Open Access
Add to BibSonomy
Abstract
We report a single-mode dye-doped polymer microbottle resonator (MBR) laser. The selective single-mode lasing from different order whispering gallery modes is achieved by precisely controlling the axial and radial coupling position between a tapered nanofiber and an MBR, respectively. The side-mode suppression ratio is above 20 dB. By doping different fluorescence dyes into the MBR, single-mode lasers at various colors are demonstrated.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Whispering gallery mode (WGM) based optical microcavities with ultra-small mode volume and ultra-high quality (Q) factor can great enhance light-matter interactions and have garnered tremendous attention as an exceptionally powerful tool for microlasers [1], cavity opto-mechanics [2,3], nonlinear optics [4,5] and ultrasensitive sensing [6,7]. Recently, microbottle resonators (MBRs) with highly prolate shapes are widely studied [8–12]. The MBRs support nondegenerate modes, which are well-separated along the axial direction. The WGM eigenvalues in an MBR are characterized by three quantum numbers: azimuthal number m, radial number p and axial number q, respectively. Due to the conservation of angular momentum, high-order axial modes or “bottle modes” sustain two spatially separated caustics with field enhancement. The two “turning points” avoid the light leakage along the bottle neck and provide a feasibility to selectively excite the bottle modes, which usually exhibit ultrahigh quality factors up to 108 [13–15].
However, the MBR lasers still suffer the “multimode-lasing” issue since the gain bandwidth of active media is usually larger than the free spectral range (FSR) of a WGM microcavity laser. Based on Vernier effect, the enlarged FSRs in coupled resonators enable only one longitudinal mode within the gain spectrum for the realization of single-mode lasing [16–18]. As an alternative approach, the manipulation of the gain and loss in WGM microlaser systems can also lead to stable single-mode operation [19–21]. The first selective mode lasing in active polymer MBRs was achieved by a selective tapered-microfiber-pumping scheme [22]. The groups of lasing equatorial modes (q = 0) and bottle modes are identified. By shrinking the polymer MBR down to 10 μm, interference pattern of the pumping laser was used to achieve single-mode lasing in polymer MBRs, where only several transverse modes lie in the effective gain interval [23]. The loss-engineering approaches by using a tapered fiber [24] or milled micro-grooves on the surface of MBRs [25] have also been provided for lasers with high side-mode suppression ratio (SMSR). However, loss-engineering method is only applicable to single fundamental mode (q = 0) lasing.
In this paper, we investigate selective single-mode lasing in active polymer MBRs with size smaller than 10 μm by using the evanescent field of the tapered nanofiber as an ultras-mall pump source [Fig. 1(a)]. Compared with microspheres or microtoroids, MBRs have much larger axial radii [10]. Thus, the different axial modes are spatially well-separated along the axial direction as shown in Figs. 1(b) and 1(d), where we calculate the field distribution of different axial modes (q = 0-4) of an MBR and a microsphere for comparison, respectively. For the axial (or polar) modes in the microsphere, the field distribution along axial direction is relatively more compact. Here, this comparison is only to illustrate that resonator with larger axial radii supports spatially well-separated axial modes. The resonant modes are studied using the finite element method with COMSOL Multiphysics 3.5a [26]. The diameter of this calculated MBR is Db = 8 μm, the diameter of the bottle stem is Da = 6 μm and neck-to-neck distance is L = 16 μm [22]. The diameter of the microsphere is the same size as the MBR. The size of the antinodes for different bottle modes is also shown in Fig. 1(c). The size of the fundamental mode in MBR is as large as 3.0 μm and obviously larger than that in microsphere. The size of axial modes is also larger than 1.3 μm even when the q is over 10, which provides a good platform for selecting excitation of different axial modes. By coupling pump laser into the tapered nanofiber, which acts as ultra-small pump source, the single fundamental mode and single axial mode lasing are achieved. The controllable method by changing the distance between the tapered nanofiber and MBR is investigated for single-mode lasing, for the first time to our knowledge. Single-mode lasers at various colors are also realized by doping various dyes into the MBR.
