# Lead Monoxide Nanostructures for Nanophotonics: A Review
## 3 Properties and Applications in Nanophotonics
### Nonlinear Photonics  
In the past decade, nonlinear photonics based on black-phosphorus-analog nanostructures, such as Bi, Se, SnS, PbO, etc., have drawn extensive attention due to their strong spin-orbit interaction, unique electronic transport, and excellent stability [92; 93; 94; 95; 96; 4; 17; 18; 33; 92; 93; 94; 96]. For example, in 2019, [PERSON] et al. [18] reported that ultrashort pulse generation in a ring-cavity fiber laser based on a few-layer PbO could be employed as a mode-locker in erbium-doped fiber lasers. Figure 9a provides the schematic diagram of the few-layer PbO-decorated microfiber. The dispersion containing the few-layer PbO NSs as saturable absorbers (SAs) was directly deposited onto a piece of microfiber by a flame taper method [95] to form a nonlinear optical device. The few-layer PbO-based microfiber was incorporated into a fiber laser cavity (Figure 9b), and the fiber laser was reversely pumped by a 980 nm laser diode with a maximum pump power of 1.2 W to overcome the propagation of the pump light. The mode-locking activity of the as-fabricated erbium-doped fiber laser displayed a mode-locked pulse of \(\sim\)1557.68 nm with a 3-dB spectral bandwidth of \(\sim\)2.9 nm (Figure 9c). The Kelly sidebands in the optical spectrum suggest that the mode-locked pulses are shaped in the manner of an optical soliton. The mode-locked pulse train obtained by a real-time oscilloscope (Figure 9d) demonstrates that the repetition rate is consistent with the cavity length, verifying that the pulses indeed originate from the mode-locking function. Additionally, a commercial autocorrelator was employed to investigate the pulse duration of the mode-locked pulse, and a pulse width of \(\sim\)900 fs was achieved (Figure 9e). Moreover, the stability of the pulse train evaluated by the radio frequency (RF) spectrum presents a signal-to-noise ratio (SNR) of \(\sim\)50 dB, indicating that the fiber laser operation is quite stable in this system (Figure 9f). The optical spectrum of the mode-locking pulses lasting over 16 h confirms that the employed few-layer PbO NSs are capable of supporting stable mode-locking in ultrafast lasers (Figure 9e), which provide fundamental guidance on the functional ultrafast photonics based on the few-layer PbO for practical applications. In addition, our group successfully fabricated for the first time a novel nanocomposite film composed of PbO QDs and polystyrene (PS) by solution blending for ultrafast photonics in harsh conditions such as marine environments [17]. The as-fabricated PbO QDs with an average diameter and thickness of 4.8 \(\pm\) 1.1 nm and 2.3 \(\pm\) 0.5 nm, respectively, are well distributed in the PbO QD/PS nanocomposite film, and the as-obtained composite film is transparent and flexible without apparent bubbles or other physical defects that will severely affect the device performance in electronics and photonics. The open aperture \(Z\)-scan test of the as-prepared composite film was carried out to investigate the performance after water immersion (Figure 10a,b). Before and after the water immersion, a stable nonlinear absorption at 1064 nm was obtained (Figure 10a). The nonlinear transmission, which strongly depends on the incident laser intensity, was obtained by extracting the \(Z\)-scan results (Figure 10b). This combines with the nonlinear properties observed in the pure PS film, indicating that the nonlinear absorption effect indeed originates from the PbO QD/PS nanocomposite film. To evaluate the mode-locking performance, the as-prepared PbO QD/PS nanocomposite film employed as a SA was incorporated into the ring cavity of an ytterbium-doped fiber laser (YDFL) (Figure 10c). It can be observed that the laser output power has a linear relationship with the pump power in the range from 55 mW to 180 mW (Figure 10d).