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The beam self-cleaning phenomenon is theoretically predicted by the two-dimensional nonlinear Schrödinger equation, which describes self-focusing, and is observed in the case of femtosecond laser filamentation in the collimated regime of propagation. However, the impact of external focusing on the self-cleaning has not been investigated so far. In this paper we systematically study this impact in a wide range of focusing conditions. We show that the energy range, in which self-cleaning can be observed, shrinks monotonically with the numerical aperture growth at some point vanishing at all. [ABSTRACT FROM AUTHOR]

In this paper, a terahertz range hybrid structured with square hollow-core PCF-based edible oil sensor is presented and statistically examined. For the purpose of determining this proposed hollow core fiber's propagation characteristics, the finite element method is used with a circular perfectly matched layer boundary condition. In ideal geometrical circumstances, the recommended detector exhibits 98.45% relative sensitivity to various edible oils at optimum frequency of 1.8 THz. An extensive simulation of that microstructure fiber across THz frequency range reveals that it is possible to concurrently achieve very low effective material loss of 0.004632 cm−1 and a very low confinement loss of 1.07 × 10–15 dB/m. In addition, the proposed fiber is thoroughly studied for other crucial factors like total loss, spot size, and numerical aperture. This optical waveguide's outstanding achievements will make it possible to use it in a variety of practical terahertz applications. [ABSTRACT FROM AUTHOR]

This work synthesizes a five-zonal steering-based PCF structure for detecting tuberculosis cells. The proposed steering-based five-zonal structure (SBFZ_PCF) comprises silica along with circular air holes in the core and an array of circular air holes with five polygons arranged in an alternative fashion in the cladding area. This paper aims to provide an in-depth and lucid view of the structure designing and simulation using COMSOL Multiphysics_Version_5.3 and perform elaborative numerical analysis by full-vector finite element method. The performance metrics yield the investigated wavelength region between 1.7 to 2.6 μm for computing optical parameters including Sensitivity, confinement loss (CL), Numerical Aperture (NA), Effective Area (Aeff), and Divergence. The numerical analysis demonstrates the maximum relative sensitivity of 89% at a very low confinement loss. The numerical aperture is investigated for assessment of the light acceptance of the proposed tuberculosis sensor. Hence our presented sensor quantitatively supports better sensitivity with all improved parameters. The proposed structure empirically proves and accomplishes label-free bio-sensing in tuberculosis cell detection along with other bio-medical applications. [ABSTRACT FROM AUTHOR]

The paper proposes polarization insensitive metalens at mid-infrared wavelength (λ = 4 μm), comprising silicon as the emitter and silicon dioxide as the substrate. A 100 μm radius, 302 μm focal length, and 70.72% focal efficiency are achieved for metalens. The difference between the theoretical and simulated focal length is 0.0013%. Multiple metalenses with different radius are designed with the minimum difference between them. These results, including focal length, numerical aperture, focal efficiency, improve the design efficiency for the metalens based on the same structure. The proposed method provides the idea for metalens selection with the minimum difference between focal lengths and the development of an efficient mid-infrared metalens. [ABSTRACT FROM AUTHOR]

In this paper, we study changes in the phase shift and length, as well as the spatial structure of birefringent microtracks induced in the bulk of fused silica in the tight focusing regime (numerical apertures NA = 0.45 and 0.55) by laser pulses with a wavelength of 1030 nm, durations of 0.3 and 0.6 ps, and variable energy. It is shown that the phase shift and the length of the microtracks increase monotonically with the pulse energy, while their spatial structure is transformed from an array of longitudinal channels with transverse subwavelength periodicity along the polarization vector into a three-dimensional hierarchical structure with additional periodicity about the wavelength along the radiation propagation axis. For the first time, it has been found that the period ΛE in three-dimensional hierarchical structures decreases by almost a factor of 2, which corresponds to an increase in the refractive index difference induced in the microtracks to . [ABSTRACT FROM AUTHOR]

While optical tweezers (OT) are mostly used for confining smaller size particles, the counter-propagating (CP) dual-beam traps have been a versatile method for confining both small and larger size particles including biological specimen. However, CP traps are complex sensitive systems, requiring tedious alignment to achieve perfect symmetry with rather low trapping stiffness values compared to OT. Moreover, due to their relatively weak forces, CP traps are limited in the size of particles they can confine which is about 100 μm. In this paper, a new class of counter-propagating optical tweezers with a broken symmetry is discussed and experimentally demonstrated to trap and manipulate larger than 100 μm particles inside liquid media. Our technique exploits a single Gaussian beam folding back on itself in an asymmetrical fashion forming a CP trap capable of confining small and significantly larger particles (up to 250 μm in diameter) based on optical forces only. Such optical trapping of large-size specimen to the best of our knowledge has not been demonstrated before. The broken symmetry of the trap combined with the retro-reflection of the beam has not only significantly simplified the alignment of the system, but also made it robust to slight misalignments and enhances the trapping stiffness as shown later. Moreover, our proposed trapping method is quite versatile as it allows for trapping and translating of a wide variety of particle sizes and shapes, ranging from one micron up to a few hundred of microns including microorganisms, using very low laser powers and numerical aperture optics. This in turn, permits the integration of a wide range of spectroscopy techniques for imaging and studying the optically trapped specimen. As an example, we will demonstrate how this novel technique enables simultaneous 3D trapping and light-sheet microscopy of C. elegans worms with up to 450 µm length. [ABSTRACT FROM AUTHOR]

In this paper, a high brightness fiber-coupled module with a central wavelength of 520 nm is simulated and designed by ray-tracing software ZEMAX and then is experimentally implemented. Three 1-W continuous-wave green LD single emitters based on TO-9-package are successively collimated, spatially combined, and focused into an optical fiber with a core diameter of 50 μm and a numerical aperture of 0.22. The final output power of 1.53 W is obtained, corresponding to an optical–optical conversion efficiency of 51% and an electro-optical conversion efficiency of 10%. The tolerances between the simulation and the experimental result are analyzed and explained. [ABSTRACT FROM AUTHOR]

