New Liquid Crystal Opens Optical Applications
A group at the University of Central Florida in Orlando has developed a type of polymer-dispersed liquid crystal that offers a gradient droplet size distribution. The gradient refractive index profile can act as switchable prism gratings, as well as positive and negative lenses with tunable focal lengths, suggesting an array of applications not attainable with other devices.
The study grew out of a concept for tunable eyeglasses, in which a voltage would adjust the prescription to meet the changing demands of the wearer. In addition, the researchers plan to develop an electro-optical zoom lens for cameras.
Two types of polymer-dispersed liquid crystal devices have been developed for use in displays and in tunable-wavelength filters. Those developed for the former application feature liquid crystal droplets approximately 1 µm in size to allow light scattering to dominate in the visible range. Those devised for the latter -- in particular, for telecom switches -- include nanometer-size droplets so that the polymer matrix in which the droplets are dispersed is optically isotropic with respect to the incident light. In both types of devices, the droplets generally are homogeneous in size.
The researchers achieved an inhomogeneous size distribution by shining 350-mW/cm2 ultraviolet radiation onto the liquid crystal/ monomer mixture through an A41-960 continuously variable density filter from Edmund Industrial Optics of Barrington, N.J., which acted as a photomask. Varying the photomask patterns produced the desired gradient in the polymer-dispersed liquid crystal film.
To produce a film with a steep phase gradient, they employed a photomask with a compressed distance from the smallest to the largest droplets. In the device's voltage-off state, the gradient phase profile is present. As voltage is applied, the phase profile decreases until it disappears.
The new type of crystal is broadband, highly transparent and polarization-independent, and displays a fast response time. However, before it can be applied to tunable focal eyeglasses, the researchers will have to shorten the focal length. "We need to increase the optical path length and sharpen the gradient index," said professor Shin-Tson Wu. "We need to develop very high birefringence liquid crystals and improve our lens design."
In the near term, the method can be used to produce microlens arrays.
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