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Abstract
We present an optofluidic droplet dye laser that is generated by an array of microfluidic nozzles fabricated on a polycarbonate chip. A droplet resonator forms upon pressurizing the nozzle backside microfluidic channel. Multimode low-threshold lasing is observed from individual microdroplets doped with dye. Additionally, droplets can be conveniently released from the nozzle by water rinsing from the top microfluidic channel and subsequently regenerated, and thus achieve optofluidic lasers on-demand. Our work demonstrates a new approach to generating on-chip laser source and laser arrays in a simple, reproducible, reconfigurable, and low-cost fashion.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Optofluidic lasers hold great promise in the development of miniaturized coherent light source on chip. The liquid in the laser system not only offers the unique capability in laser spectral tuning and reconfiguration [1–8], but also opens up a wide variety of new applications in biosensing and medical imaging [9–12]. A number of cavity configurations have been studied in optofluidic lasers, including Fabry-Perot cavities [7,11–13], distributed feedback gratings [4, 5, 14], capillary ring resonators [9, 15–17], droplets [1–3, 6, 18–22], and photonic crystals [23]. Droplet microcavities can be conveniently created by using two-phase fluids, liquid droplet in air [2, 18, 19] or liquid droplet in another immiscible liquid [1, 3, 6, 20–22], which possess a proper refractive index contrast between the droplet and its surrounding medium. Due to surface tension, the droplet has smooth surface at the two-phase interface (air-liquid or liquid-liquid) and a perfect spherical shape, which supports whispering gallery modes (WMGs) to achieve lasing.
Lasing from droplets was first observed in a pioneering study by Chang et al. from a stream of free-falling dye-doped ethanol droplets in air [18]. To prevent droplet evaporation in air, later on, droplet-based optofluidic lasers were commonly made with immiscible two-phase liquids. Lasing from individual stationary droplets that are manipulated by levitation [20] and optical trapping [3, 24], and are suspended on hydrophobic or hydrophilic surface [2, 19, 22] have been demonstrated in recent years. Lasing emission from the same stationary droplet can be continuously monitored over a prolonged period of time, which offers a unique capability when utilizing droplet lasers for biosensing applications. However, stationary droplets generated in these studies often have a large variation in droplet size. The generation process is not repeatable. Additionally, instrumentation used in droplet levitation and optical trapping is complex, which is challenging to be integrated with a droplet laser on chip. In contrast to stationary droplets, another type of droplet laser is based on a stream of droplets suspended in carrier liquid that flow in microfluidic channels [6, 7]. These monodispersed droplets can be generated with flow-focusing [25], T-junctions [26], and co-flowing structures [27] at frequencies up to 100 kHz. The advantages of this laser system are the capability of high-speed droplet generation and switching, and the potential to perform high-throughput on-chip spectroscopy and bioanalysis. However, droplet-on-the-flow limits the capability of tracking lasing emission from the individual droplet. High droplet generation and switching frequency pose a limiting factor that individual droplet is only interacting with the excitation light for a time duration in the range of microsecond to millisecond, which is not suitable for biosensing applications such as the development of micro-total analysis system using droplet laser-based assays.
Here, we demonstrate an optofluidic droplet laser with combined advantages of stationary droplet laser and droplet-on-the-flow laser, which is capable to reproducibly generate stationary droplets with well-controlled size and regenerate on-demand. The optofluidic droplet laser is generated by the microfluidic nozzle structure with two-phase immiscible liquids. As illustrated in Fig. 1(a), an array of four identical nozzle structures are fabricated on a chip. The microfluidic channel is created on the backside of the chip to deliver the oil phase to the nozzles. A separate microfluidic channel is created on the topside of the chip, where aqueous phase is flowed in and out of the channel. The topside and backside microfluidic channels are connected through the nozzle holes. When a positive pressure is applied to the backside channel, the oil phase emerges from individual nozzles and simultaneously form an array of oil droplets in water (aqueous phase). The microdroplet has a refractive index (e.g., immersion oil, n = 1.515) higher than that of the surrounding liquid (water, n = 1.334), and thus forms an optical microcavity that supports whispering gallery modes (WGMs) through total internal reflection at the two-phase liquid interface. When a gain material (e.g., organic dye) is added to the oil phase, upon excitation, fluorescence emission that is coupled into the WGMs is amplified in the droplet and thus achieve lasing when the gain surpasses the loss. Due to the low absorption coefficient of the oil phase (α <10−3 cm−1), the smooth surface formed at the liquid-liquid interface, and minimal contact region between droplet and nozzle, low-threshold lasing can be achieved in the droplet microcavity. Droplets generated on the nozzles can be conveniently removed by the initiation of water flow in the topside microfluidic channel and subsequently regenerated. The process of generation and regeneration is simple, reproducible, and well-controlled. The micro-nozzle chip with integrated microfluidics presents a versatile platform to achieve reconfigurable and regenerable optofluidic laser array.
