 103 to 104. On this basis, unidirectional output of WGM lasers was achieved based on a wire-ring coupled structure, which was constructed by an optimized one-step method. Furthermore, such strategy enabled the outcoupling of single-mode lasing based on mode selection effect in the self-assembled double-ring coupled system. These results offer a novel understanding on the self-assembly mechanism of organic crystalline structures, and provide a valuable guidance to the realization of WGM-based photonic components with unique optical performances. The crystal of 3-[4-(dimethylamino)phenyl]-1-(2-hydroxy-4-fluorophenyl)-2-propen-1-one (HDFMAC, Figure 1A and S1) was adopted owing to its relative loose crystal lattice, which would provide good tolerance for structural distortions in the formation of curved single crystal structures.35,36 The theoretically predicted equilibrium morphology of HDFMAC (Figure 1A and Table S1) showed that the π···π interaction along a specific direction would facilitate the formation of one-dimensional (1D) nanostructures. In dichloromethane solutions, the molecules


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aggregated into microbelts with a large height to width (h/w) ratio of ~10 (Figure S2), which is consistent with our calculated results. The XRD pattern of microbelts verifies high crystallinity of the microbelts (Figure S3). These crystalline belts grow along the [010] direction (Figure 2B), which is consistent with the simulation results. The molecular planes are perpendicular to the b axis and have a large space between neighboring molecules, which means that the b parameter has a considerably variable range within the tolerance of weak intermolecular interactions. As shown in Figure 1C, the microbelts exhibit remarkable flexibility, inspiring us to apply an external driven force during the self-assembly to construct the ring-shaped structures. Here, a surface tension-assisted self-assembly was developed to fabricate organic crystalline microrings

(Figure

1D).

In

a

typical

preparation,

the

HDFMAC

solution

in

ethanol/dichloromethane (volume ratio 1:1) was freshly mixed and drop-casted on the glass substrate. During the evaporation of solvent, the HDFMAC first grew along one-dimensional direction. As dichloromethane evaporated faster than ethanol,37 the rest of the solution on substrate mainly consisted of ethanol, which gradually formed quasi-hemispherical microdrops (Figure S4). These microdrops confined the assemblies and served as circular templates for ringshaped structures. With the evaporation of solvent, driven by strong surface tension, the assemblies grew into a curved belt that embraced the microdrop and finally formed the microrings without noticeable defects or joints. Following this growth process, we can adjust the diameter of the microring at certain level to modulate the optical mode by changing the concentration of HDFMAC units. For instance, the diameter of microrings can be tuned from ~13 to 40 μm by changing the concentration of HDFMAC molecules from 0.5 to 2.5 mg/mL (Figure S5).


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Figure 1. (A) Theoretically predicted equilibrium morphology of the HDFMAC crystal. (B) TEM image and SAED pattern (inset) of a HDFMAC microbelt. Scale bar: 1 μm. (C) SEM images of a microbelt and its magnified bent part (Inset). Scale bars are 5 and 1 μm, respectively. (D) The fabrication process of microrings with temporal optical microscopy images. Scale bars: 20 μm. (E) SEM, (F) cross-polarized optical, and (G) PL microscopy images of the selfassembled microrings. Scale bars: 20 μm. As shown in Figure 1E, the as-prepared microrings possess an ideal circular boundary, which would reduce the scattering loss and provide the continuous total internal reflection for optical oscillations. The typical cross-polarized optical image shows a pronounced optical birefringence around the circled boundary of the microring (Figure 1F), exhibiting a single crystalline domain behavior with highly ordered molecular alignment.38 The single-crystalline microrings with close


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molecular packing exhibit a large Stokes shift (Figure S6), which was resulted from the excitedstate intramolecular proton transfer.35 Such a process endows these crystals with high fluorescence quantum yield (Figure S7), which is highly favorable for the optical gain and amplification.39 Under UV (330−380 nm) excitation, the microscale HDFMAC rings emitted uniform red fluorescence (Figure 1G). This indicates that the microrings are free of optical defects, which is beneficial for optical waveguide and spatial confinement. These microrings with efficient light reflection and active optical properties provide an opportunity to investigate the lasing behavior. When a pulsed laser beam (400 nm, ∼150 fs, see Figure S8 for the setup) was focused on the whole microring, a series of sharp cavity-mode peaks were found in the PL spectra collected from the microring (Figure 2A). With increasing pump fluence, the PL intensity of the 647 nm peak in the gain region was dramatically amplified. The plot of the corresponding PL peak intensity versus pump fluence reveals a clear knee characteristic at the threshold of ∼14.2 μJ cm−2 (Figure 2B), which confirms lasing action in the microring. The optical image below threshold only exhibited weak PL, while an impressive laser emission emerged along the circled boundary above the lasing threshold (Figure 2A, inset) which is a typical characteristic of the WGM microresonator. The lasing spectra of HDFMAC microrings with different diameters were collected to further study the microcavity effects (Figure 2C). The relationship between the mode spacing (Δλ) and the wavelength (λ) satisfied the equation of WGM theory, λ2/Δλ = nπD, where n is the group refractive index, D is the microdisk diameter. Based on the linear fitting relationship between λ2/Δλ and D (Figure 2D), the n around 647 nm was identified with a value of 3.28, which is high enough to induce strong self-cavity optical confinement. The resulted Q factors were on the order of 103 (Figure 2E), which is pretty high for organic resonators. Therefore,


