Based on creep-induced deformation of a polyvinyl chloride (PVC) gel in an electric field, a Fresnel zone plate (FZP) is fabricated. The employed electrode is constructed with a series of concentric zones, and the soft PVC gel coated on the electrode forms a flat membrane. By applying a DC voltage to the electrode, electrons are injected to the PVC gel. Due to the generated electrostatic force, the surface of the PVC membrane is undulated. The configuration of the PVC membrane resembles the pattern of the electrode. As a result, the PVC membrane functions as an FZP. The results show that incident light passing through the FZP can be diffracted, producing a focused spot due to constructive interferences. The maximum diffraction efficiency can reach ∼32.5 % , and the focal length is switchable. The advantages of this FZP are the ease of manufacturability, compactness, large size, and optical isotropy.
Tight focusing of laser light using a polarization converter and an adaptive lens is demonstrated. The polarization converter is used to convert the linearly polarized light into a radially polarized light. The adaptive lens is used to focus the radially polarized light on a spot. At the focused spot, a large component of the electric field is along the longitudinal direction, and the focused power density is enhanced. The size of the focused spot can be changed by changing the focal length of the lens. When the focal length of the lens becomes shorter, the focused spot becomes tighter. The mechanisms of the two devices are introduced, and the theory of tight focusing is experimentally supported. Tight focusing of radially polarized laser light with a controllable spot size has potential applications in super-resolution imaging, material processing, and microparticle trapping.
A liquid crystal (LC) lens is prepared by controlling the alignment of a LC using a homogeneous polyimide (PI) layer and a homeotropic PI layer. The rubbed homogeneous PI layer has a concave surface and the homeotropic PI layer is flat. The LC sandwiched between the two PI layers obtains a hybrid alignment which has the largest gradient of refractive index (GRIN) distribution. The LC layer exhibits a lens character because of its convex shape. Since the effective refractive index of the LC is larger than that of the homogeneous PI, the LC lens can focus a light with the shortest focal length in the voltage-off state. By applying an external voltage, the LC molecules can be reoriented along the electric field. As a result, the focal length of the LC lens is reduced. The focal length of the LC lens can be tuned from ~30 to ~120 μm when the voltage is changed from 0 to 7 Vrms. This LC lens has the advantages of no threshold, low operating voltage, and simple fabrication.
We prepared a lenticular microlens array (LMA) using a polyvinyl chloride (PVC) gel and an interdigitated electrode. By applying a DC voltage to the electrode, the surface of the PVC gel can be waved with an LMA character. When the voltage is removed, the wavy PVC gel can recover its flat surface gradually. With the aid of a polarity-inverted voltage, the recovering time can be largely reduced. The LMA can present a stable dynamic response when it is repetitively impacted by a pulse voltage. The experimental results are given, and the mechanism of reducing the dynamic response time is explained. Our LMA with improved response time has potential applications in sensing, beam steering, biometrics, and displays.
A tunable-focus liquid crystal (LC) lens is prepared. The orientation of the LC is controlled using a homeotropic polyimide (PI) layer and a homogeneous PI layer. The homeotropic PI layer has a concave shape. The LC in the concave space presents a hybrid alignment and has a convex shape. As a result, the LC has a lens character with the shortest focal length in the voltage-off state. The focal length can be tuned from 9 to 15 cm when an applied voltage is changed from 0 to 8 Vrms. Our LC lens has no threshold voltage, and the fabrication procedure is relatively simple.
We prepared a polymeric microlens array (MLA) using ultraviolet (UV) light to cure photosensitive monomers through a photomask. After a short-time UV exposure, the uncured monomers experience a process of partial wetting and self-development on the surface of cured monomers. As a result, a geometric relief with a lens character is generated. Depending on the pattern of the photomask, either a convex or concave MLA can be fabricated. The mechanism of forming the MLA is explained and the concept is proved experimentally. Owing to the merits of simple fabrication, good flexibility, and high optical performance, the MLA has potential applications in light diffusers, fiber/organic light-emitting diode couplers, biomedical imaging, and displays.
When a liquid droplet is filled in the hole of a dielectric elastomer (DE) film, a liquid lens is prepared. By applying a DC voltage to the DE film, the liquid lens can be actuated. As a comparison, two liquid lenses, one in a millimeter scale and the other in a submillimeter scale, are demonstrated. In a relaxed state, the focal length of each liquid lens is the longest. In an actuated state, the diameter of each lens is reduced. As a result, their focal length is tuned. Here, the DE film functions as an actuator. Due to the biconvex shape and smooth liquid surface, each liquid lens can provide good optical performance. They also possess the merits of simple fabrication, compact structure, and easy operation. In contrast to the bigger liquid lens, the smaller one can present a better mechanical stability without the concern of the gravitational effect.
By filling a liquid droplet in the hole of a dielectric elastomer (DE) film directly, we prepared two small liquid lenses. The aperture of one lens is macro size and the other is micro size. The liquid droplet in each hole of the DE film exhibits a lens character due to its biconvex shape. In relaxed state, the focal length of each liquid droplet is the longest. When a sufficiently high DC voltage is applied, the diameter of each DE hole is decreased by the generated Maxwell stress, causing the curvature of its droplet to increase. As a result, the focal length of each lens is reduced. Here the DE film functions as an actuator. In contrast to previous approaches, our miniature liquid lenses possess the advantages of simple fabrication, fast response time (~ 540 ms), and high optical performance (~ 114 lp/mm). Moreover, the micro-sized liquid lens presents good mechanical stability.
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