Water photonics, non-linearity, and anomalously large electro-optic coefficients in poled silica fibers
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Commentaries
Water photonics, non-linearity, and anomalously large electro-optic coefficients in poled silica fibers John Canning, Interdisciplinary Photonics Laboratories, School of Electrical & Data Engineering, University of Technology, Sydney (UTS) & School of Chemistry, The University of Sydney, Sydney NSW 2007 & 2006, Australia Address all correspondence to John Canning at [email protected] (Received 12 December 2017; accepted 24 January 2018)
Abstract A review of the literature offers an explanation for the large anomalous electro-optic (e.o.) effect reported by Fujiwara et al. in 1994. It is based on the large e.o. coefficient of ordered water at an interface measured in recent years >1000 pm/V. More broadly, the concept of water-based photonics, where water could be a new platform material for devices and systems, is introduced, suggesting that liquid states of matter can allow ready shaping and exploitation of many processes in ways not previously considered. This paper is a commentary on the significance of this new understanding and the broader interest of water in photonics, particularly its consideration as a new platform material.
Introduction It is more than 22 years since the highest induced non-linearity within a silica (SiO2) optical fiber has been reported—5.8 pm/ V when poling with 193 nm ArF exciplex emission in late 1994.[1] Ultraviolet (UV) poling had achieved an order of magnitude over anything else reported in Refs. 2–7, and references therein. It was groundbreaking if correct, introducing practical non-linearity onto silicon (Si)-based platforms potentially making LiNbO3 obsolete overnight. Unfortunately, at first glance, the result appeared to defy the laws of physics in a material with no inversion symmetry. The failure to reproduce such results effectively consigned the work to the dustbin. Given the significance of those results, both at the time and in the future if it can be reproduced, this work provides an important commentary on a plausible mechanism that suggests a potential optoelectronic platform: water. To unravel the mystery requires a brief review of how symmetry in these materials can be broken. The instantaneous and parametric non-linear optical phenomena induced within an optical fiber can, in the case where applied optical fields are not too large, be artificially described by a Taylor series expansion of the dielectric polarization density P(t) at time t in terms of the applied electrical field E(t): P(t ) = 10 [x(1) E(t ) + x(2) E2 (t) + x(3) E 3 (t ) + · · ·],
(1)
where χ represents linear and higher order non-linear susceptibilities. The key photonic platform materials of Si, SiO2, and most silicate materials are centrosymmetric (no asymmetry in the overall electric field distribution giving rise to sufficiently large polarizability that can be easily
manipulated through electronic field displacement with voltage). For example, the second-order term in Cartesian space, Pi (t ) = 2 j,k di,j,k Ej (t )Ek (t), where d is the non-linear tensor, associated with the Pockel’
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