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Cecilia O’Brien, a Lippert lab summer intern, demonstrates the prototype 3-D Light PAD. [Image: Courtesy of Southern Methodist University]

Chemists from Southern Methodist University (SMU), USA, have created and demonstrated a light-based volumetric 3-D display using a new chemical “photoswitch,” with a photoactivatable dye that can be turned on and off with ultraviolet (UV) light (Nat. Commun., doi: 10.1038/ncomms15239). The researchers say that their desktop-sized display, which they call the 3-D Light PAD, overcomes complex fabrication requirements—including expensive and potentially dangerous energy beams—that have plagued previous volumetric 3-D display designs. The 3-D Light PAD could find use in medical imaging, engineering, architecture, education and military applications, according to the team.

A chemical on/off switch

The 3-D Light PAD’s key technology is its photoswitchable and photoactivatable dye. N-substituted spirolactam rhodamines are dyes that, via a thermally reversible reaction, switch from a fluorescing to a non-fluorescing isomer when exposed to UV light. By adding triethylamine to a solution of N-phenyl spirolactam rhodamine B dye molecules, the researchers were able to speed up the photoactivatable dye’s thermal-fading speed, essentially giving the dye an instantaneous and reversible on/off switch. (Without this chemical switch, thermal fading takes anywhere from minutes to hours.)

Team leader Alexander Lippert and his colleagues Shreya Patel and Jian Cao demonstrated their chemical on/off switch in cuvettes filled with a solution containing triethylamine and N-phenyl spirolactam rhodamine B. When the cuvettes were exposed to UV light, the solution fluoresced pink. When the UV light was switched off, the solution immediately went from pink to clear. They were able to reproduce the photoswitching reaction across 250 on/off cycles.

The 3-D Light PAD

Lippert’s team then built the prototype 3-D Light PAD with a commercially available picoprojector, UV and green LED projectors, and a customized quartz glass imaging chamber containing the triethylamin-spiked photoactivatable dye solution. The researchers say they were able to assemble the prototype for less than US$5,000, not including the two laptop computers that serve as the graphics engine.

For the first prototype demonstration, the researchers beamed a square of green LED light into the quartz chamber. Another LED projector positioned 90 degrees from the green LED projector emitted a series of 385-nm UV light bars into the quartz chamber. At the intersection of the two beams, the researchers observed a pattern of 2-D squares. The addition of optical filters between the UV projector and the quartz chamber, and between the quartz chamber and the CMOS camera, increased image contrast and removed the blue background light in the chamber, letting only red light to pass through.

By changing the spatial pattern of the light projected by the LEDs, the researchers were able to create more shapes, including 3-D triangles and cubes, and a real-time 3-D animation of a running mustang horse (the SMU mascot). During these demonstrations, the 3-D Light PAD provided a minimum voxel size of 0.68 mm3, with 200-µm resolution.

For future iterations of the 3-D Light PAD, the researchers hope to move from a liquid to a solid dye matrix in the quartz glass chamber. They also want to expand the system’s color palate from red to true color, using red, green and blue light.