Brillouin
Spectroscopy
Specto’s offerings
At Specto, we aim to make Brillouin spectroscopy accessible to
everyone interested in measuring micromechanical properties.
We are developing groundbreaking solutions, such as
ON-CHIP BRILLOUIN SPECTROMETERS
At Specto, we aim to make Brillouin spectroscopy accessible to
everyone interested in measuring micromechanical properties.
We are developing groundbreaking solutions, such as
HIGH-REJECTION BIPD FILTER
What is
Brillouin
Spectroscopy?
All-optical brillouin spectroscopy is a non-contact and label-free technique used to explore the mechanical properties of materials at microscopic scales.
In Brillouin spectroscopy, light exchanges energy with acoustic phonons of materials so that a small portion of the scattered light undergoes tiny frequency shifts, resulting in a Stokes and an anti-Stokes Brillouin peak. By measuring the frequency shift and linewidth of the Brillouin peaks, one can determine the intrinsic elastic moduli of the material analyzed, providing fundamental information about the material stiffness and viscosity. When applied to biological samples, such as cells and tissues, this technique offers unique capabilities in analyzing their associated biomechanics non-destructively and at micron-scale spatial resolution in 3D, as opposite to conventional elastography methods, such as AFM. As such, Brillouin spectroscopy holds great potential to be employed as an essential tool in the biomedical domain, where the biomechanical changes are responsible for the onset
How it all
started?
Fun fact
Around the same time as Léon Brillouin, Soviet physicist Leonid Mandelstam is believed to have recognised the possibility of this scattering process in 1918, but he did not publish his findings until 1926. To give credit to Mandelstam, the effect is sometimes referred to as Mandelstam-Brillouin scattering.
Brillouin spectroscopy began with the brilliant work of French physicist Léon Brillouin, who first explained how light interacts with acoustic waves in a material back in 1922. Léon was one of the pioneering physicists who lived in the early 20th century, and he is considered one of the founders of modern solid-state physics.
He predicted the interaction between acoustic phonons and photons, where a photon interacts with density fluctuations in matter, resulting in a negative and a positive frequency shift (Stokes or anti-Stokes, respectively) as shown schematically below:
A schematic of the Brillouin spectrum
Brillouin Spectroscopy: A history lesson
The phenomenon of Brillouin light scattering, named after Léon Brillouin, was first described in 1922 when he detailed the interaction between optical waves and acoustic waves within a medium.
Therefore the scientific community refers to the phenomenon as “Brillouin scattering” or “stimulated Brillouin scattering (SBS),” depending on whether the scattering occurs spontaneously or through external stimulation.
Key phases of Brillouin scattering evolvement:
1922
1929
1972
1980
Brillouin light scattering was first applied to study the elastic properties of the lens and cornea of the eye.
1997
A VIPA (Virtually Image Phased Array) introduced by Shirasaki as a nonscanning version of the Fabry-Perot interferometer for wavelength demultiplexing
2008
Scarcelli and Yun introduced a confocal Brillouin microscope integrating a VIPA etalon to acquire Brillouin maps of biological tissues.
2015
High-resolution Brillouin microscopy was demonstrated in living cells
2020
Stimulated Brillouin scattering microscopy was demonstrated in living biological samples.
2024
A birefringent-induced phase delay (BIPD) filter was introduced to suppress strong Rayleigh background light, enabling the acquisition of Brillouin images of turbid samples such as bone
Reference
1929 – link Reference
1972 – link Reference
1980 – link Reference
1997 – link Reference
2008 – link Reference
2015 – link Reference
2015 – link Reference 2
2015 – link Reference 3
2020 – link Reference
2024 – link Reference
What does it measure?
In Brillouin spectroscopy, the inelastic Brillouin spectrum is acquired and fitted using a Lorentzian or DHO function. This frequency shift is known as Brillouin frequency shift (ν_B). A measure of the Brillouin frequency shift and linewidth provides fundamental information about the viscoelastic properties, and in particular on the elastic moduli forming the full elastic tensor of a material. In the backscattering geometry commonly used in confocal microscopy, Brillouin spectroscopy determines the complex Longitudinal modulus \( M(\nu) = M'(\nu) + iM”(\nu) \).
The frequency shift of the Brillouin peak is linked to the real part of M, also known as the storage modulus \( M’ \). This part reflects the material’s elastic behaviour, or the energy stored within the material. This Brillouin frequency shift \( \nu_B \) is given by \( \nu_B = Vq \), where \( q \) is the wavevector exchange defined as \( q = \frac{2n}{\lambda_0} \sin{\left( \frac{\theta}{2} \right)} \), with \( n \) being the material’s refractive index, \( \lambda_0 \) the incident wavelength, \( \theta \) the angle between incident and scattered light, and \( V \) the acoustic velocity of the medium. \( V \) is related to the material’s density \( \rho \) and the real part of the complex longitudinal modulus \( M’ \) through \( V = \sqrt{ \frac{M’}{\rho} } \).
How does it work?
Brillouin spectroscopy functions similarly to other spectroscopy techniques, such as Raman spectroscopy, that use a monochromatic laser beam to illuminate a sample and collect the inelastically scattered light. However, detecting the Brillouin spectrum poses severe challenges. The Brillouin frequency shift, typically less than 30 GHz (1 cm\(^{-1}\)) in biological samples, occurs very close to the pump laser frequency and is almost indistinguishable from the Rayleigh scattered light. Compounding this difficulty is the fact that Brillouin scattering is approximately nine orders of magnitude weaker than Rayleigh scattering.
Detecting the Brillouin spectrum presents several challenges and requires critical components. First, a laser with a narrow linewidth and low amplified spontaneous emission (ASE) is essential to minimize noise. Unlike Raman spectroscopy, where the signal is well-separated (typically \( > 1{,}000\ \mathrm{cm}^{-1} \)) from the Rayleigh scattered light, Brillouin detection depends on highly selective optical filtering. This demands ultra-narrow, high-rejection Rayleigh filters capable of suppressing elastically scattered light while transmitting the frequency-shifted Brillouin signal. Additionally, a high-resolution spectrometer is needed to accurately resolve the Brillouin peaks, allowing precise measurement of the Brillouin frequency shift and peak linewidth. These challenges often result in current Brillouin spectroscopy solutions either having extremely long acquisition times, requiring significant optical engineering expertise to operate, or both.