Introduction

Owing to its high-beam quality and spectral purity, single-mode lasing is of fundamental importance for various scientific and technical applications. To achieve single-mode operation, some additional optical elements and strategies are required to suppress other competing modes in a lasing cavity. Owing to their high-quality factors and small-mode volumes, whispering-gallery mode (WGM) microresonators fabricated with different geometries (for example, microdisks, microspheres and microtoroids) and materials (for example, glass, semiconductor and polymer) have attracted increasing attention in various areas, including in lasing, sensing and optical communications1, 2, 3, 4, 5, 6, 7, 8. Generally, WGM lasers, which are generated through total internal reflection at the external cavity interface, are usually multimodal due to the lack of mode-selection strategies. To obtain single-mode WGM lasing, one possible strategy is to decrease the cavity dimensions and expand the free-space range (FSR) of the multimodes until only one resonant mode is left; however, this method will reduce the round-trip gain and increase the lasing threshold9. Other strategies, such as using two coupled cavities based on the Vernier effect10, 11, 12, 13 and the parity-time symmetry effect14, 15, have been reported; however, these methods usually require complicated and rigorous fabrication and precise manipulation.

Recently, prolate-shaped bottle microresonators, fabricated from optical glass fibers and capillaries, have attracted much attention and have been applied in interesting ways, such as for compact optical delay lines, cavity optomechanics, electromagnetically induced transparency-like phenomena and optical frequency comb generation16, 17, 18, 19, 20, 21, 22, 23. Compared with other WGM resonator shapes, the unique advantage of bottle microresonators is that their prolate shapes support highly non-degenerated WGMs with spatially well-separated intensity along the long-axis direction, making it possible to select one desired mode as the dominant lasing mode according to their axial-mode numbers and thus suppress all other modes24. In recent years, the direct modification of the surfaces of passive glass bottle microresonators, such as adding liquid drops and inscribing microstructures on the resonator surfaces, to reduce the number of WGMs has been performed25, 26. Nevertheless, it is difficult to incorporate gain materials into these types of glass microresonators for lasing. To our knowledge, single-mode lasing and selection have not yet been achieved. Free-space pumping is a stable and easy approach to coupling light into resonators and generating the lasing action in various narrow and harsh environments, such as a low-temperature cryostat. However, until now, most of the reported WGM lasers that incorporate free-space pumping are multimodal due to the lack of mode-selection strategies.

Polymers are good hosts for various lasing gain dopants because of their advantages of easy processing, mechanical flexibility and low cost12, 27, 28, 29, 30. In this work, we use laser-interference excitation to develop a simple and general approach to realizing single-mode WGM lasing in polymer bottle microresonators. When spatially overlapping the pump stripes with the intensity profiles of single-bottle WGMs, single-mode lasing can be efficiently generated and selected; attractive advantages of this system include a high side-mode suppression factor (SMSF), large tunability and low-lasing threshold.

Materials and methods

Sample fabrication

The gain material used is Rhodamine 6G (R6G)-doped epoxy resin with high viscosity and a refractive index (n) of ~1.53 (Refs 9, 13). Silica microfibers and fiber tapers are drawn from standard optical fibers (SMF-28, Corning, Hickory, NC, USA) using a flame-heating method27, 31, 32. The polymer bottle microresonators are fabricated using a self-assembly procedure, as shown in Figure 1a and 1b. First, a silica fiber taper is used to pick up a microdroplet of R6G-doped epoxy resin solution via micromanipulation27, 31, 32. When the microdroplet touches the suspended silica microfiber, the taper is drawn back, and a thin layer of the solution is left on the microfiber, and the solution shrinks to rapidly form a spindle-like shape due to surface tension (as shown in Supplementary Fig. S1). Finally, the droplet is solidified by heating at 60°C.

Figure 1
figure 1

Fabrication and characterization of polymer bottle microresonators. (a) Schematic and (b) microscope images of fabricating polymer bottle microresonators using a self-assembly procedure. Scale bar=10 μm. (c) Definitions of L, Dout and Dfiber. (d) SEM image of a fabricated polymer bottle microresonator. (e) Shape tuning of bottle microresonators using a fiber taper before heating solidification. Scale bar=10 μm. (f) Microscope image of polymer bottle microresonators with different Dout values from <2 μm to nearly 10 μm.