This paper has modelled and quantitatively analyzed a hollow-core PCF to evaluate its performance as a biosensor. This biosensor is specifically designed to sense four types of cancerous cells, namely Jurkat, HeLa, MCF-7, and Basal, representing the names of blood, cervical, breast, and skin cells. The designed PCF has maintained a minimum of 0.2827 numerical aperture at 1.3 µm wavelength for the four types of cells. The effective absorption and confinement loss values have been exceptionally low for this PCF. The maximum values of these two parameters are only 2.43 × 10–7 cm−1 and 1.91 × 10–8 dB/m correspondingly at 1.3 µm. At the same wavelength, this biosensor has offered a higher value of relative sensitivity for the four types of cells that vary from 88.12% to 89.65%. In addition to these traditional values of performance indices, the simple PCF structure offers a broad likelihood of implementation using the persisting fabrication process. [ABSTRACT FROM AUTHOR]

In this paper, a wheel structured Zeonex-based hexagonal packing photonic crystal fiber (PCF) sensor has been proposed for sensing camel milk with a refractive index of 1.3423 and cow milk with a refractive index of 1.3459. This sensor has been investigated for porosities of 85%, 90%, and 98% within a terahertz (THz) region ranging from 0.2 to 2.0 THz. At an operating frequency of 2 THz, this sensor has shown a maximum sensitivity of 81.16% and 81.32% for camel and cow milk, respectively. EML of 0.033013 cm−1 and 0.03284 cm−1 has been found for camel and cow milk, respectively, at the same operating conditions with negligible confinement losses of 8.675 × 10–18 cm−1 1.435 × 10–18 cm−1. Several other parameters, such as the effective area, flattened dispersion, and numerical aperture, have also been obtained during the investigation. Since considerable attention has not been given yet in detecting various types of dairy products using PCF terahertz sensors, this design will pave a whole new path in further implementing THz sensing in the dairy industry. [ABSTRACT FROM AUTHOR]

This research increases the ability and value of a traditional vertical lathe and applies to manufacture a precise lens cell for the optical industry. The optical performance is limited by the residual centration error and position accuracy of conventional assembly methods. Recently, the development of a poker-chip assembly system with high-precision lens barrels has overcome these limitations and provided a solution for ultra-high-performance optical systems. To develop a high-precision lens cell by using poker-chip assembly, an alignment turning system (ATS), is developed based on a vertical lathe and equipped with tactile and optical measurement modules. Inside the ATS, the building-in the vibration/temperature monitoring sensors, which help to self-monitoring and network communication with the intelligent manufacturing techniques, to understand and master the reliability and efficiency of ATS. This system can manufacture precise lens cells, applied for optical metrology, high numerical aperture objective lenses, and lithography projection lenses. This paper describes the design and development of the ATS and its capabilities. The ATS is composed of measurement, alignment, and turning modules. After the ATS completes the measurement, alignment, and turning processes, the centration error of a lens cell, which is 200 mm in diameter, can be controlled to within 10 arcsec. Here, a lens cell with three subcells was assembled through the poker-chip method; each subcell was measured and then it underwent alignment and turning processes. The lens assembly test was performed five times by three technicians, and the average transmission centration error of the assembly lens was 12.45 arcsec. The results demonstrate that the ATS can achieve considerable assembly efficiency for high-precision optical systems. [ABSTRACT FROM AUTHOR]

In this paper, a calibration method is proposed which eliminates the zeroth order effect in lateral shearing interferometry. An analytical expression of the calibration error function is deduced, and the relationship between the phase-restoration error and calibration error is established. The analytical results show that the phase-restoration error introduced by the calibration error is proportional to the phase shifting error and zeroth order effect. The calibration method is verified using simulations and experiments. The simulation results show that the phase-restoration error is approximately proportional to the phase shift error and zeroth order effect, when the phase shifting error is less than 2° and the zeroth order effect is less than 0.2. The experimental result shows that compared with the conventional method with 9-frame interferograms, the calibration method with 5-frame interferograms achieves nearly the same restoration accuracy. [ABSTRACT FROM AUTHOR]

Optical metasurfaces have enabled analog computing and image processing within sub-wavelength footprints, and with reduced power consumption and faster speeds. While various image processing metasurfaces have been demonstrated, most of the considered devices are static and lack reconfigurability. Yet, the ability to dynamically reconfigure processing operations is key for metasurfaces to be used within practical computing systems. Here, we demonstrate a passive edge-detection metasurface operating in the near-infrared regime whose response can be drastically modified by temperature variations smaller than 10 °C around a CMOS-compatible temperature of 65 °C. Such reconfigurability is achieved by leveraging the insulator-to-metal phase transition of a thin layer of vanadium dioxide, which strongly alters the metasurface nonlocal response. Importantly, this reconfigurability is accompanied by performance metrics—such as numerical aperture, efficiency, isotropy, and polarization-independence – close to optimal, and it is combined with a simple geometry compatible with large-scale manufacturing. Our work paves the way to a new generation of ultra-compact, tunable and passive devices for all-optical computation, with potential applications in augmented reality, remote sensing and bio-medical imaging. Researchers demonstrate that image-processing metasurfaces can be dynamically reconfigured by using phase-change materials. The work might lead to novel tunable devices for compact optical computing for applications in AR/VR and bio-medical imaging. [ABSTRACT FROM AUTHOR]

The demand for high-resolution and large-area imaging systems for non-destructive wafer inspection has grown owing to the increasing complexity and extremely fine nature of semiconductor processes. Several studies have focused on developing high-resolution imaging systems; however, they were limited by the tradeoff between image resolution and field of view. Hence, computational imaging has arisen as an alternative method to conventional optical imaging, aimed at enhancing the aforementioned parameters. This study proposes a method for improving the resolution and field of view of an image in a lens-less reflection-type system. Our method was verified by computationally restoring the final image from diffraction images measured at various illumination positions using a visible light source. We introduced speckle illumination to expand the numerical aperture of the entire system, simultaneously improving image resolution and field of view. The image reconstruction process was accelerated by employing a convolutional neural network. Using the reconstructed phase images, we implemented high-resolution topography and demonstrated its applicability in wafer surface inspection. Furthermore, we demonstrated an ideal diffraction-limited spatial resolution of 1.7 μm over a field of view of 1.8 × 1.8 mm2 for the topographic imaging of targets with various surface roughness. The proposed approach is suitable for applications that simultaneously require high throughput and resolution, such as wafer-wide integrated metrology, owing to its compact design, cost-effectiveness, and mechanical robustness. [ABSTRACT FROM AUTHOR]

