CC BY
22LM-I-10
Improvement of fabrication resolution in two-photon polymerization by using GHz burst mode
Koji Sugioka, Kotaro Obata, and Francesc Caballero-Lucas
RIKEN Center for Advanced Photonics, RIKEN, Wako, Saitama 351-0198, Japan Main author email address: ksugioka@riken.jp
Two-photon polymerization ( 2PP) of photo -sensitive polymer materials using femtosecond (fs) lasers can create true three-dimensional micro- and nano structures with almost arbitrary shapes in an additive manner. One of the important feat ures of 2PP is to achieve a high fabrication resolution fai beyond the diffraction limit of the incident laser beam due to nonlinear absorption at the focal volume. Meanwhile, in order to explore more efficient process with higher quality, the ablation usi ng a burst mode that oscillates groups of high -repetition rate pulse train with an extremely short time interval is currently widely investigated. The GHz bursts of fs laser enables to induce ablation before the residual thermal energy deposited by previous pulses diffuses away from the processed area, resulting in the reduction of the ablation threshold and enhancement of ablation efficiency [1]. In this study, we apply the fs laser beam operated with the GHz burst mode to 2PP of photo-sensitive polymers to investigate the effect of the burst on photochemical reactions.
2PP of a Zr-based organic-inorganic hybrid material mixed with photo-initiator has been performed using an fs laser system with a central wavelength of 1030 nm and a pulse duration of 220 fs. The number of pulses in the burst can be adjusted up to 25. Figure 1 shows dependence of polymerization threshold of the intra-pulse energy on the number of intra -pulses for the GHz burst mode. For comparison, the threshold for single pulse irradiation (single mode) is also shown. The polymerization threshold decreases as the number of intra -pulses increases, and 16 nJ of the threshold for 10 intrapulses reduces by 55% compared to that for the single mode. However, it should be noted that the bursi mode requires higher total power (dose) for the polymerization, indicating decrease of polymerization efficiency. More importantly, w e have found that the GHz burst mode significantly improve s the fabrication resolution, in particular the resolution along the beam axis , probably due to the reduced threshold associated with efficient heat accumulation by the burst mode.
[1] C. Kerse et al., "Ablation-cooled material removal with ultrafast bursts of pulses", Nature 537, 84-88, (2016).
50
20
+ Single mode + GHz burst (4 .8 GHz)
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53° Ko 55%
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fir.......II
Single mode
2 pulses burst
5 pulses burst
Number of intra pulse
10 pulses burst
Fig. 1
Number of Intra Pulses
Dependence of polymerization threshold of the intra-pulse energy on the number of intra-pulses at GHz burst mode. The threshold
for the single mode is also shown for comparison.