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Each shape of the as-fabricated bottle microresonators can be determined on the basis of three parameters: bottle outer diameter (Dout), microfiber diameter (Dfiber) and neck-to-neck length (L), as denoted in Figure 1c. Figure 1d shows a typical scanning electron microscope (SEM) image of a bottle microresonator with dimensions of Dout=4.9 μm, Dfiber=2.6 μm and L=9.2 μm, in which excellent surface smoothness is clearly seen. The shapes of the bottle microresonators can be tuned by adding or removing polymer solution using a fiber taper before heating solidification. Figure 1e shows that the Dout of a bottle microresonator is changed from 3.2 to 9.5 μm. Thus, many bottle microresonators with different shapes can be fabricated along the silica microfiber. Figure 1f shows a picture of several bottle microresonators, with Dout changed from <2 μm to nearly 10 μm. In this work, the bottle microresonators with Dout ranging from 3 to 6 μm are investigated.

Laser-interference excitation

The laser-interference excitation approach is illustrated in Figure 2a. First, a 532-nm pulsed laser (5 Hz, 10 ns) is divided into two collimated beams by a beam splitter. After being reflected by several reflection mirrors, the two counterpropagating beams are reflected by the corner of a knife-edge, right-angle prism (MRAK25-F01, Thorlabs, Newton, NJ, USA), which is actuated by a differential micrometer with 0.1 μm sensitivity (DM-25L, Newport, Irvine, CA, USA), to form two parallel beams. Then, the two beams are focused onto a spot with a diameter of ~30 μm through a microscope objective (× 100, NA=0.7) to produce interference patterns on the microresonator surface (the front focal plane of the objective). The bottle microresonators are suspended across a glass channel with a width of ~500 μm. Their photoluminescence (PL) signals are collected using the same objectives and then delivered to a spectrometer (QE65 Pro, Ocean Optics, Winter Park, FL, USA, spectral resolution: 0.7 nm) for spectral analysis and to a CCD camera for image capture31.

Figure 2
figure 2

(a) Experimental setup for laser-interference excitation of polymer bottle microresonators. By moving the differential micrometer (shown in pink), Spump of the interference stripes can be adjusted. By rotating the reflection mirror (shown in pink), the position of the interference stripes along the long-axis direction of the bottle microresonator can be adjusted. (b) Microscope images of interference patterns obtained for four typical Spump values.

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By moving the differential micrometers, the lateral displacement of the two collimated beams can be tuned, which finally determines the incident angle of the two beams at the focal plane and the spacing between interference patterns. Experimentally, the spacing (Spump) between the interference stripes can be precisely adjusted from ~0.7 to 3 μm. Figure 2b shows typical microscope images of interference patterns for Spump values of 0.85, 1.32, 2.15 and 2.89 μm, in which the intensity distribution along the long-axis direction follows sine functions with a visibility close to 1, as is expected (Supplementary Fig. S2). By rotating a reflection mirror (shown in pink), the position of the interference patterns along the long-axis direction of bottle microresonators can also be precisely adjusted.

Results and discussion

Figure 3a shows the PL spectrum of the R6G-doped epoxy resin, indicating a broad gain spectrum with a full width at half maximum (FWHM) of ~48 nm. For a uniform pump that is commonly used, Figure 3b shows typical emission spectra versus the peak-power density of pump light in a bottle microresonator with dimensions of Dout=4.8 μm, Dfiber=2.3 μm and L=7.5 μm. As the pump power increases above the threshold, a multimode-lasing action is observed. The measured FWHM is ~1.6 nm at the dominant peak wavelength (λpeak) of 595.9 nm, which is limited by the spectral resolution of the spectrometer (0.7 nm). In Figure 3c, the measured threshold at λpeak=595.9 nm is 0.09 MW cm−2 and is comparable to most values obtained for WGM microresonators with diameters larger than 10 μm (Supplementary Fig. S3)9, 13, 33, suggesting that decreasing the dimensions of our bottle microresonators will not increase the lasing threshold, which is related to the low optical loss and high quality of bottle microresonators.

Figure 3
figure 3

(a) PL spectrum of R6G-doped epoxy resin. (b) Emission spectra and microscope images of a polymer bottle microresonator (Dout=4.8 μm, Dfiber=2.3 μm and L=7.5 μm) under uniform-pump excitation with different peak-power densities. Upper inset: bright-field microscope image of the polymer bottle microresonator; bottom inset: dark-field microscope image of its lasing generation. (c) Emission intensity at the 595.9-nm dominant peak versus the pump peak-power density.

