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Ultrafast Laser Achieves Quill-like Writing

Photonics Spectra
Jun 2007
Femtosecond pulse is used on transparent material.

Michael A. Greenwood

Lasers have been used for years to inscribe words and other designs. But things are starting to get a little fancy.

By using ultrashort light pulses on transparent materials such as silica glass, researchers have achieved a calligraphic style of writing similar in appearance to that inked with the bygone quill pen. The findings could have applications in direct writing, materials processing and optical trapping.

TWQuill_LaserWriting_Fig1.jpg

Figure 1. This scanning electron microscope image shows cross sections of the calligraphylike structures written on glass using an ultrafast laser. The distance between the lines is 7 μm. Images reprinted with permission of Applied Physics Letters.


The researchers, led by Peter G. Kazansky of the Optoelectronics Research Centre at the University of Southampton in the UK, investigated the interaction of ultrashort light pulses with optical materials. They were surprised by the results.

A mode-locked Coherent Ti:sapphire laser system with a pulse duration of 150 fs and a repetition rate of 250 kHz was used in the experiment. It operated at 800 nm and was focused via a Nikon 0.55-NA 50× objective into the sample. Lines were written by scanning in alternating directions at a depth of 0.5 mm. The lines were written at a speed of 200 μm/s and with a pulse energy of 0.9 μJ.

After polishing the structure and examining it with a JEOL scanning electron microscope, the researchers found etchings resembling calligraphy strokes, with a broad, rounded top that gradually tapered to a pointed end (Figure 1). They also discovered that strokes written in one direction had a different appearance than those written in the opposite direction. The technique resulted in small differences in the lengths of the strokes (Figure 2) and, in a separate experiment, variations in texture. The subwavelength periodic structure of the strokes defined by laser polarization is also responsible for the self-organized form birefringence of these structures.

TWQuill_LaserWriting_Fig2.jpg
Figure 2. Small variations in length were produced by writing in opposite directions.


The findings were attributed to a new physical phenomenon — the anisotropic interaction of matter with an ultrashort light pulse. The researchers said that femtosecond pulses allow energy to be rapidly and accurately deposited in a solid.

In a second experiment, an amplified Imra America ytterbium fiber laser system was used with a pulse duration of <500 fs and a repetition rate ranging from 100 kHz to 1 MHz. The high stability of this system is crucial for systematic studies, which has been proved also by creating high-quality structures with form birefringence.

It operated at 1045 nm, and lines, as in the first experiment, were written in alternating directions. The laser’s alignment was perpendicular to the writing surface, and pulse energies ranged from 0.2 to 1.8 μJ. Higher energy levels resulted in both sets of lines becoming increasingly uneven.

The researchers found that the appearance of directional dependence was strongest between 0.8 and 0.9 μJ. They also observed that, when the beam was turned by 90°, different textures emerged, depending on the direction in which the line was written. The textural difference was clearly evident in lines written at a repetition rate of 100 kHz. The scientists said that asymmetry in the structure of the beam, in particular pulse front tilt, could be responsible.

Moreover, materials can be modified by light with methods ranging from photosynthesis and photography to materials processing and laser writing, and there are only a few parameters of the light beam that control material transformations, in particular wavelength, intensity, exposure time and pulse duration. The results of this work add one more parameter to this list — direction of beam movement or pulse front tilt. The associated degree of freedom will result in achieving additional control over light-induced material transformations.

Kazansky said that additional research is needed to understand the phenomena and to see how ultrashort light pulses behave with materials other than silica glass. He said that his team would like to eventually commercialize the technique.

Applied Physics Letters, April 9, 2007, Vol. 90, 151120.


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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