1. Field
The present invention relates generally to reflector shapes for light emitting devices and specifically to shapes designed to directly generate vertically emitted polarized light.
2. Description of the Related Art
High efficiency and power, long lifetime, small size, and the wide range of wavelengths available are reasons why light-emitting diodes (LEDs) are becoming increasingly important in areas such as sensing and imaging, general illumination and liquid crystal display (LCD) backlighting. A key benefit provided by LEDs is the ability to tune properties such as wavelength or color temperature to meet the needs of specific applications. However, the control of one property in particular—the optical polarization—has remained elusive. Polarized LEDs would be extremely useful particularly for LCD backlighting but also for imaging and communications. Accordingly, there is a need for a polarized LED source, which is enabled by a polarization-enhancing reflector design matched to the emission characteristics of GaInN LEDs so that light incident upon the reflector is redirected, and through selective polarization rotation by the reflector exhibits an enhanced polarization ratio.
Previously it has been reported that the light emitted in certain directions by 20 GaInN LEDs epitaxially grown on (0001) oriented sapphire substrates shows some degree of polarization. Polarization effects have also been demonstrated with LEDs grown on non-polar or semi-polar substrates. Studies of GaInN LEDs on sapphire substrates with multiple quantum well (MQW) active regions emitting at 460 nm revealed that light emitted from the side facets of the LED chips is dominantly polarized in the plane of the quantum wells, with values as high as 7:1 for the ratio of in-plane polarized light to normal-to-plane polarized light's. However, despite the measured polarization characteristics of unpackaged chips, conventional packaged LEDs were found to be completely unpolarized. This is attributed to the use of reflectors which do not preserve the inherent polarization properties of the LED chips. These reflectors typically exhibit continuous rotational symmetry and simply take the rays emitted in different directions and reflect them upwards. When two orthogonally polarized beams of equal intensity are combined, the result is unpolarized light.