Fig. 1 (a) Illustration of optofluidic droplet laser. Oil phase and water phase are delivered through Inlet 1 and Inlet 2 into the backside and top microfluidic channels, respectively. Upon pressurizing backside channel, oil droplets are generated on top of the micro-nozzle structures in the top channel. Droplets can be released from the nozzle by water rinsing in the top channel and regenerated subsequently. Inset: zoom-in image of one micro-nozzle with droplet. (b) The schematic illustration of the experimental setup. Pulsed optical parametric oscillator (OPO) (repetition rate: 20 Hz, pulse width: 5 ns, wavelength: 532 nm) is used for optical excitation. BS: beam splitter. L1, L2, L3, and L4 are lenses with a focal length of 5 cm, 5cm, 2.5 cm, and 2.5 cm, respectively. The fluorescent and lasing emissions from the droplets are sent to a spectrometer. The droplet is imaged by a camera, from which the droplet size is characterized. Green lines indicate pump light. Red lines indicate the emission from the droplet. Yellow lines indicate the illumination light.
To demonstrate the concept of microfluidic nozzle droplet platform, polycarbonate (PC) is used in this work. However, the concept and the device design can be readily extended to silicon, with devices made by MEMS fabrication to achieve a reduced device footprint. PC is a commonly used material in microfluidic devices in biomedical and bioanalytical applications [28, 29]. Additionally, PC is selected in prototyping our laser system because of its low cost, good machining properties, high glass transition temperature (Tg~145 °C), and transparency in the visible spectral range [30]. Due to good machining properties of PC, the fabrication of microfluidic nozzle structures is simple and straightforward. First, oil phase microfluidic channel is created on the backside of the device, as illustrated in Fig. 2(a), using a milling machine. Second, four identical nozzle holes are drilled from device front side to reach the backside channel, as shown in Fig. 2(b). Third, nozzle ring surrounding each nozzle hole are created through milling by carving out a ring-shaped trench at the outer boundary of the nozzle ring, as shown in Fig. 2(c). The nozzle ring is designed to minimize the contact between droplets and the nozzle substrate. Fourth, the backside microfluidic channel inlet hole is drilled from the device front side as shown in Fig. 2(d). Afterwards, the nozzle PC chip and another PC substrate are sonicated in ethanol for 30 minutes, then in deionized (DI) water for 15 minutes. After air dry, both PC chips are exposed to dichloromethane (DCM) (Sigma-Aldrich) to swell the surface in a vacuum chamber for 30 minutes to facilitate thermal bonding in the next step [30]. The PC nozzle and PC substrate are subsequently bonded together at 125 °C for 45 minutes, as depicted in Fig. 2(e). At last, the bonded device is immersed in ethanolic solution of SnCl2 [20% (w/w)] [31] for 8 hours and flushed with water for 5 minutes. Through this treatment, the surface of microfluidic channels becomes hydrophilic, thus decreasing the interfacial tension between microdroplet and the nozzle hence minimizing the contact area between them, which helps maintain a good Q-factor of the droplet microcavity. The surface remains hydrophilic and stable for at least 10 days when the microfluidic channels are immersed in DI water. Similarly, the topside microfluidic channel and the channel inlet and outlet are created on another PC chip and is bonded to the device shown in Fig. 2(e). The final device structure is shown in Fig. 2(f).
Fig. 2 Fabrication process flow of the micro-nozzle optofluidic laser device made of polycarbonate chip.
