Integrated platform based on high refractive index contrast waveguide for optical guiding and sorting Balpreet Singh Ahluwaliaa∗, Olav Gaute Hellesøa, Ananth Z. Subramanianb, James S. Wilkinsonb Jie Chenc and Xuyuan Chenc a Department of Physics and Technology, University of Tromsø, Tromsø 9037, Norway b Optoelectronics Research Centre, University of Southampton, Southampton, UK c Institute for Microsystems Technology, Vestfold University College, 3103 Tonsberg, Norway ABSTRACT The high-refractive index contrast (Δn ~0.65 as compared to silicon oxide) of Tantalum pentoxide (Ta2O5) waveguide allows strong confinement of light in waveguides of sub-micron thickness (200 nm). This enhances the intensity in the evanescent field, which we have employed for efficient propelling of micro-particles. The feasibility of opto-fluidics sorting of different sized micro-particles based on their varying optical propulsion velocity is suggested. Optical propulsion of fixed red blood cells (RBC) with velocity higher than previously obtained is also reported. The optical propulsion velocities of RBC in isotonic solution (0.25 M sucrose) and water have been compared. Keywords: Optical propelling, optical sorting, optical trapping of cells, Tantalum pentoxide waveguides
1. INTRODUCTION The capability of optical fields to exert a mechanical force on mass was demonstrated experimentally by Beth [1] even before the discovery of the laser. Optical forces generated by the focused laser beam for micro-manipulation of particles and biological cells have been previously reported [2-4]. The drive to non-invasively guide and sort micro-particles and cells en-masse by non-focused laser beam led to the investigation of optical waveguides [5-10] contrary to optical tweezers and optical fibre based trapping [11]. In optical tweezers the trapped micro-particle interacts with the entire beam. Contrary to this, in an optical waveguide configuration the trapped micro-particle interacts only with the evanescent beam. The waveguide materials used so far for micro-propulsion of particles are limited to the following:- a) low refractive index contrast (Δn ~0.03) Cs+ or K+ ion-exchanged waveguides [6]; b) medium Δn (~0.101) SU-8 polymer waveguides [7]; c) high Δn (~0.52) silicon nitride waveguides [8]. Sub-wavelength optical wires (SWOWs) or tapered fibers have also been used for micro-propulsion [9]. Higher refractive index waveguides (like silicon nitride) imparts higher optical forces for micro-particle propelling as compared to lower refractive index contrast waveguides. In this paper we report optical propulsion of micro-particle and cells on submicron thick (200 nm) Ta2O5 waveguides that have low propagation losses (~ 1 dB/cm @ 1070nm). Maximum optical propulsion velocity of 50µm/sec for an 8 µm particle was obtained for guided power of only 20 mW. The feasibility of optical sorting of micro-particles based on their varying optical propulsion velocities on a waveguide is also reported. Optical propulsion of red blood cells (RBC) on optical waveguide surface has been reported. Red blood cells were optically trapped and propelled in water and isotonic sucrose solutions and their propulsion velocities were compared in two mediums.
2. FABRICATION Optical propulsion of micro-particles on a waveguide surface is dependant on available power in the evanescent beam. To enhance the optical propulsion velocity of a micro-particle it is imperative to reduce the insertion loss (coupling and propagation losses). Fabrication and design optimization of Ta2O5 waveguide were done to reduce the propagation losses. The coupling losses in such submicron thickness (200 nm) waveguides are high (~6-10 dB). As such, reducing *
[email protected] phone +47 776451162
propagation loss is even more crucial. Ta2O5 is compatible with conventional silicon technology enabling efficient and simple processing. We have optimized sputtering parameters, photolithography and the ion-beam milling processes to reduce the propagation loss. Post-fabrication annealing and KOH bath (Potassium hydroxide 30% by weight) was employed to further reduce the propagation loss. Annealing removes the stress built up in the waveguides from processes like deposition, etching etc. Annealing also helps filling oxygen deficiencies to further reduce the losses and to improve the stoichiometric ratio of the sputtered Ta2O5 thin film. The propagation loss was around 1 dB/cm @ 1070 nm, determined by the cut-back method. The fabrication details and optimized parameters can be found elsewhere [11, 12].
