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Abstract
Miniaturized lasing with dynamic manipulation is critical to the performance of compact and versatile photonic devices. However, it is still a challenge to manipulate the whispering gallery mode lasing modes dynamically. Here, we design the quasi-three-dimensional coupled cavity by a micromanipulation technique. The coupled cavity consists of two intersection polymer microfibers. The mode selection mechanism is demonstrated experimentally and theoretically in the coupled microfiber cavity. Dynamic manipulation from multiple modes to single-mode lasing is achieved by controlling the coupling strength, which can be quantitatively controlled by changing the coupling angle or the coupling distance. Our work provides a flexible alternative for the lasing mode modulation in the on-chip photonic integration.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Miniaturized lasing has quite a lot of excellent properties, such as small mode volume [1–3], high quality (Q) factor [4–5], high sensitivity [6–7], manipulation [8–10] and monochromaticity [11–12]. Optimizing these properties, especially the manipulation and monochromaticity, is critical to improve performance of the whispering gallery mode (WGM) lasing [13–15] and versatile photonic devices [16]. In recent years, the single-mode lasing is vital for various photonic devices and physical properties [17–19]. Some strategies have been developed to achieve the single-mode lasing. The single-mode lasing can be achieved by reducing the resonator cavity diameter to micro/nanoscale [20–21]. The free spectral range (FSR) exceeds the photoluminescence (PL) emission range of the gain materials. So that only one lasing mode exists in the cavity. The micro/nanoscale diameter cavity will increase the laser threshold. This is not conducive to reducing laser energy consumption. Normally, the coupled cavity is common microstructure to achieve the single-mode lasing. The single-mode lasing can be achieved based on the Vernier effect [22–29], the parity-time symmetry breaking [30–31], filters [32–34], loss mechanism and passive cavity [35–36], spatially optical pumping [37], distributed feedback structures [38–40] specific cavity structures [41–42] and self-coupled resonator [43–45]. Generally, the coupled cavity microstructures are fabricated by the electron beam lithography (EBL) [46–48] and femtosecond laser direct writing [49–50]. Most of these coupling cavities are two-dimensional plane microstructures, which cannot be dynamically manipulated in real time. Besides, there are two elements affect the coupling strength: coupling angle and coupling distance. Most reports focus on manipulation of the lasing modes by controlling the coupling distance [51–52]. However, the synergistic effect of the coupling distance and the coupling angle on the coupling strength is rarely studied in the coupled cavity.
Here, a designable approach is used to manipulate the WGM lasing in the quasi-three-dimensional coupled cavities. The coupling microstructure is fabricated by self-assembly. The coupled cavity consists of two polymer microfibers. The coupling microstructure can be adjusted dynamically by carrying out micromanipulation. In our experiment, the WGM lasing is successful dynamic manipulation from multiple modes lasing to single-mode lasing by manipulating the coupling angle or controlling the coupling distance in the coupled microfiber cavity.
2. Materials and method
The chemical reagents serve as matrix, which including light-emitting molecules Disodium 4, 4'-Bis (2-sulfonatostyryl) biphenyl (S420, D-36543, Heowns Biochem Technologies, LLC, Tianjin, China), Polyvinyl alcohol (PVA) (S27039-500 g, Beijing HongHu Lianhe Huagong Chanpin Co., Ltd.,), and Cetyltrimethyammonium Bromide (CTAB, 66497D, Adamas, Beijing InnoChem Science & Technology Co., Ltd). The detail of polymer microfiber fabrication progress is as shown in Fig. S1.
Micromanipulation. The micromanipulation is carried out by using a taper probe mounted on a precisely controlled three-dimensional moving stage. The polymer Microfiber A and polymer microfiber B can be controllably by using the taper prober (in Fig. S1). The coupling strength is enhanced in the coupled polymer microfiber A + B cavity. The coupling distance d can be manipulated by using a microprobe (Supporting information in Fig. S2). Finally, the coupled polymer microfiber cavity is obtained as shown in Fig. 1(c). More detail of the micromanipulation can be demonstrated (Supporting information in Fig. S1).
