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.
The work that I will present focuses on the fabrication of non-perturbing E-field sensors based on the electro-optic effect. Lithium Niobate, combined with photonic crystals can increase considerably the material sensitivity to electric fields leading to ultra-compact devices. The target structure exhibits high sensitivity, THz bandwidth ans micrometric spatial resolution. In addition, since the sensor is only fabricated with dielectric materials, it does not perturb the electric field to be measured.
In my presentation I will focus on the simulation, fabrication and characterization of the fiber-tip electric field sensor. We have performed simulations in order to study the feasibility where the different fabrication errors are considered.The fabrication is divided into two parts: a first one where we overcome the problem of micromachining photonic crystals on thin film lithium niobate of 700 nm of thickness. In the second part, the integration of the photonic crystal within the fiber facet will be explained.
Optical characterization and electrical performance will be shown verifying its different features such as spatial resolution, linearity, electrical sensitivity and bandwidth. The fabricated device shows performances nevev achieved beofre and open up a high spectrum of applications like cold plasma, military, and telelcommunications applications.
Isotropic sillenite crystals such as Bismuth Silicon Oxide (BSO) present highly interesting opto-electronics properties including electro-optic effect and photorefractivity. BSO is also a highly suitable candidate for sensitive temperature-independent electric field sensors [1]. Then the production of low cost BSO-based optical-waveguides is becoming a major challenge. However, BSO high density (> 7 g.cm3) and non-standard dimensions are a hurdle for standard fabrication approaches such as ion diffusion or exchange and standard clean-room technologies.
Here we report for the first time the successful fabrication of low loss BSO ridge waveguides with high index contrast. The proposed technique is based on optical-grade dicing [2], which allows low cost and massive production of photonic devices in different types of material. Ridge waveguides are made in a 15-µm thick chemical mechanical polished thin layer of BSO bonded on a lithium-niobate wafer. Propagation losses, group velocity and modal birefringence of optical modes have been measured by Optical-Coherence-Tomography. The waveguides support both TE and TM guided modes at telecom wavelength (1.55 µm) and present propagation losses lower than 2 dB/cm. This approach promises to be powerful for shaping single crystal thin films even in exotic formats. We expect low loss optical-waveguide in BSO will pave the way toward compact and highly sensitive electric-field sensors, scintillators, LED and laser applications.
[1] I. Saniour et al, NMR in Biomedicine,31, (2018).
[2] N. Courjal et al, Journal of Physics D: Applied Physics, 305101,(2011).
Lithium niobate (LiN bO3) microresonators have attracted much interest over the last decade, due to the electrooptical, acousto-optic and non-linear properties of the material, that can advantageously be employed in combination with thin resonances of optical microcavities for applications as varied as integrated gyrometers, spectrometers or dynamic filters. However the integration of micrometer scale cavities with an input/output waveguide is still a critical issue. Here we propose an innovative approach, allowing low insertion losses and easy pigtailing with SMF fibers. The approach consists in producing and optimizing separately a membrane-based LiNbO3 waveguide with Spot-Size Converters, and a thin microdisk. The two elements are dynamically assembled and fixed in a second step. Additionally to the proposed integrated microresonator, this approach opens the way to the production of 3D hybrid photonic systems.
BSWs are non-radiative electromagnetic waves confined at the interface between a truncated periodic dielectric multilayer and a surrounding media. As an alternative to SPPs (Surface Plasmon Polaritons), BSWs show dramatically enhanced propagation lengths up to several millimeters range and provide new optical opportunities such as the possibility to obtain TE or TM-polarized surface waves. They have found numerous applications in vapor sensing, biosensing, fluorescence detection and imaging, and integrated optics.
In this work, we propose a 1DPhC with a thin film of LiNbO3 (TFLN) as the top layer of the multilayer structure. The bonding of LiNbO3 into the 1DPhC structure brings anisotropy and nonlinear properties into the whole crystal allowing the tunability of the BSW devices.
Here we present 1DPhCs, which are able to sustain surface waves at the LiNbO3/air interface. Two different geometries have been studied, fabricated and optically characterized. The first one is based on the LiNbO3 membrane suspended in air and the second one is held by a stable glass platform.
