FEMTO-ST-Franche Comté Electronique Mécanique Thermique et Optique - Sciences et Technologies
Video de présentationFrom basic research to industrial partnership
Micron-scale phononic crystals
- Phononic crystals
- Millimeter-sized phononic crystals
- Micron-scale phononic crystals
Micron-scale phononic crystals
Surface Acoustic Waves (SAW's) are a specific type of elastic waves that are guided along the surface of a material and confined in the direction normal to the surface. SAW-based components are already extensively used for high-frequency applications, such as wireless telecommunication systems. In this case, they are usually directly generated and detected at the surface of a piezoelectric surface by use of interdigital transducers (IDT's). A phononic crystal for SAW can be realized by fabricating an array of holes or dots on a piezoelectric semi-infinite medium. As the SAW's are naturally guided and confined to the surface, the phononic structure brings a means to control the elastic wave propagation in the surface plane. This can lead to the realization and the integration of complex signal processing functions that cannot currently be achieved through conventional acoustics, hence leading to a new generation of SAW devices.
The group has recently obtained two results worth of notice on the realization of such phononic devices: the demonstration of a phononic band gap for SAW in a square lattice lithium niobate phononic crystal and the realization of novel surface wave devices based on IDTs deposited on a mirror exhibiting an omnidirectional band gap.
Lithium niobate is a piezoelectric material widely used in the field of radio-frequency acoustics but also in integrated optics. A theoretical study performed within the group has has given evidence for the first time of the existence of a complete band gap for SAW in a square lattice phononic crystals. We have fabricated and tested several lithium niobate phononic crystal and have obtained an experimental demonstration of the phenomenon. The next step is then to demonstrate the possibility to realize resonant cavities or phononic waveguides to confine even further the elastic energy.
Lithium niobate phononic crystal fabricated using RIE (pitch: 10µm)
Another example of a phononic device is based on IDT's fabricated atop a an omnidirectional mirror. This last type device could lead to a real breakthrough in the field of SAW components. The main idea lies in preventing any leakage in the generated elastic energy by fabricating a perfect elastic mirror on the surface of a given substrate (e.g. silicon). This will prohibit elastic wave propagation towards the bulk substrate, whatever the wave polarisation or direction. The deposition of a piezolectric layer over the mirror allows to generate SAW's using standard IDT's, hence allowing to obtain localized, propagative modes along the surface. These modes can exhibit very high resonance frequencies while the corresponding IDT spatial frequency remains at a very reasonable level. As an example, we have experimentally observed a resonance frequency at 7.5 GHz excited by an IDT with a period equal to 3.6 µm, meaning that the "equivalent" phase velocity was measured to be around 54000 m.s-1.
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