Shear-Horizontal Acoustic Plate Mode Sensors
Shear-horizontal acoustic plate mode (SH-APM) sensors are quartz plates with thickness of a few wavelengths, where shear-horizontal (SH) waves are generated by means of two IDTs positioned on one surface of the plate.
What is a Shear-Horizontal Acoustic Plate Mode (SH-APM) Sensors?
SH waves have particle displacement predominantly parallel to the plate surface and perpendicular to the propagation direction along the separation path between the two IDTs and hence are suited for operation in contact with liquid. Typical operation frequencies of SH-APM sensors are 20-200 MHz. APMs are a series of plate modes with slightly different frequencies. The difference between these frequencies decreases with decreasing plate thickness. To select a dominant SH mode, material and crystal cut, IDT design and oscillator electronics must be optimized. APMs have antinodes on both device surfaces so that each of them can be used as a sensing surface. In particular, the electrode-free face can be made (bio)chemically active and analysis in solution can be performed with a complete separation between the electric side and the liquid side.
Struktur Of Shear-Horizontal Acoustic Plate Mode (SH-APM) Sensors
Surface Transverse Wave Sensors
Surface transverse wave (STW) sensors are devices in which shear vibrations are confined in a thin surface area on the face where the IDTs are placed. This wave confinement is obtained by inserting a metallic grating between the IDTs that introduces a periodic perturbation in the wave path and lowers the wave velocity at the surface.
Since the vibration energy density is concentrated on a thin layer near the surface, the device is very responsive to surface perturbations and, in particular, it provides a high mass sensitivity. As shear vibrations are predominant, STW sensors (also called SH-SAW) are indicated for in-liquid applications and are mainly used with chemically-modified surfaces for analysis in solutions.
Love Wave Sensors
Love wave (LW) sensors are rather similar to STW sensors in that they involve shear vibrations confined in the upper surface.
The wave confinement is in this case obtained by depositing a thin layer of a material with low acoustic-wave velocity over a quartz plate where two IDTs are realized. Such an added overlayer, typically of silicon dioxide or polymethylmethacrylate (PMMA), works as a waveguide and keeps most of the vibration energy localized close to the surface, regardless of the plate thickness. This has the same positive effect on the mass sensitivity as the gratings in STW sensors and, once again, in-liquid operation is permitted by the shear-mode vibrations.
Love-mode sensors are mainly used in (bio)chemical analysis in solutions. A generalized Love-wave theory considers APMs and Love waves as the two solutions of the dispersion equation of a substrate with finite thickness.
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