Chunlei Guo

Colorizing metals with femtosecond laser pulses

For centuries, it had been the dream of alchemists to turn inexpensive metals into gold. Certainly, it is not enough from an alchemist’s point of view to transfer only the appearance of a metal to gold. However, the possibility of rendering a certain metal to a completely different color without coating can be very interesting in its own right. In this work, we demonstrate a femtosecond laser processing technique that allows us to create a variety of colors on a metal that ultimately leads us to control its optical properties from UV to terahertz.

Periodic ordering of random surface nanostructures induced by femtosecond laser pulses on metals

In this paper, we performed a detailed study of the formation of femtosecond laser-induced periodic surface structures (LIPSSs) on platinum and gold at near-damage threshold fluences. We find a unique type of LIPSS entirely covered with nanostructures. A distinctive feature of the nanostructure-covered LIPSS is that its period is appreciably less than that of the regular LIPSS. We show that the reduced period is caused by an increase of the real part of the effective refractive index of the air-metal interface when nanostructures develop and affect the propagation of surface plasmons.

Multifunctional surfaces produced by femtosecond laser pulses

In this study, we create a multifunctional metal surface by producing a hierarchical nano/microstructure with femtosecond laser pulses. The multifunctional surface exhibits combined effects of dramatically enhanced broadband absorption, superhydrophobicity, and self-cleaning. The superhydrophobic effect is demonstrated by a falling water droplet repelled away from a structured surface with 30% of the droplet kinetic energy conserved, while the self-cleaning effect is shown by each water droplet taking away a significant amount of dust particles on the altered surface. The multifunctional surface is useful for light collection and water/dust repelling.

Ultrafast dynamics of femtosecond laser-induced periodic surface pattern formation on metals

In this paper, we perform a comparison study of periodic structures on the surfaces of three different noble metals, Cu, Ag, and Au, following femtosecond laser radiation. Under identical experimental conditions, laser-induced surface patterns show distinctly different level of morphological clearness on the three different metals. Simple calculations based on metal melting fail to explain the pattern differences. We show that our observation result from the competition of two ultrafast processes, electron-phonon energy coupling and hot electron diffusion, following femtosecond laser heating of metals.

Femtosecond laser nanostructuring of metals

In this paper, we report on various nanostructures produced through direct surface modification on metals using femtosecond laser pulses. We show, for the first time, that these nanosctructures are natural consequence following femtosecond laser ablation. The optimal conditions for producing various nanostructures are determined.

Femtosecond laser-induced periodic surface structure formation on tungsten

In this paper, we demonstrate the generation of periodic surface structures on a technologically important material, tungsten, at both 400 and 800 nm, despite that the table values of dielectric constants for tungsten at these two wavelengths suggest the absence of surface plasmons, a wave necessary for forming periodic structures on metals. Furthermore, we find that the structure periods formed on tungsten are significantly less than the laser wavelengths. We believe that the dielectric constants of tungsten change significantly due to intense laser pulse heating and surface structuring and roughening at nanometer scales, permitting surface plasmon excitation and periodic structure formation.

Metal pumps liquid uphill

The behavior of liquids on a solid surface is determined by the surface wettability. In this work, by structuring metal surfaces with high-intensity femtosecond laser pulses, we engineer a unique surface pattern that dramatically modifies surface wetting properties. In a gravity-defying way, the treated metal surfaces make liquids sprint vertically uphill at an unprecedented speed of 1 cm/s. Furthermore, the surface structures we create here rapidly transport a significant amount of liquid against gravitation to an elevated point above the reservoir level, thus bringing this effect to potential real-life applications.

Laser turns silicon superwicking.

Using high-intensity femtosecond laser pulses, we create a novel surface pattern that transforms regular silicon to superwicking. Due to the created surface structure, water sprints vertically uphill in a gravity defying way. Our study of the liquid motion shows that the fast self-propelling motion of water is due to a supercapillary effect from the surface structures we created. The wicking dynamics in the produced surface structure is found to follow the classical square root of time dependence.

Femtosecond laser blackening of platinum

Using a femtosecond laser processing technique, we produce the black platinum with absorptance of about 95% over a broad wavelength range from ultraviolet to infrared. From scanning electron microscopy and energy dispersive x-ray spectroscopy studies, we find that the enhanced absorption of the black metal is due to a variety of nano- and microscale surface structures. Using a unique calorimetry technique, we perform a shot-to-shot comparison study of the metal absorption change in air and vacuum. Our study shows that the blackening process for platinum is more efficient in vacuum.

Antireflection effect of femtosecond laser-induced periodic surface structures on silicon.

Following direct femtosecond laser pulse irradiation, we produce a unique grating structure over a large area superimposed by finer nanostructures on a silicon wafer. We study, for the first time, the antireflection effect of this femtosecond laser-induced periodic surface structures (FLIPSSs) in the wavelength range of 250 - 2500 nm. Our study shows that the FLIPSSs suppress both the total hemispherical and specular polarized reflectance of silicon surface significantly over the entire studied wavelength range. The total polarized reflectance of the processed surface is reduced by a factor of about 3.5 in the visible and 7 in the UV compared to an untreated sample. The antireflection effect of the FLIPSS surface is broadband and the suppression stays to the longest wavelength (2500 nm) studied here although the antireflection effect in the infrared is weaker than in the visible. Our FLIPSS structures are free of chemical contamination, highly durable, and easily controllable in size.