Leonard G. Hale

Thin-film birefringent devices based on form birefringence

The twisted nematic liquid crystal display (TN-LCD) is the leading technology for high performance flat panel displays. However, the region of high contrast for TN-LCDs is limited. Birefringent elements, of compensators, may be used to provide improved contrast at high viewing angles. The phenomenon of form birefringence has been used to design a compensator that can be fabricated by physical vapor deposition of silica and titania, two common coating materials. Improved viewing angle characteristics, particularly in the horizontal direction, have been demonstrated using the compensator. The 20:1 isocontrast region has been extended to +/- 50 degree(s) in displays incorporating the compensator, an improvement of 10 degree(s) or more relative to uncompensated displays. The compensator design includes integrated antireflection coatings to reduce glare. In this way, display legibility is maximized in the presence of both high and low ambient illumination.

Binary optics thin-film microlens array

Binary optics can produce microlenses and lens arrays with theoretical diffraction efficiency as high as 95% for eight-phase level devices. Due to shadowing, mask misalignment, and etching errors that accumulate during fabrication, the actual diffraction efficiency can be reduced to less than 70%. Advances in mask design and e-beam writing have reduced mask misalignment errors to less than 0.2 micrometers but the major issue is the accuracy of the RIE process that is used to transfer a lithographic pattern into the substrate. RIE has two limitations for binary optic applications. First, it cannot be readily employed for the wide range of possible optical substrates of interest (Al2O3 for example), and second, since the pattern is etched directly into the substrate, there is no simple means to calibrate the etch depth during the process. Thin film deposition of the binary structure addresses both of these limitations. It is applicable to a wide range of materials, and accurate in process monitoring of the deposit permits precise control of the feature height. In this paper, we report on eight-phase level binary optic microlenses processed by deposition of SiO2 on fused silica and Al2O3 on sapphire using a projection lithography system. Photoresist processing was achieved by image reversal and lift-off technique. The microlens arrays (in a square format) were designed for (lambda) equals 0.632 micrometers with two microlens sizes of 120 micrometers X 120 micrometers and 240 micrometers X 240 micrometers having speeds of F/12 and F/6 (at the corners), respectively. Optical characterization has demonstrated that the microlens arrays are near diffraction limited and diffraction efficiency is in excess of 80%.

Ellipsometric measurements applied to liquid crystal display technology

Variable angle transmission ellipsometry has been used to characterize the various elements of the liquid crystal display (LCD) architecture. Ellipsometric data, which are in the form of polarization ellipses as a function of incident angle, are analyzed using the 2 X 2 extended Jones matrix formalism. Information which can be deduced from the ellipsometric data includes the birefringence, cell gap, twist angle, and pretilt angle of the liquid crystal cell, polarization efficiency of the polarizers, as well as the retardation values of birefringent compensators. The ellipsometric method is capable of complete characterization of the polarization state of the transmitted light.

Optical testing and characterization of mirolens arrays

An automated visible-infrared optical measurement system has been used to characterize the imaging performance of binary optic microlenses. Measurements of the point spread function (PSF) were made, from which the modulation transfer functions (MTFs) were derived. Diffraction efficiencies may also be measured using the same system. PSF measurements on both infrared and visible microlenses are in close agreement with theoretical predictions. Data on both IR and visible microlens arrays are presented.