Fig. 4: Spectra of group velocity dispersion (GVD) and absorption.

a Full GVD spectrum of the O-band, C-band, and 2-μm-band samples. b–d The variations of GVD with temperature for these three sets of samples near the epsilon-near-zero (ENZ) frequency, respectively. e Full absorption spectrum of the three sets of samples. f–h The corresponding absorption variations with temperature near the ENZ frequency. In a and e, the colored vertical lines indicate the exact ENZ frequencies of the corresponding curves.