Ultrafast All-Solid-State Laser Ablation

Conventional continuous wave and long pulse (nanosecond) laser ablation is used in many fields, such as materials processing and medicine. In these regimes the dominant process involved is the heating of the target material through the liquid phase to the vapour phase, resulting in expansion and expulsion of the desired target material. This is accompanied by heating and collateral damage to the surrounding area, the degree of which is determined by the rate of energy absorption and the rate of energy loss through thermal conduction in the material. This collateral damage is often detrimental and is a limiting factor when high precision ablation is required or when it may present a hazard, as is often the case for laser surgery.

Due to their high peak intensities, ultrashort (picosecond and femtosecond) pulses ablate material via the rapid creation of a plasma that absorbs the incident energy resulting in direct vaporisation from the target surface. This produces negligible collateral heating and shock-wave damage, features providing great potential for application in surgery e.g.[1]. In metals it becomes possible to drill holes and machine features with unprecendented precision e.g.[2]. In dielectrics the plasma generation may be initiated by multi-photon ionisation rather than resonant absorption, resulting in an ablation threshold that, for sufficiently short pulses, becomes solely intensity dependent, regardless of local material properties e.g.[3]. This intensity dependence permits the ablation of features smaller than the focal spot size when operating near threshold, since only a fraction (the central region) of the focused light exceeds the threshold intensity [4].

The aim of this work is to demonstrate the utility of all-solid-state diode-pumped laser technology, with its advantages of low cost and straightforward deployment, to ultrafast ablation of different materials including metals and dielectrics.

To date, we have successfully demonstrated the ablation of micron scale holes using both an argon ion based Cr:LiSAF laser system [providing 10 ps pulses of up to 10 microjoules energy at up to 25 kHz] and a diode-pumped Cr:LiSGAF system delivering up to 1 microjoule pulses with durations ranging from 200 fs to 45 ps, both systems being tunable in the near infra-red. The experiments were performed with pulses centered at 830nm.

Argon Ion Laser Pumped Cr:LiSAF Regenerative amplifier System

This system consists of a commercial Ti:Sapphine femtosecond oscillator (Spectra-Physics 'Tsunami') and a 'home made' Cr:LiSAF regenerative amplifier both pumped by an Argon ion laser (488 nm). The amplifier consists of a 10 mm Cr:LiSAF rod end pumped in a four mirror cavity. A Medox Pockels Cell allows the swithing in of a seed pulse and the switching out of an amplified pulse at a repetition rate up to 25kHz. The system is described in more detail in [5]. This system has been used to demonstrate the sub-m J threshold energy required for ablation in glass. The two figures to the left (figures 3 & 4) show images of ablated holes in a microscope slide. Figure 3 is an electronmicrograph of a hole ablated with approximately 20,000 pulses well above threshold. The two distinctly different shapes of hole in figure 4 are caused by focusing at the surface of the slide and then moving the focus into the glass (smoother holes) and focusing in the slide and then moving the focus back beyond the surface (rough holes).
This system has been experimentally applied to the restoration of paintings, providing a means to remove unwanted glue paste from the back of canvas with high precision (work carried out with C. R. Young and J. C. Shepard at the Courtauld Institute of Art, London).

Diode-Pumped Cr:LiSGAF Regenerative amplifier System

This system consists of a three mirror amplifier cavity with a 5 mm plane/brewster face Cr:LiSGAF rod. This is pumped by two 'Coherent' 100 micron stripe 500 mW (@672 nm) diodes, one from each face of the rod (peltier cooled). The diode beams are each shaped by two spherical and two cylindrical lenses. The seed pulses for the amplifier are provided by the diode-pumped Cr:LiSGAF femtosecond oscillator described below. Switching in of the seed pulses and switching out of the amplified pulses was done using a Medox Pockels Cell.

