Shear horizontal surface acoustic waves (SH SAWs) are suitable for liquid- phase operation as they do not radiate energy into liquids avoiding any damping effects due to the fact that these waves have particle displacements that are transverse to the propagation direction and parallel to the surface plane (Figure 2.11 illustrates the propagation of a SH surface acoustic wave). By the use of a different crystal cut of a substrate, a shear horizontal surface acoustic wave (SH-SAW) instead of a vertical Rayleigh wave could be yielded. The transverse displacements of SH-SAWs, which
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are polarized parallel to the sensing surface, prevent the SH-SAW energy being radiated into the liquid. Out of the several substrate types such as LiTaO3, La3Ga5SiO14
and Quartz that exhibit SH-SAWs, the liquid sensing systems using LiTaO3 substrate
have been explored extensively. In general, the SH-SAW is sensitive to mass-loading, viscosity, conductivity and permittivity of the adjacent liquid.
Figure 2.11 Shear Horizontal Surface Acoustic Wave propagation. Adapted from [68]
Comparable to the SH-SAW is another type of acoustic wave called the surface transverse wave (STWs, i.e., shear bulk modes confined to the device surface by periodic metallic gratings), which is also a horizontally polarized shear wave (HPSW) developed by Baer et al. In order to solve the problem of high dielectric mismatch, a thick shielding layer was added on the sensor surface to reduce the high DC influence of water on the IDTs. These SH-SAW devices in ST-cut quartz crystal have a thin solid film or grating added to the surface to prevent wave diffraction into the bulk of the plate, the mass of which slightly reduces the wave velocity and these are also called STW sensors. Using this approach, lower attenuation values were achieved that potentially allowed the sensor to operate with a simple oscillator circuit in a bio sensing investigation [55]. As the STW device operates with surface shear horizontal particle displacements, it can be used for detection in both gas and liquid media. The spacing of the IDTs determine the resonance frequency of this device and its typical operating frequency is in the range of 30-300 MHz.
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Figure 2.12. STW propagation employing an energy trapping grating between IDTs
The particle displacement solution for a shear horizontal (SH) or acoustic plate mode (APW) is simply a plane shear wave having parallel propagation to the surface, with its amplitude independent of x3 (see Figure 2.12) inside the material. The phase
velocity is denoted by vt and the particle displacement associated with the nth order SH
mode (propagating in the x1direction) has only an x2-component and is given by
2 0cos 2 exp ( 1) 2 n n b u u x j t x b 2.35where b is substrate thickness, u2 is the surface particle displacement, n is the
transverse modal index (0,1,2,3…) and t is time. The propagation of the displacement profile down through the waveguide length (in the x1 direction) is described by the
exponential term in the equation with angular frequency
, wave number n and unperturbed propagation velocity of the lowest-order modev
0 and is given as2 2 0 b n v n 2.36 2.4.2.3 Love waves
A special class of SH-SAW sensors, called Love wave sensors, have proven to be the most sensitive acoustic sensor for liquid based sensing applications [74]. The Love wave effect is the acoustic resonance of a deposited waveguide layer on the top of an SH-SAW device, such that the energy of the shear horizontal wave is focused in that coating. Originally, the related physical effect was discovered by Love, who proposed that these waves were SH waves confined to a superficial layer of an elastic
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half-space deposited on a substrate made of another material with different acoustic properties and infinite thickness in comparison to the original wave guide layer. These transverse waves brought shear stresses into action as shown in Figure 2.13.
Overlaying a thin film on the surface of a substrate having the appropriate properties of a guiding layer, Love waves propagate near the surface which supports SH waves. The propagating energy is located in the waveguide layer and in the substrate face close to the interface because it is a surface wave. The shear acoustic wave velocity within the waveguide layer is lower compared to that of the substrate, which results in the acoustic wave being guided through the layer. Also, a reduction in attenuation can be obtained by adjusting the waveguide layer thickness to an optimum value. The first Love wave device for biosensor measurements based on quartz with a PMMA wave-guiding layer was presented in 1992 by Gizeli et al. The adsorption of IgG was investigated at the sensor surface, and anti-IgG and protein A were used as analyte [89]. Recently, devices based on Love waves have received increasing interest. A Love wave device based on sputtered SiO2 with an optimal layer
thickness was first presented by Du et al. in 1996 [55]. In comparison with the SH- SAW devices, SH-SAWs are limited by high noise levels and background interference from reflection off the lower surface. The waveguide layer (usually SiO2) in a Love
mode device confines the wave over the device top layer, eliminating the disadvantages associated with a similar SH device.
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With the exception that the Love waves propagate in waveguide layer on the IDT top surface, these waves have the same principle of generation as that of the SH- SAW waves. These devices’ resonance frequency is determined by the IDT finger spacing and the shear velocity in the waveguide layer. Obviously, the variation in the layer’s electrical and mechanical properties caused by the measurand, can be determined by monitoring the love waves with a high concentration of energy in the layer [90].