Abstract
We present a numerical and experimental study of a guided-mode-resonance (GMR) device for detecting surface-bound light-absorbing thin films. The GMR device functions as an optical resonator at the wavelength strongly absorbed by the thin film. The GMR mode produces an evanescent field that results in enhanced optical absorption by the thin film. For a 100-nm-thick lossy thin film, the GMR device enhances its absorption coefficients over 26 × compared to a conventional glass substrate. Simulations show the clear quenching effect of the GMR when the extinction coefficient is greater than 0.01. At the resonant wavelength, the reflectance of the GMR surface correlates well with the degree of optical absorption. GMR devices are fabricated on a glass substrate using a surface-relief grating and a titanium-dioxide coating. To analyze a visible absorbing dye, the reflection coefficient of dye-coated GMR devices was measured. The GMR-based method was also applied to detecting acid gases, such as hydrochloric vapor, by monitoring the change in absorption in a thin film composed of a pH indicator, bromocresol green. This technique potentially allows absorption analysis in the visible and infrared ranges using inexpensive equipment.
Highlights
Absorption spectroscopy is a powerful technique used in a wide variety of analytical applications in chemistry, biology, and physics [1, 2]
It can range from tens to hundreds of nanometers, resulting in an absorption signal that is below the detection limit of standard laboratory techniques
The exposure to HCl vapor increases the FWHMs of the resonant peak (Fig. 5(b)), which indicates that the acid gas diminishes the quenching effect on the GMR mode
Summary
Absorption spectroscopy is a powerful technique used in a wide variety of analytical applications in chemistry, biology, and physics [1, 2]. The ability to measure such weak absorption is especially important for detecting surface-absorbed analytes. Sophisticated techniques, such as ellipsometry, interferometry, and photoacoustics, have been applied to measure absorption in thin films [3,4,5]. Cavity-enhanced techniques effectively increase the optical path length by placing the sample within an optical cavity, where excitation light undergoes multiple passes through the sample [6,7,8]. In this paper we demonstrate and characterize a guided-mode resonance (GMR) substrate for measuring optical absorption by thin films.
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