3. OPTICAL PROPULSION OF MICROPARTICLE 3.1 Optical guiding
Propulsion Velocity (µm/s)
Optimized low-loss strip waveguides were employed for optical propulsion experiments. The experimental apparatus for optical propulsion consists of a 5 W single mode Ytterbium fiber laser @1070 nm and the light was coupled by an IR coated objective lens (Nachet 0.9 N.A 80X). The output light from the waveguide was collected by a 10X objective lens. Another 10X objective lens and a CCD camera attached to a microscope were employed to capture images. Polystyrene micro-particles (refractive index 1.59, Polyscience Europe) of diameter 3µm and 8 μm were used in the experiments. Tantalum pentoxide waveguides were found to be very efficient in propelling micro-particles. Figure 1 shows optical propulsion velocities of 8 µm particles as a function of input power on 8 µm wide waveguide. Optical propulsion velocity increase linearly with the input power. With an input power of 700mW and output power (guided power) of only 20mW the optical propulsion velocity achieved was 50 µm/s. The optical propulsion velocity was determined by taking the average velocity of the same micro-particle at different locations over the waveguide. 50 40 30 20 10 0 100 200 300 400 500 600 700 800 Input Power (mW)
Figure 1:- Optical propulsion velocity of 8 µm diameter particles with input power on an 8 µm wide waveguide.
3.2 Feasibility of optical sorting For a given input power, the optical propelling force on a micro-particle (of a given diameter) differs for waveguides of different widths. Similarly the optical propulsion velocities of different sized micro-particles on a waveguide (of a given width) are different. It is thus imperative to study the optical propulsion velocities of different micro-particles on waveguides with varying widths. This knowledge of optical propulsion velocities experienced by the micro-particles with varying waveguide widths will be useful in sorting micro-particles based on optical fractionalization techniques [14, 15]. The insertion loss of wider waveguides (5-10 µm) is less as compared to narrower width waveguides (1-4 µm), as shown in Table 1. This is mainly due to high coupling loss and sidewall scattering loss for narrow waveguides [11-13]. Due to varying insertion losses for different widths, the comparison of optical propulsion velocity is done with respect to the input power and not the guided or output power. For the same input power, the guided power in different waveguides will be different, still the comparison (of propulsion velocity) based on input power is more reliable, straightforward and
practical for various application like optical sorting. Moreover it is easier to maintain constant input power than the guided power (in the waveguide) during experiments. Table 1 Insertion loss on 2.5 cm long strip waveguides of different widths. Width of waveguide
10 - 7 µm
7 - 5 µm
5 - 2 µm
Insertion Loss
8.23 dB
8.73 dB
12.25 dB
Propulsion Veloctiy (µm/s)
The optical propulsion velocities of 3 µm particles on waveguides with different widths (3-8 µm) for 600 mW and 400 mW input power are shown in Figure 2. The maximum optical propulsion velocity of 3 µm particles was observed on a 3 µm wide waveguide for both input power. The optical propulsion velocities of the 3 µm particles decreased for wider waveguides for both the cases (input power). The average propulsion velocities of 3 µm diameter particle at 600 mW input power were 9 µm/s and 3.33 µm/s on 3 µm and 8 µm wide waveguides respectively. It is also worth highlighting that wider waveguides (6- 8 µm) guided more light as compared to the thinner waveguides (3-5 µm wide), as explained above. Still the optical propulsion velocity of 3 µm diameter particles