Fig. 1. (a) Schematic of the structure for the double layer graphene modulator (reproduced with permission^[44], Copyright 2012 American Chemical Society); (b) single-molecule optical switch (reproduced with permission^[45], Copyright 2005 American Physical Society); (c) two-photon absorption in bilayer graphene and four possible transitions in bilayer graphene (reproduced with permission^[46], Copyright 2011 American Chemical Society); (d) experimental setup for transmittance measurements of GCMs (reproduced with permission^[47], Copyright 2014 American Chemical Society).

Fig. 2. (a) Schematic of coherent nonlinear optical response measurement setup (reproduced with permission^[53], Copyright 2010 The American Physical Society); (b) transmission spectra of transmissive SWCNT saturable absorbers (reproduced with permission^[55], Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; (c) experimental setup of the ultrafast-laser based on SWNT SAs (reproduced with permission ^[56], Copyright 2016 Springer Nature); (d) schematic diagram of the Z-scan setup (reproduced with permission^[54], Copyright 2019, Springer Nature).

Fig. 3. Saturable absorber based on graphene-Bi₂Te₃ heterojunction (reproduced with permission^[103], Copyright 2015, American Chemical Society): (a) Schematic of graphene-Bi₂Te₃ heterostructure on the end-facet of fiber connector; (b) schematic diagram of the twin-detector measurement experimental setup; (c) schematic diagram showing the optical transitions in graphene-Bi₂Te₃ heterostructure; (d) Raman spectrum of the graphene-Bi₂Te₃ heterostructure.

Fig. 4. (a) Third harmonic generation in MoS₂ (reproduced with permission^[113], Copyright 2014 American Chemical Society); (b) 2D (TMD) optical emitter (reproduced with permission^[114], Copyright 2018 American Physical Society); (c) different nonlinear optical phenomenon of few-layer MoS₂ (reproduced with permission^[35], Copyright 2016, American Chemical Society); (d) polar plots of normalized SHG and THG (reproduced with permission^[115], Copyright 2018 American Physical Society).

Fig. 5. (a) Switch and signal of carriers in the linearly dispersive valence and conduction bands of bismuthine (reproduced with permission^[124], Copyright 2017, American Chemical Society); (b) superposition principle of switch and signal light (reproduced with permission^[124], Copyright 2017, American Chemical Society); (c) output polarization characteristics of BP Q-switched fiber laser (reproduced with permission^[127], Copyright 2015, Springer Nature); (d) relationship between transmittance of the phosphorene dispersions and intensity of the femtosecond laser (reproduced with permission^[104], Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Fig. 6. (a) Photograph of the microfiber deposited with PQDs, (b) saturable absorption property of the PQD-SA device (reproduced with permission^[138], Copyright 2017 Springer Nature); (c) the linear, nonlinear and total refractive index changes with photon energy for 0s–1p transitions (reproduced with permission^[139], Copyright 2011 American Institute of Physics); (d) asymmetric quantum dot in a microcavity as a nonlinear optical element (reproduced with permission^[140], ©2012 American Physical Society).