As a kind of non-diffracting beam, optical needle is gaining increased attention for its interesting properties and various potential applications. In this paper, we theoretically investigate the optical trapping force and torque on a Rayleigh spheroidal particle by an optical needle. A strategy to realize a non-diffracting optical needle by focusing a radially polarized vortex (RPV) beam with a high numerical aperture (NA) lens system is introduced and a diffractive optical element (DOE) is adopted to control the field distribution of the optical needle. Based on the field expressions of the optical needle derived through the Richards and Wolf theory, the factors affecting the properties of the optical needle are examined. Within the framework of dipole approximation, the optical trapping force and torque of an optical needle on a prolate spheroid with any spatial orientation are calculated. Numerical results show that the trapping force and torque are significantly affected by the field distribution of the optical needle, as well as the ellipticity and orientation of the spheroid. The present study may provide useful insights into the dynamic behavior during an actual optical trapping. [ABSTRACT FROM AUTHOR]

Metalenses are flat lenses, where sub-wavelength, so-called meta-atoms manipulate the electric field to perform a given lens function. Compared to traditional lenses, the two main drawbacks of metalenses are their achromatic limitations and low efficiencies. While an abundance of simulations show that efficiencies above 90% are attainable for low numerical apertures (NA), experimental reports showing such high efficiencies are limited. Here, we use electron-beam lithography (EBL) to realize a set of lenses with varying NA from 0.08 to 0.93. The low NAs were expected to fit the model, and the higher NAs determine the validity range of the model. We find that measured efficiencies above 92% for NA = 0.24 are achievable, and that a slight modification of the simulation model extends its validility to NA = 0.6. Based on our results, we discuss that the lower efficiencies reported in the literature are caused by low-fidelity manufacturing, closing the efficiency gap between measurements and simulation in metalens fabrication. While numerical simulations for metalenses frequently show efficiencies above 90% at low numerical apertures, the experimental counterpart struggles to reach such efficiencies. The authors modify the model for prediction and systematically realise a set of high-precision meta-lenses with high efficiencies across the whole numerical aperture range. [ABSTRACT FROM AUTHOR]

Terahertz (THz) biosensors have been widely used to detect bio-molecules or proteins because of their high detection accuracy, flexible design and easy fabrication. However, detecting bio-molecules at low concentration is difficult because of the strong absorption of water in the THz region. In this work, a THz anti-resonant fiber biosensor (ARF-BS) was investigated theoretically and verified experimentally. The sensing region of the ARF-BS consists of an elliptical core within a cylindrical cladding, supported by rectangular pillars, and surrounded by semi-elliptical tubes. Detailed numerical simulations and theoretical calculations are provided to evaluate the sensor's characteristics, including sensitivity, birefringence, transmission loss, dispersion, effective mode field area, numerical aperture, and power fraction. The modal electric field distribution of the THz fiber indicates that the electromagnetic fields are well confined inside the fiber core where contains sensing molecular analytes. The THz fiber was fabricated using 3D printing with digital light processing (DLP). The sensing ability of the THz fiber to detect different concentrations of Tau protein solutions was analyzed and verified using THz time-domain spectroscopy (THz-TDS). Experimental results demonstrated that the sensor achieves a maximum detection sensitivity of 2.726 AU/(μmol/L) for Tau protein solutions within the frequency range of 0.1–1.1 THz. The proposed fiber structure can be easily fabricated using 3D printing and has potential applications in the fields of biomedicine and biosensing with THz waveguides. [ABSTRACT FROM AUTHOR]

The strong interaction of light with micro- and nanostructures plays a critical role in optical sensing, nonlinear optics, active optical devices, and quantum optics. However, for wavefront shaping, the required local control over light at a subwavelength scale limits this interaction, typically leading to low-quality-factor optical devices. Here, we demonstrate an avenue towards high-quality-factor wavefront shaping in two spatial dimensions based on all-dielectric higher-order Mie-resonant metasurfaces. We design and experimentally realize transmissive band stop filters, beam deflectors and high numerical aperture radial lenses with measured quality factors in the range of 202–1475 at near-infrared wavelengths. The excited optical mode and resulting wavefront control are both local, allowing versatile operation with finite apertures and oblique illumination. Our results represent an improvement in quality factor by nearly two orders of magnitude over previous localized mode designs, and provide a design approach for a new class of compact optical devices. Wavefront manipulation with metasurfaces is typically limited to low quality factors. Here, the authors show how higher-order Mie modes can be leveraged to design high quality factor optical metasurfaces for wavefront manipulation in two dimensions. [ABSTRACT FROM AUTHOR]

Off-axis parabolic mirrors (OAPMs) are widely used in the THz and mm-wave communities for spectroscopy and imaging applications, as a result of their broadband, low-loss operation and high numerical apertures. However, the aspherical shape of an OAPM creates significant geometric aberrations: these make achieving diffraction-limited performance a challenge, and lower the peak electric field strength in the focal plane. Here, we quantify the impact of geometric aberrations on the performance of the most widely used spectrometer designs, by using ray tracing and physical optics calculations to investigate whether diffraction-limited performance can be achieved in both the sample and the detector plane. We identify simple rules, based on marginal ray propagation, that allow spectrometers to be designed that are more robust to misalignment errors, and which have minimal aberrations for THz beams. For a given source, this allows the design of optical paths that give the smallest THz beam focal spot, with the highest THz electric field strength possible. This is desirable for improved THz imaging, for better signal-to-noise ratios in linear THz spectroscopy and optical-pump THz-probe spectroscopy, and to achieve higher electric field strengths in non-linear THz spectroscopy. [ABSTRACT FROM AUTHOR]

