Light Coupling Research Articles

Enhancing flexographic printing process with laser-induced microstructured printing forms for adjustable material transfer

Polymer optical waveguides are efficiently produced using flexographic printing, enabling fast and cost-effective manufacturing. However, the small print height results in a cross-sectional area too small for effective light coupling. To address this, multiple layers are printed to achieve the required waveguide dimensions. This sequential printing process prolongs processing time as material is repeatedly fed into the machine, potentially leading to issues like dust inclusions and oxygen inhibition. This study demonstrates modifying flexographic printing forms to increase transferred material height by up to 20 % in a single pass. Different microstructure geometries (hexagon, triangle, and square) with varying depths were inserted into the printing form to analyze their effects on material transfer. The use of a UV-nanosecond laser and a mask-based process allowed for precise insertion of the microstructures, resulting in high geometric accuracy and homogeneous ablation.

All-Optical Self-Heated Sensing Platform for Water Content Monitoring in Soil

In this contribution, we propose the use of an advanced all-optical self-heated sensing platform for highly sensitive monitoring of the volumetric water content (VWC) in the soil. The sensor is realized by properly integrating, inside a standard metallic needle, a fiber optic heating device based on core offset light coupling method and a Fiber Bragg Grating (FBG) acting as thermal monitor. The performance of the proposed device was validated through a series of measurements in the range [9 - 36] %VWC in the soil temperature range between 5 and 40 °C. Collected results have demonstrated that the presented platform is able to perform highly stable, fast and robust measurements, with time response lower than 10 seconds. The possibility of tuning the value of the injected power according to the application makes the proposed sensing solution extremely flexible and versatile for its exploitation in large areas and/or long-distance.

Large Fluorescence Enhancement via Lossless All-Dielectric Spherical Mesocavities.

Nano- and microparticles are popular media to enhance optical signals, including fluorescence from a dye proximal to the particle. Here we show that homogeneous, lossless, all-dielectric spheres with diameters in the mesoscale range, between nano- (≲100 nm) and micro- (≳1 μm) scales, can offer surprisingly large fluorescence enhancements, up to F ∼ 104. With the absence of nonradiative Ohmic losses inherent to plasmonic particles, we show that F can increase, decrease or even stay the same with increasing intrinsic quantum yield q0, for suppressed, enhanced or intact radiative decay rates of a fluorophore, respectively. Further, the fluorophore may be located inside or outside the particle, providing additional flexibility and opportunities to design fit for purpose particles. The presented analysis with simple dielectric spheres should spur further interest in this less-explored scale of particles and experimental investigations to realize their potential for applications in imaging, molecular sensing, light coupling, and quantum information processing.

Extra thin sidelight-emitting polymer optical fiber narrow fabrics for UV structural adhesive joints: design, preparation and performance experiments

Ultraviolet (UV) bonding technology is widely used across industries. As the curing reaction is initiated via an external light source, the use of fast-curing structural UV acrylates has so far been limited to joint designs with at least one transparent joining part. By introducing a side-emitting polymer optical fiber textile into the adhesive layer, which acts as an embedded light source, the technology can be made accessible for applications with nontransparent substrates such as metals. This paper describes the development of extra thin polymer optical fiber narrow fabrics designed to enable curing of structural adhesive joints. Polymer optical fibers (warp) were combined with different low-titer polyester weft yarns in two fabric design types with different primary out-coupling mechanisms: undulation and surface modification of the polymer optical fibers. The effect of yarn properties, thread densities and weave on fabric quality, geometry and lateral light intensity of each type were investigated. Results showed that narrow fabric production within the tight geometric limits required is feasible. Fabrics manufactured with nontextured yarns at low titers and low weft densities exhibited the highest lateral light intensities for the surface modification type fabric. To compensate for intensity loss over fabric length, investigating light coupling from both sides is recommended for this fabric type. For the undulation type fabric, the lateral light intensity characteristics indicated that the high tenacity yarns are promising candidates for the fabrication of low sidelight attenuation fabrics through modulation of the weft density.

