Evanescent fields in attenuated total reflection (ATR) spectroscopy have enabled molecular analysis in the mid-infrared (MIR) region for decades. Recently, thin-film single-mode waveguides have been introduced, significantly improving sample interactions through evanescent fields along their surfaces. However, their implementation demands precise coupling of an infrared laser beam into a sub-wavelength-thick layer, posing design challenges. Here, we introduce gradient refractive index waveguides that achieve robust coupling, substantially enhanced evanescent fields and absorption efficiency, surpassing the performance of conventional single-mode waveguides. Using in-house Finite-Difference Time-Domain (FDTD) simulations validated against commercial software, we demonstrate that these waveguides enhance sample interactions by an order of magnitude. Moreover, their increased robustness simplifies optical system design and broadens their applicability across a wide wavelength range, making them compatible with broadband tunable lasers, LEDs (light-emitting diodes), and they have the potential to accommodate less collimated light sources, such as thermal light sources. This novel waveguide platform promises to advance mid-infrared ATR sensor technologies, particularly in lab-on-a-chip systems and compact devices targeting biomedical and environmental applications.
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