Thresholdless behavior and linearity of the optically induced metallization of NbO_{2}

The optical excitation of NbO_{2} is known to induce a transient metallicity that partially recovers on a subpicosecond speed and only fully recovers after several nanoseconds. Here, through low-fluence pump-probe spectroscopy, the short-lived and long-lived components of NbO_{2}’s optically induced modulation are found to be linearly dependent on pump fluence in the low-fluence regime. Through an in-depth analysis, the short-lived and long-lived modulations are found to be caused by photocarrier-induced refractive index changes and photothermally induced changes in the lattice temperature of NbO_{2}, respectively, and not an insulator-metal transition.