Optical metasurfaces performing analog image processing – such as spatial differentiation and edge detection – hold the potential to reduce processing times and power consumption, while avoiding bulky 4 F lens systems. However, current designs have been suffering from trade-offs between spatial resolution, throughput, polarization asymmetry, operational bandwidth, and isotropy. Here, we show that dispersion engineering provides an elegant way to design metasurfaces where all these critical metrics are simultaneously optimized. We experimentally demonstrate silicon metasurfaces performing isotropic and dual-polarization edge detection, with numerical apertures above 0.35 and spectral bandwidths of 35 nm around 1500 nm. Moreover, we introduce quantitative metrics to assess the efficiency of these devices. Thanks to the low loss nature and dual-polarization response, our metasurfaces feature large throughput efficiencies, approaching the theoretical maximum for a given NA. Our results pave the way for low-loss, high-efficiency and broadband optical computing and image processing with free-space metasurfaces. Here the authors demonstrate a path to design metasurfaces that perform broadband, high-NA, high-efficiency and dual-polarization edge detection without using bulky 4 f systems. This work introduces new approaches towards passive, ultra-compact optical computing and image processing. [ABSTRACT FROM AUTHOR]

This work presents a high-efficiency achromatic meta-lens based on inverse design with topology optimization methodology. The meta-lens design with high numerical aperture values (NA = 0.7, NA = 0.8, and NA = 0.9) optimized along wavelength range starts from 550 to 800 nm, then the direct solver along the full extended wavelength band from 400 to 800 nm that applied to the final optimized structures under the three conditions of the high numerical apertures have high focusing efficiency for the all conditions. The optimization problem is based on Kreisselmeier–Steinhauser (k-s) objective function, leading to approximately stable response over the broadband bandwidths of the three designs. [ABSTRACT FROM AUTHOR]

In this study, a photonic crystal fiber (PCF) sensor is proposed for refractometric sensing. The proposed PCF sensor includes a single rectangular core and 32 rectangular air holes in the cladding region that have the same width and height as the core rectangle. The fiber substance used is Zeonex. The finite element-based COMSOL 5.6 program is used to numerically analyze the model. The simulation confirms the suggested PCF capability to identify the analyte samples with extremely high sensitivity. A frequency range of 1.5–3.0 THz was adopted. At an operating frequency of 2.8 THz, many performance metrics are calculated. Extremely low effective area (6.3575 × 104 µm2), effective material loss (0.00222 cm− 1) and confinement loss (0.52772 × 10− 14) have been obtained with the proposed sensor. Extremely high-power factor (98.128%), birefringence (3.3 × 10− 3), numerical aperture (0.23319) and relative sensitivity (98.876%) have been also obtained with the current PCF sensor. Moreover, the fabrication possibility of the proposed sensor is further assured by the PCF design's simplicity. [ABSTRACT FROM AUTHOR]

The overall size of an optical system is limited by the volume of the components and the internal optical path length. To reach the limits of miniaturization, it is possible to reduce both component volume and path length by combining the concepts of metasurface flat optics and folded optics. In addition to their subwavelength component thickness, metasurfaces enable bending conventional folded geometries off axis beyond the law of reflection. However, designing metasurfaces for highly off-axis illumination with visible light in combination with a high numerical aperture is non-trivial. In this case, traditional designs with gradient metasurfaces exhibit low diffraction efficiencies and require the use of deep-subwavelength, high-index, and high-aspect-ratio semiconductor nanostructures that preclude inexpensive, large-area nanofabrication. Here, we describe a design approach that enables the use of low-index (n ≈ 1.5), low-aspect ratio structures for off-axis metagratings that can redirect and focus visible light (λ = 532 nm) with near-unity efficiency. We show that fabricated optical elements offer a very large angle-of-view (110°) and lend themselves to scalable fabrication by nano-imprint lithography. The authors show how concepts from metasurface flat optics and folded optics can be combined to push the limits in miniaturization of optical systems. Such optical systems can be fabricated using low-index materials, opening up a path for large-area fabrication using nanoimprint lithography. [ABSTRACT FROM AUTHOR]

Ultrashort laser pulses are used to generate Laser-Induced Periodic Surface Structures (LIPSS). To apply LIPSS on small confined areas, the laser source must be focused to a small spot diameter. Contrary to commonly used high numerical aperture objectives, we present a Bessel beam-based approach to produce LIPSS covered lines with a width of 4 to 10 µm. Therefore, an 800 nm Ti:Sa-laser with a pulse duration of 110 fs and a repetition rate of 5000 Hz was used. The setup convinces with an extended depth of focus. Thus, high reproducibility of the process was proven, while no focus control and no specific requirements for the surface quality are needed. The applicability of this method is demonstrated by structuring the shell surface of nickel–titanium wires. [ABSTRACT FROM AUTHOR]

Metasurfaces can modulate light with periodically arranged subwavelength scatterers, and they can generate arbitrary wavefronts. Therefore, they can be used to realize various optical components. In particular, metasurfaces can be used to realize lenses, so-called metalenses. In the last decade, metalenses have been actively studied and developed. In this review, we firstly introduce the fundamental principles of metalenses in terms of materials, phase modulation method, and design method. Based on these principles, the functionalities and the applications can consequently be realized. Metalenses have a much larger number of degrees of freedom compared with that of existing refractive or diffractive lenses. Thus, they afford functionalities such as tunability, high numerical aperture, and aberration correction. Metalenses with these functionalities can be applied in various optical systems such as imaging systems and spectrometers. Finally, we discuss the future applications of metalenses. [ABSTRACT FROM AUTHOR]

By the use of the vector diffraction theory, the focusing properties of radially polarized Bessel-like beam with radial cosine wave front phase through a high numerical-aperture (NA) lens are investigated theoretically and numerically in this work. The wave front phase distribution is a cosine function of radial coordinate. Calculation results show that focus shift is considerably influenced by changing frequency parameter C and topological charge. An increase on the focus shift C, the total intensity pattern changes remarkably and it focuses along the optical propagation axis. Thus, the focal shift direction can be adjusted by the change of the frequency parameter in cosine function. In this paper, the generation of the focal spot allows an increase in focal length in the axial direction of the incident beam propagation. Under higher numerical aperture (NA = 0.95), the effect of the frequency parameter and topological charge on the focal pattern gets stronger. [ABSTRACT FROM AUTHOR]

