Optoelectronic properties of materials are frequently examined through spectroscopic tools based on the absorption and emission of (visible) photons. Typically, one assumes that these transient characterization processes do not cause observable change of these properties because of the low power of the employed light sources. However, for flexible conjugated polymers in a medium of low viscosity, it is decidedly unclear whether photoexcitation of electrons on the backbone of a long polymer chain has an influence on macromolecular conformations and the corresponding intermolecular interactions. Here, we provide clear experimental evidence that continuous absorption of photons progressively causes persistent changes in the spectroscopic properties of conjugated polymers, accompanied by microscopically observable changes in their spatial distribution within a low‐viscosity matrix. By contrast, all these changes were completely absent in nonilluminated parts of the same sample. Quantitative spectral analysis allowed to identify the fraction of molecules that underwent such metamorphosis, which increased with the dose of photoexcitation, that is, with the number of absorbed photons. Complementary experiments revealed the reversibility of this behavior and so demonstrated the integrity of the polymers. Our results depict new pathways for tailoring properties of conjugated polymers and widening their spectrum of advanced engineering applications.
Flexible ultrasonic transducers are important for wearable medical imaging and therapeutic applications, yet combining high electromechanical performance with structural conformity and array uniformity remains difficult. Here, a 2-1-2-type piezoelectric composite consisting of PZT-4, epoxy resin, and silicone rubber is prepared through a monolithic dice-fill technique. A series-parallel equivalent model is employed to guide the structural optimization of the composite. Based on the theoretical analysis, representative samples with ceramic volume fractions of vc = 50 % and 60 % and a substrate volume fraction of vp = 20 % are selected for fabrication and experimental validation. These composites exhibit high-purity thickness vibration, a thickness electromechanical coupling coefficient (kt) of 0.62, and an acoustic impedance (Z) of 9.41 MRayl, indicating efficient energy conversion and favorable acoustic matching. The composite sustains a maximum tensile load close to 20 N and endures 400 cycles under a 5 N cyclic load without performance degradation. Resonance characteristics remain stable from 20°C to 60°C, showing strong fatigue and thermal stability for long-term wearable use. The fabricated arrays display high inter-element uniformity, with relative mean deviation (RMD) below 1 % and maximum deviation ratio (MDR) below 3 %. These results confirm the 2-1-2 composite as a promising material platform for conformal ultrasonic imaging and wearable therapeutic ultrasound systems.
Materials of engineering and construction. Mechanics of materials
Abstract The quantum properties of matter and radiation can be leveraged to surpass classical limits of sensing and detection. Quantum optics does so by creating and measuring nonclassical light. However, better performance requires higher photon-number states, which are challenging to generate and detect. Here, we combine photons and free electrons to solve the problem of generating and detecting high-number states well beyond those reachable with light alone and further show that an unprecedented level of sensitivity and resolution is gained based on the measurement of free-electron currents after suitably designed electron–light interactions. Our enabling ingredient is the strong electron–light coupling produced by aloof electron reflection on an optical waveguide, leading to the emission or absorption of a high number of guided photons by every single electron. We establish through rigorous theory that, by combining electron-beam splitters with two electron–waveguide interactions, the sensitivity to detect optical-phase changes can be enhanced dramatically using currently attainable technology. These results inaugurate a disruptive quantum technology relying on free electrons and their strong interaction with waveguided light.
In this paper, we demonstrate a free space optical (FSO) wavelength-division-multiplexing passive optical network (WDM-PON) architecture with short-reach to long-reach fiber transmission to overcome different environments and geographical restrictions. Based on the periodic nature of the 2×N arrayed waveguide grating (AWG), the downstream and upstream wavelengths in the proposed fiber access network are offset by one channel to enable symmetric FSO data transmission. Thus, the Rayleigh backscattering (RB) beat noise can be mitigated fully. Four WDM channels are selected to transmit wireless FSO signals over the proposed PON architecture, each utilizing different modulation rates and fiber transmission distances for demonstration. In addition, the corresponding bit error rate (BER), detected power sensitivity and power budget of each WDM FSO signal are also analyzed and discussed.
