A HIGHLY EFFICIENT THREE-DIMENSIONAL (3D) LIQUID-LIQUID WAVEGUIDE LASER BY TWO FLOW STREAMS

Y. Yang1, C. D. Ohl2, H. S. Yoon3 and A. Q. Liu1† 1School of Electrical & Electronic Engineering 2School of Physical & Mathematical Sciences

3School of Biological Sciences Nanyang Technological University, SINGAPORE 639798

(†E-mail: eaqliu@ntu.edu.sg; Tel: +65-6790-4336; Fax: +65 6793-3318) ABSTRACT This paper reports a tunable three-dimensional (3D) liquid-liquid waveguide laser constructed by two flow streams in a microchannel. It comprises a 3D dye dissolved liquid-liquid waveguide as gain medium and gold (Au) mirrors to provide feedback. The 3D liquid-liquid waveguide is highly efficient to overcome the optical loss as compared with liquid-liquid three-dimensional (2D) waveguide. The lasing wavelength is tuned by the refractive index of the liquid core flow stream, and the output energy can be varied by changing the volume ratio between the two flow streams. It has wide applications in biomedical, biological and chemical analyses in the near future.

INTRODUCTION Optofluidics dye lasers are microfabricated dye lasers integrated in the microfluidic channels [1]. Besides providing the optical gain, the liquid medium in the microchannel plays a more flexible role in varying the optical properties such as changing the lasing wavelengths, tuning the spatial modes, and controlling the output energy, which do not exist in solid-state or bulk dye laser systems. Different kinds of optofluidic dye lasers have been developed, such as microcavity dye lasers, microdroplet dye lasers [2], DBF dye lasers [3] and waveguide dye lasers [4]. Among of these devices, liquid waveguide dye laser is typically attracting many interests because of the achievable low threshold and high efficiency.

Liquid waveguides retain all the functionalities associated with the waveguiding capability while providing a natural synergy with lab-on-a-chip systems. 2D liquid-liquid waveguide using three flow streams was published and is subsequently a breakthrough research [5,6]. However, the 2D liquid-liquid waveguide cannot be regarded as a complete 3D liquid waveguide. The absence of the liquid cladding on the vertical direction produces serious restriction to the optical property of the liquid waveguides. First, the refractive index of the solid substrate is usually much higher than the liquid claddings. Therefore, some important optical parameters such as cut-off frequency or band gap of the liquid-liquid waveguide are still determined by the solid materials but not as expected by the liquid materials. Many propagation modes should be kept in the liquid-liquid waveguide will leak from the vertical direction,

making the liquid cladding in the parallel direction is not fully functional. Second, the actual profile of the cross-section in the 2D liquid-liquid waveguide can only tuned in one dimension with limited flexibility as compared to a pure fluidic systems [7]. As a result, traditional liquid-liquid waveguide is still a solid–fluid hybrid. Thus, the efficiency of the liquid waveguide dye laser is also reduced by this drawback. This paper reports a tunable 3D liquid-liquid waveguide laser constructed by two flow streams in a microchannel. The 3D liquid-liquid waveguide is achieved based on the centrifugal force [8, 9] and is highly efficient to overcome the optical loss as compared with liquid-liquid 2D waveguide.

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Figure 1: The schematic illustration of 3D liquid waveguide laser. (a) The work principle: the lasing wavelength is determined by the length of the waveguide and the refractive index of the liquid core. (b) The output energy can be tuned by the volume ratio between liquid core and cladding. (c) The microchip design: 3D dye dissolved liquid-liquid waveguide as gain medium, a pump laser to produce stimulated emission, and two mirrors to form a cavity.

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978-1-4244-9634-1/11/$26.00 ©2011 IEEE 1071 MEMS 2011, Cancun, MEXICO, January 23-27, 2011

Figure 2: Simulation results of a common 2D liquid waveguide and 3D liquid waveguide. The cross-sectional intensity filed of (a) 2D waveguide, and (b) 3D waveguide. The intensity field along the waveguide length of (c) 2D waveguide, and (d) 3D waveguide (d). The refractive index of the core and the cladding is 1.410 and 1.332. The refractive index of PDMS is 1.410.

