The researchers used ultrafast lasers to create nanoscale photonic crystals

The optical properties of photonic crystals are closely related to their lattice constants, which are usually required to be in the same order of magnitude as the operating wavelength. In a crystal material, the photonic crystal structure is formed by the periodic arrangement in space of units whose dielectric constant is different from that of the crystal itself, and whose lattice constant depends on the size of the unit and the gap between adjacent units.

Therefore, to achieve light control in the near infrared and visible range, it is necessary to precisely control the photonic crystal unit structure and gap at the nanoscale.

Femtosecond laser is one of the best methods to construct photonic crystal structures in crystalline materials, which can fabricate three-dimensional micro-nano structures directly inside transparent materials. However, the existing femtosecond laser processing techniques of photonic crystals usually adopt a single-beam point-by-point scanning strategy, which is limited in the preparation of nanoscale unit structures due to the overlap of processing trajectory and motion control accuracy.

Microlens array machining technology and laser interference machining technology provide solutions to the above problems to a certain extent. However, the former is not flexible enough, and different microlens arrays need to be designed and fabricated for different target structures. Although the latter has high flexibility, it is usually only used for machining planar two-dimensional structures and lacks three-dimensional customization capabilities.

Therefore, a new femtosecond laser processing technology is urgently needed to prepare the nanometer three-dimensional space photonic crystal structure inside the crystal.

In a new paper published in the journal Light: Science and Applications, a team of scientists led by Professor LAN Jiang of the School of Mechanical Engineering at the Beijing Institute of Technology has developed a fabrication method for photonic crystal structures based on nanoscale femtosecond laser multi-beam lithography, by tightly focusing multiple light fields with a controllable three-dimensional spatial distribution inside the crystal and combining them with chemical etching.

On the one hand, by designing optical phase and tight focusing methods, it is possible to control the size and gap of the manufactured structural units at the sub-wavelength level. On the other hand, with multi-beam light field, optical control can be used instead of electrical control, effectively avoiding the problems of laser spot overlap and component motion accuracy in single-beam laser processing.

The one-to-one correspondence between spatial phase and optical field distribution provides the feasibility of the method. In this paper, the researchers found that the binary phase period and the laser flux together affect the size and gap of the processed structure, and achieved the preparation of sub-wavelength scale photonic crystal structure units.

Based on the above results, by adjusting the gray level of the binary phase and the superposition of the final phase, the multi-beam optical field with controllable laser flux distribution and three-dimensional spatial structure can be customized, and the corresponding complex structure photonic crystals can be fabricated.

Raman spectroscopy and X-ray photoelectron spectroscopy test show that the structural unit obtained by this method is the same as that obtained by single beam scanning point by point in non-overlapping state, and has high stability and reliability.

The long period and subwavelength grating structures are prepared by this method. The experimental results are in agreement with the theoretical calculation, which further verifies the machining capability of this method.

The scientists summarized the benefits and promise of their technique:

"(1) Simple operation, low cost, no need to design different optical components to process different target structures; (2) The precise control of the structure size and gap can realize the manufacture of nanoscale photonic crystal cells; (3) The ability to process three-dimensional complex spatial structures, which can prepare three-dimensional photonic crystal structures inside the crystal."

"The flexible control of nanostructures makes the reported method an alternative method for weaving complex photonic crystals with subwavelength structures." The potential of multi-beam processing methods may open up possible ways to fabricate nanostructures for optical communication and optical manipulation applications."

Source: Laser Network