A test photo, placed at differing distances from the lens, shows the focusing ability of the diffractive liquid crystal lens when no voltage is applied (a) and when focal length is 40 cm (b), 20 cm (c), 13.3 cm (d), 10 cm (e) and 6.6 cm (f).
An adaptive liquid crystal (LC) diffractive lens developed at the University of Arizona (U.S.A.) focuses with high efficiency and zooms with millisecond-fast switching times. Doctoral student Pouria Valley and colleagues at the University of Arizona (U.S.A.) reported a lens based on a 3-µm-thick layer of nematic liquid crystals (P. Valley et al. “Tunable-focus flat liquid crystal diffractive lens,” Opt. Lett. 35, 336).
Potential applications include zoom lenses with no moving parts. His team can change the optical power in a relatively large range (40 diopters or more) with a very low power source (± 2.4 V ac) and very fast (20-150 ms).
The moving parts in regular zoom lenses are bulky, expensive and fragile, and the process of physically moving lenses is slow compared to the speed of electronic switching. This explains the lack of zoom capacity in camera phones, for example.
Previous LC lenses have shown lower efficiency, and some have required notably higher voltages. Compared to the Arizona group’s previous lenses developed for ophthalmic applications, “the new lenses have higher optical powers and higher diffraction efficiency so we can tune them in a wider range,” explains Valley.
The LC lens consists of a flat diffractive optical element and a layer of LC sandwiched between two glass substrates coated with transparent and conductive indium tin oxide. One of the substrates is patterned like a diffractive optical element. The effective refractive index of the LC varies with applied voltage. One type of diffractive element is a binary Fresnel zone plate, with each zone divided into several sub-zones to digitize the phase profile.
With 12 phase levels, the diffraction efficiency (the fraction of light intensity passing through the lens that is focused into a specific diffraction order) reached 95 percent. High efficiencies are easier to achieve with small-aperture systems, although lens apertures of 10 mm in diameter or more are possible.
The test images were made using 540-nm light. Later work will be done with broadband white light. “The chromatic aberration can be reduced either by a proper hybrid design of a diffractive and refractive lens or by adoption of multi-order or harmonic diffraction,” Valley says. “We plan to show this in future articles.”d
The images were captured by placing the test photo at different distances from the lens and bringing it into focus. Although the efficiency drops as the focal length shortens, the image quality remains good.
