Fig. 1. Schematic diagram of single-photon absorption and two-photon absorption of femtosecond laser^[31-32]

Fig. 2. Schematic diagram of femtosecond laser two-photon polymerization processing system. (a) Single-focus processing^[36]; (b) multi-focus processing^[38]; (c) incorporation of spatial light modulator for optical field modulation^[40]; (d) processing system based on spatial and temporal focusing^[52]

Fig. 3. Femtosecond laser two-photon polymerization for micro-nano robot processing with different driving mechanisms. (a) Ultrasonic drive^[69]; (b) magnetic drive^[79]; (c) thermal drive^[84]

Fig. 4. Two types of micro robots with different driving mechanisms. (a) Magnetic driven micro spiral robots can easily adjust parameters such as height, radius, pitch, and cone angle^[56]; (b) photothermal driven humanoid robot with joints composed of hydrogel with a gradient change in crosslinking density and deposited silver nanoparticles^[57]

Fig. 5. Micro-lens fabricated by femtosecond laser technology. (a) Achromatic meta-lens based on holographic diffraction in visible light spectrum^[59]; (b) refractive index gradient change lens for aberration-free focusing of infrared light^[96]

Fig. 6. Applications of femtosecond laser processing in field of integrated optical micro-devices. (a) Adiabatic twist of waveguides for polarization encoding^[60]; (b) fiber endface lens with ultra-high numerical aperture^[92]; (c) miniature biomimetic compound eye integrated with CMOS camera^[61]

Fig. 7. Fabrication of chiral micro-nano structures. (a), (b) Solely based on femtosecond laser direct writing^[99-100]; (c), (d) combination of femtosecond laser direct writing and surface metal deposition^[101-102]; (e), (f) combination of femtosecond laser direct writing and electrochemical deposition^[103-104]

Fig. 8. Helical dichroism response of different chiral structures and its applications. (a) Response of planar chiral micro structures to left-handed/right-handed vortex light^[62]; (b) flexible modulation of helical dichroism spectra using rotating stacked planar chiral structures^[63]; (c) response of three-dimensional spiral structures to left-handed/right-handed vortex light^[64]; (d) applications of chiral spiral arrays in information display, hiding, and encryption^[65]

Fig. 9. Applications of femtosecond laser processing in field of biomedicine. (a) Microtube arrays with variable cross-sections fabricated by femtosecond laser processing for regulating yeast cell growth^[66]; (b) porous microtube arrays can accelerate growth and directional connection of neurons^[67]; (c) magnetically controlled robots for noninvasive puncture therapy of cells^[68]; (d) capillary networks fabricated by femtosecond laser dynamic holography^[16]