Taek Yong Hwang

Angular effects of nanostructure-covered femtosecond laser induced periodic surface structures on metals

In this paper, we study nanostructure-covered laser induced periodic surface structures (NC-LIPSSs) on metals produced by femtosecond laser pulses incident at various angles. For the first time, we show that the dependence of NC-LIPSS period on the incident beam angle deviates significantly from that of regular LIPSSs studied previously using longer laser pulses. Our study shows that this deviation is due to the nanostructures on LIPSSs that can be explained by the Maxwell–Garnett theory of effective media.

Ultrafast dynamics of femtosecond laser-induced nanostructure formation on metals

We perform a comparison study on femtosecond laser-induced nanostructures on three noble metals, Cu, Ag, and Au. Under identical experimental conditions, the three metals each gain a different amount of surface area increase resulting from nanostructuring. We show that the different surface area increase from nanostructuring directly relates to the competition of two ultrafast processes, electron-phonon coupling and hot electron diffusion, following femtosecond laser heating of metals.

Polarization and angular effects of femtosecond laser-induced conical microstructures on Ni

In this paper, through femtosecond laser pulse irradiation with various polarizations and incident angles, we create arrays of nanostructure-covered conical microstructures (NC-CMs) on Ni. We show that the shape of CMs depends significantly on the polarization and incident angle of the laser beam, and find that the size of nanostructures is distributed asymmetrically on the CMs at off normal incidence. We suggest that nonuniform energy deposition owing to the polarization and the incident angle of laser beam plays an important role in the asymmetric nanostructure distribution and shape of CMs on Ni.

Enhanced efficiency of solar-driven thermoelectric generator with femtosecond laser-textured metals

Through femtosecond laser irradiation, we produce in this work a unique type of surface nanostructure on Al that have enhanced absorption at UV and visible but a relatively small emissivity in infrared. By integrating this laser-treated Al to a solar-driven thermoelectric generator, we show that the thermoelectric generator integrated with the femtosecond laser-treated Al foil generates a significantly higher power than the ones without. Our study shows that our technique can dramatically enhance the efficiency of solar-driven thermoelectric devices that may lead to a leap forward in solar energy harnessing.

Observation of femtosecond laser-induced nanostructure-covered large scale waves on metals

Following femtosecond (fs) laser pulse irradiation, we produce a type of periodic surface structure with a period tens of times greater than the laser wavelength and densely covered by an iterating pattern that consists of stripes of nanostructures and microscale cellular structures. The morphology of this large scale wave pattern crucially depends on laser fluence and the number of laser pulses, but not on the laser wavelength. Our study suggests that this large scale wave is initiated by fs laser induced surface unevenness followed by periodically distributed nonuniform surface heating from fs pulse irradiation.

Femtosecond laser-induced blazed periodic grooves on metals

In this Letter, we generate laser-induced periodic surface structures (LIPSSs) on platinum following femtosecond laser pulse irradiation. For the first time to our knowledge, we study the morphological profile of LIPSSs over a broad incident angular range, and find that the morphological profile of LIPSSs depends significantly on the incident angle of the laser beam. We show that LIPSS grooves become more asymmetric at a larger incident angle, and the morphological profile of LIPSSs formed at an incident angle over 55° eventually resembles that of a blazed grating. Our study suggests that the formation of the blazed groove structures is attributed to the selective ablation of grooves through the asymmetric periodic surface heating following femtosecond pulse irradiation. The blazed grooves are useful for controlling the diffraction efficiency of LIPSSs.

Polarization and angular effects of femtosecond laser-induced nanostructure-covered large scale waves on metals

In this paper, we find that nanostructure-covered large scale waves (NC-LSWs) can be produced on metals using both s- and p-polarized femtosecond laser pulses. We show that the period of NC-LSWs can be controlled by laser fluence, the number of irradiating pulses, and the incident beam angle. By modeling angular dependence of NC-LSW period induced by s-polarized light, we reveal that inhomogeneous energy deposition plays an important role in the formation of NC-LSWs. This allows us to establish a three-step model for NC-LSW formation, the formation of laser-induced surface unevenness, inhomogeneous energy deposition due to the interference of the incident light with the scattered light, and nonuniform energy deposition due to shielding by LSWs.

Femtosecond laser-induced asymmetric large scale waves on gold surfaces

With femtosecond (fs) pulse irradiation, we investigate the morphological evolution of a unique type of fs laser-induced periodic surface structure, called nanostructure-covered large scale waves (NC-LSWs), covered by iterating stripe patterns of nanostructures and microstructures with a period of tens of microns. By monitoring the morphological profile of NC-LSWs following fs laser heating of Au, we show that the NC-LSWs are highly asymmetrically formed and propagate on a gold surface. We believe that the selective laser ablation of Au surface and the subsequent mass transfer of liquid Au following nonuniform energy deposition result in the asymmetric NC-LSW propagation on metals.

Surface plasmon enhanced photoelectron emission

In this work, we show that the resonant angle of surface plasmons (SPs) excited on a unique type of nanostructured metal can be significantly different from the calculated values. We also find that the excitation of SPs can significantly enhance photoelectron emission on the structured metal surfaces.

Enhanced Nonlinear Photoelectron Emission by Surface Plasmons from Nanostructure-Covered Periodic Grooves

We find that surface plasmon excitation on nanostructure-covered periodic grooves can significantly enhance photoelectron emission, leading to a 4-photon process that is absent without surface plasmons within the intensity range in our experiment.