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Sub-micron pitch variable diffraction grating using nanoporous electrodes and electrophoresis of dye ions Wong, Richard Tik Fai


One approach to achieve non-mechanical beam steering is to use variable diffraction devices, whose intensity and direction of the outgoing diffractive orders can be actively (typically electronically) altered by modifying the optical properties of the device. Devices with sub-micron features are of particular interest when working with visible wavelengths, since such gratings can, when not activated, produce only an undeflected beam as the output and, when activated, produce diffractive orders at large deflection angles that appear and disappear entirely. Such sub-micron pitch variable diffraction gratings can be achieved by electrophoretically moving dye ions into and out of appropriately patterned transparent electrically-conductive nanoporous electrodes. This study aimed to explore their potential for use in practical devices. One significant technical hurdle associated with these devices has been the irreversible electrochemical reactions that can be induced with an applied voltage difference between the two electrodes exceeding a critical value on the order of 1 V. These reactions are undesirable as they limit the lifetime of the device. However, it was observed that when operating within such a voltage limit the optical response time is undesirably long - on the order of seconds. To better optimize the lifetime and response time of these devices, an electron tunneling model of the electrochemical reaction threshold was developed and verified. This enabled the use of a high speed drive circuit that was able to improve the response time by a factor of 50 without compromising the lifetime of the device. A prototype grating with nanoporous zinc antimonate electrodes and methylene blue dye-methanol solution was fabricated using focused ion beam milling. Modulation of a diffractive order was observed with this prototype, where the exit beam geometry was in good agreement with the model. The diffractive efficiency of the diffractive order was observed to be 3.9×10-3 when in the charged state and 7.9×10-4 when in the uncharged state with an optical switching time of 250 ms. These observations demonstrated the potential of this type of device to achieve large angular deflection and fast operating speed for non-mechanical beam steering applications.

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