decreased with increasing width of the waveguide as shown in Fig. 2. When the particle diameter is smaller than the width of a waveguide, the particle does not interact with full available evanescent field and its lower propulsion velocity can be understood. Furthermore, a small diameter particle on a wide waveguide (multiple mode waveguide) experiences enhanced meandering effect (due to mode beating). The waveguides (3-8 µm wide) used in these experiments are multiple-moded waveguides. Thus the beating of the intensity pattern exists in the waveguides. Optical propulsion of gold nanometer was reported to follow intensity beat patterns in dual-moded waveguide [16]. Similarly Mie particle of diameter 4 µm was also reported to follow intensity beating during optical propulsion on a waveguide [17]. We have also noticed similar beating effects on 3 µm diameter particle. The meandering effect of a 3 µm particle was more prominent in the wider waveguides, resulting slower propulsion of the particle. 600mW Input Power
10
400 mW Input Power
8 6 4 2 0 1
3
5
7
9
Width of Waveguide (µm)
Figure 2: - Optical propulsion velocity of 3 µm particles over waveguides of different widths with 600 mW and 400 mW input power. Figure 3 shows the optical propulsion velocities of 3 µm and 8 µm sized particles on different waveguides with constant input power of 600mW. Contrary to 3 µm particle, the optical propulsion velocity of an 8 µm particle increases with the width of the waveguide. The increase of optical propulsion velocity with the increase of particle diameter is also reported earlier [6]. The lower propulsion velocity of a 8 µm particle by the smaller width waveguide (3-5 µm) as compared to wider waveguides (6-8 µm) can be partially associated with higher insertion losses of narrower waveguides. The meandering effect due to mode beating will be less when the size of particle is bigger than the width of the waveguide. The 8 µm particles were noticed to vary its optical propulsion velocity at different locations over the waveguide due to
Propulsion Veloctiy (µm/s)
intensity beating. However meandering effect was not observed for 8 µm particle, which is apparent from the fact, that the particle diameter is bigger than the width of the waveguide. The big variation in the optical propulsion velocities of 8 µm diameter particles was noticed, this is in agreement to reported literature [6].
35
3 µm particle
30
8 µm particle
25 20 15 10 5 0 0
2 4 6 8 Width of Waveguide (µm)
Figure 3 Comparison of optical propulsion velocity for 3 µm and 8 µm particle over waveguides of different widths at an input power of 600mW. The difference in optical propulsion velocity (optical forces) experienced by different size particles on a waveguide can be used for micro-particle sorting. Like on an 8 µm wide waveguide, the optical propulsion velocity of a 3 µm particle is around ten times less than that of an 8 µm particle, as noticed from Fig. 3. The optical propulsion velocities of 3 µm and 8 µm micro-particles on an 8 µm wide waveguide were 3.33 µm/s and 32 µm/s respectively. Thus it should be possible to sort micro-particles by the optical forces when introducing an additional force, for example that associated with micro-fluidics [14, 15]. It is also worth mentioning that for a 3 µm wide waveguide; there is a small difference in the optical propulsion velocities of 3 µm and 8 µm sized micro-particles. On a 3 µm wide waveguide the optical propulsion velocities of 3 µm and 8 µm micro-particles were 9 µm/s and 12 µm/s respectively. Thus to implement efficient optical sorting, a waveguide of correct dimensions should be chosen, giving significant differences in the optical propulsion forces (velocity) for the chosen particle diameters.