Adaptive-optics-based turbulence mitigation in a 400 Gbit/s free-space optical link by multiplexing Laguerre–Gaussian modes varying both radial and azimuthal spatial indices

In general, atmospheric turbulence can degrade the performance of free-space optical (FSO) communication systems by coupling light from one spatial mode to other modes. In this Letter, we experimentally demonstrate a 400 Gbit/s quadrature-phase-shift-keyed (QPSK) FSO mode-division-multiplexing (MDM) coherent communication link through emulated turbulence using four Laguerre Gaussian (LG) modes with different radial and azimuthal indices (LG10, LG11, LG−10, and LG−11). To mitigate turbulence-induced channel cross talk and power loss, we implement an adaptive optics (AO) system at the receiver end. A Gaussian beam at a slightly different wavelength is co-propagated with the data beams as the probe beam. We use a wavefront sensor (WFS) to measure the wavefront distortion of this probe beam, and this information is used to tune a spatial light modulator (SLM) to adaptively correct the four distorted data-beam wavefronts. Using this adaptive-optics approach, the power loss and cross talk are reduced by ∼10 and ∼18 dB, respectively. © 2023 Optical Society of America

Reconfigurable phase change chalcogenide grating couplers with ultrahigh modulation contrast

In photonic integrated circuits, efficient coupling of light between fibers and waveguides is challenging due to mode area mismatch. In-plane grating couplers (GC) have become popular for their low cost, easy alignment, and design flexibility. While most GC designs have fixed coupling efficiencies, with a view to emerging adaptive neuromorphic and quantum integrated circuits and interposers that need ultra-compact memory/modulation components, we introduce a CMOS-compatible GC based on phase-change chalcogenide alloy germanium antimony telluride. The GC design optimized utilizing inverse design techniques achieves over 50% coupling efficiency at 1550 nm when amorphous, and near-zero efficiency when switched to a crystalline state. This design is non-volatile, reversible, and provides ultra-high transmission modulation contrasts of up to 60 dB. While the operational range can be adjusted across the telecommunication band by modifying the GC's etch depth or thickness. We show that such devices do not need global switching of their entire phase change volume and can achieve maximum modulation contrasts through switching precisely positioned phase change inclusions hinting at low-power and ultrafast modulation potential.

Asymmetric vortex beam emission from a metasurface-integrated microring with broken conjugate symmetry

Vortex beams that carry orbital angular moment (OAM) have recently attracted a great amount of research interest, and metasurfaces and planar microcavities have emerged as two prominent, but mostly separated, methods for Si chip-based vortex beam emission. In this work, we demonstrate in numerical simulation for the first time the hybridization of these two existing methods in a Si chip-based passive emitter (i.e., a light coupler). A unique feature of this device is its broken conjugate symmetry, which originates from introducing a metasurface phase gradient along a microring. The broken conjugate symmetry creates a new phenomenon that we refer to as asymmetric vortex beam emission. It allows two opposite input directions to generate two independent sets of OAM values, a capability that has never been reported before in Si chip-based passive emitters. In addition, we have also developed here a new analytical method to extract the OAM spectrum from a vector vortex beam. This analytical method will prove to be useful for vector vortex beam analysis, as mode purity analysis has rarely been reported in literature due to the complexity of the full-vector nature of such beams. This study provides new approaches for both the design and the analysis of integrated vortex beam emission, which could be utilized in many applications such as free-space optical communications and microfluidic particle manipulation.

Boosting solar photothermal synergy for efficient overall water splitting based on Mg, Al codoped and Rh/Cr2O3/CoOOH coloaded SrTiO3

The synergistic coupling of light and heat in solar energy has attracted increasing research attention. In this work, both codoping and coloading were applied to SrTiO3-based catalysts to improve their solar photo and thermal conversion, respectively, and maximum productivities of 51.55 and 25.59 mmol·g−1·h−1 were achieved for H2 and O2 in photothermal synergistic water splitting. The yield of H2 was nearly double that achieved via pure photocatalysis. In addition to many experiments and characterizations, DFT calculations were also performed to clarify the photothermal synergy. It was found that both the light absorbance and mobility of photoinduced carriers could be promoted by solar thermal effects in the material. Meanwhile, the energy barriers of the key steps were reduced with increasing temperature in the hydrogen production driven by solar photo effects. This work offers new ideas for designing catalysts for solar hydrogen production with photothermal synergistic strategies.

