Nanoscale This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online REVIEW View Journal | View Issue Excitation and detection of acoustic phonons in Open Access Article. Published on 09 September 2022. Downloaded on 5/12/2023 11:34:11 AM. Cite this: Nanoscale, 2022, 14, 13428 nanoscale systems Ryan C. Ng, *a Alexandros El Sachat, *b Francisco Cespedes, a,c Martin Poblet, a Guilhem Madiot, a Juliana Jaramillo-Fernandez, a Omar Florez, a,c Peng Xiao, a,c Marianna Sledzinska, a Clivia M. Sotomayor-Torres a,d and Emigdio Chavez-Angel *a 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 motiv- ating 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 Received 25th July 2022, of acoustic phonons and vibrations in solid materials, as well as advantages, disadvantages, and additional Accepted 15th August 2022 considerations related to their application. We then provide perspectives towards open challenges in DOI: 10.1039/d2nr04100f nanophononics and how the additional understanding granted by these techniques could serve to enable rsc.li/nanoscale the next generation of phononic technological applications. Introduction surface waves; and Sezawa,4 who demonstrated the existence of surface waves in layered structures. The propagation of SAW Pioneering studies of confined acoustic waves were first per- modes in these types of systems has been widely studied by formed by Lord Rayleigh1 in 1885. He demonstrated the exist- the seismology community, who were the first to study the con- ence of surface acoustic waves (SAWs) propagating along the finement of acoustic waves, although their relevance is perva- surface of an isotropic solid half-space. These waves are non- sive and extends throughout modern devices.5 Years later, the dispersive and elliptically polarized in the sagittal plane, with acoustic properties of other finite systems and geometries were velocities that are slightly smaller than that of bulk shear also studied. Pochhammer6 and Chree7 described the propa- waves, and with amplitudes that decay exponentially, confining gation of waves in infinite cylindrical rods. Lamb developed them to roughly within a wavelength of the surface. Following theoretical models to describe the natural vibration of his results, other scientists further developed this topic, par- spheres8 and suspended thin films.9 While all the aforemen- ticularly: Stoneley,2 who described waves that propagate along tioned models were originally applied in “bulk-like” or macro- the interface between two half-spaces, i.e., a generalization of scale systems, the results are scale invariant and still hold Rayleigh modes; Love,3 who described horizontally polarized down to the nano-10 and molecular11 scales. The quantization of the vibrational energy of an atom or a group of atoms in matter is a quasiparticle known as a a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, phonon. As phonons are bosonic quasiparticles, they obey Campus UAB, Bellaterra, 08193 Barcelona, Spain. E-mail: ryan.ng@icn2.cat, Bose–Einstein statistics and have an energy ħω and a pseudo- emigdio.chavez@icn2.cat momentum p = ħq, where ω = 2πf, f is the phonon frequency, ħ b National Center for Scientific Research “Demokritos”, 15310 Athens, Greece. is the reduced Planck constant (h/2π), and q is the wavevec- E-mail: alexandros.elsachat@icn2.cat c tor.12 Similar to other quasiparticles, the wavelength depen- Departamento de Física, Universidad Autónoma de Barcelona, Bellaterra, 08193 Barcelona, Spain dence of the phonon energy can be represented by a dis- d ICREA, Passeig Lluis Companys 23, 08010 Barcelona, Spain persion relation, or a relationship between the phonon fre- 13428 | Nanoscale, 2022, 14, 13428–13451 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Review quency and its wavevector. The phonon group velocity, vg = phononics has received less attention, in part due to the dω/dq, can be determined from the slope of a dispersion difficulty associated with the experimental excitation and relation curve. For bulk material, the dispersion relation of detection of acoustic phonons. Beyond characterization of acoustic phonons with short wavevector (i.e., near the Γ-point phonons, even the development and fabrication of platforms This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. in the first Brillouin zone) is linear and the group velocity in that operate at higher energies (i.e., GHz) are challenging due the material is constant.13 Upon decreasing the characteristic to limitations in nanofabrication and the difficulties associ- dimensions of the material, this linear dependence no longer ated with pushing fabrication resolution to a few nanometer holds, and the phonon energy is quantized into many discrete feature size. While phonons have traditionally been considered modes. This spatial confinement resulting from the reduction to be an inconvenience in most materials and devices, recent Open Access Article. Published on 09 September 2022. Downloaded on 5/12/2023 11:34:11 AM. of bulk material down to “finite” nanometer sizes affects a wide research has shown that phonons can be exploited for a wide array of physical properties such as the phonon density of variety of applications. Existing reviews have focused on states, group velocity, specific heat capacity, and electron– thermal phonons25,30,33–36 or have provided a holistic analysis phonon and phonon–phonon interactions, among others.14–19 of the interaction and transduction between photons, elec- Phonon statistics and the interaction of phonons with other trons, and phonons within electro- and opto-mechanical particles sets a limit to properties such as the electrical and systems,37,38 though a comparison of existing experimental thermal conductivity, sound transmission, reflectivity of ionic methods available to probe and directly measure mechanical crystals, the linewidth of semiconductor quantum dot emission, modes and acoustic phonons (i.e., low energy phonons,