Achromatic Lens Research Articles

Tunable Liquid Crystal Twisted‐Planar Fresnel Lens for Vortex Topology Determination

AbstractThe development of straightforward and effective methods to determine the topological charge of phase singular beams is crucial for broadening the applications of optical vortices. Here, a straightforward and elegant method for determining the topological charge of an optical vortex using an innovative switchable achromatic nematic liquid crystal Fresnel lens is proposed. The approach allows for the unambiguous determination of both the absolute value and the sign of the topological charge of a singular beam across a wide range of the visible spectrum, as demonstrated both experimentally and theoretically. The research outcomes are promising for applications in optical information transmission systems, cryptography, and quantum computing.

A Simple Doublet Lens Design for Mid-Infrared Imaging.

Wide-field mid-infrared (MIR) hyperspectral imaging offers a promising approach for studying heterogeneous chemical systems due to its ability to independently characterize the molecular properties of different regions of a sample. However, applications of wide-field MIR microscopy are limited to spatial resolutions no better than ∼1 μm. While methods exist to overcome the classical diffraction limit of ∼λ/2, chromatic aberration from transmissive imaging reduces the achievable resolution. Here we describe the design and implementation of a simple MIR achromatic lens combination that we believe will aid in the development of resolution-enhanced wide-field MIR hyperspectral optical and chemical absorption imaging. We also examine the use of this doublet lens to image through polystyrene microspheres, an emerging and simple means for enhancing spatial resolution.

Intensity-tunable achromatic cascade liquid crystal Pancharatnam-Berry lens

In the current solution for multiwavelength achromatic flat lenses, a one-to-one correspondence exists between the number of writing phase distributions and the number of achromatic wavelengths. Breaking this correspondence requires a complex phase design and parameter optimization. Here, we show that a dual-layer cascade liquid crystal Pancharatnam-Berry lens (CLCPBL) with two writing phase distributions and a specific coupled phase distribution between the layers can achieve three wavelength achromaticity without any parameter optimization process. Similarly, in a three-layer cascade, the number of achromatic wavelengths increases to seven through the permutations of the layers, with adjustable amplitude factors. We fabricate a three-layer CLCPBL with the design wavelengths of 396.8 nm, 1064 nm, and 1550 nm, which theoretically allows the light at 632.8, 532.8, 3383 and 450 nm to form a common focus, and test such structure. Our CLCPBL enables a wider range of applications than conventional achromatic flat lenses.

Design of an Achromatic Graphene Oxide Metalens with Multi-Wavelength for Visible Light

Dispersion control is a critical aspect in nano-optical systems. Moreover, chromatic aberration significantly impacts image quality. Despite metasurfaces being a novel approach to tackle chromatic aberration in diffractive lenses, numerous challenges hinder their practical implementation due to the complexity of 3D fabrication techniques and high manufacturing costs. In contrast, ultra-thin graphene oxide lenses are simpler and less expensive to manufacture. The optical performance of graphene oxide lenses, such as high focusing efficiency, large depth of field, wide bandwidth, and zooming capability, depends on the design of the positional arrangement of reduced graphene oxide regions. In this study, we utilized the self-constructed datasets to train machine learning models based on the structure of the graphene oxide lens and combined it with intelligent optimization algorithms. This approach facilitated the design of the graphene oxide achromatic lens in multi-wavelengths with high-performance. Experimental results substantiate that the designed ultra-thin graphene oxide lens, with a thickness of ~200 nm, effectively controls dispersion across multiple incident wavelengths (450, 550, and 650 nm) and achieves super resolution with consistent intensity at the focal point. Our graphene oxide lens holds the potential for integration into micro-optical systems that demand dispersion control, providing broad applications in optical imaging, optical communication, the biomedical field, and beyond.

‘Excellentissimo tubo Dollondiana’: The Stockholm Observatory’s 10-foot Dollond achromatic refractor

The instrument collection at the Royal Swedish Academy of Sciences houses a historically significant 10-foot achromatic refractor crafted by London instrument maker John Dollond. The telescope came into use at the Academy’s Observatory in Stockholm in 1761 and remained in service into the 1820s. This paper aims to add to the biography of this instrument, encompassing its six decades of active service and, after 150 years in storage, its transformation into an exhibition showpiece. The paper begins by introducing the telescope, its maker and the conflict involving Dollond and the Swedish mathematician and physicist Samuel Klingenstierna over the discovery of the achromatic lens. This dispute ultimately resulted in the telescope finding its way to the Stockholm Observatory. Subsequently, the paper delves into how Academy astronomer Pehr Wargentin perceived and utilized this state-of-the-art refractor, along with brief mentions of its use by his successors. The final section narrates how the telescope ended up in the permanent exhibition of the Stockholm Observatory Museum.

