Fig. 1. (A) Schematic of SHG experimental setup with flat-top beam illumination. HWP, half-wave plate; L1–L3, convex lenses; Obj.; microscope objective; OF, optical filter; P, polarizer; sCMOS, scientific camera with complementary metal-oxide semiconductor detector. (B) Beam shaper realizes the transformation from Gaussian beam to flat-top beam. (C) Illustration of the polarization direction of laser and the orientation of myosin fibers. s^ is the symmetric axis of the myosin fibril.

Fig. 2. The simulation of two-dimensional transverse intensity distribution for different types of beams. (A) Gaussian beam; (B) super-Gaussian beam; (C) ideal flat-top beam; (D) intensity profiles of corresponding beams at the position indicated by red dotted line. Profiles of the laser beam at the focal plane: (E) Gaussian beam with intensity profiles along the (G) x axis and (I) y axis; (F) flat-top beam with intensity profiles along the (H) x axis and (J) y axis. (K) Standard deviation corresponding to the intensity profiles, respectively. Scale bar: 2 µm in (E) and (F).

Fig. 3. Wide-field SHG images corresponding to two kinds of beams with different excitation and detection polarization directions. (A) Gaussian beam illumination; (B) flat-top beam illumination. Red and blue arrows denote the excitation and detection polarization directions, respectively (red, α angle; blue, ψ angle). Scale bar: 5 µm.

Fig. 4. Polarization measurement results of myosin fibrils from rat muscle. Overall intensities U with the orientation of the myosin fibrils indicated by white arrows. (A) Gaussian beam illumination; (B) flat-top beam illumination; (C) and (D) magnified view inside the dashed box area corresponding to the previous images; (E) and (F) histograms of the orientation angle. Scale bar:5 µm in (A) and (B); 1 µm in (C) and (D).