The fabricated nozzle chip before bonding to the topside microfluidic channel is shown in Fig. 3(a), where an array of 1x4 nozzles are created. The dimensions of nozzle and backside channel are shown in Figs. 3(b) and 3(c). The diameter of nozzle hole is 282 μm measured at nozzle top surface and 239 μm measured at the bottom surface. The slightly larger diameter at the top surface is caused by the tapered shape of the drill bit when drilling is conducted from the topside of the device to the backside. The nozzle ring outer diameter is 1.239 mm. The backside microfluidic channel width is 566 μm. To generate the droplet array, a home-built stepwise pump, consisting of a micrometer (Newport) and a 1 μL syringe (Hamilton), is used to deliver the oil phase to the nozzle. The pressure applied to the nozzle can be fine-tuned with a single step injection volume of nL to control and maintain the droplet to the desired size. Organic dye Nile Red (Sigma-Aldrich) dissolved in immersion oil (Sigma-Aldrich) is used as the gain material in the droplet laser. As shown in Fig. 3(d), with the proper control of the injection volume, an array of dye-doped droplets are generated simultaneously on top of each nozzle. The spherical shape of the droplet is well maintained with minimized contact area between droplet and nozzle substrate. Contact angle of these four droplets are characterized using a side-view camera with a customized setup [22], which reveals an average contact angle of 151.1° ± 2.4°. The size of four droplets are identical with a diameter of 1.584 ± 0.058 mm. Additionally, different sizes of droplets can be generated by controlling the injection volume to the backside channel, as shown in Fig. 4(a). When injection volume increases from 50 nL to 300 nL, the diameter of the droplet increases accordingly. To demonstrate the generation reproducibility, a total of 40 droplets are generated in sequence, with 10 droplets in each size group. The variation in size for these droplets are shown in Fig. 4(b). The size measurement for these droplets are 429 ± 53 μm, 564 ± 32 μm, 646 ± 18 μm, 777 ± 7.7 μm, respectively, in diameter.
Fig. 3 (a) An image of 1x4 micro-nozzles made on polycarbonate chip before bonding to the topside microfluidic channel. (b) Top view of the device (image of the area marked in (a)), showing the nozzle hole diameter of 282 μm and the nozzle ring outer diameter of 1239 μm. (c) Backside view of the device (image of the area marked in (a)), showing the backside microfluidic channel width of 566 μm and the nozzle hole diameter of 239 μm. (d) Generation of a droplet array. The average contact angle of the four droplets is 151.1° ± 2.4°.
Fig. 4 Characterization of droplet generation. (a) The relationship between injection volume applied to the backside channel and the size of the droplet generated on the nozzle. The error bar is obtained on ten droplets generated in sequence for each injection volume. (b) The droplet diameter variation at different injection volumes. A total of 40 droplets were measured.
To study the lasing performance, 500 μM Nile Red in immersion oil is used for laser characterization. As shown in Fig. 1(b), a confocal setup is used to excite the microdroplets with a pulsed optical parametric oscillator (OPO) (repetition rate: 20 Hz, pulse width: 5 ns, wavelength: 532 nm, Continuum Surelite). Pump energy is adjusted by a variable neutral density filter. The pump laser beam spot size is 10 mm2, focused by an objective lens (40x, NA = 0.60). The laser emission signal from the microdroplet is collected via the same objective and sent to a 90/10 beam splitter. 90% of the emission light goes to the spectrometer (Horiba iHR320) for spectrum analysis and 10% of the emission light goes to the CCD camera for droplet monitoring and imaging. The emission spectrum of the droplet is obtained with single pulse excitation from the OPO laser to minimize photobleaching effect. Figure 5(a) shows the lasing spectra measured by the spectrometer using 600 g/mm grating from a droplet of 563 μm in diameter under pump energy densities varied from 5.7 μJ/mm2 to 100 μJ/mm2. Initially, fluorescence emission is observed at low pump power. Lasing emission is observed when pump energy density is at 15.1 μJ/mm2 and above. The lasing peaks occur at the longer wavelength side of the Nile Red fluorescence spectrum. The maximal intensity is centered around 635 nm for lasing emission as opposed to 610 nm for fluorescence emission. The absorption spectrum of Nile Red has a peak at 542 nm and tails around 620 nm and beyond. Thus, the laser gain profile maximum is at the low absorption region of 620 – 650 nm of Nile Red, where lasing emission should emerge first. According to the size of the droplet, free spectral range (FSR) of the WGMs supported by the droplet cavity can be calculated as
where λ is the center wavelength of lasing emission spectrum (), is the refractive index of immersion oil (), and D is the diameter of the microdroplet (). The FSR cannot be fully resolved due to the limited spectral resolution of the spectrometer (spectral resolution 0.7 nm), which agrees with the measured lasing spectra in Fig. 5(a). After subtracting fluorescent background from the lasing spectrum, the integrated lasing intensity of each lasing spectrum is plotted as a function of pump energy density, as depicted in Fig. 5(b). The lasing threshold derived from the linear fitting is approximately 12 μJ/mm2.Fig. 5 (a) Fluorescence and lasing emission spectra from a 563 μm diameter microdroplet under different pump energy densities. All spectra are taken under a 600 g/mm grating. (b) The plot of integrated lasing intensities as a function of pump energy density. The lasing threshold derived from the linear fitting is approximately 12 μJ/mm2. (c) Lasing emission spectra from a 544 μm diameter regenerated microdroplet under different pump energy densities. All spectra are taken under a 1200 g/mm grating. Traces are shifted vertically for clarity. (d) The plot of integrated lasing intensities as a function of pump energy density. The error bar is obtained on three spectra excited under the same pump energy density. The lasing threshold derived from the linear fitting is approximately 10 μJ/mm2. Insets in (b) and (d) are the side-view optical microscope images of the measured microdroplets. The scale bar is 250 μm in both images.
