Material modification occurs when energy is absorbed by the material causing a change in structure of the area being processed by the laser beam.
For long pulse (nanosecond duration) laser machining, the energy is deposited over a relatively long period of time, allowing heat to be conducted into the material and cause undesirable effects such as splatter and re-solidification of melted material, discoloration, cracks, and voids due to thermal stress. The size of the “Heat Affected Zone” (HAZ) limits the minimum feature size that can be created. For ultrashort pulse laser processing (pulse durations of 10-12 seconds and below), the duration of the laser pulse is shorter than the time required for energy to be transferred to the surrounding area of the laser interaction volume, or transferred from electrons to atoms, when the initial incident light is absorbed by electrons. This minimizes the HAZ and maximizes the precision of the material processing.
Another important advantage of femtosecond laser processing is that the peak power is very high, allowing nonlinear absorption within the bulk of transparent materials. Because of nonlinear absorption, it is possible to limit the region of material modification to the area around the laser focus. Any energy absorbed above or below the focus will generally be below the material modification threshold, so no damage is produced. This allows for novel 3-dimensional patterning within optically transparent materials. For longer pulses (greater than several picoseconds, depending on the material), the peak power is relatively low so the resulting intensity at the laser focus is not sufficient to produce enough nonlinear absorption for material modification. Therefore, to produce any material change, high average powers are needed which is dominated by linear absorption, resulting in excess thermal damage both at the focus and in the surrounding material.
IMRA’s FCPA µJewel femtosecond fiber lasers offer the advantages of ultrashort pulse processing in a robust, reliable fiber-based package. In addition to their stability and compactness, these lasers have high pulse repetition rates (up to 5 MHz) for faster processing.
Benefits of femtosecond fiber lasers for materials processing:
- Minimal heat affected zone (HAZ)
- Very fine, high resolution microprocessing is possible
- Ability to machine or modify transparent materials, even below the surface
- Fast processing due to high repetition rates, while keeping the average power low
Several types of material processing are discussed below.
The high peak intensity of femtosecond laser pulses is essential for ablating a high band gap material such as diamond. Even the hardest materials can be precisely ablated with the FCPA µJewel.
Transparent Materials Processing
The high peak intensity of femtosecond laser pulse can induce nonlinear absorption in transparent materials such as glass or clear polymers. The absorption occurring at the focal spot inside the transparent material allows the creation of 3D structures by material modification. The material modification fluence threshold is generally deterministic so that features smaller than the focused spot size can be produced.
In certain optical materials, processing with femtosecond lasers causes change of the refractive index at the laser focus. This change can be used to create optical waveguides and diffractive optic elements inside the bulk of a material. In the example on the right, IMRA’s femtosecond fiber laser was used to create a nano-aquarium for studying the movement of micro-organisms.
In-volume Selective Laser Etching
The area modified with a femtosecond laser has different chemical and mechanical properties than the bulk of the material. For some substances, the modified material is much more reactive with an etchant compared to the unmodified material. This process, called In-volume Selective Laser Etching (ISLE), was developed at IMRA’s Premier Applications Lab in Aachen, Germany and can be used to make extremely high aspect ratio cuts.
Transparent Material Processing Papers
Glass / Plastic Welding
Another important technique enabled by femtosecond laser pulses is welding of transparent materials, e.g. glass or polymers. The laser beam is focused at the interface between the two materials to be welded. Because of nonlinear absorption, only the region around the laser focus melts to create the weld. Even though a single laser pulse deposits minimal heat in the material, rapid, overlapping pulses can be used to produce precise thermal effects at the laser focus through heat accumulation. Precise control of the thermal effect can be achieved by varying the translation speed and pulse repetition rate.
3D Laser Lithography
Also known as two-photon polymerization. Light absorption in this specifically designed material via two-photon absorption causes bonding and hardening. With tight focusing, the required high intensity is present only at the laser focus, which is about the size of the wavelength (< 1 micron). By scanning the focal spot, 3D structures can be built up with very fine resolution.
Pulsed Laser Deposition
The unique ablation process of femtosecond lasers can be used to produce uniform coatings. At high repetition rates, the femtosecond laser creates a droplet-free plasma of ablated material which is deposited onto a substrate, forming a uniform thin film.
Femtosecond laser ablation can be used to generate nanoparticles, with dimensions of 10′s of nanometers (nm), directly in liquid solvents. The properties of the nano-material can be controlled by varying the laser and processing parameters. IMRA has developed a technique for generating nanoparticles in solution, without the use of surfactants or other additives. With a clean, uncontaminated surface, it is possible to conjugate different molecules to the particle surface to produce bio-active nanoparticles. Contact Us for more information.
Important Nanoparticle Generation Papers
|2012||Stable Gold nanocolloids with controllable surface modification and functionalization|
|2011||Highly efficient and controllable PEGylation of Gold nanoparticles prepared by femtosecond laser ablation in water|