Infrared optical filters based in macroporous silicon for espectroscopic gas detection

Aplicat embargament des de la data de defensa fins el 31 de desembre de 2021Gas detection is of great importance in areas as diverse as industry, health or safety in domestic environments or public spaces, among others, and it is highly specific to each application. The detection method depends on factors such as the species of gas to be detected, concentration range, required resolution, sensitivity, specificity, response time, operating environment (temperature, humidity, interfering species, etc.), size and cost, among other considerations. Optical gas sensors are an attractive solution for gas detection. Most of them rely on molecular absorption and offer fast responses, minimal drift and are intrinsically reliable thanks to perform self-referenced measurements. Sensitivity and selectivity depend on the characteristics of the device. For example, laser-based gas sensors are highly selective with zero cross response to other gases and also with first-in-class sensitivity. The downside is that they are expensive. Non-dispersive infra-red (NDIR) sensors are a widespread alternative for cost-effective optical detection. They have inferior performances in terms of sensitivity and selectivity than laser-based sensors, but are two or three orders of magnitude less expensive. This thesis is dedicated to improving the selectivity and sensitivity of NDIR devices through the use of macroporous silicon technology. More specifically, it studies how photonic crystals manufactured by electrochemical etching can be used as narrow mid-infrared filters for gas detection purposes. That is, the photonic crystals are designed in such a way that only a small range of frequencies from an external source are transmitted while the surroundings are blocked. These filters are narrower than those available on the market and can be used to improve the selectivity and the sensitivity of NDIR devices as well as to reduce cross detection with other gases. In addition, the study shows how macroporous silicon photonic crystals can be heated to work as selective emitters. This can be used to reduce the complexity of the NDIR device while maintaining similar optical characteristics. Furthermore, it is proven that photonic molecules can be employed to perform dual detection in both transmission and emission, giving a new approach to self-referenced measurements. Conclusions of the work show that macroporous silicon technology is a versatile platform to provide solutions in the mid-infrared range for developing compact, sensitive and selective optical gas sensing.La detecció de gasos és de gran importància en àrees tan diverses com la indústria, la salut o la seguretat en entorns domèstics o espais públics, entre d'altres, i és altament específica per a cada aplicació. El mètode de detecció a utilitzar depèn de factors com ara el gas a detectar, el rang de concentració, la resolució requerida, la sensibilitat, l'especificitat, el temps de resposta, l'entorn operatiu (temperatura, humitat, espècies interferents, etc. .), la mida i el cost, entre altres consideracions. Els sensors òptics de gas són una solució atractiva per a la detecció de gas. La majoria d'ells es basen en l'absorció molecular i ofereixen respostes ràpides, deriva mínima i són intrínsecament fiables gràcies a la realització de mesures auto-referenciades. La sensibilitat i la selectivitat depenen de les característiques del dispositiu. Per exemple, els sensors de gas basats en tecnologia làser són altament selectius, no presenten resposta creuada a altres gasos i són altament sensibles. El desavantatge és que són cars. Els sensors d'infrarojos no dispersius (NDIR) són una alternativa molt estesa per a la detecció òptica de baix cost. Tenen un rendiment inferior en termes de sensibilitat i selectivitat que els sensors basats en làser, però són dos o tres ordres de magnitud més barats. Aquesta tesi està dedicada a millorar la selectivitat i la sensibilitat dels dispositius NDIR mitjançant la tecnologia de silici macroporós. Més específicament, estudia com els cristalls fotònics fabricats mitjançant el gravat electroquímic poden ser usats com a filtres estrets d'infraroig mitjà per a la detecció de gasos. És a dir, els cristalls fotònics estan dissenyats de tal manera que només un petit rang de freqüències d'una font externa es transmet mentre que els voltants estan bloquejats. Aquests filtres són més estrets que els disponibles en el mercat i poden utilitzar-se per millorar la selectivitat i la sensibilitat dels dispositius NDIR, així com per reduir la detecció creuada amb altres gasos. A més, l'estudi mostra com els cristalls fotònics de silici macroporós poden funcionar com a emissors selectius si són escalfats. Això pot ser usat per reduir la complexitat dels dispositius NDIR alhora que es mantenen característiques òptiques similars. A més, s'ha demostrat que les molècules fotòniques poden emprar-se per realitzar una detecció dual tant en la transmissió com en l'emissió, donant un nou enfocament a les mesures auto-referenciades. Les conclusions del treball mostren que la tecnologia de silici macroporós és una plataforma versàtil que proporciona solucions en el rang d'infraroig mitjà per al desenvolupament de sensors de gas òptics compactes, sensibles i selectius.Postprint (published version

