The material processing with a femtosecond double-pulse laser beam has been demonstrated since 1990s. Only a few papers have discussed the possibility of ablation suppression mechanism. The mechanism of laser ablation with the double pulse beam is still open question due to two pulses consist of same laser wavelength because it is difficult to distinguish between the effects of the first and second pulses. In this study, the ablation rate has been investigated for titanium with a two-color double-pulse laser beam in the delay time (Δt) from 0 to 600 ps. The double pulse laser beam consisted of 800 nm with 150 fs pulse and 400 nm with < 150 fs pulse in cross polarization. The fluence of the first pulse was kept above the ablation threshold, while the fluence of the second pulse was kept below the ablation threshold. The ablation rate was clearly suppressed at the delay time of Δt ~ 60 ps in case of second pulse of 400 nm. On the other hand, in case of second pulse of 800 nm, the ablation rate was suppressed at the delay time of Δt ~ 200 ps. The delay time was approximately three times difference for both irradiation case. The difference of the delay time might be suggested that the ablation rate was effectively suppressed when the expanding surface plasma produced by first laser pulse should be close to the critical density for the second laser pulse.
We have discovered that the pre-irradiation of a below-ablation-threshold femtosecond laser pulse (first pulse) suppressed titanium ablation rate by subsequent irradiation of another above-ablation-threshold femtosecond laser pulse (second pulse). To ablate titanium targets, we used linearly polarized femtosecond laser pulses (810nm wavelength, 45fs pulse duration, 10Hz repetition rate). Ablation suppression was transiently observed with the delay of several hundreds of picosecond. With these delays, the ablation rate with 10% below-ablation-threshold first-pulse pre-irradiation was suppressed to half of that without pre-irradiation. This result indicated that the below-ablation-threshold first-pulse preirradiation does not ablate the titanium target but changes the optical property of the surface. To understand the physics of the ablation suppression, we have developed and performed a new measurement to estimate the temporal changes of the effective laser penetration length (LPL) induced by the below-ablation-threshold first-pulse pre-irradiation. We developed a new description of the dependence of the ablation rate on the laser fluences and delay by considering the absorption and decay of the laser pulse energy in the target. We have demonstrated that below-ablation-threshold firstpulse pre-irradiation reduced LPL with the delay of several hundreds of picosecond. The result indicated that the preirradiation of a below-ablation-threshold first-pulse changed the titanium surface into a novel state with reduced LPL, which is not explained by simple heating of surface.
We have demonstrated the suppression of ablation rate on a silicon surface irradiated by a double-pulse beam with two color laser in time delays of Δt = -900 - 900 ps. The double pulse beam consists of 810nm with 40fs pulse and 405nm with > 40fs pulse. The fundamental-pulse fluence F810 is kept below ablation threshold (Fth, 810nm = 0.190 J/cm2 ) while the second harmonic pulse fluence F405 are kept above the ablation threshold (Fth, 405nm = 0.050 J/cm2 ). We find that ablation rate of silicon is drastically decreased at delay times of 600ps.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.