The multilayer of the membrane based crystal is as following: air/6 pairs of Si3N4(200nm) and SiO2(215nm)/TFLN(1.1μm) – polished from bulk LN/air. The multilayer of the glass supported crystal is as following: glass/UV glue/6 pairs of Si3N4(220 nm) and SiO2(490nm)/TFLN(386nm)/air. 1DPhCs were characterized in Kretschmann configuration at visible and IR wavelengths.
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.
Bloch surface waves (BSWs) are electromagnetic surface waves which can be excited at the interface between periodic dielectric multilayer and a surrounding medium. In comparison with surface plasmon polaritons these surface states perform high quality factor due to low loss characteristics of dielectric materials and can be exited both by TE and TM polarized light. A platform consisting of periodic stacks of alternative SiO2 and Si3N4 layers is designed and fabricated to work at the wavelength of 1.55 µm. The platform has an application in sensing and in integrated optics domain. A standard way of BSW excitation is coupling via Kretschmann configuration, but in this work we investigate a grating coupling of BSWs. Grating parameters are analytically and numerically optimized by RCWA and FDTD methods in order to obtain the best coupling conditions. The light is launched orthogonally to the surface of the photonic crystal and the grating. Due to a special grating configuration we demonstrate directionality of the BSW propagation depending on polarization of the incident light. The structure was experimentally realized on the surface of the photonic crystal by FIB milling. Experimental results are in a good agreement with a theory. The investigated configuration can be successfully used as a BSW launcher in on-chip all-optical integrated systems and work as a surface wave switch or modulator.
We present easy-to-implement technologies to produce LiNbO3 PhCs in confined optical waveguides. Ti-indiffusion or
Annealed Proton Exchange (APE) are combined with optical grade dicing to fabricate ridge waveguides with
propagation losses that can be lower than 0.2 dB/cm. Firstly we show how a PhC inscribed in a confined ridge
waveguide can be exploited as a temperature sensor with an unexpectedly high 8 nm/°C temperature sensitivity. LiNbO3
PhCs with high aspect ratio are also demonstrated. The performance is achieved by properly tilting the ridge before
patterning its walls by Focused Ion Beam (FIB). A eight micrometer long 1D-PhC on a Ti:LiNbO3 ridge waveguide has
been fabricated and its reflectivity has been evaluated using an optical coherence tomography (OCT) system: it is
measured to be 53 % for the TM wave and 47 % for the TE wave. The period can be optimized in order to increase the
reflection of the 1D-PhC up to 80 %. These developments open the way to the dense integration of compact dynamic
devices such as modulators, spectral filters or electric field sensors.
KEYWORDS: Photonic crystals, Lithium niobate, Waveguides, Thin films, Sensors, Electro optics, Thin film devices, Crystals, Electric field sensors, 3D modeling
We report on photonic crystal electro-optic devices formed in engineered thin film lithium niobate (TFLN™) substrates.
Photonic crystal devices previously formed in bulk diffused lithium niobate waveguides have been limited in performance by the depth and aspect ratio of the photonic crystal features. We have overcome this limitation by implementing enhanced etching processes in combination with bulk thin film layer transfer techniques. Photonic crystal
lattices have been formed that consist of hexagonal or square arrays of holes. Various device configurations have been
explored, including Fabry Perot resonators with integrated photonic crystal mirrors and coupled resonator structures. Both theoretical and experimental efforts have shown that device optical performance hinges on the fidelity and sidewall profiles of the etched photonic crystal lattice features. With this technology, very compact photonic crystal sensors on the order of 10 μm x 10 μm in size have been fabricated that have comparable performance to a conventional 2 cm long bulk substrate device. The photonic crystal device technology will have broad application as a compact and minimally invasive probe for sensing any of a multitude of physical parameters, including electrical, radiation, thermal and chemical.
We report two novel kinds of LiNbO3 electro-optic modulators. The first one is oriented toward long haul high bit rate telecommunication systems. An original single-ended structure with a poled section and phase reversal electrodes is proposed to prevent the intensity modulation from chirp, without sacrifice on the driving voltage. We also show that improvements can be performed with the use of several poled sections. To remain attracting, LiNbO3 modulators should also exhibit a lower size. The second configuration described here is a new generation of LiNbO3 modulators based on photonic crystals, with a micrometric active length. We theoretically show that the optimal photonic structures for an efficient electro-optical tuning are based on a triangular array of holes integrated on a X-cut substrate. The first optical characterizations confirm the theoretical predictions, and exhibit a -12dB extinction ratio in the transmission response.
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