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WGM resonance modes were well confined in self-assembled microrings, as indicated by the simulated electric field intensity (|E|2) distribution for transverse electric (TE) and transverse magnetic (TM) modes (Figure 2F). Interestingly, the width of the microring was smaller than the propagating wavelength, leading to more energy leakage of TE modes (horizontally polarized) to the air than that of TM modes (vertically polarized).40 As a result, TM modes were selectively amplified in the microring (Figure 2A, inset). Due to the considerable h/w ratio, the TM modes were confined away from the substrate, which would eliminate the substrate-induced propagation loss and thus benefit the coupling with other optical components.

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Figure 2. (A) PL spectra of an individual microring as a function of pump energy. Left inset: PL image of the microring below (bottom) and above (top) threshold. Scale bar: 10 μm. Right inset: Polarization profile of the lasing emission (647 nm). (B) Plots of PL peak intensities (black line)


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and FWHM (red line) vs pump fluence. (C) PL spectra of microrings with different diameters. Scale bars: 20 μm. (D) Relationship between λ2/Δλ and D. (E) Plot of experimental Q factor vs D. (F) |E|2 distribution for the cross-section of the microring resonator (w=250 nm, h=2.5 μm). White arrows denote the polarization directions of the optical modes. High-quality microring lasers, as an ideal coherent light source for next-level devices, usually require the outcoupling with 1D waveguides,11 which would greatly benefit the highly directional collection and processing of microring resonance modes (Figure 3A). The proposed ring-shaped self-assembly, strongly guided by the surface tension of microdrops, provides a great chance to realize waveguide-coupled microrings by controlling the external driven force. As shown in Figure 3B, the proportion of ethanol in the mixed solution was reduced (ethanol/dichloromethane at a volume ratio = 1:3), which led to the rapid evaporation of the solvent and thus induced the nucleation and growth of the microbelts. Driven by strong surface tension, the microbelts grew into the curved structures. However, the diameter of the microdrop template decreased, making it difficult to confine the whole curved microbelt. The microdrops preferred to move to the tips of microbelts to reduce the interfacial energy, and trigger the circle assembly, resulting in the microrings coupled with the curved microbelts. As shown in Figure 3C and 3D, the as-prepared structures are single-crystalline with smooth surfaces. In the fluorescence microscopy image (Figure 3E), the PL spots from the wire tips and weaker PL from the wire bodies can be observed, which confirms the typical characteristic of the optical waveguide.


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A

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Coupling Waveguide

C2H5OH/CH2Cl2 (volume ratio=1:3) Shrinking

Output

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tip

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640 660 680 Wavelength (nm)

Figure 3. (A) Schematic illustration of a microbelt-coupled microring for efficient outcoupling of WGM lasers. (B) The fabrication process of waveguide-coupled microrings with temporal optical microscopy images. Scale bars: 10 μm. (C) SEM, (D) cross-polarized optical, and (E) PL microscopy images of the microbelt-coupled microring. Scale bars: 10 μm. (F) |E|2 distribution in the microstructure. (G) Bright-field and PL images of a microbelt-coupled microring. Scale bar: 10 μm. PL spectra collected from the microring (H) and the tip (I) as a function of pump fluence. The |E|2 distribution (Figure 3F) reveals that the optical fields are well-confined inside the microring to form WGMs, and the corresponding optical modes can be efficiently outputted from the microbelt. When a 400 nm femtosecond-pulsed laser was focused on the whole microring resonator, the strong emission emerged along the circle boundary (Figure 3G). With the increase


