Giant absorption of light by molecular vibrations on a chip

Overtone Spectroscopy for Sensing─Recent Advances and Perspectives

In this perspective, we report an opinion on overtone spectroscopy for sensing and discuss the nature of the opportunities perceived for specialists. New spectroscopic strategies can potentially be extended to detect other common toxic byproducts in a chip-scale label-free manner and to enhance the functionality of chemical and biological monitoring. Nevertheless, the full potential of overtone spectroscopy is not yet exhausted, challenges must be overcome, and new avenues await. Within this Perspective, we look at where the field currently stands, highlight several successful examples of overtone spectroscopy based sensors and detectors, and ask what it will take to advance current state-of-the-art technology. It is our intention to point out some potential blind spots and to inspire further developments.

Fingerprint detection in the mid-infrared region based on guided-mode resonance and phonon-polariton coupling of analyte

Mid-infrared absorption spectroscopy is an effective method for detecting analyte fingerprints without labeling, but the inherent loss of metals in current methods is a main issue. Here, a sensing scheme was proposed that uses an all-dielectric grating metasurface and angular scanning of polarized light, and then it was verified by numerical simulation. The proposed fingerprint detection scheme could effectively couple a guided-mode resonance spectrum peak with the characteristic peak of the analyte's phonon-polariton in the mid-infrared region, significantly enhancing the interaction between light and the analyte. The novel scheme would realize broadband enhancement to detect a variety of substances, and facilitate mid-infrared sensing and analysis of trace substances.

Dry and Wet Mechanochemical Synthesis of Piroxicam and Saccharin Co-Crystals and Evaluation by Powder X-Ray Diffraction, Thermal Analysis and Mid- and Near- Infrared Spectroscopy

The purpose of this study is to investigate the effects of dry and wet mechanochemical synthesis on piroxicam (PX) and saccharin (SA) mixtures. For this purpose, PX and SA mixtures prepared by wet mechanochemical processes using three solvents and by dry mechanochemical synthesis were evaluated by mid-and near-infrared spectroscopy, powder X-ray diffraction (PXRD), and differential scanning calorimetry (DSC). The mixtures of wet-type products were transformed into PX/SA 1:1 co-crystals. The effect of the solvent was key to the co-crystallization of PX and SA. The products from the dry process were transformed into the amorphous phase. For the sample of the amorphous mixture, two exothermic peaks due to crystallization were observed in the thermal analysis. Bulk PX was ground for the same number of times for transformation, but was not successfully transformed to the amorphous bulk; the same was observed for SA. It is suggested that the mutual existence of PX and SA promotes mutual amorphization.

Towards a Glass New World: The Role of Ion-Exchange in Modern Technology

Glasses, in their different forms and compositions, have special properties that are not found in other materials. The combination of transparency and hardness at room temperature, combined with a suitable mechanical strength and excellent chemical durability, makes this material indispensable for many applications in different technological fields (as, for instance, the optical fibres which constitute the physical carrier for high-speed communication networks as well as the transducer for a wide range of high-performance sensors). For its part, ion-exchange from molten salts is a well-established, low-cost technology capable of modifying the chemical-physical properties of glass. The synergy between ion-exchange and glass has always been a happy marriage, from its ancient historical background for the realisation of wonderful artefacts, to the discovery of novel and fascinating solutions for modern technology (e.g., integrated optics). Getting inspiration from some hot topics related to the application context of this technique, the goal of this critical review is to show how ion-exchange in glass, far from being an obsolete process, can still have an important impact in everyday life, both at a merely commercial level as well as at that of frontier research.

Broadband transparency with all-dielectric metasurfaces engraved on silicon waveguide facets: effect of inverted and extruded features based on Babinet's principle

Building blocks of photonic integrated circuitry (PIC), optical waveguides, have long been considered transparent. However, the inevitable Fresnel reflection from waveguide facets limits their transparency. This limitation becomes more severe in high-index waveguides in which the transparency may drop to 65%. We overcome this inherent optical property of high-index waveguides by engineering an appropriate facet landscape made of sub-wavelength artificial features unit cells. For this, we develop a semi-analytical formalism for predicting the metasurface parameters made of high-index dielectric materials, to be engraved on the facets of optical waveguides, based on Babinet's principle: either extruded from the waveguide facet or etched into it. Our semi-analytical model predicts the shape of anti-reflective metasurface unit cells to achieve transmission as high as 98.5% in near-infrared from 1 μm to 2 μm. This new class of metasurfaces may be used for the improvement of PIC devices for communication and sensing, where device transparency is crucial for high signal-to-noise ratios.

Lattice Rayleigh Anomaly Associated Enhancement of NH and CH Stretching Modes on Gold Metasurfaces for Overtone Detection

Molecular overtones stretching modes that occupy the near-infrared (NIR) are weak compared to the fundamental vibrations. Here we report on the enhancement of absorption by molecular vibrations overtones via electromagnetic field enhancement of plasmonic nanoparallelepipeds comprising a square lattice. We explore numerically, using finite element method (FEM), gold metasurfaces on a transparent dielectric substrate covered by weakly absorbing analyte supporting N-H and C-H overtone absorption bands around 1.5 μ m and around 1.67 μ m, respectively. We found that the absorption enhancement in N-H overtone transition can be increased up to the factor of 22.5 due to the combination of localized surface plasmon resonance in prolonged nanoparticles and lattice Rayleigh anomaly. Our approach may be extended for sensitive identification of other functional group overtone transitions in the near-infrared spectral range.