Fig. 1 (a) Schematic of tapered nanofiber pumped MBR laser. (b) 1-D and (d) 2-D of field distributions of different axial modes in MBR and microsphere. (c) FWHMs of different axial modes in MBR.
2. Fabrication and single fundamental mode laser
Microbottles were fabricated by transferring the SU-8 droplet doped with 0.3 wt% Rhondamine B (RhB) on the tapered fibers according to the self-assembly method previously reported [22,27,28]. The Q factor of the passive MBR with SU-8 material is about 105-106, which was measured via near-field coupling method in Ref [22]. and is higher than that in MBR with ultraviolet (UV)-curable adhesive [29]. The diameter of the stem fiber is 2-5 μm and the active microbottles with diameters ranging from 5 to 10 μm are chosen for study.
Another tapered nanofiber is fabricated with diameter as thin as below 1 μm as a pump source. The diameter of the tapered nanofiber attached directly on a glass slide is measured by an atomic force microscope (AFM, JPK NanoWizard III). The height, i.e., the diameter of the tapered nanofiber is about 550 nm as shown in the inset of Fig. 2(b). Measurement of the diameter of another tapered nanofiber fabricated under the same conditions is also performed by scanning electron microscope (SEM) and its diameter is 629 nm, which indicates that the size of the tapered fiber is nanoscale. The 532 nm ns pulsed laser (Ekspla NT340) at a repetition rate of 10 Hz is coupled to this tapered nanofiber via an objective. The pump laser is polarized along the axis direction of the MBR. An attenuator is inserted before the objective to tune the pump laser power. The tapered nanofiber with an evanescent component that extends into the surrounding medium provides a convenient and efficient means to inject pump light and also retrieve lasing output spectrum. The other end of the tapered nanofiber is directly connected into the monochromator (HORIBA iHR550) equipped with a cooled CCD detector to collect the cavity emission spectra. A 1200/mm grating is installed in the monochromator. The integration time is 1 s unless otherwise specified.
Fig. 2 Experimental characterization of the single fundamental mode (q = 0) laser. (a) Output laser intensity as a function of the pump power (light-light curve) around the single-mode lasing wavelength. The inset shows the optical microscopy image of the tapered nanofiber attached with the center of the MBR. (b) Evolution of the normalized output intensity as a function of wavelength and pump power. The left inset is the image of the tapered nanofiber measured by an AFM and the right inset shows the height, i.e., diameter of the tapered nanofiber.
We firstly study the lasing emission pumped by the tapered nanofiber which is placed in contact with the center of the active MBR. The size parameters of this MBR are Db = 8 μm, Da = 6 μm and L = 16 μm. The plot for energy of the cavity emission collected by the tapered nanofiber versus the total power coupled into the tapered nanofiber is shown in Fig. 2(a). A clear laser action with threshold power of about 0.18 μW is observed from Fig. 2(a), and above the threshold a linear behavior can be seen.
A plot of the evolution of the output power as a function of both the pump power and the wavelength is shown in Fig. 2(b). At a low pump power, only broadband fluorescence signal is observed. While increasing the pump power, one obvious narrow peak wins out from the fluorescence spectrum, which is called single-mode lasing. The single-mode lasing is achieved for the following reasons: the relatively larger azimuthal FSR (> 10 nm), the spatial overlap between the tapered nanofiber and the emission fundamental WGM, and along with the mode competition. The SMSR, which is defined as the ratio of the lasing power between the main mode and side mode, can reach to 17 dB with pump power at 0.32 μW. However, at higher pump power (> 0.4 μW), other side mode laser around the original single-mode laser peak emerges, that is multi-mode lasing. The SMSR also drops to about 11 dB. Note that the ratio of side-mode lasing and the main-mode lasing threshold Ps/Pth ~0.4 μW/0.18 μW is above 2, which is even larger than that in coupled ring resonators [30,31] for single-mode lasing.
