Human vision excels at perceiving depth in a three-dimensional (3D) world, yet most modern display technologies still rely on two-dimensional (2D) screens. [1] These screens emit light pixels with specific colors and brightness but lack depth information, limiting our ability to fully utilize our stereoscopic vision. While advanced technologies like virtual reality (VR) and stereoscopic displays attempt to bridge this gap, they often fall short. These systems reconstruct 3D visuals from 2D images, which can result in unnatural experiences, limited viewing areas, and discomfort from headgear. [2] Furthermore, such systems typically require high computing power and are often accessible to a single user at a time. Holographic and volumetric displays (VDs) offer true 3D imagery by structuring light into volumetric pixels (voxels), enabling dynamic visuals with wider viewing angles and natural depth perception. Laser-based VDs stand out in this field for their ability to deliver vivid colors, high contrast, and a broad color gamut using the unique properties of lasers. [3] However, challenges such as material efficiency, color purity, and scalability remain to be addressed before laser-based VDs can see widespread adoption in fields like medicine, engineering, and education. Despite decades of research utilizing various materials such as nanomaterials and crystal ceramics, the desired success in the aforementioned areas has yet to be achieved. [4-20]

Our study introduces a novel solution: specially designed monolithic glasses doped with rare-earth ions (Ho, Tm, Nd, Yb). These glasses leverage the up-conversion process to produce tunable full-color emissions under dual laser excitation at 808 nm and 980 nm. Yb³⁺ ions act as energy donors, transferring absorbed photons to radiative energy levels in Ho³⁺ and Tm³⁺ ions, resulting in red, green, and blue emissions. Additionally, Nd³⁺ ions enhance emission versatility by populating intermediate states. By integrating these components within a single material, we achieved a broad spectrum of RGB colors, overcoming the limitations of previous methods. Using these rare-earth doped glasses, we developed prototypes capable of displaying intricate 2D and 3D static and dynamic images. The glasses’ high thermal and chemical stability, low phonon energy, and superior mechanical resistance make them ideal for large-scale production and long-term use. The simplicity of fabrication and control over emission wavelengths further highlight their potential as next-generation VD materials.

Sample Preparation

Based on previous experience [21], glasses with the composition 25Li₂O-25WO₃-47.65TeO₂+(0.05Ho₂O₃-0.25Tm₂O₃-0.05Nd₂O₃-2Yb₂O₃) mol% were selected for their low phonon energy, high lanthanide solubility, and thermal, chemical, and mechanical advantages. The samples were fabricated via melt-quenching, with continuous stirring to ensure homogeneity and transparency. After melting, the glasses were poured into molds and annealed at 290 °C for 3 hours. The resulting 5 cm x 5 cm x 2 cm glass samples exhibited optimal transparency and structural integrity for 3D VD applications.

Laser Path Design and Laser Penetration Depth Investigation

To demonstrate the feasibility of our approach, we designed a laser system using 808 nm and 980 nm lasers, directing the beams through optical components to focus on the glass sample and create images. The two laser beams pass through various optical components, with their frequencies and modulation adjusted via waveform and wavelength generators, while the galvanometer's mirrors are controlled independently along the laser path using the EasyWaveX application. Glass samples with varying Yb₂O₃ concentrations (0.5% to 2% mol) were produced to evaluate how Yb³⁺ affects laser penetration depth while maintaining constant base glass composition and RE³⁺ concentrations. Higher Yb₂O₃ concentrations enhance the 980 nm absorption band but reduce laser penetration due to energy quenching, with 2% mol Yb₂O₃ enabling 3D-like images by diffusing laser-induced surface images into the sample.

The characteristic transitions of Ho³⁺, Tm³⁺, Nd³⁺, and Yb³⁺ ions doped into the glass sample are depicted in Figure 1a. The critical aspect of this analysis is the ability to excite these lanthanide ions at specific wavelengths where they exhibit their characteristic emissions. To achieve this, two distinct lasers operating at 808 nm and 980 nm were utilized. Figure 1b illustrates all potential transitions observed. Under continuous excitation at 980 nm, the Ho³⁺ ions predominantly emit red luminescence, whereas when the same 980 nm laser is applied in pulsed mode, the dominant emission shifts to green, also originating from the Ho³⁺ ions. Additionally, under continuous 808 nm excitation, the Tm³⁺ ions exhibit dominant blue luminescence. The corresponding emission spectra are presented in Figure 1c. In addition, all possible energy transfer events that may occur within the lanthanide ions incorporated into the structure under 980 nm and 808 nm excitation, potential emission colors, and partial Jablonski diagrams are presented in Figure 1d and 1e.

In addition to the three primary colors obtained, further studies were conducted to generate intermediate colors. By exciting the 980 nm and 808 nm lasers at varying frequencies, pulse widths, and excitation powers using a wavelength generator, a range of distinct colors was achieved. These images are displayed in Figure 2a. Figures 2b and 2c present the photoluminescence (PL) spectra and the CIE chromaticity diagram corresponding to these colors. Furthermore, to analyze emission behavior at different frequencies, the lifetime decay curves are provided in Figure 2d.