The focus-shaping technique of the cylindrically polarized Bessel-Gaussian-like beam, referred to the cylindrically polarized Bessel-Gaussian beam modulated by Bessel gratings, propagating through a high numerical-aperture lens is investigated theoretically and numerically in this paper, using the vector diffraction theory method. Results show that the intensity distribution in the focal region can be influenced considerably by: the beam topological charges ( m, n), the beam parameter β and the polarization angle ϕ. The intensity distribution in focal region can be tailored considerably by appropriately adjusting the polarization angle. Peak-centered, donut and flattop focal shapes with extended focal depth which are potentially useful in optical tweezers, material processing and laser printing can be obtained using this technique. [ABSTRACT FROM AUTHOR]

A light-field camera is an optical system that provides multi-field-angle information in one shot. The microlens array (MLA) in a light-field camera system is the key to provide multi-field-angle information. However, its numerical aperture limits the microlens' spatial resolution and the image's quality. We proposed a new microlens array for the light-field camera in this work. This MLA consists of one larger primary microlens and 72 satellite microlenses, simultaneously providing high-resolution image information from different angles. Moreover, this MLA can be easily fabricated by nanoimprint at a low cost. The measured spatial resolution of the primary microlens is 53.74 lp/mm higher than satellite microlens when MTF = 0.3. Finally, a one-shot photograph of a light-field camera shows one high-resolution and other views from different angles of the bear doll. The high-resolution light-field camera has great potential to apply in object ranging and face recognition in the future. [ABSTRACT FROM AUTHOR]

Through filling CS 2 into the core, a liquid-core photonic crystal fiber(LCPCF) with a fully circular air hole structure was designed. The finite element method(FEM) and numerical analysis are combined to simulate the structure and optimize the parameters of the fiber. When λ = 1550 nm ,the geometric optimal parameters are Λ = 0.75 μ m, d 1 / Λ = 0.86 , d 2 / Λ = 0.96 , and d 3 / Λ = 0.20 . Meanwhile, the LCPCF can achieve a large negative dispersion of - 2697.06 ps/nm/km and a high nonlinearity of 50677.77 W - 1 km - 1 . The numerical aperture and light acceptance are 0.674 and 65.37%, respectively. By comparison, we believe that this LCPCF has obvious advantages in optical communication compensation and supercontinuum generation. [ABSTRACT FROM AUTHOR]

Brain tumors, abnormalities, and malignancies are considered fatal. Early detection techniques could save lives if they are utilized effectively for brain cancers. They should have remarkable mobility, high accuracy, high response speed, and low losses. The presented study shows what is believed to be a step toward achieving these objectives. A novel biomedical photonic crystal fiber sensor that can accurately detect and discriminate between different brain cells is proposed. The anomalous brain cells include injuries, tumors, and malignant cells. The proposed sensor can detect these various types with high relative sensitivity and negligible losses in comparison to other photonic crystal fiber-based biomedical sensors. The presented sensor exhibits a relative sensitivity of 97.5%, an effective area of 4.17 × 10−8 m2, a negligible confinement loss of 1.76 × 10−11 cm−1, a remarkably low effective material loss of 0.005942 cm−1, and a numerical aperture of 0.3524. The presented sensor is eligible to work in the terahertz regimes with a wide range of 1–3 THz. Also, a detailed comparison between the presented sensor and associated photonic crystal fiber literature is carried out to authenticate the effectiveness and veracity of the presented structure. [ABSTRACT FROM AUTHOR]

In this paper, a circular core shaped photonic crystal fiber (PCF) is proposed and the optical propagation characteristics are investigated and simulated by applying the finite element method (FEM) with the help of COMSOL Multiphysics software. The cladding of this PCF is composed of fused silica including a large center air-hole. The simulation process is performed within 1000–2000 nm wavelength. The raised PCF supports up to 38 OAM modes with larger bandwidth (1000 nm) as well as flat dispersion variations. The refractive index difference can exceed 10 - 4 for each OAM mode. The confinement loss of this PCF remains low approximately around 10 - 9 to 10 - 8 dB/m, comparatively better the nonlinearity, the numerical aperture, and the dispersion variation evolves into smoother. So, all these optimizing optical properties prove that our designed PCF is a promising candidate for the OAM mode transmission and other relevant optical communications. [ABSTRACT FROM AUTHOR]

Filamentation is favorable for many long-range outdoor laser applications, some of which require propagation to or at high altitudes. Understanding how the filamentation process and filament properties are impacted by the low pressure conditions present at high altitudes is essential in designing effective applications. The scaling of filament preconditions with pressure is considered. An increase in critical power and decrease in transition numerical aperture (NA) is predicted to occur with a drop in pressure, indicating that nonlinear pulse propagation and filamentation at high altitudes requires higher energy and a longer assisted focal length than sea level filamentation. A summary of pressure-scaled filament properties is also presented. New simulations demonstrate filamentation at pressures as low as 0.0035 atm (38.5 km altitude) is possible. [ABSTRACT FROM AUTHOR]

Optical fiber bundle-based microendoscope, which is significant in clinical diagnosis and industrial detection, calls for miniaturization of the probe and high-resolution observation. Here, we propose a double-layer metasurface array borrowing the structures of insect compound eyes to meet both requirements instead of traditional optical components. Each unit in the array aims for an incident field of view, focusing light at the center of the fiber end face with no chromatic aberration at the wavelengths of 470 nm, 530 nm and 630 nm. The metasurface array is composed of a series of isotropic TiO2 nanopillars which are special selected after considering resonance mode and angular dispersion characteristics, etched on both sides of a silica substrate, with the individual functions of deflecting and focusing. In image space, numerical aperture (NA) is 0.287 and the particular layout of two layers achieve zero telecentricity theoretically, which meet the requirements of optical fiber bundle coupling. A unit for incident angle of 20° is shown to validate our design approach numerically, which obtains a focused spot close to the diffraction limit. The compact and ultrathin metasurface could greatly reduce the size of the probe in optical fiber bundle based microendoscope while ensuring the imaging quality. [ABSTRACT FROM AUTHOR]

