Capture of femtosecond plasmon excitation on transient nonequilibrium states of the metal surface

Understanding of laser-material interactions is a scientific evergreen in the fundamental research of physics and optics. We report here that the ultrafast dynamics of the Cu(110) crystal surface is permanently captured by the formation of subwavelength periodic structures using two collinear femtosecond laser irradiations with different linear polarizations. Surprisingly, such periodic structures are found to have slantwise orientation that is anomalously change as a function of the time delay between two laser beams. In the case of the shorter time delays, the time-dependent slantwise orientations oscillated with terahertz frequency, depending on the pulse width and the intersection angle of two polarization directions, whereas it only presents monotonic change for the larger time delays. Analyses suggests that the former case is attributed to the surface plasmon excitation of the temporally delayed femtosecond laser irradiation on the transient state of the metal surface, which is consequently modulated by some nonthermal effects such as shock wave and bond hardening, while the latter situation is predominated by thermal relaxation of the material lattice. The simulation results agree with the experimental measurements. This investigation not only allows us to sensitively record the transient spatiotemporal evolution on superheated metal surfaces, but also provides insights for the control of material microprocessing.

Figure 1

Schematic diagrams for the observed orientation change of the LIPSSs on copper surface. (a) Experimental setup with two time-delayed collinear femtosecond laser beams linearly polarized in different directions. (b) Sketch of ultrafast dynamic processes occurring on the metal surface upon irradiation of two femtosecond laser beams. The first laser irradiation introduces some transient physical effects, including a transient grating pattern of the optical index (shaded green regions) on the surface and the longitudinal standing shock-wave strain within the bond-hardening layer. This subsequently affects the surface plasmon excitation of the second laser irradiation, thereby changing the spatial orientation of the subwavelength LIPSS (the small springs represent the potential energy force among the metal lattices, which can be increased by the higher electronic temperature).

Figure 2

Formation of the slantwise-oriented LIPSSs by two different linear polarization of femtosecond lasers at zero time-delay irradiation. (a) SEM pictures of the LIPSSs formed on Cu (110) surface by two femtosecond laser irradiations with the intersection angle of $\theta ={45}^{\circ }$ between their polarization directions (left panel); the results obtained from the two individual laser irradiations with higher energy fluence are also shown (right panel) for comparison, where the yellow double arrows ($E_1$ and $E_2$) denote the electric fields of the laser irradiations; both blue and red dashed lines represent the spatial orientations of the LIPSSs. (b) The measured variation of SOA (black squares) and total energy fluence for the LIPSSs formation (blue circles) as a function of the polarization intersection angle θ , where the blue dashed curve is for the fitting by $1/cos{\left(\theta /2\right)}^2$.

Figure 3

Observation of the slantwise-oriented LIPSSs on the sample surface for two laser irradiations having an intersection angle of $\theta ={45}^{\circ }$ between the polarization directions, where the time duration of two lasers is $\tau =50\mathrm{fs}$ and their total energy fluence is $F=0.35\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$. The solid red line is for the direction perpendicular to the angular bisector of θ.

Figure 4

(a) Evolution of the SOA of the LIPSSs with the time delay between two femtosecond laser irradiations ($\tau =50\mathrm{fs}$, $\theta ={45}^{\circ }$), where the solid black squares represent the experimental data, and the solid red line is for the fitting curve. The inset picture shows the details of the oscillatory component within the time delays less than $\mathrm{\Delta }t=12\mathrm{ps}$. (b) The obtained Fourier transform spectra for the measured oscillations in time domain, where the red line represents a numerical fitting based on Eq. (1).

Figure 5

Measured influence of the other laser parameters on the oscillation change of the SOA. (a) Time-delay-dependent oscillations of the SOA induced by different polarization intersection angles of $\theta ={70}^{\circ }$, 60°, and 30°, where the total laser energy fluences are $F=0.385\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$, $0.385\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$, and $0.315\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$, respectively. The solid black squares represent the experimental data, and the solid red lines are for the fitting curves. (b) Dependence of the maximum magnitude (black squares) and the frequency (blue circles) for the SOA oscillation on the θ angle. The dashed line represents the simulation result of Eq. (3). (c) Dependence of the maximum magnitude (black circles) and the frequency (blue squares) for the SOA oscillation on the laser pulse width.

Figure 6

Theoretical analyses of the temporal evolution of the LIPSSs orientation on copper surfaces. (a) Proposed physical model for the observed SOA change by two femtosecond laser irradiations with the linear polarization in different directions (left and right panels represent different time-delay regimes), where $\mathrm{\Delta }\overrightarrow{k_g^{\prime }}$ represents the modulation of the transient grating vector based on the shock-wave effect, and $\overrightarrow{k_{sp}}$ (red lines) for the excited SP vector of the second laser irradiation. Inset pictures located at the upper-right corner of each panel map the physical process of the SOA oscillation, with $\overrightarrow{r_1}$ and $\overrightarrow{r_2}$ being the structure orientations formed by two individual high energy fluence of laser irradiations $S_1$ and $S_2$, respectively. (b) Simulation results of the lattice density fluctuation (black lines) and the shock-wave reflectivity (blue lines) at the boundaries of bond hardening for two different laser pulse widths.