Abstract
We report on a highly sensitive measurement of the relative humidity (RH) of moist air using both the surface plasmon resonance (SPR) and Bloch surface wave resonance (BSWR). Both resonances are resolved in the Kretschmann configuration when the wavelength interrogation method is utilized. The SPR is revealed for a multilayer plasmonic structure of SF10/Cr/Au, while the BSWR is resolved for a multilayer dielectric structure (MDS) comprising four bilayers of TiO/SiO with a rough termination layer of TiO. The SPR effect is manifested by a dip in the reflectance of a p-polarized wave, and a shift of the dip with the change in the RH, or equivalently with the change in the refractive index of moist air is revealed, giving a sensitivity in a range of 0.042–0.072 nm/%RH. The BSWR effect is manifested by a dip in the reflectance of the spectral interference of s- and p-polarized waves, which represents an effective approach in resolving the resonance with maximum depth. For the MDS under study, the BSWRs were resolved within two band gaps, and for moist air we obtained sensitivities of 0.021–0.038 nm/%RH and 0.046–0.065 nm/%RH, respectively. We also revealed that the SPR based RH measurement is with the figure of merit (FOM) up to 4.7 × 10−4 %RH, while BSWR based measurements have FOMs as high as 3.0 × 10 %RH and 1.1 × 10−3 %RH, respectively. The obtained spectral interferometry based results demonstrate that the BSWR based sensor employing the available MDS has a similar sensitivity as the SPR based sensor, but outperforms it in the FOM. BSW based sensors employing dielectrics thus represent an effective alternative with a number of advantages, including better mechanical and chemical stability than metal films used in SPR sensing.
Highlights
Surface plasmon resonance (SPR) based optical sensors as the heart of a mature technology in the field of optical sensing have a number of applications in physics [1,2], chemistry [3], biology [4], and other fields [5,6,7]
Bloch surface waves (BSWs) based sensors employing dielectrics represent an effective alternative with a number of advantages, including better mechanical and chemical stability than metal films used in SPR sensing
To demonstrate that both multilayer structures have potential to be employed in sensor applications, the setup shown in Figure 7 and the wavelength interrogation were utilized in the relative humidity measurements at a temperature of 22.5 ◦ C
Summary
Surface plasmon resonance (SPR) based optical sensors as the heart of a mature technology in the field of optical sensing have a number of applications in physics [1,2], chemistry [3], biology [4], and other fields [5,6,7]. The SPR sensors utilize the interaction of light with free electrons at a metal-dielectric interface [8], and are very sensitive to the refractive index changes in a very thin layer at the sensor surface. The current trend in the sensing approaches prefers to utilize the wavelength interrogation and so the reflectance responses computation of the SPR and BSWR based systems is desirable. As in both of the mentioned cases, one deals with thin film structures, the transfer matrix method (TMM), often used in optics of layered media [71,72], is used in most cases [73].
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