Generation and Manipulation of Spin‐Orbit Coupling mode via Four‐Wave Mixing with Quantum Interference

AbstractRecently, the vectorial nonlinear optical processes driven by spin‐orbit coupling (SOC) light have come to the fore, leading to striking optical phenomena and important applications both in classical and quantum optics. However, research on the SOC‐mediated light‐atom interactions is still in its infancy. Here, the generation and manipulation of SOC mode through a vectorial four‐wave mixing (FWM) process in 85Rb vapor is demonstrated. Under the excitation of two SOC pump beams, multiple FWM paths can be simultaneously established due to the rich atomic Zeeman sublevels coupling with different circularly polarized components of SOC fields. A higher‐order cylindrical or more general SOC FWM signal is observed in the vectorial FWM process, which obeys angular momentum conservation and Gouy phase matching. In particular, quantum interference between different FWM paths plays a crucial role in manipulating the SOC mode of the generated FWM signal, revealing that the conversion of SOC mode is intrinsically quantum. This work provides a pathway toward deeper insight into the vectorial nonlinear optical processes and may advance technology for shaping spatially structured light, which is essential in applications such as nonlinear polarization imaging and the optical communication realm.

Continuously wavelength tunable, continuous wave laser ideal for UV Raman spectroscopy

AbstractWe utilize a novel, high‐power, tunable, continuous wave (CW) deep UV laser to measure resonance Raman spectra of phenolate solutions with high signal‐to‐noise ratios (SNR). In UV resonance Raman (UVRR), increased coupling of the excitation light with a chromophore can transfer molecules into excited states that cause increased heating and photochemistry. Deep UV lasers have traditionally utilized high peak powers to enable efficient single‐pass nonlinear conversion from visible into near infrared light. Nonlinear phenomena such as the formation of transient radical species, Raman saturation, thermal heating, and dielectric breakdown can introduce extraneous light sources that can complicate the interpretation of the Raman spectrum. Dielectric breakdown can increase the baseline, increase noise, and sometimes saturate the detector, preventing Raman detection. Spontaneous Raman scattering intensities should scale linearly with the excitation light intensity. However, this linear behavior does not always occur with pulsed laser excitation. This occurs because stimulated Raman scattering can cause a superlinear intensity response, or transient absorption can cause sublinear intensity responses. CW laser excitation excites samples with electric fields that are much lower than typical pulsed laser excitation. This eliminates the nonlinear responses. The geometry of our new CW laser enables high gain in the harmonic generation cavities that achieve high harmonic generation efficiencies. Average power in the deep UV is >30 mW for wavelengths as short as 206 nm. In the work here, we demonstrate that CW excitation is ideal for resonance Raman measurements in general to reduce spectral complexity.

Heavy water coupling gel for short-wave infrared photoacoustic imaging.

Changes in lipid, water, and collagen (LWC) content in tissue are associated with numerous medical abnormalities (cancer, atherosclerosis, and Alzheimer's disease). Standard imaging modalities are limited in resolution, specificity, and/or penetration for quantifying these changes. Short-wave infrared (SWIR) photoacoustic imaging (PAI) has the potential to overcome these challenges by exploiting the unique optical absorption properties of . This study's aim is to harness SWIR PAI for mapping LWC changes in tissue. The focus lies in devising a reflection-mode PAI technique that surmounts current limitations related to SWIR light delivery. To enhance light delivery for reflection-mode SWIR PAI, we designed a deuterium oxide (, "heavy water") gelatin (HWG) interface for opto-acoustic coupling, intended to significantly improve light transmission above 1200nm. HWG permits light delivery up to 1850nm, which was not possible with water-based coupling ( light delivery up to 1350nm). PAI using the HWG interface and the Visualsonics Vevo LAZR-X reveals a signal increase up to 24dB at 1720nm in lipid-rich regions. By overcoming barriers related to light penetration, the HWG coupling interface enables accurate quantification/monitoring of biomarkers like LWC using reflection-mode PAI. This technological stride offers potential for tracking changes in chronic diseases (in vivo) and evaluating their responses to therapeutic interventions.

Silicon-tapered waveguide for mode conversion in metal-insulator-metal waveguide-based plasmonic sensor for refractive index sensing.