Fig. 1. The concept of dynamic manipulation lasing modes by tailoring the coupling strength in the coupled polymer microfiber cavity. (a-c) The evolution process of WGM lasing is from multiple modes lasing (top) to single-mode lasing (bottom) with decreasing the coupling distance d from coupled microfiber A + B cavities. (d-f) The dynamic manipulation process of WGM lasing is from multiple modes lasing to single-mode lasing with altering the coupling angle α from 90° to 0°.
Figure 1 presents the schematic diagram of the design principle for achieving lasing mode manipulation. The lasing modes can be dynamically manipulated by tailoring the coupling strength. The coupling coefficient is controlled by the coupling distance (in Fig. 1(a)) and the coupling angle (in Fig. 1(d)) in the coupled polymer microfiber cavity. The lasing emission spectra are from multiple modes lasing to single-mode lasing when the polymer microfiber A gradually approaches the polymer microfiber B as shown in Fig. 1(a)-(c). The quasi-three-dimensional coupled cavity is designed by manipulating the taper probe as shown in Fig. 1(d). The lasing modes are evolution from multiple modes lasing (in Fig. 1(d)) to single-mode lasing (in Fig. 1(d)-(f)) by altering coupling angle α from 90° to 0°.
3. Results and discussion
In order to further understand the effect of the coupling strength on the spectral properties, we investigate the evolution of WGM lasing spectra. The coupling strength can be tailored by controlling the coupling distance and manipulating the coupling angle (in Fig. 2). Figure 2(a) demonstrates that the coupling distance can be manipulated by using a microprobe in the coupled polymer microfiber cavity, the more details on manipulation can be shown in video S1. Bright-field microscopy of the polymer microfibers at different preparation stages can be shown in Fig. S1. Figure 2(b) demonstrates the numerically simulated electric field distributions of the lasing modes, which reveals the mode modulation in the microfiber coupled cavity.
Fig. 2. Controllable the micromanipulation fabrication in the coupled polymer microfiber cavity. (a) Bright-field microscopy of the polymer microfibers at different preparation stages (see Visualization 1). Scale bars are 30 µm. (b) The numerically simulated electric field distributions of the lasing modes, which demonstrate the mode modulation in the coupled cavity. (c) The lasing spectrum is from multiple modes lasing (top) to single-mode lasing (bottom). Insets: corresponding PL images of the individual resonator and the coupled microfibers under laser excitation. Scale bars are 30 µm. (d) The lasing spectra in the isolated polymer microfiber. The evolution of lasing mode is from multiple modes lasing (e) with 90° and few modes lasing (f) with 45° to single-mode lasing (g) with 0° in the coupled microfiber A + B cavities. Insets: controllable the micromanipulation fabrication progress in the coupled microfiber cavity.
The PL spectrum of blue-emission in microfiber isolated resonant cavity exhibits a series of multiple mode lasing (in Fig. 2(c), top). One of the lasing modes from microfiber cavity is selected when another microfiber is coupled with the cavity, leading to the blue single-mode lasing (in Fig. 2(c), bottom). The insets show the PL images of the individual microfiber and the coupled microfibers.
Figure 2(d) exhibits the lasing spectra above the lasing threshold in the isolated polymer microfiber. The wavelength of one lasing mode is 445.14 nm. In the experiment, the evolution of lasing mode is from multiple modes lasing (in Fig. 2(e)), a few modes lasing (in Fig. 2(f)) to single-mode lasing (in Fig. 2(g)) in the coupled microfiber A + B cavities. The results demonstrate that the lasing mode can be manipulated by controlling the coupling angle. And the number of lasing mode decreases with altering the coupling angle from 90° to 0°. It is an effect way to achieve the single-mode lasing.
The coupled microfiber microstructure breaks the original propagating modes and instigates their mutual interaction. As a result, the strong coupling between each other microfiber will be described by the coupling coefficient κ. We discuss the coupled cavity frequency of the modes ωa and ωb in each microfiber cavity. In general, the two microfibers are of different diameters. The intermodulation of modes in the coupled cavity can be described by the coupled-mode theory [53]:
where the a and b are their mode amplitudes of microfiber A and microfiber B, respectively. d presents the coupling distance. L is the coupling wavelength∼450 nm.Considering the resonant frequency of ω, the coupling coefficient can be demonstrated in the coupled system.
