The Inorganic Waveplate – a thin-film optical component

Waveplates polarize the direction of light

A waveplate is an optical component used primarily to efficiently utilize light. It is used in optical devices such as projectors to adjust the direction of light in the desired direction. Let’s take a closer look at its structure and functions below.

Polarizers, described here , take unpolarized light and transmit it in a specific direction. In contrast, a waveplate can change the state of polarization by shifting the phase (the timing of light) of incoming light.

The following two illustrations show the light waves  passing through a half-wave plate and a quarter-wave plate, which are the two most common types of waveplates.

Light can be regarded as a composite of two orthogonal vectors. As shown in the illustration below, when the phase is shifted by half a wavelength, linear polarization at an angle of 90 degrees to the unshifted phase is obtained.

The illustration below shows the light passing through a quarter-wave plate, which shifts the phase timing by a quarter. This is called circular polarization because the vector of the synthesized light forms a spiral arc. A quarter-wave plate can generate linear polarization when circularly polarized light is introduced.

Waveplates efficiently use light

Next, the following will show an example of how waveplates are used in optical devices, such as projectors. The waveplate is inserted in an optical component called a PS converter. The PS converter transmutes the light emitted from the light source into polarized light and aligns the polarization to match that of the liquid crystal, thereby increasing the efficiency of light utilization.

The illustration below shows how the light passes through the PS converter and follows the path inside the projector to project an image. The unpolarized light emitted by the light source on the left is a mixture of P-polarized light (horizontal light waves) and S-polarized light (vertical light waves). The PS converter (polarization converter) transmits S-polarized light as it is, while P-polarized light is converted to S-polarized light after passing through a half-wave plate.

The S-polarized light that passes through the PS converter is reflected by a polarizing beam splitter. When the light enters the reflective LCD panel, only the output pixels are converted into P-polarized light. The image is then projected through the polarizing beam splitter.

The main feature required for  a waveplate is high polarized light transmittance. Polarized light transmittance refers to the percentage of incident light on a waveplate. This indicates how efficiently the light is converted into polarized light. The following are two graphs showing the  polarized light transmittance of the broadband half-wave plate developed by Dexerials. The first graph shows the wavelength on the horizontal axis, and the second graph shows the incident angle on the horizontal axis. In order to use light from a light source as effectively as possible, it is necessary to maintain a polarized light transmittance over a wide range of wavelengths and incident angles as close to 100% as possible.

Features of the Dexerials’ inorganic waveplates

Most conventional waveplates are made of organic materials. While such materials are inexpensive, there is a major disadvantage as they are vulnerable to heat. Therefore, in recent years, inorganic materials that can withstand long-term use are being used in projectors with high brightness capabilities.

The inorganic waveplates developed by Dexerials have the following three main features.

1. Excellent heat and light resistance due to inorganic materials used in their design
2. Small phase difference due to thin substrate to meet customer requirements.
3. Low dependence from the incident light angle due to thin substrate

There are also inorganic waveplates made of quartz. However, quartz waveplates are less accurate since they use a mechanical polishing process to achieve ½ or ¼ phase differences.

In contrast, Dexerials developed the thin-film waveplates with a column structure by orthotopically depositing inorganic materials on glass substrates to produce phase differences. This enables high-precision manufacturing.

Dexerials will continue its research to improve the brightness level and image quality of projectors.