Abstract
In this study, we present a new configuration of the recently reported optofluidic platform exploiting liquid crystals reorientation in lithium niobate channels. In order to avoid the threshold behaviour observed in the optical control of the device, we propose microchannels realized in a x-cut crystal closed by a z-cut crystal on the top. In this way, the light-induced photovoltaic field is not uniform inside the liquid crystal layer and therefore the conditions for a thresholdless reorientation are realized. We performed simulations of the photovoltaic effect based on the well assessed model for Lithium Niobate, showing that not uniform orientation and value of the field should be expected inside the microchannel. In agreement with the re-orientational properties of nematic liquid crystals, experimental data confirm the expected thresholdless behaviour. The observed liquid crystal response exhibits two different regimes and the response time shows an unusual dependence on light intensity, both features indicating the presence of additional photo-induced fields appearing above a light intensity of 107 W/m2.
Highlights
A key role is played by the bulk photovoltaic effect induced by light in LiNbO3 crystals [10,11] and successfully used to reorient the molecular liquid crystals (LC) director in properly designed nematic liquid crystal cells [7,8,9,12]
The subsequent charge redistribution that takes place inside the crystal gives rise to an electric field, the photovoltaic field [11], that has been exploited to realize photorefractive optics in LiNbO3 [13,14]
This effect is strongly enhanced by doping LiNbO3 with iron [10,15], which introduces electron donor (Fe2+ ions) and acceptor (Fe3+ ions) centers and makes it possible to reach a photovoltaic field up to 107 V/m
Summary
The possibility of combining these two materials for advanced all-optical devices was demonstrated [7,8,9] In these investigations, a key role is played by the bulk photovoltaic effect induced by light in LiNbO3 crystals [10,11] and successfully used to reorient the molecular LC director in properly designed nematic liquid crystal cells [7,8,9,12]. The subsequent charge redistribution that takes place inside the crystal gives rise to an electric field, the photovoltaic field [11], that has been exploited to realize photorefractive optics in LiNbO3 [13,14] This effect is strongly enhanced by doping LiNbO3 with iron [10,15], which introduces electron donor (Fe2+ ions) and acceptor (Fe3+ ions) centers and makes it possible to reach a photovoltaic field up to 107 V/m
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