Control of structural modifications inside silica glass by changing the front tilt of an ultrashort pulse is demonstrated, achieving a calligraphic style of laser writing. The phenomena of anisotropic bubble formation at the boundary of an irradiated region and modification transition from microscopic bubbles formation to self-assembled form birefringence are observed, and the physical mechanisms are discussed. The results provide the comprehensive evidence that the light beam with centrosymmetric intensity distribution can produce noncentrosymmetric material modifications

Hirao, Kazuyuki

Kazansky, Peter G.

Miura, Kiyotaka

Sakakura, Masaaki

Shimotsuma, Yasuhiko

Yang, Weijia

Applied Physics Letters

English

Weijia Yang

Peter G. Kazansky

Yasuhiko Shimotsuma

Masaaki Sakakura

Kiyotaka Miura

Kazuyuki Hirao

Crossref

Ultrashort-pulse laser calligraphy

Kyoto University Research Information Repository

Phase-change functionality in gallium nanoparticles offers an innovative conceptual basis for the development of high density, low energy, nonvolatile optical memories

Yang, W.

Kazansky, P.G.

Shimotsuma, Y.

Hirao, K.

Southampton (e-Prints Soton)

Ultrashort-pulse laser calligraphyWeijia Yang,1 Peter G. Kazansky,1,a Yasuhiko Shimotsuma,2 Masaaki Sakakura,2Kiyotaka Miura,2 and Kazuyuki Hirao21Optoelectronics Research Centre, University of Southampton, SO17 1BJ, United Kingdom2Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura,Nishikyo-ku, Kyoto 615-8510, JapanReceived 19 September 2008; accepted 9 October 2008; published online 29 October 2008Control of structural modifications inside silica glass by changing the front tilt of an ultrashort pulseis demonstrated, achieving a calligraphic style of laser writing. The phenomena of anisotropicbubble formation at the boundary of an irradiated region and modification transition frommicroscopic bubbles formation to self-assembled form birefringence are observed, and the physicalmechanisms are discussed. The results provide the comprehensive evidence that the light beam withcentrosymmetric intensity distribution can produce noncentrosymmetric material modifications.© 2008 American Institute of Physics. DOI: 10.1063/1.3010375Modification of transparent materials with ultrafast la-sers has attracted considerable interest due to a wide range ofapplications including laser surgery,1 integrated optics,2 op-tical data storage,3 and three-dimensional micro-4,5 andnanostructuring.6,7 Three different types of structural changescan be induced with ultrafast laser irradiation in the bulk of atransparent material, silica glass in particular: an isotropicrefractive index change type 1,2 a form birefringence asso-ciated with self-assembled nanogratings and negative refrac-tive index change type 2,7 and a void type 3.3 The type ofmodification depends on several factors including the laserparameters wavelength, pulse duration, energy, and repeti-tion rate, the numerical aperture NA of a focusing lens andthe material parameters band gap, thermal properties. Infused silica, the transition from type 1 to type 2 and finally totype 3 modification is observed with an increase of pulseenergy. More recently, a remarkable phenomenon in ultrafastlaser processing of transparent materials has been reportedmanifesting itself as a change in material modification byreversing the writing direction.8 It has been speculated thatthe pulse front tilt could be added to the list of laser param-eters defining the light-matter interactions in material pro-cessing. One of the main reasons why this peculiar physicalphenomenon has not been observed until recently is ex-plained by the difficulty in the measurement and control ofthe tilt of intensity front of an ultrashort pulse. Sophisticatednonlinear optical techniques such as frequency resolved op-tical gating and its modifications GRENOUILLE have beendeveloped for characterization of the pulse front tilt.9In this letter, we provide experimental evidence, whichshows that indeed the pulse front tilt can be used to controlmaterial modifications and in particular as a tool for laserprocessing and optical manipulation, e.g., for achieving cal-ligraphic style of laser writing, when the appearance of a“stroke” varies in relation to its direction. Moreover, aniso-tropic bubble formation and bubble-nanograting transitionwere observed at high pulse energies. Our experiments usingpulse front tilt characterization and control provide the ex-perimental evidence that the tilt in intensity front of an