Mid-Infrared Opto-Nanofluidic Slot-Waveguide for Label-Free On-Chip Chemical Sensing

A mid-infrared sensor for label-free on-chip chemical detection was developed using an engineered nanofluidic channel consisting of a Si-liquid-Si slot-structure. A sensitivity with 75 times improvement was achieved compared to conventional evanescent-wave sensing. Mid-infrared spectroscopy is a detection technique commonly used for identifying biochemicals and tracing of toxic molecules, and is free of target labels and sensor surface functionalization. The use of mid-IR spectrum circumvents the need for labeling the sample, because the characteristic wavelength of absorption by many functional groups present in chemical or biological molecules falls within this region of the spectrum. Herein, we present a new chip-scale optofluidic device that utilizes mid-IR techniques for label-free and surface functionalization-free chemical sensing. The optofluidic platform is built using CMOS processes, and is capable of accomplishing broad mid-IR spectral sensing. Fig. 1 schematically illustrates the structure of the mid-IR opto-nanofluidic device. The sensing element is a nanofluidic-channel slot-waveguide with its two ends connected to Si-SiO₂-Si slot-waveguides. We embed the entire nanofluidic channel and part of the silicon-oxide slot-waveguides in the PDMS chamber. Upon filling the interior of the chamber with liquid analyte, the solution inside the nanofluidic channel converts the fluid-filled channel into a fluidic slot-waveguide. The mid-IR probe light, after passing through the nanofluidic channel, propagates into the second SiO2 slot-waveguide at the other end. The transmitted light is encoded with the absorption spectrum of the analyte in the fluid because the absorption of probe light by the analyte that fills the nanofluidic channel heavily modulates the intensity of the guided light at the characteristic absorption wavelengths. The enhancement of chemical sensitivity of our fluidic slot-waveguide is evaluated. Fig. 2 (a) compares the predicted optic- l-field profiles for propagating mid-IR (? = 3.3 μm) radiation within a rectangular-strip waveguide, to that of a nanofluidic slot waveguide. In the case of the rectangular strip-waveguide, the optical field is mainly retained inside the Si core and its penetration as an evanescent wave into the surrounding fluid is small. In the slot-waveguide the optical field is highly concentrated at the center of the fluidic channel and interacts strongly with the liquid inside the channel. Thus, even a slight change in the concentration of analyte will result in a significant modulation of intensity to the guided mid-IR that consequently boosts the sensitivity when sensing chemicals. From the plot in Fig. 2 (b), the enhancement-factor Sslot/Sstrip rises to 75 times as the slot-width narrows to d = 80 nm.