In order to understand the effect of the coupling angle on the coupling strength, Fig. 3(a) reveals the evolution of coupling coefficient with altering the coupling angle calculating with the MATLAB. We set the coupling distance to zero and the resonant wavelength is ranging from 400 nm to 600 nm. At the same wavelength, the coupling strength decreases with the increase of the coupling angle.
Fig. 3. The relationship between the coupling strength and the coupling angle in the coupled microfiber cavity. (a) The coupling angle evolution of changing the coupling angle with different wavelength using the MATLAB. (b) The numerically simulated electric field distributions of the lasing modes with different coupling angel in microfiber coupled cavity. (e) The evolution of lasing spectra with different pump power density in isolated microfiber cavity. Inset: corresponding bright-field microscopy of the microfiber. The evolution of lasing modes is from multiple modes lasing (f) with 90° and few modes lasing (g) with 45° to single-mode lasing (h) with 0° in the coupled microfiber A + B cavity. Insets: corresponding bright-field microscopy of the coupled microfiber cavity. Scale bars are 100 µm.
The simulation and analysis are accomplished by combining the commercial software COMSOL and three-dimensional mode as shown in Fig. 3(b)–3(d) (Supporting information, in Fig. S3, in Fig. S4). The numerically simulated results demonstrate that the coupling strength almost disappears when coupling angle is 90° (in Fig. 3(b)). Moreover, the strong coupling is existed when the coupling angle is 0° (Fig. S3(d)).
Figure 3(e) exhibits the lasing spectra with intensity-dependent in the isolated polymer microfiber. We can obtain the fluorescence emission (∼25 nm) at low pump density fluence. A series of separating laser peaks are observed with narrow linewidth (< 0.1 nm). The inset shows the bright-field microscopy of the microfiber. The emission spectrum depends on the coupling angle. Here, the coupling angles are fixed at 90°, 45° and 0°, the microfiber diameters are fixed at 21.8 µm and 24.5 µm. The evolution of lasing mode is from multiple modes lasing (in Fig. 3(e)) with 90° and few modes lasing (in Fig. 3(f)) with 45° to single-mode lasing (in Fig. 3(g)) with coupling angle 0° in the coupled microfiber A + B cavity. The coupling strength is enhanced due to the large coupling angle between microfiber pair, which agrees with the theoretical calculation and numerical simulation. The results demonstrate that the lasing mode can be manipulation by controlling the coupling angle.
In order to further investigate the effect of the coupling distance on the coupling strength, the coupling coefficient is calculated with altering the coupling distance as shown in Fig. 4(a). Here, the resonant wavelength is fixed at 450 nm, the coupling strength decreases with the increase of the coupling distance. With the two relative coupled polymer microfibers, Fig. 4(b) presents the numerical simulation electric field distributions with different coupling distance. The results demonstrate that the coupling strength decreases with the increase of the coupling distance (Supporting information, in Fig.S5), which is the consistency of calculation. To construct different gap distances in the coupled polymer microfiber cavities, we can exert a lateral force to control the coupling distance. First, we push two polymer microfibers together by using a taper probe to obtain the coupled polymer microfiber A + B with no/little space between two polymer microfibers. Then, the longer polymer microfiber is pushed away from the axial direction to a certain angle (δ) by the taper probe. Finally, we can get the coupled cavity with different distance. More details of controlling the coupling distance are shown in Fig. S6. The normalized PL emission spectra and absorption of the light-emitting molecules are shown in Fig. S7. To examine the emission spectra of the coupled polymer microfiber, the micro-photoluminescence (µ-PL) system is used (Supporting information, in Fig. S8). The optical multichannel analyzer is with a 2400 g/mm grating.
Fig. 4. The relationship between the coupling strength and the coupling distance in the coupled microfiber cavity. (a) The coupling strength evolution with changing the coupling distance with different wavelength using the MATLAB. (b) The numerically simulated electric field distributions of the lasing modes with different coupling distance in microfiber coupled cavity. (c-k) The evolution of lasing spectra from multiple modes lasing to single-mode lasing with different coupling distance. Scale bars are 60 µm.