Deflected Talbot-Mediated Overtone Spectroscopy in Near-Infrared as a Label-Free Sensor on a Chip

Rapid, sensitive, and reliable detection of aromatic amines, toxic manufacturing byproducts, has been previously achieved with molecular vibrations in the mid-infrared (Mid-IR) region. However, Mid-IR spectroscopic tools are hampered by a need to prepare the samples and the sensor cost. Here, we develop an affordable label-free sensor on a chip, operating in near-infrared (NIR) for ultrasensitive detection of absorption line signatures based on molecular vibrations overtones of the aromatic amine N-methylaniline probe molecule. We design a perforated silicon rib waveguide and fabricate it by milling cylindrical inclusions through the waveguide core. The molecular signatures were monitored when waveguides are embedded in toxic N-methylaniline, experiencing a deflected Talbot effect. We observed that when the Talbot effect is deflected, the absorption lines in NIR are enhanced despite the weakly absorbing nature of the probe molecules. This new spectroscopic strategy can potentially be extended to detect other common toxic byproducts in a chip-scale label-free manner and to enhance the functionality of chemical monitoring.

Differential extinction of vibrational molecular overtone transitions with gold nanorods and its role in surface enhanced near-IR absorption (SENIRA)

Resonant coupling between plasmonic nanoantennas and molecular vibrational excitations is employed to amplify the weak overtone transitions that reside in the near-infrared. We explore for the first time the differential extinction of forbidden molecular overtone transitions coupled to the localized surface plasmons. We show a non-trivial interplay between the molecular absorption enhancement and suppression of plasmonic absorption in a coupled system. When the resonance conditions are met at 1.5 μm, two orders of magnitude enhancement of differential extinction as compared to the extinction of the same amount of free probe molecules is achieved. Our results pave a road toward a new class of surface enhanced near-infrared absorption-based sensors.

Spatial eigenmodes conversion with metasurfaces engraved in silicon ridge waveguides

We explore the discrete nature of waveguide modes and the effective medium concept to achieve an ultra-compact highly efficient mode conversion device in a high-index platform such as a silicon waveguide. The proposed device is based on a co-directional coupler that has a periodic variation in its refractive index along the propagation direction. The transverse variation of the index profile is calculated based on the interference pattern between the modes of interest. We show that mode conversion can be realized with dielectric metasurfaces engraved in the silicon waveguide. We derive the equation for effective index and show proof-of-concept numerical results of the device performance. We obtain conversion efficiencies of 95.4% between the TE₀-TE₁ modes over 8.91 μm interaction distance and 96.4% between the TE₀-TE₂ over 6.32 μm. The resulting coupling coefficient changes as a function of the interaction distance in a sinusoidal manner, which is crucial for constructive energy transfer from one mode to another. Such mode coupling devices have the potential for application in dispersion compensations, wavelength division multiplexing systems, and sensing.

Si Nanostrip Optical Waveguide for On-Chip Broadband Molecular Overtone Spectroscopy in Near-Infrared

The ability to probe the molecular fundamental or overtone (high harmonics) vibrations is fundamental to modern healthcare monitoring techniques and sensing technologies since it provides information about the molecular structure. However, since the absorption cross section of molecular vibration overtones is much smaller compared to the absorption cross section of fundamental vibrations, their detection is challenging. Here, a silicon nanostrip rib waveguide structure is proposed for label-free on-chip overtone spectroscopy in near-infrared (NIR). Utilizing the large refractive index contrast (Δ n > 2) between the silicon core of the waveguide and the silica substrate, a broadband NIR lightwave can be efficiently guided. We show that the sensitivity for chemical detection is increased by more than 3 orders of magnitude when compared to the evanescent-wave sensing predicted by the numerical model. This spectrometer distinguished several common organic liquids such as N-methylaniline and aniline precisely without any surface modification to the waveguide through the waveguide scanning over the absorption dips in the NIR transmission spectra. Planar NIR Si nanostrip waveguide is a compact sensor that can provide a platform for accurate chemical detection. Our NIR Si nanostrip rib waveguide device can enable the development of sensors for remote, on-site monitoring of chemicals.

Invisibility Cloaking Scheme by Evanescent Fields Distortion on Composite Plasmonic Waveguides with Si Nano-Spacer

A new, composite plasmonic waveguide based electromagnetic cloaking scheme is proposed with Si nano-spacer. Here we show, that the scattering fields of an object located on the cloak do not interact with the evanescent field, resulting in object's invisibility. Finite difference time domain (FDTD) numerical calculations were performed to extract the modal distributions and surface intensities on a composite plasmonic waveguide with a metasurface overlayer. Spatially varying effective permittivity was analytically calculated using transformation optics. Cloaking was demonstrated for a cylindrical object with diameter of 70% from the waveguide width on a high index ridge waveguide structure with silicon nitride guiding layer on silica substrate. Our results open the door to new integrated photonic devices, harnessing from evanescent fields distortion on composite plasmonic waveguides and dielectric nano-spacers for the variety of applications from on-chip optical devices to all-optical processing.