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제안 방법
- We fabricated ablated craters smaller than the diffraction limit and investigated the influence of film thickness and irradiation beam spot size. Topographies of the ablated features were obtained by atomic force microscopy (AFM) and crater diameters were measured and utilized to calculate ablation thresholds.
- These attenuation ratios were utilized to obtain the pulse energies in the course of the nanoablation experiment that were too low to measure via the energy meter. Using the computer-controlled system, single pulse ablation with quartz only and PEG films of various thicknesses on quartz was performed. The ablation craters were examined by AFM (Nanoscope IIIa, Veeco) with high aspect ratio tips (Nanosensors, AR5-NCHR, Aspect ratio ≥ 5:1).
- It is noted that the transient pressure generated by vigorous intramolecular interactions and vibration excited by the laser irradiation is an important mechanism of polymer thin film ablation [27,28]. In this study, the film thickness (10 ~ 70 nm) is thinner than the optical penetration depth of the nonlinear absorption of 100 fs laser pulses at 400 nm wavelength, which means electron-photon energy transition takes place through the whole thickness of the films. While the electron excitation energy relaxes, mechanical stress is accumulated in the film due to transient pressure generated by the molecular motion.
- Femtosecond laser ablation of PEG brush layers with various thicknesses was investigated by examining the profiles of single pulse craters with AFM. The ablation threshold of the thin film was not influenced by the film thickness when the ablation occurs mainly by the nonthermal process.
대상 데이터
- The sample was precisely aligned perpendicular to the incident laser beam by adjusting the tilting angle of the sample. To investigate the influence of beam spot size and film thickness, long working distance objective lenses with three different numerical aperture sizes were used: 10x Mitutoyo M Plan Apo (N.A.= 0.28), 50x Mitutoyo M Plan Apo (N.A.= 0.55) and 100x Nikon CFI 60 LU Plan Epi ELWD Infinity-Corrected (N.A.= 0.8). The pulse energy was measured by a pyroelectric energy meter (Molectron J5-09) placed between the attenuator and the ND filter.
이론/모형
- 3 shows the linear relation between the squared ablated spot diameter and the logarithm of the average laser fluence for quartz and films of different thickness with 10x and 100x objective lenses. A square diameter-regression technique was used to calculate the ablation threshold and the Gaussian laser beam spot diameter[23, 24]. The method was necessary for thin film ablation studies, since the direct ablation depth measurement to determine ablation threshold [7] cannot be implemented when the ablation depth exceeds the film thickness.
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