Plasmonic semiconductor nanocrystals (NCs) show great potential for use as SAs in ultrafast photonics because of the nonlinear optical enhancement effect at the localized surface plasmonic resonance (LSPR) frequency region [17,
18]. In comparison to traditional noble plasmonic metals (e.g., gold, silver, and copper), plasmonic semiconductor NCs (e.g., Cu
2−xS, Cu
2−xSe, Cu
2−xTe) [
19,
20] have a long LSPR wavelength of 1000 nm to 7
mm, which is caused by their lower carrier concentrations [
21,
22] and tunable LSPR frequency obtained by simply adjusting their element composition [
23]. Self-doped colloidal copper phosphide (Cu
3−xP) nanocrystals, which are an emerging member of 2D plasmonic nanomaterials, were recently found to have an ultrafast dynamic response (~130 fs) and a strong optical nonlinearity in terms of a relatively large modulation depth (>18%) [
19]. Accordingly, high-energy and stable Q-switching pulses at 1.5 µm were generated by drop-casting its solution onto the end-facet of an optical fiber as a new kind of saturable absorber. Cu
2−xS nanocrystals were also later proved to be effective saturable absorbers for mode-locking pulse generation at 1.0, 1.5, and 2.0 µm wavebands [
24]. However, the LSPR peak of Cu
2−xS nanocrystals is around 1200 nm, while that of Cu
3−xP nanocrystals can be precisely adjusted to 1550 nm, which is more suitable for telecommunication optical applications. The advantages of semiconductor 2D NCs over other nanomaterials, especially their repeatability and potential for scalable industrial production, have not yet been fully investigated.