Enhancement of Numerical Aperture of Liquid Crystal Microlenses Using a Stacked Electrode Structure

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Abstract
A liquid crystal (LC) microlens with a stacked electrode structure is fabricated by attaching a transparent flat electrode outside a conventional LC microlens. Numerical aperture (NA) properties in two driving methods (an addition type and a subtraction type) are discussed. NA of LC microlens in the addition-type driving method is almost the same as that of the conventional LC microlens, while in the subtraction type, NA is much larger. The enhancement of NA is explained by considering the spatial distribution of the electric field in the LC layer. The dependence on thickness of the insulating layer (t i) of the maximum NA (NAmax) when the applied voltage is varied is also discussed. In the subtraction type, NAmax strongly depends on t i and takes a maximum value when t i is varied. The enhancement of NA leads to the application of LC microlens as a collimating lens of a light source with a large divergence angle such as a laser diode.