Transparent, Polycrystalline Upconverting Nanoceramics: Towards 3-D Displays

Although the concept of transparent, polycrystalline ceramics is now 40 years old, only recently has control of processing parameters been sufficient to produce photonic quality materials, Nd:YAG and Yb:Yttria lasers in particular. In all instances, average grain sizes (AGSs) in these fully dense materials are in the range 10–50 mm. Furthermore, photonic quality transparency has only been achieved for materials with cubic crystal structures thereby avoiding scattering due to birefringence. Recent work by Krell points to the possibility of also obtaining photonic transparency in materials with submicron AGSs, in a-Al2O3 in particular. [8–10] In principle, the smaller the final AGS and average defect size, the higher the expected transparency even for non-cubic crystal systems such as titanium doped sapphire (0.5 at % Ti3þ in a-Al2O3) or ruby (0.5–3.0 at%Cr3þ in a-Al2O3). Access to such materials is best served by sources of high quality nano-oxides that permit lowtemperature densification without coincident grain growth. If transparency can indeed be achieved with very fine-grained ceramics, the potential exists to create threedimensional emissive displays using the ‘‘inverted planetarium’’ concept described below. Transparent, upconverting phosphor pixels uniformly arrayed in thin walled spherical, cylindrical or even box shapes could provide 3-D displays in which color would be achieved by computer controlled rastoring of an IR laser (or lasers) in the interior of the display across the pixels at rates fast enough to generate 3-D images. Upconverting phosphors typically capture two IR photons (e.g., 960 nm) and subsequently emit visible red, green and blue light depending on the phosphor, thus no visible beam is observed from a rastored IR source. Quantum efficiencies can be upwards of 5%.