3. Selective single-mode laser with different q number
Different coupling positions along the axial surface of MBR are also investigated as shown in Fig. 3. Insets in the right panels of Figs. 3(b)-3(e) show the field distributions of different axial modes (transverse-electric polarization) and provide an intuitive view of the relative coupling positions between the tapered nanofiber and the lasing mode. We also calculate the resonant wavelengths for different m and q numbers which are shown in Fig. 3(a). The relative wavelength of fundamental mode is set to zero. Single-mode lasing can be achieved in almost all the conditions.
Fig. 3 Selective single fundamental mode and single axial mode lasing when changing the tapered nanofiber position along the z-axis. (a) calculated relative wavelength of the resonant modes with different axial mode numbers q and azimuthal numbers m within one FSR. Single-mode laser spectra as the coupling positions are 5.8 μm (b), 3.8 μm (c) and, 2.5 μm (d) and 1 μm (e) away from the center of the MBR, respectively. Right panels show the corresponding optical microscopies of the MBR laser, insets are the field distribution profiles of the corresponding lasing mode.
When the tapered nanofiber is 1 μm away from the center, the single-mode lasing wavelength plotted in Fig. 3(e) is the same with that shown in Fig. 2, where the tapered nanofiber is placed at the center of the MBR. That means, in this condition, the single lasing mode is still the fundamental mode. Thus, the single-mode lasing pumped by the evanescent field of the tapered nanofiber is robust and shows high tolerance with the coupling position. When the tapered nanofiber is 2.5 μm away from the center, the field of tapered nanofiber overlaps with that of the q = 4 mode mostly shown in Fig. 3(d). This single-mode lasing spectrum with an SMSR higher than 20 dB can be obtained [Fig. 3(d)], which is comparable with that in Ref [24]. Further moving the tapered nanofiber far away from the center, higher order axial modes are excited [14]. When the tapered nanofiber is placed at 3.8 μm away from the center of the MBR, the axial mode with q = 8 is excited and the single -mode laser is shown in Fig. 3(c). When the tapered nanofiber is placed at 5.8 μm away from the center of the MBR [Fig. 3(b)], the lasing of the axial mode with q = 19 is also demonstrated. Little mismatch between the measured lasing wavelength and the calculated resonant wavelength may result from the measurement error of the MBR size.
4. Controllable single-mode laser
One of the advantages by using the tapered nanofiber as the pumping source is the additional flexibility to control the distance between the pump source (the tapered nanofiber) and the resonator [32]. Thus we next control the distance between the tapered nanofiber and MBR to investigate the laser performance. An MBR with Db = 6 μm, Da = 4 μm and L = 15 μm is mounted on a nano-positioner, while the tapered nanofiber is fixed. The nano-positioner is driven by a function generator to set the scan rate of 1 step (16 nm)/1.5 s and the spectrum is also acquired automatically per step with integration time of 1.5 s.
We firstly place the tapered nanofiber in touch with the center of the MBR and tune the pump light to achieve multimode lasing. Then, the pump power is fixed and the tapered nanofiber is moved far away (> 300 nm) from the MBR. In this condition, there is no signal as shown in Fig. 4. Decreasing the gap distance d, a sharp peak arises, i.e., single-mode lasing is achieved via the strong evanescent field of the tapered nanofiber even a relatively large gap between the tapered nanofiber and MBR exists. The lasing mode is the fundamental mode with q = 0. When the gap is less than 20 nm, multimode lasing is observed as shown in Fig. 4. The new emerging mode is high order bottle mode with q = 2, which is secondly most overlapped with the tapered nanofiber [Fig. 5(b)). The lasing intensity of the fundamental mode decreases due to the photo-bleaching effect for long time excitation. To the best of our knowledge, this is the first time for investigating the single-mode laser by controlling the distance between the tapered nanofiber and the microcavity.