In this study, we have undertaken a comprehensive numerical investigation of a refractive index sensor designed around a metal-insulator-metal (MIM) plasmonic waveguide. Our approach utilizes the finite element method to thoroughly analyze the sensor's performance. The sensor's configuration utilizes a ring resonator design, which has been slightly modified at the coupling segment. This modification enhances the efficiency of light coupling between a bus waveguide and the ring resonator, particularly at the resonance wavelength. This strategic adjustment significantly improves the device's extinction ratio, a critical factor in its functionality. Remarkably, the sensitivity of this sensor is determined to be approximately 1155.71nm/RIU, while it possesses a figure of merit of 25.9. Furthermore, our study delves into the intricate mechanism governing the injection of light into the nanoscale MIM waveguide. We achieve this through the incorporation of silicon-tapered waveguides, which play a pivotal role in facilitating the transformation of a dielectric mode into a plasmonic mode, and vice versa. Ultimately, the findings of this research hold significant promise for advancing the field of plasmonic sensing systems based on MIM waveguide technology. The insights gained here pave the way for the practical realization and optimization of highly efficient and precise plasmonic sensors.

Miniature fiber-optic near-surface gap-coupled cladding waveguide Mach-Zehnder interferometric refractive index sensor inscribed by femtosecond laser

A miniature fiber-optic gap-coupled cladding waveguide Mach-Zehnder interferometer (MZI) is proposed and demonstrated for high-sensitive refractive index measurement. Two straight cladding waveguides respectively coupled with the fiber core are inscribed by a femtosecond laser (fs) to form the MZI structure. At their far ends, the two cladding waveguides are very close to the cladding surface, and a short separation was intentionally designed for the coupling of light between them and generating light field with evanescent wave expanded outside the cladding surface, which makes the MZI structure very sensitive to ambient refractive variations. In a refractive index range of 1.34–1.42, a high sensitivity of −59.90238 nm/RIU is achieved simultaneously with good linearity. The sensing structure also shows good characteristic of temperature insensitivity, which may find practical applications in chemical or biological sensing.

Overview of light coupling methods to optical planar waveguides

Overview of light coupling methods to optical planar waveguides

Compact on-chip THz circular polarization detectors with high discrimination based on chiral plasmonic antennas

THz circular polarization detection is an important technology in many applications of THz waves. With the ongoing miniaturization of optoelectronic systems, there is an increasing demand for compact on-chip THz circular polarization detectors. Here, we propose what we belive to be a novel device of this kind based on the composite structure of quantum well (QW) infrared detection material sandwiched by a chiral plasmonic antenna array and a metal plane. Due to the circular polarization dependent light coupling discrimination provided by the cavity enhanced chiral antenna and the second polarization selection of the QWs, a circular polarization extinction ratio as high as 25 is achieved, surpassing all reported on-chip THz circular polarization detectors. Due to the field enhancement at the QWs, the absorption for the principle circular polarization is 15 times higher than a standard reference. The absorption peak can be tuned over the range from 6.41 to 6.56 THz, while considerable absorption enhancement and high circular polarization discrimination are preserved even under non-normal incidence. Our proposed device's structure is compatible with the QWIP focal plane array and has far-reaching application prospects opens a new avenue to the development of high-performance compact on-chip THz circular polarization detectors.

New proposal for a two-channel optical multiplexer based on photonic crystal fibers

An innovative idea for a photonic crystal fiber-based 2 × 1 power beam combiner is presented. This research introduces an optical multiplexer (MUX) that supports wavelength multiplexing at 1.46 and 1.48 µm using a photonic crystal fiber topology. The multiplexer is a component that combines the specific wavelengths into the group of wavelengths in a network. This paper aims to create a new design structure for an optical Multiplexer based on photonic crystal fibers while optimizing the geometrical parameters and the multiplexer device and illustrating how a PCF structure could combine several optical signals into a group of signals in a single fiber. High-performance waveguides known as photonic crystal fibers (PCF) are constructed from a microstructured combination of substances with varying refractive indices. The PCF Mux design relies on appropriate PCF structure improvements and the replacement of specific air-hole sections with pure silica to manage the light coupling between the cores over the PCF axis; in comparison to traditional fiber, they have a shorter coupling length between close core and more design flexibility.