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As illustrated in Figure 4a, the prolate shape of a bottle microresonator supports highly non-degenerated WGMs (transverse modes) with spatially well-separated intensity along the long-axis direction16, 17, 18, 24, 25, 26. Under the action of a uniform pump, the modes within the gain curve of resonators can be excited and a multimode-lasing behavior is observed (as demonstrated in Figure 3b). By using small-bottle microresonators with Dout <6 μm, the large FSR can keep only several transverse modes within the whole-cavity resonance range by pushing all other modes to the edge or out of the gain range, as shown in the right schematic of Figure 4a. Because different transverse modes have markedly different axial distributions, by engineering the pump intensity such that it produces an interference distribution on the microresonator surface (Figure 4b), the spacing between and axial positions of pump patterns can be spatially overlapped with the intensity profile of a desired WGM. Therefore, only the excited WGMs can lase, and other competing modes are suppressed. It is evident that such a bottle WGM laser is intrinsically single mode (right schematic of Figure 4b).

Figure 4
figure 4

Principle of single-mode WGM lasing in a polymer bottle microresonator. (a) Multimode-lasing behavior under the action of a uniform pump. Right schematic shows that by using small-bottle microresonators, the large FSR can keep only several transverse modes within the entire cavity resonance range by pushing all other modes to the edge or out of the gain range. (b) By engineering the pump intensity to modify the spatial gain profiles of bottle WGMs, the mode intensity profiles of single-bottle WGMs can be spatially overlapped with the pump stripes, intrinsically enabling single-mode lasing (right schematic).

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To demonstrate this principle, the black line in Figure 5a (bottom) shows a representative lasing spectrum under the action of a uniform pump, measured in a bottle microresonator with dimensions of Dout=4.7 μm, Dfiber=2.7 μm and L=7.5 μm, in which four strong lasing peaks are denoted. Its corresponding microscope image is shown in Figure 5b. Each bottle’s WGMs can be defined by the axial number q and the azimuthal number m, which represent the number of intensity maxima along the axis and the half number of intensity maxima in the plane perpendicular to the axis, respectively18, 24. By using a three-dimensional finite-difference time-domain (FDTD) method34, we calculate the electromagnetic field distribution in the cavity. The simulations reveal that the peak of 1 (597.9 nm) corresponds well to the transverse-magnetic polarization mode , and the peaks of 2 (600.5 nm), 3 (605.2 nm) and 4 (609.6 nm) correspond well to and , respectively (Supplementary Fig. S4). The FSR of the bottle microresonator is calculated as 17.2 nm, which fits the function FSR=λpeak2/πnDout well and confirms the WGM lasing mechanism. Figure 5c shows the electric field-intensity distributions of modes 1−4 on the cross-plane of the bottle microresonator along its axis direction. The intensity of the mode is concentrated at the centerline, and the intensities of the and modes are distributed symmetrically at both sides of the centerline, with distances of 0.9, 1.4 and 1.8 μm between the two maximum intensity positions, respectively.

Figure 5
figure 5

Observation of single-mode WGM lasing in a polymer bottle microresonator. (a) Lasing spectra and (b) their corresponding microscope images of a polymer bottle microresonator (Dout=4.7 μm, Dfiber=2.7 μm and L=7.5 μm) under the action of a uniform and a laser-interference pump. The colored solid lines are recorded for different P1 patterns, and the colored dashed lines are recorded for different P2 patterns. The black dotted line in b denotes the centerline of the bottle microresonator. (c) Electric field-intensity distributions of and modes on the cross-plane of the bottle microresonator along its axis direction.

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For the case in which the spacing between and axial positions of interference patterns are changed, the lasing spectra and their corresponding microscope images are as shown in Figure 5a and 5b. For Spump=2.06 μm, when one stripe is located at the microresonator centerline (which can be simply called the P1 pattern), a pronounced single-mode lasing emission with λpeak=597.6 nm emerges. The SMSF is ~9.3 dB, and the FWHM is ~1.6 nm. The single bright spots at both the top and bottom edges of the bottle microresonator (Figure 5b) are attributed to the light scattering due to the imperfect surface quality and are expected to reveal the electromagnetic field-intensity distribution inside the bottle microresonator (Figure 5c). Obviously, the scattering pattern matches the intensity distribution of the mode well. When Spump=2.25 μm, a single-mode emission with λpeak=597.4 nm and a SMSF of 16.2 dB are achieved, which is also due to the mode. Another transverse mode can be selected when two stripes are symmetrically located on both sides of the centerline (which can be simply called the P2 patterns). For Spump=1.89 μm, an obvious single-mode lasing emission emerges, with λpeak=608.9 nm and a SMSF of 9.9 dB. The four bright light spots at both the top and bottom edges share the same signature as the intensity distribution of the mode. When Spump=2.06 μm, a single-mode emission with λpeak=609.3 nm and a SMSF of 6.7 dB is observed, which also arises from the mode.

