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Circulating cell-free mitochondrial DNA (ccf-mtDNA) has emerged as a promising biomarker, with potential implications for disease diagnosis. Changes in mtDNA, such as deletions, mutations or variations in the number of copies, have been associated with mitochondrial disorders, heart diseases, cancer and age-related non-communicable diseases. Previous methods, such as polymerase chain reaction-based approaches, next-generation sequencing and imaging-based techniques, have shown improved accuracy in identifying rare mtDNA variants or mutations, but they have limitations. This article explains the basic principles and benefits of using planar optical waveguide-based detection devices, which represent an advanced approach in the field of sensing.

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Titanium dioxide (TiO2) thin films prepared by the sol-gel technique have been shown to be optically isotropic and, unlike the films obtained by competitive methods, do not exhibit measurable birefringence. A series of submicrometer-thin titanium dioxide films were prepared using the sol-gel technique and then thermally annealed at different temperatures. The samples were analyzed by spectroscopic ellipsometry using the Mueller matrix formalism, X-ray diffractometry and scanning electron microscopy. The conversion of amorphous titanium dioxide to polycrystalline anatase occurred at 400 °C or higher. Crystallites of a few percent of the film thickness were observed. Nevertheless, the crystallization process did not trigger the appearance of birefringence. These observations demonstrate that high-quality planar optical waveguides can be successfully fabricated on flexible substrates, in particular those composed of efficient polymers that can withstand the aforementioned temperatures.

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Novel poly(dimethylsiloxane) (PDMS) doped with two different spiropyran derivatives (SP) were investigated as potential candidates for the preparation of elastomeric waveguides with UV-dependent optical properties. First, free-standing films were prepared and evaluated with respect to their photochromic response to UV irradiation. Kinetics, reversibility as well as photofatigue and refractive index of the SP-doped PDMS samples were assessed. Second, SP-doped PDMS waveguides were fabricated and tested as UV sensors by monitoring changes in the transmitted optical power of a visible laser (633 nm). UV sensing was successfully demonstrated by doping PDMS using one spiropyran derivative whose propagation loss was measured as 1.04 dB/cm at 633 nm, and sensitivity estimated at 115% change in transmitted optical power per unit change in UV dose. The decay and recovery time constants were measured at 42 and 107 s, respectively, with an average UV saturation dose of 0.4 J/cm2. The prepared waveguides exhibited a reversible and consistent response even under bending. The sensor parameters can be tailored by varying the waveguide length up to 21 cm, and are affected by white light and temperatures up to 70 ℃. This work is relevant to elastomeric optics, smart optical materials, and polymer optical waveguide sensors.

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Polymorphism plays a pivotal role in generating a range of crystalline materials with diverse photophysical and mechanical attributes, all originating from the same molecule. Here, we showcase two distinct polymorphs: green (GY) emissive and orange (OR) emissive crystals of 5'-(4-(diphenylamino)phenyl)-[2,2'-bithiophene]-5-carbaldehyde (TPA-CHO). These polymorphs display differing optical characteristics, with GY exhibiting thermally activated delayed fluorescence (TADF) and OR showing room temperature phosphorescence (RTP). Additionally, both polymorphic crystals display mechanical flexibility and optical waveguiding capabilities. Leveraging the AFM-tip-based mechanophotonics technique, we position the GY optical waveguide at varying lengths perpendicular to the OR waveguide. This approach facilitates the exploration of the interplay between TADF and RTP phenomena by judiciously controlling the optical path length of crystal waveguides. Essentially, our approach provides a clear pathway for understanding and controlling the photophysical processes in organic molecular crystals, paving the way for advancements in polymorphic crystal-based photonic circuit technologies.

