NCLA - Ultrafast Laser Materials Processing



Introduction to femtosecond laser micro-machining at the NCLA Femtosecond laser workstation development Laser micro-machining applications developed Detailed results on femtosecond laser micro-machining of semi-conductors

Ultrafast laser trepanned hole in a 200 micron thick steel material with spinning polarisation technique (© NCLA 2003)


For further details please contact: Dr Ger O'Connor NCLA NUI, Galway Galway Ireland Tel: +353 91 750469 Fax: +353 91 750594 Email:

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Introduction to femtosecond laser micro-machining at the NCLA
In November 2000, the NCLA opened a state of the art Ultrafast laser micromachining laboratory to investigate precision machining of extremely tiny features from a wide range of materials. The femtosecond laser source was funded under the Programme for Research in Third Level Institutions (PRTLI) and is part of the new National Centre for Biomedical Engineering Science (see http://www.nuigalway.ie/ncbes ) recently established at NUI, Galway. Femtosecond lasers are capable of producing machined features with no heat affected zone or recast layer. As such the machining is referred to as “cold,” and the bulk of the material experiences no compositional changes. The facility features a CLARK – MXR CPA 2001 femtosecond laser and associated equipment, an air conditioned laboratory with a stable floor, and a vibration isolating compressed air driven optical bench. The facility is available for research and development projects with industry and academia.

Figure 1. The NCLA Femtosecond laser ( © NCLA 2000)
In Ultrafast laser materials processing it is useful to know accurately what pulse length is being used. An autocorrelator is a device which can measure such short pulse lengths. One femtosecond is 1 x 10^-15 seconds which is a very short period of time indeed. A femtosecond is a million times shorter than a nanosecond. The ratio of one femtosecond to one second is the same as the ratio of the width of a human hair to a journey 800 times around the circumference of the earth. Nanosecond pulses are used routinely for excimer laser machining. A femtosecond corresponds to a million billionth of a second. During such a small time scale the material is instantaneously raised above the melting, boiling, and vaporisation temperatures into the plasma regime. During such a small time frame the energy does not have time to diffuse from the area of application and as such there are no problems with a “plume” blocking further laser energy from striking the machining zone – a common problem with longer pulse machining. The very high power density over a very short timescale facilitates high quality machining of materials, which are otherwise difficult to machine without damage with longer pulses, such as high conductivity or refractory metals. It is suitable for machining transparent, metallic and polymer materials. An example pulse length of 172 fs is shown in the autocorrelator trace in Figure 2.

Figure 2. A 240 fs Autocorrelator pulse trace which corresponds to a laser pulse length of 172 fs for a Gaussian beam profile. (© NCLA 2000)
The average power of commercially available femtosecond lasers is relatively low, less than 1.3W in many instances. As the pulse length is so short however, the peak power is extremely high, and the system shown in Figure 3. routinely reaches between 5 and 10 Giga watts. At such high power it is possible to cause air breakdown when the beam is focussed through a lens to a small spot size, as evidenced in Figure3, if no special measures are taken.

Figure 3. Air breakdown at the focal point at the end of the optical path ( © NCLA 2000)


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