# Efficient Multifocal Structured Illumination Microscopy Utilizing a Spatial Light Modulator
## 1 Introduction  
The fluorescence microscopy technique has been utilized as a powerful tool for a wide variety of biological applications such as cell biology and neuroscience [1]. However, owing to the wave nature of visible light, the Abbe diffraction limit imposes a critical barrier on the spatial resolution of optical microscopy, meaning that the maximum available resolution in a conventional WF microscope is approximately close to one-half of the emission wavelength of the fluorophores [2], which severely constrains the observation of the detailed structure of biological samples. Hence, high-resolution microscopy is vital for the study of biological structures and accurate diagnosis of medical diseases. To overcome the Abbe limit and achieve super-resolution, impressive developments in the field of super-resolution microscopy have come up in the past two decades [3; 4; 5; 6], enabling observations within spatial regimes that were previously not accessible, such as photo-activated localization microscopy (PALM) [7; 8; 9], stochastic optical reconstruction microscopy (STORM) [10; 11; 12], stimulated emission depletion microscopy (STED) [13; 14; 15] and structured illumination microscopy (SIM) [16; 17; 18]. As one of the versatile super-resolved microscopy technologies, SIM utilizes spatially patterned illumination to excite the fluorescence of the samples. Linear SIM could double the spatial resolution of the conventional microscopy while nonlinear SIM can improve the performance of that further by exploiting the nonlinear behavior of the sample in response to the illumination light [19; 20; 21]. Conventional SIM with sinusoidal illumination patterns, does not require sophisticated samplepreparation procedures while such a WF illumination even has its own limitation. It is incompatible with the situations that require a high intensity of incident light, which is only practical when focused laser beams are scanned through the focal plane. Furthermore, WF illumination typically limits the imaging depth and only thin samples show great imaging performances [22]. Utilizing point-scanning techniques, the quality of the images can be better maintained, while it might be not adequate for rapid acquisition due to its scanning mechanism and the size of the area to be scanned. Since two-dimensional (2D) SIM was described out [23], its theoretical aspects have been gradually proposed [24] and applied [25] in true super-resolution imaging. As a kind of 2D SIM, multifocal structured illumination microscopy (MSIM), with a sparse lattice of excitation foci, would dramatically increase the acquisition speed and show excellent performance. One way to generate multifocal patterns by a digital micro-mirror device (DMD) proved highly inefficient. Only a small fraction (<1%) of the pixels were in the on state during a given raw profile, wasting the majority of the excitation light [26]. Alternatively, the micro-lens arrays used to generate patterns are even complex to manufacture, of which parameters need to be carefully optimized to ensure the quality of the excitation foci [27]. Meanwhile, the number of the raw images used for reconstruction would affect the efficiency of the reconstruction, and reconstruction utilizing hundreds of raw images might be time-consuming [26; 28]. In this paper, we proposed an efficient MSIM system utilizing a spatial light modulator (SLM). Nine phase profiles of chessboard phase plates loaded on the SLM in sequence are used to generate nine multifocal arrays on the focal plane. Then nine raw multifocal images are acquired. Finally, by extracting the parameters of the illumination patterns from the raw images precisely, we can reconstruct the final image. We carry out experiments on several kinds of samples including microspheres and other biological samples and the results show enhanced resolution by a factor of about 1.504 compared with WF microscopy. The proposed system shows its high efficiency and flexibility, which might exploit potential application in the field of super-resolution microscopy.