ul-trashort pulse can reveal itself directly in laser induced ma-terial modifications.An amplified, mode-locked Ti:sapphire laser operating at800 nm wavelength with 70 fs pulse duration and a 250 kHzrepetition rate was used in the first experiment. The linearlypolarized laser beam was focused via a 50 NA=0.8 ob-jective at a depth of 60 m beneath the surface of the fusedsilica sample. Line structures were written inside the bulkmaterial by translating the sample perpendicularly to thelight propagation direction, using a linear motorized stageAerotech ALS-130. After irradiation, the sample was in-spected using an optical microscope. The first group of lineswas written by scanning in alternating directions inside thesample with the pulse energy of 2.6 J and the scan speed of50 m /s Fig. 1a. The temporal characteristic of thepulses, in particular the pulse front tilt before the focusingobjective, were characterized using a GRENOUILLE device8.50 Model. The measured pulse front tilt for the first groupof lines was 410−2 fs /mm. As shown in Fig. 1a, thedirectional dependence can be clearly observed in the mor-phology of the lines written in opposite directions, with aline written in one direction being rougher than a line writtenin the reversed direction. The directional dependence canalso be revealed by imaging the lines between crossed polar-izers, in which only the smooth lines written in one directionshow birefringence. Next the pulse front tilt was reversedaElectronic mail: pgk@orc.soton.ac.uk.FIG. 1. Color online Microscope bright field image of the line structureswritten using femtosecond pulses with a positive pulse front tilt or bnegative pulse front tilt. The distance between the lines is 25 m. Thewriting direction is shown by the arrow. The respective screen shots con-taining measured laser pulse parameters by GRENOUILLE device areshown.APPLIED PHYSICS LETTERS 93, 171109 20080003-6951/2008/9317/171109/3/$23.00 © 2008 American Institute of Physics93, 171109-1Downloaded 11 Nov 2009 to 152.78.208.72. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspby tuning the pulse compressor to the value of −8.6410−2 fs /mm and the second group of lines was imprintedby scanning in alternating directions Fig. 1b. After com-paring the structures written with the opposite sign of thepulse front tilt, the mirror change in the induced modifica-tions is evident Figs. 1a and 1b. This experiment unam-biguously demonstrates that the directional dependence ofthe writing process and the induced modification is deter-mined by the pulse front tilt of the femtosecond laser pulses.Smooth line structures written in one direction Fig. 1correspond to type 2 modifications with the evidence of formbirefringence, while the rough structures produced by writ-ing in the opposite direction belong to type 3 modifications,with the evidence of a void formation. At pulse energiesbelow the threshold value of 2 J, both lines written in op-posite directions reveal type 2 modifications. Moreover,when the pulse energy was increased to 3 J and above, bothlines revealed type 3 modifications and the birefringence inthe induced structures disappeared. The latter result indicatesthat the threshold energy for creating type 3 modificationinside fused silica depends not only on the pulse energy butalso on the writing direction. For example, the line struc-tures, written toward the top in Fig. 1a, reveal higherthreshold energy for the type 3 modifications than the struc-tures written in the opposite direction. This threshold depen-dence reverses when the sign of the pulse front tilt ischanged Fig. 1b. Furthermore, no directional dependencecould be observed when the pulse front tilt was minimizedby tuning the pulse compressor.It should be highlighted that the quill writing effectdepends strongly on the focusing depth of the laser irradia-tion under the sample surface. Changing the focusing depthby only 10%, which is from 60 to 55 or 65 m, completelyeliminated the quill writing effect and produced type 2 ortype 3 structures in both directions.An alternative experiment was also carried out to pro-vide additional evidence that the pulse front tilt is respon-sible for the quill writing effect. A laser source, producingpulses of 150 fs duration at 250 kHz repetition rate and800 nm, was used in this experiment. The laser beam wasfocused via a 50 NA=0.55 objective at a depth of120 m beneath the surface of the sample. The optimumdepth for the quill effect in this experiment was differentfrom the previous one