A perspective on inelastic light scattering spectroscopy for probing transport of collective acoustic excitations

TL;DR

Brillouin scattering methods (BLS) and impulsive stimulated scattering (ISS) address the challenge of probing sub-THz acoustic excitations that dominate nanoscale energy transport. The paper reviews the operating principles, instrumentation, and key applications of BLS and ISS, including phonon and magnon dispersions, lifetimes, and spin/valley dynamics, and discusses data analysis and interpretation strategies. It positions BLS/ISS as complementary to INS/IXS and Raman spectroscopy for bridging length- and time-scales in energy transport, enabled by relations such as the Brillouin shift $ $ \nu = \dfrac{2 n v_s}{\lambda_i} \sin\left(\tfrac{\theta}{2}\right) $ and by transient-grating techniques for measuring MFPs and diffusion. The authors outline challenges (signal strength, noninvasiveness) and opportunities (sub-micrometer grating periods, attosecond timing, Brillouin microscopy) with potential impact on thermoelectrics, thermal management, and spintronics.

Abstract

Understanding and manipulating nanoscale energy transport and conversion processes are essential for diverse applications, ranging from thermoelectrics and energy harvesting to thermal management of microelectronics. While it has long been recognized that acoustic and thermal properties in condensed matters are primarily due to microscopic transport of phonons as quasiparticles, probing thermal acoustic excitations particularly at sub-THz remains a challenge primarily due to limitations in experimental techniques with spatiotemporal resolutions pertinent to probing them. Brillouin light scattering (BLS) and its variant, impulsive stimulated Brillouin scattering (ISS), provide access to these thermal acoustic excitations, enabling measurement of quantities such as acoustic dispersions along with relaxation dynamics occurring in ultrasonic as well as hypersonic frequencies. In this perspective, we provide a brief overview of the operational principles of BLS and ISS, and highlight their applications in probing acoustic, thermal, and magnetic excitations in emerging and low-dimensional materials. We conclude by discussing current challenges and future opportunities for advanced material characterization using Brillouin light scattering spectroscopy techniques.

Figures (5)