Photonic molecules for improving the optical response of macroporous silicon photonic crystals for gas sensing purposes

In this paper, we report the benefits of working with photonic molecules in macroporous silicon photonic crystals. In particular, we theoretically and experimentally demonstrate that the optical properties of a resonant peak produced by a single photonic atom of 2.6 µm wide can be sequentially improved if a second and a third cavity of the same length are introduced in the structure. As a consequence of that, the base of the peak is reduced from 500 nm to 100 nm, while its amplitude remains constant, increasing its Q-factor from its initial value of 25 up to 175. In addition, the bandgap is enlarged almost twice and the noise within it is mostly eliminated. In this study we also provide a way of reducing the amplitude of one or two peaks, depending whether we are in the two- or three-cavity case, by modifying the length of the involved photonic molecules so that the remainder can be used to measure gas by spectroscopic methods.Postprint (published version

Study of resonant modes in a 700 nm pitch macroporous silicon photonic crystal

In this study the modes produced by a defect inserted in a macroporous silicon (MP) photonic crystal (PC) have been studied theoretical and experimentally. In particular, the transmitted and reflected spectra have been analyzed for variations in the defect’s length and width. The performed simulations show that the resonant frequency is more easily adjusted for the fabricated samples by length tuning rather than width. The optimum resonance peak results when centered in the PC bandgap. The changes in the defect geometry result in small variations of the optical response of the PC. The resonance frequency is most sensitive to length variations, while the mode linewidth shows greater change with the defect width variation. Several MPS photonic crystals were fabricated by the electrochemical etching (EE) process with optical response in the range of 5.8 µm to 6.5 µm. Results of the characterization are in good agreement with simulations. Further samples were fabricated consisting of ordered modulated pores with a pitch of 700 nm. This allowed to reduce the vertical periodicity and therefore to have the optical response in the range of 4.4 µm to 4.8 µm. To our knowledge, modes working in this range of wavelengths have not been previously reported in 3-d MPS structures. Experimental results match with simulations, showing a linear relationship between the defect’s length and working frequency inside the bandgap. We demonstrate the possibility of tailoring the resonance peak in both ranges of wavelengths, where the principal absorption lines of different gases in the mid infrared are placed. This makes these structures very promising for their application to compact gas sensors.Postprint (author's final draft

Transmission and thermal emission in the NO2 and CO absorption lines using macroporous silicon photonic crystals with 700 Nm pitch

Postprint (author's final draft

Controlling Plateau-Rayleigh instabilities during the reorganization of silicon macropores in the Silicon Millefeuille process

The reorganization through high-temperature annealing of closely-packed pore arrays can be exploited to create ultra-thin (<20 µm) monocrystalline silicon layers that can work as cheap and flexible substrates for both the electronic and the photovoltaic industries. By introducing a periodic diameter modulation along deep etched pores, many thin layers can be produced from a single substrate and in a single technological process. Besides the periodicity, the exact shape of the modulation also has a profound impact on the process and subtle profile changes can lead to important differences on the process outcome. In this paper we study both theoretically and experimentally the effect of the initial profile on the pore reorganization dynamics and the morphology of the thin layers obtained through annealing. We show that process reliability, annealing time and final layer characteristics, all can be engineered and optimized by precisely controlling the initial pore profile.Postprint (published version

Enhanced geometries of macroporous silicon photonic crystals for optical gas sensing applications