Fluorescence super-resolution microscopy has, over the last two decades, been extensively developed to access deep-subwavelength nanoscales optically. Label-free super-resolution technologies however have only achieved a slight improvement compared to the diffraction limit. In this context, we demonstrate a label-free imaging method, i.e., hyperbolic material enhanced scattering (HMES) nanoscopy, which breaks the diffraction limit by tailoring the light-matter interaction between the specimens and a hyperbolic material substrate. By exciting the highly confined evanescent hyperbolic polariton modes with dark-field detection, HMES nanoscopy successfully shows a high-contrast scattering image with a spatial resolution around 80 nm. Considering the wavelength at 532 nm and detection optics with a 0.6 numerical aperture (NA) objective lens, this value represents a 5.5-fold resolution improvement beyond the diffraction limit. HMES provides capabilities for super-resolution imaging where fluorescence is not available or challenging to apply. The authors demonstrate a label-free superresolution imaging method by using a hyperbolic material as a substrate for tailored light-matter interactions. The hyperbolic material enhanced scattering, combined with dark-field detection, result in 5.5-fold resolution improvement beyond the diffraction limit. [ABSTRACT FROM AUTHOR]

On the basis of two optical fibers with an optional lateral and longitudinal displacement in a homogeneous background medium, we describe a general full-vectorial Wilson-basis discretized mode-matching method that evaluates the converged electromagnetic fields after all resonances in the possible gap cavity have settled. Wilson basis functions feature strong localization in both the spatial and the spectral domain, which allows for efficient modal electromagnetic field expansions, adequate truncation of field propagation operators, and sparse translation operators, which in turn allow to make ad hoc electromagnetic field-matching operators. For physical contact connections between single-mode fibers with a mode-field diameter mismatch, we obtain attenuation curves that are right between those obtained from approximation methods that either effectively match the electric field or the magnetic field under the assumption of a vanishing reflection. For fibers separated by a growing gap, constructive and destructive interference patterns in the cavity are computed by the successive application of Love's equivalence principle and the propagation operator. By leveraging the physical width of the Wilson basis functions and the stepsize of the propagation operator, the initial operators may be reused in solving the interface problem for other wavelengths. For multi-mode fiber connections, we provide attenuation curves on a modal electromagnetic field level, as well as for overfilled and core-confined target encircled-flux compliant launches. A comparison to geometrical-optics based approaches shows attenuation differences in the order of several hundredth of a dB, and although that is small, it is significant for modern connection attenuation specifications. In the final example of connections between regular and trench-assisted multi-mode fibers, we notice that the relative change in the cumulative near-field power distribution can be significant, despite a marginally small attenuation. The influence of a core diameter and/or numerical aperture mismatch can be examined with a deliberate lateral misalignment. [ABSTRACT FROM AUTHOR]

The absorption and heat accumulation of successive laser pulses with nanosecond pulsed width and small numerical aperture focused in the glasses results in the melting of the glass materials. The features of the inscribed internal modification marks within the trace along the focus axis are analyzed under different operational parameters. A dynamic absorptivity evaluation measured the ratio of the transmitted energy to the incident energy is conducted. It is found that the feature of mark extends from a thread to a baseball bat, which includes three individual features of a thread, a cascade, and a molten region appeared clearly with the increase of the laser input energy. The mark length increases with the increases of both the pulsed energy at the fixed frequency and pulsed frequency at a specific energy. The mark length increases significantly with the number of pulses below the 20 shots, but approaches the plateau when the number of pulses are over 40 shots. The mark width increases with the number of pulse increase, but decreases when a black spot appears at near top of the modification mark. The intensity of peaks measured at 580 and 1118 cm−1 of the irradiated modification marks is smaller than that of unprocessed area revealed by the Raman spectrum. The peak position is found to slightly shift to low frequency for the irradiated mark. The dynamic absorptivity increases abruptly with increase of the pulsed energy and reaches a plateau. The maximum allowable pulsed energy to evaluate the dynamic absorptivity decreases with the increase of the pulsed frequency. The thermal diffusion model is provided to interpret qualitatively the observed scenery of the variation of mark width with the number of pulses in the experiment for a pulsed energy and a frequency. A mechanism is proposed to interpret the collaboration of the multi-pulses into a modification mark and the increase of interspacing distance between the marks with the scanning velocity. Finally, a relationship between the fracture shear strength and the focused depth beneath the interface of the glass substrates in the spot welding is experimentally reported. [ABSTRACT FROM AUTHOR]

Based on vector Debye integral, focusing properties of linearly polarized Bessel–Gaussian beam modulated by Bessel gratings combining a radial phase shift modulated spiral zone plate (RSSZP) passing through a large numerical aperture lens system are investigated. The data obtained lead that the intensity and the phase distributions in the focal section are significantly controlled by the order of the beam (m, n), the topological charge p and the shifting parameter β of the RSSZP. Simulations show that by increasing the topological charge, the radius of the ring increases and that the shifting parameter has an effect on the ring size. Also, the linear polarized incident beam lost its polarization after crossing a high-numerical-aperture objective. [ABSTRACT FROM AUTHOR]

Based on the Richards–Wolf vector diffraction theory, the strong focusing properties of a 4Pi focusing system under the illumination of the azimuthally polarized Laguerre–Gaussian vortex beam, which is modulated by the diffracted optical elements (DOE), are investigated numerically. We mainly study the influence trend of three parameters, which are the ratio (β ), the numerical aperture ( NA ) of the objective lens and the fourth ring parameter of the DOE, on the focal field in the 4Pi focusing system. The results show that three kinds of focusing patterns (the class optical needle, the flattop optical field and the dark channel) can be obtained according to the influence trend of the three parameters on the focal field. And three focal patterns can be converted by changing three parameters. The conversion between these special focal field patterns has potential application value in the optical micro-control field. [ABSTRACT FROM AUTHOR]

This study suggested a novel hexa-sectored square photonic crystal fiber (HS-SPCF) for blood serum and blood plasma sensing. The proposed HS-SPCF depicts an eminent sensitivity of blood plasma 66.7% and blood serum73.4% with ultralow low confinement loss 1.55 × 10–12 and 10.55 × 10–12 at the wavelength of 1.33 µm for blood plasma and serum. The operating wavelength to measure the optical properties using FEM was 0.6–1.6 µm. The proposed HS-SPCF showed ameliorating performance in confinement loss and relative sensitivity than the previous structures for blood components sensing. In addition, other optical characteristics like high birefringence of 2.6 × 10–3 and 2.6 × 10–3, lower EML of 0.21247 (cm−1) and 0.2170 (cm−1), effective area of 6.1 µm2 and 6.4 µm2, nonlinearity of 21.1(W−1 km−1) and 19.6 (W−1 km−1), numerical aperture (NA) of 0.286 and 0.281 has been achieved for proposed PCF at the wavelength of 1.33 µm. The proposed PCF will be used for biosensing or blood-sensing purposes and a broad diversity of chemical sensing functions. [ABSTRACT FROM AUTHOR]