This system was used to investigate the relative ablation threshold for femtosecond (as low as 140 fs) and picosecond (5ps) pulses in glass and the ablation threshold of 'long'(45ps) pulses in thin films of aluminium. Running at 5kHz, the pulses emerging from the amplifier had energies up to one microjoule with a duration of 45ps. This pulse stretching was due to the group velocity dispersion experienced by the initial 30fs seed pulse. With a pulse switch out time of approximately 1.2 microseconds the pulse 'experiences' 180 round trips of the cavity, equivalent to about 10 m of glass. Since this process dominates any self-phase-modulation in the amplifier, the bandwidth/spectrum of the pulses allows them to readily be recompressed using a simple double pass two grating compressor. (We did not correct for higher order dispersion and so the compressed pulses were limited to 140 fs duration)

A simple set-up was used to compress, measure, ablate and image during the experiments. This is shown above (figure 6). A rotating diffuser was used in conjunction with the HeNe laser to reduce speckle in the images. The lenses were chosen to provide approximately 50 times magnification of the sample surface on the CCD. The pulse energy was attenuated using an ND wheel and the pulse duration was changed by altering the grating seperation in the compressor. The pulses were autocorrelated using a background free Spectra-Physics autocorrelator.

The picture to the left (figure 7) shows four series of holes made in glass using two different pulse durations. No drastic change in threshold is predicted when using pulses of the order of 100 fs to several ps and the threshold energies in the two series of holes ablated in this preliminary experiment concur with this. Each hole was made using approximately 20,000 pulses and two series were ablated to test the consistency of the process.

A thin film of Aluminium on glass was exposed to series of 45ps pulses of decreasing energy and the results are shown in figure 8 (below). The images are the same magnification as the images of the ablation in glass and the same lens was used to focus the pulses.

It can be seen however that the size of the features is much larger and the threshold energy for a surface alteration is very low. The interaction is bound to be different since we are dealing with a thin film and relatively 'long' pulses with an associated increase in target heating.

Diode-Pumped Cr:LiSGAF Femtosecond Oscillator

The figure to the right shows the diode-pumped oscillator used to pump the regenerative amplifier. Two 670 nm diodes, one 100 micron stripe (500 mW) device and one 50 micron (350 mW) stripe device, are used to endpump a 10 mm Cr:LiSGAF rod in a 4-mirror resonator cavity. Dispersion compensation is provided by two prisms and the oscillator is Kerr-lens mode-locked. An acousto-optic modulator on one of the prisms and some regenerative electronics provide stabilisation. Pumping with just the 50 micron stripe diode resulted in pulses as short as 90 fs and an average power of 23 mW. Pumping with both diodes gave an average power of 58 mW with pulse durations as short as 25 fs.

Conclusions:

We have demonstrated the application of an all-solid-state cheap portable ultrafast laser system to the ablation of metal films and glass slides using picosecond and femtosecond pulses.
Applications include micromachining, surgery and art restoration.

Future Work:

• Further characterisation of the laser system
• Single pulse experiments
• Ablation of metals

References

[1] A. A. Oraevsky, L. B. Da Silva, A. M. Rubenchik, M. D. Freit, M. E. Glinsky, M. D. Perry, B. M. Mammini, W. Small, IV, and B. C.Stuart: 'Plasma mediated ablation of biological tissues with nanosecond-to-femtosecond laser pulses: relative role of linear and nonlinear absorption', IEEE Journal of Selected Topics in Quantum Electronics, Vol2 No.4, pp. 801-809 (1996)

[2] C.Momma, S. Nolte, B. N. Chichkov, F. V. Alvensleben, A. Tunnermann: 'Precise laser ablation with ultrashort pulses', Applied Surface Science., 109, pp. 15-19 (1997)

[3] B. C. Stuart, M. D. Feit, A. M. Rubenchik, B. W. Shore and M. D. Perry: 'Laser-Induced damage in dielectrics with Nanosecond to Subpicosecond Pulses', Physical Review Letters, 74, pp. 2248-2251 (1995)

[4] P. P. Pronko, S. K. Dutta, J. Squier, J. V. Rudd, D. Du, G. Mourou: 'Machining of sub-micron holes using a femtosecond laser at 800nm', Optics Comm., 114, pp. 106-110 (1995)

[5] S. C. W. Hyde, N. P. Barry, R. Mellish, R. Jones, P. M. W. French, J. R. Taylor, C. J. van der Poel and A. Valster: 'Argon-ion-pumped and diode-pumped all-solid-state femtosecond Cr:LiSrAlF6 regenerative amplifiers', Optics Letters, 20, pp. 160-162 (1995)