The photoexcitation, and especially photoionization, is one of the most important manifestations of the light-matter interaction in nature, ranging from photosynthesis in plants and vision in biology to photography and laser processing of materials. It is generally accepted that the change in a substance is weaker, the less light is absorbed. Here we found that this is not always the case.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Peter G. Kazansky from Optoelectronics Research Centre, University of Southampton, United Kingdom and co-workers have demonstrated efficient ultrafast laser nanostructuring with elliptical polarization in silica glass. Contrary to intuition, despite the nonlinear absorption being about 2.5 times weaker, elliptically polarized pulses result in about twice the birefringence of linearly polarized light. Anisotropic nanopores with a larger concentration are observed with elliptically polarized pulses. The phenomenon is interpreted in terms of enhanced interaction of circularly polarized light with a network of randomly oriented bonds and hole polarons in silica glass, as well as efficient tunnelling ionization of defects with low excitation potentials by circular polarization.
"It is commonly believed that the multiphoton ionization dominates in ultrafast laser writing in transparent materials, but we revealed that tunneling excitation of laser induced defects, such as self-trapped holes, is a key for nanostructruing in silica glass" the scientists added.
"In 5D optical data storage, information can be recorded by elliptically polarized pulses with lower energy and higher writing speed. Moreover, our demonstration allows production of large area geometric phase optical elements and vector beam converters with ultrahigh transmittance for high power and UV lasers." they forecast.