An efficient method for evaluating the spectral irradiance properties of the white light of white LEDs is conducted. The method includes two main steps. The first step is to build up spectral irradiance modeling for the blue and yellow emission bands. The photometric parameter of the spectral irradiance of white light which is generated by yellow and blue light mixing is determined based on the photometry and colorimetry theories. The correlated color temperature value strongly depends on the power ratios of blue and yellow light. In addition, the result indicates that the emission bandwidth of yellow phosphor is also an important factor for increasing the color performance of output light. The selection of material with a broader bandwidth of yellow light can control a slower variation in color property compared to the case of using a material with a narrower bandwidth. In addition, the blue light hazard ratio of the spectral irradiance of white light can be extracted, which is helpful for designing the white light with moderate blue and yellow power ratios before fabricating the white LEDs product.
Line-field confocal optical coherence tomography (LC-OCT) is an optical technique based on low-coherence interference microscopy with line illumination, designed for tomographic imaging of semi-transparent samples with micrometer-scale spatial resolution. A theoretical model of the signal acquired in LC-OCT is presented. The model shows that a refractive index mismatch between the sample and the immersion medium causes a dissociation of the coherence plane and the focal plane, leading to a decrease in the signal amplitude and a degradation of the image’s lateral resolution. Measurements are performed to validate and illustrate the theoretical predictions. A mathematical condition linking various experimental parameters is established to ensure that the degradation of image quality is negligible. This condition is tested experimentally by imaging a phantom. It is verified theoretically in the case of skin imaging, using experimental parameters corresponding to those of the commercially available LC-OCT device.
One challenge of using nonlinear optical phenomena for practical applications is the need to perform phase-matching. Recently, epsilon-near-zero materials have been shown to demonstrate strong optical nonlinearities, in addition to their other unique properties. As suggested by their name, the permittivity of the material is close to zero for a certain wavelength range. We demonstrate that this small permittivity allows for efficient three-wave mixing interactions to take place in indium–tin–oxide thin films without the need for phase matching the pump and signal beams. The efficiency of the second-order nonlinear interactions is characterized, and cascaded three-wave mixing is demonstrated.
Adrien Bouscal, Malik Kemiche, Sukanya Mahapatra
et al.
Novel platforms interfacing trapped cold atoms and guided light in nanoscale waveguides are a promising route to achieve a regime of strong coupling between light and atoms in single pass, with applications to quantum non-linear optics and quantum simulation. A strong challenge for the experimental development of this emerging waveguide-QED field of research is to combine facilitated optical access for atom transport, atom trapping via guided modes and robustness to inherent nanofabrication imperfections. In this endeavor, here we propose to interface Rubidium atoms with a photonic-crystal waveguide based on a large-index GaInP slab. With a specifically tailored half-W1 design, we show that a large chiral coupling to the waveguide can be obtained and guided modes can be used to form two-color dipole traps for atoms down to 115 nm from the edge of the structure. This optimized device should greatly improve the level of experimental control and facilitate the atom integration.
Abstract Computer-generated holography is a promising technique that modulates user-defined wavefronts with digital holograms. Computing appropriate holograms with faithful reconstructions is not only a problem closely related to the fundamental basis of holography but also a long-standing challenge for researchers in general fields of optics. Finding the exact solution of a desired hologram to reconstruct an accurate target object constitutes an ill-posed inverse problem. The general practice of single-diffraction computation for synthesizing holograms can only provide an approximate answer, which is subject to limitations in numerical implementation. Various non-convex optimization algorithms are thus designed to seek an optimal solution by introducing different constraints, frameworks, and initializations. Herein, we overview the optimization algorithms applied to computer-generated holography, incorporating principles of hologram synthesis based on alternative projections and gradient descent methods. This is aimed to provide an underlying basis for optimized hologram generation, as well as insights into the cutting-edge developments of this rapidly evolving field for potential applications in virtual reality, augmented reality, head-up display, data encryption, laser fabrication, and metasurface design.