Meta-optics has achieved major breakthroughs in the past decade; however, conventional forward design faces challenges as functionality complexity and device size scale up. Inverse design aims at optimizing meta-optics design but has been currently limited by expensive brute-force numerical solvers to small devices, which are also difficult to realize experimentally. Here, we present a general inverse-design framework for aperiodic large-scale (20k × 20k λ2) complex meta-optics in three dimensions, which alleviates computational cost for both simulation and optimization via a fast approximate solver and an adjoint method, respectively. Our framework naturally accounts for fabrication constraints via a surrogate model. In experiments, we demonstrate aberration-corrected metalenses working in the visible with high numerical aperture, poly-chromatic focusing, and large diameter up to the centimeter scale. Such large-scale meta-optics opens a new paradigm for applications, and we demonstrate its potential for future virtual-reality platforms by using a meta-eyepiece and a laser back-illuminated micro-Liquid Crystal Display. The authors present a general inverse-design framework for large-area 3D meta-optics that show engineered focusing. Such meta-optics, in combination with a laser-illuminated micro-LCD, open a path towards a future virtual reality platform. [ABSTRACT FROM AUTHOR]

In an optical fiber based SPR sensor, a segment of metal clad silica fiber is used as the sensing element and resonant coupling occurs to the surface plasmon mode excited at the interface of the metal and ambient which surrounds the sensing element. When polychromatic light is transmitted through the sensing element, the spectrum of light detected at the other end shows minima at a resonance wavelength. In wavelength interrogation, the shift in the resonance wavelength is a measure of change in the refractive index of the ambient. Alternatively, intensity interrogation can be used in which the change in transmitted power at a given wavelength is a measure of change in refractive index of the ambient. The sensitivity and detection accuracy of the sensor depend on the numerical aperture (NA) of the multimode optical fiber used to fabricate the sensor. In the present work, we have carried out a study of the dependence of the sensitivity and detection accuracy on numerical aperture to obtain the optimal value for a multimode optical fiber which is used to fabricate the sensing element under both wavelength and intensity interrogation. For a gold clad sensing element with l/a = 33.33 for the refractive index of the ambient in the range 1.33 to 1.38, the optimized numerical aperture is obtained as NA = 0.2 for spectral interrogation mode. At this optimal value, the sensitivity is estimated to be 4040 nm/RIU and detection accuracy lies between 20 and 48 μm−1. In single wavelength intensity interrogation, at the He–Ne laser wavelength (633 nm), the optimized numerical aperture is obtained as NA = 0.42. Experimental results on a sensing element fabricated with gold coating on the core of a plastic clad silica fiber are also reported. [ABSTRACT FROM AUTHOR]

A newly developed pentagonal photonic crystal fiber (P-PCF) with high nonlinearity and birefringence has been introduced in this work. Numerical analysis is carried out across the broader wavelength spectrum from 1.0 to 2.0 μm within the inferred region (IR). Furthermore, using the finite element method (FEM) with smaller mesh, a numerical examination of several optical characteristics such as effective mode area (EMA), nonlinearity, numerical aperture (NA), birefringence, and V-parameter was carried out. By using a circular completely matched layer (PML), the proposed P-PCF's optical transmission qualities have been guided. For the x-polarization mode, the suggested P-PCF exhibits high nonlinearity of 1.6 × 104 W−1 Km−1 at the working wavelength of λ = 1.4 µm. Furthermore, the proposed structure offers strengthening birefringence 2.5 × 10–6 at 1.6 μm wavelength including numerical aperture (NA) of 0.44 for the X-polarization mode. Based on the outstanding advantages, it can be asserted that the suggested model will find a promising domain of application in the area of all optical signal processing, sensing applications, and coherent optical communications. [ABSTRACT FROM AUTHOR]

Chloropicrin, ethyl-bromide, and methyl-bromide are the three primary chemical malignant anatomical stuff found in poultry feeds. The risk of cancer is found in some cases for the association of this stuffs. Hence, an effective detection method is necessary for identifying this malignant stuff. Here, we have suggested a square core-based PCF sensor that is operable in the 1–2 THz regime. The well-known polymer zeonex is selected as the hosting material and the investigation is carried out by the FEM method. After completing the numerical analyses, a higher sensitivity of 94.6%, 92.7%, and 93.3% is, respectively, obtained for chloropicrin, ethyl-bromide, and methyl-bromide. Also, an imperceptible CLoss and effective material loss are obtained from this simulation. Again, the measurements of the effective area, numerical aperture, core power fraction, and propagation constant are done for the three malignant stuff. The present existing technologies confirm the manufacturing chances of this recommended sensor. Our proposed sensor can be a deft aspirant in the areas of poisonous chemical sensing along with various application areas of bio-sensing. [ABSTRACT FROM AUTHOR]

Single-molecule localization microscopy provides insights into the nanometer-scale spatial organization of proteins in cells, however it does not provide information on their conformation and orientation, which are key functional signatures. Detecting single molecules' orientation in addition to their localization in cells is still a challenging task, in particular in dense cell samples. Here, we present a polarization-splitting scheme which combines Stochastic Optical Reconstruction Microscopy (STORM) with single molecule 2D orientation and wobbling measurements, without requiring a strong deformation of the imaged point spread function. This method called 4polar-STORM allows, thanks to a control of its detection numerical aperture, to determine both single molecules' localization and orientation in 2D and to infer their 3D orientation. 4polar-STORM is compatible with relatively high densities of diffraction-limited spots in an image, and is thus ideally placed for the investigation of dense protein assemblies in cells. We demonstrate the potential of this method in dense actin filament organizations driving cell adhesion and motility. Single-molecule localisation microscopy does not give orientation information. Here the authors combine Stochastic Optical Reconstruction Microscopy (STORM) with single molecule orientation and wobbling measurements using four-polarisation image splitting, 4polar-STORM. [ABSTRACT FROM AUTHOR]

