Ultrafast Microscopic Movie Revealing Complete Structural Dynamics
Femtosecond (fs) laser-material interactions have been extensively studied in the past. However, there still lacks a clear understanding for even the simplest morphological responses; a direct visualization of the complete ultrafast structural dynamics has never been realized. Interestingly, fs laser-induced morphological changes often drastically alter a material’s physical properties. For example, surface nano- and micro-structuring induced by fs laser pulses has led to the creation of novel material effects, such as black silicon, black and colored metals, superhydrophilic, superhydrophobic, and multifunctional surfaces [1,2]. These highly functionalized surfaces can find a wide range of applications across physical and biological sciences. Following irradiation by a high-intensity fs laser pulse, a material surface undergoes transitions to liquid and supercritical states. The transient morphological surface fluctuations in these states govern the formation of final permanent surface structures rendering new properties to materials. Therefore, imaging the transient surface structures is critically important for fundamentally understanding the formation of laser-induced surface structures and creating novel materials with unique functionalities.
Optical microscopy is one of the most powerful methods for studying surface morphology of materials. Conventional ultrafast optical imaging based on collecting specular reflections [3,4] results in a strong background that makes it difficult to discern the small changes in reflection, which are often associated with nano/microstructure formation. In a recent work of ours , we developed an ultrafast optical imaging technique that utilizes scattered light. The scattered-light imaging provides a near-zero background and high contrast, essentially allowing us to obtain an ultrafast movie that resolves, for the first time, the complete temporal and spatial evolution of fs laser-induced morphological surface structure formation. An illustration of our work is shown in figure 1.
The ultrafast movie developed through our optical microscopy technique uses a pump-probe configuration with an amplified fs Ti:sapphire laser system lasing at a central wavelength of 800 nm.
The laser beam is split into a pump and probe beams. A pump laser pulse induces the formation of surface structures on a metal through direct irradiation. The probe laser pulse frequency is doubled to a wavelength of 400 nm, which is used for ultrafast stroboscopic illumination of the surface structures induced by the pump pulse. The optical delay line allows various time delays, ranging from 0 to 408 ns, with fs temporal resolution. The imaging optics are capable of resolving both microscale clusters of nanostructures and individual microstructures. At a fixed laser fluence, we capture a sequence of snapshots of the metal surface undergoing morphological transient fluctuations at various delay times. By capturing such sequences of surface images at different laser fluence, we can resolve the dynamics of both the formation of the transient surface structures and their resolidification.
Samples of Results
As an example, figure 2 shows a set of time-resolved surface images acquired at various delays following pump pulse irradiation of zinc. Under this experimental condition, we can see that, somewhat counter-intuitively, transient surface structures first appear at the edge of the irradiated spot and form a ring at 300 ps after the pump pulse. Over time, the transient surface structures move toward the center. At 5 - 9.3 ns, the center of the irradiated spot is populated with structures. Through a range of systematic studies , we find that transient surface structures first appear at a delay time on the order of 100 ps, and this is attributed to ablation driven by pressure relaxation in the surface layer. At lower laser fluences that favor nanostructure formation, the transient surface structures first appear in the central portion of the ablated spot. At higher laser fluences that favor microstructure formation, nanostructures first emerge at the periphery, and the center region is later populated with microstructures. Our study shows that the predicted cooling time is two orders of magnitude shorter than the time found in our observations, and we speculate that this slower cooling process is due to an enhanced thermal coupling phenomenon.
The time-resolved optical imaging technique allows us, for the first time, to capture and visualize the complete temporal and spatial evolution of the femtosecond laser-induced surface structural dynamics of metals. The visualization and control of surface structural dynamics are not only of fundamental importance for understanding the femtosecond laser-induced responses of materials but also for paving the way for the design of new material functionalities through surface structuring.
 Vorobyev AY, Guo C. Direct femtosecond laser surface nano/microstructuring and its applications. Laser Photonics Rev 2013; 7: 385-407.
 Ahmmed KMT, Grambow C, Kietzig AM. Fabrication of micro/nano structures on metals by femtosecond laser micromachining. Micromachines 2014; 5: 1219-1253.
 Downer MC, Fork RL, Shank CV. Femtosecond imaging of melting and evaporation at a photoexcited silicon surface. J Opt Soc Am B 1985; 2: 595-599.
 Sokolowski-Tinten K, Bialkowski J, Cavalleri A, von der Linde D, Oparin A et al. Transient states of matter during short pulse laser ablation. Phys Rev Lett 1998; 81: 224-227.
 R. Fang, A. Vorobyev, and C. Guo, Direct visualization of the complete evolution of femtosecond laser-induced surface structural dynamics of metals. Light: Science & Applications (2017) 6, e16256.
Ranran Fang1, Anatoliy Vorobyev1, Chunlei Guo1
1 The Institute of Optics, University of Rochester, Rochester, USA