With integrated top microfluidic channel, the droplets can be conveniently removed from the nozzle by initiation of a water flow. After water rinsing, another droplet is subsequently regenerated, as shown in the inset of Fig. 5(d). The size of this droplet is 544 μm in diameter. The lasing emission from the droplet is characterized with the spectrometer using 1200 g/mm grating (spectral resolution 0.3 nm). The lasing spectra under various pump energy densitiesare presented in Fig. 5(c). Three lasing spectra are taken for each pump energy density to characterize the mean and standard deviation of the lasing emission intensity. The lasing threshold curve is plotted in Fig. 5(d). Saturation effect is observed for pump energy density above 120 μJ/mm2. The lasing threshold derived from the linear fitting below 120 μJ/mm2 is approximately 10 μJ/mm2. The lasing threshold demonstrated here is on par with that of other droplet optofluidic lasers reported previously [6, 19–21]. The Q-factor of the droplet is estimated to be in the range of 104-105 from theoretical modeling [22] and by comparing our lasing threshold to the ones reported in the previous work in which Q-factor is characterized experimentally [13]. The contact angle of the droplet measured is around 120°, which is smaller than that of the droplets shown in Fig. 3(d). The decrease in contact angle is due to relatively large size of the nozzle hole in comparison to the droplet size. The contact region can be further minimized (increase the contact angle) by decreasing the nozzle hole size either through improvement of micromachining precision or fabricating nozzle devices on a silicon chip using MEMS technologies.
As demonstrated in Fig. 4, droplets of a variety of sizes can be generated by controlling the injection volume to the backside channel. To characterize the lasing emission from different sizes of droplets, the emission spectra are measured when the droplets are excited at the same pump energy density, which is at ten times above the lasing threshold, while gradually increasing the droplet size. Similar lasing spectra are observed from different sizes of droplets, as presented in Fig. 6. Only fluorescence emission is observed from a droplet of 491 μm in diameter, which is due to relatively large nozzle hole size in comparison to the droplet. When droplet size increases, lasing emission starts to emerge. The intensity of the lasing emission increases gradually with the size of the droplet. However, lasing emission spectra is observed in the range of 620 – 650 nm, regardless of the droplet size.
Fig. 6 The emission spectra measurement of droplets with different sizes. The diameter of the droplets are 491μm, 588 μm, 642 μm, 670 μm, 694 μm, 706 μm, and 747 μm, respectively.
In conclusion, we have demonstrated integrated optofluidic droplet laser on PC chip through microfluidic nozzle array structures. Through independent control of oil phase and aqueous phase in separate microfluidic channels, the droplet laser can be generated, removed, and re-generated in a simple and robust manner. The droplet laser design demonstrated here is transferrable to devices made of other materials, such as silicon, to further reduce device footprint. A lasing threshold of 10 μJ/mm2 is achieved, which can be further improved through increasing the contact angle of the droplet, and hence cavity Q-factor.
Funding
National Science Foundation (NSF) (1554013) and the ORAU Ralph E. Powe Junior Faculty Enhancement Award.
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