Refractive-Index-Enhanced Raman Spectroscopy and Absorptiometry of Ultrathin Film Overlaid on an Optical Waveguide

Waveguide-based spectroscopic techniques utilizing evanescent wave as a probe are ideal tools for surface and interface analysis. However, practical applications of these techniques are largely limited by the weak effective signals resulting from the restricted strength of interaction between the evanescent wave and the ultrathin sample immobilized at the waveguide surface; this is especially the case in light of the evanescent-wave excited waveguide Raman spectroscopy. Hence, we present a non-surface-enhanced Raman spectroscopy (SERS) method in this work to enhance the Raman spectroscopy of ultrathin film overlaid on the waveguide surface by intensifying the evanescent wave–sample interaction via manipulating the refractive index (RI) of the waveguide superstrate. Polarization-controllable and highly reproducible Raman enhancement can be achieved for the 10-nm-thick copper phthalocyanine (CuPc) ultrathin film deposited on the surface of a composite optical waveguide (COWG), with the enhancement factors being five and eight for transverse electric (TE) and transverse magnetic (TM) polarization, respectively. Additionally, enhanced absorption of the CuPc ultrathin film was also observed, with the absorbance increments being 0.155 and 0.165 for TE and TM polarization, respectively. Polarization controllability and good reproducibility are two great advantages of our method, and the implementation of this method should promote the practical applications of waveguide-based Raman and absorbance spectroscopic techniques effectively.