Direct VCSEL interconnection with a hollow core fiber

The direct interconnection of a hollow-core fiber (HCF) to a laser diode or photodiode is essential to full exploitation of the unique characteristics of HCFs. In this paper, we investigate two specific methods for HCF interconnection (i.e., butt coupling and simple two-lens imaging) for coupling light from a single-mode vertical-cavity surface-emitting laser (VCSEL) operating at 850 nm. Our research findings indicate that direct (lensless) butt coupling leads to poor coupling efficiency (∼10 %) due to the significant mode field diameter (MFD) mismatch between the waveguide and the HCF. However, the efficiency can be significantly improved (up to approximately 69 %) by using a graded-index fiber based all-fiber MFD converter. On the other hand, a two-lens system enables an extremely high coupling efficiency of ∼ 96 %.

One-step rolling fabrication of VO2 tubular bolometers with polarization-sensitive and omnidirectional detection.

Uncooled infrared detection based on vanadium dioxide (VO2) radiometer is highly demanded in temperature monitoring and security protection. The key to its breakthrough is to fabricate bolometer arrays with great absorbance and excellent thermal insulation using a straightforward procedure. Here, we show a tubular bolometer by one-step rolling VO2 nanomembranes with enhanced infrared detection. The tubular geometry enhances the thermal insulation, light absorption, and temperature sensitivity of freestanding VO2 nanomembranes. This tubular VO2 bolometer exhibits a detectivity of ~2 × 108 cm Hz1/2 W-1 in the ultrabroad infrared spectrum, a response time of ~2.0 ms, and a calculated noise-equivalent temperature difference of 64.5 mK. Furthermore, our device presents a workable structural paradigm for polarization-sensitive and omnidirectional light coupling bolometers. The demonstrated overall characteristics suggest that tubular bolometers have the potential to narrow performance and cost gap between photon detectors and thermal detectors with low cost and broad applications.

Enhanced performance of solution processed OLED devices based on PFO induced TADF emission layers

The internal quantum efficiency (IQE) of the OLED devices based on thermally activated delayed fluorescence (TADF) materials was thought to nearly 100 % by assuming the light outcoupling efficiency of 20–25 %. However, the external quantum efficiency (EQE) is still limited by the ηout. One crucial factor that affects the light output coupling efficiency is the molecular transition dipole moment of the emission molecule. In this study, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) was chosen as an orientation inducer to enhance the horizontal orientation ratio of a TADF emitter, 2-Phenyl-4,6-bis[4-(9,9-dimethyl-9,10-dihydroacridine) phenyl] pyrimidine (DMAc-PPM), prepared using a solution method. With doping PFO in weight ratios ranging from 1 wt% to 9 wt%, the orientation factor of TADF emitter increased, which testified the inducing function of PFO. The TADF-OLED devices using PFO as an inducer exhibited an EQE increasing with the PFO doping weight ratio. The maximum EQE of 7.3 % was achieved at the doping weight ratio of 7 wt%, which increase about 1.7 times compared to the device without the PFO inducer. This observed variation of EQE differs from the trend observed in maximum luminance intensity. The increase of EQE can be attributed to the improvement of light output efficiency caused by the variation of orientation factor. The lower carrier mobility lead to the lower maximum luminance intensity. The synergistic effect of light output efficiency and carrier mobility enhanced the performance of OLED devices.

A spatially uniform illumination source for widefield multi-spectral optical microscopy.

Illumination uniformity is a critical parameter for excitation and data extraction quality in widefield biological imaging applications. However, typical imaging systems suffer from spatial and spectral non-uniformity due to non-ideal optical elements, thus require complex solutions for illumination corrections. We present Effective Uniform Color-Light Integration Device (EUCLID), a simple and cost-effective illumination source for uniformity corrections. EUCLID employs a diffuse-reflective, adjustable hollow cavity that allows for uniform mixing of light from discrete light sources and modifies the source field distribution to compensate for spatial non-uniformity introduced by optical components in the imaging system. In this study, we characterize the light coupling efficiency of the proposed design and compare the uniformity performance with the conventional method. EUCLID demonstrates a remarkable illumination improvement for multi-spectral imaging in both Nelsonian and Koehler alignment with a maximum spatial deviation of ≈ 1% across a wide field-of-view.