Interference illumination of three nonzero-order beams for LCOS-based structured illumination microscopy

To avoid the need for a mask and polarisation-adjusting devices, and to solve the problem of low fringe contrast caused by the reflected light along with 0th-order diffraction beam, this paper presents an illumination method using three nonzero-order diffraction beams in liquid crystal on silicon (LCOS)-based structured illumination microscopy. Here, a LCOS-based spatial light modulator (SLM) is used to diffract the collimated light and a rotating frosted film is used to reduce the spatial coherence of the laser; then, the fringe is produced by adjusting the SLM angle to allow three nonzero-order diffraction beams to interfere on the sample surface. Interference fringes with high contrast in all directions can be obtained without considering polarisation control and the removal of the 0th-order diffraction beam, which demonstrates that the optical setup is simple and easy to control. We carried out experiments on a photolithographic pattern on a silicon chip, and the resolution after reconstruction is 210 nm, reaching the theoretical resolution at our experiment condition and nearly half of the Rayleigh resolution limit (100× objective, NA = 0.8), which is 406 nm. LAY DESCRIPTION: SIM has been widely applied in imaging of biological sample owing to its advantage of super-resolution. Commonly the structured illumination is produced by interfering two or three diffractive beams and the fringe contrast affects the reconstruction result directly. In this study about liquid-crystal-on-silicon based structured illumination microscopy (LCOS-based SIM), we presents an illumination method using three nonzero-order diffractive beams. Our method can avoid the need for a mask and the polarisation-adjusting devices, because three-beam interference can reduce the influence of polarisation on the fringe contrast. Besides, 0th-order beam is not used, because reflected light still exists even the grey level of the picture-pixels uploaded to spatial light modulator are all 0, which means the 0th-order beam will bring obvious noise. Using our method, interference fringe with high contrast in all directions can be obtained at a relatively high utilisation rate of laser intensity without considering the control of polarisation. Our setup is simple and easy to control, because the adjustment of the deflection angle of the spatial light modulator can realise the removal of the zero diffraction order. We have analysed and discussed the reasons why the interference of three nonzero-order beams can avoid the influence of polarisation and amplitude. The experiments carried out on a photolithographic pattern on silicon chip showed that the resolution after reconstruction is 210 nm, reaching the half of the Rayleigh resolution limit (100× objective, NA=0.8), which is 406 nm.