Onde évanescente | EPFL Graph Search

L'onde plane évanescente est une onde plane particulière dont l'amplitude varie exponentiellement dans une direction orthogonale à sa direction de propagation. Expression théorique En supposant l'onde monochromatique, on peut en donner une expression mathématique utilisant la notation exponentielle : Le vecteur d'onde \mathcal{\vec k} est un vecteur complexe. Sa partie réelle, \vec k{'}, et sa partie imaginaire \vec k{'}{'}, sont deux vecteurs orthogonaux. La direction de propagation de l'onde est celle de la partie réelle. L'amplitude de l'onde évanescente tend exponentiellement vers zéro dans la direction donnée par la partie imaginaire. L'ondes plane n'existe que dans le monde mathématiques, puisque son extension est infinie. Une Difficulté supplémentaire de l'onde évanescente est son amplitude qui tend vers l'infini dans une direction. Ces ondes peuvent se présenter comme solutions d'équation de propagation pour de

Contactless optical trapping and manipulation of nanoparticles utilizing SIBA mechanism and EDL force

Mahdi Sahafi

On-chip optical tweezers based on evanescent fields overcome the diffraction limit of the free-space optical tweezers and can be a promising technique for developing lab-on-a-chip devices. While such trapping allows for low-cost and precise manipulation, it suffers from unavoidable contact with the device surface, which eliminates one of the major advantages of the optical trapping. Here, we use a 1D photonic crystal cavity to trap nanoparticles and propose a novel method to control and manipulate the particle distance from the cavity utilizing a self-induced back-action (SIBA) mechanism and electrical-double-layer (EDL) force. It is numerically shown that a 200 nm radius silica particle can be trapped near the cavity with a potential well deeper than 178k(B)T by 1 mW of input power without any contact with the surface and easily moved vertically with nanometer precision by wavelength detuning. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

