The Application of Femtosecond Laser in Precision Photonics Manufacturing

The femtosecond laser emits ultra short optical pulses with a duration of less than one picosecond, reaching the femtosecond level (1fs=10-15s). The characteristics of femtosecond laser are extremely short pulse width and high peak intensity.

Ultra short pulse trains can minimize residual heat, ensure precise material processing, and minimize incidental damage. Its high peak intensity can induce nonlinear optical interactions such as multiphoton ionization and plasma formation, providing precise spatial control of laser energy for various applications.

The nonlinear confinement effect of femtosecond laser can achieve nanoscale resolution, with characteristics smaller than the diffraction limit of light. These lasers have a wide range of applications and can be used in various materials, including metals, semiconductors, ceramics, polymers, and composite materials, without the need for masks or photoresists. The focusing ability of femtosecond lasers in transparent materials also helps to create complex three-dimensional (3D) structures, which is crucial for manufacturing integrated photonic chips.

In short, femtosecond laser is an ideal choice for precision microfabrication and photon manufacturing.

The main applications of femtosecond laser in precision photon manufacturing are as follows:

01
Photolithography of photonic crystals
It is crucial to accurately control the unit structure and gaps at the nanoscale in order to effectively control the light in photonic crystals in the near-infrared and visible light ranges. Femtosecond lasers can directly manufacture three-dimensional micro/nanostructures in transparent materials, utilizing their ultra short pulse duration to achieve ultra-high precision, and perform excellently in manufacturing these structures.

A study published in "Light: Science and Applications" confirms this by introducing a method for manufacturing photonic crystal structures using nanoscale femtosecond laser multi beam lithography technology. Researchers focused a controllable multi beam light field on the interior of the crystal and combined it with chemical etching. This method can precisely control the structural units and gaps of sub wavelength sizes, overcoming the limitations of single beam processing.

The proposed method is both economical and simple, and can achieve three-dimensional photonic crystal structures within crystals, with the potential to be applied in the fields of optical communication and manipulation.

02
Simplify the manufacturing of periodic nanostructures
With the advancement of materials science and nanomanufacturing technology, people have begun to explore periodic nanostructured surfaces for advanced photonics applications, such as plasma and dielectric element surfaces. Traditionally, the processing of these periodic surface structures (PSS) using photolithography methods is both complex and time-consuming.

However, focusing on femtosecond lasers provides a one-step, mask free, and efficient alternative method suitable for various materials. In this way, laser induced PSS (LIPSS) can be used to create features smaller than the wavelength of the laser.

Recent research, particularly on broadband gap transparent crystals such as lithium niobate, has demonstrated the potential of femtosecond lasers in manufacturing large-area LIPSS with enhanced light absorption through controlled heating strategies. This provides a promising approach for the precise manufacturing of dielectric crystals other than lithium niobate.

03
Design a three-dimensional photonic integrated structure
The femtosecond laser direct writing technology provides enormous potential for manufacturing three-dimensional photonic integrated circuits (PICs) on transparent substrates. However, a key challenge faced by this technology is how to achieve smooth and significant refractive index changes within the laser irradiation area, which hinders the development of compact photonic integrated circuits.

A study published in Science China Physics, Mechanics&Astrology has solved this problem by proposing a significant method to suppress the bending loss of small curvature radius waveguides, paving the way for reducing the size of three-dimensional photonic integrated circuits.

The proposed method includes the use of femtosecond laser direct writing technology to engrave multiple modified tracks in fused silica, thereby enhancing the refractive index contrast and successfully reducing the bending loss in the bent waveguide. This breakthrough is expected to improve the integration density and flexibility of three-dimensional photonic devices.

04
Three dimensional micro nano structures in dielectric materials
Femtosecond laser induced chemical etching (FLICE) selectively etches laser modified areas by utilizing laser-induced changes in chemical properties. This allows complex three-dimensional microstructures and nanostructures to be directly written into the interior of dielectric materials. FLICE has been used to create embedded hollow microstructures for microfluidics and three-dimensional optofluids in glasses.
Recent work has achieved over 100000 ultra-high etching selectivity in crystals such as YAG and sapphire. This enables the realization of three-dimensional photonic lattices, waveguides, and nanopores at the nanoscale without the need for crystal damage.

05
Surface lithography technology
As a maskless and high-precision 3D processing technology, femtosecond laser processing can be used for surface lithography on materials such as thin films of lithium niobate. This breakthrough has successfully overcome the challenges in material integration and achieved the manufacturing of high-performance photonic components.

For example, researchers have used femtosecond laser assisted chemical mechanical polishing (CMP) lithography technology to manufacture low loss waveguides and high Q-value microresonators on lithium niobate chips. This processing strategy has strong potential to functionalize different crystal platforms for integrated photonics.

06
High speed, high-quality silicon ablation
The use of femtosecond laser for silicon ablation refers to the precise removal of materials on silicon substrates using an ultra short pulse group. This process is crucial in precision photonics as it can create complex structures with minimal thermal damage, thereby producing high-quality optical devices such as optical waveguides.

Researchers from the Advanced Photonics Center of the Institute of Physics and Chemistry have developed a new technology called BiBurst mode, which uses GHz femtosecond laser pulse quenching grouped by MHz envelope lines to achieve efficient and high-quality silicon ablation. These research findings are published in the International Journal of Extreme Manufacturing.

The research team has demonstrated that using the BiBurst mode, the rate of silicon ablation is 4.5 times faster than that of single pulse mode, and the quality is better. The mechanism involves the absorption of absorption points generated by subsequent pulses on previous pulses, thereby improving efficiency. This breakthrough will have a significant impact on the basic research and industrial applications of femtosecond laser processing, thereby improving throughput and micro machining accuracy.

07
Manufacturing quantum photon processors
Femtosecond laser writing (FLW) stands out in the field of passive and reconfigurable integrated photonic circuits due to its low cost, simplicity, and rapid prototyping capabilities. The rapid reconfigurability of this technology makes it of great value for the initial evaluation of optical laboratories.

A study published in Applied Physics Letters used FLW technology to manufacture a programmable dual qubit quantum photon processor. The FLW quantum processor manufactured has achieved high fidelity, with single qubit gates reaching 99.3% and double qubit CNOT gates reaching 94.4%.
Despite challenges such as propagation loss and low refractive index comparison, the coupling loss between FLW chips and standard single-mode fibers is naturally very low, which provides advantages for quantum photon experiments.

Conclusion
Femtosecond laser processing is rapidly becoming a key technology for advancing photon manufacturing, bringing new possibilities for design and structure. The current development indicates that the influence of femtosecond laser processing in industry and academia will continue to expand in the coming years.

Source: Guangxing Tianxia