Magnetic effects at optical frequencies are notoriously weak, so magneto-optical devices must be large to create a sufficient effect. In graphene, it has been shown that inhomogeneous strains can induce ‘pseudomagnetic fields’ that behave in a similar manner to real ones. Here, we show experimentally and theoretically that it is possible to induce such a field at optical frequencies in a photonic lattice. To our knowledge, this is the first realization of a pseudomagnetic field in optics. The field yields ‘photonic Landau levels’ separated by bandgaps in the spatial spectrum of the structured dielectric lattice. The gaps between these highly degenerate levels lead to transverse optical confinement. The use of strain allows for the exploration of magnetic-like effects in a non-resonant way that would be otherwise inaccessible in optics. It also suggests the possibility that aperiodic photonic-crystal structures can achieve greater field enhancement and slow-light effects than periodic structures via high density of states at the Landau levels. Generalizing these concepts to systems beyond optics, such as matter waves in optical potentials, offers new intriguing physics that is fundamentally different from that in purely periodic structures. Magnetic effects are fundamentally weak at optical frequencies. Now, by applying inhomogeneous strain in photonic band structures of a honeycomb lattice of waveguides, scientists show experimentally and theoretically that it is possible to induce a pseudomagnetic field at optical frequencies. The field yields 'photonic Landau levels', which suggests the possibility of achieving greater field enhancements and slow-light effects in aperiodic photonic crystal structures than those available in periodic structures.
In hybrid plasmonic Si waveguides integrated with ultrathin Au stripes, mode hybridization effects between quasi-TE modes of different orders and various bound modes are first presented. Enabled by nearly complete mode-selective absorption attributed to the enlarged mode propagation attenuation (MPA) coefficient for lower-order quasi-TE modes under circumstances of mode hybridizations, multi-segment tapered structures are theoretically proposed to constitute high order mode (HOM) pass filters. It is believed that these innovative observations would have potential applications in the ultra-compact on-chip mode division multiplexing (MDM) systems, and provide inspirations for the development of multimode photonics.
Inhomogeneity and low efficiency are two important factors that limit the application of laser-induced periodic surface structures (LIPSSs), especially on glass surfaces. In this study, two-beam interference (TBI) of femtosecond lasers was used to produce large-area straight LIPSSs on fused silica using cylindrical lenses. Compared with those produced using a single circular or cylindrical lens, the LIPSSs produced by TBI are much straighter and more regular. Depending on the laser fluence and scanning velocity, LIPSSs with grating-like or spaced LIPSSs are produced on the fused silica surface. Their structural colors are blue, green, and red, and only green and red, respectively. Grating-like LIPSS patterns oriented in different directions are obtained and exhibit bright and vivid colors, indicating potential applications in surface coloring and anti-counterfeiting logos.
Thorben Kewitz, Christoph Regula, Maik Fröhlich
et al.
Abstract The influence of different nozzle head geometries and, therefore, the variation of the excitation and relaxation volume on the energy flux from an atmospheric pressure plasma jet to a surface have been investigated. Measurements have been performed by passive calorimetric probes under variation of the gas flow through the nozzle. The results show that the geometry of the nozzle head has a significant impact on the resulting energy flux. The relaxation volume affects the dependence of the energy flux on the gas flow. While there is no significant influence of the working gas flow on the energy flux without a relaxation volume, utilizing a relaxation volume leads to a decrease of the energy flux with increasing working gas flow. Within the analyzed parameter range, the energy flux reveals for both nozzle heads a linear dependency on the applied primary voltage.
In this work, we propose a high-resolution optical system for Earth remote sensing, operating at 200-3300-nm wavelengths and providing a 4º field of view. Parameters of the system's structural elements are calculated and presented. Dot charts of the spots of confusion for the center, intermediate zones, and the edge of the field are considered. Over most of the operating wavelengths, the optical system is shown to be diffraction-limited, which provides attaining the highest possible spatial resolution. This system is considered as a tool for monitoring of the Earth's surface and collection of information in the ultra-violet, visible and near infrared spectral ranges (200-3300 nanometers).
We investigate the spatial nonlinear localization of light on a quasi-plane-wave background with a harmonic perturbation induced by modulation instability in a quadratic nonlinear optical medium. In particular, we demonstrate experimentally the excitation of deterministic Akhmediev breathers and thus the growth-decay dynamics of modulation instability in a LiNbO_{3} slab waveguide. The results should stimulate new interest in modulation instability, extreme events, turbulence, recurrence, and supercontinuum generation in quadratic nonlinear optics.
