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${}_2$/SiO${}_2$ with a rough termination layer of TiO${}_2$. 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${}^{-1}$, while BSWR based measurements have FOMs as high as 3.0 × 10${}^{-3}$ %RH${}^{-1}$ and 1.1 × 10⁻³ %RH${}^{-1}$, 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.

A one-dimensional photonic crystal (1DPhC) with a defect layer is utilized as an optical filter in a simple realization of narrow linewidth LED-based sources. The 1DPhC comprising TiO2 and SiO2 layers is characterized by two narrow defect mode resonances within the 1DPhC band gap, or equivalently, by two peaks in the normal incidence transmittance spectrum at wavelengths of 625.4 nm and 697.7 nm, respectively. By combining the optical filter with LEDs, the optical sources are employed in interferometry experiments, and the defect mode resonances of a Lorentzian profile with linewidths of 1.72 nm and 1.29 nm, respectively, are resolved. In addition, a simple way to tune the resonances by changing the angle of incidence of light on the optical filter is demonstrated. All-dielectric optical filters based on 1DPhCs with a defect layer and combined with LEDs thus represent an effective alternative to standard coherent sources, with advantages including narrow spectral linewidths and variable output power, with an extension to tunable sources.

Spectral-domain resonances for cavities formed by two distributed Bragg reflectors (DBRs) were analyzed theoretically and experimentally. We model the reflectance and transmittance spectra of the cavity at the normal incidence of light when DBRs are represented by a one-dimensional photonic crystal (1DPhC) comprising six bilayers of TiO${}_2$/SiO${}_2$ with a termination layer of TiO${}_2$. Using a new approach based on the reference reflectance, we model the reflectance ratio as a function of both the cavity thickness and its refractive index (RI) and show that narrow dips within the 1DPhC band gap can easily be resolved. We revealed that the sensitivity and figure of merit (FOM) are as high as 610 nm/RIU and 938 RIU${}^{-1}$, respectively. The transmittance spectra include narrow peaks within the 1DPhC band gap and their amplitude and spacing depend on the cavity’s thickness. We experimentally demonstrated the sensitivity to variations of relative humidity (RH) of moist air and FOM as high as 0.156 nm/%RH and 0.047 %RH${}^{-1}$, respectively. In addition, we show that, due to the transmittance spectra, the DBRs with air cavity can be employed as spectral filters, and this is demonstrated for two LED sources for which their spectra are filtered at wavelengths 680 nm and 780 nm, respectively, to widths as narrow as 2.3 nm. The DBR-based resonators, thus, represent an effective alternative to both sensors and optical filters, with advantages including the normal incidence of light and narrow-spectral-width resonances.

A spacer-based cavity with distributed Bragg reflectors (DBRs) is employed in a high-performance gas sensing utilizing the wavelength interrogation. The DBR is formed by a one-dimensional photonic crystal (1DPhC) comprising six bilayers of TiO2/SiO2 with a termination layer of TiO2, and the spacer is a Ti layer deposited partially on one of the DBRs. The reflectance spectra of the sensor are measured in visible and near-infrared spectral regions at a near-normal incidence of light for a humid air, and narrow dips within the 1DPhC band gap due to the cavity modes are resolved. Two thicknesses of the spacer are considered and it is revealed that the spacer thickness change is accompanied by nearly five-fold sensitivity enhancement. The sensitivity to relative humidity (RH) of a humid air, figure of merit (FOM) and the limit of detection reached 0.444 nm/%RH, 0.178 %RH−1 and 0.022 %RH, respectively. The theoretical analysis showed that the RH response of the sensor is due to the porous termination layer of the 1DPhC, and strong dependence of the sensor sensitivity and FOM on both the spacer thickness and the position of the resonance dip in the band gap was confirmed. A sensor employing a spacer-based cavity with DBRs thus represents an effective alternative with advantages such as sensing at normal incidence of light with high sensitivity and FOM.

Bloch surface waves (BSWs), generated at the interface of a truncated one-dimensional photonic crystal (1DPhC) and the adjacent medium (analyte), are accompanied by narrow resonance dips that are very advantageous compared to too wide resonance dips associated with the surface plasmon resonance (SPR) phenomenon. Consequently, BSW-based sensors have been thoroughly studied and applied in the field of optical sensors, but their sensitivity to gaseous analytes does not outperform the sensitivity of the SPR-based sensors. One of the possible solutions to enhance the sensitivity represents a metal-dielectric 1DPhC. We report on a sensing concept for gaseous analytes based on the wavelength interrogation and resonances supported by a metal-dielectric 1DPhC in the Kretschmann configuration. For a metal-dielectric 1DPhC comprising bilayers of TiO $$_2$$ /Au with a termination layer of TiO $$_2$$ , we show that the Bloch-like SW-based resonances are resolved for both TE and TM waves. For the TE wave and the refractive index (RI) in a range of 1–1.0015, a sensitivity of 10,900 nm/RIU, a figure of merit (FOM) of 474 RIU $$^{-1}$$ , and a limit of detection (LOD) of 9.3 $$\times$$ 10 $$^{-6}$$ RIU were reached. The analysis extended to the 1DPhC with the modified thicknesses of TiO $$_2$$ layers and gas, whose RI changes in a range of 1.0002–1.0022, leads to the sensitivity and FOM in a range of 10,680–28,000 nm/RIU and 434–1217 RIU $$^{-1}$$ , respectively, and to a very low LOD of 3.6 $$\times$$ 10 $$^{-6}$$ RIU for the TM wave. This research is the demonstration of exceptional properties of the Bloch-like SW-based sensors employing metal-dielectric 1DPhCs that can be used in a simple sensing of a wide range of gaseous analytes.

A new concept of an extrinsic fiber-optic gas sensor employing a cavity with distributed Bragg reflectors (DBRs) is proposed and experimentally demonstrated using the wavelength interrogation. The DBR is represented by a one-dimensional photonic crystal (1DPhC) comprising six bilayers of TiO _² /SiO ₂ with a termination layer of TiO ₂ , and the cavity of the adjustable thickness is realized by means of springs separating two parts of the fiber connector adapter to which the DBRs are glued. Two different extrinsic fiber-optic gas sensors are experimentally analyzed with results manifesting different technologies with which the DBRs are manufactured. The transmission spectra of the fiber-optic sensors are measured in visible and near-infrared spectral regions for a humid air, and we revealed, in accordance with the theory, narrow peaks within the 1DPhC band gap due to the cavity modes. The first sensor employs porous DBRs and has a maximum sensitivity at low relative humidities (RHs) of air, and the sensitivity to the RH of air, figure of merit (FOM) and the limit of detection (LOD) reached at 25 %RH values of 0.566 nm/%RH, 0.094 %RH ^-1 and 0.018 %RH, respectively. The second sensor is characterized by the sensitivity increasing with the RH, and the sensitivity and FOM attained at 70 %RH values of 0.109 nm/%RH and 0.017 %RH ^-1 , respectively. An extrinsic fiber-optic gas sensor employing a cavity with DBRs of adjustable thickness has the potential to be used in a great number of gas sensing applications.