due to the difference in laser beamparameters in the two experiments. The first group of fourlines with alternating writing directions was imprinted withthe pulse energy of 1.4 J and scan speed of 50 m /s Fig.2a. One additional mirror was added in the setup to re-verse the direction of the pulse front tilt before writing thenext group of lines Fig. 2b. In this writing, configurationthe second group of four lines was imprinted with all otherwriting parameters identical to the previous experiment. Itwas observed that the structural modifications in the lines ofthe second group were mirrored when compared with thelines of the first group. This result provides further evidencethat the pulse front tilt is responsible for the directional de-pendence in the ultrafast laser writing.We also observed strong white light emission 400–700nm corresponding to type 3 structural modifications and aweaker emission corresponding to the nanogratings forma-tion, which were reproducible in different writing experi-ments. Supercontinuum generation can be excluded from theexplanation of this emission because the length of lightpropagation in the focus of high NA objective is too small toproduce a significant spectral broadening effect. The ob-served white emission can be explained by thermal radiation,e.g., bremsstrahlung radiation, emitted by electron plasmawhich is heated to thousands of degrees at high light inten-sities of about 1014 W /cm2.10Another intriguing result was the observation of aniso-tropic bubble formation, when the pulse energy was in-creased to the level of 2.4 J. Neither birefringence struc-tures type 2, nor rough features with evidence of voidformation type 3 could be observed in the modified region.Instead, bubbles with diameters varying from 1–3 m wererevealed at the boundary of the irradiated region, which isevident from the cross-sectional image Fig. 3c and thecorresponding top view images of the line structure Figs.3a and 3b. The color of bubbles visible under white lightillumination in the optical microscope Figs. 3a and 3b isattributed to the Fabry–Perot effect. It is well known thatvoids type 3 can be produced on the axis of a focusedfemtosecond laser beam inside transparent material as a re-sult of a microexplosion.4,11 However, in our case, thebubbles are formed not in the region of highest intensitywhich is in the centre of a Gaussian beam but are shiftedabout 5 m toward one side of this center Figs. 3b and3c. It should be noted that the movement of bubbles alongthe direction of writing has been previously demonstrated infused silica.12 Surprisingly, in our experiment, we observedthat the bubbles are shifted perpendicular to the writing di-rection. In addition, we observed bubbles at both sides of theirradiated region when the writing direction was reversedFig. 3a. The mechanism of anisotropic bubble formationis a puzzle. Bubbles can be formed on the axis of the focusedlaser beam as a result of microexplosion and subsequentlymove from the molten center to the boundary of irradiatedregion. Alternatively, they can be formed directly at theboundary region of a light affected zone, where tensile stresscauses the formation of cavitation bubbles when the rupturestrength in molten glass is exceeded.13 Clarification of theFIG. 2. Color online Microscope images without polarizers light part andin crossed polarizers dark part of the line structures fabricated with twomirrors a and three mirrors b in the writing setup at 1.4 J pulse energy.The pulse front is shown by the red line in the setup schematic. k is thewave vector of the writing laser.171109-2 Yang et al. Appl. Phys. Lett. 93, 171109 2008Downloaded 11 Nov 2009 to 152.78.208.72. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspphenomenon’s mechanism needs further investigation. How-ever, we believe that the light pressure at the tilted front ofthe pulse could be responsible for the asymmetry of bubbleformation and its dependence on writing direction.Another interesting observation was an unusual transi-tion from the regime of bubble formation to self-assembledform birefringence, which occurred during the continuouswriting process Fig. 3d. The phenomenon revealed itselfas follows during single line writing. First, bubbles began toappear almost from the start of writing process. At a certaindistance, typically about 500 m from the start of writing,the formation of bubbles abruptly terminated. Instead evi-dence of self-assembled form birefringence appeared andcontinued until the end of line writing