Direct laser writing of optical waveguides in transparent materials using a high repetition rate femtosecond laser / Ng Kok Bin

Ng, Kok Bin (2024) Direct laser writing of optical waveguides in transparent materials using a high repetition rate femtosecond laser / Ng Kok Bin. PhD thesis, Universiti Malaya.

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Abstract

Direct laser writing (DLW) is an advanced nanofabrication technology where high-power laser is tightly focused into a transparent material to induce modifications. Arbitrary structure can be produced by moving the sample across the laser focal spot in the 3-D space. During the fabrication process, several laser writing parameters are manipulated depending on the material properties, desired structure, dimensions, and applications. Among the many types of lasers, high repetition rate femtosecond laser (>100 kHz) has attracted much attention due to its unique interactions with the transparent material. DLW with high repetition rate laser typically gives rise to the heat accumulation effect. The pulse energy is deposited to the material faster than the thermal energy can dissipate between the arrival of successive pulses. This causes thermal energy to build up at the laser focal spot, leading to intense heat accumulation effect. In this work, the interaction between the heat accumulation effect with different transparent materials is investigated. The first study involves the waveguide fabrication in soda lime glass with a 100 kHz femtosecond laser at 515 nm. The waveguide produced has a core index of 4.7 x 10-3, among the highest recorded value compared with the waveguide written in borosilicate glass and fused silica glass. However, the fabricated waveguide exhibits high propagation loss of 2.5 dB/cm and coupling loss of 2.8 dB due to the non-uniformity of the core refractive index and the ellipticity mismatch, respectively. Further work is necessary to improve the performance of the waveguide. The second study involves laser ablation of CR-39 using 80 MHz femtosecond laser at 800 nm. The result shows that continuous modification is impossible when the laser is focused inside the material due to the inherent characteristics of thermoset plastics. However, straight trenches were successfully ablated when the laser is focused on the surface of the CR-39. The study highlights that the surface ablation efficiency is influenced by the dynamic interplay between the plasma iv shielding effects and beam profile distortion at the laser focal spot. The third study involves the laser-induced polymerization in SU-8 for optical waveguide fabrication. Straight channel waveguide was first fabricated in the SU-8 film using an 80 MHz femtosecond laser operating at 780 nm. The processing window for complete polymerization is obtained in terms of the laser fluence, which is in the range of 1.8 – 10 kJ/cm2. Thermal damage observed during the suspended waveguide writing suggesting that beam truncation effect is involves during the fabrication of 3D waveguide, where the variation of the voxel dimension at different focal position changes the local intensity of the laser. This study demonstrates the potential of SU-8 in the creation of 3D optical waveguide for future photonics integrated circuit applications. As a conclusion, the experimental works included in this dissertation provide valuable insights of the interaction between high repetition rate femtosecond laser with various transparent materials, which in turns benefit future research not only in the field of direct laser writing, but also applications in the diverse field of femtosecond laser interactions with materials.

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