**2. SPRs properties of single ZnO layer surfaces**

This section deals with the correlation between SPR phenomena and evanescent fields on metallic ZnO: Ga film surfaces. The fabrication of ZnO: Ga film is introduced

#### *Biological Sensing Using Infrared SPR Devices Based on ZnO DOI: http://dx.doi.org/10.5772/intechopen.104562*

as follows [13]. ZnO: Ga films with 5% Ga content were grown on BK-7 glass substrates at 260°C using the pulsed laser ablation (PLD) method. ArF excimer laser pulses (193 nm, 3 Hz, and 1 J/cm2 ) were irradiated on a ZnO: Ga ceramic target in an O2 atmosphere of 10−4 Pa. The film thickness (*t*) was controlled within 41 to 180 nm range. A BK-7 glass substrate with a refractive index of 1.517 was applied using an ATR system with a BK-7 optical prism. SPR reflectance (*R*p/*R*s) was measured using a Kretschmann-type ATR system connected to a Fourier-transform NIR spectrometer, where *R*p and *R*s indicate the *p*- and *s-*polarized reflection lights, respectively. The spectral intensity of SPR reflectance was acquired in the wavenumber range of 7000– 4000 cm−1. For theoretical calculations, SPR reflectance spectra and their electric-field (*E*-field) depths were simulated by Fresnel relations of an *N*-multilayer model to calculate reflection coefficients under *s*- and *p*-polarized configurations [13, 24].

**Figure 2(a)** shows SPR reflectance spectra of a 174 nm-thick film as a function of incident angle (*θ*) from 62o to 72o in 2o increments. The peak position gradually shifted to higher wavenumbers with increasing incident angle, also confirmed by calculated SPR reflectance spectra (**Figure 2(b)**). The SPR behavior was not observed in the film samples with small thicknesses below 100 nm. The screened bulk plasmon resonances appeared in place of SPRs. **Figure 2(c)** shows a dispersion curve of a 164 nm-thick film sample, revealing consistency between the experimental

#### **Figure 2.**

*(a) Experimental and (b) calculated SPR reflectance spectra of a 174 nm-thick ZnO: Ga film. Water was selected as the dielectric medium. The incident angle of light (*θ*) was changed from 62o to 72o . (c) Experimental and calculated dispersion curves of SPRs. A dotted line indicates a light line in a water medium. (d) a depthdependent mean square evanescent field at calculated at an SPR peak position of θ = 72.*

(black circles) and calculated (black line) data, which were derived from the single SPR mode at the water-ZnO: Ga film interface. The result of *E*-field depth also evidenced this result. **Figure 2(d)** shows a depth profile of a mean-square evanescent field <*E*zz2 > at 4500 cm−1 of the *p*-polarized component along the z-direction. Here, the *x* and y directions are parallel to the film, while the *z* direction is normal to the film. The film sample had a < *E*zz<sup>2</sup> > value of 7.3 with a penetration depth (*δ*W) of 200 nm into the water medium, where *δ*W was determined at the depth at which the field decays by a factor of 1/*e*. The ZnO: Ga film provided a narrow field depth in the IR range, which was essentially different from Au film-based SPRs [25].

The SPR performance was evaluated by examining the bulk sensitivity using a change in the refractive index of glucose-water mixed solution. **Figure 3(a)** shows the SPR reflectance spectra at *θ* = 75o for a 107 nm-thick ZnO: Ga film using different glucose contents (0, 1 and 5 g/dL) [24]. The peak position systematically shifted to lower wavenumbers with increasing glucose content in water, showing a peak shift (Δ*v*) of 22 cm−1 at 1 g/dL. Furthermore, we experimentally and theoretically evaluated the sensitivity (*S*exp and *S*cal) at a wavenumber of 4500 cm−1. The detection sensitivity was expressed by the following expression:

$$S = \Delta \nu / \Delta n \tag{1}$$

where Δ*n* indicates the per unit change in the refractive index. The experimental *S*exp was determined to be 8300 cm−1/RIU (RIU: refractive index unit), which was similar to the calculated value (8600 cm−1/RIU) (**Figure 3(b)**).

This section introduced the SPR responses of single ZnO: Ga film surfaces in the IR range. We could clearly observe the SPR spectra and their field distributions. The bulk sensitivity was evaluated using the glucose-water solution to estimate sensing performance. However, the bulk sensitivity requires improvement for real-time monitoring of biological interactions.

#### **Figure 3.**

*(a) SPR reflectance spectra of the single film measured with varying glucose content in water from 0, 1 and 5 g/dL. (b) Correlation between peak shift (Δv) and glucose content in water for the single film. The change in refractive index (n) of a mixed solution consisting of glucose and water is also described in the upper horizontal axis. Black line and dots indicate calculated and experiment data (***Figure 4(b)** *and* **(c)** *of [24]). Copyright by the American Institute of Physics.*