4. OPTICAL PROPULSION OF RED BLOOD CELLS 4.1 Introduction to optical trapping of red blood cells The human red blood cell (RBC) has a biconcave shape with an average diameter of about 8 µm. A RBC circulates in the body almost half a million times during its lifetime. The elasticity of the RBC is its inherent property which allows it to undergo deformation while flowing through narrow capillaries (sometimes less than half the diameter of RBC ~ 3 µm). After the passage, the RBC recovers its original shape. Non-contact, far field and non-invasive micro-manipulation of RBC employing optical trapping techniques have been widely studied [18-26]. The inception paper [16] on optical trapping of RBC in infra-red laser based optical tweezers in 1987, (Ashkin et. al) also reported the distortion of the trapped RBC. Due to the extreme flexibility (high elasticity) of RBC, its distortion and folding to a rod-like shape has been since reported by various researchers [18-21]. Even at lower power (20 mW) the shape of RBC is distorted by the focused laser beam at focal vicinity in optical trapping [19, 20]. The distorted or folded RBC in optical trap regains its normal shape after blocking the laser beam. The distorted RBC was also reported to possess additional properties like form birefringence [19, 21]. The trapped RBC (folded rod-like geometry) aligns its long axis with the electric field vector distribution of the linearly-polarized laser beam. Furthermore, a trapped RBC with folded geometry can be rotated by the spin angular momentum of the incident circularly polarized light. The elasticity of a RBC makes trapping and
manipulation of RBC without distorting its shape in a conventional optical trapping set up. In this paper, we report optical trapping and propulsion of ‘un-distorted’ RBC on an optical waveguide surface. Furthermore, we have investigated optical propulsion of RBC in different mediums. Optical propulsion velocity of RBC in water and isotonic solution (0.25 M sucrose) with respect to incident power is reported. We have employed fixed red blood cells, which were dead but retain its shape and properties in this work. 4.2 Optical propulsion of red blood cells in water Optimized high-refractive index Ta2O5 waveguides of width 10 µm were employed for optical propulsion of RBC. In conventional optical trapping the laser light interacts with the entire cells which are trapped at the focal vicinity. Contrary to this, for waveguides the optical trapping and propulsion is based on the evanescent beam. The red blood cells are trapped and propelled on an optical waveguide and does not show any visible surface distortion or folding. The gradient force attracts the RBC cells towards the waveguide and radiation force propels it along the propagation of the light. Figure 4 shows optical propulsion of RBC on Ta2O5 waveguides in water. Fig. 4 (a) highlights trapped and untrapped (reference cells) RBC. The laser is turned off in Fig 4(a) and turn on for Fig 4(b-d). It is evident from Fig 4 that there is no folding or distinct distortion of RBC as it is trapped and propelled on the waveguide. Furthermore, the shape of the reference RBC cell (un-trapped) and the trapped RBC are almost identical; suggesting no distinct distortion or folding of RBC to a rod like structure. In other experiments red blood cells were propelled for more than 500 µm and with high input power (1800 mW) without any noticeable distortion of its shape. This is contrary to the published results [19-21] where distortion or folding of RBC to a rod-like structure was reported in optical tweezers set-up.
Figure 4:- Optical propulsion of un-distorted RBC on Ta2O5 waveguide. We have also noticed that group of cells propel faster than a single cell. Figure 5 shows optical propulsion of group of red blood cells. On comparing Fig. 5(a-c) it can be noticed that the group of three red blood cells are propelled faster than individual cells. This is similar to reported literature, where group of polystyrene micro-particles were propelled faster than a single micro-particle [27]. Table 2 highlights the optical propulsion velocity of a single RBC and a group of RBC with different input power. At 1800 mW input power the maximum optical propulsion velocities were 2.83 µm/s and 1.5 µm/s for a group of RBC and a single cell respectively. We have also noticed the formation of red blood cells chain on a waveguide surface over an extended period of time. This is similar to extended chains of micro-particles formed on a waveguide surface [27].
Figure 5:- Optical propulsion of a single and a group of red blood cells on Ta2O5 waveguide.