Following the successful generation of the three primary colors and various intermediate colors, efforts were directed toward creating desired images within the glass sample. Initially, the intended images were designed on paper and subsequently transferred to a Cartesian coordinate system to identify the corner points of the shapes. These coordinates were then input into the wavelength generator using the EasyWaveX application. While one wavelength generator controlled the lasers, another controlled the galvanometer mirrors to project the desired shapes. The schematic representation of this process is provided in Figures 3a and 3b. Examples of 2D and 3D images created in different colors within the glass sample are shown in Figures 3c and 3d.

The final step required for utilizing the designed sample as a display screen involves enabling motion of the images created within the glass matrix. Using the galvanometer and wavelength generator with varying parameters, the images formed in different colors were moved along the x and y axes and subjected to rotational motion within the glass matrix. Additionally, the scalability of the images in two dimensions was tested, allowing for enlargement and reduction. Complex shapes were also animated, with specific parts (e.g., the flapping of a bird's wings) brought to motion. The resulting images are detailed in Figure 4, and accompanying videos demonstrating the generated images and their movements are provided below.

Figure 4 presented the images obtained under various laser excitations and excitation conditions, while the links below provide the video files of these images.

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Conclusion

In this study, we developed a proof-of-concept 3D VD using monolithic tellurite glass as the host material, incorporating RE³⁺ ions and 980 nm and 808 nm laser stimulation to create images in RGB colors. The precise control of laser pulses and galvanometer mirrors allows for the manipulation of generated images, enabling translation, rotation, expansion, and sequential movement on the glass. The unique properties of the glass material support the production of high-quality displays in various sizes and the creation of controlled 3D shapes, highlighting its potential for true 3D color imaging applications.

References

1. Geng, J. Three-dimensional display technologies. Advances in Optics and Photonics 5, 456-535 (2013).
2. Kumagai, K. & Hayasaki, Y. Volumetric graphics in liquid using holographic femtosecond laser pulse excitations. Proceedings of SPIE 10335, Digital Optical Technologies 2017. Munich, Germany: SPIE, 2017,
3. Miyazaki, D., Lasher, M. & Fainman, Y. Fluorescent volumetric display excited by a single infrared beam. Applied Optics 44, 5281-5285 (2005).
4. Zhu, B. et al. A volumetric full-color display realized by frequency upconversion of a transparent composite incorporating dispersed nonlinear optical crystals. NPG Asia Materials 9, e394 (2017).
5. Downing, E. et al. A three-color, solid-state, three-dimensional display. Science 273, 1185-1189 (1996).
6. Yang, Z., Feng, Z. M. & Jiang, Z. H. Upconversion emission in multi-doped glasses for full colour display. Journal of Physics D: Applied Physics 38, 1629-1632 (2005).
7. Chen, X. B. & Nie, Y. X. Principle demonstration of 3D volumetric display achieved by PrYb co-doped material. Proceedings of SPIE 4221, Optical Measurement and Nondestructive Testing: Techniques and Applications. Beijing, China: SPIE, 2000, 3-77.
8. Gu, Y. et al. Luminescent materials for volumetric three-dimensional displays based on photoactivated phosphorescence. Polymers 15, 2004 (2023).
9. Wan, S. et al. A prototype of a volumetric three-dimensional display based on programmable photo-activated phosphorescence. Angewandte Chemie International Edition 59, 8416-8420 (2020).
10. Deng, R. et al. Temporal full-colour tuning through non-steady-state upconversion. Nature Nanotechnology 10, 237-242 (2015).
11. Yin, X. et al. Three primary color emissions from single multilayered nanocrystals. Nanoscale 10, 9673-9678 (2018).
12. Hong, A. R. et al. Orthogonal R/G/B upconversion luminescence-based full-color tunable upconversion nanophosphors for transparent displays. Nano Letters 21, 4838-4844 (2021).
13. Huang, K. et al. Orthogonal trichromatic upconversion with high color purity in core-shell nanoparticles for a full-color display. Angewandte Chemie International Edition 62, e202218491 (2023).
14. Mun, K. R. et al. Elemental-migration-assisted full-color-tunable upconversion nanoparticles for video-rate three-dimensional volumetric displays. Nano Letters 23, 3014-3022 (2023).
15. Huang, J. et al. Engineering orthogonal upconversion through selective excitation in a single nanoparticle. Advanced Functional Materials 33, 2212037 (2023).
16. Zhang, N. et al. Upconversion nano-composites for orthogonal trichromatic luminescence aiming at high color purity volume display. Advanced Materials Technologies 9, 2301942 (2024).
17. Jia, H. et al. Full-color upconversion luminescence nanoplatform for real three-dimensional volumetric color displays. Chemical Engineering Journal 488, 150790 (2024).
18. Wang, F. et al. Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping. Nature 463, 1061-1065 (2010).
19. Jia, H. et al. Multicolor tunable upconversion luminescence via near-infrared manipulation of population pathways of Er³⁺ ions excited-state levels for volumetric color displays. Advanced Optical Materials 12, 2302583 (2024).
20. Zhao, S. et al. Tricolor upconversion phosphors of LiYO₂:RE³⁺/Yb³⁺ (RE = Tm, Ho, Eu) for metamerism anti-counterfeiting and 3D volumetric display. Advanced Optical Materials 12, 2302408 (2024).
21. Vahedigharehchopogh, N. et al. Color tunability and white light generation through up-conversion energy transfer in Yb³⁺ sensitized Ho³⁺/Tm³⁺ doped tellurite glasses. Journal of Non-Crystalline Solids 525, 119679 (2019).