[1] R. W. Boyd. Nonlinear Optics(2003).

[2] O. Reshef, I. De Leon, M. Z. Alam, R. W. Boyd. Nonlinear optical effects in epsilon-near-zero media. Nat. Rev. Mater., 4, 535(2019).

[3] M. Z. Alam, I. De Leon, R. W. Boyd. Large optical nonlinearity of indium tin oxide in its epsilon-near-zero region. Science, 352, 795(2016).

[4] M. Z. Alam, S. A. Schulz, J. Upham, I. De Leon, R. W. Boyd. Large optical nonlinearity of nanoantennas coupled to an epsilon-near-zero material. Nat. Photonics, 12, 79(2018).

[5] K. Wu, Z. W. Wang, J. H. Yang, H. Ye. Large optical nonlinearity of ITO/Ag/ITO sandwiches based on Z-scan measurement. Opt. Lett., 44, 2490(2019).

[6] I. Liberal, N. Engheta. Near-zero refractive index photonics. Nat. Photonics, 11, 149(2017).

[7] A. Capretti, Y. Wang, N. Engheta, L. Dal Negro. Comparative study of second-harmonic generation from epsilon-near-zero indium tin oxide and titanium nitride nanolayers excited in the near-infrared spectral range. ACS Photonics, 2, 1584(2015).

[8] S. Suresh, O. Reshef, M. Z. Alam, J. Upham, M. Karimi, R. W. Boyd. Enhanced nonlinear optical responses of layered epsilon-near-zero metamaterials at visible frequencies. ACS Photonics, 8, 125(2021).

[9] R. W. Boyd, J. E. Sipe. Nonlinear optical susceptibility of layered composite materials. J. Opt. Soc. Am. B, 11, 297(1994).

[10] P. B. Johnson, R. W. Christy. Optical constants of the noble metals. Phys. Rev. B, 6, 4370(1972).

[11] M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan, E. W. Van Stryland. Sensitive measurement of optical nonlinearities using a single beam. IEEE J. Quantum Electron., 26, 760(1990).

[12] T. Ning, H. Lu, Y. Zhou, B. Man. Decrease and enhancement of third-order optical nonlinearity in metal-dielectric composite films. Appl. Phys. Lett., 112, 151904(2018).

[13] T. Ning, Y. Zhou. Effect of structure on nonlinear optical properties in CaCu3Ti4O12 films. J. Appl. Phys., 118, 233103(2015).

[14] H. I. Elim, W. Ji, F. Zhu. Carrier concentration dependence of optical Kerr nonlinearity in indium tin oxide films. Appl. Phys. B, 82, 439(2006).