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Direct laser-writing of ferroelectric single-crystal waveguide architectures in glass for 3D integrated optics, Scientific Reports  5:10391, (2015) DOI: 10:1038/srep10391, A. Stone, H. Jain, V. Dierolf, M.Sakakura, Y. Shimotsuma, K. Miura, K. Hirao, J. Lapointe & R. Kashyap,

Direct three-dimensional laser writing of amorphous waveguides inside glass has been studied intensely as an attractive route for fabricating photonic integrated circuits. However, achieving essential nonlinear-optic functionality in such devices will also require the ability to create high-quality single-crystal waveguides. Femtosecond laser irradiation is capable of crystallizing glass in 3D, but producing optical-quality single-crystal structures suitable for waveguiding poses unique challenges that are unprecedented in the field of crystal growth. In this work, we use a high angular-resolution electron diffraction method to obtain the first conclusive confirmation that uniform single crystals can be grown inside glass by femtosecond laser writing under optimized conditions. We confirm waveguiding capability and present the first quantitative measurement of power transmission through a laser-written crystal-in-glass waveguide, yielding loss of 2.64 dB/cm at 1530 nm. We demonstrate uniformity of the crystal cross-section down the length of the waveguide and quantify its birefringence. Finally, as a proof-of-concept for patterning more complex device geometries, we demonstrate the use of dynamic phase modulation to grow symmetric crystal junctions with single-pass writing.

Multilayer aberration correction for depth-independent three-dimensional crystal growth in glass by femtosecond laser heating, A. Stone, H. Jain, V. Dierolf, M. Sakakura, Y. Shimotsuma, K. Miura, and K. Hirao,  J. Opt. Soc. Am. B, Vol. 30, 1234 (2013).

Focused femtosecond lasers are known for their ability to modify transparent materials well below the surface with 3D selectivity, but spherical aberration causes degraded focal intensity and undesirable absorption conditions as focal depth increases. To eliminate such effects we have implemented an aberration correction procedure that accounts for multiple refracting layers in order to crystallize LaBGeO5 glass inside a temperature-controlled microscope stage via irradiation through a silica glass window. The correction, applied by a spatial light modulator, was effective at removing the focal depth-dependent degradation and achieving consistent heating conditions at different depths, an important consideration for patterning single-crystal architecture in 3D. Additional effects are noted, which produce a range of crystal cross-section shapes and varying degrees of partial crystallization of the melt.

Unexpected influence of focal depth on nucleation during femtosecond laser crystallization of glass, A. Stone, M. Sakakura, Y. Shimotsuma, K. Miura, K. Hirao, V. Dierolf, and H.Jain, Optical Materials Express, Vol. 1, Issue 5, pp. 990-995 (2011).

Three-dimensional (3D) space-selective crystallization by femtosecond laser irradiation was investigated in LaBGeO5 glass. Heat modification could be induced space-selectively, but crystal nucleation showed an unexpected sensitivity to focal depth. Laser-induced heat modification profiles were inspected with optical microscopy and Raman spectroscopy in order to explain this phenomenon. We propose a mechanism based on heterogeneous nucleation at the surface of laser-induced defects and suggest strategies for achieving space-selective crystal nucleation.

Formation of ferroelectric single-crystal architectures in LaBGeO5 glass by femtosecond vs.continuous-wave lasers,    A. Stone, M. Sakakura,Y. Shimotsuma, G. Stone, P. Gupta, K. Miura, K. Hirao, V. Dierolf, H. Jain, Journal of Non-Crystalline Solids 356 [52-54], 3059  (2010).

    We have succeeded in crystallizing LaBGeO5 glass with an 800 nm, 250 kHz repetition rate femtosecond laser and observed different crystallization behavior when varying focal depth. Despite glass and crystal having the same composition, an unidentified off-stoichiometric phase appeared before crystalline LaBGeO5 when focusing deep inside the glass. Crystal lines were written in three dimensions inside the glass, and the results were compared to 1064 nm continuous-wave laser crystallization of Sm-doped LaBGeO5 by samarium atom heat processing. Notable differences were found in the structure of the lines and crystal growth rates, with femtosecond laser crystallization requiring much higher writing speeds. Raman spectra were highly consistent, suggesting that the lines were highly oriented with a consistent alignment between the crystal lattice and the growth direction.

Directionally controlled 3D ferroelectric single crystal growth in LaBGeO5 glass by femtosecond laser irradiation , A. Stone, M. Sakakura, Y. Shimotsuma, G. Stone, P. Gupta, K. Miura, K. Hirao, V. Dierolf, H. Jain, Optics Express 17, 23284 (2009)

    Laser-fabrication of complex, highly oriented three-dimensional ferroelectric single crystal architecture with straight lines and bends is demonstrated in lanthanum borogermanate model glass using a high repetition rate femtosecond laser. Scanning micro-Raman microscopy shows that the c-axis of the ferroelectric crystal is aligned with the writing direction even after bending. A gradual rather than an abrupt transition is observed for the changing lattice orientation through bends up to ~14°. Thus the single crystal character of the line is preserved along the bend through lattice straining rather than formation of a grain boundary.