process. The transfor-mation in the type of modification was correlated with thechange in intensity of white light emission during the in-scription process, which was similar to the emission de-scribed above. Strong white light emission during the bubbleformation dropped at the modification transition and onlyweak emission could be observed during the formation ofbirefringent structures. It should be noted that all writingparameters, including the pulse energy and scanning speed,were kept constant during the encryption process and thephenomenon could be repeated in different areas of thesample. However, there is one parameter which is not con-stant, in particular the temperature of the whole glass sample,which could increase as a result of light absorption duringthe writing process. It should be noted that self-assembledform birefringence structures and related nanogratings disap-pear at temperatures above the glass transition temperature.This indicates that local temperature in the irradiated regionshould drop at modification transition from the glass meltingtemperature 1715 °C, corresponding to the bubble forma-tion, to the temperature below the glass transition tempera-ture 1175 °C, corresponding to the nanograting formation.This local temperature decrease can be explained by an in-crease of the heat capacity caused by the heating of thewhole glass sample. Indeed, given that T=Q /mcp, whereT is the temperature difference, Q is the heat energy, andm is the mass of the substance, cp is the heat capacity, thetemperature drops when heat capacity increases. The nan-ograting starts to form once the local temperature hasdropped below the glass transition temperature. The energyis consumed by a nanograting formation process, whichlocks temperature below the glass transition temperature andexplains why formation of nanogratings does not stop untilthe end of writing process.In conclusion, a change in structural modification wasdemonstrated in silica glass sample by controlling the direc-tion of pulse front tilt, achieving a calligraphic style of laserwriting which is similar in appearance to that inked with thebygone quill pen. Moreover, anisotropic bubble formation,which could be controlled by writing direction, was observedat high pulse energies. The phenomenon of modificationtransition in a continuous encryption process was observed.Our experiments provide the comprehensive evidence thatthe light beam with centrosymmetric intensity distributioncan produce noncentrosymmetric material modifications. Weanticipate that the possibility of achieving control of light-matter interactions by adjusting the tilt of ultrashort pulsefront could lead to advances in material processing, opticalmanipulation and data storage.The work was supported by the Physical Sciences Re-search Council EPSRC and New Energy and IndustrialTechnology Development Organization NEDO.1R. Birngruber, C. A. Puliafito, A. Gawande, W. Z. Lin, R. W. Schoenlein,and J. G. Fujimoto, IEEE J. Quantum Electron. 23, 1836 1987.2K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, Opt. Lett. 21, 17291996.3E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P.Callan, and E. Mazur, Opt. Lett. 21, 2023 1996.4B. Poumellec, L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, Opt.Express 11, 1070 2003.5W. J. Yang, P. G. Kazansky, and Y. P. Svirko, Nat. Photonics 2, 99 2008.6Y. Shimotsuma, P. G. Kazansky, J. R. Qiu, and K. Hirao, Phys. Rev. Lett.91, 247405 2003.7W. J. Yang, E. Bricchi, P. G. Kazansky, J. Bovatsek, and A. Y. Arai, Opt.Express 14, 10117 2006.8P. G. Kazansky, W. J. Yang, E. Bricchi, J. Bovatsek, A. Arai, Y. Shimot-suma, K. Miura, and K. Hirao, Appl. Phys. Lett. 90, 151120 2007.9S. Akturk, M. Kimmel, P. O’Shea, and R. Trebino, Opt. Express 11, 4912003.10R. Graf, A. Fernandez, M. Dubov, H. J. Brueckner, B. N. Chichkov, andA. Apolonski, Appl. Phys. B: Lasers Opt. 87, 21 2007.11S. Juodkazis, K. Nishimura, S. Tanaka, H. Misawa, E. G. Gamaly, B.Luther-Davies, L. Hallo, P. Nicolai, and V. T. Tikhonchuk, Phys. Rev. Lett.96, 166101 2006.12W. Watanabe and K. Itoh, Opt. Express 10, 603 2002.13A. Vogel, J. Noack, G. Huttman, and G. Paltauf, Appl. Phys. B: LasersOpt. 81, 1015 2005.FIG. 3. Color online a and b Bight field images with different mag-nifications of the line structures fabricated in opposite directions a and bwith 2.4 J pulse energy and scan speed of 50 m /s. c Image of a crosssection of the line structure shown in b. d Images of the transition regionwithout and with crossed polarizers. V is the writing velocity, k is the wavevector of the writing laser, and E is the electric field of the writing laser.171109-3 Yang et al. Appl. Phys. Lett. 93, 171109 2008Downloaded 11 Nov 2009 to 152.78.208.72. