Preparation of synthetic micro- and nano plastics for method validation studies.

Microplastic (MP) pollution is a persisting global problem. Accurate analysis is essential in quantifying the effects of microplastic pollution and develop novel technologies that reliably and reproducibly measure microplastic content in various samples. The most common methods for this are FTIR and Raman spectroscopy. Coloured, standardized beads are often used for method validation tests, which limits the conclusions to a very specific case rarely observed in the natural environment. This study focuses on the preparation of reference micro- and nanoplastics via cryogenic milling and shows their use for FTIR and Raman method validation studies. MPs can now be reproducibly milled from various plastics, offering the advantages of a better representation of MPs in real environment. Moreover, this study highlights issues with the current detection methods, up to now considered as the most reliable ones for MP detection and identification. Such issues, e.g. misidentification, will need to be addressed in the future. Additionally, milled MPs were used in experiments with commercial high-resolution imaging device, enabling a possible in-situ optical detection of microplastics. These experiments represent a step forward in understanding MPs in a water sample and provide a basis for a more accurate detection and identification directly from water, which would considerably reduce the time of analysis.

Strip-loaded Mach-Zehnder interferometer for absolute refractive index sensing.

This paper presents the design, fabrication, and characterization of a Mach-Zehnder interferometer (MZI) on a strip-loaded platform specifically developed for the measurement of refractive index variations in liquids. A novel approach of large opening in the sensing region enhances the sensitivity by using the analyte as the loading strip. This allows surpassing the performances of the conventional perforations and trenched-based MZIs and prevent protecting the reference arm. The implementation of this design resulted in a high confinement factor of approximately 23[Formula: see text] in the cladding, enabling an effective interaction between the evanescent field and analyte. Ethanol-water solutions with varying concentrations were used as analyte for the characterization of the device. The strip-loaded waveguide was a 200 nm TiO[Formula: see text] thin film on a silicon dioxide wafer and a 1.2 [Formula: see text]m wide for 800 nm-thick nLOF resist served as loading strip. The sensing area featured a 10 [Formula: see text]m wide open-ing in the loading material with a sensing length of 5 mm. The homogeneous sensitivity was experimentally determined to be 0.128, demonstrating the effectiveness of the proposed design, which enabled a refractive index change monitoring of 10[Formula: see text].

Dielectric Cavity-Insulator-Metal (DCIM) Metamaterial Absorber in Visible Range.

For many years, metamaterial absorbers have received much attention in a wide range of application fields. There is an increasing need to search for new design approaches that fulfill more and more complex tasks. According to the specific application requirements, design strategy can vary from structure configurations to material selections. A new combination of a dielectric cavity array, dielectric spacer, and gold reflector as a metamaterial absorber is proposed and theoretically studied in this work. The complexity of the dielectric cavities leads to a more flexible optical response than traditional metamaterial absorbers. It gives a new dimension of freedom for a real three-dimensional metamaterial absorber design.

Bloch Surface Waves in Open Fabry-Perot Microcavities.

Thanks to the increasing availability of technologies for thin film deposition, all-dielectric structures are becoming more and more attractive for integrated photonics. As light-matter interactions are involved, Bloch Surface Waves (BSWs) may represent a viable alternative to plasmonic platforms, allowing easy wavelength and polarization manipulation and reduced absorption losses. However, plasmon-based devices operating at an optical and near-infrared frequency have been demonstrated to reach extraordinary field confinement capabilities, with localized mode volumes of down to a few nanometers. Although such levels of energy localization are substantially unattainable with dielectrics, it is possible to operate subwavelength field confinement by employing high-refractive index materials with proper patterning such as, e.g., photonic crystals and metasurfaces. Here, we propose a computational study on the transverse localization of BSWs by means of quasi-flat Fabry-Perot microcavities, which have the advantage of being fully exposed toward the outer environment. These structures are constituted by defected periodic corrugations of a dielectric multilayer top surface. The dispersion and spatial distribution of BSWs' cavity mode are presented. In addition, the hybridization of BSWs with an A exciton in a 2D flake of tungsten disulfide (WS2) is also addressed. We show evidence of strong coupling involving not only propagating BSWs but also localized BSWs, namely, band-edge and cavity modes.

Distance-controlled surface-enhanced Raman spectroscopy of nanoparticles.

