MP3L David Keng, Monica Agarwal 03/17/2008 A method for constructing a compact flowcell – using short taper coupling MP3L David Keng, Monica Agarwal 03/17/2008

Overview Flowcell requirements Previous What to change? How to change? New design Fabrication Result Conclusion

Flowcell Requirements Contains Microsphere Coupling Fiber Inlet / Outlet Dimension Small size overall Small overall dimension tolerance Previous flowcell design

Flowcell – previous To contain the coupling fiber Flowcell has to cover the effective length of the fiber This length is usually > 10 mm Overall volume ~ 20 mm3 minimum Contains only one microsphere Difficult to contain more Dips only and high Q not easy to track Require exact phase matching to couple Difficult to couple high index microspheres What to change?

What to change? Design Goal Smaller size Multiple microsphere An easy way to locate high Q WGM Possible solution for high refractive index microsphere coupling (why?) How?

How to change? Use a fiber taper instead Short effective length ~ 300 micron Phase matching not as crucial Another pick up fiber to detect peaks Use an array of these fiber tapers Multiple microsphere coupling Should have at least one reference sphere

Reference microsphere Common noise rejection Thermal drift of cell Difficult to compensate Thermal drift of laser Require a high Q wavelength reference TTL triggering delay Can be compensated, but cannot be eliminated Non-specific binding Can only be detected by another microsphere Reference microsphere Should solve the problems above New design

Fiber taper coupling Pump – probe configuration Pump fiber Probe fiber Excite WGM Probe fiber Sample WGM

Flowcell – new design Multiple pump-probe pairs In this case, 2 Allows two microspheres to be coupled Within 800 microns of each other Why this configuration?

Flowcell – Planar configuration Planar design Low profile = small flowcell volume Semi-automatic alignment Microsphere coupling relatively easy

Flowcell - Fabrication Meniscus Forms between 2 liquids Height = Radius-1 Creates a taper This technique Well established Near field probe Silicone oil (PDMS) 48% HF acid

Flowcell – Fabrication detail How to build this? (a) (b2) (b1) (c1) (c2) 30 min 5 min LIFT

Fabrication result Smooth linear taper Only the cladding is etched! Propagation unchanged Low loss <50% Possibly due to the meniscus profile 8 micron /div 4 micron

Assembling Check fiber diameter UV glue under microscope Fine etch to adjust diameter UV glue under microscope Allows time to align Apprx. 50 micron gap between the two pairs

Result Q ~ 106 with zero background in water Peaks can be easily tracked Detector gain offset

Conclusion Low loss taper fabrication SOP established (15 days) Multiple fiber SOP established (10 days, with Monica) Assembling SOP established (3 days) Fabrication and assembling takes ~1hr <20 micron tolerance un-jacketed region <1 micron tolerance on effective region diameter Same coupling apparatus as before Original concept 02/8/2008 (40 days) To do: Testing flow noise Attempting higher refractive index microsphere coupling