The Art of Femtosecond Laser Writing
The Art of Femtosecond Laser Writing
P.G. Kazansky1,2, R. Drevinskas1, A. Cerkauskaite1, A. Patel1, S .S. Fedotov2, S V. Lotarev2, A.G. Okhrimchuk2 and V.N. Sigaev2
1Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK
2International Advanced Laser Technology Centre, Mendeleev University of Chemical Technology of Russia, 125047 Moscow, Russia
Abstract: Formation of sub-wavelength periodic structures in bulk transparent materials during irradiation with intense ultrashort light pulses remains a mystery. Nevertheless the phenomenon has enabled unique applications ranging from printed flat optics to eternal data storage.
Modification of transparent materials with ultrafast lasers has recently attracted considerable interest due to new science and a wide range of applications ranging from 3D integrated optics and microfluidics to geometrical phase optics and durable data storage . The key advantage of using femtosecond pulses for direct laser writing, as opposed to longer pulses, is that they can rapidly deposit energy in solids with high precision. The light is absorbed and the optical excitation ends before the surrounding lattice is perturbed, which results in highly localized nanostructuring without collateral material damage.
A decade ago it has been discovered the creation of sub-wavelength periodic structures with record small features of tens of nanometers self-organized along the polarization of light in the volume of silica glass, which is renowned for its high optical quality and chemical stability. On a macroscopic scale the self-assembled periodic nanostructure behaves as a uniaxial optical crystal with negative birefringence and slow axis oriented along nano-platelets of the structure. The optical anisotropy, which results from the alignment of the nano-platelets, referred to as form birefringence, is of the same order of magnitude as positive birefringence in crystalline quartz. More recently giant birefringence was also produced by ultrashort light pulses in amorphous hydrogenated silicon films .
A uniform birefringent layer can be imprinted by continuously scanning the material with a tightly focused laser beam. The control of the polarization orientation allows direct writing elements of flat optics with spatially variant anisotropy, which exploits Pancharatnam-Berry phase . Unlike with refractive or diffractive elements the phase is not defined by optical path difference but results from the geometrical phase that accompanies space-variant polarization manipulation. The S-waveplate is one of the examples of such birefringent optical component, which can be used for producing optical vortexes and axially symmetric polarization states, e.g. radial or azimuthal, with applications ranging from optical trapping to microscopy and material processing [4, 5]. Another examples of the geometrical phase optical elements imprinted by ultrashort light pulses are Airy beam converters in silica glass (Fig. 1) and geometrical phase holograms in amorphous hydrogenated silicon (Fig. 2).
The two independent parameters describing self-assembled form birefringence in quartz glass, the slow axis orientation (4th dimension) and the strength of retardance (5th dimension), were also explored for the optical encoding of information in addition to three spatial coordinates (Fig. 3). The slow axis orientation
and retardance were independently manipulated by the polarization and intensity of the femtosecond laser beam. The data optically encoded into five dimensions is successfully retrieved by quantitative birefringence measurements. The storage allows unprecedented parameters including hundreds of terabytes per disc data capacity and thermal stability up to 1000° . Even at elevated temperatures of 190 oC, the extrapolated decay time of nanogratings is comparable with the age of the Universe - 13.8 billion years. The demonstrated recording of the first digital documents including the eternal copy of Newtons Opticks, which will survive the human race, is a vital step towards an eternal archive (Fig. 4). Also the projects such as Time Capsule to Mars (http://www.timecapsuletomars.com/) or Moon Mail (https://www.astrobotic.com/moon-mail) could benefit from the extreme durability of data imprinted by femtosecond laser in quartz glass, which is necessary for storage on Moon or Mars. The search of new materials and applications for ultrafast laser nanostructuring is currently in progress .
 M. Beresna, M. Gecevičius, and P. G. Kazansky, “Ultrafast laser direct writing and nanostructuring in transparent materials,“ Advances in Optics and Photonics 6, 293 (2014).
 R. Drevinskas, M. Beresna, M. Gecevičius, M. Khenkin, A. G. Kazanskii, I. Matulaitienė, G. Niaura, O. I. Konkov, E. I. Terukov, Yu. P. Svirko, and P. G. Kazansky, “Giant birefringence and dichroism induced by ultrafast laser pulses in hydrogenated amorphous silicon,” Appl. Phys. Lett. 106, 171106 (2015).
 B. Y. S. Pancharatnam, Proc. Indian Acad. Sci. A 44, 247 (1956).
 R. Drevinskas, J. Zhang, M. Beresna, M. Gecevičius, A. G. Kazanskii, Yu. P. Svirko, and P. G. Kazansky, “Laser material processing with tightly focused cylindrical vector beams,” App. Phys. Lett. 108, 221107 (2016).
 J. Zhang, M. Gecevičius, M. Beresna, and P. G. Kazansky, “Seemingly unlimited lifetime data storage in nanostructured glass,” Phys. Rev. Lett. 112, 033901 (2014).
 S. S. Fedotov, R. Drevinskas, S.V. Lotarev, A. S. Lipatiev, M. Beresna, A. Čerkauskaitė, V. N. Sigaev, and P. G. Kazansky, “Direct writing of birefringent elements by ultrafast laser nanostructuring in multicomponent glass”, Appl. Phys. Lett. 108 (7), 071905 (2016).