Biological particles, e.g., viruses, lipid particles, and extracellular vesicles, are attracting significant research interest due to their role in biological processes and potential in practical applications, such as vaccines, diagnostics, and therapies. Their surface and interior contain many different molecules including lipids, nucleic acids, proteins, and carbohydrates. In this Letter, we show how distance-controlled surface-enhanced Raman spectroscopy (SERS) is a promising method to extract essential information from the spatial origin of the signal. This is a highly important parameter in the analysis of these biological particles. The principle of the method is demonstrated by using polystyrene (PS) beads as a biological particle model conjugated with gold nanospheres (AuNSs) functioning as distance-controlled SERS probes via biotin-streptavidin binding. By tuning the size of AuNSs, the Raman signal from the PS beads can be weakened while the signal from the biotin-streptavidin complex is enhanced.

Low Loss Vertical TiO2/Polymer Hybrid Nano-Waveguides.

This article proposes a novel demonstration of a low-loss polymer channel hybridized with a titania core leading to a nano-waveguide elongated in the normal direction to the substrate. It is aimed at using the quasi-transverse magnetic (TM) mode as the predominant mode in compact photonic circuitry. A detailed design analysis shows how a thin layer of a higher-refractive index material in a trench within the core of the waveguide can increase the confinement and reduce the propagation losses. This thin layer, produced by atomic layer deposition, covers the entire polymer structure in a conformal manner, ensuring both a reduction of the surface roughness and a stronger field confinement. The trench can be made at any place within the polymer channel and therefore its position can be tuned to obtain asymmetric modal distribution. The waveguide is demonstrated at telecom wavelengths, although the material's properties enable operation over a large part of the electromagnetic spectrum. We measured propagation losses as low as 1.75 ± 0.32 dB/cm in a 200 nm × 900 nm section of the waveguide core. All processes being mass-production compatible, this study opens a path towards easier integrated-component manufacture.

Pulse modulation by Bloch surface wave excitation.

Considering dielectric multilayers with N identical bilayers and an additional terminating layer, we address the effect of Bloch surface wave excitation on the temporal characteristics of short optical pulses. When such a resonant excitation occurs within the spectrum of the incident pulse, the reflected pulse splits into leading and trailing parts, the latter having an exponentially decaying tail. The role of the number of bilayers and the level of absorption in the multilayer stack is illustrated.

Time-gated Raman spectroscopy and proteomics analyses of hypoxic and normoxic renal carcinoma extracellular vesicles.

Extracellular vesicles (EVs) represent a diverse group of small membrane-encapsulated particles involved in cell-cell communication, but the technologies to characterize EVs are still limited. Hypoxia is a typical condition in solid tumors, and cancer-derived EVs support tumor growth and invasion of tissues by tumor cells. We found that exposure of renal adenocarcinoma cells to hypoxia induced EV secretion and led to notable changes in the EV protein cargo in comparison to normoxia. Proteomics analysis showed overrepresentation of proteins involved in adhesion, such as integrins, in hypoxic EV samples. We further assessed the efficacy of time-gated Raman spectroscopy (TG-RS) and surface-enhanced time-gated Raman spectroscopy (TG-SERS) to characterize EVs. While the conventional continuous wave excitation Raman spectroscopy did not provide a notable signal, prominent signals were obtained with the TG-RS that were further enhanced in the TG-SERS. The Raman signal showed characteristic changes in the amide regions due to alteration in the chemical bonds of the EV proteins. The results illustrate that the TG-RS and the TG-SERS are promising label free technologies to study cellular impact of external stimuli, such as oxygen deficiency, on EV production, as well as differences arising from distinct EV purification protocols.

Optical Monitoring of Microplastics Filtrated from Wastewater Sludge and Suspended in Ethanol.

The abundance of microplastics (MPs) in the atmosphere, on land, and especially in water bodies is well acknowledged. In this study, we establish an optical method based on three different techniques, namely, specular reflection to probe the medium, transmission spectroscopy measurements for the detection and identification, and a speckle pattern for monitoring the sedimentation of MPs filtrated from wastewater sludge and suspended in ethanol. We used first Raman measurements to estimate the presence and types of different MPs in wastewater sludge samples. We also used microscopy to identify the shapes of the main MPs. This allowed us to create a teaching set of samples to be characterized with our optical method. With the developed method, we clearly show that MPs from common plastics, such as polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), and polyethylene (PE), are present in wastewater sludge and can be identified. Additionally, the results also indicate that the density of the plastics, which influences the sedimentation, is an essential parameter to consider in optical detection of microplastics in complex natural environments. All of the methods are in good agreement, thus validating the optics-based solution.

