Ultraviolet single-frequency lasing is realized in a coupled optofluidic ring resonator (COFRR) dye laser that consists of a thin-walled capillary microfluidic ring resonator and a cylindrical resonator. The whispering gallery modes (WGMs) in each resonator couple to each other and generate single-frequency laser emission. Single-frequency lasing occurs at 386.75 nm with a pump threshold of 5.9 μJ/mm2. The side-mode-suppression ratio (SMSR) is about 20 dB. Moreover, the laser emits mainly in two directions, and each of them has a divergence of only 10.5°.
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Ultraviolet (UV) microcavity lasers have been under wide investigation in the past few years because of their broad applications in high-tech industrial and medical applications, such as optical data storage, nano-fabrication, high-resolution printing, biomedical instrumentation and fluorescence spectroscopy [1–5]. Ideally, a high-performance UV laser possessing of good monochromaticity, i.e. narrow spectral linewidth, is desired to detect the specific proteins by accurate measurement of spectral absorption, differentiate small molecular rotational energy in Raman spectroscopy, enhance the visibility of the interference fringes in the interferometry and focus beam to smaller spot in high-density storage or acousto-optic deflector. To achieve a narrow spectral linewidth, much more attention has been paid to develop the single-frequency microcavity lasers. Generally, a single-frequency operation can be realized by reducing the length of a resonator to make the free spectral range (FSR) of the cavity exceed the spectral width of the gain medium, such as the vertical-cavity surface-emitting lasers (VCSELs) and nanocavity lasers [6–9]. Other methods include the use of distributed Bragg reflectors (DBRs) and distributed feedback (DFB) gratings [10, 11]. Moreover, the photonic-crystal defect microcavities are carried out by controlling of photonic band gaps to confine only one lasing mode . Unfortunately all these approaches rely on sophisticated fabrication techniques, and sometimes on materials. Moreover, the low cavity quality (Q) factor results in a relatively high lasing threshold.
Whispering gallery mode (WGM) lasers use total internal reflection to trap light in round-shaped cavity boundary and form high-Q (>106) resonances. The cavity structure works generically for all wavelengths and materials. However, single WGM lasing occurs only in very small cavities . In our previous work, we demonstrated that by using two coupled cavities that have slightly different sizes, it is possible to generate a single-frequency laser emission .
A great challenge in generating single-frequency UV laser emission by using coupled WGM resonator is the control of the coupling [14, 15]. It is much more difficult to control the coupling in UV region compared to that in the visible region. For example, the coupling coefficient between two 1st order radial WGMs in the contacted 125 μm-diameter and 140 μm-diameter cylindrical resonators, κ, is calculated to be 8.9 × 10−3 (at 600nm) and 4.1 × 10−3 (at 390nm) by the coupled mode theory respectively [16, 17]. On the other hand, if there is a tiny gap of 200 nm between the resonators, κ600nm decreases to 1.3 × 10−4 while κ390nm decreases to 4.8 × 10−6, which is 68 and 850 times lower than that of contacted resonators. This demonstrates that κ is much more sensitive to the change of coupling region in the range of shorter wavelength (UV region).
We found that the coupled cavity structure described in our earlier work not only has excellent coupling performance even in UV region, but also enables cavity structure varieties. As an example, in this paper, we demonstrated the single-frequency UV laser emission in a coupled optofluidic ring resonator (COFRR). It works on the same principle of single frequency selection via Vernier effect, but has additional advantages of ultralow lasing threshold and much better photo-stability, which allows integration with micrototal analysis systems (μTAS) for biochemical analysis.
2. Experiment and result
Figure 1 shows a schematic of the COFRR laser [Fig. 1(a)] and the measurement [Fig. 1(b)]. The COFRR laser consists of a thin-walled capillary ring resonator and a cylindrical resonator. The capillary ring resonator has the gain medium (liquid dye) in the tube, and the cylindrical resonator works as a wavelength-selective attenuator. In the fabrication process, a segment of commercial glass fiber (125 μm diameter, 2 cm long) is stacked in parallel with a fused silica capillary (140 μm outside diameter, 20 μm thick wall). The capillary is then etched with low concentration of HF for several hours to achieve a wall thickness of about 2 μm, and serves as an OFRR dye laser. Since the wall is thin, the WGM profile in the circular cross section of capillary overlaps with the gain medium and is easily coupled to the nearby cylindrical resonator (glass fiber) by the evanescent wave. When both of the resonators are on resonance, the attenuation is at minimum and a single-frequency laser emission emerges when the effective FSRs of the coupled cavity is large enough to suppress the multimode resonances within the gain spectrum.