Experimental analysis of the near-nozzle direct injection spray for gasoline and ethanol-gasoline blended fuels

The current work analyses the spray during the early stage of development in the near-nozzle region using a single-hole gasoline direct injection system for gasoline (G100) and ethanol-gasoline (E85) blended fuels. The present work combined a Galilean-type beam expander with an achromatic lens, using an LED light and a high-speed camera to visualize a magnified view of the near-nozzle microscopic aspect of the emerged jet. Three different shapes, spheroids, cylindrical, and steeple, were visualized experimentally during the evolution of the fuel jet under different fuel injection and chamber pressures. A transition in shape from the steeple-to-spheroid shape was detected. It was observed that the velocity of the emerged jet is affected by its evolved shapes, attributing to differences in the initial breakup characteristics and shot-to-shot variation in the spray at a given condition. The velocity of the cylindrical-shaped jet was 14% and 42% higher than the steeple and spheroid-shaped jets, respectively. The Rayleigh-Plesset equation describes the reason for an increase in the spheroidal-shaped jet velocity by 84% when the chamber pressure is reduced from 0.6 to 0.1 MPa. The Ohnesorge and Reynolds number of the emerged liquid jets were estimated to describe the breakup regime of the jet during the initial stage. It was observed that the cylindrical and the steeple-shaped jets pertain to a higher tendency to atomize during the initial stage of evolution compared to the spheroidal jet. The average jet velocities and liquid penetrations in the near-nozzle region for E85 and G100 were estimated.

3D-printed multilayer structures for high-numerical aperture achromatic metalenses.

Flat optics consisting of nanostructures of high-refractive index materials produce lenses with thin form factors that tend to operate only at specific wavelengths. Recent attempts to achieve achromatic lenses uncover a trade-off between the numerical aperture (NA) and bandwidth, which limits performance. Here, we propose a new approach to design high-NA, broadband, and polarization-insensitive multilayer achromatic metalenses (MAMs). We combine topology optimization and full-wave simulations to inversely design MAMs and fabricate the structures in low-refractive index materials by two-photon polymerization lithography. MAMs measuring 20 μm in diameter operating in the visible range of 400 to 800 nm with 0.5 and 0.7 NA were achieved with efficiencies of up to 42%. We demonstrate broadband imaging performance of the fabricated MAM under white light and RGB narrowband illuminations. These results highlight the potential of the 3D-printed multilayer structures for realizing broadband and multifunctional meta-devices with inverse design.

Design of achromatic diffractive lenses.

Diffractive lenses can be very thin and light. They usually suffer from chromatic aberration and work only over a narrow range of wavelengths but so-called achromatic diffractive lenses have recently attracted attention. Ways in which the profile of such lenses can be chosen to optimize either the Strehl ratio or the efficiency are compared and the extent to which the performance of the resulting lens designs approaches theoretical limits is investigated. Simple rules are given for the average Strehl ratio and efficiency expected in certain conditions. In other cases they provide approximate guidelines. Some reported simulated and measured efficiencies greatly exceed those that appear credible. This is attributed to failure to take into account radiation scattered to large off-axis angles or to inadequate sampling of the radial profile.

Experimental characterization of early, quasi-steady and post-injection evolution of single-hole direct injection sprays for spark-ignited engines

The atomization characteristics of the ethanol-gasoline (E85) blended fuel using a single-hole GDI atomizer were studied. The spray at early development, quasi-steady, and post-injection stages were studied in the near-nozzle region. A Galilean-type beam expander and an achromatic lens were used to achieve the desired magnification. In the early development stage, the jet velocity was measured to estimate the breakup regime based on the Ohnesorge (Oh) and Reynolds (Re) numbers. Spray during the initial stage at 2 MPa injection pressure shows second wind-induced breakup under 0.6 MPa chamber pressure (Pch). A transition between second-wind and atomization breakup for jets under 0.03 and 0.05 MPa Pch. The jet experiences atomization breakup under 0.1 MPa Pch. The effect of the injection pressure and chamber temperature were also studied. The post-injection liquid sheet formation near the nozzle exit was observed. Results show that the size of the liquid sheet depends on the liquid-to-gas density ratio (ρlg). The size of the sheet for E85 fuel at ρlg of 2363 was found to be 6 mm, which was reduced by 75 % with a reduction in ρlg by 95 %. The chamber temperature increased from 303 K to 363 K, ρlg increased from 702 to 842, and a reduction in the liquid sheet was observed from 2.5 mm to 1.8 mm, respectively. The necking phenomenon of the liquid sheet was observed at the nozzle tip before the sheet detached and disintegrated into droplets. The necking length for the E85 was compared with gasoline fuel.

