STSAW · Sub-THz Surface Acoustic Waves

Sub-THz Surface Acoustic Waves

As counterparts of natural earthquakes at the scale of km, microscale to nanoscale vibrations can be created and monitored in solids in laboratories through electronical or optical means. When the vibration is confined at the surface, namely, surface acoustic waves (SAWs), it is able to interact with other entities such as molecules, excitons, and magnons in the vicinity of the surface. Due to the confinement close to the surface, the surface condition including defects or inhomogeneities can be characterized by SAWs. Not only exhibit SAWs promising scientific applications, but also enable widespread usages in industry and our everyday life. The modern communication devices such as mobile phones and global positioning system (GPS) require SAWs for Radio frequency signal processing and filtering. With the development of smart home, sensors (e.g. pressure, humidity, chemicals) are also demanded elements. For satisfying the rapid growth of the communication market in the future, the bandwidth is required to be broadened and higher frequency should be accessed. The SAW devices are commonly realized by interdigital transducer (IDT) which consists of zipper-like metallic electrodes deposited on piezoelectric materials. In consequence, the frequency is limited by the electronics up to a few GHz. To break the electronic limitation, optically-controlled SAWs were realized by shining laser beams on the periodic structure of metallic grating deposited on a substrate, where the SAW frequency is constrained by the metallic grating pitch size produced by lithography or focused ion beam. The state-of-the-art frequency of SAWs is still below 100 GHz by current technology. In contrast, the bulk acoustic waves achieved in the superlattice (SL) which consists of stacked alternating two or more layers were already in the THz range. In fact, the SL can be sliced or cleaved along the layering direction to produce a nanostructured surface that shares the same periodicity and quality as the bulk structure. The epitaxy growth of SL enables the atomic-scale pitch precision and interface quality. That is to say, the cleaved SL surface provides a desired phononic periodic structure that is no longer confined by periodicity limitation to reach THz range for SAWs. Despite the existing theoretical investigation on such a structure, the experimental demonstration of SAWs on cleaved SLs has never been reported. This project combines nanostructure engineering and picosecond laser ultrasonics techniques, along with the aid of numerical calculation, to explore the SAWs in the earlier unreachable regimes. We aim to experimental realization of sub-THz (100 GHz – 1THz) SAWs (STSAWs) by applying femtosecond laser pump-probe spectroscopy on cleaved SLs (see Figure 1). The overall objectives of this project include: (1) Vibration − develop the first SL-based capabilities to optically control coherent SAWs and extend accessible frequencies to the 100 GHz - 1 THz band; (2) Transmission − STSAWs transmission between the emitter and the receiver; and (3) Application - showcase experiments over that frequency band to demonstrate classical SAW experiments can be conducted with STSAWs for the characterization of materials at nm to sub-nm distances from the surface. The project has achieved the spectral breakthrough for SAWs.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз