Realizing near-infrared directional absorption by epsilon-near-zero yttrium-doped cadmium oxide thin films

Recent development of epsilon-near-zero (ENZ) thin films in the infrared region has intrigued the unprecedented directional manipulation of thermal radiation relying on directional absorption. However, the effect is rarely studied in the short-wavelength region. Here, we realize near-infrared directional broadband absorption by stacking Y-doped cadmium oxide (Y-CdO) thin films. Material characterization reveals that Y-doped blueshifts the ENZ wavelength of Y-CdO thin films by efficiently enhancing their carrier concentration, until its doping concentration >10%. The retrieved dispersion of complex permittivity fits well with the Drude model calculated by the tested static electrical parameters. Then, four gradient-doped Y-CdO thin films form the multilayer structure, with their corresponding ENZ absorption modes merging in the spectral domain. High directional (incident angle ranging from 55° to 75°) absorbance (>0.6) for p-polarized incident light in a relatively broad range of wavelengths (from 1.25 to 1.93 μm) is thus achieved. Moreover, such performance is robust to the stacking order of Y-CdO thin films. Our work provides not only near-infrared directional absorbers that can extend the spectral region of thermal radiation control but also a material platform for the design of optical ENZ devices.