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High-index contrast lithium niobate waveguides, fabricated by the High Vacuum Vapor-phase Proton Exchange (HiVac-VPE) technique, are very promising for increasing both the optical nonlinear and electro-optical efficiencies of photonic integrated devices. So as to play this role effectively, it is mandatory to know the crystallographic phase composition of waveguides and the position of protonated layers for appropriate tailoring and optimization based on the intended applications. In addition, the estimation of structural disorder and electro-optical properties of the waveguides are also of high interest. Benefiting from Raman spectroscopy, IR reflection, IR absorption, and UV-VIS absorption, the HxLi1-xNbO3 phase compositions, as well as the structural disorder in waveguides, were determined. Based on experimental data on the shift of the fundamental absorption edge, we have quantitatively estimated the electro-optic coefficient r13 in as-exchanged waveguides. The electro-optical properties of the waveguides have been found to be depending on the phase composition. The obtained results allow for reconsidering the proton exchange fabricating process of photonic nonlinear devices and electro-optic modulators based on high-index contrast channel waveguides on the LiNbO3 platform.

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The dense and ordered molecular arrangements endow dynamic molecular crystals with fast response, rapid energy conversion, low energy dissipation, and strong coupling between heat/light and mechanical energy. Most of the known dynamic crystals can only respond to a single stimulus, and materials that can respond to multiple stimuli are rare. Here, we report an organic crystalline material that can be bent plastically and is also thermosalient, as its crystals can move when they undergo a reversible phase transition. The crystals transmit light regardless of their shape or crystalline phase. The combination of light transduction and reversible thermomechanical deformation provides an opportunity to switch the waveguiding capability of the material in a narrow temperature range, which holds a tremendous potential for applications in heat-averse electronic components, such as central processing units. Unlike existing electronics, the material we report here is completely organic and therefore much lighter, potentially reducing the overall weight of electronic circuits.

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Multicolor luminescence of organic fluorescent materials is an essential part of lighting and optical communication. However, the conventional construction of a multicolor luminescence system based on integrating multiple organic fluorescent materials of a single emission band remains complicated and to be improved. Herein, organic alloys (OAs) capable of full-color emission are synthesized based on charge transfer (CT) cocrystals. By adjusting the molar ratio of electron donors, the emission color of the OAs can be conveniently and continuously regulated in a wide visible range from blue (CIE: 0.187, 0.277), to green (CIE: 0.301, 0.550), and to red (CIE: 0.561, 0.435). The OAs show analogous 1D morphology with smooth surface, allowing for full-color waveguides with low optical-loss coefficient. Impressively, full-color optical displays are easily achieved through the OAs system with continuous emission, which shows promising applications in the field of optical display and promotes the development of organic photonics.

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With the advent of the first laser sources and suitable detectors, optical sensor applications immediately also came into focus. During the last decades, a huge variety of optical sensor concepts were developed, yet the forecast for the future application potential appears even larger. In this context, the development of new sensor probes at different scales down to the atomic or molecular level open new avenues for research and development. We investigated an iron based triazole molecular spin-crossover complex changing its absorption characteristics significantly by varying environmental parameters such as humidity, temperature, magnetic or electric field, respectively, with respect to its suitability for a new class of versatile molecular sensor probes. Hereby, besides the investigation of synthesized pure bulk material using different analyzing methods, we also studied amorphous micro particles which were applied in or onto optical waveguide structures. We found that significant changes of the reflection spectra can also be obtained after combining the particles with different types of optical waveguides.The obtained results demonstrate the suitability of the material complex for a broad field of future sensor applications.

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This paper presents composite beam splitters realized with polymer materials for developing photonic integrated circuits. We used organic-inorganic hybrid polymer materials to form this composite beam splitter realized with step-index (SI) core profiles. We used the alternating direction implicit technique of the Rsoft CAD BeamPROP solver to design and analyze these beam splitters. We successfully examined and manipulated the beam splitter's polarization dependency to obtain a 99% output efficiency with a 50:50 splitting ratio. The SI beam splitter exhibits an excess loss of 0.014 dB. When we apply polarized light in this beam splitter, the excess loss increases to 2 dB, and this loss gradually decreases as the angle of incident light increases. The excess loss reduces to 0.05 dB at the 31-degree angles of the incident polarized light. We also investigated the crosstalk of this beam splitter by varying the wavelength, and it is evident that the lowest crosstalk is -19.77 dB at the polarized angle of 31°.

