A brief review is given regarding ultrafast laser micromachining of materials. Some general experimental observations are first provided to show the characteristics of ultrafast laser micromachining. Apart from empirical research, mathematical models also appear to allow for a further and systematic understanding of these phenomena. A few fundamental ultrafast laser micromachining mechanisms are addressed in an attempt to highlight the physics behind the experimental observations and the mathematical models. It is supposed that a vivid view of ultrafast laser micromachining has been presented by linking experimental observations, mathematical models and the behind physics.

A review of ultrafast laser materials micromachining

We review the most important fabrication techniques for glass and plastic refractive microlenses and we quantitatively characterize in a systematic way the corresponding state-of-the-art microlenses, which we obtained from selected research groups. For all our measurements we rely on three optical instruments: a non-contact optical profiler, a transmission Mach–Zehnder interferometer and a Twyman–Green interferometer. To conclude, we survey and discuss the different fabrication techniques by comparing the geometrical and optical characteristics of the microlenses, the range of materials in which the lenses can be produced, their potential for low-cost fabrication through mass-replication techniques and their suitability for monolithic integration with other micro-optical components.

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Comparing glass and plastic refractive microlenses fabricated with different technologies

Femtosecond laser micromachining has excited vivid attention in various industrial fields and in medicine owing to the advantages of ultrashort laser pulses compared to long-pulse treatment. These are mainly the reduction of the laser fluence needed to induce ablation and the improvement of the contour sharpness of the laser-generated structures. Recently, special attention was paid to femtosecond laser experiments on nonabsorbing inorganic dielectrics. This is due to the fact that optical damage in dielectric optical elements limits the performance of high-power laser systems. Despite the fact that a large variety of organic polymers can be machined with excimer lasers successfully, the involvement of thermal processes can lead to an unsatisfactory quality of the structures. Ultrashort, fs-laser pulses might be an alternative for the treatment of polymers. Therefore, femtosecond laser machining investigations of dielectrics and polymers are reviewed in this paper. Similarities and differences of the ablation behavior of both material classes are discussed. The influence of the bandgap on the ablation threshold in dependence on the pulse duration, the enhancement of the machining precision with a shortening of the pulse duration, incubation phenomena, and morphological features appearing on the surface after femtosecond laser treatment are mentioned. Possible applications, e.g., in medicine and biosensors, are described.

Ultrashort pulse laser interaction with dielectrics and polymers

Injection molding could be used as a mass production technology for microlens arrays. It is of importance, and thus of our concern in the present study, to understand the injection molding processing condition effects on the replicability of microlens array profile. Extensive experiments were performed by varying processing conditions such as flow rate, packing pressure and packing time for three different polymeric materials (PS, PMMA and PC). The nickel mold insert of microlens arrays was made by electroplating a microstructure master fabricated by a modified LIGA process. Effects of processing conditions on the replicability were investigated with the help of the surface profile measurements. Experimental results showed that a packing pressure and a flow rate significantly affects a final surface profile of the injection molded product. Atomic force microscope measurement indicated that the averaged surface roughness value of injection molded microlens arrays is smaller than that of mold insert and is comparable with that of fine optical components in practical use.

Replication of microlens arrays by injection molding

The damage in fused silica and CaF2 crystals induced by wavelength tunable femtosecond lasers is studied. The threshold fluence is observed to increase rapidly with laser wavelength lambda in the region of 250-800 nm, while it is nearly a constant for 800 <lambda < 2000 nm. The ultrafast electronic excitation is also studied by a pump and probe method. The reflectivity increases rapidly in the latter half of pump pulse, which supports that impact ionization plays an important role in the generation of conduction band electrons (CBEs). We study the CBEs absorption via subconduction-band (sub-CB) transition, and develop a coupled avalanche model. Our results indicate that the CBEs absorption via sub-CB transition plays an important role in the damage in dielectrics irradiated by the visible and near ultraviolet femtosecond lasers. Our theory explains well the experiments.

Ultraviolet-infrared femtosecond laser-induced damage in fused silica and CaF2 crystals

Micro-lenses with well-defined optical parameters are generated on polymethylmethacrylate (PMMA) substrates doped with diphenyltriazene (DPT) by controlled use of a swelling effect generated under conditions of subablative excimer laser illumination. The surface profiles depend on the laser spot size and energy density. A sensitively balanced combination of matrix softening, substrate volume expansion due to photochemical nitrogen release, and surface tension is responsible for the final shape of the lenses. Complete arrays of identical lenses with 15 μm diameters and a focal length of 30 μm are produced by irradiation of (0.25 wt. %) DPT-PMMA with a single laser pulse at a wavelength of 308 nm and a fluence of 3 J/cm2. It is shown experimentally and theoretically that appropriate volume expansion is possible without introducing internal light scattering due to the formation of small bubbles.

Micro-lens arrays generated by UV laser irradiation of doped PMMA

Laser ablation of polyimide (PI) and polymethyl-methacrylate (PMMA) at 248 nm with pulse lengths τ ranging from 200 fs to 200 ps was investigated. The measured ablation rates show minima for pulse lengths of about 5 ps (PMMA) or 50 ps (PI).

F. Beinhorn

Papers

Micro-lens arrays generated by UV laser irradiation of doped PMMA

Plasma effects in picosecond-femtosecond UV laser ablation of polymers