Highly Sensitive Sensor Structure Based on Sol-Gel Waveguide Films and Grating Couplers

Abstract

The technologies of optical planar evanescent wave chemical and biochemical sensors require chemically resistant, high refractive index waveguide films having very good optical transmission properties. In this paper we present such two-compound SiOx:TiOy waveguide films fabricated by using the sol-gel method and the dip-coating technique. These films not only have high optical quality and low propagation losses but also an extremely high refractive index of >1.90 (λ = 632.8 nm). Further we demonstrate efficient and simple sensing structures, designed and fabricated based on these films. For this purpose, grating couplers with a period of Λ = 417 nm were fabricated on the interface between a waveguide film and cover using the single-step nanoimprint method. These sensing structures were tested as planar refractometers. The results of the theoretical analysis on the basis of which the structures were designed as well as results of their experimental characterization are presented in this work. Consequently, the relationship between parameters and the sensitivity of investigated sensing structures is discussed. As a result, the profitable properties of the designed grating coupler sensors are verified and excellent consistency between the results of the theoretical analysis and experimental results is achieved.

High Refractive Index Silica-Titania Films Fabricated via the Sol–Gel Method and Dip-Coating Technique—Physical and Chemical Characterization

“…For the waveguide shown in Figure 14 ( n 1 = 1.9383, d = 145.7 nm), the determined optical losses are, respectively, a TM = 4.343 m = 1.1 dB/cm for mode TM 0 and a TE = 4.343 m = 2.7 dB/cm for mode TE 0 (at λ = 676.7 nm). These losses are several times higher than those for the previously reported SiO x :TiO y waveguide films with lower refractive indices [ 40 , 42 ]. This result does not come as a surprise to us, because in the composite SiO x :TiO y films presented here we have a much higher content of titanium.…”

“…For this purpose, the reflection spectra of the film and those of the substrate were compared. When the film is optically homogeneous, then in the spectral range away from the absorption edge, the interference minima of the reflection spectrum of the film lie on the reflection characteristics of the substrate [ 41 , 42 ].…”

“…The subject of this work involves composite films SiO x :TiO y of high refractive index, fabricated by the sol–gel method and dip-coating technique. Binary composite films SiO x :TiO y with the proportion of components SiO x :TiO y = 1:1 have been investigated by us for many years [ 38 , 39 , 40 , 41 , 42 ]. The films developed by us have high refractive index n ~1.8, low optical losses, they remain stable over many years [ 40 ] and are characterized by high chemical and optical homogeneity [ 41 ].…”

“…In this work, we present the results of our research on SiO x :TiO y composite films with even higher refractive indexes ( n ~1.94 at wavelength 632.8 nm), also fabricated by the sol–gel method and dip-coating technique. In the waveguide films of the refractive index of n ~1.91 at wavelength 632.8 nm, we produced grating couplers whereof sensor properties were the subject of our earlier work [ 42 ]. The optical properties of the SiO x :TiO y composite waveguide films presented here were investigated using spectrophotometric and ellipsometric methods.…”

Mode Sensitivity Exploration of Silica–Titania Waveguide for Refractive Index Sensing Applications

“…This fabrication procedure has several fundamental advantages, including: The first four of the above-mentioned advantages have been demonstrated in our previous research [6]. The possibility of using the NIL technique was partially demonstrated with the development of shallowly patterned vertical grating coupler sensors [17]. The scalability will be investigated in our forthcoming research after the successful development of a fully efficient WG film fabrication and patterning technique when the technology will reach the industrial maturity standard.…”