Calculations of second harmonic generation with radially polarized excitations by elliptical mirror focusing

Abstract

Summary Second harmonic generation (SHG) polarization intensity distribution illuminated with radially polarized beams by lens focusing appears two peaks, when the nonlinear optical coefficients dominate that is relevant to the transverse electric field components. Such two peaks pattern may result in ghosting and the decrement of imaging resolution. In this paper, an elliptical mirror based system is proposed in the case of radially polarized beams illumination for SHG. The calculated predictions and numerical simulations demonstrate that for radially polarized beams, the proportion of transverse field components at the focal plane under the condition of elliptical mirror focusing is 2.6 times smaller than that with lens focusing when its numerical aperture (NA) is 1. Furthermore, the full width at half maximum (FWHM) of the total field intensity profile is approximately 81% of that in a lens focusing system. Due to the enhancement of longitudinal components of incident field, the distribution of SHG polarization presents a single‐peak pattern, in which two peaks can be observed with lens focusing. The SHG polarizations in collagen fiber, KTiOPO4, and LiNbO3 have been numerical simulated and discussed in detail to verify the validity of the proposed method. Lay Description A novel focusing mode for second harmonic generation (SHG) microscopy is described in this paper. In the case of radially polarized excitations, the SHG polarization distributions of some specimens appear two peaks pattern with traditional lens focusing. Such two peaks pattern may result in ghosting and the decrement of imaging resolution. By introducing elliptical mirror, the modulation of electric field at the focal plane can be realized. Thus, the distribution of SHG polarization is converted into a single‐peak pattern which eliminates the ghosting and improves the spatial resolution on SHG images. This proposed method can be used for SHG in various materials and the full width at half maximum (FWHM) of SHG polarization will compress in different degrees. Besides, there is a significant improvement on signal‐to‐noise ratio in SHG imaging when an annular aperture illumination is applied. The focusing mode with elliptical mirror can also be utilized in other nonlinear optics areas such as polarized third harmonic generation (THG) measurements and two‐photon fluorescence (TPF) microscopy.

Advanced Label-Free Laser Scanning Microscopy and Its Biological Imaging Application

“…Radially-polarized laser illumination is an effective and easy way for dramatically enhancing the resolution of SHGM [74][75][76]. Wang et al demonstrated that spatial resolution was improved more than 21% by using radially-polarized beam focused by elliptical mirror compared with lens [77]. The work of Tian et al showed that the subtractive imaging method could been extended to SHGM that increased the resolution to 210 nm and obtained a better contrast simultaneously [78].…”

Nonlinear-optical microscopy of asymmetric-shaped nanoantennas

“…Thus, radially polarized vector beams with a doughnut-shaped intensity distribution can provide a smaller focal spot due to the appearance of a strong longitudinal field component along the optical axis of the beam, which is absent in plane light waves. In turn, light focusing increases the lateral resolution in the laser microscopy [19,20]. Longitudinal electric field component can be of particular importance in the studies of a plenty of fundamental and applied scientific problems, e.g.…”

Second harmonic generation microscopy using pixel reassignment

“…SHG imaging with cylindrical vector beams provided contrast for structural features that could not be resolved by conventional techniques (Bautista et al ., 2012). An elliptical mirror based system was also proposed in the case of radially polarised beams illumination for SHG microscopy (Wang et al ., 2019). In spite of the enhancement of longitudinal components of incident field, the spatial resolution improvement was slight.…”