Pdlc-Based Device for Optical Limiting
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active protection filter. EXPERIMENT Preparation and basic characterization of PDLC films Standard polymer and liquid crystal have been supplied from Merck Ltd. with matched refractive indices (npoly = no, CL). Liquid crystal and polymer precursor are mixed with controlled respective concentrations6 . In order to optimize the performances, we use 10 Pm to 50 pm thick cells composed of two glass plates coated with indium tin oxide (ITO) transparent electrodes and polyimide alignment layers. Phase separation is performed inside the cells by photoinduced phase separation under controlled UV irradiation (between 320 and 390 nm), resulting in the formation of the PDLC. *
Present address : Centre d'61ectronique et de microoptodlectronique, Universit6 Montpellier 2, Place E. Bataillon, 34094 Montpellier, France.
239 Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
The transmission spectra of empty cells and voltage-driven PDLC-filled cells are recorded on a dual beam spectrometer. The driving voltage is measured as a function of the PDLC thickness and the response time is evaluated according to the operating voltage. The angular distribution of the scattered light is measured at 633 nm (He-Ne laser). With 0.8 degree angular acceptance, the scattered intensity is sampled every degree using a photodiode linked to a lock-in amplifier. PDLC light-valve The transmission state of a PDLC film can be optically addressed by replacing one of the glass plate of the cell by a photoconductive substrate acting as an optical switch (see figure 1). This device is electrically biased so that, under low flux, the driving voltage is applied on the PDLC, then maintained in its transparent state. Because of the direct nature of the PDLC, we need a three ITO electrodes structure in order to short-circuit the applied voltage on the PDLC under laser irradiation. Under local laser threat onto the photoconductor, its resistivity decreases by several order of magnitude the driving voltage is no longer applied onto the PDLC which then becomes diffusive. Alignment layers
0 00
Photoconductor
Figure 1: Structure of PDLC-based optical valve
4 . 0-
PDLC
Transparent electrodes The optical limiting properties are evaluated in a 6'5 optical bench using an Ar laser at 514 nrm. The transmitted power is measured while controlling the incident energy by the combination of a half-wave plate and a polarizer. In such an optical limiter, the two different functions are separated on two different elements. The photoconductor switch ensures alert detection and determines the activation threshold. The PDLC layer is the protection filter and determines the induced optical density and the response time. For these reasons, the key parameters of the PDLC layer have first been optimized (materials, thickness of the cell...) independently of the photoconductor. Then a preliminary demonstration of the PDLC optical valve has been made. RESULTS PDLC characteristics The spectrum presented in figure 2 shows that the transmission of a 25 gim thick P
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