Transient optical properties in fused silica measured by time-resolved high-power laser photometer Download: 1106次
The power of a laser system was limited by the laser damage resistance of optical components, especially the fused silica irradiated by the ultraviolet laser[1,2]. During the high-power nanosecond laser irradiation in thick fused silica, various physics processes, such as multi-photon ionization, self-focusing, and stimulated Brillouin scattering (SBS), occurred and coupled with others[3]. Multi-photon ionization helped electron excite from the valence band to the conduction band. Accompanied with impact ionization, the density of free electrons increased sharply[4]. Self-focusing occurred while the natural divergence due to diffraction was compensated by the focusing due to a higher refractive index in the middle of the beam. The Gaussian profile beam will self-focus to a tiny diameter, usually making a filamentary damage in fused silica[5,6]. SBS was driven by the electrostriction of the bulk medium, which tends to become denser in the regions of high optical density. The transmitted light could be converted to Stokes wave scattering mainly in the backward direction, which often initiates front surface damage in silica glasses due to SBS[7,8].
As the above physical processes were generated in a nanosecond or even shorter timescale[9], the transient material responses were difficult to obtain, and the detailed response sequence during high-power nanosecond laser irradiation in the bulk of fused silica is still unclear. However, understanding these transient modifications is of importance for attractive applications such as inertial confinement fusion[1] and laser micromachining[10].
In order to understand the transient material responses, some dynamic detection techniques were developed, which can be roughly divided into two categories: high-speed sampling technique and pump–probe technique. The high-speed sampling technique directly detects changes in the optical properties of the material with high temporal resolutions. Carr
In this study, based on the high-speed sampling technique, a time-resolved photometer, which enables a simultaneous measurement of the transient transmission, reflection, and scattering with picosecond time resolution, was developed. Using this time-resolved photometer, the material response sequence during the entire nanosecond laser irradiation in fused silica was investigated. The results revealed that the transmission decreased first at the rising edge of the pulse, accompanied by an increase of the internal reflection, owing to the SBS. While there was no macroscopic damage, the transmission would recover after the peak of the pulse. While the damage occurred, following the occurrence of the reflection, the scattering increased immediately, and the transmission did not recover. The intense SBS process, leading to high localized pressure, was believed to assist the plasma formation and nonlinear self-focusing during the damage event.
The schematic of the time-resolved photometer is shown in Fig.
Fig. 1. Schematic of the time-resolved photometer: , half-wave plate; , quarter-wave plate; D1/D2/D3/D4, photodiode detectors.
Before entering the sample, the laser beam was split by a wedge plate; the reflected beam was scattered and detected using a fast photodiode detector D1. The detected signal was defined as the reference signal
An optical isolator, consisting of a polarizer and quarter-wave plate in front of the sample, was used to acquire the internal reflection or back-scattering. The incident angle of the laser beam on the sample was
In addition, the scattered signal
It is worth noting that all of the signals were scattered prior to their detection. This reflected the variation of the entire laser beam and avoided beam focusing into the detector, which could easily lead to detector damage. The diffusing screen was made of a polytetrafluoroethylene (PTFE) plate, which would not emit luminescence pumped by a 355 nm laser. The signals detected with D1, D2, D3, and D4 were captured with an oscilloscope (Tektronix DPO7040 C) operating at a bandwidth of 4 GHz and a sampling rate of 25 GS/s, i.e., sample signals were recorded every 40 ps. It is noted that in order to obtain the variation sequence of these signals, the influence of optical path difference and electronic timing difference among these signals was calibrated using the sub-threshold pulses.
Using the above setup, the time-resolved reference signal
Figure
Fig. 2. While the laser intensity was , (a) the time-resolved signal variations and (b) the variations of the optical properties during the irradiation.
While the laser intensity was up to
Fig. 3. While the laser intensity was , (a) the time-resolved signal variations and (b) the variations of the optical properties during the irradiation.
As the laser intensity increased to
Fig. 4. While the laser intensity was , (a) the time-resolved signal variations and (b) the variations of the optical properties during the irradiation.
During the damage event in Fig.
Fig. 5. (a) Broadband emission spectrum associated with the damaged fused silica. (b) Plasma emission image integrated for 500 μs. (c) The optical microscopy image of the damage site.
As described above, the material response sequence during the irradiation of a high-power nanosecond laser in fused silica can be summarized as follows: (1) a significant decrease in the transmission emerged first at the rising edge of the pulse, accompanied with the increase of the internal reflection; (2) while there was no damage, the transmission would recover with the transient power decreasing; (3) while the damage occurred, following the increase of the reflection, the transmission did not recover with an increase of the scattering, and the scattering ratio kept on increasing during the irradiation. The underlying physics processes were discussed to interpret the material response sequence.
In order to ascertain the cause of decline in transmittance, the reflected and transmitted energies at different laser intensities were measured. As shown in Fig.
Fig. 6. (a) Reflected energy and energy loss [with subtracted surface loss ( ) and transmitted energy from the incident energy] as a function of the laser intensity. (b) Reflectivity as a function of the laser intensity.
There are two potential physical explanations for the sharp decrease of transmission, accompanied by the internal reflection. One is the formation of plasma. When the electronic density is over-critical, the plasma formed, and it would reflect the beam[17]. However, no plasma emission was captured while laser damage did not occur. Therefore, the internal reflection did not result from the plasma. The other one is due to the SBS in the fused silica. As we know, the nanosecond laser pulses can excite an acoustic wave, and a Stokes wave scatters in the backward direction, mainly in silica glasses[7]. The SBS reflectivity increased nonlinearly with the increase of the incident energy, and it could exceed 90%[18], which was consistent with the reflection characteristics we measured. In addition, our statistical time-resolved measurements revealed that the critical power for the increase of reflection was
During high-power laser irradiation in thick fused silica, while the transient laser power exceeded the threshold, the transmission significantly decreased first, owing to the increase of SBS. The SBS resulted from the electrostriction of the fused silica, and the electrostrictive pressure is proportional to the electric field[19]. Thus, after the peak of the pulse, with the decrease of the incident electric field, the SBS would decrease, leading to the recovery of the transmission, as shown in Figs.
As shown in Fig.
After the generation of the damage, as shown in Fig.
In summary, a time-resolved photometer, which can simultaneously measure transient transmission, reflection, and scattering, was developed to investigate the material response sequence during a high-power nanosecond irradiation into thick fused silica. Our results revealed that the transmission significantly decreased first at the rising edge of the pulse accompanied by the increase of the reflection, mainly owing to the SBS. While the damage did not occur, the transmission would recover after the peak of the pulse. However, with higher intensity laser irradiation, after the occurrence of a more intense SBS, a spike in the scattering would appear immediately, revealing the generation of damage, and the transmission would not recover. The damage continued to grow during the irradiation, which is confirmed by the increase in the scattering ratio. The intense SBS could assist the plasma formation and self-focusing, contributing to the damage formation.
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Zhen Cao, Hongbo He, Guohang Hu, Yuanan Zhao, Liujiang Yang, Jianda Shao. Transient optical properties in fused silica measured by time-resolved high-power laser photometer[J]. Chinese Optics Letters, 2019, 17(5): 051601.