The static volumetric 3D display technology displays 3D objects by volume pixels in 3D space, presenting real stereoscopic images. It can provide physiological and psychological depth clues for human visual systems to perceive 3D objects and can meet all-around observation needs. Additionally, it is the most likely 3D display technology to achieve high spatial resolution, multi-angle, and simultaneous observation of multiple people, real-time interaction, and large size. Among them, the static volume 3D display technology based on dual beam addressing has attracted much attention due to its unique advantages such as fine voxel, high spatial resolution, easy realization of full-color display, and meanwhile the image is no shaking and does not require auxiliary equipment (such as glasses) to view. By employing the energy of two infrared photons to pump a material into an excited energy level, the electrons in the excited energy level will transition to a lower energy level and produce visible light, which is an effective way to achieve dual-beam addressing. The material that can implement this luminescence process is also known as the two-step two-frequency (TSTF) up-conversion luminescence (UCL) material, and it can have great potential applications in static volumetric 3D display technology due to its rapid response, high contrast, and high color purity. Despite this, the material has received few reports in volumetric 3D display applications because of its low UCL efficiency and small display volume. Additionally, some literature focuses on the properties of materials, with less introduction of 3D display systems. The above two points greatly limit the application and research interest of the 3D volume display of TSTF UCL materials. Thus, we develop a 3D imaging system based on the TSTF UCL mechanism of rare earth ions, and meanwhile build a projection imaging optical path based on digital optical processing (DLP) and a line laser shaping optical path based on scanning galvanometer and cylindrical mirror. The display system is based on the TSTF UCL technology, which employs a dual infrared laser excitation, and adopts the digital micromirror display (DMD) and scanning galvanometer to achieve rapid scanning of image volume at high resolution. It has lower material performance requirements and cost, and more simple method than dual DLP imaging mode. This system is very suitable for the preliminary study of the stereoscopic display effect of TSTF UCL materials and also provides an effective idea for the imaging schemes of other addressing media materials. Additionally, the TSTF UCL material utilized for the display is a cyclohexane solution of core-shell NaYF₄∶0.5%Er@NaGdF₄∶2%Yb@NaYF₄∶1%Er (NYF@NGF@NYF) nanocrystals, which has great potential for large-scale imaging.

We present a static volumetric 3D display system with wide wavelength and fast response, which includes three parts of display medium, control system, and laser system. In the experiment, nanocrystals NaYF₄∶Er@NaGdF₄∶Yb@NaYF₄∶Er with dual-step dual-frequency up-conversion ability are selected as imaging medium. The control system employs 1024×768 DMD and scanning galvanometer to project the infrared laser. By the appropriate design of imaging optical software, the two-dimensional slice of the stereoscopic image is converted into the control signal of the DMD/scanning galvanometer. The laser system adopts 1550 nm and 850 nm infrared lasers as the addressing and imaging light source and adjusts the beam and optical path with appropriate parameters.

The upconversion emission spectra of NYF@NGF@NYF are measured [Fig. 3(b)]. After integrating the emission spectrum in the visible range (500-700 nm), it can be concluded that contrast is I_1550+850/(I₁₅₅₀+I₈₅₀)=28.69, where I₁₅₅₀+I₈₅₀, I₁₅₅₀, and I₈₅₀ are the emission intensity under co-excitation of 1550 nm and 850 nm lasers, under excitation of 1550 nm laser, and under excitation of 850 nm laser respectively. With self-made display materials and the self-built static volumetric display system, a variety of 3D images can be demonstrated at a refresh rate of 40 Hz, and the images are clear and bright (Fig. 7). The maximum luminous power of a single point measured by the power meter can reach 0.5 mW, the theoretical maximum resolution can be 30×1024×768, and the number of voxels is close to 23 million.

We report a two-beam scanning 3D imaging system based on the dual-frequency upconversion luminescence mechanism of rare earth ions. The DLP and scanning galvanometer in the optical path are controlled by the computer to build a 3D dynamic model in the liquid medium. The images presented by the system feature stability, high resolution, fast scanning speed, and maximum voxels of 23 million, without observation angle limitations. The various parts of the system such as the light source, the light path, and the display medium are independent and can be quickly replaced and flexibly adjusted to adapt to the excitation properties of different materials. The material adopted for the display is a cyclohexane solution of the core-shell structure NYF@NGF@NYF nanocrystals, which has great potential for large-scale imaging. The system has certain reference significance for the development of volumetric 3D display and provides support for the preliminary research on 3D display capability of display media such as TSTF materials. Meanwhile, this display system is characterized by convenient building and obvious display effect, without the requirement for high material properties. It assists with the preliminary research on up-conversion materials in 3D display and serves as references for exploring large-size 3D volume display technology.