Paper Title
The Application of Semi-Metal 2D Hafnium Ditelluride in Nonlinear Optics and Laser Technologies

Abstract
2D semi metallic materials are pivotal for cutting-edge electronics and energy devices due to their superior conductivity and quantum properties, finding use in transistors, sensors, and energy storage systems. Due to the zero-bandgap characteristics, strong absorption in the near-infrared band, high sensitivity, and unique electron configuration along with the thermal and dynamic stability, semi-metal 2D Hafnium Ditelluride (HfTe2) garnered interest for various applications such as solar steam generation, gas sensing, and acting as a catalyst. However, the nonlinear optical (NLO) properties and the opportunity of utilizing HfTe2 material in laser technologies has not been explored that much. In this study, the HfTe2 nanosheets were synthesized by the liquid-phase exfoliation technique. The resulting nanosheets exhibited exceptional NLO properties, as indicated by the obtained modulation depths of 11.9% at 800 nm and 6.35% at 1560 nm. Furthermore, the observed shift from saturable to reverse saturable absorption demonstrates the wide range of applicability of the material in the field of nonlinear optics. A mode-locked laser with a pulse width of 724 fs and a signal-to-noise ratio (SNR) of 56.63 dB has been successfully developed by integrating a side polished fiber-based HfTe2-saturable absorber (SA) into the cavity of an Er-doped fiber laser. The generated laser with this SA has the second lowest mode-locking pump threshold (18.35 mW), among the other 2D material based-SAs, thus paving the way for future laser technologies with improved efficiency and reduced thermal impact. Utilizing HfTe2-SA, a remarkably steady single-frequency fiber laser (SNR = 74.56 dB; linewidth ≈ 1.268 kHz) is produced for the first time, demonstrating its potential for ultranarrow photonic applications.