Biomedical engineering (BME), electrophysiology, Electromagnetic Compatibility (EMC) or aerospace and defense fields demand compact electric field sensors with very small spatial resolution, low sensitivity and large bandwidth. We show that the electro-optical property of lithium niobate coupled with the tunability of photonic crystals can answer this request through Lab-on-Fiber technology.
First, band diagram calculations and Finite Difference Time Domain (FDTD) simulations analysis lead to the design of the most suitable two-dimensional photonic crystal geometry. We show that light normal incidence on rectangular array of air holes in free standing X-cut thin film lithium niobate produces a very sharp and E-field sensitive Fano resonance at a wavelength of 1550nm. Then, in order to concentrate the E-Field to be detected in the photonic crystal area (20μm*20μm*0.7μm) we design a thin metallic antenna, scaled down them in such a way that it does not produce any disturbances while increasing the sensitivity.
The LN membrane with the antenna is fabricated by standard clean room processes and Focused Ion Beam (FIB) is used to mill the photonic crystal. Then, by means of a flexible/bendable transparent membrane, we were able to align and to attach the photonic crystal onto a ferrule ending polarization maintained optical fiber.
Optical characterizations show that the Fano resonance is easily modulated (wavelength shifted) by the surrounding E-field. The novel non-intrusive E-field sensor shows linearity, low sensitivity, large bandwidth (up to 100GHz) and a very small spatial resolution (≈20μm). To the best of our knowledge, this spatial resolution has never been achieved in E-field optical sensing before.