KEYWORDS: Sensors, Antennas, Crystals, Waveguides, Terahertz radiation, Optical sensors, Electric field sensors, Plasmas, Optical microsystems, Near field
The measurement of microwave electric-field (E-field) exposure is an ever-evolving subject that has recently led the International Commission on Non-Ionizing Radiation Protection to change its recommendations. With frequencies increasing toward terahertz (THz), stimulated by 5G deployment, the measurement specifications reveal ever more demanding challenges in terms of bandwidth (BW) and miniaturization. We propose a focus on minimally invasive E-field sensors, which are crucial for the in situ and near-field characterization of E-fields both in harsh environments such as plasmas and in the vicinity of emitters. We browse the large varieties of measurement devices, among which the electro-optic (EO) probes stand out for their potential of high BW up to THz, minimal invasiveness, and ability of vector measurements. We describe and compare the three main categories of EO sensors, from bulk systems to nanoprobes. First, we show how bulk-sensors have evolved toward attractive fibered systems that are advantageously employed in plasmas, resonance magnetic imagings chambers or for radiation-pattern imaging up to THz frequencies. Then we describe how the integration of waveguides helps to gain robustness, lateral resolution, and sensitivity. The third part is dedicated to the ultra-miniaturization of components allowing ultimate steps toward electromagnetic invisibility. This review aims at pointing out the recent evolutions over the past 10 years, with a highlight on the specificities of each photonic architecture. It also shows the way to future multi-physics and multi-arrays smart sensing platforms.
Optical nanofibers have recently emerged as attractive nanophotonic platforms for several applications ranging from quantum technologies to nonlinear optics due to both the tight optical confinement and their wide evanescent field. In this work, we report on a theoretical and experimental investigation of the optical Kerr effect in the evanescent field of silica nanofibers immersed in several highly nonlinear liquids such as ethanol, acetone and water and we further compare them with air cladding. We provide formula of the effective nonlinear coefficients including the contribution of the nanofiber silica core and of the evanescent field for varying nanofiber diameter and for different surrounding media. Our results show that the contribution of the silica core inversely scales with the effective mode area, while the cladding contribution via the evanescent field depends both on the taper diameter and the nonlinear properties of the liquid. More specifically, it is shown in a silica nanofiber immersed in acetone that the evanescent field contribution to the total Kerr effect is greater than that of the silica core for a taper diameter smaller than 560 nm. We further report the observation of a strong evanescent Kerr effect through measurements of the stimulated Raman-Kerr scattering in a silica nanofiber immersed in acetone. The evanescent Kerr effect is shown to give rise to a strong asymmetric spectral broadening of the first Raman order generated in the nonlinear liquid. Finally, the evanescent Kerr and Raman effects demonstrated in this work may find potential applications to ultra-sensitive liquid sensing and Raman spectroscopy, as the optical mode propagating in the nanofiber essentially interacts with the outer environment without any major contribution from the nanofiber itself.
We investigate both theoretically and experimentally the polarization properties of Brillouin light scattering in silica optical nanofibers. Our results show that while all hybrid acoustic waves scatter light without altering the state of polarization, one of the surface acoustic wave generates a depolarized Stokes light. Because of the slight ellipticity of the nanofiber, the surface wave is actually split into two torso-radial modes which give rise to polarization scrambling of the backward Brillouin Stokes signal. Our model also predicts that the polarization of the scattered light can be restored for one specific pump polarization.
We demonstrate a simple and efficient technique that allows for a complete characterization of silica-based tapered optical fibers with sub-wavelength diameters ranging from 0.5 μm to 1.2 μm. The technique is based on Brillouin reflectometry using a single-ended heterodyne detection. It has a high precision sensitivity down to 1% owing to the strong dependence of the Brillouin spectrum on the taper diameter. We further investigate the tensile strain dependence of the Brillouin spectrum for an optical microfiber up to 5% of elongation. The results show strong dependences of several Brillouin resonances with different strain coefficients ranging from 290 MHz/% to 410 MHz/% with a specific nonlinear deviation at high strain. Those results therefore show that optical micro and nanofibers could find potential application for sensitive strain optical sensing.
Fabrication and characterization of submicron optical waveguides is one of the major challenges in modern photonics, as they find many applications from optical sensors to plasmonic devices. Here we report on a novel technique that allows for a complete and precise characterization of silica optical nanofibers. Our method relies on the Brillouin backscattering spectrum analysis that directly depends on the waveguide geometry. Our method was applied to several fiber tapers with diameter ranging from 500 nm to 3 μm. Results were compared to scanning electron microscopy (SEM) images and numerical simulations with very good agreement and similar sensitivity.
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