# A Novel Love Wave Mode Sensor Waveguide Layer with Microphononic Crystals
## 1 Introduction  
Surface acoustic wave (SAW) sensors, with their many advantages of small size, high sensitivity and wireless and passive form, have been applied in many areas of radar, television, phone and microelectromechanical systems (MEMS). Common types of SAW waves include the Rayleigh wave [1], shear horizontal wave [2], Love wave [3] and Lamb wave [4]. Rayleigh wave propagating in the surface of the piezoelectric is the earliest mode to be studied and widely used in the remote sensors of temperature and strain with high Q and low attenuation. However, when it is used in the liquid, it will have a serious loss of acoustic energy, as it has a large vertical displacement component. Different to the Rayleigh wave, the energy of the Love wave is focused on the waveguide layer and can propagate in the liquid; thus, the Love wave mode sensor can be used in the area of biochemistry [5, 6, 7]. Love wave mode is a kind of horizontal shear wave, of which the particle vibrates perpendicularly to the direction of propagation and the direction of the surface normal. The energy of the wave is trapped in the thin waveguide layer without radiating into a semi-infinite medium [8; 9]. The Love wave is sensitive to surface disturbance, and thus, it has application potential in a liquid environment due to its only shear displacement parallel to the substrate. The common Love wave mode sensor consists of a delay line structure with an emission transducer, receiving transducer, waveguide layer and sensitive layer [10], as shown in Figure 1. Usually, the wave in the piezoelectric wafer propagates perpendicularly to the transducers. When it approaches the waveguide layer, which has lower shear wave velocity, the Love wave mode will be excited with the vibration perpendicular to both the direction of wave propagation and the normal direction of the surface. When there are different target liquids (such as different concentrations and different permittivity) on the surface of the waveguide layer, the velocity of the Love wave mode will be changed. Thus, the detection signal will have different characteristics, which can represent different target liquids. Through the principle of the Love wave mode sensor, it can be observed that the performance of the Love wave mode sensor will be significantly influenced by the waveguide layer. Much research has been conducted on the waveguide layer materials [11; 12; 13; 14; 15; 16; 17; 18]. The ideal waveguide layer material should have low shear velocity, low density and also excellent elastic characteristics and low acoustic absorption; otherwise, it will lead to a large insertion loss, causing a large amount of noise. Thus, much research focuses on the waveguide layer materials with excellent uniformity, high density, high density, high surface flatness and low stress to improve the performance of devices. The employed waveguide layer materials include organic waveguide, inorganic waveguide, composite waveguide and gradient waveguide, such as SiO\({}_{2}\)[11; 12], ZnO [13; 14; 15], PMMA [16] and SUS [17; 18]. On the other hand, piezoelectric phononic crystals (PnCs) [19] are periodic elastic structures consisting of two different elastic materials, which have band gaps. This property has led to many explorations regarding the use of piezoelectric PnCs in SAW devices [20; 21; 22; 23; 24; 25]. These application explorations include SAW waveguides studied by [PERSON] in 2006 and [PERSON] in 2014, and SAW resonators studied by [PERSON] in 2010. They demonstrate the potential for using piezoelectric PnCs in SAW devices to improve their characters. However, most prior studies have focused on the Rayleigh wave devices. As the developing on-chip micro-nanosystems need many microstructures to achieve microchannels and microcontrol [26; 27; 28], to promote on-chip micro-nanosystems combined with Love wave mode sensors, this paper proposed a new Love wave mode delay line waveguide layer structure with microphononic crystals, which can be extended to be used as microchannels.