We demonstrate an efficient sensing of both gaseous and aqueous analytes utilizing Bloch surface waves (BSWs) and guided waves (GWs) excited on a truncated one-dimensional photonic crystal (1DPhC) composed of six TiO${}_2$/SiO${}_2$ bilayers with a termination layer of TiO${}_2$. For the gaseous analytes, we show that 1DPhC can support the GW excited by an s-polarized wave and the theoretical shift of the resonance wavelength is linear for small changes in the analyte refractive index (RI), giving a constant RI sensitivity of 87 nm per RI unit (RIU). In addition, for the aqueous analytes, the GW excited by s-polarized and BSW by p-polarized waves can be resolved and exploited for sensing applications. We compare two designed and realized 1DPhCs with termination layer thicknesses of 60 nm and 50 nm, respectively, and show experimentally the differences in their very narrow reflectance and phase responses. An RI sensitivity and figure of merit as high as 544.3 nm/RIU and 303 RIU${}^{-1}$, respectively, are obtained for the smaller thickness when both s- and p-polarized BSWs are excited. This is the first demonstration of both very deep BSW-based resonances in two orthogonal polarizations and a very narrow resonance in one of them.

Spatial phase modulation in an imaging interferometer is utilized in surface plasmon resonance (SPR) based sensing of liquid analytes. In the interferometer, a collimated light beam from a laser diode irradiating at 637.1 nm is passing through a polarizer and is reflected from a plasmonic structure of SF10/Cr/Au attached to a prism in the Kretschmann configuration. The beam passes through a combination of a Wollaston prism, a polarizer and a lens, and forms an interference pattern on a CCD sensor of a color camera. Interference patterns obtained for different liquid analytes are acquired and transferred to the computer for data processing. The sensing concept is based on the detection of a refractive index change, which is transformed via the SPR phenomenon into an interference fringe phase shift. By calculating the phase shift for the plasmonic structure of SF10/Cr/Au of known parameters we demonstrate that this technique can detect different weight concentrations of ethanol diluted in water, or equivalently, different changes in the refractive index. The sensitivity to the refractive index and the detection limit obtained are −278 rad/refractive-index-unit (RIU) and 3.6 $\times {10}^{-6}$ RIU, respectively. The technique is demonstrated in experiments with the same liquid analytes as in the theory. Applying an original approach in retrieving the fringe phase shift, we revealed good agreement between experiment and theory, and the measured sensitivity to the refractive index and the detection limit reached −226 rad/RIU and 4.4 $\times {10}^{-6}$ RIU, respectively. These results suggest that the SPR interferometer with the detection of a fringe phase shift is particularly useful in applications that require measuring refractive index changes with high sensitivity.

Interferometric methods of optical sensing based on the phase shift of the Bloch surface waves (BSWs) and guided waves (GWs) supported by a one-dimensional photonic crystal are presented. The photonic crystal, composed of six SiO₂/TiO₂ bilayers with a termination layer of TiO₂, is employed in the Kretschmann configuration. Under resonance condition, an abrupt phase change is revealed, and the corresponding phase shift is measured by interferometric techniques applied in both the spectral and spatial domains. The spectral interferometric technique employing a birefringent quartz crystal is used to obtain interference of projections of p- and s-polarized light waves reflected from the photonic crystal. The phase shifts are retrieved by processing the spectral interferograms recorded for various values of relative humidity (RH) of air, giving the sensitivity to the RH as high as 0.029 rad/%RH and 0.012 rad/%RH for the BSW and GW, respectively. The spatial interferometric technique employs a Wollaston prism and an analyzer to generate an interference pattern, which is processed to retrieve the phase difference, and results are in good agreement with those obtained by sensing the phase shift in the spectral domain. In addition, from the derivative of the spectral phase shifts, the peak positions are obtained, and their changes with the RH give the sensitivities of 0.094 nm/%RH and 0.061 nm/%RH for the BSW and GW, respectively. These experimental results demonstrate an efficient optical sensing with a lot of applications in various research areas.

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.

One of many possible applications of a one-dimensional photonic crystal (1DPhC) is sensing utilizing the Bloch surface wave (BSW). In this paper, we present a method of relative humidity (RH) sensing based on the phase shift of the BSW supported by a truncated 1DPhC represented by a multilayered structure. The structure is composed of six TiO₂/SiO₂ bilayers with a termination layer of TiO₂. The BSW is excited by a total internal reflection of light in the Kretschmann configuration. A spectral interferometric technique is used to obtain a channeled spectrum due to projections of both reflected s and p polarized light waves. The phase is retrieved using a Windowed Fourier transform and spectral derivatives forming peaks are calculated. The sensing concept is based on tracking the derivative peak as a function of RH. A sensitivity to humidity of 0.028 nm/%RH and figure of merit of 0.0042 %RH₋₁ were determined.