08 Juin – Thesis defense - Chang-Hyun Park

16 h Full videoconferencing

Study on nonlinear multi-dimensional - Direct laser writing by using ultrashort high power laser.

In the past 30 years as the pulse width of lasers has been narrowed and high-power lasers have been developed, researches on the interaction between photon and materials using femtosecond lasers have been actively conducted. The high energy density of femtosecond pulsed lasers enables nonlinear photoionization processes in several ways depending. This paper reports a study of a type Argentum direct laser writing in silver containing zinc phosphate glasses by inducing a nonlinear absorption deformation of femtosecond laser pulses. When silver-containing zinc phosphate glasses are irradiated with femtosecond laser pulses, ring-shaped clusters are formed due to non-linear absorption. The fluorescence properties and the refractive index of the silver cluster induced by this deformation are different from those of the original glass.
Simultaneous comparisons of chemical micro probes, NSOM and numerical modeling were used to analyze the laser-induced silver species distribution. The results significantly strengthen the understanding of material modifications in such glasses in a non-thermal interaction regime. In particular, it has been found that the spatial distribution of species in silver-containing glasses produced by femtosecond laser irradiation has a significant effect on chemical etching selectivity.
The Y-shaped beam splitters, 2D structures made by using type A DLW, was fabricated and its performance was measured. It was confirmed that a waveguide of a general shape in which the refractive index of the core is larger than that of cladding can be produced through type A DLW. Since the type A DLW always induces positive refractive index changes from 2.7x10-3 to 5.1x10-3, it is very suitable for making waveguide. A symmetric Y-junction and various asymmetric Y-junctions were designed and fabricated using double line waveguides, and the output ratios were measured according to the transition of the inject position. It was confirmed that the output ratio could be from 96%-4% to 57%-43% due to the different irradiance in the process of writing between the upper branch and lower branch. So DLW in silver containing zinc phosphate glasses can be utilized easily and quickly to fabricate the desired type of optical device with only the writing process.
Finally, by researching 5D optical data storage (ODS) using type A DLW, the scope of application was further expanded. We have demonstrated 5D optical data storage encoded in orientated type A DLW modifications by using a relatively low laser irradiance compared to conventional DLW. Five dimensions were achieved by adding the orientation of ellipse pattern and fluorescence intensity to 3D position. The ellipse pattern was created by anamorphic focusing, and the orientation was adjusted to 16 levels by employing SLM. In addition, AOM device was used to adjust the femtosecond laser irradiance of 16 levels.
To confirm the possibility of the proposed 5D ODS, two different images were simultaneously embedded in one image by type A DLW. And the two different original images of 4-bit bitmap format were successfully restored. The corresponding reading fidelities of 60.5% and 25.1% were obtained for the orientation direction and fluorescence intensity levels, respectively. In addition, it is shown that the reading accuracy can be greatly improved to 85.0% and 47.1% when 3-bit bitmap format was applied. Using the proposed this technology, we have reached a maximum data density of 14.9 Gb/cm3, and we believe that data storage densities of up to 119.2 Gb/cm3 (using NA = 1.3 oil target) can be achieved.
In conclusion, the fluorescence characteristics of type A DLW in silver containing zinc phosphate glasses were studied, and its utility as a multi-dimension application was confirmed. We believe this technology has great potential for nano-scale patterning in semiconductor and fabrication of micro-scale optical devices.

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