Overlayer thickness

For overlayer thicknesses of a few atomic or molecular layers, the supporting metal can produce surface-enhanced fields at the surface of the overlayer. Then, composition and structure of the overlayer surface can be analyzed by SERS spectroscopy [4.291]. 

From these figures it is apparent how the HRI overlay stretches the cladding mode fields toward itself for both modes and both overlay thicknesses. In addition, for the same cladding mode, the field content within the overlay increases as the overlay thickness is increased (see Fig. 3.4a). Finally, for a given overlay thickness, the field content within the overlay of higher order modes is higher compared with low order modes (see Fig. 3.4b)31. 

The same observations are true for the evanescent wave of the modes in the surrounding medium. Moreover, these changes in the field distributions are accompanied by an increase of the effective refractive indices of the cladding modes, as will be clarified later. For this reason and by virtue of (3.1), the resonance wavelengths of the coated LPG are expected to blue-shift in response to an overlay thickness change. 

The mode transition is shown in Fig. 3.5b-d for different overlay thicknesses, namely 200,250, and 300 nm. As observed in the figures, the transition region moves to lower SRIs as the overlay thickness increases, approaching the ambient index of 1.33 where bare LPGs demonstrate a significantly lower sensitivity. As a secondary effect, the SRI sensitivity becomes smaller as the overlay thickness increases. However, even if the absolute sensitivity decreases, the relative sensitivity in comparison to the bare device still increases as will be shown experimentally. 

The coupling coefficients vs. the SRI are reported in Fig. 3.6 b-d for overlay thicknesses of 200, 250, 300 nm, respectively, always with the same coating index of 1.578. From these figures, it can be clearly inferred that increasing the overlay thickness the coupling coefficients curves shift toward lower SRIs as it happens for the effective refractive index curves. 

Fig. 3.8 Effective refractive index sensitivity to overlay RI changes vs. overlay thickness for different cladding modes...

An overlay can be deposited onto the LPG filling up the chamber by means of a syringe with a solution of sPS in chloroform (typ. 2% by weight) and then emptying it out in few seconds. Different overlay thicknesses can be obtained by different extraction speeds and/or solution viscosities. The clathrate sPS film thus formed should be exposed to air for several hours and at a temperature not higher than 50°C, in order to extract chloroform and obtain the empty 8 form sPS layer35. 

The overlays thickness estimation is possible exploiting a twin fiber placed in the same deposition chamber and subject to the same coating procedure. The coated twin fiber can be cut by a precision cleaver and analyzed by scanning electron microscopy (SEM). One of the deposited thin overlay is clearly observable in the SEM image of the fiber section reported in Fig. 3.13. 

The use of the dip-coating technique allows to obtain different overlay thicknesses by acting on the solution viscosity and extraction speed as stated by the Landau-Levich equation (see (3.14)). In particular, thicker overlays can be obtained by increasing the extraction speed and/or by increasing the solution viscosity. 

It is worth noting that in good agreement with the numerical analysis, the transition is slower and less marked as the overlay thickness increases. However, much higher sensitivities than those of the bare device can be obtained in correspondence of air (n = 1), water (n = 1.33), or other solutions as surrounding media, by using the suitable overlay thickness. 

In this section, the sensitivity characteristics of HRI-coated LPGs have been investigated to outline their dependence on the overlay thickness and mode order. In addition, the experimental results here presented provide the basic design criteria for the development of highly sensitive in fiber refractometers and chemical sensors for specific SRI ranges. 

The mechanism that induces such a change in the resonance wavelength behavior is the transition of the lowest order cladding mode into an overlay guided mode and the consequent mode redistribution. Here, the attention is focused on the analysis of the transition curves to determine the novel sensitivity characteristics of HRI-coated LPGs and to outline the dominant role of the overlay thickness and the mode order. 

Fig. 3.22 Sensitivity characteristics of the mode LPos for different overlay thickness values...

It is worth noting that, although the maximum sensitivity decreases for thicker overlays, the sensitivity gain with respect to the bare device increases. In fact, the sensitivity gain functions are obtained as the ratio between the sensitivity characteristics of coated and bare LPGs. They are reported for different mode orders and overlay thickness values in Fig. 3.23. 

On the basis of the reported results, it is evident how the selection of the overlay thickness and of the specific cladding mode offers a certain degree of flexibility to design chemical sensors with optimized sensitivity in the desired SRI range. 

Fig. 3.23 Sensitivity gain between coated and bare LPG for different mode orders and overlay thickness values...

In this experiment, we optimize the sensitivity of the coated LPG to the overlay RI changes for an SRI = 1.33 by acting on the overlay thickness. 

Here, a response time (10 90%) of about 21 min (tj) for 10 ppm chloroform concentration was measured. As the dynamic of the chemical sorption relies on the diffusion of the analyte through the sensitive overlay, the overlay thickness determines the time response of the sensor. Moreover, the dependence of the response times on the analyte concentrations can be attributed to the dependence of the diffusivity on the concentration of the absorbed analyte. 

A great feature of the proposed configuration is the possibility to select the overlay thickness and mode order to meet specific requirements in terms of sensitivity and response time. In addition, with commercially available and low cost spectrometers, a wavelength shift resolution of 0.05 nm is easily available, allowing sub-ppm chemical detection. 

Relative humidity changes were measured with a nano-scale HRI-coated LPG in a wide range from 38.9% to 100% RH with a sensitivity of 0.2 nm/%RH, and an accuracy of 2.3% RH59. The material used was a hydro-gel layer composed by, among other chemicals, acrylic acid and vinyl pyridine. Fine adjust of the components proportion made possible to obtain a refractive index of the gel of about 1.55. The overlay thickness was estimated to be in that case 600 nm. 

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