Table 2:- Optical propulsion velocity of a single and group of RBC with different input power. Maximum Input power Propulsion (800 mW) Velocity Single RBC 0.75µm/s Group of RBC
1.3 µm/s
Input power (1200 mW)
Input power (1800 mW)
1.15µm/s
1.5 µm/s
2 µm/s
2.83 µm/s
4.3 Optical propulsion of red blood cells in isotonic solution The long term goal of optical waveguide propulsion is to enable point-of-care testing and diagnosis for bio-medical labon-a-chip application. However to work with live cells it is necessary to maintain various parameters within a constant level (pH, isotonic medium and temperature) and keep the environment sterile. One of the important parameter is to have an isotonic solution so that cells do not swell up or collapse due to the miss-match in the osmotic pressure. An osmotic pressure difference can eventually burst and thus kill the cells. An isotonic solution enables the ionic concentration within a cell and the solution equal avoiding cell burst. The commonly used isotonic solutions are phosphate buffer solution (PBS) and 0.25 molar sucrose solutions. Red blood cells were found to stick to waveguide surface in PBS solution. The electrostatic attraction of cells with the waveguide surface in PBS solution was presumably higher than the optical forces. Thus the optical propulsion of RBC in PBS solution was not achieved. RBC was successfully propelled in 0.25 M isotonic sucrose solution. Only small difference in the optical propulsion velocities of RBC in water and isotonic sucrose mediums were noticed. RBC was propelled with slightly less velocity in isotonic sucrose solution as compared to that in water. Average optical propulsion velocities of RBC at 1200 mW input power were 0.73 µm/s and 0.95 µm/s in isotonic sucrose and water mediums respectively.
5. CONCLUSION The high-refractive index contrast, submicron thick (200 nm) Tantalum pentoxide waveguides are employed for micromanipulations of particles and cells. The enhanced intensity in the evanescent field has been employed for efficient propelling of micro-particles with a propulsion velocity higher than previously reported for micro-particles and cells. The feasibility of opto-fluidics sorting of different sized micro-particles based on their varying optical propulsion velocities is reported. It is highlighted that to enable such a sorting of micro-particles, the correct width of the waveguide should be chosen. Optical propulsion of RBC without any distortion is reported. The optical propulsion of RBC in isotonic sucrose solution is also shown. No significant difference between the optical propulsion velocities of RBC in water and isotonic sucrose solutions were noticed. The feasibility of optical propulsion of RBC in isotonic solution, allows propulsion of live cells, this will be carried out in the near future.
Acknowledgements The authors wish to acknowledge David A. Sager and Neil P. Sessions for their assistance in waveguide fabrication, Peter McCourt for his help with cell preparation and Pål Løvhaugen for his useful advice. This work is supported by the Research Council of Norway under FRINAT-programme.
REFERENCES [1] R. Beth, “Mechanical detection and measurement of the angular momentum of light,” Phy. Rev. 50, 115 (1936). [2] Ashkin, A.; Dziedzic, J. M.; and Yamane, T., “Optical trapping and manipulation of single cells using infrared laser beams”, Nature 330, 769 (1987). [3] D. G. Grier, “A revolution in optical manipulation”, Nature 424, 810 (2003). [4] P. J. Rodrigo, V. R. Daria, and J. Glückstad, “Real-time three-dimensional optical micromanipulation of multiple particles and living cells," Opt. Lett. 29, 2270 (2004). [5] S. Kawata and T. Tani, “Optically driven Mie particles in an evanescent field along a channeled waveguide,” Opt. Lett. 21, 1768 (1996). [6] K. Grujic, O. G. Hellesø, J. S. Wilkinson and J. P. Hole, “Optical propulsion of microspheres along a channel waveguide produced by Cs+ ion-exchange in glass,” Opt. Commun. 