If we further increase the spacing, the widths of the stripes become so large that the discrepancy of overlapping with different modes vanishes, which induces multimode lasing. As shown in Figure 5a and 5b, when using the P1 pattern with Spump=2.39 μm, two strong peaks corresponding to the and modes are observed for P1 patterns, whereas when using the P2 pattern with Spump=2.25 μm, three strong and transverse modes are observed. Similarly, when using a small spacing, such as Spump=1.77 μm, the dense stripes induce multimode lasing (Supplementary Fig. S5).

By carefully tuning the spacing between and axial positions of pump patterns, a single-mode lasing with a SMSF as high as 21.7 dB is obtained from a bottle microresonator with dimensions of Dout=4.5 μm, Dfiber=1.6 μm and L=5.1 μm (Figure 6a). The selection of single modes can be further improved by using ultranarrow-linewidth pump lasers or by using optimized pump patterns that have been used in single-mode random lasers35, 36, 37. The thresholds of the selected single-mode lasing are also investigated. As shown in Figure 6b, the measured threshold of a bottle microresonator (Dout=4.4 μm, Dfiber=2.1 μm and L=5.4 μm) is ~1.62 MW cm−2 under the action of a uniform pump and reduces to ~1.07 MW cm−2 under the action of a P1-pattern pump, which occurs due to the high overlap of the pump stripes and the WGM intensity profiles that make the pump energy usage more efficient.

Figure 6
figure 6

(a) Single-mode lasing with a SMSF as high as 21.7 dB, obtained from a bottle microresonator (Dout=4.5 μm, Dfiber=1.6 μm and L=5.1 μm). Inset shows its lasing microscope image. (b) Lasing threshold comparison between the uniform and the P1-pattern pump conditions. Insets show the lasing microscope images for (upper) the uniform and (bottom) the P1-pattern pump. (c) Higher-order single transverse-mode lasing from a more prolate bottle microresonator (Dout=4.6 μm, Dfiber=3.7 μm and L=10.4 μm). Inset shows its lasing microscope image. (d) Spectral shift of a lasing peak from 595.8 to 587.4 nm for a tensile strain increase from 0 to ∼7.3%. Upper inset shows a schematic of strain sensing; bottom inset shows its lasing microscope image.

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This laser-interference excitation approach is very general and reproducible and, in principle, valid for almost arbitrary small-bottle microresonators (here, Dout is <6 μm). For example, Figure 6c shows a higher-order single transverse-mode lasing from a more prolate bottle microresonator with dimensions of Dout=4.6 μm, Dfiber=3.7 μm and L=10.4 μm, for which the SMSF is measured to be 14.1 dB at λpeak=590.5 nm. In addition, the microfiber that supports the bottle microresonators is not limited to a silica material. For example, Figure 6d shows single-mode lasing with λpeak=595.8 nm from a bottle microresonator (Dout=4.9 μm) with a 2.6-μm-diameter poly(vinyl chloride) (PVC) microfiber32. Benefitting from the high-mechanical pliability, the lasing emission can be tuned by pulling the PVC microfiber along its length direction (Supplementary Fig. S6)38. For a tensile strain increase from 0 to ∼7.3%, the 595.8-nm lasing peak shifts to 587.4 nm while maintaining its FWHM, which corresponds to a change of over 50% of the FSR (~15.3 nm) and suggests a feasible approach to broad, tunable single-mode lasers.

Conclusions

In conclusion, we have demonstrated intrinsic single-mode WGM lasing in polymer bottle microresonators by engineering the pump intensity to modify the spatial gain profiles of bottle WGMs. When their mode intensity profiles are spatially overlapped with the pump stripes, single-bottle WGMs can be efficiently selected to lase, with attractive advantages including a high SMSF (over 20 dB), large tunability (over 8 nm) and low-lasing threshold. Only two parameters, that is, the spacing between and positions of interference stripes, need to be adjusted. In addition, this approach is precise and reversibly controllable and does not need a complex facility design and expensive components. Moreover, the mechanism can, in principle, be adopted for other types of bottle microresonators, such as hollow-bubble or tube resonators20, 23, 24, 25, and for other gain media, such as graphene quantum dots39, up-conversion nanocrystals40 and perovskite nanoparticles41 ranging from their solid form to their liquid form42. Therefore, our simple and general approach may have a variety of promising applications, ranging from tunable single-mode lasing and sensing to nonlinear optics43.