Typical L-type photonic crystal (PC) microcavities have a dynamic range of approximately 3-4 orders of magnitude in biosensing. We experimentally demonstrated that multiplexing of PC sensors with different geometry can achieve a wide dynamic range covering 6 orders of magnitude with potential for 8 or more orders with suitable optimization.
Integrating photonic waveguide sensors with microfluidics is promising in achieving high-sensitivity and
cost-effective biological and chemical sensing applications. One challenge in the integration is that an air gap would
exist between the microfluidic channel and the photonic waveguide when the micro-channel and the waveguide
intersect. The air gap creates a path for the fluid to leak out of the micro-channel. Potential solutions, such as oxide
deposition followed by surface planarization, would introduce additional fabrication steps and thus are ineffective in
cost. Here we propose a reliable and efficient approach for achieving closed microfluidic channels on a waveguide
sensing chip. The core of the employed technique is to add waveguide crossings, i.e., perpendicularly
intersecting waveguides, to block the etched trenches and prevent the fluid from leaking through the air gap. The
waveguide crossings offer a smooth interface for microfluidic channel bonding while bring negligible additional
propagation loss (0.024 dB/crossing based on simulation). They are also efficient in fabrication, which are patterned and
fabricated in the same step with waveguides. We experimentally integrated microfluidic channels with photonic crystal
(PC) microcavity sensor chips on silicon-on-insulator substrate and demonstrated leak-free sensing measurement with
waveguide crossings. The microfluidic channel was made from polydimethylsiloxane (PDMS) and pressure bonded to
the silicon chip. The tested flow rates can be varied from 0.2 μL/min to 200 μL/min. Strong resonances from the PC
cavity were observed from the transmission spectra. The spectra also show that the waveguide crossings did not induce
any significant additional loss or alter the resonances.
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