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DESIGN AND THEORY Liquids traveling through curved microchannels

experience forces which are more complex than that in straight microchannels [9]. Besides inertial forces acting in the axial motion, the centrifugal force is also acted along the radius of curvature of the conduit. The interplay between them establishes a radial pressure gradient whose magnitude can be sufficient to generate a transverse flow field (Fig. 1 inset), which is known as the Dean flow [10 - 12]. It can be characterized in terms of a dimensionless Dean number (De) that expresses the relative magnitudes of the inertial and the centrifugal forces to the viscous forces

De= 0.5Re (1) where = d / R is a geometrical parameter and R is the flow path radius of curvature. Re = Vd / , V is the average flow velocity, d is the channel hydraulic diameter and v is the kinematic viscosity of the fluid. Figure 1 shows the schematic of the 3D liquid-liquid waveguide laser. The working principle of the 3D liquid waveguide laser is shown in Fig. 1 (a). High concentration of fluorescent dyes is dissolved in the liquid core of the 3D waveguide as the gain medium and light is confined to enhance the efficiency. The lasing wavelength is selected by the effective cavity length, which is finally determined by the refractive index of the core flow stream. Fig. 1(b) shows the flexibility of the 3D waveguide laser. The output lasing energy can be tuned by the volume ratio between the two flow streams. Fig. 1 (c) shows the chip design of the 3D waveguide laser, which consists of a 3D dye dissolved liquid-liquid waveguide, a pump laser to produce the stimulated emission, and Au mirrors to provide feedback. The 3D liquid waveguide consists of two flow streams in a microchannel as shown in Fig. 1(c). The two flow streams are pumped into the microchannel from the inner and the outer inlets. When the two flow streams are transported in the curved section of the microchannel, the inner flow stream is surrounded by the outer flow stream by centrifugal force. The refractive index of the inner flow streams is higher than that of the outer flow stream and light is projected into the 3D liquid waveguide from the outlet of the microchannel. This 3D liquid waveguide is divided into two regions. One is the reconfiguration region (I), and the other one is the waveguide region (II). Based on the designed microchannel geometry and the chosen liquids, the 3D liquid-liquid waveguide can be formed by tuning the flow rates according to the Eq. (1). In traditional liquid core-liquid cladding waveguides, the core liquid was horizontally sandwiched by the two cladding liquids. However, the core flow is contacted with the channel wall in the vertical direction (top and bottom), so that the waveguides could not be fully characterized as pure liquid waveguides. More importantly, the refractive index of the cladding liquid was usually lower than that of the polydimethylsiloxane (PDMS), so that optical loss was enhanced at the interface between the core liquid and the PDMS channel wall. Fig. 2 shows the simulated field intensity in the common 2D liquid-liquid waveguides [5,6]

and the 3D liquid waveguides. The refractive index of the liquid core and the one of the liquid cladding are 1.410 and 1.332, respectively. The intrinsic loss of the 2D liquid waveguide is shown in Fig. 2 (a) and (c), which poses a fundamental problem in the laser application. Compared with the 2D liquid waveguide, the 3D liquid waveguide overcome the drawback and light is well kept in the liquid core as shown in Fig. 2 (b) and (c) [13]. Different from the bulk liquid handing system, the mixing, replacing and transporting of liquids in the microchannel can be achieved in a highly flexible fashion. This feature can make the liquid waveguide laser as a tunable device by tuning the refractive index contrast between the liquid core and the claddings. In liquid waveguide lasers, the refractive index of the core flow stream determines the optical path in the microcavity. Different effective cavity length L can select different lasing modes. However, in traditional liquid waveguides, the tuning range is limited by the condition that the refractive index of the core liquid must be higher than the PDMS to confine light in the vertical directions. In 3D liquid waveguides, it can keep light well even the refractive index in smaller than PDMS as demonstrated in Fig. 2. Thus, a more flexible waveguide laser is produced. Fig. 3 shows the relationship the longitudinal mode spacing (•v) and the refractive index of the core flow stream. The liquid flowing into the core can be tuned from DI water (n = 1.332) to benzylalcohol (n = 1.530).

