Inkjet Deposition of Sorbent Polymers on Photonic Integrated Circuit Sensing Elements for Waveguide-Enhanced Raman Spectroscopy

Photonic integrated circuit (PIC) sensing techniques have the potential to lead to new compact and low-power sensor systems for a variety of chemical and biological sensing applications, such as detection of low-concentration hazardous vapors. Waveguide-enhanced Raman spectroscopy (WERS) is one type of PIC sensing technique that takes advantage of the high level of chemical specificity of vibrational spectroscopy. Raman spectroscopy is a weak phenomena, however, and is generally used to analyze condensed phases solids and liquids. To increase the signal levels to allow for detection of vapors, two different enhancement mechanisms are used. Long pathlength single-mode evanescent optical spiral waveguides with lengths of several centimeters are used to obtain larger signal levels, while sorbent polymers designed to absorb and concentrate classes of vapors provide further enhancement of the signal.

Deposition of sorbent polymers on the WERS sensing elements is a critical element in developing a WERS sensor system. It is necessary to obtain an optimal and uniform thickness in the range of 300-700 nm in order to obtain the best sensor performance. Coatings that are too thin lead to optical losses, while coatings that are too thick lead to slow sensor response due to the time it takes for the absorbed analytes to diffuse through the polymer into the evanescent region above the waveguide. Deposition of different sorbent polymers on different sensing elements on the same chip is of interest for creating a multiplexed sensor that can target different classes of analytes simultaneously. This can be achieved using inkjet deposition to precisely deposit polymer coatings at controlled locations on a sensing element. Characterization of inkjet-deposited films will be shown, and the effects of film thickness and uniformity on WERS spectral analysis will be presented.