抑制材料应力分布技术为皮秒激光脉冲精微加工打开新的大门

高重复率超短脉冲激光器,由于可以实现局部熔化,从而获得了广泛关注。与此同时,它也促进了用于透明材料(如玻璃基板)微焊接的核心技术的发展。

局部熔化是由非线性光激发和局部热量持续累积造成的,其与激光脉冲的持续时间密切相关。其中,光激发电子后产生的等离子体的运动,更是直接影响着皮秒激光脉冲的加工精度(因为通常运动和热化的时间尺度范围恰巧在10 ps左右)。

与飞秒激光器相比,皮秒激光器凭借体积小、成本低的优点,受到了工业应用的青睐。但传统皮秒激光加工中有一个棘手的问题,即等离子体的不连续运动会造成应力分布不规则。因此要想使用皮秒激光器完成均匀的加工过程,必须要解决应力分布不均匀的问题。

针对该问题,京都大学Yasuhiko Shimotsuma博士所在的课题组提出了一种抑制皮秒激光直写玻璃基板熔化区域时应力产生的方法,即实时调制皮秒激光脉冲的能量。此项研究成果发表在 Chinese Optics Letters 2020年第12期上(Akinao Nakamura, Tomoki Mizuta, Yasuhiko Shimotsuma, Masaaki Sakakura, Tomohito Otobe, Masahiro Shimizu, Kiyotaka Miura. Picosecond burst pulse machining with temporal energy modulation [Invited][J]. Chinese Optics Letters, 2020, 18(12): 123801.

氮化铝晶体经施加和不施加实时能量调制的皮秒激光加工后,加工轨迹的光学显微图

通过使用实时能量调制的皮秒脉冲进行加工,可以抑制不规则应力的集中,从而提高出现玻璃裂纹的能量阈值。接着,通过配备了由光子晶体制成的像素偏振片阵列的高速相机直接与CMOS传感器相连的方式,观察了施加和不施加实时能量调制下,皮秒激光加工过程中等离子体运动引起的应力分布情况。

此外,为了探讨皮秒激光脉冲可以利用实时调制脉冲能量的方法抑制应力分布背后的物理机制,该研究基于产生的等离子体是完全导体的假设,对等离子体的运动进行了仿真模拟。

根据材料的热扩散系数,对脉冲能量进行适当的频率调制,可以将皮秒激光脉冲产生的应力分布降低到飞秒激光脉冲的程度。调制后的皮秒脉冲可以改善应力分布的关键原因是等离子体产生及其沿激光传播方向振荡引起的应力弛豫。这种抑制应力分布的技术为使用皮秒激光脉冲的激光焊接和激光切片等精微加工技术打开了新的大门。

Picosecond burst pulse machining with temporal energy modulation

High repetition rate ultrashort pulse lasers have been attracting much interest as a tool for local melting, leading to elemental technologies for micro-welding of transparent materials such as a glass substrate.

Local melting is triggered by the non-linear photoexcitation and the successive local heat accumulation. It is well known that such phenomena depend to a larger extent on the laser pulse duration. Particularly, the behavior of photoexcited electron plasma directly effects on a machining accuracy using picosecond laser pulses, because the typical boundary for time scale of motion and thermalization is estimated to be approximately 10 ps.

A crucial issue to be solved for the conventional picosecond laser processing is discontinuous movement of plasma resulting in an irregular stress distribution. Although a picosecond laser pulse system is more attractive for industrial applications due to its compact and low-cost compared to a femtosecond laser system, it should overcome the disadvantage to form homogeneous modification.

The research group of Dr. Shimotsuma from Kyoto University proposed a method to suppress the stress generation during picosecond laser direct writing of molten region in a glass substrate by temporally modulating pulse energy of picosecond laser pulses.

It was published in Chinese Optics Letters, Vol. 18, Issue 12, 2020 (Akinao Nakamura, Tomoki Mizuta, Yasuhiko Shimotsuma, Masaaki Sakakura, Tomohito Otobe, Masahiro Shimizu, Kiyotaka Miura. Picosecond burst pulse machining with temporal energy modulation [Invited][J]. Chinese Optics Letters, 2020, 18(12): 123801.)

Optical micrographs of modified tracks in AlN crystal by picosecond laser pulses with / without temporal energy modulation.

By using the picosecond burst pulse machining with temporal energy modulation, the irregular stress concentration can be suppressed, leading to enhance the energy threshold for crack generation in glass. The stress distribution caused by the plasma movement during picosecond laser machining with and without the temporal modulation was directly observed by using high-speed camera equipped with a pixelated polarizer array which is made from photonic crystal bonded directly to the CMOS sensor.

Furthermore, to discuss the mechanism of the suppression of stress distribution by the temporal energy modulation of picosecond laser pulses, the plasma motion was also simulated, based on the assumption of the generated plasma acting as a complete conductor.

By modulating the pulse energy with adequate frequency according to the thermal diffusion coefficient of material, the induced stress distribution of picosecond laser pulses can be intentionally relaxed to the degree of that for femtosecond laser pulses. The key point of the modification induced by picosecond laser pulses with temporal energy modulation is the stress relaxation caused by the plasma generation and its oscillation along with laser propagation direction. This technique for preventing the stress distribution opens a new possibility for precise micromachining such as laser-welding and laser-slicing using picosecond laser pulses.