Holography, which can record and reconstruct the amplitude and phase information of light waves, is one of the most promising technologies for achieving three-dimensional scene reconstruction. This technology is widely used in three-dimensional displays, beam shaping, microscopy, optical tweezers, and other directions in the optics field. Computer-generated Holography (CGH) refers to the replacement of the holographic recording process by computers, avoiding complex optical systems, and also calculating holograms of virtual objects, which greatly expands its application range. At present, the fabricating process of CGH mainly relies on standard nanofabrication processes, including electron beam lithography, ultraviolet lithography, etc. However, these methods are usually complex, time-consuming, and expensive, not suitable for large-area processing, and the experimental conditions need to be strictly controllable. Therefore, researchers have explored the use of a femtosecond laser direct writing system to fabricate CGH, but previous experimental studies have mostly used this method to achieve two-dimensional CGH imaging, but there has been little exploration in three-dimensional imaging, which limits its application.

In this paper, we propose that the three-dimensional holographic imaging is realized by fabricating the CGH inside the glass through the femtosecond laser direct writing system, and this method makes full use of the ability of femtosecond laser three-dimensional processing to fabricate the hologram inside the glass, effectively avoiding external mechanical damage. First, we build a multi-plane image with the letters "SJTU" of the school name, and as our three-dimensional model, then calculate the complex amplitude distribution of each layer of the image reaching the holographic surface according to Fresnel diffraction and superimpose, for the total complex amplitude distribution is encoded by the calculation method of interference CGH to obtain a binary hologram suitable for femtosecond laser processing, the calculation process is shown in Figure 1, and the optical microscopic image of the hologram written directly by the femtosecond laser is shown in Figure 2. After that, we built an optical path to reconstruct the CGH image, as shown in Figure 3: different letters achieved clear reconstruction in their corresponding reconstruction planes, and the quality and numerical simulation results of different letter reconstruction images were compared and analyzed, which verified the three-dimensional reconstruction ability of the holographic imaging.

Figure 1 A three-dimensional image composed of the letters "SJTU" generates CGH.

Figure 2 Optical microscopic image of a hologram fabricated by a femtosecond laser.

Figure 3 The optical reconstruction of CGH.

The research was published in “Honghuan Tu, Tingge Yuan, Zhiwei Wei, Yuping Chen and Xianfeng Chen. Fabrication of 3D computer-generated hologram inside glass by femtosecond laser direct writing, Optical Materials, 135, 113228 (2023)”.

Link: https://doi.org/10.1016/j.optmat.2022.113228