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This research introduces a novel methodology of harnessing liquids to facilitate the realization of parity-time (PT)-symmetric optical waveguides on highly integrated microscale platforms. Additionally, we propose a realistic and detailed fabrication process flow, demonstrating the practical feasibility of fabricating our optofluidic system, thereby bridging the gap between theoretical design and actual implementation. Extensive research has been conducted over the past two decades on PT-symmetric systems across various fields, given their potential to foster a new generation of compact, power-efficient sensors and signal processors with enhanced performance. Passive PT-symmetry in optics can be achieved by evanescently coupling two optical waveguides and incorporating an optically lossy material into one of the waveguides. The essential coupling distance between two optical waveguides in air is usually less than 500 nm for near-infrared wavelengths and under 100 nm for ultraviolet wavelengths. This necessitates the construction of the coupling region via expensive and time-consuming electron beam lithography, posing a significant manufacturing challenge for the mass production of PT-symmetric optical systems. We propose a solution to this fabrication challenge by introducing liquids capable of dynamic flow between optical waveguides. This technique allows the attainment of evanescent wave coupling with coupling gap dimensions compatible with standard photolithography processes. Consequently, this paves the way for the cost-effective, rapid and large-scale production of PT-symmetric optofluidic systems, applicable across a wide range of fields.

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The use of mechanically flexible molecular crystals as optical transuding media is demonstrated for a plethora of applications; however, the spectral peaks of optical outputs located mainly in the range of 400-600 nm are insufficient for practical telecommunication and full-color display applications. Herein, two elastically bendable organic crystals are reported that show red emission of the rec.709 gamut under 365 nm UV light irradiation yet generate rec.2020 gamut red optical waveguides and amplified spontaneous emissions when irradiated by a 355 nm laser. Capitalizing on the extended π-conjugation and donor-acceptor character, as well as mechanical elasticity, these organic crystals exhibit flexible optical waveguides with Commission Internationale de L'Eclairage (CIE) coordinates of (0.70, 0.29), nearly identical to the red chromaticity of the rec.2020 gamut required for ultrahigh-definition (UHD) displays. Notably, one of the elastic crystals functions as a soft resonance cavity, resulting in amplified spontaneous emission waveguides with CIE coordinates of (0.71, 0.29) and the standard red chromaticity of the rec.2020 gamut, both in straight and bent states. This study presents a new avenue for the development of high-purity red-emissive crystalline materials to create all-organic, lightweight, and mechanically compliant optical telecommunication and UHD display devices.

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This review article provides an in-depth exploration of the role of gels in the fields of organic electronics and photonics, focusing on their unique properties and applications. Despite their remarkable potential, gel-based innovations remain relatively uncharted in these domains. This brief review aims to bridge the knowledge gap by shedding light on the diverse roles that gels can fulfil in the enhancement of organic electronic and photonic devices. From flexible electronics to light-emitting materials, we delve into specific examples of gel applications, highlighting their versatility and promising outcomes. This work serves as an indispensable resource for researchers interested in harnessing the transformative power of gels within these cutting-edge fields. The objective of this review is to raise awareness about the overlooked research potential of gels in optoelectronic materials, which have somewhat diminished in recent years.

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In order to achieve a multifunctional compound with potential application in organic photonics and electronics, a multidonor benzothiadiazole derivative was rationally designed and synthesized employing microwave irradiation as energy source, increasing the process efficiency about yields and reaction times in comparison with conventional conditions. This powerful compound displayed solvatochromism and showed efficient behavior as red optical waveguide with low OLC around 10-2  dB µm-1 and with the capacity of light transmission in two directions. In addition, the proposed derivative acted as efficient p-type semiconductor in organic field-effect transistors (OFETs) with hole mobilities up 10-1  cm2  V-1 s-1 . This corroborates its multifunctional character, thus making it a potential candidate to be applied in hybrid organic field-effect optical waveguides (OFEWs).

