Ultrafast All-Optical Complete Phase Modulation at Near-Infrared Wavelengths in an Epsilon-Near-Zero Material by Femtosecond Frequency-Domain Interference

Ultrafast optical phase modulation plays a key role in modern high-speed optical processing and integrated photonic devices. However, most existing transient phase modulation strategies are implemented through indirect phase measurements. Although several works on direct estimation of phase change in microstructured semiconductors via time-resolved interferometers have been reported, the small modulation depth (<0.01π) and the limited operation wavelengths (visible region) strongly restrict their applications. Here, by developing a near-infrared femtosecond frequency-domain interferometer, we directly measure the transient optical phase change at near-infrared (NIR) wavelengths and achieve a complete phase modulation (0–2π) within 150 fs under a moderate pump intensity of 40 GW/cm² in an epsilon-near-zero (ENZ) material. Compared to the transient phase modulation under non-ENZ conditions in the same material, the application of ENZ effects reduces the pump intensity for efficient phase modulation by more than 1 order of magnitude. The direct measurements of femtosecond complete phase modulation in the NIR range in this study provide an efficient pathway for telecom-based high-speed, accurate phase modulators and integrated photonic devices.