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Title Ultrashort-pulse laser calligraphyAuthor(s)Yang, Weijia; Kazansky, Peter G.; Shimotsuma, Yasuhiko;Sakakura, Masaaki; Miura, Kiyotaka; Hirao, KazuyukiCitationAPPLIED PHYSICS LETTERS (2008), 93(17)Issue Date2008-10-27URL http://hdl.handle.net/2433/84591RightCopyright 2008 American Institute of Physics. This article maybe downloaded for personal use only. Any other use requiresprior permission of the author and the American Institute ofPhysics.Type Journal ArticleTextversionpublisherKyoto UniversityUltrashort-pulse laser calligraphyWeijia Yang,1 Peter G. Kazansky,1,a Yasuhiko Shimotsuma,2 Masaaki Sakakura,2Kiyotaka Miura,2 and Kazuyuki Hirao21Optoelectronics Research Centre, University of Southampton, SO17 1BJ, United Kingdom2Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura,Nishikyo-ku, Kyoto 615-8510, JapanReceived 19 September 2008; accepted 9 October 2008; published online 29 October 2008Control of structural modifications inside silica glass by changing the front tilt of an ultrashort pulseis demonstrated, achieving a calligraphic style of laser writing. The phenomena of anisotropicbubble formation at the boundary of an irradiated region and modification transition frommicroscopic bubbles formation to self-assembled form birefringence are observed, and the physicalmechanisms are discussed. The results provide the comprehensive evidence that the light beam withcentrosymmetric intensity distribution can produce noncentrosymmetric material modifications.© 2008 American Institute of Physics. DOI: 10.1063/1.3010375Modification of transparent materials with ultrafast la-sers has attracted considerable interest due to a wide range ofapplications including laser surgery,1 integrated optics,2 op-tical data storage,3 and three-dimensional micro-4,5 andnanostructuring.6,7 Three different types of structural changescan be induced with ultrafast laser irradiation in the bulk of atransparent material, silica glass in particular: an isotropicrefractive index change type 1,2 a form birefringence asso-ciated with self-assembled nanogratings and negative refrac-tive index change type 2,7 and a void type 3.3 The type ofmodification depends on several factors including the laserparameters wavelength, pulse duration, energy, and repeti-tion rate, the numerical aperture NA of a focusing lens andthe material parameters band gap, thermal properties. Infused silica, the transition from type 1 to type 2 and finally totype 3 modification is observed with an increase of pulseenergy. More recently, a remarkable phenomenon in ultrafastlaser processing of transparent materials has been reportedmanifesting itself as a change in material modification byreversing the writing direction.8 It has been speculated thatthe pulse front tilt could be added to the list of laser param-eters defining the light-matter interactions in material pro-cessing. One of the main reasons why this peculiar physicalphenomenon has not been observed until recently is ex-plained by the difficulty in the measurement and control ofthe tilt of intensity front of an ultrashort pulse. Sophisticatednonlinear optical techniques such as frequency resolved op-tical gating and its modifications GRENOUILLE have beendeveloped for characterization of the pulse front tilt.9In this letter, we provide experimental evidence, whichshows that indeed the pulse front tilt can be used to controlmaterial modifications and in particular as a tool for laserprocessing and optical manipulation, e.g., for achieving cal-ligraphic style of laser writing, when the appearance of a“stroke” varies in relation to its direction. Moreover, aniso-tropic bubble formation and bubble-nanograting transitionwere observed at high pulse energies. Our experiments usingpulse front tilt characterization and control provide the ex-perimental evidence that the tilt in intensity front of an ul-trashort pulse can reveal itself directly in laser induced ma-terial modifications.An amplified, mode-locked Ti:sapphire laser operating at800 nm wavelength with 70 fs pulse duration and a 250 kHzrepetition rate was used in the first experiment. The linearlypolarized laser beam was focused via a 50 NA=0.8 ob-jective at a depth of 60 m beneath the surface of the fusedsilica sample. Line structures