Brillouin – Mandelstam Light Scattering Spectroscopy: Applications in Phononics and Spintronics

Scattering of light by surface phonons: In opaque and semitransparent materials, the BMS 
spectrum is dominated by the scattering of light by surface phonons via the ripple 
mechanism.
2,44,45,52,53
The process is illustrated schematically in Fig. 1e,f. Owing to high optical 
extinction in these types of materials, the penetration depth of light is limited to the surface and 
only the in-plane component (parallel to the surface in the scattering plane) of the phonon 
momentum conservation law is satisfied. Therefore, only the phonons with the in-plane wave-
vector component, 
𝑞
∥
= 𝑘
𝑖
sin(𝜃
𝑖
) − 𝑘
𝑠
sin(𝜃
𝑠
), where 𝜃
𝑖
and 
𝜃
𝑠
are the incident and scattered 
light angle with respect to the normal to the surface, contribute to the light scattering (Fig. 1f). In 
a complete backscattering geometry where 
𝜃
𝑖
= 𝜃
𝑠
= 𝜃, the in-plane phonon momentum is 𝑞
∥
=
(4𝜋 𝜆
⁄ )sin⁡(𝜃). Note that the phonon wave-vector depends only on the incident angle and 
excitation laser wavelength. Under such conditions, one can probe phonons with different wave-
vectors by changing the incident angle of the laser light. The latter allows one to obtain the phonon 

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Brillouin – Mandelstam Light Scattering Spectroscopy: Applications in Phononics and Spintronics - UCR, 2020 
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P a g e
dispersion that is energy (frequency) of the phonons as a function of wave-vector. Obtaining the 
energy dispersion of the elemental excitations is a distinctive advantage of BMS technique over 
Raman spectroscopy. Modifications in the phonon dispersion, and correspondingly, in the phonon 
density of states, contain a wealth of information on confinement and proximity effects in low-
dimensional material systems.
54,55