• Figure 1: (a) Map showing the accessible frequency and momentum transfer ranges of different inelastic scattering spectroscopy techniques. Shaded regions indicate typical accessible windows for each technique, and the Brillouin scattering shows the appropriate range to probe excitations with relatively low frequency and momentum. Adapted from Ref. Bencivenga_APX_2023 with permission under CC BY license. (b) Representative spectra for Brillouin scattering, showing inelastic Stokes (downshifted) and anti-Stokes (upshifted) peaks displaced from the central elastic Rayleigh scattering peak by Brillouin shift. The shifted peaks exhibit relatively low intensity compared to Rayleigh scattering, and the spectral separation is determined by the wavelength and angle of incident light, together with the refractive index and acoustic velocity of a material. The dotted lines represent the measurement range of BLS, Raman, and low-wavenumber Raman (LWNR) spectroscopy. Reprinted with permission from Karger, F. et al., Nature Photon.15, 720-731 (2021). Copyright 2021 Springer Nature. (c) Measured mean free paths versus frequency for thermal acoustic vibrations in vitreous silica from IXS (triangles, Ref. baldi_prl_2010Masciovecchio_prb_1997), INS (open circles, Ref. Wischnewski_prb_1998), BLS (5-pointed stars, Ref. Masciovecchi_prl_2006Benassi_prb_2005Vacher_prb_2006Levelut_prb_2006).
• Figure 2: Typical experimental setup for (a) spontaneous Brillouin light scattering spectroscopy, adapted with permission from DOI:10.1088/0022-3727/45/27/275302, J. Phys. D: Appl. Phys., 45, 275302. Copyright 2012 IOP Publishing Ltd BLS_setup, and (b) impulsive stimulated scattering spectroscopy.
• Figure 3: (a) Comparison of Brillouin spectra for hybrid halide perovskites with different cations and halides compositions, showing larger Brillouin shifts with increasing MA and Cl content. Reproduced from Ref. kabakova2018effect with permission from the Royal Society of Chemistry. (b) Phonon dispersion relation in a holey Si membrane obtained from BLS compared with simulations, showing an omnidirectional band gap Florez_2022_NatNanotech. Reprinted with permission from Florez, O. et al., Nature Nanotechnol.17, 947-951 (2022). Copyright 2022 Springer Nature. (c) Measured Brillouin spectra of GaAs nanowires array (red) compared to bulk GaAs substrate (blue), showing additional low-frequency peaks corresponding to confined acoustic phonon modes. Reproduced from Ref. Kargar_2016_Natcom_BLSGaAs, licensed under a Creative Commons Attribution 4.0 International License. (d) Wavenumber-resolved BLS signal from a YIG thin film, showing field positions of maximum BLS amplitude, which corresponds to hybridized magnon-phonon modes Holanda_2018_natphys. Adapted with permission from J. Holanda et al., Nature Phys.14, 500-506 (2018). Copyright 2018 Springer Nature.
• Figure 4: (a) Measured (solid circles) and calculated (line) dispersion relation of the lowest frequency surface acoustic mode in a silicon thin film (Inset: dispersion curves of the four lowest frequency modes) maznev_apl_2009. Reprinted from A. A. Maznev et al., Appl. Phys. Lett.95, 011903 (2009), with the permission of AIP Publishing. (b) TGS signal from a micro-patterned 1D phononic structure, showing acoustic oscillations associated with non-leaky, long-lived surface acoustic modes. (Inset: the same signal over a longer time scale) maznev_japplphys_2009. Reprinted from A. A. Maznev et al., J. Appl. Phys.105, 123530 (2009), with the permission of AIP Publishing. (c) Acoustic dispersion of a 2D granular crystal, where red and black markers represent the measured frequencies with and without silica microspheres, respectively. A horizontal dotted line indicates the resonance frequency arising from microsphere-substrate adhesion boechler2013interaction. Reprinted figure with permission from N. Boechler et al., Interaction of a Contact Resonance of Microspheres with Surface Acoustic Waves, Phys. Rev. Lett.111, 036103 (2013). Copyright 2013 by the American Physical Society. (d) Measured spin helix lifetimes versus wavevector in GaAs/AlGaAs quantum wells, showing enhanced lifetime peaks to higher wavevectors with increasing doping asymmetry koralek2009emergence. Reproduced with permission from J. D. Koralek et al., Emergence of the persistent spin helix in semiconductor quantum wells, Nature458, 7238 (2009). Copyright 2009 Springer Nature.
• Figure 5: (a) Thermal decay signals of silicon at several transient grating periods. The inset shows the complete wave form for a representative grating period of 7.5 $\mu$m. The thermal signal decays slower as the grating period increases. Reprinted figure with permission from Johnson et al., Phys. Rev. Lett., 110, 025901 2013. Copyright 2013 by the American Physical Society Johnson_2013_PRL_TGsignals. (b) Thermal decay rate versus grating wavevector squared, $q^2$Johnson_2013_PRL_TGsignals. As grating period decreases, the thermal decay rate deviates from what is predicted from heat diffusion theory. (c) Measured thermal conductivity versus grating period at 350 K in silicon thin film. As grating period decreases, the measured thermal conductivity approaches specular limit Navaneetha_2018_PRX_specularity. (d) Reconstructed phonon specularity versus phonon wavelength from grating period dependent thermal conductivity measurements including (c). As wavelength of phonon gets larger, specularity increases Navaneetha_2018_PRX_specularity. (e) Reconstructed thermal conductivity accumulation of partially ordered polyethylene thin films Andrew_2019_PNAS_DrawRatio. (f) Reconstructed frequency dependent MFPs of thermal acoustic excitations in aSi. Reprinted figure with permission from Kim et al., Phys. Rev. Materials, 5, 065602 2021. Copyright 2021 by the American Physical Society Kim_2021_PRM_aSi_TG.