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Polymer-based surface plasmon resonance (SPR) sensors can be used to realize simple, small-size, disposable, and low-cost biosensors for application in several fields, e.g., healthcare. The performance of SPR sensors based on optical waveguides can be changed by tuning several parameters, such as the dimensions and the shape of the waveguides, the refractive index of the core, and the metal nanofilms used to excite the SPR phenomenon. In this work, in order to develop, experimentally test, and compare several polymer-based plasmonic sensors, realized by using waveguides with different core refractive indices, optical adhesives and 3D printed blocks with a trench inside have been used. In particular, the sensors are realized by filling the blocks' trenches (with two plastic optical fibers located at the end of these) with different UV-cured optical adhesives and then covering them with the same bilayer to excite the SPR phenomenon. The developed SPR sensors have been characterized by numerical and experimental results. Finally, in order to propose photonic solutions for healthcare, a comparative analysis has been reported to choose the best sensor configuration useful for developing low-cost biosensors.

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In this review, we present a short overview of the development of sol-gel glasses for application in the field of photonics, with a focus on some of the most interesting results obtained by our group and collaborators in that area. Our main attention is devoted to silicate glasses of different compositions, which are characterized by specific optical and spectroscopic properties for various applications, ranging from luminescent systems to light-confining structures and memristors. In particular, the roles of rare-earth doping, matrix composition, the densification process and the fabrication protocol on the structural, optical and spectroscopic properties of the developed photonic systems are discussed through appropriate examples. Some achievements in the fabrication of oxide sol-gel optical waveguides and of micro- and nanostructures for the confinement of light are also briefly discussed.

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A novel hybrid multimode interferometer for sensing applications operating with both TE and TM polarizations simultaneously is proposed and numerically demonstrated. The simulations were performed assuming an operating wavelength of 633 nm with the goal of future use as a biosensor, but its applications extend beyond that area and could be adapted for any wavelength or application of interest. By designing the mutimode waveguide core with a low aspect ratio, the confinement characteristics of TE modes and TM modes become very distinct and their interaction with the sample in the sensing area becomes very different as well, resulting in high device sensitivity. In addition, an excitation structure is presented, that allows good control over power distribution between the desired modes while also restricting the power coupled to other undesired modes. This new hybrid TE/TM approach produced a bulk sensitivity per sensor length of 1.798 rad · RIU - 1 · µ m - 1 and a bulk sensitivity per sensor area of 2.140 rad · RIU - 1 · µ m - 2 , which represents a much smaller footprint when compared to other MMI sensors, contributing to a higher level of integration, while also opening possibilities for a new range of MMI devices.

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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.

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[This corrects the article DOI: 10.3389/fchem.2022.916553.].

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Photonic circuit systems based on optical waveguiding heteroarchitectures have attracted considerable interest owing to their potential to overcome the speed limitation in electronic circuits by modulating the optical signal at the micro- or nanoscale. However, controlling the parameters, including the wavelength and polarization of the light outcoupling, as well as the sequence among different building blocks, remains a key issue. Herein, supramolecular heteroarchitectures made by phosphorescent organometallic complexes of Pt, Pd, Cu, and Au are applied as photonic logic gates that show continuously variable emission colors from 475 to 810 nm, low waveguide losses down to 0.0077 dB µm-1 , and remarkable excitation-light polarization-dependent photoluminescence with anisotropy ratios up to 0.68. The sequences among Pt, Pd, Au, and Cu building blocks in the heteroarchitectures are controlled by living supramolecular polymerization or crystallization-driven self-assembly synthetic approaches. The results indicate the prospects for using organometallic complexes and supramolecular synthetic approaches to prepare photonic circuit systems with tunable emission color and controllable sequences among different blocks that achieve modulation of the optical signal in the visible-to-near-infrared spectral region.

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Integrating plasticity and elasticity in anisotropic molecular crystals is theoretically possible and is beneficial in enabling rich and complex deformations; however, it is much harder to implement in practice. Herein, we report a centimeter-long organic crystal that is two-dimensionally (2D) elastically bendable not only at room temperature but also at ultralow temperatures (-196 °C). The straight crystal can also be manually twisted and reconfigured to form arbitrary right-handed or left-handed helical structures. The integration of low-temperature-resistant (LTR) 2D elastic-bending and reconfigurable plastic-twisting deformations into one organic crystal expands the perspectives of the emerging crystal flexibility. Taking advantage of the unique multiple flexibility characteristics, spatial controllability of optical transmission for cryogenic applications and reusability of light-polarization rotations have been implemented simultaneously in an organic crystal.