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Part A
Basic Principles and Materials
optical communication, optical distance measurement and space navigation. In a binary-phase planar Fresnel lens, the phase difference between the adjacent zones 0V 8 can be tuned by the applied voltage. Maximum diffrac7 tion occurs when the phase difference is equal to an odd 6 multiple of π while no diffraction occurs for even mul2V 5 tiple of π . Through diffraction, the Fresnel zone plate 4 works as a focusing element. 3 Due to higher-order Fourier components, Fresnel lens has multiple foci at f , f /3, f /5, etc. However, the 2 4V majority of the incident light diffracts into the primary 1 7V focus. The primary focal length can be expressed by 0 f = r 12 /λ, where r 1 is the innermost zone radius. The –1.0 –0.5 0.0 0.5 1.0 Position (cm) radius of the n -th zone (r n ) is given by r n2 = nr 12 , n is the Fig. 5.75 Spatial phase profile of the positive lens at varizone number. Theoretically, the diffraction efficiency of ous voltages the primary focus for the binary phase Fresnel lens is 41%. creases so that the focal length increases. Due to the A method of fabricating LC Fresnel lens was long focus, this lens is suitable for satellite imaging proposed by Patel and Rastani [5.391]. The idea to and astronomy applications. To shorten the focal length, fabricate the lens is to make the neighboring zones several methods can be chosen, such as improving the with orthogonal LC directors on one substrate, and photomask pattern, using high birefringence LC mater- homeotropic alignment on the other. Another method ial, increasing the cell gap, or reducing the diameter of of fabricating LC Fresnel lens is to etch the ITO electhe curved spot. trode by photolithographic to form a zone plate [5.392]. To fabricate a negative lens, one can choose a photo- However, the two fabrication methods are rather commask with reversed optical density. By using patterned plicated. UV exposure method, both positive and negative lenses Using UV pattern irradiation technique, one can be fabricated. Since the lens is a single pixel de- can simplify the device fabrication process [5.393]. vice, the fabrication process is simple and the lens size A method of fabricating Fresnel lens using patterned depends upon the photomask design. photomask is similar to that as shown in Fig. 5.73, exLike the aforementioned LC lens, PNLC lens is cept that the photomask was produced with transparent polarization dependent too. To overcome this polariza- odd zones and opaque even zones. tion dependence, two cells with orthogonal alignment To fabricate a nanoscale PDLC Fresnel lens, directions should be considered. The PDLC with sub- 26wt% LC E48 and 74wt% UV curable prepolymer wavelength droplet size is another technique for solving NOA81 were mixed and injected into a homogeneous the polarization dependent problem for LC lens [5.394]. empty cell. The cell gap is 15 µm. A photomask was The drawbacks of the nano-PDLC lens are the high op- produced by etching a chromium oxide layer using elecerating voltage and small refractive index change which tron beam lithography. The radius of the innermost zone results in a long focal length. is 0.5mm, and the zone plate consists of 80 zones in Besides the patterned photomask, exposure using 1 cm aperture. The photomask is in proximity contact a UV laser beam with Gaussian intensity distribution with the LC cell substrate during exposure to the UV to the LC /monomer cell has been studied [5.396]. The light. cured spot presents a lens-like character under the acTo evaluate the image quality of the PDLC Frestion of a homogeneous electric field. The concern for nel lens, a black cardboard with transparent alphabet this technology is that a high power laser is needed for R was placed before the sample. A collimated He–Ne scaling up the lens aperture. laser has 1 cm diameter which fills the entire zone plate. A CCD camera was set at ≈ 25cm behind the samFresnel Zone Plate. In comparison with the above re- ple. Figure 5.76a,b shows the photos without and with fractive lens, Fresnel zone plate (FZP) lens belongs to sample, respectively. When the sample is absent, no fothe diffraction lens. It can be fabricated with large aper- cusing effect occurs. Once the LC Fresnel lens is in ture size. Fresnel lenses are suitable for long distance position, a clear but smaller image is observed, although Phase (π ) 9
P a r t A 5 . 1 0