Magnetic resonators based on metamaterials are valuable for numerous applications including perfect absorber, sensing and medical imaging. However, due to the existence of the difficulty in magnetic excitation in planar metasurface, the study of magnetic metamaterial resonators is less reported, particularly in contrast to the electrical resonators. In this paper, the three-dimensional split ring resonator (SRR) metamaterials featuring dual-band magnetic plasmonic resonance modes are proposed and numerically analyzed. We calculate the electromagnetic field distributions of the three-dimensional metamaterials at the resonant frequencies to elucidate the resonant characteristics of the dual modes (fundamental LC mode and high-order magnetic plasmonic resonance mode). The influences of geometric parameters of the constituent meta-atoms on the resonance frequencies of the two resonance modes are analyzed by numerical calculations. The results show that the resonance frequency of the high-order magnetic resonance mode is nearly independent of the standing columns of the metamaterials owing to the special resonant characteristic of the high-order mode. Benefiting from the independence, the frequency interval between the dual modes can be customized by adjusting the dimensions of the column. In addition, by tuning the dimension of the columns, the quality factors of the resonances can reach the highest value of 176 for LC mode and 270 for the high-order mode, respectively. In light of practical application, we evaluate refractive index sensing performance of the proposed metamaterials, the maximum sensitivity can reach 474 GHz/RIU for LC mode and 446 GHz/RIU for high-order resonance, respectively. We also explore the universality of the special resonant characteristic of the high-order mode in other similar three-dimension SRR metamaterials, which shows all of these three-dimension magnetic plasmonic metamaterials provide potential platforms for multiband magnetic metamaterial applications.
Abhishek Upadhyay, Michael Lengden, David Wilson
et al.
A calibration-free first harmonic (1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>) wavelength modulation spectroscopy (1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>-WMS) technique for gas species parameter measurement is demonstrated. In this technique, the total magnitude of the 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>-WMS signal is normalized by a component of the 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula> residual amplitude modulation signal. This method preserves the advantages of the traditional <inline-formula><tex-math notation="LaTeX">$nf\!/\!1f$</tex-math></inline-formula>-WMS (<inline-formula><tex-math notation="LaTeX">$n\geq 2$</tex-math></inline-formula>) technique, such as the immunity to the non-absorbing systematic losses and the accurate recovery of gas parameters, without the requirement for non-absorbing regions for normalization at high pressure or high modulation index values (<italic>m</italic>-values). The proposed technique only requires the 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula> signal, which has the largest magnitude of all the harmonics signals, and, therefore, fundamentally has a higher sensitivity to the <inline-formula><tex-math notation="LaTeX">$nf\!/\!1f$</tex-math></inline-formula> technique. Furthermore, since only the 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>-WMS signal is used, the technique is less complex in terms of signal processing and data acquisition. This paper also shows a comparison of the proposed technique and 2<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>/1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula> for measuring CO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> in the exhaust of a combuster. The data highlight how nonlinearities in the optical detection system as a function of frequency have a considerable effect on the recovered <inline-formula><tex-math notation="LaTeX">$2f\!/\!1f$</tex-math></inline-formula> spectra, causing variation in the recovered gas concentrations. This effect is not seen in the methodology proposed in this paper.
Proteins are a class of biomaterials having a vast array of functions, including the catalysis of metabolic reactions, DNA replication, stimuli response and transportation of molecules. Recent progress in laser-based fabrication technologies has enabled the formation of three-dimensional (3D) proteinaceous micro- and nano-structures by femtosecond laser cross-linking, which has expanded the possible applications of proteins. This article reviews the current knowledge and recent advancements in the femtosecond laser cross-linking of proteins. An overview of previous studies related to fabrication using a variety of proteins and detailed discussions of the associated mechanisms are provided. In addition, advances and applications utilizing specific protein functions are introduced. This review thus provides a valuable summary of the 3D micro- and nano-fabrication of proteins for biological and medical applications.
Visible light positioning (VLP) is widely believed to be a cost-effective answer to the growing demand for real-time indoor positioning. However, due to the high computational cost of image processing, most existing VLC-based systems fail to deliver satisfactory performance in terms of positioning speed and accuracy, both of which are crucial for the performance of indoor navigation. This paper proposes a novel VLP solution that provides accurate and high-speed indoor navigation via the designs of an elaborate flicker-free line coding scheme and a lightweight image processing algorithm. In addition, this solution has the advantage of supporting flicker mitigation and dimming, which are important for illumination. An Android-based system prototype has been developed for field tests on an off-the-shelf smartphone. Experimental results show that it supports indoor positioning for users moving at a speed of up to 18 km/h. In addition, it can achieve a high accuracy of 7.5 cm, and the computational time is reduced to 22.7 ms for single-luminaire and to 35.7 ms for dual-luminaries, respectively.