We first measured the Q-factor of the thin-walled capillary resonator to characterize the roughness of the inner surface of the capillary. Since the straight tapered fiber cannot function well in the shorter wavelength regime , we coupled the light from a 1550 nm tunable diode laser to the capillary through a tapered fiber, which indirectly quantify the fabrication quality. The spectral linewidth is about 0.6 pm, corresponding to a Q-factor of 2.6 × 106, as shown in Fig. 2(a) . The high Q-factor demonstrates that the inner surface of the capillary is still very smooth after HF etching. Then dimethyl sulfoxide (DMSO, refractive index (RI) n = 1.501 at 385 nm) with 1 mM laser dye LD390 flows through the capillary by a syringe pump at a rate of 15 μl/min. The dye laser is optical pumped by a 355 nm pulsed light coming from a mode-locked Nd:YAG laser (EKSPLA, 30 ps pulse width and 10Hz repletion rate). Laser emission is collected through free space by a multimode fiber bundle and is delivered to a monochromator (Acton spectrapro 2750) equipped with a scientific CCD-array detector (Andor iXonEM). The measurement system has a resolution of 0.03 nm. Figure 2(b) shows the OFRR laser emission spectrum in the wavelength range from 385 nm to 400 nm. The average FSR of WGMs is Δλ~0.25 nm, corresponding to an effective RI n1eff = λ2/(ΔλπD1) = 1.405, where D1 = 140 μm is the outer diameter of the ring resonator. Figure 2(c) is the radial intensity profile of the WGM of the 8th order near 390 nm using a theoretical model based on Mie theory . This mode has an effective RI of 1.402, which agrees well with the experiment result. The strong electric field concentrated within 4 μm near the inner wall surface interacts with the dye and provides excellent optical feedback for lasing. Figure 2(d) plots the relation between the output light intensity and pump energy density. The lasing threshold is about 0.77 μJ/mm2, which is an order of magnitude higher than the reported lowest lasing threshold of the OFRR laser, because UV dye (LD390) has much lower fluorescence quantum yield than that of Rh6G that was used in Ref .
When the capillary optofluidic dye laser is coupled to a cylindrical resonator and forms a COFRR dye laser, the laser emission spectra totally change. Figure 3(a) presents the schematic of the coupled WGMs in the COFRR laser. At a high pump level of 19.1 μJ/mm2 [Fig. 3(b)], the emission spectrum has multimodes with clear wavelength modulation that comes from Vernier effect and wavelength dependence of the coupling . The spectral separation between the modulated spectrum envelopes is 3.3 nm, which agrees well with the FSR calculated based on the Vernier equation16], an effective Vernier effect based coupling is guaranteed. When the pump energy density decreases to 8.8 μJ/mm2, the laser runs in single longitudinal mode, and the side-mode-suppression ratio (SMSR, = 10lg(Im/Is), where Im and Is are the laser power of the main mode and laser power of the side modes respectively) is 19.6 dB [Fig. 3(c)]. The stable single-frequency laser emits at about 386.75 nm with a linewidth of 0.12 nm. Figure 3(d) shows the relation between the output light intensity and pump energy density. The single-frequency lasing threshold is Pth = 5.9 μJ/mm2, and a side-mode starts to emerge at a pump of Psth = 8.9 μJ/mm2. Therefore the ratio of the side- and main-mode lasing threshold (Psth/Pth) is as large as 1.5, even higher than that we reported earlier in the visible region , which demonstrates that the coupled cavity structure has excellent coupling performance in the UV region. On the other hand, the lasing threshold is obviously larger than that of the single-cavity OFRR laser, because light is scattered from the coupling region. We will show later that the scattering contributes to strong directional laser emissions, which is one advantage of the coupled cavity.
We also measured the far field and near field emission patterns of the COFRR laser when it is in single-frequency emission. Far field emission pattern was measured by placing a detector 3 cm away from the sample and rotating 0°-180° in the horizontal plane at 5° intervals. Since the coupled cavity has one symmetric axis (x axis), the laser emission in the other half space is its mirror-image. Figure 4(a) shows the measured far-field emission pattern. The emission consists of two main narrow emission peaks at θ = ± 7°; each one has a divergence of only 10.5°. Define U as the emission fraction of light in the two main directions, we have U = 52.3%. In addition, at 2.2 μJ pump intensity, the single-frequency laser output energy in free space is measured to be about 0.25 nJ, corresponding to a 0.011% light-light efficiency. Near field emission pattern was obtained by using an objective lens and a charge coupled device (CCD) camera. Figure 4(b) is the near-field image of the single-frequency emission viewed at the angle of 7°. We found that the UV light comes out through the cylindrical resonator. Combining the far field and near field emission patterns, we may conclude that the directional emissions possibly come from scattered laser light at the coupling region of the COFRR and are focused by the cylinder to more or less collimated lasing beams, as illustrated in Fig. 4(c).
The generation of single-frequency laser emission sensitively depends on the cavity size difference. When two cavities have very close sizes, i.e. D1 ≈D2, we have Δλ >> FSR1, 2 = λ2/πD1,2n1,2eff from Eq. (1). Therefore the single-frequency resonance inside the gain spectrum is better guaranteed. Meanwhile both cavities have similar resonant mode fielddistributions that provide sufficient overlap, leading to stronger coupling. Figure 5(a) shows the single-frequency laser emission from a COFRR laser in which the sizes of the OFRR (capillary) and the cylindrical resonator (glass fiber) are both 125 μm. Since the high-order radial WGM in the capillary has slightly different effective RI from that of the fiber, Vernier effect is still observed but the multimode suppression is stronger. Figure 5(b) plots the lasing threshold curve with a higher ratio of the side- and main-mode lasing threshold of 1.7.
In conclusion, we demonstrated single-frequency UV whispering gallery lasing from a COFRR laser. Coupled microcavity structure provides a simple and effective way to avoid the challenging fabrication of DBR mirror of high reflectivity in the deep UV region and nanostructures.
This work is supported in part by National Natural Science Foundation of China (grant # 60977047, 60907011, 61078052, 11074051), National Basic Research Program of China (973 Program) (grant # 2011CB921802) and Natural Science Foundation of Shanghai (grant # 09ZR1402800).
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