Towards the Development of Portable and In Situ Optical Devices for Detection of Micro and Nanoplastics in Water: A Review on the Current Status.

The prevalent nature of micro and nanoplastics (MP/NPs) on environmental pollution and health-related issues has led to the development of various methods, usually based on Fourier-transform infrared (FTIR) and Raman spectroscopies, for their detection. Unfortunately, most of the developed techniques are laboratory-based with little focus on in situ detection of MPs. In this review, we aim to give an up-to-date report on the different optical measurement methods that have been exploited in the screening of MPs isolated from their natural environments, such as water. The progress and the potential of portable optical sensors for field studies of MPs are described, including remote sensing methods. We also propose other optical methods to be considered for the development of potential in situ integrated optical devices for continuous detection of MPs and NPs. Integrated optical solutions are especially necessary for the development of robust portable and in situ optical sensors for the quantitative detection and classification of water-based MPs.

Double resonant plasmonic lattices for Raman studies.

We demonstrate radiation induced enhancement of both the in coupling of Raman excitation wavelength and Raman signal in plasmonic nanoparticle lattices. Rectangular nanoparticle lattices show two independently controllable lattice resonances, which we tune to be resonant with both the Raman excitation wavelength and the Raman transitions of rhodamine 6G molecules. We demonstrate that these narrow and intense resonances produced by the nanoparticle lattices allow for Raman transition specific enhancements. The system allows for independent tuning of both resonance conditions, enabling an efficient and versatile platform for Raman studies of various molecules.

On optical sensing of surface roughness of flat and curved microplastics in water.

The growth of microplastic (MP) pollution is of increasing concern and represents a global challenge. In situ detection of these small particles is difficult because of their sizes, shapes, transparency or translucency, surface texture and ambient conditions. We propose and demonstrate the use of a prototype optical sensor to detect flat, nearly flat, curved and rough MPs prepared from commercial polyethylene terephthalate (PET) plastics and PET bottles in water. The prototype measures the specular reflection of a laser radiation incident on MPs, with a photodiode, and the transmitted laser speckle pattern, with a charge-coupled device (CCD) camera. The presence of the MPs as well as the optical surface roughness are determined from the specular reflection. Additionally, the so-called speckle contrast calculated from the speckle pattern, as a promising tool, is used to rank the rough MPs according to the different average surface roughness, to approximately twice the wavelength of the probing light. The novel application of laser speckle contrast and the optical roughness estimation allows the description of MP surface roughness in water. Moreover, in combination with earlier studies, these results, therefore, pave a way for the complete and a relatively easier description of MPs properties optical and also advances our step towards the development of simple and robust optical monitoring techniques for micro and nanoplastics in open and wastewater.

Optical screening for presence of banned Sudan III and Sudan IV dyes in edible palm oils.

Due to the proven carcinogenicity of Sudan III and IV dyes, they are considered global public health issues. They are banned in all forms as food colourants. We propose the monitoring of simple and easy-to-measure optical properties of palm oils, such as the refractive indices and spectrophotometric properties, as efficient indicators to detect adulteration. Coupling these results with principal component analysis, excess refractive index, and integration of transmittance introduces a novel detection tool for the authentication of edible palm oil. This opens a new opportunity for accurate handheld devices to detect adulteration and provide control in the field. This work assessed in total of 49 samples, some collected from different parts of Ghana and others, in-house adulterated samples. The Ghana Food and Drugs Authority, who performed a complex and expensive chemical analysis of the samples, confirmed our results with good agreement.

A prototype of a portable optical sensor for the detection of transparent and translucent microplastics in freshwater.

Microplastic pollution in water bodies is an alarming problem which needs to be addressed. However, issues such as size, shape and their appearance to light (transparent or translucent) make it difficult to be optically detected. Here, a feasibility study of a portable prototype optical sensor with the capability of measuring simultaneously specular laser light reflection and transmission from microplastic particles is presented. The specular reflection signal and the transmitted interference pattern were recorded with a photodiode and a CCD camera, respectively. With the combination of these two modes of detection, it is possible to screen the type, size, and nonplanarity of two microplastics types, i.e., transparent polyethylene terephthalate (PET) and translucent low-density polyethylene (LDPE), in a volume of freshwater, with high confidence. In principle, the prototype could be used for the detection of both floating microplastics as well as microplastics experiencing sedimentation in natural water bodies, and in water filtration in water treatment plants.

