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Customize chiral optical properties with femtosecond laser direct writing of silica


Recently, Matthieu Lancry’s team from the University of Paris, France, proposed a phenomenological model for inducing optical chirality in silica materials using laser direct writing technology, and realized the on-demand chirality induction of silica materials in experiments, which is of great value to the theory of micro-nano optics and opens up a new way for photonics applications.

Research background

From the fundamental physics of the interaction of light and matter to the fabrication of target optical properties in highly complex optical engineering, femtosecond lasers play a key role in laser manufacturing. Ultrashort light pulses can precisely focus light energy in a given transparent material region through controlled focusing conditions. The nonlinear absorption of high-density photon energy allows local modifications or even breakdown of the material to occur without causing surrounding damage. This nonlinear process facilitates the use of lasers to engrave a wide variety of structures, which often lead to innovative applications.

Recent studies have shown that a femtosecond laser beam can be written directly by laser light to create optical chirality inside a non-chiral material. This concept clarifies a new way to customize chiral optical properties in three-dimensional materials, offering broader prospects for laser manufacturing. A theoretical model to explain this phenomenon believes that the combined action of stress field and direct current electric field leads to the chiral atomic arrangement of matter, but this model’s understanding of the origin of optical chirality brought by laser direct writing is still vague, and it cannot explain the inherent chiral phenomenon. Therefore, if an effective model of laser direct writing induced chirality effect can be established, the on-demand laser manufacturing process can be realized, which can further promote the development of laser manufacturing technology and open up a new way for the application of ultrafast photonics.

Innovative research

In this study, the researchers propose that the circular optical properties induced by femtosecond lasers may result from the accompanying contributions of morphological birefringence and stress-induced birefringence (Figure 1). The researchers “decomposed” the dependence of morphology and stress contribution on the direction of laser polarization, such that the slow/fast axis of morphological birefringence is controlled by the direction of laser polarization, and the associated delay amplitude is controlled by the laser energy density (Figures 2 and 3). In addition, the research team also studied the polarization dependence of stress-induced birefringence through a simple method based on stress engineering waveplates, and proposed a bilayer model to quantitatively explain the generation of linear and circular anisotropic optical properties (Figure 4). Finally, the research team used the model to design chiral optical properties according to two different designs, namely multilayer “nanograting-based waveplates” and “stress-engineered waveplates”, and successfully customized chiral optical properties in glass using femtosecond laser direct writing (Figure 5).

This study establishes a theoretical basis for the application of ultrafast laser customized optical chirality, which plays an extremely important role in the theoretical development of ultrafast photonics and the improvement of technical level, and provides a new path for subsequent research.

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Figure 1: Femtosecond laser-induced circular properties and their contribution. (a) Conceptual scheme for femtosecond laser lettering with circular nanogratings with two contributions; TEM image of a nanograting perpendicular (a1) and parallel (a2) to the direction of laser propagation; (a3) Microscope image of the cross-polarizer of the stress engineering wave plate between the two irradiated regions (called the stress bar). (b) Circular birefringence (top) and circular dichroism (bottom) measured at a wavelength of light at 550 nm according to the evolution of laser polarization.

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Figure 2: Laser polarization dependence of anisotropic properties of silica samples irradiated with femtosecond lasers. (a) Linear birefringence and 45° linear birefringence according to the evolution of laser polarization. (b) Line dichroism and 45° line dichroism are based on the evolution of laser polarization. (c) Linear birefringence fast axis evolution and its deviation (difference from laser polarization α, -90°< deviation ≤90°). (d) Linear dichroic “low attenuation axis” evolution and polarizability (DoP). (e) SEM image of the area irradiated by the femtosecond laser using X+45° writing using X+45° written at a wavelength of 550 nm.

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Figure 3: Laser polarization independence for stress-induced birefringence. (a) Fast axis orientation and delay of linear birefringence stress according to the direction of laser polarization; The illustration refers to the cross-polarization microscope image of the stress bar (blue), which creates a pure stress region with a measurement area marker (0° reference is the direction of the laser straight line, i.e. along the x-axis). (b) “Ideal nanograting” (blue), measured femtosecond laser-induced evolution of the nanograting (pink) and pure stress (green) in the direction of the total linear birefringence fast axis direction of the laser polarization. (c) Evolution of the linear birefringence and stress birefringence/morphological birefringence ratios at different laser pulse energies. All measurements were made at 550 nm.

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Figure 4: Two-layer model and simulation results. (a) Schematic diagram of two contributions leading to laser-induced circular optical properties. (b) The schematic diagram of the two-layer model consists of two linear delayers. The simulation of the slow axis (e) curves of circular birefringence (c), circular dichroism (d) and bus birefringence with respect to the laser polarization angle is compared with the experimental results. Both simulation and measurement are done at 550 nm.

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Figure 5: Circular anisotropic optical property engineering using multi-layer write strategy. (a) Two-layer nanograting diagram, 45° dislocation. (b) Circular spectral characteristics of “nanograting-based waveplates” in the 450-1000 nm range. (c) Schematic diagram of the stress of two layers, 45° dislocation. (d) The spectral circular characteristics of “stress engineering waveplates” in the 450-1000 nm range. (e) Thermal stability of linear and circular characteristics. Measurements were made at 550 nm. The illustration depicts the evolution of circular birefringence spectroscopy with two layers of “nanograting-based waveplates”.

The article was published in the top international academic journal Light: Science & Applications, entitled “Tailoring chiral optical properties by femtosecond laser direct writing in silica”, with Jiafeng Lu as the first author and Matthieu Lancry as the corresponding author of the paper.

Related paper information:https://www.nature.com/articles/s41377‍-023-0‍1080-y

(Source: LightScience Applications WeChat public account)
 
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