Fig. 1. Time-resolved transient transmission trace ΔT/T0 of (a) 6.8 nm and (b) 20 nm PtTe2 films under various pump fluences with pumping and probing at 780 nm. Blue circles denote experimental data; red solid lines are the fitting curves with convoluted biexponential decay function. The peak ΔT/T0 versus pump fluence for (c) 6.8 nm and (d) 20 nm PtTe2 films; red solid lines are the linear fitting.

Fig. 2. Fitting fast lifetime τf of (a) 6.8 nm and (b) 20 nm PtTe2 with a convoluted biexponential decay function with respect to pump fluence. (c) The extracted coherent acoustical phonon oscillatory signal by subtracting the fitted incoherent carrier dynamics from total transient trace under pump fluence of 994  μJ/cm2. Inset shows the frequency domain after FFT. (d) Schematic diagrams of CAP generation induced by temperature gradient. The left and right drawings respectively express the temperature response of 6.8 and 20 nm PtTe2 films upon laser irradiation.

Fig. 3. Time- and spectrum-resolved ultrafast TA mapping of (a) 6.8 nm PtTe2 and (c) 20 nm PtTe2 photoexcited by 780 nm fs pump pulse with constant 1  mJ/cm2 pump fluence. The unit OD denotes optical density. The TA spectrum of (b) 6.8 nm and (d) 20 nm PtTe2 at several selected time delays.

Fig. 4. Probe wavelength-dependent COP dynamics of PtTe2 film. TA kinetics in the first 5 ps of (a) 20 nm and (c) 6.8 nm PtTe2 at selective representative probe wavelengths. Extracted typical optical phonon oscillation at 525 nm and the fitting curve of (b) 20 nm and (d) 6.8 nm PtTe2, in which the inset denotes the FFT amplitude spectrum. (e) The 532 nm Raman spectrum of 20 nm PtTe2; the red arrows indicate the motion direction of the Te atom. (f) The FFT amplitude of residual oscillations of 6.8 nm PtTe2 as a function of the probe wavelength.

Fig. 5. The TA plotting of (a) 6.8 nm and (b) 20 nm PtTe2 film in the initial 5 ps of time delay.

Fig. 6. Residual periodical oscillatory signals of 6.8 nm PtTe2 film with respect to the probe wavelength in the time domain. The individual signal is obtained by subtracting the incoherent carrier dynamics from the total transient trace.

Fig. 7. Damping time of coherent optical phonon of 6.8 nm PtTe2 film with respect to probe wavelength.