Ultrafast Laser Inducing Continuous Periodic Crystallization in the Glass Activated via Laser‐Prepared Crystallite‐Seeds

Abstract

In particular, ultrafast laser direct writing based glasscrystal optics has been widely reported to realize 3D nonlinear photonic crystals, [6,7] second harmonic generation, [8,9] optical waveguides, [10,11] and perovskite quantum dots in the glass. [12,13] However, the reported ultrafast laser-patterned microstructures mainly rely on direct laser energy deposition, and the processing precision and efficiency are very limited. Recently, functional crystal arrays (CAs) with periodically varied optical properties and nanoscale feature sizes have been demonstrated to possess promising applicable prospects in frontier studies, [14,15] but remain very difficult to be obtained with conventional technologies. Therefore, novel approaches for realizing highresolution periodic nanopatterning in transparent dielectrics are highly desirable to solve these problems. Extensive studies have demonstrated that periodic field distribution (PFD) generated by ultrafast laser-matter interaction process can periodically modify transparent media at the nanometer-scale level and generate polarization-dependent grating structures with a periodic change in component, accompanied with strong polarization-dependent birefringence. [16][17][18] Such modification breaks the diffraction limit and is applicable to several transparent dielectrics like silica glass, borosilicate glass, and GeO 2 glass, [19][20][21] which provides a highly promising approach to create CAs. However, despite nearly 20 years of study and a large number of published works, only a very limited number of materials are able to support the formation of CAs and the reason remains a mystery. [22][23][24] The mechanism bridging PFD and CAs remains largely unclarified. From these studies, we note two key points. First, the formation capability of crystalline grating structures strongly depends on the thermal effect and the glass composition, which is in line with the capability of phase transition from glass to ceramic. Second, the crystal nucleus is generally a critical starting unit for crystal formation and growth.Here, we present an ultrafast laser-induced CPC process in the La 2 O 3 -Nb 2 O 5 glass (LN glass) that allows creating periodic structures made of self-assembled CAs. We reveal that this CPC process is catalyzed by the local defective crystalliteseeds that can offer a unique auxiliary effect to activate and Tailoring ultrafast light-matter interaction as an enabler to create functional periodic crystalline patterns inside transparent dielectrics is of great scientific and technological significance but remains a tremendous challenge because of plenty of unclarified process details and physical mechanisms. Here, a dynamic process of ultrafast laser-induced continuous periodic crystallization (CPC) to generate self-organized crystal arrays (CAs) inside the glass is reported. The crystallite-seeds induced by ultrafast laser are revealed to offer an auxiliary effect that can initiate and maintain the CPC process, which promises the high-efficiency writing of CAs. The...

Ultrafast Laser Direct Writing in Glass: Thermal Accumulation Engineering and Applications

“…As the thermal accumulation effect generated by fs pulses is much weaker than that by ps laser pulses in the repetition rate range of 100–200 kHz, heat accumulation is suggested to be necessary for formation of crystallite based nanogratings. [ 34 ] Similar pulse dependence with the threshold at around 800 fs is also reported in the 22Na 2 O–78GeO 2 glass. [ 69 ] The phase retardation of birefringence from the periodic nanogratings in the GeO 2 glass is also dependent on the pulse duration and the modified region induced by thermal accumulation can be observed.…”

“…Reproduced with permission. [ 34 ] Copyright 2019, Wiley‐VCH. e) Optical microscope images of modified regions irradiated by 500 kHz fs laser pulses for different time (top view).…”

“…[ 39,52 ] In addition, it is interesting to note that the pulse duration is reported to be an important parameter affecting generation of periodic nanogratings, especially, the periodic phase transition from amorphous to crystal in some glasses that are relatively easily transformed to crystal structures. [ 34,68,69 ] For example, Figure 4d reveals that pulses with ps duration are necessary for birefringence creation in the multicomponent La 2 O 3 –Ta 2 O 5 –Nb 2 O 5 glass. As the thermal accumulation effect generated by fs pulses is much weaker than that by ps laser pulses in the repetition rate range of 100–200 kHz, heat accumulation is suggested to be necessary for formation of crystallite based nanogratings.…”

“…[ 99 ] Furthermore, the induction of crystallization promoters can facilitate the crystal growth. [ 34,65 ] Jain and co‐workers demonstrated that 200 kHz fs laser can produce single crystal (e.g., LiNbO 3 , LaBGeO 5 ) with unlimited lengths using optimal parameters in the glass. [ 100,101 ] By optimally manipulating the laser power density and scanning speed, they suggest that the nucleation and growth of the crystal line happen during thermal accumulation heating at the leading end of the laser focus, which would align with the c ‐axis parallel to the temperature gradient.…”

“…The thermal accumulation effect has also been reported to be critical for the formation of periodic nanogratings in some glass systems. [ 34 ] Moreover, the thermal accumulation induced high temperature can produce thermally excited free electrons, which will seed the avalanche ionization and significantly enhance absorptivity. [ 35 ] As a result, more energy can be absorbed and this leads to increase of thermal accumulation.…”