Fabrication of buried nanostructures by atomic layer deposition.

We present a method for fabricating buried nanostructures by growing a dielectric cover layer on a corrugated surface profile by atomic layer deposition of TiO2. Selecting appropriate process parameters, the conformal growth of TiO2 results in a smooth, nearly flat-top surface of the structure. Such a hard surface can be easily cleaned without damage, making the nanostructure reusable after contamination. The technique has wide applicability in resonance-domain diffractive optics and in realization of quasi-planar metamaterials. We discuss design issues of such optical elements and demonstrate the method by fabricating narrow-band spectral filters based on the guided-mode resonance effect. These elements have strong potential for, e.g., sensing applications in harsh conditions.

Silicon slot waveguide Fano resonator.

The growing interest for Fano resonators during the past decade is due to the narrow line shape observable in their optical spectra. The drastic phase shift occurring at the resonance yields a steep drop from a high to low amplitude. Fano resonances can be obtained by a combination of nanostructures. Such a system is extremely sensitive in terms of both geometrical parameters and environmental conditions. Here we study a complex arrangement of photonic crystal cavities and slot waveguides on a silicon chip. Our structure, composed of several cavities in parallel, has a particular response superimposing a shallow photonic bandgap and a resonance with a Fano line shape. It provides a low noise and a clear asymmetric resonance. We demonstrate it experimentally and show the potential of such a device for sensing. A sensitivity of 92 nm/RIU is measured.

Multi-wavelength add-drop filter with phase-modulated shifted Bragg grating.

We present, for the first time to the best of our knowledge, a multichannel add-drop operation with a phase-modulated shifted Bragg grating based filter. The device is realized in a silicon-on-insulator waveguide platform with TiO2 as a coating material to reduce the refractive index contrast. The operation is shown for three and five wavelength channels within the telecom C-band. A line width of 0.6 nm with an extinction ratio of 20 dB is achieved. The shifted Bragg grating is modulated maintaining a modal phase-matching condition for multiple wavelengths. The phase function is calculated with an iterative Fourier transform algorithm. The experimental results are in very good agreement with the design.

Multi-wavelength filtering with a waveguide integrated phase-modulated Bragg grating.

We present a multi-wavelength band-rejection filter on a titanium dioxide-coated silicon-on-insulator platform. The concept rests on the use of a finely tuned waveguide-based Bragg grating for which the periods are slightly varied from one to another. This phase-modulated Bragg grating enables precise customization of integrated waveguide filters. The number of rejection bands and the center-to-center separation between them are tailored by dividing the grating into several super-periods and coding an optimal phase function onto each super-period. The optimal phase function is obtained by employing an iterative Fourier transform algorithm. The design is supported by an experimental demonstration.

Influence of an Al2O3 surface coating on the response of polymeric waveguide sensors.

The responses of a polymer ridge waveguide Young interferometer with and without a bilayer of Al2O3/TiO2, fabricated by atomic layer deposition, are studied and compared when applied as an aqueous chemical sensor. The phase shift of the guided mode, as a result of the change in refractive index of the cover medium, is monitored. The results indicate that the over-coating affects the linearity of the sensor response. The effect of concentration on the linearity of the sensor response is investigated by applying different concentrations of water-ethanol solution. Although the performance of the sensor is improved by the additional layers, the study reveals a non-monotonic behavior of the device. We show that it comes mainly from the adsorption of ethanol molecules on the surface of the films. Such an understanding of the platform is crucial for sensing of analytes involving polar molecules.

Dielectric V-ridge gratings: transition from antireflection to retroreflection.

We present a systematic study of the optical properties of dielectric gratings with symmetric V-shaped ridges having a 90 degree apex angle and refractive index n. Such structures exhibit completely different optical properties if the dimensions are scaled with respect to the vacuum wavelength λ0 of light. In the subwavelength domain, where the grating period d is less than λ0/n, the grating behaves as an antireflection layer. In the large-period domain d≫λ0 (with normal incidence from the dielectric side), the grating turns into a micro-retroreflector array. The transition between these well-known domains is studied using rigorous diffraction theory. The results are verified experimentally by fabricating and characterizing V-profile gratings with the aid of wet etching of silicon using a process that defines a 90 degree apex angle and replication into a polymer.