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Efficient direct writing of elliptically polarized femtosecond lasers


Light excitation is one of the most important manifestations of light-matter interaction in nature, such as photosynthesis in plants, vision in living things, photography and laser processing of materials. It is generally believed that the more light a substance absorbs, the stronger the material modification produced. However, this is not the case in the processing of glass by femtosecond lasers.

Femtosecond (1 fs=10-15 s) laser pulse has the characteristics of ultra-short pulse time and ultra-high peak power, so it can quickly and accurately deposit energy in transparent materials and obtain ultra-high three-dimensional spatial resolution, which is widely used in ophthalmic surgery, three-dimensional integrated optics, quantum optics, microfluidic devices, optical device preparation and optical storage. However, many of the observed physical phenomena are not fully explained. On the other hand, the relatively low processing speed and high processing cost limit the practical application of femtosecond laser processing technology.

Birefringence, that is, the incident of light waves into an anisotropic medium and decomposes into two polarized lights with vibration directions perpendicular to each other and different refractive indices, is a common phenomenon in optics. Commonly used optical components, such as half-wave plates and polarizers, are prepared based on the birefringence properties of crystals.

Glass itself does not have birefringence properties. Researchers have demonstrated that focused femtosecond laser pulses can induce periodic nanograting structures with birefringence properties in quartz glass.

This flexible and controllable birefringence has been applied to (1) high-density multi-dimensional light storage with unlimited life; (2) Preparation of geometric phase devices and vector light converters with high damage thresholds. The physical mechanism of nanostructure formation and how to improve its preparation efficiency has always been a hot topic in research.

Innovative research

Recently, a research group led by Professor Peter G. Kazansky of the University of Southampton in the United Kingdom demonstrated that elliptically polarized femtosecond laser pulses can produce stronger birefringence modification in quartz glass (Figure 1). Although the absorption rate of elliptically polarized light pulses (elliptic polarization of 0.6) is only about 40% of linearly polarized light, it can produce 1. 5x phase delay. In other words, with elliptically polarized femtosecond laser pulses, we can produce stronger material modifications with less energy. Inside this birefringence modification are randomly distributed anisotropic nanopores with up to 99% optical transmittance, which is much higher than traditional nanograting-based birefringence modifications.

Fig. 1 Birefringence modification of femtosecond laser pulse writing with different elliptic polarization degrees. (a) birefringence voxels written at different ellipsometries and (b) phase delay measured. (c) Absorption rates of different elliptically polarized light pulses.

Compared to linearly polarized pulses, elliptically polarized light pulses can create nanopores with larger duty cycles in quartz glass (Figure 2). The formation of anisotropic nanopores consists of two parts: the generation of nanopores and the stretching of nanopores. The authors propose that the strongest material modification produced by elliptically polarized light pulses is the result of the generation and tensile equilibrium of nanopores.

On the one hand, the stretching of nanopores is based on near-field enhancement effects, which means that linearly polarized light can maximize the stretching of nanopores. Circularly polarized light, on the other hand, can produce more nanopores. Because through tunnel ionization, circularly polarized light can more effectively excite defects with low ionization energy in quartz glass, resulting in nanopores with larger duty cycles.

It is generally believed that multiphoton ionization dominates the processing of transparent materials by femtosecond lasers. But this study reveals tunneling ionization of laser-induced defects with low excitation energy, which is key to the formation of nanostructures in quartz glass.

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Figure 2 Birefringence structure of ultrafast laser writing. (a) Birefringence images. (b) Scanning electron microscopy image of anisotropic nanopores.

Elliptically polarized femtosecond laser writing has been applied to five-dimensional permanent optical storage, and its data readout accuracy is close to 100%. Compared to traditional linear polarized light writing, elliptically polarized light can write data points of the same mass with fewer pulses and less energy, thereby improving data storage speed.

On the other hand, the preparation time of geometric phase devices with ultra-high transmittance and high damage threshold and vector light converters has also been greatly reduced, and it is expected to be used in high-power lasers and ultraviolet lasers.

The results were published in Light Science & Applications under the title “Efficient ultrafast laser writing with elliptical polarization”. The main author of this paper is Lei Yuhao, a doctoral student at the University of Southampton, UK, and the co-corresponding author is Professor Peter G. Kazansky. Key collaborators include Professor Linards Skuja of the University of Latvia and Professor Yuri Svirko of the University of Eastern Finland. Associate Professor Wang Lei and Engineer Yanhao Yu, who are currently working at Jilin University, also made important contributions to this paper. (Source: LightScience Applications WeChat public account)

Related paper information:https://www.nature.com/articles/s41377‍-023-0‍1098-2

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