The diffraction is in charge of the beam divergence in the free-space propagation and propagation into the shadow region. Strong diffraction is more pronounced for long waves such as water or sound waves, while being less appreciable in optics. Strong diffraction appears when the transverse dimensions are comparable with the wavelength. The diffraction is defined as any deviation from rectilinear propagation. The light diffraction is the consequence of the violation of the rectilinear propagation. Light propagates rectilinearly and experiences diffraction in daily life. Nonetheless, with the illumination of dual Bessel beams at non-identical orders, the transverse resolution can also be improved by the concept of image subtraction, which would provide clearer images in neuronal imaging. These additional features to the existing microscopy directly realize a great advantage over the field, especially for recording the ultrafast neuronal activities, including the calcium voltage signal recording. Moreover, the fine axial resolution is enabled by the self-accelerating Airy beams combined with deep learning algorithms. This is a significant advancement in imaging applications with sparse sample structures, especially in neuron imaging. The non-diffracting Bessel beam and the Airy beam have been successfully used in volumetric imaging applications with faster speed since a single 2D scan provides information in the whole volume that adopted 3D scan in traditional scanning microscopy. The non-diffracting photons may provide scattering resilient imaging and fast speed in the volumetric two-photon fluorescence microscopy. Then, we discuss the biomedical applications of the non-diffracting beams, focusing on linear and nonlinear imaging, e.g., light-sheet fluorescence microscopy and two-photon fluorescence microscopy. We start from the fundamental concepts of the non-diffracting beams, and examples of the non-diffracting beams include but are not limited to the Bessel beam, Airy beam, and Mathieu beam. Studying the light propagation is a century-old problem as the photons may attenuate and wander. The light propagation in the medium normally experiences diffraction, dispersion, and scattering. 3Advanced Biomedical Instrumentation Centre, Hong Kong Science Park, Shatin, Hong Kong, SAR China.2Department of Electrical and Electronic Engineering, University of Hong Kong, Pokfulam, Hong Kong, SAR China.1Institute for Translational Brain Research, Shanghai Medical School, Fudan University, Shanghai, China.Yu-Xuan Ren 1*, Hongsen He 2, Huajun Tang 2 and Kenneth K.
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