Fig. 4 Evolution of the normalized output intensity as a function of distance between the tapered nanofiber and MBR.
Fig. 5 Calculated coupling factor (κ). (a) κ vs d when the tapered nanofiber is at center of the MBR. (b) κ vs q when the tapered nanofiber is at the center of MBR (z = 0). (c) κ vs q when the tapered nanofiber is 5 μm away from the center of MBR (z = 5 μm).
In order to explain the experimental phenomenon above, the coupling factor κ is analyzed. The square of the coupling factor κ, which represents the proportion of power coupled from the waveguide mode to one WGM per revolution, can be written as [33]:
where coupling efficiency Ko is written as:k is the wavevector of the WGM; βf is the propagation constant of the tapered nanofiber waveguide Δβ is difference between βf and the propagation constant of the WGM β; γf is the waveguide mode of the tapered nanofiber decay constant away from the waveguide surface; nm and no are the refractive index of the MBR and surrounding material, respectively; Fo and Ψp,m,q are the normalized waveguide mode field and WGM field in r-z cross-sectional plane, respectively. As the pumping configuration is assumed as non-resonant excitation and the thickness of the gain material is as thin as ~1 μm, the lasing first radial order mode (p = 1) has the lowest threshold. The coupling factor κ can be used to determine the coupling efficiency Ko between the lasing mode and the tapered nanofiber waveguide mode.The calculation results of the coupling factor κ are shown in Fig. 5. In this case, the diameter of the tapered nanofiber is set as 600 nm and the diameter of the MBR Db is set as 6 μm with surface curvature of 0.012 μm−1 in the calculations. As shown in Fig. 5(a), the coupling factor increases exponentially as d decreases. This can explain why the lasing spectrum intensity increases dramatically when the tapered nanofiber is closer to the MBR (Fig. 4).
κ as a function of q number when the tapered nanofiber is placed at the center of the MBR (z = 0) and axially 5 μm away from the center of the MBR (z = 5 μm) are also plotted in Figs. 5(b) and 5(c), respectively. κ for the fundamental mode (q = 0) has the largest value among all the modes [Fig. 5(b)]. When the tapered nanofiber is placed at the center of the MBR, the lasing q = 0 mode achieved firstly as shown in Figs. 2(b), 3(e) and 4. The value of κ for the q = 2 mode is the second-largest [Fig. 5(b)]. When the distance between the tapered nanofiber and MBR decreases (i.e., the pump power density increases), the new lasing q = 2 mode emerges as shown in Fig. 4. When d decreases further, the higher order (both for radial and axial) modes lase (small peaks in Fig. 4) as the pump power is relatively high. When the tapered nanofiber is axially placed at z = 5 μm, the value of κ for q = 3 mode is the largest [Fig. 5(c)] as the field overlap between the q = 3 mode and waveguide mode is largest. So, this results in Fig. 5(c) indicate that when the tapered nanofiber is axially placed, one certain axial mode possesses largest κ depending on the coupling position and this axial mode can lase firstly with the help of mode competition. As a result, single-mode lasers with different q numbers can be obtained by changing the coupling positions in experiment (see Fig. 3).
5. Single-mode lasing at various colors
As known, polymer material is popular for its convenience of doping various dyes [34], rare earth ion [35,36], quantum dots [37–40] and upconversion nanoparticle [41] as gain mediums, which providing a broad range of laser emission band [32]. Thus we finally dope various fluorescence dyes for investigating single-mode laser at various colors.
Doping another three fluorescence dyes: Rhodamine 123 (Rh123), Rhodamine 6G (Rh6G), and LDS 698 into the SU-8 respectively, MBRs with different gain mediums are fabricated. Under the excitation wavelength of 488 nm, 532 nm and 480 nm, single-mode lasers at various colors with wavelength of 545 nm, 595 nm, 673 nm (Fig. 6) is realized respectively. In principle, this method provides a simple and efficient scheme to achieve any color single-mode laser.