An enhanced chromatic dispersion probe for simultaneous measurement of dual-axis absolute and relative displacement with nanometric resolutions

This paper presents an enhanced chromatic dispersion probe for simultaneous measurement of dual-axis absolute and relative displacement with nanometric resolutions based on a 4f optical system and a signal processing algorithm. In the 4f optical system, a light source is generated by a pinhole in such a way that the employed pinhole is placed on the focal planes of the two achromatic lenses, aimed at generating a low-pass filter in the frequency domain. With a smaller pinhole, side lobes of the measured spectra are effectively suppressed and the signal-to-noise ratio is optimized, resulting in an expanded absolute measurement range and improved resolution of the two axes, and an enhanced relative measurement resolution of the dual-axis. A signal processing algorithm is proposed in such a way that in the spectral curvature region of the dual-axis output spectra, the original fitted spectrum is replaced by a linear fitting line and a newly-intersected wavelength can be obtained, resulting in an expanded measurement range of the relative displacement of the dual-axis. Numerical simulation and experiments have indicated that the absolute measurement range has been expanded to 225 μm with an enhanced resolution of 40 nm, and the measurement range of the relative displacement of the dual-axis has been expanded to 370 μm with an enhanced relative measurement resolution of 90 nm.

3D nanoprinting for fiber-integrated achromatic diffractive lens.

Achromatic performance is crucial for numerous multi-wavelength optical fiber applications, including endoscopic imaging and fiber sensing. This paper presents the design and nanoprinting of a fiber-integrated achromatic diffractive lens within the visible spectrum (450-650 nm). The 3D nanoprinting is achieved by a high-resolution direct laser writing technology, overcoming limitations in the optical performance caused by the lack of an arbitrary 3D structure writing capability and an insufficient feature resolution in the current manufacturing technology for visible light broadband achromatic diffractive lenses. A three-step optimization algorithm is proposed to effectively balance optical performance with writing difficulty. The characterization results demonstrate excellent achromatic focusing performance, paving the way towards the development of nanoprinted flat optical devices for applications such as optical fiber traps, miniature illumination systems, and integrated photonic chips.

Deep learning enhanced achromatic imaging with a singlet flat lens.

Correction of chromatic aberration is an important issue in color imaging and display. However, realizing broadband achromatic imaging by a singlet lens with high comprehensive performance still remains challenging, though many achromatic flat lenses have been reported recently. Here, we propose a deep-learning-enhanced singlet planar imaging system, implemented by a 3 mm-diameter achromatic flat lens, to achieve relatively high-quality achromatic imaging in the visible. By utilizing a multi-scale convolutional neural network (CNN) imposed to an achromatic multi-level diffractive lens (AMDL), the white light imaging qualities are significantly improved in both indoor and outdoor scenarios. Our experiments are fulfilled via a large paired imaging dataset with respect to a 3 mm-diameter AMDL, which guaranteed with achromatism in a broad wavelength range (400-1100 nm) but a relative low efficiency (∼45%). After our CNN enhancement, the imaging qualities are improved by ∼2 dB, showing competitive achromatic and high-quality imaging with a singlet lens for practical applications.

