Periodic Reporting for period 1 - STSAW (Sub-THz Surface Acoustic Waves)
Reporting period: 2022-02-01 to 2024-01-31
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.
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.
Through this project, overall, we have successfully experimentally demonstrated optically-monitored Rayleigh SAWs and surface skimming acoustic waves on the unconventional platform of cleaved SLs with deeply sub-optical-wavelength nanometer periodicities (Figure 1). The proof-of-principle experiments on a 71 nm GaAs/AlAs SL revealed Rayleigh SAWs and skimming surface bulk waves in the range of 40−70 GHz by a 405/810 nm setup. Subsequently, this pioneer report together with the theoretical analysis ensured the further exploration in sub-THz range. On a platform of Al0.7Ga0.3As/GaAs SLs with only ~18 nm periodicity, a triplet of well-separated ~ 150, 200, 300 GHz coherent acoustic waves were observed by a 354/343 nm setup. They are consistent to the first-order zone-center Rayleigh, transverse and longitudinal modes in the dispersion relation and increase with the diminishing of the SL periodicity. This means that Rayleigh SAWs above 100 GHz, namely, sub-THz SAWs, have been for the first time achieved on the untraditional platform of cleaved SLs, satisfying this project’s main objective. It was also exciting that we even observed surface skimming longitudinal acoustic waves up to 1 THz on cleaved In0.2Ga0.8N/GaN SLs with an 8 nm periodicity. This encourages further development of sub-THz Rayleigh SAWs project towards higher frequency and the understanding of SAW behavior at such regime. Besides, the results revealed the dominant surface skimming transverse mode in opaque SL materials, which was not previously reported. This platform enabled the means to generate and detect surface skimming transverse waves which exhibit promising applications such as inspection of material elasticity and cracks. Importantly, for the SAW transmission at the sub-THz range, they have shown a few µm propagation distance on the Al0.7Ga0.3As/GaAs SLs. Lastly, considerable efforts have been also made on exploring new or optimized optical systems, on optimizing SL constituent layers for SAW transmission, on understanding the impact of oxidation and surface roughness. Our observations build a bridge from GHz towards THz opto-acoustic/acousto-optic transducers. For dissemination, communication, and exploitation, 5 conferences, 1 publication (so far), 1 summer school, 1 european researchers' night, several exhibitions, and further project collaboration were involved.
As a reminder, the highest reported frequencies of coherent Rayleigh SAWs monitored by metal gratings was ~90 GHz. The shortest reported period of metallic nanopatterns employed for the optical control of SAWs was 45 nm. The highest frequency of surface acoustic skimming longitudinal waves was up to 360 GHz realized by light gratings with a period of 28 nm induced extreme UV (EUV) radiation from the free electron laser. This project has enabled striking spectral progress beyond the state of the art. Firstly, we eventually achieved Rayleigh SAWs above 150 GHz on the platform of cleaved Al0.3Ga0.7As/GaAs SLs with ~18 nm periodicities. Secondly, surface skimming acoustic longitudinal waves up to 1 THz on the platform of cleaved In0.2Ga0.8N/GaN SLs, with 8 nm periodicities. Thirdly, the dominant surface skimming acoustic transverse waves were observed in the cleaved AlxGa1-xAs/ AlyGa1-yAs SLs, which were rarely reported before in opaque materials to the best our knowledge. Lastly, the propagation of Rayleigh SAWs in the sub-THz regime were observed on the cleaved Al0.3Ga0.7As/GaAs SLs.
The observation of the Rayleigh SAWs above 150 GHz and surface skimming bulk waves up to 1 THz paves a way for the future realization of the THz SAW opto-acoustic/acousto-optic transducers. In general, the vibration and propagation of sub-THz SAWs established a solid foundation for future development in fundamental science and industry applications. Especially, for the sustainable development of economy and society, the progress of sub-THz Rayleigh SAWs achieved in this project encourages the next generation of information and communication technology devices beyond the current 5G standard (600 MHz to 6 GHz).
The observation of the Rayleigh SAWs above 150 GHz and surface skimming bulk waves up to 1 THz paves a way for the future realization of the THz SAW opto-acoustic/acousto-optic transducers. In general, the vibration and propagation of sub-THz SAWs established a solid foundation for future development in fundamental science and industry applications. Especially, for the sustainable development of economy and society, the progress of sub-THz Rayleigh SAWs achieved in this project encourages the next generation of information and communication technology devices beyond the current 5G standard (600 MHz to 6 GHz).