6. Summary
In conclusion, dye-doped polymer MBRs with size smaller than 10 μm are fabricated. Laser action is observed by launching pump light via the evanescent field of tapered nanofiber into the active MBRs. Combining the relatively large azimuthal FSR and small size of the tapered nanofiber as the pump source, both single fundamental mode and high-order axial mode lasing are achieved by precisely selecting the coupling position between the tapered nanofiber and the resonator. By controlling the distance between the pumping tapered nanofiber and the MBR, the switch from fluorescence to single-mode lasing, then to multimode lasing, can be efficiently controlled. Finally, single-mode lasing at various colors was also demonstrated by doping various dyes into the polymer resonator.
Funding
National Key Basic Research Program of China (973 project) (2015CB352006); National Natural Science Foundation of China (61705039, 61378080 and 61335011); China Postdoctoral Science Foundation (2017M610389); Fujian Provincial Program for Distinguished Young Scientists in University; Fujian Provincial Key Project of Natural Science Foundation for Young Scientists in University (JZ160423); Program for Changjiang Scholars and Innovative Research Team in University (No. IRT_15R10); Special Funds of the Central Government Guiding Local Science and Technology Development (2017L3009).
Acknowledgments
We thank Dr. Yuhua Wang for the measurement of diameter of the tapered nanofiber.
References
1. H. M. Tzeng, K. F. Wall, M. B. Long, and R. K. Chang, “Laser emission from individual droplets at wavelengths corresponding to morphology-dependent resonances,” Opt. Lett. 9(11), 499–501 (1984). [CrossRef] [PubMed]
2. M. Aspelmeyer, T. J. Kippenberg, and F. Marquardt, “Cavity optomechanics,” Rev. Mod. Phys. 86(4), 1391–1452 (2014). [CrossRef]
3. C. Dong, V. Fiore, M. C. Kuzyk, and H. Wang, “Optomechanical Dark Mode,” Science 338(6114), 1609–1613 (2012). [CrossRef] [PubMed]
4. X. Jiang, L. Shao, S. X. Zhang, X. Yi, J. Wiersig, L. Wang, Q. Gong, M. Lončar, L. Yang, and Y. F. Xiao, “Chaos-assisted broadband momentum transformation in optical microresonators,” Science 358(6361), 344–347 (2017). [CrossRef] [PubMed]
5. S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, “Ultralow-threshold Raman laser using a spherical dielectric microcavity,” Nature 415(6872), 621–623 (2002). [CrossRef] [PubMed]
6. W. Chen, Ş. Kaya Özdemir, G. Zhao, J. Wiersig, and L. Yang, “Exceptional points enhance sensing in an optical microcavity,” Nature 548(7666), 192–196 (2017). [CrossRef] [PubMed]
7. F. Vollmer and S. Arnold, “Whispering-gallery-mode biosensing: label-free detection down to single molecules,” Nat. Methods 5(7), 591–596 (2008). [CrossRef] [PubMed]
8. M. Sumetsky, “Whispering-gallery-bottle microcavities: The three-dimensional etalon,” Opt. Lett. 29(1), 8–10 (2004). [CrossRef] [PubMed]
9. Y. Louyer, D. Meschede, and A. Rauschenbeutel, “Tunable whispering-gallery-mode resonators for cavity quantum electrodynamics,” Phys. Rev. A 72(3), 031801 (2005). [CrossRef]
10. M. Sumetsky, “Lasing microbottles,” Light Sci. Appl. 6(10), e17102 (2017). [CrossRef]
11. Q. Lu, S. Liu, X. Wu, L. Liu, and L. Xu, “Stimulated Brillouin laser and frequency comb generation in high-Q microbubble resonators,” Opt. Lett. 41(8), 1736–1739 (2016). [CrossRef] [PubMed]
12. M. Sumetsky, “Delay of light in an optical bottle resonator with nanoscale radius variation: dispersionless, broadband, and low loss,” Phys. Rev. Lett. 111(16), 163901 (2013). [CrossRef] [PubMed]
13. M. Pöllinger, D. O’Shea, F. Warken, and A. Rauschenbeutel, “Ultrahigh-Q tunable whispering-gallery-mode microresonator,” Phys. Rev. Lett. 103(5), 053901 (2009). [CrossRef] [PubMed]
14. G. Senthil Murugan, J. S. Wilkinson, and M. N. Zervas, “Selective excitation of whispering gallery modes in a novel bottle microresonator,” Opt. Express 17(14), 11916–11925 (2009). [CrossRef] [PubMed]
15. C. Junge, D. O’Shea, J. Volz, and A. Rauschenbeutel, “Strong coupling between single atoms and nontransversal photons,” Phys. Rev. Lett. 110(21), 213604 (2013). [CrossRef] [PubMed]
16. L. Shang, L. Liu, and L. Xu, “Single-frequency coupled asymmetric microcavity laser,” Opt. Lett. 33(10), 1150–1152 (2008). [CrossRef] [PubMed]
17. X. Wu, H. Li, L. Liu, and L. Xu, “Unidirectional single-frequency lasing from a ring-spiral coupled microcavity laser,” Appl. Phys. Lett. 93, 1710 (2008).
18. X. Wu, Y. Sun, J. D. Suter, and X. Fan, “Single mode coupled optofluidic ring resonator dye lasers,” Appl. Phys. Lett. 94(24), 241109 (2009). [CrossRef]
19. L. Feng, Z. J. Wong, R. M. Ma, Y. Wang, and X. Zhang, “Single-mode laser by parity-time symmetry breaking,” Science 346(6212), 972–975 (2014). [CrossRef] [PubMed]
20. H. Hodaei, M. A. Miri, M. Heinrich, D. N. Christodoulides, and M. Khajavikhan, “Parity-time-symmetric microring lasers,” Science 346(6212), 975–978 (2014). [CrossRef] [PubMed]
21. H. Wang, S. Liu, L. Chen, D. Shen, and X. Wu, “Dual-wavelength single-frequency laser emission in asymmetric coupled microdisks,” Sci. Rep. 6(1), 38053 (2016). [CrossRef] [PubMed]
22. Q. Lu, X. Wu, L. Liu, and L. Xu, “Mode-selective lasing in high-Q polymer micro bottle resonators,” Opt. Express 23(17), 22740–22745 (2015). [CrossRef] [PubMed]
23. F. Gu, F. Xie, X. Lin, S. Linghu, W. Fang, H. Zeng, L. Tong, and S. Zhuang, “Single whispering-gallery mode lasing in polymer bottle microresonators via spatial pump engineering,” Light Sci. Appl. 6(10), e17061 (2017). [CrossRef]
24. F. Xie, N. Yao, W. Fang, H. Wang, F. Gu, and S. Zhuang, “Single-mode lasing via loss engineering in fiber-taper-coupled polymer bottle microresonators,” Photon. Res. 5(6), B29–B33 (2017). [CrossRef]
25. S. B. Gorajoobi, G. S. Murugan, and M. N. Zervas, “Mode-selective spectrally-cleaned-up microbottle resonator laser,” in Photonics Conference (2016), pp. 105–106.