were written inside the bulkmaterial by translating the sample perpendicularly to thelight propagation direction, using a linear motorized stageAerotech ALS-130. After irradiation, the sample was in-spected using an optical microscope. The first group of lineswas written by scanning in alternating directions inside thesample with the pulse energy of 2.6 J and the scan speed of50 m /s Fig. 1a. The temporal characteristic of thepulses, in particular the pulse front tilt before the focusingobjective, were characterized using a GRENOUILLE device8.50 Model. The measured pulse front tilt for the first groupof lines was 410−2 fs /mm. As shown in Fig. 1a, thedirectional dependence can be clearly observed in the mor-phology of the lines written in opposite directions, with aline written in one direction being rougher than a line writtenin the reversed direction. The directional dependence canalso be revealed by imaging the lines between crossed polar-izers, in which only the smooth lines written in one directionshow birefringence. Next the pulse front tilt was reversedaElectronic mail: pgk@orc.soton.ac.uk.FIG. 1. Color online Microscope bright field image of the line structureswritten using femtosecond pulses with a positive pulse front tilt or bnegative pulse front tilt. The distance between the lines is 25 m. Thewriting direction is shown by the arrow. The respective screen shots con-taining measured laser pulse parameters by GRENOUILLE device areshown.APPLIED PHYSICS LETTERS 93, 171109 20080003-6951/2008/9317/171109/3/$23.00 © 2008 American Institute of Physics93, 171109-1Downloaded 29 Jun 2009 to 130.54.110.22. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspby tuning the pulse compressor to the value of −8.6410−2 fs /mm and the second group of lines was imprintedby scanning in alternating directions Fig. 1b. After com-paring the structures written with the opposite sign of thepulse front tilt, the mirror change in the induced modifica-tions is evident Figs. 1a and 1b. This experiment unam-biguously demonstrates that the directional dependence ofthe writing process and the induced modification is deter-mined by the pulse front tilt of the femtosecond laser pulses.Smooth line structures written in one direction Fig. 1correspond to type 2 modifications with the evidence of formbirefringence, while the rough structures produced by writ-ing in the opposite direction belong to type 3 modifications,with the evidence of a void formation. At pulse energiesbelow the threshold value of 2 J, both lines written in op-posite directions reveal type 2 modifications. Moreover,when the pulse energy was increased to 3 J and above, bothlines revealed type 3 modifications and the birefringence inthe induced structures disappeared. The latter result indicatesthat the threshold energy for creating type 3 modificationinside fused silica depends not only on the pulse energy butalso on the writing direction. For example, the line struc-tures, written toward the top in Fig. 1a, reveal higherthreshold energy for the type 3 modifications than the struc-tures written in the opposite direction. This threshold depen-dence reverses when the sign of the pulse front tilt ischanged Fig. 1b. Furthermore, no directional dependencecould be observed when the pulse front tilt was minimizedby tuning the pulse compressor.It should be highlighted that the quill writing effectdepends strongly on the focusing depth of the laser irradia-tion under the sample surface. Changing the focusing depthby only 10%, which is from 60 to 55 or 65 m, completelyeliminated the quill writing effect and produced type 2 ortype 3 structures in both directions.An alternative experiment was also carried out to pro-vide additional evidence that the pulse front tilt is respon-sible for the quill writing effect. A laser source, producingpulses of 150 fs duration at 250 kHz repetition rate and800 nm, was used in this experiment. The laser beam wasfocused via a 50 NA=0.55 objective at a depth of120 m beneath the surface of the sample. The optimumdepth for the quill effect in this experiment was differentfrom the previous one due to the difference in laser beamparameters in the two experiments. The first group of fourlines with alternating writing directions was imprinted withthe pulse energy of 1.4 J and scan speed of 50 m /s Fig.2a. One additional mirror was added in the setup to re-verse the direction of the pulse front tilt before writing thenext group of lines Fig. 2b. In this writing, configurationthe second group of