Polarization-independent achromatic Huygens’ metalens with large numerical aperture and broad bandwidth

Abstract Achromatic lenses, which have the same focal length regardless of the illumination frequency, find strong applications in imaging, sensing, and communication systems. Making achromatic lenses with metasurfaces is highly desirable because they are flat, ultrathin, relatively light, and easily fabricable. However, existing metalenses experience combinations of limitations which include single polarization operation, narrow bandwidth, and small numerical aperture (NA). In this work, we propose a dual polarized, broadband and high NA achromatic metalens based on the Huygens’ metasurface. We use Huygens’ metasurface unit cells with three tunable resonances to realize a stable group delay over a large bandwidth, while also achieving high transparency and large phase tunability. With these cells, we construct a dual-polarized achromatic Huygens’ metalens with an NA of 0.64 that works from 22 to 26 GHz. Our achromatic metalens achieves diffraction-limited focusing with 2 % maximum focal length deviation and 70 % average focusing efficiency over a bandwidth of 16.7 %. Most key performance metrics for this lens surpass or are comparable with the best of previous metalenses. An achromatic metalens simultaneously achieving broad bandwidth, large NA, and polarization-independent operation will open wide-ranging opportunities for microwave and mm-wave imaging and communication applications.

Dual-wavelength UV-visible metalens for multispectral photoacoustic microscopy: A simulation study

Photoacoustic microscopy is advancing with research on utilizing ultraviolet and visible light. Dual-wavelength approaches are sought for observing DNA/RNA- and vascular-related disorders. However, the availability of high numerical aperture lenses covering both ultraviolet and visible wavelengths is severely limited due to challenges such as chromatic aberration in the optics. Herein, we present a groundbreaking proposal as a pioneering simulation study for incorporating multilayer metalenses into ultraviolet-visible photoacoustic microscopy. The proposed metalens has a thickness of 1.4 µm and high numerical aperture of 0.8. By arranging cylindrical hafnium oxide nanopillars, we design an achromatic transmissive lens for 266 and 532 nm wavelengths. The metalens achieves a diffraction-limited focal spot, surpassing commercially available objective lenses. Through three-dimensional photoacoustic simulation, we demonstrate high-resolution imaging with superior endogenous contrast of targets with ultraviolet and visible optical absorption bands. This metalens will open new possibilities for downsized multispectral photoacoustic microscopy in clinical and preclinical applications.

Design of achromatic hybrid metalens with secondary spectrum correction.

Metasurface can be used in combination with singlet refractive lens to eliminate chromaticity, in which the metasurface usually works as a dispersion compensator. Such a kind of hybrid lens, however, usually has residual dispersion due to the limit of meta unit library. Here, we demonstrate a design method that considers the refraction element and metasurface together as a whole to achieve large scale achromatic hybrid lens with no residual dispersion. The tradeoff between the meta-unit library and the characteristics of resulting hybrid lenses is also discussed in detail. As a proof of concept, a centimeter scale achromatic hybrid lens is realized, which shows significant advantages over refractive lenses and hybrid lenses designed by previous methods. Our strategy would provide guidance for designing high-performance macroscopic achromatic metalenses.

An Improved Automation System for Destructive and Visual Measurements of Cross-Sectional Geometric Parameters of Microdrills

Microdrills are specific cutting tools widely used to drill microholes and microvias. For certain microdrill manufacturers, a conventional sampling inspection procedure is still manually operated for carrying out the destructive and visual measurements of two essential cross-sectional geometric parameters (CSGPs), called the cross-sectional web thickness (CSWT) and the cross-sectional outer diameter (CSOD), of their straight (ST) and undercut (UC) type microdrill products. In order to comprehensively automate the conventional sampling inspection procedure, a destructive and visual measuring system improved from an existing vision-aided automation system, for both the hardware and the automated measuring process (AMP), is presented in this paper. The major improvement of the hardware is characterized by a machine vision module consisting of several conventional machine vision components in combination with an innovative and lower cost optical subset formed by a set of plano-concave achromatic (PCA) lenses and a reflection mirror, so that the essential functions of visually positioning the drilltip and visually measuring the CSGPs can both be achieved via the use of merely one machine vision module. The major improvement of the AMP is characterized by the establishment of specific image processing operations for an auto-focusing (AF) sub-process based on two-dimensional discrete Fourier transform (2D-DFT), for a web thickness measuring (WTM) sub-process based on an iterative least-square (LS) circle-fitting approach, and for an outer diameter measuring (ODM) sub-process based on integrated applications of an iterative LS circle-fitting approach and an LS line-fitting-based group-dividing approach, respectively. Experiments for measuring the CSGPs of microdrill samples were conducted to evaluate the actual effectiveness of the developed system. It showed that the developed system could achieve good repeatability and accuracy for the measurements of the CSWTs and CSODs of both ST and UC type microdrills. Therefore, the developed system could effectively and comprehensively automate the conventional sampling inspection procedure.