The coupling gap distance has a significant influence on the optical coupling and the mode selection modulation in the coupled cavity. To demonstrate the influence of gap distance on mode selection modulation, we record the emission spectra with different coupling distance in the coupled polymer microfiber cavities (Supporting information, in Fig. S9 and in Fig. S10). The monochromaticity of emission spectrum can be improved by manipulating the coupling distance between the two polymer microfibers. Figure 3(c)-(k) show that the evolution of lasing spectra is from multiple modes lasing to single-mode lasing with different coupling distance. The inset shows the dark field microscopy in the coupled microfiber cavity.
The side-mode suppression factor (SMSF) is defined [54].
Miniaturized lasing with linear polarization and reliable stability are important to the photonic devices. To study the polarization anisotropy of the polymer microfiber lasing. The emission spectra are recorded as a function of rotation angle (θ) below lasing threshold (Supporting information, in Fig. S18 and in Fig. S19) and above lasing threshold (Supporting information, in Fig. S20, in Fig. S21). The results indicate that there is a strongly polarization ratio above the threshold [55]. To study the stability of the polymer microfiber lasing, Fig. S22 shows the lasing spectra evolution of peak wavelength, emission intensity and the full width at half maximum (FWHM) with different pump time in the microfiber cavity. The results demonstrate that the polymer microfiber lasing has excellent optical characteristics (Supporting information, in Fig. S23).
The monochromaticity of WGM lasing can be improved by manipulating the coupling angle and controlling the coupling distance in the coupled microfiber cavity. Figure 5(a) shows the coupling strength at different coupling distance and coupling angle. Furthermore, the individual cavities will be enhanced while the other modes will be suppressed in coupled microfibers [56].
Fig. 5. Synergistic effect of the coupled cavity. (a) The coupling strength evolution with changing the coupling distance and the different coupling angle with different wavelength in the coupled microfiber cavity. (b) Schematic diagram of the coupling between two relative coupled microfiber cavity. One microfiber cavity can be served as an optical filter of another microfiber cavity to realize the single-mode lasing. (c) The numerically simulated electric field distributions of the lasing modes in microfiber coupled cavity. (d) The lasing spectra of isolated microfiber A (DA = 21.8 µm) and microfiber B (DB = 24.5 µm). (e) Lasing spectra of the coupled microfiber cavity under different pump power density. Inset: corresponding PL image of the presented microfibers. (f) The normalized intensity and the FWHM of lasing peaks as a function of different pump density. The threshold is 115.2 µJ/cm2.
Here, we propose the coupled microfiber cavity to achieve single-mode lasing operation through the Vernier effect as shown in Fig. 5(b). Figure 5(c) demonstrates the numerically simulated electric field distributions of the lasing modes in microfiber coupled cavity. Figure 5(d) presents the lasing spectra of each isolated microfiber at a pump fluence of 194.2 µJ/cm2. The lasing spectra and threshold of isolated microfiber A (in Fig. S24) and isolated microfiber B (in Fig. S25) can be shown. The diameters of two microfibers are 21.8 µm (DA) and 24.5 µm (DB), respectively. The FSR of the couple polymer microfiber cavity can be calculated by the Vernier equation (DB > DA) [57]:
where λ is resonant wavelength, neff presents the effective refractive index of polymer microfiber, D is the cavity diameter. The resonant wavelength is about 444 nm. The theoretically calculated FSR is about 15.5 nm, which has already exceeded the gain spectral range. Figure 5(e) presents lasing spectra of the coupled microfiber cavity under different pump power density. The inset is the PL image of the presented microfiber cavity. Figure 5(f) shows the normalized intensity and the FWHM of lasing peaks as a function of different pump density. The lasing threshold is 115.2 µJ/cm2. The single-mode lasing is reported by manipulating the coupling angle or controlling the coupling distance as shown in Fig. S26. The polarization of the single mode WGM lasing is confirmed from experimental wise (in Fig. S27).4. Conclusions
In summary, we have demonstrated the dynamic manipulation of WGM lasing by manipulating the coupling angle and controlling the coupling distance in the coupled microfiber cavity. The coupled cavity consisted of two polymer microfibers is adjustable in real time by a micromanipulation technique. The lasing mode can be dynamic manipulation from multiple modes lasing to single-mode lasing by quantitatively controlling the coupling angle or controlling the coupling distance. These results may shed light on the flexible manipulation of the coupling strength of WGM lasers.
Funding
Beijing Municipal Natural Science Foundation (4222066, Z180015); National Natural Science Foundation of China (61822501).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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