Femtosecond laser direct writing technology is a kind of micro-nano processing technology that can focus a pulsed laser beam on the surface or inside of a material, and cause a change in the local properties of the material through the nonlinear interaction of the laser with the material in the focal region, which has been widely used in many fields such as microfluidics, micro-nano photonics, integrated optics, and so on. The traditional femtosecond laser direct writing technology has the problem of asymmetry between transverse processing resolution and axial resolution, and the axial resolution is obviously elongated, which limits the application of femtosecond laser in three-dimensional processing to some extent. In recent years, in order to balance the difference between the lateral and axial resolutions of femtosecond laser direct writing, several beam shaping techniques have been proposed, such as slit shaping technique, astigmatism shaping technique, and cross-beam irradiation technique. However, none of these techniques can achieve three-dimensional isotropic processing based on a single objective lens.
Spatiotemporal focusing techniques were originally developed for bio-imaging applications and have been used in the field of femtosecond laser micromachining. Femtosecond laser spatiotemporal focusing technology provides a new dimension of temporal focusing, allowing it to excel in improving axial fabrication resolution and eliminating nonlinear self-focusing effects. The mechanism of spatiotemporal focusing technology is that: the different spectral components of femtosecond laser are dispersed spatially through grating pairs, the spatially dispersed light is then focused through the objective lens, the different spectral components are recombined at the focal point, and the pulse width is restored to the femtosecond order of magnitude.
At present, most of the existing studies on three-dimensional micromachining with femtosecond laser spatiotemporal focusing are based on wide bandwidth, low repetition frequency titanium gemstone lasers, and the low repetition frequency restricts the speed of laser processing, so the application of spatiotemporal focusing technology to a high-repeating-frequency femtosecond laser light source is an inevitable requirement to meet the requirements of high-efficiency, three-dimensional anisotropic processing at the same time. However, the bandwidth of high-repeat-frequency femtosecond laser sources is usually narrow, the spatial dispersion volume introduces a large number of negative time chirps, and the laser itself cannot provide sufficient time compensation, resulting in the pulse width at the focal point not being able to be restored to the femtosecond order of magnitude, which restricts the application of spatio-temporal focusing technology to high-repeat-frequency laser processing. Therefore, three-dimensional isotropic processing based on high heavy-frequency femtosecond laser spatiotemporal focusing technology needs to provide additional time compensation.
Research Highlights
The team of Prof. Yangjian Cai from Shandong Normal University and Prof. Ya Cheng from East China Normal University have collaborated to propose a scheme of extra-cavity time compensation for high-frequency lasers, which realizes high-efficiency, three-dimensional isotropic machining based on the spatiotemporal focusing technique of high-frequency femtosecond laser light sources. In this work, the Martinez pulse broadener built outside the laser is used to introduce a large number of time-positive chirps to broaden the pulse width to the picosecond order of magnitude, and then the spatial dispersion of the single-pass grating compressor (grating pair) and the focusing of the objective lens ensures the recombination of the different spectral components at the focal point with a pulse width in the femtosecond order of magnitude. The experimental system is shown in Fig. 1.
Fig. 1 Schematic diagram of three-dimensional isotropic processing device based on high-frequency femtosecond laser spatio-temporal focusing technology
It is well known that the effect of femtosecond laser processing is affected by the processing direction, pulse energy and processing depth, etc. In order to verify whether the spatiotemporal focusing device has the ability of three-dimensional isotropic processing, Prof. Yangjian Cai's team and Prof. Cheng Ya's team demonstrated the optical cross-section of the device in different directions, at different depths, and processed by different pulse energies inside the photosensitive glass (as shown in Fig. 2). The experimental results show that the resolution along different directions is the same and circular, and the 3D isotropic processing resolution (8-22 μm) is proportional to the pulse energy and insensitive to the processing depth. The significance of this work mainly lies in the combination of high processing efficiency and continuously adjustable 3D isotropic processing resolution, which provides a new technical means for laser processing.
Fig. 2 Influence of different directions, pulse energies, and processing depths on the processing resolution of the temporal focusing system.
In order to more intuitively demonstrate the three-dimensional fabrication capability of the space-time focusing device, the research team combined the space-time focusing technology with the post-chemical corrosion method to fabricate a variety of three-dimensional isotropic microfluidic structures inside the photosensitive glass. Compared with traditional laser processing, the device has the advantages of high efficiency, continuously adjustable 3D isotropic processing resolution, insensitivity to processing depth, etc. The results of this research are expected to be applied to 3D microfluidic chip, photonic chip fabrication as well as laser 3D printing and other fields.