26. M. Oxborrow, “Traceable 2-D finite-element simulation of the whispering-gallery modes of axisymmetric electromagnetic resonators,” IEEE. T. Microw. Theory 55(6), 1209–1218 (2007). [CrossRef]
27. I. Grimaldi, S. Berneschi, G. Testa, F. Baldini, G. N. Conti, and R. Bernini, “Polymer based planar coupling of self-assembled bottle microresonators,” Appl. Phys. Lett. 105(23), 231114 (2014). [CrossRef]
28. J. M. Ward, Y. Yang, and S. Nic Chormaic, “Glass-on-Glass Fabrication of Bottle-Shaped Tunable Microlasers and their Applications,” Sci. Rep. 6(1), 25152 (2016). [CrossRef] [PubMed]
29. G. Gu, C. Guo, Z. Cai, H. Xu, L. Chen, H. Fu, K. Che, M. Hong, S. Sun, and F. Li, “Fabrication of ultraviolet-curable adhesive bottle-like microresonators by wetting and photocuring,” Appl. Opt. 53(32), 7819–7824 (2014). [CrossRef] [PubMed]
30. X. Tu, X. Wu, M. Li, L. Liu, and L. Xu, “Ultraviolet single-frequency coupled optofluidic ring resonator dye laser,” Opt. Express 20(18), 19996–20001 (2012). [CrossRef] [PubMed]
31. C. Y. Gong, Y. Gong, W. L. Zhang, Y. Wu, Y. J. Rao, G. D. Peng, and X. Fan, “Fiber optofluidic microlaser with lateral single mode emission,” IEEE. J. Sel. Top. Quantum Electron. 24, 0900206 (2017).
32. M. Cai, O. Painter, and K. J. Vahala, “Observation of critical coupling in a fiber taper to a silica-microsphere whispering-gallery mode system,” Phys. Rev. Lett. 85(1), 74–77 (2000). [CrossRef] [PubMed]
33. B. E. Little, J. P. Laine, and H. A. Haus, “Analytic Theory of Coupling from Tapered Fibers and Half-Blocks into Microsphere Resonators,” J. Lightwave Technol. 17(4), 704–715 (1999). [CrossRef]
34. T. Reynolds, N. Riesen, A. Meldrum, X. Fan, J. M. M. Hall, T. M. Monro, and A. François, “Fluorescent and lasing whispering gallery mode microresonators for sensing applications,” Laser Photonics Rev. 11(2), 1600265 (2017). [CrossRef]
35. Y. Yang, F. Lei, S. Kasumie, L. Xu, J. M. Ward, L. Yang, and S. Nic Chormaic, “Tunable erbium-doped microbubble laser fabricated by sol-gel coating,” Opt. Express 25(2), 1308–1313 (2017). [CrossRef] [PubMed]
36. M. Pöllinger, D. O’Shea, F. Warken, and A. Rauschenbeutel, “Ultrahigh-Q tunable whispering-gallery-mode microresonator,” Phys. Rev. Lett. 103(5), 053901 (2009). [CrossRef] [PubMed]
37. H. Liu, J. B. Edel, L. M. Bellan, and H. G. Craighead, “Electrospun Polymer Nanofibers as Subwavelength Optical Waveguides Incorporating Quantum Dots,” Small 2(4), 495–499 (2006). [CrossRef] [PubMed]
38. C. H. Dong, F. W. Sun, C. L. Zou, X. F. Ren, G. C. Guo, and Z. F. Han, “High-Q silica microsphere by poly(methyl methacrylate) coating and modifying,” Appl. Phys. Lett. 96(6), 061106 (2010). [CrossRef]
39. P. Bianucci, X. Wang, J. G. Veinot, and A. Meldrum, “Silicon nanocrystals on bottle resonators: mode structure, loss mechanisms and emission dynamics,” Opt. Express 18(8), 8466–8481 (2010). [CrossRef] [PubMed]
40. Y. Wang, V. D. Ta, K. S. Leck, B. H. Tan, Z. Wang, T. He, C. D. Ohl, H. V. Demir, and H. Sun, “Robust whispering-gallery-mode microbubble lasers from colloidal quantum dots,” Nano Lett. 17(4), 2640–2646 (2017). [CrossRef] [PubMed]
41. H. Zhu, X. Chen, L. M. Jin, Q. J. Wang, F. Wang, and S. F. Yu, “Amplified Spontaneous Emission and Lasing from Lanthanide-Doped Up-Conversion Nanocrystals,” ACS Nano 7(12), 11420–11426 (2013). [CrossRef] [PubMed]
