Optofluidic Microparticle Splitters Using Multimodeinterference-based Power Splitters Reporter: Nai-Chia Cheng ( 鄭乃嘉 ) Advisor: Ding-Wei Huang 2012/5/30 1

Abstract  In this paper, they report an optofluidic microparticle splitter on a silicon chip using a silicon nitride (SiN) multimode-interference (MMI) based 1×2 power splitter embedded in a silica microfluidic channel.  They demonstrate transportation of polystyrene particles to the two MMI output-ports with various branching ratios by tuning the MMI output-power ratio upon various laser wavelengths around the 1550nm range. 2

Introduction  Optofluidics – A technology area that combines the advantages of microfluidics and optics.  Optofluidics enables on- chip optical manipulation of nano/micro-sized objects in microfluidic channels, thereby providing a non- intrusive method to optically manipulate microparticles in a small fluidic sample volume. 3

Introduction  Optical manipulation of particles on a chip using the evanescent wave on an integrated waveguide-based device embedded in a microfluidic channel has been gaining increasing interest in the research area of optofluidics for lab-on-a-chip applications. 4 Free space optical tweezing Waveguide based near-field optical trapping

Optofluidic Waveguide Fabrication  The microfluidic channel is lithographically integrated with the silicon nitride (SiN) waveguide of 0.7 μm thicknesses.  The silica-based microfluidic channel is 6 μm in height and bonded with a cover glass. 5 Fig 1: (a) Schematic cross-sectional view of the optofluidic device. (b) Optical micrograph of the fabricated optofluidic device. (c) Schematic of longitudinal forces on the microbead along the waveguide. F flow : flow-induced force, F light optical scattering force.

Optofluidic Waveguide Fabrication  They adopt SiN material as the optical layer because: 1. Relatively wide transparency window from the visible to the near-IR bands. 2. High refractive index contrast between SiN and water (~2/1.33). 3. Complementary metal-oxide-semiconductor (CMOS) compatible fabrication process. 6

Microbeads Transportation by Optofluidic Waveguide  Two 1 μm-sized polystyrene particles were driven along an illuminated waveguide for the duration of 60s. 7 Fig 2: (a)-(c) Optically driven 1-μm-sized polystyrene microbeads by a SiN waveguide integrated with a microfluidic channel. (d)-(f) Releasing of the microbeads after the laser switched off.

Multimode-interference (MMI) Power Splitters  Particles are optically transported from the single-mode input-waveguide to the multimode waveguide.  It is possible to route particles from the input-port via the MMI region to the output-ports according to the power splitting ratio. 8 Fig. 3 (a) Schematic of particle manipulation on a MMI power splitter. (b) Optical micrograph of the optofluidic device with air cladding before filling with colloidal solution.

Multimode-interference (MMI) Power Splitters  About 191mW TM-polarized laser light in the 1550nm wavelength range into the input-waveguide (port 1) using a polarization- maintaining lensed fiber.  Given the ~18dB - ~21dB insertion loss, the estimated optical power in the MMI region is around 13 mW.  The transmission powers from ports 2 and 3 vary with laser wavelength, and wavelengths A, B and C for the particle manipulation experiments. 9

Microbeads Splitter Demonstration 10 Fig. 4 (a)-(c) Optical micrographs of the MMI-based device with 1μm-sized polystyrene particles upon laser wavelength A at various times (a) 0 s, (b) 24 s and (c) 32 s. (d) Accumulated trajectories of 41 particles in the multimode waveguide region upon laser wavelength A. (e) Simulated TM-polarized mode-field intensity pattern in the multimode waveguide region at wavelength A using finite-element method (COMSOL).  Upon wavelength A at 1550 nm, the transmission power from output-port 2 is ~1.5dB higher than that from output-port 3.

Trapping Potential In the Input-junction  The mode-field intensity drop in the junction between the tapered- waveguide and multimode waveguide suggests a longitudinal (z direction) gradient force pointing to the higher field intensity. 11 Fig. 5. Schematic of particles manipulation of a MMI power splitter. Inset: Schematic of trapping potential in the vicinity of the input-junction.

Different Routing Conditions  Zoom-in images of various sized particles in the input- junction at various times: 1. (b)-(f) 1 μm-sized particles. 2. (g)-(k) 2 μm-sized particles. 3. (l)-(p) 110 nm-sized particles.  (q)-(t) zoom-in images of particles in the output ports with laser wavelength at (q) and(s) A, and (r) and (t) B. 12

Particle Branching Ratio  Upon wavelength A at 1550 nm, their experiment shows ~97 % of the 64 trapped particles are routed to output-port 2 while ~3% are routed to output-port 3.  Upon wavelength B at nm, their experiment shows ~67 % of the 60 trapped particles are routed to output-port 3 while ~33% are output-port 2.  Upon wavelength C at 1560 nm, their experiment shows the 73 trapped particles have almost equal probability to be routed to output-port 2 (52%) and output-port 3 (48 %). 13