OPTICAL MATERIALS

42 Optics & Photonics News ■ December 2002

OPTICAL MATERIALS Photorefractive PolymersFor Real-Time HolographyFabricated by Injection Molding Bernard Kippelen, Seth R. Marder and N. Peyghambarian

P hotorefractive polymers, whichemerged in the 1990s, provide low-

cost alternatives to their inorganic crys-talline counterparts. They have been usedin real-time holographic applications, in-cluding storage, time-average interferome-try, imaging through scattering media,optical correlation, optical limiting, ho-modyne detection of ultrasounds andnovelty filtering. During the past decade,significant progress has been achieved inimproving their dynamic range, responsetime, gain coefficient and phase stability.Recently, we demonstrated that thesephotorefractive polymers have the poten-

tial to be mass produced at lowcost using standard techniquesfor the manufacturing of plastics,such as injection molding.

Precision injection moldingof synthetic organic polymers isan important technology for pro-ducing objects with variousshapes at low cost and in highvolume. Injection molding ofhighly transparent optical poly-mers is playing an increasinglyimportant role in enabling thefabrication of numerous low-costoptical elements, including opti-cal disks for storage, lightpipes

for liquid crystal displays or connectors forsingle-mode fibers. The replication func-tions that injection molding provides havebeen used in recent years to fabricatewaveguide couplers and diffractive opticalelements.

An injection-molding process involvesthe following steps: the raw plastic materi-al is fed into a heated barrel, where it ismelted. The temperature of the melt isspecific to the material used to meet theflow requirements for the fabrication of agiven component. By applying pressure tothe melt in the barrel, the plastic is then in-jected through a nozzle into a mold. Themold is a custom-tooled cavity represent-ing the negative volume of the componentto be manufactured. In the mold, the plas-

tic cools rapidly and solidifies. In a finalstep, the mold is opened and the finishedpart is released and removed.

In our experiments,1 we used a 22-tonvertical travel press. The mold cavities al-lowed the fabrication of bulk samples withdimensions of 60 mm �13 mm �3 mm or10 mm �15 mm �1 mm. For our demon-stration, we used photorefractive polymercompositions comprised of a low-birefrin-gence copolymer that provided sampleswith good optical quality. Photorefractivi-ty was obtained by adding a sensitizer anda bifunctional dopant that provided si-multaneously for charge transport andelectro-optic activity. An example of a pre-form fabricated by injection molding isshown in the inset of Fig. 1. Photorefrac-tive samples were fabricated by melting asmall piece of the preform between twoindium-tin-oxide-coated glass slides. Inthe best samples, diffraction efficiencies of20% were obtained (see Fig. 1). Net gainwas also demonstrated. The response timein these materials is quite slow at this stage(330 s) and requires optimization. Ourwork shows that multifunctional polymersare amenable to high-volume productionthrough well-established plastic manufac-turing techniques and can meet the lowcost targets required for many new emerg-ing optical applications.

AcknowledgmentThis work was partially supported by NSF,ONR and AFOSR.

References 1. J.A. Herlocker, C. Fuentes-Hernandez, J. F.Wang, Q.

Zhang, S. R. Marder, N. Peyghambarian and B. Kip-pelen,Appl. Phys. Lett. 80 (7), 1156, 18 February2002.

Bernard Kippelen (kippelen@u.arizona.edu), Seth R.Marder and N. Peyghambarian are with the OpticalSciences Center, University of Arizona,Tucson.

Figure 1. Field dependence of the diffraction effi-ciency measured in a photorefractive polymersample fabricated by injection molding. (Inset) Pho-tograph of a thick photorefractive polymer pre-form fabricated by injection molding.