Fig. 1. Schematics of chiral SFG microscope. (a) Conventional noncollinear setup, which uses two separated linear-polarized beams, has sub-optimum NA. (b) Collinear setup, which uses linearly and radially polarized beams, can utilize the full NA with rotational symmetry. The top insets show the cross-sections of the excitation beams and the polarization arrangement at the objective aperture. ESFG,E1 and E2 are the electric fields of the SFG signal and the two incident beams, respectively.

Fig. 2. Numerical simulation of the foci and the chiral SFG PSF. The first row depicts collinear modality: (a) laser intensity at the objective lens pupil, (b) foci, and (c) chiral SFG PSF. The second row depicts noncollinear modality: (d) laser intensity at the objective lens pupil, (e) foci, and (f) chiral SFG PSF. The colors green, red, and purple are used to represent wavelengths of 520, 820, and 318.2 nm, respectively.

Fig. 3. Dependence of the chiral SFG PSF FWHM on wavelength. The wavelength of visible beam is tuned to scan the SFG wavelength across a typical electronic resonance range.

Fig. 4. Dependence of a far-field chiral SFG signal on the sample size. (a) Focusing on the interface between a chiral liquid and an achiral medium. (b) Focusing inside the bulk of a uniform chiral liquid.

Fig. 5. Spectroscopic and imaging demonstration. (a) Experimental light path, where RPC denotes the radial polarization converter, PBS denotes the pellicle beam splitter, and Spec. denotes spectrometer with a liquid-nitrogen-cooled camera. (b) SEM image of the cubic liquid cell, with a scale bar of 5μm. (c) Spectral profile of the SFG signal from R-BINOL solution and a racemic mixture. (d) Collinear chiral SFG image of the cubic liquid cell.

Fig. 6. Knife-edge measurement of the collinear and noncollinear configuration. The cumulative distribution of the PSF along the x, y, and z directions for collinear (a)–(c) and noncollinear (d)–(f) chiral SFG. The spatial resolutions of collinear chiral SFG along the y and z directions are enhanced by ∼1.4 times, while the spatial resolutions along the x direction are comparable. The error bars represent the standard deviation.