Shear Brillouin light scattering microscope

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.

Fig. 1

Schematic of the off-axis Brillouin confocal microscope. Flip-mirrors are used to choose from the sample arm or calibration arm. Obj: objective lens (UPLSAPO, 60XW, 1.2 NA).

Fig. 2

Water-Polystyrene optical spectra acquired with the two-stage VIPA spectrometer. (a) Spectrogram of two calibration materials (water and polystyrene cuvette). Color bar, number of photons. (b) 1-D plot of the Brillouin spectrum. Blue line: measured data, red dotted line: Lorentzian curve fit to the measured data.

Fig. 3

Brillouin spectra of polycarbonate for VV input-output polarization (a-c) and VH polarization (d-f). (a), (d): Spectrograms at 0.1 s and at 1 s, respectively. Color bar, number of photons (counts) (b), (c) 1-D plot of the spectra at 0.1 s and at 5 s, respectively. The measured Brillouin frequency shifts and linewidths are 11.68 ± 0.03 GHz and 0.36 GHz. (e), (f) 1-D plot of the spectra at 1 s and at 50 s, respectively. The measured Brillouin frequency shifts and linewidths are 5.16 ± 0.07 GHz and 0.68 GHz. (Blue line: measured data. Sampling points are interpolated by ten times. Red dotted line: Lorentzian curve fit to the measured data, x axis: pixel index, y axis: number of photons (counts) in (b), (c), (e) and (f)).

Fig. 4

Brillouin spectra of fused quartz for VV (a-b) and VH (c-d). (a), (c): Spectrograms at 1 s and at 10 s, respectively. Color bar, number of photons (counts) (b): 1-D plot of the spectra of (a). The measured Brillouin frequency shifts and linewidths are 26.03 ± 0.15 GHz and 0.18 GHz. (d): 1-D plot of the spectra of (c). The measured Brillouin frequency shifts and linewidths are 15.9 ± 0.11 GHz and 0.57 GHz.

Fig. 5

Brillouin spectra of borosilicate glass for VV (a-b), and VH (c-(d). (a), (c): Spectrograms at 1 s and at 10 s, respectively. Color bar, number of photons (counts) (b): 1-D plot of the spectra of (a). The measured Brillouin frequency shifts and linewidths are 24.47 ± 0.11 GHz and 0.11 GHz. (d): 1-D plot of the spectra of (c). The measured Brillouin frequency shifts and linewidths are 14.73 ± 0.13 GHz and 0.68 GHz.

Fig. 6

Schematic of the confocal microscope reconfigured for back-scattering measurement. The flippable mirror in front of the single mode fiber is shifted 6mm along the direction of the arrow to change the beam path. Note that this configuration allows only the longitudinal phonons to be characterized.

Fig. 7

Optical spectra acquired with two-stage VIPA spectrometer. (a), (b), (c): Spectrograms of polycarbonate (0.1 s), fused quartz (1 s), borosilicate glass (2 s) for VV scattering, respectively. Color bar, number of photons (counts) (d), (e), (f): 1-D plot of the spectra of (a), (b), and (c), respectively experimental data (blue line); The curve fit based on Lorentzian functions (red dotted line); The measured Brillouin frequency shifts and linewidths are 14.55 GHz and 0.61 GHz for (d), 32.37 GHz and 0.18 GHz for (e), 30.56 GHz and 0.12 GHz for (f). (x axis: pixel index, y axis: number of photons (counts) in (d), (e) and (f)).