four lines was imprinted with all otherwriting parameters identical to the previous experiment. Itwas observed that the structural modifications in the lines ofthe second group were mirrored when compared with thelines of the first group. This result provides further evidencethat the pulse front tilt is responsible for the directional de-pendence in the ultrafast laser writing.We also observed strong white light emission 400–700nm corresponding to type 3 structural modifications and aweaker emission corresponding to the nanogratings forma-tion, which were reproducible in different writing experi-ments. Supercontinuum generation can be excluded from theexplanation of this emission because the length of lightpropagation in the focus of high NA objective is too small toproduce a significant spectral broadening effect. The ob-served white emission can be explained by thermal radiation,e.g., bremsstrahlung radiation, emitted by electron plasmawhich is heated to thousands of degrees at high light inten-sities of about 1014 W /cm2.10Another intriguing result was the observation of aniso-tropic bubble formation, when the pulse energy was in-creased to the level of 2.4 J. Neither birefringence struc-tures type 2, nor rough features with evidence of voidformation type 3 could be observed in the modified region.Instead, bubbles with diameters varying from 1–3 m wererevealed at the boundary of the irradiated region, which isevident from the cross-sectional image Fig. 3c and thecorresponding top view images of the line structure Figs.3a and 3b. The color of bubbles visible under white lightillumination in the optical microscope Figs. 3a and 3b isattributed to the Fabry–Perot effect. It is well known thatvoids type 3 can be produced on the axis of a focusedfemtosecond laser beam inside transparent material as a re-sult of a microexplosion.4,11 However, in our case, thebubbles are formed not in the region of highest intensitywhich is in the centre of a Gaussian beam but are shiftedabout 5 m toward one side of this center Figs. 3b and3c. It should be noted that the movement of bubbles alongthe direction of writing has been previously demonstrated infused silica.12 Surprisingly, in our experiment, we observedthat the bubbles are shifted perpendicular to the writing di-rection. In addition, we observed bubbles at both sides of theirradiated region when the writing direction was reversedFig. 3a. The mechanism of anisotropic bubble formationis a puzzle. Bubbles can be formed on the axis of the focusedlaser beam as a result of microexplosion and subsequentlymove from the molten center to the boundary of irradiatedregion. Alternatively, they can be formed directly at theboundary region of a light affected zone, where tensile stresscauses the formation of cavitation bubbles when the rupturestrength in molten glass is exceeded.13 Clarification of theFIG. 2. Color online Microscope images without polarizers light part andin crossed polarizers dark part of the line structures fabricated with twomirrors a and three mirrors b in the writing setup at 1.4 J pulse energy.The pulse front is shown by the red line in the setup schematic. k is thewave vector of the writing laser.171109-2 Yang et al. Appl. Phys. Lett. 93, 171109 2008Downloaded 29 Jun 2009 to 130.54.110.22. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspphenomenon’s mechanism needs further investigation. How-ever, we believe that the light pressure at the tilted front ofthe pulse could be responsible for the asymmetry of bubbleformation and its dependence on writing direction.Another interesting observation was an unusual transi-tion from the regime of bubble formation to self-assembledform birefringence, which occurred during the continuouswriting process Fig. 3d. The phenomenon revealed itselfas follows during single line writing. First, bubbles began toappear almost from the start of writing process. At a certaindistance, typically about 500 m from the start of writing,the formation of bubbles abruptly terminated. Instead evi-dence of self-assembled form birefringence appeared andcontinued until the end of line writing process. The transfor-mation in the type of modification was correlated with thechange in intensity of white light emission during the in-scription process, which was similar to the emission de-scribed above. Strong white light emission during the bubbleformation dropped at the modification transition and onlyweak emission could be observed during the formation ofbirefringent structures. It should