A macroporous silicon photonic crystal is designed and optimized theoretically for its use in gas sensing applications and IR optical filters. Light impinges perpendicularly onto the sample surface (vertical propagation) so a three-dimensional (3d) structure is used. For gas sensing, a sharp resonance is desired in order to isolate an absorption line of the gas of interest. The high Q-factors needed mandate the use of a plane defect inside the PhC to give rise to a resonant mode inside the bandgap tuned to the gas absorption line. Furthermore to allow gas passage through the device, an open membrane is required. This can affect the mechanical resilience. To improve the strength of the photonic crystal the pores are extended after the “active” 3d part. The number of modulations, and the extension length have been optimized to obtain the largest Q-factor with reasonable transmitted power. These proposed structures have been experimentally performed, probing an enhancement of almost an order of magnitude in the Q-factor in respect with the basic case. Simulations considering CO2 have been performed showing that the proposed structures are promising as precise optical gas sensors.Peer ReviewedPostprint (author's final draft

Impact of the absorption in transmittance and reflectance on macroporous silicon photonic crystals

The characteristics of reflection and transmission peaks in the spectra of photonic crystals have been studied theoretically and the results compared to measurements performed in fabricated samples. The aim of this work is to investigate the relation between material losses and the effective Q factors that can be obtained in photonic crystals made with it. Photonic crystals have been designed with defects of periodicity to introduce states in the band gap that give place to reflectance and transmittance peaks at adjustable specific wavelengths. The fabricated structures are described together with their reflection and transmission spectra. The influence of losses in the material in these spectra is evaluated.Postprint (published version

Macroporous silicon filters, a versatile platform for NDIR spectroscopic gas sensing in the MIR

© The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution 4.0 License (CC BY,http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in anymedium, provided the original work is properly cited.This paper describes the spectroscopic detection of gases using macroporous silicon photonic crystals as narrow filters. The study begins by demonstrating the feasibility of photoelectrochemical etching to fabricate narrow filters along the mid infrared band. Next, we focus on the filter centered on the carbon dioxide fingerprint. The filter response is described for three different cell lengths and concentrations below 1%. Results show a concordance with the reformulated Beer-Lambert law. This can be used to predict the response of the filter for longer path lengths and higher concentrations, showing broad working ranges and compact sizes for CO2. In addition, optical robustness to external variations and long-term stability are also reported. Results are extrapolated to other macroporous silicon filters centered on the absorption spectra of N2O, OCS, NO2 and SO2. Finally, high sensitivity and selectivity is demonstrated by comparing them with some commercial filters.Postprint (published version

Empirical demonstration of CO2 detection using macroporous silicon photonic crystals as selective thermal emitters

This study describes the detection of CO2 using macroporous silicon photonic crystals as thermal emitters. It demonstrates that the reduction of structural nonhomogeneities leads to an improvement of the photonic crystals’ emission. Narrow emission bands (¿~120) located within the R-branch of carbon dioxide were achieved. Measurements were made using a deuterated triglycine sulfate photodetector and the photonic crystals, heated to 400°C, as selective emitters. A gas cell with a CO2 concentration between 0 ppm and 10,000 ppm was installed in the center. Results show high sensibility and selectivity that could be used in current nondispersive infrared devices for improving their features. These results open the door to narrowband emission in the mid-infrared for spectroscopic gas detection.Postprint (author's final draft

Bandgap widening in macroporous silicon photonic crystals by multiperiodic structures

Large bandgaps with low transmission in 3D macroporous silicon photonic crystals have been proved as an interesting technology for the development of optical filters and spectroscopic MIR gas sensors. The aim of this study is the investigation of different bandgap widening methods based on multiperiodic structures for 3D macroporous silicon photonic crystals. To do so, chirped modulations and structures with different periodicity groups have been modelled and theoretically analysed by means of 3D FDTD simulations. They have revealed that by using different decreasing periodicity groups, bandgaps with null transmission and widths as high as 1800 nm, 4 times the original single periodicity photonic crystal bandgap, can be obtained. Furthermore, it has been shown that a resonant cavity with a 20% transmission can be placed in a 1 µm wide bandgap. The results open a way to use this type of structures not only for gas sensing but also for other purposes such as wide stop-band filters, selective filters or broadband mirrors.Peer ReviewedPostprint (published version