This paper reviews a new field of direct femtosecond laser surface nano/microstructuring and its applications. Over the past few years, direct femtosecond laser surface processing has distinguished itself from other conventional laser ablation methods and become one of the best ways to create surface structures at nano- and micro-scales on metals and semiconductors due to its flexibility, simplicity, and controllability in creating various types of nano/microstructures that are suitable for a wide range of applications. Significant advancements were made recently in applying this technique to altering optical properties of metals and semiconductors. As a result, highly absorptive metals and semiconductors were created, dubbed as the “black metals” and “black silicon”. Furthermore, various colors other than black have been created through structural coloring on metals. Direct femtosecond laser processing is also capable of producing novel materials with wetting properties ranging from superhydrophilic to superhydrophobic. In the extreme case, superwicking materials were created that can make liquids run vertically uphill against the gravity over an extended surface area. Though impressive scientific achievements have been made so far, direct femtosecond laser processing is still a young research field and many exciting findings are expected to emerge on its horizon.

Direct femtosecond laser surface nano/microstructuring and its applications

Surface Structuring with Ultra-short Laser Pulses: Basics, Limitations and Needs for High Throughput

Optical properties of silicon light trapping structures for photovoltaics

Adaptive optics are becoming a valuable tool for laser processing, providing enhanced functionality and flexibility for a range of systems. Using a single adaptive element, it is possible to correct for aberrations introduced when focusing inside the workpiece, tailor the focal intensity distribution for the particular fabrication task and/or provide parallelisation to reduce processing times. This is particularly promising for applications using ultrafast lasers for three-dimensional fabrication. We review recent developments in adaptive laser processing, including methods and applications, before discussing prospects for the future.

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Adaptive optics in laser processing.

A high-accuracy aluminum-based surface plasmon resonance (SPR) chalcogenide sensor is proposed for IR. The structure is based on widely used 2S2G chalcogenide glass with aluminum as the SPR active metal. The angular interrogation method has been used to study the performance of the sensor in terms of intrinsic sensitivity (IS) that includes the width and shifts of the SPR curve for a given refractive index of sensing layer. The IS of Al-based chalcogenide glass sensor is almost 400% more as compared with an Au-based one, which is the most widely used SPR active metal. The oxidation problem of an Al-based SPR sensor has been addressed.

http://www.icfo.eu/images/publications/J09-027.pdf

High-performance sensor based on surface plasmon resonance with chalcogenide prism and aluminum for detection in infrared.

The enhanced optical absorptance in metals was recently demonstrated using femtosecond laser-induced surface structuring. This structuring was obtained by simply focusing the light to the sample surface. Here we demonstrate more efficient absorptance enhancement using interferometric ablation. This interferometric ablation technique produces deeper surface structures and, consequently, higher absorption than structures obtained by just focusing the light to the surface. We also show the measured reflectance spectra over visible region for unaltered and structured stainless steel and copper samples.

Enhanced optical absorptance of metals using interferometric femtosecond ablation

Water-Assisted Femtosecond Laser Pulse Ablation of High Aspect Ratio Holes

A high refractive index Te-enriched bulk chalcogenide glass Ge20As20Se14Te46 (n≈3.3) has been patterned by ablation using four- and two-beam interference femto-second laser setups operating at 800 nm. The regular arrays of 0.8 μm diameter and more than 0.8 μm depth holes and/or grooves of typical size of 1×1 mm2 have been written on the surface of the glass in a time-scale of 1 second with 50 femtosecond pulses. The high photosensitivity of this narrow-gap semiconductor glass to the femtosecond irradiation is ascribed to a free electron absorption typical of metals, which is caused by laser-induced heating of the glass.

High refractive index chalcogenide glass for photonic crystal applications.

The use of computer generated holograms together with spatial light modulator (SLM) enable highly parallel laser micromachining. Usually SLM is used for splitting the original laser beam to desired number of beams with equal intensity. However, this technique also enables that the intensity of every beam can be controlled individually. Example of the hologram designing procedure for separation of the original beam to 400 beams with individually controlled intensity is presented. The proposed technique is demonstrated by femtosecond laser ablation of grayscale pictures so that grey scale of the pixel is addressed with corresponding beam intensity in the ablated picture.

Parallel femtosecond laser ablation with individually controlled intensity

We demonstrate the linear transformation of small phase excursions of a speckled wavefront into modulation of the transmitted intensity using the polarization self-modulation effect in photoconductive electro-optic crystals. The technique is more advantageous than the widely used photorefractive two-wave mixing technique.

Kimmo Paivasaari

Papers

Enhanced optical absorptance of metals using interferometric femtosecond ablation

Water-Assisted Femtosecond Laser Pulse Ablation of High Aspect Ratio Holes

High refractive index chalcogenide glass for photonic crystal applications.

Parallel femtosecond laser ablation with individually controlled intensity

Adaptive interferometer using self-induced electro-optic modulation