Fig. 1. Multiphoton reduction to achieve electrical connection. (a) Electron microscopy of silver nanowires and electrodes; (b) high magnification electron microscopy of silver nanowires

Fig. 2. Electrical connection by femtosecond laser with assistant of surfactant^[22]. (a) Schematic of gold nanowire reduction; (b) mechanism of multiphoton reduction of gold nanoparticles; (c) absorption spectra of HAuCl₄, C5, and their mixture; (d) SEM image of gold nanowires; (e) AFM diagram of gold nanowires; (f) image of microcircuits

Fig. 3. Electrical connection is achieved within polymer matrix. (a) Gold electrode interconnection within PVP^[27]; (b) reduction of gold nanowires in SU8 doped with Au^3＋[28]; (c) reduction of gold nanoparticles in composite photoresist^[29]; (d) 3D electrical connection in composite photoresist^[29]

Fig. 4. Electrical connection is realized by photodynamic assembly method^[31]. (a) Schematic of photodynamic assembly method; (b) fabrication of cross-finger electrodes, double electrodes, and multiple electrodes by photodynamic assembly method

Fig. 5. Electrical connection is realized by femtosecond laser sintering of nanoparticles. (a) Femtosecond laser sintering of silver nanoparticles^[39]; (b) femtosecond laser sintering of copper nanoparticles^[43]

Fig. 6. Two-photon induced luminescence spectra of gold nanowire and its corresponding SEM images^[53].(a) Single gold nanowire; (b) coupled gold nanowire

Fig. 7. Melted nanowire and electric field intensity distributions when laser polarization is perpendicular to the long axis of silver nanowire^[54]. (a) Melting of nanowire ends, where the double sides arrow indicates the laser polarization direction; (b) the nanowire is dispersed and melted, and the nanowire is separated into nanoparticles; (c) absorption spectrum calculated by FDTD simulation; (d) electric field distribution calculated by FDTD; (e) electric field intensity distribution after silver nanowire melting on both ends; (f) electric field intensity distribution after nanoparticles separate from the ends of the nanowire

Fig. 8. Morphologies of silver nanowire and substrate after welding. (a) Silver nanowires connected at angle of 27°^[12]; (b) silver nanowires connected at angle of 87°^[12]; (c) morphology of silver nanowires welded by femtosecond laser^[50]; (d) morphology of silver nanowires welded by nanosecond laser^[50]; (e)(f) surface morphology and AFM diagram of PET after removing welded nanowires by ultrasonic cleaning^[57]

Fig. 9. Morphologies of ZnO nanowires welded joint (laser energy is 77.6 mJ/cm² and the irradiation time is 30 s)^[59]. (a) X-shaped welded joint; (b) Y-shaped welded joint

Fig. 10. Morphology and field intensity distribution of Ag-TiO₂ nanowires^[60]. (a) Ag-TiO₂ nanowires formed solder joint at the laser energy of 17.5 mJ/cm^2; (b) electric field intensity distribution around crossed nanowires

Fig. 11. Interfacial field intensity distribution and welding morphology of Au-TiO₂ under femtosecond laser irradiation. (a) Electric field intensity distribution around Au-TiO₂ nanowires electrode connection structure under 800 nm wavelength laser irradiation, where the double sides arrow indicates the laser polarization direction^[6]; (b) microstructure of Au-TiO₂ connection structure after femtosecond laser irradiation for 5 s with energy density of 18.3 mJ/cm^2[6]; (c)(d) microstructure of copper nanowire connected with silver substrate^[64]

Fig. 12. Sketch of femtosecond laser direct writing graphene and reduced graphene oxide circuit^[71]. (a) Experimental process for fabrication of fully reduced graphene oxide FET based on femtosecond laser direct writing technique, where step ⅰ represents reduction of graphene oxide by high energy femtosecond laser direct writing, step ⅱ represents preparation of reduced graphene oxide channel by adjusting laser energy, step ⅲ represents preparation of dielectric layer by spin-coating PMMA, step ⅳ represents spin-coating graphene oxide, and step ⅴ represent preparation of electrode by reduction of graphene oxide with laser direct writing; (b) laser direct written reduced graphene oxide circuit after preparation, repair, and adjustment

Fig. 13. Type Ⅰ waveguide fabricated by ultrafast laser. (a) Photograph and structure of L-shaped waveguide arrays^[76]; (b) two-dimensional waveguide interconnection diagram^[77]; (c) three-dimensional waveguide interconnection diagram^[78]; (d) three-dimensional photonic wire bonding structure^[82]; (e) two SOI waveguide interconnection on the same chip^[82]; (f) interconnection of different chips^[82]

Fig. 14. Morphologies and mode field diagrams of the end face of type Ⅱ waveguide. (a) End face morphologies in KTN crystal^[88]; (b)　end face mode field diagrams in z-cut MgO∶LiNbO₃ crystal^[87]; (c) end face morphologies in Bi₄Ge₃O₁₂ crystal^[89]; (d) end face mode field diagrams in Bi₄Ge₃O₁₂ crystal^[89]

Fig. 15. Morphology and mode field distributions of end face of type Ⅲ waveguide in ZBLAN crystal^[92]

Fig. 16. Different couplers and their end-face mode field distribution. (a) Three-dimensional coupler^[9]; (b) 1×2 coupler^[97]; (c) 2×2 coupler^[97]; (d) 3×3 coupler^[97]

Fig. 17. Beam splitters fabricated by ultrafast lasers. (a) Structure and light splitting of 1-2, 1-4, and 1-8 beam splitters^[99]; (b) Y beam splitter based on type Ⅲ waveguide^[104]; (c) 1-4, 1-9, and 1-16 waveguide beam splitters prepared by 3D printing method^[105]

Fig. 18. Other optical discrete components. (a) Microlens array prepared by femtosecond laser combined with chemical etching^[108]; (b) Fresnel zone plate prepared by two-photon polymerization technology^[110]; (c) multi-core fiber to single-mode fiber interface device^[115]