The extremely short duration and high peak intensity of femtosecond lasers enables processing of any kind of material, even transparent materials, where light would not normally be absorbed.  These materials include glass, diamond, sapphire, polymers, and thin/thick layer coatings.  The femtosecond laser pulses are also ideal for processing light absorbing materials, such as metals, opaque polymers, and semiconductors.  In both cases, material ablation occurs with minimal HAZ (heat affected zone) enabling production of accurately clean cut edges without the need for a post process.   High-precision, micro-machined parts can be generated with IMRA’s FCPA DE and DX Series laser sources, as the examples below show.

Above: HAZ diagram; Below: FCPA laser micromachining of 50 μm diameter hole in stainless steel.

The ultrafast dynamics of the laser interaction with material is governed by the energy transfer time from the laser light to the material.  In most cases, primarily the electrons absorb the energy from the light, becoming hot.  Subsequently the electron energy is transferred to the cold lattice atoms, or molecules.  The time required for this transfer is characterized with phonon emission time.  If electrons and phonons are in a thermal equilibrium state, the state can be described by a single temperature.  This is called being thermalized.  In practice, the hot/cold and thermal/non-thermal regime can coexist, depending on the details of the interacting laser and material parameters.

For laser material processing, the important question is, how much energy of the absorbed light contributes to the ablation of the material.  Therefore, it is more practical to trace the heat generated and dissipated in the material, which is described as the heat affected zone (HAZ).  As shown in the diagram above, the extended HAZ area results in producing molten debris, casts and other undesirable formations of materials.  Many precision parts these days require processing approaches that eliminate or minimize HAZ.

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.

Phormidium bacteria gliding in a nanoaquarium created with a femtosecond laser direct writing. DOI: 10.1039/C1LC20101H

Direct Writing
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.

Cross-section of a femtosecond laser weld at the interface between two pieces of glass. A complete seal is formed, with no damage to the external surfaces of the glass.

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.

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.

Comparison of FeSi2 film produced by nanosecond PLD (left) and FCPA femtosecond PLD (right)

The combination of high repetition rate, variable pulse energy, and ultrashort pulse width provides a unique opportunity of fine tuning the PLD process for superior film quality.  One example of this is achieving droplet-free coating. Conventional nanosecond pulsed lasers often produce large liquid droplets in the ablation plasma, resulting in nonuniform coating. The ultrashort pulse width of FCPA is ideal for cold ablation which generates plasma free of droplets, while the high pulse repetition rate assures deposition rate. The above figure is a comparison of morphology between nanosecond and FCPA PLD films.

TEM image of nanoparticles generated with FCPA laser ablation directly in liquid. Average particle size is < 40 nm.

FCPA laser ablation can be used to generate nanoparticles, with dimensions of 10′s of nanometers, 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.