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A novel platform based on evanescent wave sensing in the 6.5 to 7.5 µm wavelength range is presented with the example of toluene detection in an aqueous solution. The overall sensing platform consists of a germanium-on-silicon waveguide with a functionalized mesoporous silica cladding and integrated microlenses for alignment-tolerant back-side optical interfacing with a tunable laser spectrometer. Hydrophobic functionalization of the mesoporous cladding allows enrichment of apolar analyte molecules and prevents strong interaction of water with the evanescent wave. The sensing performance was evaluated for aqueous toluene standards resulting in a limit of detection of 7 ppm. Recorded adsorption/desorption profiles followed Freundlich adsorption isotherms with rapid equilibration and resulting sensor response times of a few seconds. This indicates that continuous monitoring of contaminants in water is possible. A significant increase in LOD can be expected by likely improvements to the spectrometer noise floor which, expressed as a relative standard deviation of 100% lines, is currently in the range of 10−2A.Ueng
Sensitivity of evanescent wave sensing of gaseous species can be vastly increased by enrichment materials that locally concentrate the analyte on the sensor. Here, we investigate functionalized mesoporous silica films as versatile enrichment layer for sensing volatile organic compounds (VOCs) from gas-phase. Attenuated total reflection (ATR) crystals were coated with silica films of different pore sizes and their capability to enrich three different aromatic hydrocarbons from a vapor stream was studied by means of Fourier Transform infrared (FTIR) spectroscopy. Thereby, single-digit ppmv limits of detection (LOD) were achieved with an effective path length of only 6.3 μm. The selectivity introduced by the functionalization of the silica films effectively minimized interferences of water vapor, which gave access to the spectral fingerprint region between 1550 and 1450 cm−1. This allowed to discriminate and quantify toluene, p-xylene and 1,2,4-trimethylbenzene in multicomponent mixtures at high humidity. Fast response and regeneration times and enrichment factors up to 32 000 showcase the high potential of this material for evanescent wave sensing.
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The local structure of water on chemically and structurally different surfaces is a subject of ongoing research. In particular, confined spaces as found in mesoporous silica have a pronounced effect on the interplay between the adsorbate–adsorbate and adsorbate–surface interactions. Mid-infrared spectroscopy is ideally suited to quantitatively and qualitatively study such systems as the probed molecular vibrations are highly sensitive to intermolecular interactions. Here, the quantity and structure of water adsorbed from the gas phase into silica mesopores at different water vapor pressures was monitored using mid-infrared attenuated total reflection (ATR) spectroscopy. Germanium ATR crystals were coated with different mesoporous silica films prepared by evaporation-induced self-assembly. Quantitative analysis of the water bending vibration at 1640 cm–1 at varying vapor pressure allows for retrieving porosity and pore size distribution of the mesoporous films. The results were in excellent agreement with those obtained from ellipsometric porosimetry. In addition, different degrees of hydrogen bonding of water as reflected in the band position and shape of the stretching vibrations (3000–3800 cm–1) were analyzed and attributed to high-density, unordered bulk, low-density, and surface-induced ordered water. Thereby, the progression of surface-induced ordered water and bulk water as a function of water vapor pressure was studied for different pore sizes. Small pores of 5 nm diameter showed a number of two-ordered monolayers, whereas for pores >12 nm diameter, the number of ordered monolayers is significantly larger and agrees with the number observed on planar SiO2 surfaces.
Laser integration and photonics chip packaging are the two key challenges that require attention to drive down
the cost/bit metric for silicon photonics based optical interconnects. We try to address the latter by demonstrating optical interfaces that fit well in an overall scheme of 2.5D/3D electro-optic integration needed for a high
performance computing environment.
A through-substrate coupling interface provides the benefit of bonding a silicon photonic chip face-up on a
package substrate such that the device-side of the chip remains accessible for die-stacking and fiber-array packaging, thereby offering a promising alternative to flip-chip based packaging. In this paper, we demonstrate
three through-substrate coupling elements to enable alignment tolerant and energy-efficient integration of silicon
photonics with board-level or package-level optical interconnects : (i) a downward directionality O-band grating coupler with a peak -2.3 dB fiber-to-silicon waveguide coupling efficiency; (ii) polymer microlenses hybrid
integrated onto the substrate of a silicon photonic chip to produce an expanded collimated beam at λ=1310
nm for a distance of more than 600 µm; (iii) a ball lens placed in a through-package via to result in a 14 µm
chip-to-package 1-dB lateral alignment tolerance for coupling into a 20×24 µm squared cross-section board-level
Microlenses integrated on the substrate-side can greatly relax alignment tolerances for interfacing photonic integrated circuits. This is demonstrated on a 750um thick chip with standard grating couplers (operation wavelength 1550nm). High quality silicon microlenses (surface roughness below 20 nm) were realized by transfering reflowed photoresist into the silicon substrate using reactive ion etching. The microlens allows interfacing the chip from the backside with an expanded beam, drastically increasing lateral alignment tolerances. A 1dB alignment tolerance of ±8um and ±11um (along and perpendicular to the grating coupler direction, respectively) was experimentally found when a 40um mode field diameter beam was used.
Adsorption of molecules on high-surface-area materials is a fundamental process critical to many fields of basic and applied chemical research; for instance, it is among the simplest and most efficient principles for separating and remediating polluted water. However, established experimental approaches for investigating this fundamental process preclude in situ monitoring and thus obtaining real-time information about the ongoing processes. In this work, mid-infrared attenuated total reflection (ATR) spectroscopy is introduced as a powerful technique for quantitative in situ monitoring of adsorption processes and thus enrichment of traces of organic pollutants from aqueous solution in ordered mesoporous silica films. The synthesis, functionalization, and characterization of two silica films with 3D hexagonal and cubic pore structure on silicon ATR crystals are presented. Benzonitrile and valeronitrile as model compounds for aromatic and aliphatic water pollutants are enriched in hydrophobic films, while the matrix, water, is excluded from the volume probed by the evanescent field. Enrichment times of 200 and >100, respectively, yielding detection limits in the low ppm range. Furthermore, fast and complete desorption of the analyte, ensuring reliable regeneration of the sensor, was verified. Lastly, we derive and experimentally validate equations for ATR spectroscopy with thin film adsorption layers to quantify the absolute mass of adsorbed pollutant in the film. The excellent agreement between recorded absorptions at target wavenumbers of the target analytes and corresponding simulations corroborates the validity of the chosen approach