Excitation and detection of acoustic phonons in nanoscale systems

Abstract

Phonons play a key role in the physical properties of materials, and have long been a topic of study in physics. While the effects of phonons had historically been considered to be a hindrance, modern research has shown that phonons can be exploited due to their ability to couple to other excitations and consequently affect the thermal, dielectric, and electronic properties of solid state systems, greatly motivating the engineering of phononic structures. Advances in nanofabrication have allowed for structuring and phonon confinement even down to the nanoscale, drastically changing material properties. Despite developments in fabricating such nanoscale devices, the proper manipulation and characterization of phonons continues to be challenging. However, a fundamental understanding of these processes could enable the realization of key applications in diverse fields such as topological phononics, information technologies, sensing, and quantum electrodynamics, especially when integrated with existing electronic and photonic devices. Here, we highlight seven of the available methods for the excitation and detection of acoustic phonons and vibrations in solid materials, as well as advantages, disadvantages, and additional considerations related to their application. We then provide perspectives towards open challenges in nanophononics and how the additional understanding granted by these techniques could serve to enable the next generation of phononic technological applications.

Fig. 1. (a) Schematic of a basic Raman scattering setup. (b) An example demonstrating the band shift and band broadening of a Raman signal for diamond nanocrystals as a function of number of unit cells (uc) with Raman line shape calculated via a Gaussian confinement approximation (RCF model).

Fig. 2. Unpolarized low frequency Raman spectra of MoS₂ for A–A and A–B stacking orientations. The measurement was performed using a S&I Raman spectrometer (https://www.s-and-i.de) using a 532 nm wavelength laser with an incident power below 100 μw focused on MoS₂ crystals on a SiO₂/Si substrate grown via chemical vapour deposition.

Fig. 3. Twist-angle dependence of breathing (B) and shear (S) modes in bi-layer WSe₂. (a) Schematic illlustration of the twisted bilayer stacks. (b) Raman spectra for varying stacking angle. Asterisks indicate the interlayer breathing modes and dashed lines indicate the moiré phonons. (c) Peak positions as a function of the stacking angle. Reproduced from ref. with permission from John Wiley and Sons, copyright 2021.

Fig. 4. Schematic representation of the (a) photoelastic and (b) moving boundary scattering mechanisms. The inset indicates the normal displacement of the moving boundary surface/interface.

Fig. 5. (a) Simplified schematic of a tandem Fabry–Pérot interferometer used to measure and enhance the backscattered signal. BS: beam splitter, M: mirror, P: prism, FP: Fabry–Pérot cavity. (b) Schematic of a single-stage VIPA spectrometer. C1: cylindrical lens, α: angle inclination of VIPA (this inclination is optional, but is sometimes used to increase the contrast of the interferometer), S1f: spherical lens. (b) Reproduced from ref. with permission from Optica publishing group, copyright 2011.

Fig. 6. Layout of a typical LDV system. (a) Schematic of scanning-LDV system. (b) Working principle of the LDV: an electrical excitation causes the suspended sample to vibrate. The laser from the interferometer in the scanning head is focused on a sample. A photo detector records the interference of the back scattered light with the reference beam. An output voltage which is proportional to the velocity of the scanned point parallel to the measurement beam is provided by the vibrometer. Finally, the output signal is obtained as velocity or displacement signal using the velocity or the displacement decoder. (b) Reproduced from ref. with permission from IOP Publishing, copyright 2009.

Fig. 7. (a) Schematic of an AFM-based probe system for detecting laser induced surface acoustic waves. A photo-sensitive detector (PSD) and laser diode (LD) are indicated. (b) Schematic of an AFAM setup. The vibrations of the cantilever are excited either by a transducer below the sample (transducer 1) or by a transducer which is positioned on the top end of the cantilever (transducer 2). The low frequency components of the beam deflection signal are used to control the static deflection of the cantilever. (c) Schematic of an ultrasonic near-field optical microscopy setup using a plasmonic probe. (b) Reproduced from ref. with permission from AIP Publishing, copyright 2020 and (c) from ref. with permission from AIP Publishing, copyright 2013.

Fig. 8. Pump–probe technique. (a) Pump–probe measurement configuration. (b) Typical optical response of a sample under pump–probe excitation.

Fig. 9. Comparison of the phonon lifetimes as a function of the frequency measured for different material systems compared to the phonon lifetime values as determined by the boundary, Akhiezer, and Landau-Rumer approach for each system. The total lifetime is estimated by Matthiessen's rule.

Fig. 10. Examples of IDT systems based on: (a) Piezoelectric actuation of acoustic waves; device with metal IDT patterned onto an (Al,Ga)As-based heterostructure (top); SEM images of the IDT (bottom). (b) Photo-elastic coupling; schematic of device where modulated light is absorbed and converted into acoustic waves via thermal expansion of illuminated gold grating (top); optical microscope image of a race-track resonator waveguide and gold grating (bottom). (c) Electro-optomechanical coupling; experimental setup of the nano-electro-optomechanical platform where Al concentric IDTs are used to piezoelectrically launch phonons in a nc-Si optomechanical system which is embedded in photonic circuitry (SEM image, bottom right). (a) reproduced from ref. with permission from American Physical Society, copyright 2017, (b) from ref. with permission from Springer Nature, copyright 2019, and (c) from ref. with permission from American Chemical Society, copyright 2022.