We have been working on the tailored ablation processing of advanced materials using femtosecond lasers. Here we would like to focus on the femtosecond laser processing tailored for hydroxyapatite.
Hydroxyapatite is a key material of human tooth and human bone. The human bone is made of hydroxyapatite oriented along the collagen. The micromachining of human bone is highly required for medicine. The medical issue is how to preserve the chemical property of the laser-ablated surface. By use of pulsewidth tunable femtosecond laser (50fs - 2ps, 1.5mJ, 1kpps), we compared the relative content of calcium and phosphorus. The relative content of calcium and phosphorus is kept unchanged before and after laser ablation. For these medical applications, the intense femtosecond laser delivery through optical fibers is required. It is theoretically shown that it is possible to deliver the 900 fs pulses of 0.1 mJ/pulse through a 1 m-lohng graded index fiber with a 200 μm core diameter if the fiber has the optimum refractive index profile. We therefore conclude that graded index multimode fibers give better spatial distributions of the output transverse mode than hollow fibers or step index multimode fibers, and can deliver larger pulse energy than single mode fibers.
We have studied femtosecond laser ablation characteristics of LiNbO<sub>3</sub> for the first time. LiNbO<sub>3</sub> is ferroelectric material with large optical nonlinearity and Pockels effect. The femtosecond laser ablation is very useful to fabricate various optical devices including the optical modulator and the tunable optical filter for optical communication systems because the thermal damage around the irradiated area is small due to the short pulse width, and the sub-wavelength structures may be formed by the multi-photon excitation. In our experiments, the femtosecond Ti:Sapphire laser system (Energy 0.14 mJ/pulse, Wavelength 800 nm, Pulse duration 60 fs, Repetition rate 1 kHz) based on the chirped-pulse amplification (CPA) technique was used. The aperture with a diameter of 5 mm was imaged onto the LiNbO<sub>3</sub> surface by the objective lens in the air. We observed ablation holes by the scanning electron microscope and the profilometer. We have found no damage around the holes and the clear boundary between ablated area and non-ablated area was observed. Those features are very useful for precise material processing. The bottom face of the holes was relatively flat. The etching rate was 0.93 micrometer/pulse and proportional to the number of the laser pulse. The results showed that the femtosecond laser ablation is an innovative tool for manufacturing LiNbO<sub>3</sub>-based optical devices.
In recent years, femtosecond laser processing of human hard/soft tissues has been studied. Here, we have demonstrated ablation etching of hydroxyapatite. Hydroxyapatite (Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>) is a key component of human tooth and human bone. The human bone is mainly made of hydroxyapatite oriented along the collagen. The micromachining of hydroxyapatite is highly required for orthopedics and dentistry. The important issue is to preserve the chemical property of the ablated surface. If chemical properties of hydroxyapatite change once, the human bone or tooth cannot grow again after laser processing. As for nanosecond laser ablation (for example excimer laser ablation), the relative content of calcium and phosphorus in (Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>) is found to change after laser ablation. We used here pulsewidth tunable output from 50 fs through 2 ps at 820 nm and 1 kpps. We measured calcium spectrum and phosphorus spectrum of the ablated surface of hydroxyapatite by XPS. As a result, the chemical content of calcium and phosphorus is kept unchanged before and after 50-fs - 2-ps laser ablation. We also demonstrated ablation processing of human tooth with Ti:sapphire laser, and precise ablation processing and microstructure fabrication are realized.
We have developed an automatic pulsewidth tunable femtosecond Ti:sapphire laser system that can generate an output of 50fs-1ps and sub-mJ/pulse at a repetition rate of 1 kpps. The automatic pulse compressor enables on to control the pulsewidth in the range of 50fs-1ps by the use of a personal computer (PC). We describe our recent results of tailored ablation processing of advanced functional materials such as GaN, BN, and hydroxyapatite. By use of the femtosecond laser pulse tailored for a specific material, we have demonstrated precise processing without chemical composition change and heat affected zone.