Abstract
Brillouin spectroscopy has been used to characterize shear acoustic phonons in materials. However, conventional instruments had slow acquisition times over 10 min per 1 mW of input optical power, and they required two objective lenses to form a 90° scattering geometry necessary for polarization coupling by shear phonons. Here, we demonstrate a confocal Brillouin microscope capable of detecting both shear and longitudinal phonons with improved speeds and with a single objective lens. Brillouin scattering spectra were measured from polycarbonate, fused quartz, and borosilicate in 1-10 s at an optical power level of 10 mW. The elastic constants, phonon mean free path and the ratio of the Pockels coefficients were determined at microscopic resolution.
Highlights
Brillouin scattering spectroscopy is a useful technique for noncontact and nondestructive measurements in material sciences [1,2,3,4], mineralogy [5,6] and biology [7]
Its shear modulus at hypersonic frequency has been previously measured by Brillouin spectroscopy and is in the range of 1 GPa at room temperature [28,29]
For longitudinal Brillouin scattering, we used the positive sign because the Stokes peak appears on the right side of the anti-Stokes peak, whereas the negative sign applies to Shear Brillouin scattering
Summary
Brillouin scattering spectroscopy is a useful technique for noncontact and nondestructive measurements in material sciences [1,2,3,4], mineralogy [5,6] and biology [7]. Faster acquisition of a few seconds was demonstrated by the use of angle-dispersive Fabry-Perot interferometry at an optical power of 100-200 mW in quartz [19,20,21] This approach achieves the data acquisition speed only at the expense of the spectral resolution, as the throughput efficiency is inversely proportional to the Finesse (F). The ability to acquire all spectral components simultaneous with minimal incident loss owing to the special coating of VIPA tremendously increased data acquisition speed without the loss of spectral resolution This innovation has enabled measurement of the age-related stiffening in the crystalline lens [24], in situ and in vivo mapping of the longitudinal modulus of the human cornea [25,26]. We achieved a reduction in data acquisition time by more than an order-of-magnitude compared to previous measurement of shear phonons using scanning Fabry-Perot interferometers
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