Fig. 1. Optical elements. (a) Geometric element; (b) planar phase element

Fig. 2. Fusion design of geometric and phase elements. (a) Joint design of geometric and phase elements; (b) geometric element as substrate of phase element; (c) geometric element tunes distribution of phase function of phase element

Fig. 3. Recording and reconstruction process of HOE

Fig. 4. HOE based on photopolymer. (a) Schematic diagram of package structure of photopolymer; (b)schematic diagram of formation of holographic volume grating based on photopolymer

Fig. 5. Representation way of HOE

Fig. 6. Characterization methods of HOE. (a) Recording waves; (b) holographic phase function; (c) holographic grating vector

Fig. 7. Classification of HOE

Fig. 8. Holographic optical systems jointly designed by FOE and SHOE with planar substrate. (a) AR near-eye display system with combiner^[121]; (b) AR near-eye display system with waveguide type^[122]

Fig. 9. Retinal projection display system based on SHOE with cylindrical substrate^[123]

Fig. 10. HUD system based on HOE with curved substrate designed by Wayray company^[124-126]. (a) Design layout, inset image is schematic diagram of holographic HUD system; (b) comparison diagram of projection unit volume with conventional HUD system

Fig. 11. Fabricating FHOE based on freeform waves. (a) Fabricating FHOE using freeform wave generated by CGH^[127]; (b) fabricating FHOE using freeform waves generated by multi-level HOEs, with circles represent spherical wave^[128]

Fig. 12. Point-by-point design method of phase function^[129-130]. (a) Calculation of phase function value and phase function graph normal vector at intersection points of sample rays with holographic substrate surface; (b) analytical phase function after fitting

Fig. 13. Design method of imaging optical system using confocal flat phase element^[131]. (a) Basic idea of using confocal method to design off-axis three-mirror imaging system without intermediate image; (b) off-axis three-mirror system with various folding forms of optical path

Fig. 14. Design of optical system based on FHOE^[132]. (a) Schematic diagram of recording system of FHOE, using freeform lenses to modulate recording waves; (b) AR near-eye display system based on FHOE; (c) virtual image displayed by AR system

Fig. 15. Design of recording system of FHOE^[130]. (a) Schematic diagram of recording system of FHOE; (b) reverse design process of recording system of FHOE

Fig. 16. Schematic diagram of HUD system based on FHOE^[130]

Fig. 17. Design of HUD system based on FHOE^[130]. (a) HUD imaging system as well as recording systems of FHOE 1 and FHOE 2 after joint optimization design, three systems are plotted on same figure simultaneously; (b) recording system setup of FHOE 1; (c) recording system setup of FHOE 2

Fig. 18. Design of HUD system based on FHOE^[130]. (a) Prototype of HUD system; (b) virtual image and real scene captured

Fig. 19. Design of full-color AR near-eye display system based on FHOE^[133]. (a) Schematic diagram of AR system; (b) virtual image captured; (c) real scene captured

Fig. 20. Fabrication of LCPH element based on freeform exposure and inkjet printing^[134]. (a) Retinal projection display system; (b) virtual image and real scene captured

Fig. 21. Schematic diagram of holographic printing technology

Fig. 22. Coupled optical path of the phase SLM and 4f system

Fig. 23. Holographic printing systems. (a)(b) One-arm holographic printing systems^[136-137]; (c) two-arm holographic printing system^[132]

Fig. 24. Holographic printing technology based on amplitude SLM

Fig. 25. Holographic printing system based on amplitude SLM^[139-140]. (a) Two-arm holographic printing system; (b) holographic imaging combiner for extending eyebox in two-dimensional directions; (c) FHOE array

Fig. 26. Design of optical system based on FHOE with curved substrate^[141]. (a) HUD system based on FHOE with spherical substrate; (b) recording system of FHOE of HUD system

Fig. 27. Holographic waveguide with cylindrical substrate and one-dimensional-pupil expansion^[142]. (a) Footprint of exit pupil of imaging beam of single filed point without aberration correction; (b) recording system of injection HOE; (c) recording system of extraction HOE

Fig. 28. Research on phase element with curved substrate. (a) Co-axial imaging system based on phase element with freeform substrate^[147]; (b) off-axis three-mirror imaging system based on phase element with freeform substrate^[148]

Fig. 29. Imaging system based on metasurface with cylindrical substrate^[149]. (a) Cylindrical lens; (b) cylindrical lens attached metasurface; (c) focal spot size located at image plane

Fig. 30. Imaging system based on metasurface with freeform substrate^[150]. (a) Layout of imaging system, illustrated by resolution target imaged via metasurface; (b) metasurface of freeform substrate base; (c) scanning electron microscope image of nano-structures of metasurface