The trapping stiffness and width are two important parameters to characterize a counter-propagating dual-beam optical trap. We present two types of asymmetric counter-propagating dual-beam optical trap with the different numerical aperture (NA) and trapping power to eliminate the multi-equilibrium positions when two foci of the optical trap are not coincided. Meanwhile, the asymmetric dual-beam trap with the different NA enhances the axial trapping width and stiffness over five and three times, respectively, higher than the standard dual-beam trap with the higher and same average NA. Besides, it increases the transverse trapping stiffness when two foci are not coincided. The asymmetric dual-beam optical trap will benefit the future applications for the study of precision measurement, basic physics and biomaterials. [ABSTRACT FROM AUTHOR]

We present the modeling of a porous core dispersion flatten photonic crystal fiber (PCF) for the fruitful transmission of THz signals. The model comprises only rectangular holes in both core and cladding areas. The model is numerically studied through simulation employing the finite element method (FEM)-based software. Simulation results disclose an insignificant amount of confinement loss (CL) of 8.01 × 10–5 cm−1 at 1.1 THz in the x-polarization mode. Besides, the effective material loss (EML) is only 2.2 × 10–3 cm−1 at the point. These values ensure a negligible amount of losses present at the modeled waveguide. The model offers a high numerical aperture. The power fraction for this model is 45% in the x-polarization mode at 1.1 THz. One of the splendid properties of the presented model is the ultra-flatten dispersion of approximately ± 0.004 ps/THz/cm, which ensures an insignificant amount of pulse broadening while applying this model in communication fields. The simple structure involving rectangles as well as desired values for optical parameters enhance the fruitfulness and fabrication feasibility of the presented THz waveguide. Thereby, the proposed sensor offers compound features, for instance, low CL, low EML, high NA, high power fraction, and flatten dispersion simultaneously. [ABSTRACT FROM AUTHOR]

A new photonic crystal fiber (PCF)–based, hollow-core, optical waveguide is proposed and numerically investigated to quickly identify numerous species of cancerous cells in the human body. Typical and cancerous cells have different refractive indices (RIs), and via this characteristic, the other important optical parameters are evaluated. The guiding properties of this proposed cancer cell sensor are analyzed in the COMSOL Multiphysics environment which used the finite element method as mathematical tool to solve differential equations. Furthermore, to ensure the highest simulation accuracy, extremely fine mesh elements are introduced. The simulation studies confirm that the proposed sensor, at 2.5 THz, achieves an extremely high relative sensitivity of almost 98% with negligible loss (< 0.025 dB/cm). Furthermore, a high numerical aperture (NA) and spot size, with low modal area, enhance the propagation characteristics of the sensor to a new height. The sensor's physical structure is very simple so that it can be easily fabricated with modern fabrication technology. Thus, it seems that this sensor will open a new door in the field of detecting and diagnosing different cancer cells. [ABSTRACT FROM AUTHOR]

In this study, a simple hollow core hexagonal structured photonic crystal fiber is offered and analyzed to discern commonly used different type of alcohols in our daily life. The proposed sensor guiding properties are numerically investigated using a full vectorial finite element software-based scheme. To ensure higher accuracy, the properties of alcohols (refractive index and dielectric constant) at different wavelength are used to calculate the behaviour of the sensor. The simulation results ensure that extremely high relative sensitivity around 89% can be achieved from the sensor at optimum structural condition. In addition, other important propagation parameters such as confinement loss (CL), single-mode propagation spot size, numerical aperture, etc., are discussed in detail for various geometric conditions. The structure of that sensor is straightforward, and available commercial fabrication technology can be used to fabricate it without any complexity. [ABSTRACT FROM AUTHOR]

The presented research explores the sensing capability of the Self-Mixing Optical Feedback Interferometry (SM-OFI). The study evaluates the impact of different trade-off parameters like injection current, incident angle, target velocity, ambient temperature, the numerical aperture of collimating lens, etc. All the modes of SM signal acquisition are compared and investigate the influence of the acquisition scheme on the quality of the signal. The incident angle, wavelength, and external cavity length are also being trade-offs to increase the SNR and decrease of speckle effect. The study confirms that SNR increased linearly (R-square = 0.97) corresponds to numerical aperture. It was also analyzed that the external cavity length has a direct impact on the Doppler spectrum. The main aim of the study is to lay down the experimental narrative and to provide a simple mathematical relationship. [ABSTRACT FROM AUTHOR]

A low confinement loss of hollow-core photonic crystal Fiber (HC-PCF) for liquid-/gas-sensing applications has been proposed. Various HC-PCF backgrounds such as silica, BK7 and Teflon glass with different core materials such as glycerol, benzene and toluene have been studied using a finite element method with perfectly matched layer-based COMSOL software. The performance of the proposed PCF is analyzed using different metrics such as confinement loss, effective area, numerical aperture, relative sensitivity and effective refractive index for different combinations to compare the effects of different backgrounds in HC-PCF. The proposed HC-PCF structure offers relative sensitivity of 91.96% and 1.74 × 10−14 dB/m of confinement loss using Teflon glass as the substrate for benzene. The proposed PCF can be used for liquid- and gas-sensing applications with its low confinement loss and high sensitivity. The proposed simple cladding structure is easy to fabricate and offers 26% improved relative sensitivity compared to existing PCFs in the literature. [ABSTRACT FROM AUTHOR]

This study has reported an extremely high sensitive and nonlinear chemical sensor based on photonic crystal fiber is presented with numerical investigation. In order to reduce fabrication complexity, the proposed chemical detector is designed with circular air holes. To calculate the guiding characteristics, finite element method based Comsol software is used. Different types of commonly used materials are used as background material of that proposed sensor to ensure maximum relative sensitivity to the chemicals. The simulation results confirms that, very high relative sensitivity of 97.89%, 96.31%, 91.87% and 88.93% for benzene, chloroform, ethanol and water respectively at 1.55 µm of optical signal. Moreover the proposed chemical sensor offers negligible confinement loss of around 10–10 dB/m for all sensing analytes. In addition, other important characteristics such as numerical aperture, nonlinearity are discussed in detail. The wavelength dependent light guiding characteristics for the solid materials and the sample under test is used in simulation to ensure better accuracy and to create real life environment. [ABSTRACT FROM AUTHOR]