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RESULTS AND DISCUSSIONS For the experimental study, the microchannel system was fabricated using soft photolithography processes. First, photoresist-on-silicon master was prepared in a clean room facility with photolithography (Micro-Chem, SU-8) using transparent glass masks (CAD/Art Services, Inc. Poway, CA). Then, microchannels were molded using PDMS and sealed against flat PDMS sheets after plasma oxidation. After fabrication, the microchannel dimensions of W = 100 µm and H = 100 µm. The flow path radius of curvature R is 2 mm. A Fabry-Perot cavity with metallic mirrors is fabricated on the surface of the fibers by electron beam evaporator. The right mirror H1 is made of 30 nm Au, and have a reflectance R1 = 70% and a transmittance T1 = 5%. The left mirror H2 is made of 80 nm Au, and has a reflectance more than 80% as shown in Fig. 4. The cavity length is 500 µm. A dye solution of 2mM Rhodamine 6G in ethyl glycol (EG) is pumped from the inner inlet as the gain medium. In the experiment, EG (n = 1.410) and DI water (n = 1.332) are used to form the 3D liquid-liquid waveguides and injected

from the inner and outer inlets, respectively. Figure 5 shows the experimental confocal microscopy results of the dean flow at the cross section of the microchannel. Besides inertial forces, the centrifugal force acts along the conduit’s radius of curvature establishes a radial pressure gradient whose magnitude can be sufficient to generate a transverse flow field, which is known as the Dean flow. The positions between EG and DI water are transposed as shown in (a-f) at the flow rates v = 1000 µl/min. It implies that if the flow rates are sufficient, the liquid pumped from the inner inlet can be totally surrounded by the liquid pumped from the outer inlet. Figure 6 shows the confocal microscopy images of the 3D liquid waveguide laser at different volume ratio between EG and DI water. The flow rate of EG is about 400 µl/min and the volume ratios are (a) = 30, (b = 40, (c) = 50. The EG flow stream moves into the middle of the straight microchannel and is surrounded by DI water to form a

Figure 6: The 3D confocal microscopy images of the 3D liquid core/liquid cladding waveguides at the volume ratio are (a) = 30, (b) = 40, (c) = 50. The output lasing energy can be tuned by the volume ratio.

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Figure 5: Cross sectional view of the confocal microscopy results at the at the flow rates v = 1000 μl/min from the beginning to the end of the curvature (a-f). The positions between ethyl glycol (EG) and DI water are transposed by Dean flow.

Figure 3: The relationship between the longitudinal mode spacing •v and the refractive index of the core flow stream.

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tunable 3D liquid circular waveguide, resulting different output energy. The 3D liquid waveguide structure is steady in the straight microchannel because of the absence of the Dean flow.

Figure 7 shows the results of the 3D liquid waveguide laser. The pump source is a 532 nm pulse laser. If there is no Au mirrors to support the feedback, the spontaneous emission of the RH 6G in EG solution is shown in blue line. The full width at half maximum (FWHM) is 40 nm. However, if the F-P cavity is coated, the laser emission of the 3D liquid waveguide laser is emitted around 578 nm (red). The FWHM of the waveguide laser is only 8 nm, which is much smaller than that waveguide source. The inset is the output laser spot image after it coupled from the fiber mirror H1. CONCLUSIONS In conclusion, we construct a 3D liquid-liquid waveguide laser in a microchannel using only two flow streams. The 3D liquid-liquid waveguides is high efficiency to overcome the optical loss comparison with traditional 2D liquid core-liquid cladding waveguide. The lasing wavelength is tuning by the refractive index of liquid core flow stream, and output energy can be chosen by changing volume ratio between the two flow streams. It has wide applications in biomedical, biological and chemical analyses in the near future.

ACKNOWLEDGMENTS This work was supported by the Environmental and Water Industry Development Council of Singapore (Grant No. MEWR C651/06/171).

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[5] D. B. Wolfe, R. S. Conroy, P. Garstecki, B. T. Mayers, M. A. Fischbach, K. E. Paul, M. Prentiss and G. M. Whitesides, “Dynamic control of liquid-core/liquid-cladding optical waveguides,” Proceedings of the National Acedemy of Sciences, vol. 101, pp. 12434, 2004.

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[12] J. M. Lim, S. H. Kim, S. M. Yang, “Liquid–liquid fluorescent waveguides using microfluidic-driftinginducedhydrodynamic focusing,” Microfluid Nanofluid., vol. 1, pp. 1-7, 2010.

[13] A. W. Snyder, J. D. Love, “Optical Waveguide Theory,” Institute of Advanced Studies, Australian National University, 1983.

Figure 7: The laser emission spectra of the 3D liquid waveguide laser (red) and the emission spectra of the RH 6G in EG without Au mirrors (blue). The inset is the output laser spot image after the fiber (H1).

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