Fig. 1. Schematic illustration of the mechanism of mode-locking^[3].

Fig. 2. Schematic illustrating the collective oscillations of conduction electrons in response to an external electric field for nanoparticles^[41].

Fig. 3. LSPR frequency dependence on free carrier density and doping constraints^[49].

Fig. 4. Photoexcitation and relaxation of metallic nanoparticles: (a)−(d) Photoexcitation and subsequent relaxation processes following the illumination of a metal nanoparticle with a laser pulse, and characteristic timescales^[42].

Fig. 5. Energy-level structure of a two-energy level system and the process of stimulated absorption.

Fig. 6. Absorption spectrμm and pulse laser generation of Gold nanorods (GNRs): (a) Transmission electron microscope image, the inset of (a) shows the photograph of the GNRs solution; (b) absorption spectrum of GNRs from 400 to 3200 nm; (c) the finite-difference time-domain simulation results of the absorption cross section of one, two, three, and four GNRs concatenated; (d) experiment schematic of a tunable passively Q-switched Er³⁺:ZBLAN fiber laser using GNRs as the saturable absorber; (e) output spectrum of tunable passively Q-switched Er³⁺:ZBLAN fiber laser^[62].

Fig. 7. Experimental preparation and characterization of Q-switched mode-locked pulses at 1064 nm: (a) Schematic diagram of the experimental process; (b) cross-sectional transmission electron microscope image of the Ag:SiO₂ with Ag⁺ fluence of 1.0 × 10¹⁷ ions per cm², the selected area electron diffraction image and element mapping image are shown as the left and right insets; (c) schematic diagram of Q-switched mode-locking operation; (d) the single pulse profile (left image) and the radio-frequency spectrum (right image)^[64].

Fig. 8. Schematic representation of the common doping mechanisms in metal oxides relative to a basic lattice containing metal cations (orange spheres) and oxygen anions (red spheres)^[66].

Fig. 9. Nonlinear properties of Cu_2–xS nanocrystals and its ultrafast pulse generation: (a) Absorption spectrum of the synthesized nanocrystals; (b) typical Z-scan curves of Cu_2–xS and Cu₂S nanocrystals recorded at 1300 nm; (c) corresponding input power-dependent transmission; (d) mode-locking pulse train; (e) autocorrelation trace; (f) the radio-frequency optical spectrum at the fundamental frequency^[67].

Fig. 10. Nonlinear optical response and ultrafast transient optical response of the ITO nanocrystals in ENZ region: (a) Typical transmission electron microscope images of ITO nanocrystals, with an average diameter of about 9 nm, the inset shows a photograph of the colloidal solution of ITO nanocrystals and a high resolution transmission electron microscope image of a single ITO nanocrystals; (b) normalized optical extinction spectra of the ITO nanocrystals with different doping levels; (c) wavelength dependent real part of the permittivity of the spin-coated ITO nanocrystals thin films; (d) Z-scan trace of a PVA film containing ITO nanocrystals recorded at 1.3 μm, ITO-12 shows notable saturable absorption, as compared to the undoped In₂O₃; (e) transient bleaching dynamics of ITO-10 nanocrystals film (spin-coated on quartz slid) under different pump fluence. Solid line shows the fitting with a single exponential decay function^[70].

Fig. 11. The Q-switching at mid-infrared region band based on IZO nanoparticles: (a) Schematic illustration of laser setup; (b) typical Q-switched pulse train; (c) optical spectrum; the inset is the radio frequency spectrum, indicating a signal-to-noise ratio of ～30 dB; (d) single pulse profile^[71].

Fig. 12. Characterizations of 2D MoO₃ nanosheets: (a) Atomic force microscope image; (b) VIS-NIR absorption spectra for the colloidal dispersions of pristine MoO₃ nanosheets and plasmonic (photoactivated) MoO₃ nanosheets; the inset is the corresponding photographs; (c) dependence of transmission as a function of input power for plasmonic 2D MoO₃; (d) optical spectrum; (e) pulse train; (f) pulse duration^[72].

Fig. 13. Ultrafast pulse laser generation and Q-switched laser based on TiN: (a) Nonlinear transmittance curve of the TiN/PVA sample versus the input pulse fluence at 1550 nm; (b) optical spectrum; (c) pulse trains; (d) autocorrelation trace; (e) laser spectrum from the Q-switched laser at the maximum pumping power; (f) average output powers versus pumping power for lasing operation at 1064 nm^[74].

Fig. 14. Different plasmonic materials corresponding LSPR wavelength.

Table 1. [in Chinese]