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of pump fluence, the PL peak intensity in the gain region was dramatically amplified (Figure 3H), which reveals a clear knee behavior at the threshold of ∼15.9 μJ cm−2 (Figure S9). The PL spectrum collected from the ring indicates that the mode spacing (Δλ, ~1.75 nm) versus the perimeter (P, ~71 μm) of microring is in agreement with that given by the WGM equation. Although the additional microbelt would induce the unexpected optical loss to the microring resonator, the microbelt-coupled microring can still maintain the WGM resonance. Interestingly, in spite of the inevitable bending loss of the curved microbelt,41,42 a clear bright spot can be observed at the end of the unpumped microbelt with the aid of low optical waveguide loss (Figure S10). The emission spectra collected from the tip (Figure 3I) showed the same peaks as in the spectra obtained from the microring. These results indicate that the lasing mode in the microring was effectively outcoupled from the waveguide without obvious signal distortion, supporting the directional outcoupling and transmitting of the coherent signals from the WGM lasing. We can speculate that the configuration of the crystalline microrings can be further extended by reversely altering the volume ratio of ethanol/dichloromethane solution, which would prolong the circular growth time and thus may permit multi-ring coupled structures with the mode selection effect (Figure 4A).43 As illustrated in the Figure 4B, the increase on the proportion of ethanol in the mixed solution (ethanol/dichloromethane at a volume ratio = 3:1) induced the microdrops with larger diameters. After the formation of the first microring, the residual microdrops preferred to move to the boundary of the first microring to reduce the interfacial energy. The first microring acted as the preferential nucleation center, which gave rise to the second-ring assembly and thus generated the double-ring coupled structures. As shown in Figure 4C and 4D, the obtained structures possessed the obvious single-crystalline characteristic and


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smooth boundary. Based on these merits, both the inner and outer rings showed uniform luminescence without evident scattering points (Figure 4E), which is favorable for the light confinement during the coupling.

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0.7 0.6

10 15 20 Pump Fluence (μJ cm-2)

Figure 4. (A) Schematic illustration of a double-ring coupled resonator for optical mode coupling. (B) The fabrication process of the double-ring coupled resonator with temporal optical microscopy images. Scale bars: 10 μm. (C) SEM, (D) cross-polarized optical, and (E) PL microscopy images of the double-ring coupled resonator. Scale bars: 10 μm. (F) |E|2 distribution in the resonator. (G) Bright-field and PL images of a coupled resonator. Scale bar: 10 μm. (H) PL spectra collected from the coupled microring as a function of pump fluence. (I) Plots of PL peak intensities (black line) and FWHM (red line) vs pump fluence.


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The |E|2 distribution shown in Figure 4F indicates that the double-ring system is on resonance, making it possible to realize the outcoupling and optical modulation of the WGM modes. As illustrated in Figure 4G, when the coupled structure was uniformly pumped, the bright red emission along the boundary suggests efficient optical coupling between the two rings. Interestingly, the PL spectrum collected from the coupled microring (Figure 4H) exhibited a single-mode lasing emission, which is in sharp contrast with the multimode lasing action in the isolated one (Figure 2A). As WGMs are generated along the circular boundary of the microring, the Δλ of the double ring-coupled cavity (Δλ12) can be estimated by Vernier equation: Δλ12 = λ2/nπ(D2-D1), where D1 and D2 are the diameters of the inner and outer rings, respectively. Considering that D1 = 24.8 μm, D2 = 28.3 μm, and n = 3.28, theoretical prediction Δλ12 is obtained as 11.4 nm, which is consistent with the experimental data (11.7 nm, Figure S11). These results revealed that the Vernier effect existed in the double ringcoupled cavity and was the key factor on the single mode operation.13,44,45 The high crystallinity of the structure induced a low optical loss, which greatly contributes to the stable operation of low-threshold single-mode laser over a large range of pump intensities (Figure 4I). In addition to the reduced number of lasing modes, the double-ring coupled structure did not cause an obvious increase of the lasing threshold (~17.4 μJ cm−2), slightly higher than those of the isolated one (Figure 2B), which can be ascribed to the large optical cross section in organic materials and the coupled cavity nature. In summary, we report the construction of single-crystalline organic microrings and their derived structures by a surface tension-assisted self-assembly method to realize controlled outcoupling of WGM lasers. The self-assembled microrings, with perfect circle boundaries and molecular-smooth surfaces, served as WGM microlasers with a lasing threshold of ∼14.2 μJ cm-2


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and a quality factor on the order of 103 to 104. The assembly mechanism enabled us to design and synthesis different complex ring-shaped structures with desired outcoupling functionalities, such as the wire-ring coupled structure with unidirectional laser output and the double-ring coupled system with single-mode lasing performance. These results would provide a new avenue for the rational design of WGM-based lasers with unique applications in nanophotonic devices.