be noted that all writingparameters, including the pulse energy and scanning speed,were kept constant during the encryption process and thephenomenon could be repeated in different areas of thesample. However, there is one parameter which is not con-stant, in particular the temperature of the whole glass sample,which could increase as a result of light absorption duringthe writing process. It should be noted that self-assembledform birefringence structures and related nanogratings disap-pear at temperatures above the glass transition temperature.This indicates that local temperature in the irradiated regionshould drop at modification transition from the glass meltingtemperature 1715 °C, corresponding to the bubble forma-tion, to the temperature below the glass transition tempera-ture 1175 °C, corresponding to the nanograting formation.This local temperature decrease can be explained by an in-crease of the heat capacity caused by the heating of thewhole glass sample. Indeed, given that T=Q /mcp, whereT is the temperature difference, Q is the heat energy, andm is the mass of the substance, cp is the heat capacity, thetemperature drops when heat capacity increases. The nan-ograting starts to form once the local temperature hasdropped below the glass transition temperature. The energyis consumed by a nanograting formation process, whichlocks temperature below the glass transition temperature andexplains why formation of nanogratings does not stop untilthe end of writing process.In conclusion, a change in structural modification wasdemonstrated in silica glass sample by controlling the direc-tion of pulse front tilt, achieving a calligraphic style of laserwriting which is similar in appearance to that inked with thebygone quill pen. Moreover, anisotropic bubble formation,which could be controlled by writing direction, was observedat high pulse energies. The phenomenon of modificationtransition in a continuous encryption process was observed.Our experiments provide the comprehensive evidence thatthe light beam with centrosymmetric intensity distributioncan produce noncentrosymmetric material modifications. Weanticipate that the possibility of achieving control of light-matter interactions by adjusting the tilt of ultrashort pulsefront could lead to advances in material processing, opticalmanipulation and data storage.The work was supported by the Physical Sciences Re-search Council EPSRC and New Energy and IndustrialTechnology Development Organization NEDO.1R. Birngruber, C. A. Puliafito, A. Gawande, W. Z. Lin, R. W. Schoenlein,and J. G. Fujimoto, IEEE J. Quantum Electron. 23, 1836 1987.2K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, Opt. Lett. 21, 17291996.3E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P.Callan, and E. Mazur, Opt. Lett. 21, 2023 1996.4B. Poumellec, L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, Opt.Express 11, 1070 2003.5W. J. Yang, P. G. Kazansky, and Y. P. Svirko, Nat. Photonics 2, 99 2008.6Y. Shimotsuma, P. G. Kazansky, J. R. Qiu, and K. Hirao, Phys. Rev. Lett.91, 247405 2003.7W. J. Yang, E. Bricchi, P. G. Kazansky, J. Bovatsek, and A. Y. Arai, Opt.Express 14, 10117 2006.8P. G. Kazansky, W. J. Yang, E. Bricchi, J. Bovatsek, A. Arai, Y. Shimot-suma, K. Miura, and K. Hirao, Appl. Phys. Lett. 90, 151120 2007.9S. Akturk, M. Kimmel, P. O’Shea, and R. Trebino, Opt. Express 11, 4912003.10R. Graf, A. Fernandez, M. Dubov, H. J. Brueckner, B. N. Chichkov, andA. Apolonski, Appl. Phys. B: Lasers Opt. 87, 21 2007.11S. Juodkazis, K. Nishimura, S. Tanaka, H. Misawa, E. G. Gamaly, B.Luther-Davies, L. Hallo, P. Nicolai, and V. T. Tikhonchuk, Phys. Rev. Lett.96, 166101 2006.12W. Watanabe and K. Itoh, Opt. Express 10, 603 2002.13A. Vogel, J. Noack, G. Huttman, and G. Paltauf, Appl. Phys. B: LasersOpt. 81, 1015 2005.FIG. 3. Color online a and b Bight field images with different mag-nifications of the line structures fabricated in opposite directions a and bwith 2.4 J pulse energy and scan speed of 50 m /s. c Image of a crosssection of the line structure shown in b. d Images of the transition regionwithout and with crossed polarizers. V is the writing velocity, k is the wavevector of the writing laser, and E is the electric field of the writing laser.171109-3 Yang et al. Appl. Phys. Lett. 93, 171109 2008Downloaded 29 Jun 2009 to 130.54.110.22. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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Ultrashort-pulse laser calligraphy

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