Waveguide displays based on polymer-dispersed liquid crystals

Polymer-dispersed liquid crystals (PDLCs) are a relatively new class of materials that consist of liquid crystal droplets suspended in a solid polymer matrix. An applied voltage causes the randomly oriented liquid crystals to align, allowing light through. This ability to control the amount of light flow has led to the wide use of the materials in smart glass windows. They are also promising for flexible reflective displays that have a contrast ratio as good as newsprint. Researchers have already demonstrated active matrix PDLC displays in which pixels are addressed with thin-film transistors.¹

Figure 1. A light waveguide display showing the number 1.

Lightweight, transparent PDLCs are suitable for head-mounted displays that are worn like eye glasses and are used by the military and the gaming industry. We have demonstrated two types of novel see-through devices using these materials: a light waveguide display and an arrayed waveguide display.

Waveguide structures with liquid-crystal cladding layers are frequently used as optical switches. The group of Funayama proposed the idea of light waveguide displays and fabricated a seven-segment device using a dynamic scattering mode liquid-crystal cladding.² We chose a PDLC cladding for a higher contrast ratio.³

In our design, light from a commercial LED is coupled into a glass planar waveguide that has a PDLC cladding. The liquid crystals align when a voltage is applied, and their refractive index matches that of the cladding polymer. This traps light inside the waveguide due to total internal reflection, showing a transparent pixel. When the voltage is cut off, the random orientation of the liquid-crystal droplets causes index mismatch, scattering the light and making the pixels bright and opaque. Our seven-segment light waveguide display has a total of eight pixels including the background electrode, each of which can be electrically controlled to make it transparent or colored. Voltage is always applied to the background electrode to keep it transparent. Figure 1 shows the number 1. The paper under the display can also be seen.

We used a white light source to prove this idea, but it is possible to use any color. This PDLC-based device combined with thin-film transistors would be a promising candidate for a monochrome head-mounted display because of its fast response time, high transparency, simple fabrication steps, and low weight.

Arrayed waveguide displays can theoretically be full color and have high light-use efficiency.⁴ In such a display, light of three primary colors from an emitter array would be coupled into a waveguide array coated with liquid crystals. We have proven the feasibility of a monochrome display based on this concept using PDLCs.

Our waveguide core is made of SU-8 photoresist. The side and bottom cladding is polymethyl methacrylate, while the upper cladding is a PDLC layer. Under a high voltage, the liquid crystals align with the electric field. Their refractive index matches that of the nearby polymer, and both are less than the index of the core. Hence, the quasi-transverse electric mode beam is confined due to total internal reflection, making the pixels transparent. Without the voltage the pixels turn bright and opaque because of partial light scattering. There are multiple pixels along a waveguide, and we controlled the PDLC cladding of each separately to make it transparent or colored. Figure 2 (left) shows two vertical waveguides with the red beam confined in the left one. Figure 2 (right) shows that two horizontal electrodes with an applied voltage successfully activated the two pixels, showing the color of the underlying black paper. While we used a red light source to prove this idea, it is possible to add green and blue to make a full-color arrayed waveguide display.

Figure 2. An arrayed waveguide display without voltage (left) and with voltage (right).

We are now working on such a full color see-through display. We believe a low-cost, lightweight transparent display is possible in the near future.

This work was financially supported by National Science Council under grant NSC 99-2815-C-002-145-E.