Fig. 1. (a) X-ray diffraction (XRD) pattern of ITO film; the standard diffraction of In2O3 is plotted as comparison. The inset graph is the enlargement from 29° to 32°. (b) SEM image of the ITO film. (c) AFM characterization of ITO film. (d) Raman spectrum of ITO film.

Fig. 2. (a) Calculated real and imaginary parts of the permittivity of ITO film as a function of wavelength. (b) Calculated field enhancements of the ITO film as a function of incident laser wavelength. (c) Calculated field enhancements of the structure as a function of location at 1.03 μm. (d) Schematic of nonlinear harmonic generation on the ENZ sample composed of silicon substrate and ITO film. The pump laser is vertically incident from the ITO side. Notably, the light beam direction of harmonic waves is only a diagrammatic sketch, not the real propagating direction. (e) The harmonic spectrum under different incident pump power densities. The intensity of the third-, fourth-, and fifth-harmonic generation was enlarged for clarity.

Fig. 3. (a) Spectrum of the fundamental wave. (b) The second-harmonic spectrum of ITO film and GaAs (111) crystal with pump intensity of 42.2  GW⋅ cm−2. (c) The third-harmonic spectrum of ITO film and Si (100) with pump intensity of 51.4  GW⋅ cm−2. (d) The fourth-harmonic and (e) fifth-harmonic spectra of ITO film with pump intensity of 438 and 944  GW⋅ cm−2. (f) The generated harmonic power as a function of incident pump power. Solid lines show the power scaling law In and are plotted to guide the eye.

Fig. 4. (a) Experimental setup for 515 nm pumped second-harmonic generation in ITO film. HR and HT represent high reflectivity (>99%) and high transmittance (>95%). (b) Experimental setup for 1030 nm pumped fourth-harmonic generation in ITO film. (c) The second-harmonic power as a function of incident 515 nm pump intensity and the fourth-harmonic power as a function of incident 1030 nm pump intensity.