ASSOCIATED CONTENT Supporting Information. Materials, experimental details and theoretical simulations. Structure and morphology (singlecrystal data, XRD patterns). Experimental setup for the optical characterization. Fluorescence properties (spectrum, quantum yield, and lifetime). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported financially by the Ministry of Science and Technology of China (Grant No. 2017YFA0204502), the National Natural Science Foundation of China (Grant Nos. 21790364, 21533013 and 21603241).


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(23) Jeukens, C. R. L. P. N.; Lensen, M. C.; Wijnen, F. J. P.; Elemans, J. A. A. W.; Christianen, P. C. M.; Rowan, A. E.; Gerritsen, J. W.; Nolte, R. J. M.; Maan, J. C. Nano Lett. 2004, 4, 14011406. (24) Venkatakrishnarao, D.; Mamonov, E. A.; Murzina, T. V.; Chandrasekar, R. Adv. Opt. Mater. 2018, 6, 1800343. (25) Balzer, F.; Beermann, J.; Bozhevolnyi, S. I.; Simonsen, A. C.; Rubahn, H. G. Nano Lett. 2003, 3, 1311-1314. (26) Zhang, H.; Liao, Q.; Wu, Y.; Zhang, Z.; Gao, Q.; Liu, P.; Li, M.; Yao, J.; Fu, H. Adv. Mater. 2018, 30, 1706186. (27) Adachi, T.; Tong, L.; Kuwabara, J.; Kanbara, T.; Saeki, A.; Seki, S.; Yamamoto, Y. J. Am. Chem. Soc. 2013, 135, 870-876. (28) Ghosh, S.; Mishra, M. K.; Ganguly, S.; Desiraju, G. R. J. Am. Chem. Soc. 2015, 137, 99129921. (29) Wang, Y.; Liu, J.; Tran, H. D.; Mecklenburg, M.; Guan, X. N.; Stieg, A. Z.; Regan, B. C.; Martin, D. C.; Kaner, R. B. J. Am. Chem. Soc. 2012, 134, 9251-9262. (30) Takazawa, K.; Inoue, J.; Mitsuishi, K.; Takamasu, T. Adv. Mater. 2011, 23, 3659-3663. (31) Wei, C.; Liu, S.-Y.; Zou, C.-L.; Liu, Y.; Yao, J.; Zhao, Y. S. J. Am. Chem. Soc. 2015, 137, 62-65. (32) Han, S.; Zhang, W.; Qiu, B.; Dong, H.; Chen, W.; Chu, M.; Liu, Y.; Yang, X.; Hu, F.; Zhao, Y. S. Adv. Opt. Mater. 2018, 6, 1701077.


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(33) Fang, H.-H.; Yang, J.; Feng, J.; Yamao, T.; Hotta, S.; Sun, H.-B. Laser Photonics Rev. 2014, 8, 687-715. (34) Feng, J.; Jiang, X.; Yan, X.; Wu, Y.; Su, B.; Fu, H.; Yao, J.; Jiang, L. Adv. Mater. 2017, 29, 1603652. (35) Cheng, X.; Zhang, Y.; Han, S.; Li, F.; Zhang, H.; Wang, Y. Chem. Eur. J. 2016, 22, 48994903. (36) Ghosh, S.; Mishra, M. K.; Kadambi, S. B.; Ramamurty, U.; Desiraju, G. R. Angew. Chem., Int. Ed. 2015, 54, 2674-2678. (37) Qi, Z.; Yu, H.; Chen, Y.; Zhu, M. Mater. Lett. 2009, 63, 415-418. (38) Rigas, G. P.; Payne, M. M.; Anthony, J. E.; Horton, P. N.; Castro, F. A.; Shkunov, M. Nat. Commun. 2016, 7, 13531. (39) Park, S.; Kwon, O.-H.; Kim, S.; Park, S.; Choi, M.-G.; Cha, M.; Park, S. Y.; Jang, D.-J. J. Am. Chem. Soc. 2005, 127, 10070-10074. (40) Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269-1273. (41) Wang, W.; Yang, Q.; Fan, F.; Xu, H.; Wang, Z. L. Nano Lett. 2011, 11, 1603-8. (42) Wei, H.; Xu, H. Nanoscale 2012, 4, 7149-7154. (43) Li, H.; Shang, L.; Tu, X.; Liu, L.; Xu, L. J. Am. Chem. Soc. 2009, 131, 16612-16613.


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Nano Letters

SelfAssembly

Unidirectional laser output

WGM resonator

Single-mode lasing


Controlled Outcoupling of Whispering-Gallery-Mode Lasers Based on

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