239, 227 (2004). [7] Bradley S. Schmidt, Allen H. J. Yang, David Erickson, and Michal Lipson, “Optofluidic trapping and transport on solid core waveguides within a microfluidic device”, Opt. Exp. 15, 14322 (2007). [8] S. Gaugiran, S. Gétin, G. Colas, A. Fuchs, F. Chatelain, J. Dérouard, and J.M. Fedeli, “Optical manipulation of microparticles and cells on silicon nitride waveguides,” Opt. Exp. 13, 6956 (2005). [9] G. Brambilla, G. Senthil Murugan, J. S. Wilkinson, and D. J. Richardson, “Optical manipulation of microspheres along a subwavelength optical wire”, Opt. Letts., 32, 3041 (2007). [10] Allen H. J. Yang, Sean D. Moore, Bradley S. Schmidt, Matthew Klug, Michal Lipson & David Erickson, “Optical manipulation of nanoparticles and biomolecules in sub-wavelength slot waveguides”, Nature 457, 71 (2009). [11] B.S. Ahluwalia, A.Z. Subramanian, O.G. Helleso, N.M.B. Perney, N.P. Sessions, J.S. Wilkinson, "Fabrication of Submicrometer High Refractive Index Tantalum Pentoxide Waveguides for Optical Propulsion of Microparticles"Photon. Tech. Lett., 21, 1408, (2009). [12] Balpreet Singh Ahluwalia, Olav Gaute Hellesø, Ananth Z. Subramanian, Nicolas M. B. Perney, Neil P. Sessions and James S. Wilkinson, “Fabrication and optimization of Tantalum pentoxide waveguides for optical micropropulsion”, SPIE Photonics West 2010, OPTO, paper 7604-31, San Francisco, USA. [13] F. Grillot, L. Vivien, S. Laval, D. Pascal, and E. Cassan, “Size Influence on the Propagation Loss Induced by Sidewall Roughness in Ultrasmall SOI Waveguides”, IEEE Photon. Tech. Lett. 16, 1661 (2004). [14] Kosta Ladavac, Karen Kasza, and David G. Grier, “Sorting Mesoscopic Objects with Periodic Potential Landscapes: Optical Fractionation”, Phys.l Rev. E 70, 010901 (2004). [15] M.P. MacDonald, G.C. Splading and K. Dholakia, “Microfluidic Sorting in an Optical Lattice”, Nature 426, 421 (2003). [16] J.P.Hole, J.S.Wilkinson, K.Grujic, O.G.Helleso, “Velocity distribution of gold nanoparticles trapped on an optical waveguide”, Opt. Exp. 13, 3896 (2005).
[17] Takuo Tanaka and Sadahiko Yamamoto, “Optically Induced Meandering Mie Particles Driven by the Beat of Coupled Guided Modes Produced in a Multimode Waveguide”, Jpn. J. Appl. Phys. 41, L260 (2002) [18] A. Ashkin and J. M. Dziedzic, “Optical trapping and manipulation of single cell using infrared laser beams,” Nature 330, 769 (1987). [19] J. A. Dharmadhikari, S. Roy, A. K. Dharmadhikari, S. Sharma, and D. Mathur, "Naturally occurring, optically driven, cellular rotor," Appl. Phys. Lett. 85, 6048 (2004). [20] J. A. Dharmadhikari, S. Roy, A. K. Dharmadhikari, S. Sharma, and D. Mathur, "Torque-generating malaria infected red blood cells in an optical trap," Opt. Exp. 12, 1179 (2004). [21] A Ghosh, S. Sinha, J. A. Dharmadhikari, S. Roy, A. K. Dharmadhikari, J. Samuel, S. Sharma and D. Mathur, "Euler buckling-induced folding and rotation of red blood cells in an optical trap," Phys. Biol. 3, 67 (2006). [22] S. C. Grover, R. C. Gauthier, and A. G. Skirtach, "Analysis of the behaviour of erythrocytes in an optical trapping system," Opt. Exp.7, 533 (2005). [23] S. Hénon, G. Lenormand, A. Richert, and F. Gallet, "A new determination of the shear modulus of the human erythrocyte membrane using optical tweezers," Biophys. J. 76, 1145 (1999). [24] M. Dao, C. T. Lim, and S. Suresh, "Mechanics of the human red blood cell deformed by optical tweezers," J. Mech. Phys. Solids 51, 2259 (2003). [25] J. Guck, R. Ananthakrishnan, H. Mahmood, T. J. Moon, C. C. Cunningham, and J. Käs, "Optical deformability of soft biological dielectrics," Phys. Rev. Lett. 84, 5451 (2000). [26] S. K. Mohanty, K. S. Mohanty and P. K. Gupta, "Dynamics of Interaction of RBC with optical tweezers," Opt. Exp.13, 4745 (2005). [27] K. Grujic and O. G. Hellesø, “Dielectric microsphere manipulation and chain assembly by counter-propagating waves in a channel waveguide”, Opt. Exp, 15, 6470 (2007).
