Femtosecond laser inscription or writing has been recognized as a powerful technique to engineer various materials toward a number of applications. By efficient modification of refractive indices of dielectric crystals, optical waveguides with diverse configurations have been produced by femtosecond laser writing. The waveguiding properties depend not only on the parameters of the laser writing but also on the nature of the crystals. The mode profile tailoring and polarization engineering are realizable by selecting appropriate fabrication conditions. In addition, regardless of the complexity of crystal refractive index changes induced by ultrafast pulses, several three-dimensional geometries have been designed and implemented that are useful for the fabrication of laser-written photonic chips. Some intriguing devices, e.g., waveguide lasers, wavelength converters, and quantum memories, have been made, exhibiting potential for applications in various areas. Our work gives a concise review of the femtosecond laser-inscribed waveguides in dielectric crystals and focuses on the recent advances of this research area, including the fundamentals, fabrication, and selected photonic applications.

Copper (Cu) nanoparticles (NPs) are synthesized under the near-surface region of the Nd:Y3Al5O12 (Nd:YAG) crystal by direct Cu+ ions implantation. Subsequently, the monolithic ridge waveguide with embedded Cu NPs is fabricated by C4+ ions irradiation and diamond saw dicing. The nonlinear optical response of the sample is investigated by the Z-scan technique, and pronounced saturable absorption is observed at the 1030 nm femtosecond laser. Based on the obvious saturable absorption of Cu NPs embedded Nd:YAG crystal, 1 μm monolithic mode-locked pulsed waveguide laser is implemented by evanescent field interaction between NPs and waveguide modes, reaching the pulse duration of 24.8 ps and repetition rate of 7.8 GHz. The work combines waveguides with NPs, achieving pulsed laser devices based on monolithic waveguide chips.