2019

Evanescent wave gas sensing based on modulation spectroscopy

Daniel Salzmann

In recent years, optical gas detectors based on a tunable diode laser absorption spectrometer has become more and more important for monitoring trace gases in atmospheric, medical, and industrial processes. In the medical field, blood gases, essentially pO2 and pCO2, are vital parameters for patient respiratory status monitoring. Typically, hypoxaemia or hypercapnia can be very harmful, especially for neonates, and requires continuous monitoring for rapid clinical intervention. Transcutaneous blood gases, permeating through the skin, can be continuously measured in patients by electrochemical sensors applied to the surface of the skin. These sensors suffer specific instability and must be re-calibrated. For this reason, the present thesis explores a new concept of gas sensors, applying light absorption at the surface of an integrated optics waveguide and using the advantages of a modulated spectroscopy technique for sensitivity and selectivity enhancement. In other words, the present thesis sets a mathematical model and demonstrates experimentally a new concept of evanescent wave gas sensing based on modulation spectroscopy. The field of application considered in this book is restricted to gas detection, however the concept can also be applied to the detection of liquids. In particular, we demonstrate the potential applicability of the concept to transcutaneous blood gas monitoring, and especially carbon dioxide monitoring. The system transfer function modelization of the concept includes carbon dioxide absorption spectrum in the near-infrared region (1.6µm), laser diode modulation spectroscopy assuming optical frequency modulation (FM) as well as intensity modulation (IM) induced by current modulation, and integrated optics sensor-head transmission. The modelization evidences that the silicon nitride technology has the advantage of: high evanescent wave sensitivity (8% for the selected TE mode), low propagation loss, possibility of strong curvatures for miniaturization and availability of fiberto- waveguide mode matching technique (taper). In the modelization, we evidence that the optimal waveguide length equals the inverse of the propagation loss which is mainly limited by the waveguide scattering. Also, we demonstrate that the implementation of a fiber-to-waveguide mode matching taper at both waveguide ends, drastically decreases the resonance limitation of the system resolution. Experimentally, the transfer function of the concept is validated and characterized based on a free-space modulation spectroscopy setup whose performance, in terms of second harmonic resolution, is first measured at 3.55 · 10-6 1/√Hz corresponding to 0.275 mmHg/√Hz CO2 partial pressure. In the chosen region of 200-400MHz of frequency modulation, we confirm that the 1/f-noise is negligible and the dominating amplifier noise can be approximated by the thermal noise. In the case of high insertion loss and short interaction length the interferometric noise (or etalon-effect) becomes the system resolution limitation. By the insertion of a prism coupled planar waveguide instead of the free-space cell, we demonstrate the concept of integrated optics evanescent wave sensing based on modulation spectroscopy. The system characterization is completed by the validation of the evanescent wave sensitivity in gas of about 8% for a laterally confined waveguide. The outlook of a transcutaneous blood gas monitoring instrument is also established. We first extrapolate the expected waveguide characteristics to show that a resolution of 1mmHg CO2 partial pressure can be reached in 4.5 to 12sec response-time. Then, the requirement of a maximal sensing area of 10mm diameter is at the origin of two geometries of sensor-head: a crossing spiral and a non-crossing double spiral. We verify experimentally a propagation loss as low as 0.5dB/cm and observe a stringent tolerance for light injection. The latter requires an integrated device for mode matching and pigtailing. After implementation of a taper at both waveguide ends, we demonstrate the drastic reduction of the total coupling loss. The carbon dioxide measurements reveal a relatively deep pattern on the second harmonic signal that tends to prove that polarization and resonances are coupled together most likely at the waveguide level. Applying nitrogen (N2) as reference gas, the pattern compensation results in the carbon dioxide signature measurement in direct detection only. The next generation, now in process, is a patented pigtailed non-crossing double-spiral sensor-head building the first step towards an industrial prototype. The waveguide miniaturization allows an interaction length longer than 21cm on a 3×3mm area. The improvement towards a high resolution system concerns the propagation loss and the coupling loss on the sensor-head, and concerns the polarization control or scrambling on the modulation spectroscopy setup. We finally conclude, following the demonstration on the planar waveguide and the extrapolation of the measured characteristics, that the concept of evanescent wave gas sensing based on modulation spectroscopy, can compensate the lack of sensitivity of an integrated optics sensor-head and can be applied in gas and liquid sensing. Moreover, based on the sensor-head developments and realistic improvements, we also conclude that the perspective of high resolution sensing system, and in particular for carbon dioxide determination, evidences the potential applicability of the concept to transcutaneous blood gas monitoring. This opens a new optical outlook of continuous and non-invasive blood gas monitoring, applied in medicine for patient respiratory status monitoring.

EPFL2003

Design, Fabrication and Characterization of Mid-Infrared Strip Waveguide for Laser Spectroscopy in Liquid Environments

Yu-Chi Chang

Optical sensors based on the interaction between molecular vibrations and mid infrared radiations attracted high attentions in the last decades. In this study, we present the development of a waveguide evanescent field sensor based on the absorption spectroscopy in liquid environments. The device consists of a single mode strip waveguide integrated with a microfluidic system. With a quantum cascade laser at the wavelength of 5.8 μm, we demonstrate the detection of cocaine dissolved in tetrachloroethylene (PCE). Mid infrared germanium waveguides on silicon have been proposed but not experimentally demonstrated. Here we realize this new type of waveguide with a monocrystalline germanium layer on a silicon substrate (Ge-on-Si) with standard photolithography and reactive ion etching. It is designed to be singlemode in transverse magnetic polarization at 5.8 μm. We used a method based on the Fabry-Perot resonance to measure the propagation loss, and the achieved lowest loss is 2.4 dB/cm. The bending loss was measured to be 0.12 dB per 90∘bend with the radius of 115 μm. A microfluidic chip made from a UV-curable adhesive is integrated to the Ge-on- Si waveguide. This is the first integration of this waveguide with a microfluidic system. Solutions of cocaine in PCE flow in the microfluidic channel and absorb the light in the evanescent field. A reference waveguide and an integrated Y- junction eliminate the noise from coupling fluctuations, and the limit of detection is calculated to be 5 μg/ml. Mid infrared photonics can be further developed based on the platform of Ge-on- Si waveguides. We demonstrate a detection of refractive index changes with a Mach-Zehnder interferometer integrated with a microfluidic system. Subwavelength structures for antireflection of the waveguide facets are also experimentally tested. We foresee an increasing development and applications based on this waveguide in the mid infrared.

EPFL2012