Apochromatic X-ray focusing

Achromatic doublets are combinations of two individual lenses designed to focus different wavelengths of light in the same position. Apochromatic optics are improved versions of the achromatic schemes which extend the wavelength range significantly. Both achromatic and apochromatic optics are well-established for visible light. However, X-ray achromatic lenses did not exist until very recently, and X-ray apochromatic lenses have never been experimentally demonstrated. Here, we create an X-ray apochromatic lens system using an appropriate combination of a Fresnel zone plate and a diverging compound refractive lens with a tuned separation distance. The energy-dependent performance of this apochromat was characterized at photon energies between 6.5 and 13.0 keV by ptychographic reconstruction of the focal spot and scanning transmission X-ray microscopy of a resolution test sample. The apochromat delivered a reconstructed focal spot size of 940 × 740 nm2. The apochromatic combination shows a four-fold improvement in the chromatic aberration correction range compared to an achromatic doublet configuration. Thus, apochromatic X-ray optics have the potential to increase the focal spot intensity for a wide variety of X-ray applications.

Chromatic aberration correction employing reinforcement learning.

In fluorescence microscopy a multitude of labels are used that bind to different structures of biological samples. These often require excitation at different wavelengths and lead to different emission wavelengths. The presence of different wavelengths can induce chromatic aberrations, both in the optical system and induced by the sample. These lead to a detuning of the optical system, as the focal positions shift in a wavelength dependent manner and finally to a decrease in the spatial resolution. We present the correction of chromatic aberrations by using an electrical tunable achromatic lens driven by reinforcement learning. The tunable achromatic lens consists of two lens chambers filled with different optical oils and sealed with deformable glass membranes. By deforming the membranes of both chambers in a targeted manner, the chromatic aberrations present in the system can be manipulated to tackle both systematic and sample induced aberrations. We demonstrate chromatic aberration correction of up to 2200 mm and shift of the focal spot positions of 4000 mm. For control of this non-linear system with four input voltages, several reinforcement learning agents are trained and compared. The experimental results show that the trained agent can correct system and sample induced aberration and thereby improve the imaging quality, this is demonstrated using biomedical samples. In this case human thyroid was used for demonstration.

Achromatic polymer-dispersed liquid crystal lens with diffractive–refractive hybrid structure

The focal length of liquid crystal Fresnel lens is electrically controllable, but the disadvantage is the chromatic aberration is obvious. In this paper, the electrically controlled zoom characteristics of liquid crystal lens and the basic theory of Fresnel lens achromatic are analyzed, and a diffractive–refractive hybrid lens model made of polymer-dispersed liquid-crystal (PDLC) material is proposed. Fresnel liquid crystal lenses and PDLC hybrid lenses are tested with light at R (700.0 nm), G (546.1 nm) and B (435.8 nm) wavelengths, respectively. The simulation results show that the hybrid lens provides a greater range of adjustment of the focus position. After voltage modulation, the measured light with three different wavelengths have the same focal length, which means that the axial chromatic aberration is eliminated. In addition, with the elimination of axial chromatic aberration, the zoom range of the hybrid lens is doubled by further accurately controlling the electrode, which breaks through the limitation that the traditional liquid crystal lens can only work in monochromatic light spectrum.

A Study on the Development of a White Light Source Module for a Large-Capacity Searchlight Using a Blue Laser Diode

In this paper, a large-capacity white light source module using a high-power blue laser diode and a reflective spaced phosphor was designed. The reflective spacing phosphor ensured thermal stability. The proposed white light module is a reflective phosphor structure with a bi-directional optical system based on a rhombus prism lens. The rhombus prism optical system can greatly narrow the blue laser beam width and a long-wavelength band-pass filter of 500 nm or more is applied to change the movement path of the laser beam and transmit white light excited by the phosphor. A dichroic filter was applied to the fold mirror part and a planar convex lens was designed to focus the blue laser beam so that the phosphor was irradiated. Finally, a high-power white light source is obtained from the pre-optics unit to which the dichroic filter is applied. In order to use the proposed white light source as a searchlight, a divergence angle of 4° or less is required. For this implementation, a large-area collimating lens combining an aspheric condensing lens and an achromatic lens was applied. It was confirmed that the divergence angle of 4 degrees or less was satisfied at the focal length (FL) of 38.5 to 42.5 mm of the optical lens of the laser white light module emitter and the collimating optical system.