30 Jan 2008
Using a femtosecond laser to write patterns on a lithium niobate crystal has yielded the first evidence for a new optical phenomenon.
Physicists at the University of Southampton in the UK and the University of Joensuu in Finland have made the suprising discovery that structures written into the surface of a lithium niobate crystal using an ultrafast laser are mirrored when the direction of light propagation is reversed. This new optical effect, which the scientists have dubbed non-reciprocal photosensitivity, could be important for applications in laser machining and optical data storage.
The team used a high-intensity Ti:Sapphire laser operating at 800 nm with a 150 fs pulse duration and a repetition rate of 250 kHz. "We ran the experiment numerous times, each time changing the crystal orientation and light direction, and the observations were quite interesting," said Peter Kazansky of the Optoelecronics Research Centre at the University of Southampton.
The researchers first tested the effect of scanning the laser beam horizontally across the top surface of the lithium niobate crystal. When the laser beam was scanned in one direction (+y) and then the other (–y), distinct differences were observed in both the surface morphology and the texture of the resulting line structures.
Similar asymmetries have been observed when using ultrashort pulse lasers to write patterns in glass, an effect that has become known as the "femtosecond laser quill writing effect". But the new results are different in one key respect: no differences were observed when the laser beam was scanned back and forth in the transverse (x) direction, indicating that the directional dependence in this case has its roots in lithium niobate's non–centrosymmetric crystal structure.
More surprises were in store when the team turned the crystal over along its y-axis, which in effect reverses the direction of the laser beam. "When the crystal is flipped, the light enters from the bottom and exits from the top," said Kazansky. "So this is equivalent to moving the laser source from above the crystal to a position below it."
After flipping the crystal, the researchers observed that the patterns on the crystal appeared mirrored. In other words, when the laser direction was reversed the +y patterns appeared like –y patterns, and vice versa. "A reversal of light propagation direction had the same impact as a change in the direction of movement – which amounts to a mirroring of the structures," Kazansky added.
This finding goes against the commonly held belief that structural modifications caused by laser writing do not depend on the direction of light. "Non-reciprocal phenomena are very rare in optics and are usually associated with the presence of a magnetic field, as in the Faraday effect," commented Kazansky. "In our experiment, we observed this behaviour without the use of a magnetic field."
Further experiments have led Kazansky and his team to conclude that this new nonlinear phenomenon is caused by the bulk photothermal effect. "Every time a light beam is directed on a non-centrosymmetric crystal, like lithium niobate, it produces a thermal current perpendicular to the direction of light propagation," he explained.
It's this heat that causes different structures to be produced in the material when the direction of writing is reversed. "Strangely enough, in non-centrosymmetric materials subjected to uniform light illumination the heat can flow from cold to hot, but the second law of thermodynamics is not violated because the system is not isolated," Kazansky added.
The team now intends to study the thermal effects more closely to figure out how to control it. And while practical applications may be some way off, the phenomenon does present some interesting areas for exploration.
"There is nothing markedly special about lithium niobate, and we believe that this same behaviour must be shown by other non-centrosymmetric materials having a similar crystalline structure," said Weijia Yang, also at the ORC. "On the basis of this work, we believe that non-reciprocity of ultrafast laser writing could lead to new opportunities in laser machining and optical data storage."
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