Extreme Phononics

Ultrafast Photonics and Extreme Phononics

Over the years, Prof Sun's laboratory has successfully devised a mechanism to generate sub-terahertz (THz) acoustic waves by combining femtosecond mode-locked laser (pulsewidth < 200 fs) and semiconductor quantum well structures as optical piezoelectric transducers (Lin et al., 2005; Sun, Liang, & Yu, 2000). This technique has allowed Sun and his team to delineate the physical properties of substances that is not only previously unknown, but also unapproachable using conventional methods.

Building on this technique with various sophistications to enhance the lateral resolution and penetration depth (Lin et al., 2007; Mante, Wu, et al., 2013), Prof Sun and his team was able to explore physical phenomena which led to better understanding of heat transport and the properties of interfacial substances. This technique is often referred to as nanoultrasonics or femtosecond acoustics.

In quantum mechanics, the generated acoustic waves are a bunch of coherent acoustic phonons which are heat carriers in most non-metals. This makes femtosecond acoustics a great tool to explore heat behaviour in various scientific regimes. Sun and his team has made many important findings in this aspect, such as interfacial energy transfer from graphene to adjacent substrate longitudinally (Chen et al., 2014) as well as understanding the Kapitza anomaly (Mante, Chen, Wen, Sheu, & Sun, 2013). On the other hand, heat conductance in amorphous materials has long been of interest - as amorphous materials are abundant in our day to day lives. For instance, amorphous materials are present in various semiconductor devices, and as heat dissipation in these semiconductors is tightly linked to their performance, investigating heat conduction in amorphous materials could provide better strategies for thermal management of modern semiconductor devices. In addition to this, heat balance plays a vital role in various biological functions and processes, and as biological systems are composed primarily of amorphous water, the study of heat conduction in amorphous structures is critical in understanding the mechanistic in these biological processes. Given the importance and implication of heat conduction in amorphous materials, this area is still challenging to decipher and remain poorly understood. Prof Sun and his team has been working towards understanding the heat transfer phenomenon in amorphous materials, including the famous Boson Peak, an anomaly of thermal property observed in amorphous materials.

Nanoultrasonics/nanoacoustics (or femtosecond acoustics) has also capacitated the examination of interfacial substrates, including "interfacial water" - something which has intrigued scientists for decades. Interfacial water is of fundamental importance in various phenomena and has a broad spectrum of applications ranging from surface wetting, protein folding, electrolysis to hydrogen generation. Although the study of interfacial water has been attempted using many other strategies, the angstrom-scale feature and the picosecond relaxation dynamics of water have made the water-water and water-substrate interaction at the water-solid interface challenging to elucidate. By applying the femtosecond acoustic pulse with a subnanometer pulsewidth, Prof Sun's laboratory was the first to demonstrate the measurement of the mass-density, visco-elastic properties and hydration structure of interfacial water in a non-invasive manner with both high temporal and spatial resolution(Mante et al., 2014). Through exploration of the interfacial chemical reactions, such as photoelectrochemical water-splitting, the atomic-layer-by-layer oxidation process was in situ visualized (Shen et al., 2017) for the first time in Sun's laboratory.
Currently we aim to further develop new methods of excitation and detection of THz coherent acoustic waves based on metallic nano-films, and to establish a proper theoretical model for opto-acoustic interaction and phonon transport in the nano-scale regime. It is noted that we have recently demonstrated a record-short acoustic pulse with a pulsewidth on the order of 300 fs generated by 2DEG (Wu et al., 2020). Through further advancing the extreme phononics technology, The main scientific objective is to clarify the crucial bonding force interaction in the vdWs heterojunctions (Chen et al., 2014), and to investigate the corresponding thermal transport behaviors.

References.

C.-K. Sun, J.-C. Liang, and X.-Y. Yu, “Coherent acoustic phonon oscillations in semiconductor multiple-quantum-wells with piezoelectric fields,” Physical Review Letters 84 (1), pp. 179-182 (2000).
K.-H. Lin, G.-W. Chern, C.-T. Yu, T.-M. Liu, C.-C. Pan, G.-T. Chen, J.-I. Chyi, S.-W. Huang, P.-C. Li, and C.-K. Sun, “Optical piezoelectric transducer for nano-ultrasonics,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 52 (8), pp. 1404-1414 (2005).
K.-H. Lin, C.-M. Lai, C.-C. Pan, J.-I. Chyi, J.-W. Shi, S.-Z. Sun, C.-F. Chang, and C.-K. Sun, “Spatial Manipulation of Nanoacoustic waves with a Nanoscale Spot Size,” Nature Nanotechnology 2 (11), pp. 704-708 (2007).
P.-A. Mante, C.-C. Chen, Y.-C. Wen, J.-K. Sheu, and C.-K. Sun, “Thermal boundary resistance between GaN and cubic ice and THz acoustic attenuation of cubic ice from complex acoustic impedance measurements,” Physical Review Letters 111 (22), 225901 (2013).
P.-A. Mante, Y.-C. Wu, C.-Y. Ho, L.-W. Tu, and C.-K. Sun, “Gigahertz Coherent Guided Acoustic Phonons in AlN/GaN Nanowire Superlattice,” Nano Letters 13 (3), pp. 1139-1144 (2013).
I-J. Chen, P.-A. Mante, C.-K. Chang, S.-C. Yang, H.-Y. Chen, Y.-R. Huang, L.-C. Chen, K.-H. Chen, V. Gusev, and C.-K. Sun, “Graphene to Substrate Energy Transfer through Out-of-plane Longitudinal Acoustic Phonons,” Nano Letters 14, pp. 1317-1323 (2014).
P.-A. Mante, C.-C. Chen, Y.-C. Wen, H.-Y. Chen, S.-C. Yang, Y.-R. Huang, I-J. Chen, Y.-W. Chen, V. Gusev, M.-J. Chen, J.-L. Kuo, J.-K. Sheu, and C.-K. Sun, “Probing Hydrophilic Interface of Solid/Liquid-Water by Nanoultrasonics,” Scientific Reports 4, 6249 (2014).
Shen, C.-C., M.-Y. Weng, J.-K. Sheu, Y.-T. Yao, and C.-K. Sun, “In situ monitoring of chemical reactions at a solid-water interface by femtosecond acoustics,” Journal of Physical Chemistry Letters 8 (21), pp. 5430-5437 (2017).
C.-L. Wu, V. Gusev, L.-H. Peng, J.-K. Sheu, and C.-K. Sun, “Ultra-short photoacoustic pulse generation through hot electron pressure in two-dimensional electron gas,” Optics Express 28 (23), pp. 34045-34053 (2020).