This paper reports a new design of optofluidic Fabry–Pérot (FP) micro cavity that combines the highest reported quality factor for an on-chip FP resonator that exceeds 3600, and the highest reported sensitivity for an on-chip volume refractometer based on a FP cavity that is about 1000 nm/RIU.
For using the optical resonator as a refractometer, it is convenient to have sharp and highly selective resonance peaks for accurate measurements; thereby the quality factor (Q) of the resonator is preferred to be high. The highest reported Q factor reported by other groups is only 400 . This limitation comes from using straight mirror for the FP, which causes high diffraction loss due to beam expansion after few round trips. Our group has previously reported a cavity employing curved mirrors and a micro-tube in-between holding the analyte . The curvature of the mirrors and the micro-tube achieved better light confinement and hence high Q factors up to 2,800. On the other hand, the sensitivity was only 428 nm/RIU since the analyte doesn’t fill the entire cavity. The highest reported sensitivity by literature was 907 nm/RIU in case of the analyte occupying the whole cavity .
In this work, a novel structure for FP micro-cavity is reported, achieving both high Q factor resonator and high sensitivity refractometer. The proposed structure is schematically depicted in Fig. 1. It employs cylindrical Bragg mirrors forming the FP cavity to confine light in the in-plane direction, while an external cylindrical lens - implemented by a fiber rod lens (FRL) - is used to confine light in the out-of-plane direction before it enters the FP cavity and after it exits to be efficiently collected by the collecting fiber. The cavities are fabricated from silicon by Deep Reactive Ion Etching (DRIE) process, and then capped by a PDMS cover. The FRLs are placed later after micro fabrication. A photo of the chip combining several cavities with different lengths is shown in Fig. 2. The analyte is passed between the mirrors enabling its detection from the shift of resonance peaks of the transmission spectra. The spectra are obtained by recording the output from an optical detector while varying the input light wavelength from a tunable laser in the near infrared band. The spectrum of a cavity of 318 µm physical length filled with DI-water is presented in Fig. 3 showing the narrow line widths of the resonance peaks. The peak has a linewidth of 0.44 nm, which provides a Q factor of 3649.
Mixtures of ethanol and DI-water with different ratios are used as analytes with different refractive indices to exploit the device as a micro-opto-fluidic refractometer, which are plotted in Fig. 4. The sensitivity is obtained from the slope of the linear plot in Fig. 5 of the resonance wavelength shift versus the difference in refractive index between the analytes and the DI-water, which is taken as a reference. The cavity used has a physical length of 256 µm and the obtained sensitivity is 1000 nm/RIU.