Tunable Doppler shift using a time-varying epsilon-near-zero thin film near 1550 nm

We experimentally investigate the tunable Doppler shift in an 80 nm thick indium-tin-oxide (ITO) film at its epsilon-near-zero (ENZ) region. Under strong and pulsed excitation, ITO exhibits a time-varying change in the refractive index. A maximum frequency redshift of 1.8 THz is observed in the reflected light when the pump light has a peak intensity of ∼140 GW/cm² and a pulse duration of ∼580 fs, at an incident angle of 40^◦. The frequency shift increases with the increase in pump intensity and saturates at the intensity of ∼140 GW/cm². When the pump pulse duration increases from ∼580 fs to ∼1380 fs, the maximum attainable frequency shift decreases from 1.8 THz to 0.7 THz. In addition, the pump energy required to saturate the frequency shift decreases with the increase in pump pulse duration for ^<_∼1 ps and remains unchanged for ^>_∼1 ps durations. Tunability exists among the pump pulse energy, duration, and incident angle for the Doppler shift of the ITO-ENZ material, which can be employed to design efficient frequency shifters for telecom applications.