As early as 1977, the Star Wars envisioned a vibrant future for holographic 3D display. From medical to education, from industry to entertainment, all walks of life have been looking forward to the application of holographic 3D display for so long. Holographic 3D display, as a kind of true 3D display technology, is capable of offering all the depth information of any 3D object, therefore guaranteeing a comfortable and immersive viewing experience. However, the journey towards widespread commercial use has been hindered by certain persistent challenges. Among these, chromatic aberrations and the narrow viewing angle stand out as significant yet challenging issues to overcome.

In this work, we propose a color liquid crystal grating based holographic 3D display system with large viewing angle. Figure 1 is the concept of the proposed system. Different from the traditional liquid crystal grating that has inevitable chromatic aberration, the proposed color liquid crystal grating can perform secondary diffraction modulation on red, green and blue reconstructed images with the same diffraction angle when the voltage is applied, thus avoiding chromatic aberration. In addition, a chromatic aberration-free hologram generation mechanism is proposed to cooperate with color liquid crystal grating to achieve large viewing angle color display. The proposed system shows a color viewing angle of ~50.12º, without any chromatic aberration.

The core component of the proposed system is a customized color liquid crystal grating. In order to make the three colors of light passing through the color liquid crystal grating have the same diffraction angle, we design and fabricate a color liquid crystal grating with three different-pitch regions in one liquid crystal cell for different incident wavelengths, namely region I, region II and region III, as shown in Fig. 2. The three regions of the color liquid crystal grating are controlled by the same voltage. The color liquid crystal grating consists of a top substrate, liquid crystal layer, pixel electrodes, common electrodes and a bottom substrate. The pixel and common electrodes are etched by planar indium tin oxide electrode. The widths of common electrode of the three regions are w_r, w_g and w_b, respectively. In each region, the width of the pixel electrode is the same as that of the common electrode. The electrode arrangement is optimized by using a separate periodic electrode arrangement to eliminate the fringe electric field and diffraction crosstalk among the three regions.

The pitches of the color liquid crystal grating in region I, region II and region III are d_r, d_g and d_b, respectively. The gaps between the common electrode and pixel electrode in region I, region II and region III are l_r, l_g and l_b, respectively. The same voltage is applied to the pixel electrodes in three regions, and the common electrodes in all regions are grounded. With this design, the arrangement periods of liquid crystal molecules in regions I, II and III of the color liquid crystal grating are different from each other when the voltage is applied, so the pitches of different regions are different accordingly. The pitch of region I is the largest, followed by region II and region III is the smallest. In addition, the adjacent electrodes between different regions are all set as common electrodes, thus the fringe electric field of adjacent regions can be eliminated and the angle disturbance of the liquid crystal molecules in adjacent regions is decreased by 80%. Therefore, the control accuracy of different regions of the color liquid crystal grating can be improved. The customized color liquid crystal grating distinguished by its chromatic aberration-free quality, ease of fabrication, and lightness, contributes to the remarkable large-viewing-angle holographic color 3D display.

The generation mechanism of a hologram without chromatic aberration is as follows. Firstly, the information of red, green and blue color channels of a 3D object is extracted. Then, the signal controller is used to generate the hologram of each color channel, and the corresponding blazed grating is superimposed on the hologram. The red channel hologram and blazed grating I, the green channel hologram and blazed grating II, and the blue channel hologram and blazed grating III are superimposed respectively, and then loaded on the SLM in time sequence. The red, green and blue diffraction images pass through the three regions of the color liquid crystal grating and are modulated respectively, and finally color reconstructed image without chromatic aberration can be be generated.

Overall, we have overcome the obstacles of small viewing angle and serious chromatic aberration in the traditional holographic 3D display. Additionally, our proposed system also boasts a straightforward and easily implementable structure. We believe that our proposed system will significantly propel the commercial viability of holographic color 3D display, bringing this futuristic technology closer to daily use. This breakthrough marks a significant step forward in making the once-dreamt holographic 3D display a reality for everyone.