Optical Diffraction Research Articles

Direct laser patterning of ruthenium below the optical diffraction limit

We describe a method that can be used to produce ruthenium/ruthenium oxide patterns starting from a ruthenium thin film. The method is based on highly localized oxidation of a small surface area of a ruthenium film by means of exposure to a pulsed laser under ambient conditions. Laser exposure is followed by dissolution of the un-exposed ruthenium in a NaClO solution, which leaves the conductive, partially oxidized ruthenium area on the substrate. Spatially selective oxidation, material removal, and, by implication, patterning, are, therefore, achieved without the need for a photoresist layer. Varying the exposure laser parameters, such as fluence, focus diameter, and repetition rate, allows us to optimize the process. In particular, it enables us to obtain circular Ru/RuO2 islands with a sub-diffraction-limited diameter of about 500 nm, for laser exposure times as short as 50 ms. The capability to obtain such small islands suggests that heat-diffusion is not a limiting factor to pattern Ru by laser heating on a (sub-)micron scale. In fact, heat diffusion helps in that it limits the area where a sufficiently high temperature is reached and maintained for a sufficiently long time for oxidation to occur. Our method provides an easy way to produce metallic Ru/RuO2 (sub-)micron structures and has possible applications in semiconductor manufacturing.

Influence of Microstructure on the Mechanical Properties and Polishing Performance of Large Prehardened Plastic Mold Steel Blocks

The microstructures throughout a 696 × 1360 mm cross-section of an ISO 1.2738 prehardened steel block for a plastic mold were characterized via optical and electron microscopy and electron backscatter diffraction. The hardness, strength, and polishing performance of the steel block were also tested. The results showed that the microstructure of the steel bloom from the edge to the core consisted of tempered sorbite, tempered bainite, and pearlite microstructures. Abnormal upper bainite and coarse carbides were also found. The bloom sections with hardness values of 37.4 to 39.3 HRC comprised tempered sorbite and bainite. The hardness of the core was approximately 36.5 HRC due to the presence of pearlite. The tensile and yield strengths were the same in the edge and middle areas mainly owing to tempered sorbite. The polishing performance was affected by the microstructure. Tempered sorbite produced the best polishing performance due to its fine and uniform microstructure, whereas that of tempered bainite and pearlite, which contained large carbide particles and mixed phases, was worse.

Multi-material stainless steel fabrication using plasma wire arc additive manufacturing

Layered stainless steel samples were manufactured using Plasma Wire Arc Additive Manufacturing by depositing alternating layers of two dissimilar stainless steels onto an AISI 316L substrate. The selected materials were the M430 ferritic and M316L austenitic stainless steels. Comprehensive microstructural characterization in different areas, including interfaces between the two different materials, was achieved through optical microscopy and electron backscatter diffraction. Results revealed good weldability among the two-layered steels and on the substrate. Also, a gradient duplex structure was observed mainly at the layer interfaces, suggesting a nuanced blending of properties in the deposited zone. To establish a direct correlation with mechanical properties in the observed structures, macro and micro-hardness tests were conducted along the cross-section. These tests evidenced the presence of a harder phase along the interface compared to the individual materials. The microstructural analysis corroborated the presence of mixing between phases in conjunction with some areas of delta ferrite.

Influence of compound layer on the corrosion resistance of low alloy steel in a 3.5% NaCl solution

Samples of AISI 4140 low alloy steel were nitrided at different conditions by using plasma nitriding. The optical microscopy observation and x-ray diffraction analysis showed that a 3 ∼ 12 μm thick compound layer composed of ε-Fe2–3N and γ′-Fe4N was formed on the surface after nitriding treatments. The corrosion behavior was evaluated by measuring the anodic polarization curves in a 3.5% NaCl solution along with the observation of corroded surfaces and cross-sectional morphologies using optical and scanning electron microscope. The results indicated that the corrosion resistance of low alloy steel was significantly enhanced through the formation of compound layer. The thickness of compound layer was one important factor in determining its corrosion resistance.

Enhancing the color gamut of waveguide displays for augmented reality head-mounted displays through spatially modulated diffraction grating

Augmented reality (AR) applications require displays with an extended color gamut to facilitate the presentation of increasingly immersive content. The waveguide (WG) display technology, which is typical AR demonstration method, is a critical constraint on the color gamut of AR systems because of the intrinsic properties of the holographic optical elements (HOEs) used in this technology. To overcome this limitation, we introduce a method of spatially modulated diffractive optics that can expand the color gamut of HOE-based WG displays. This approach involves spatial modulation using sub-pixelized HOEs, which enables the diffraction of red, green, and blue rays along identical directions. The proposed structure considers both the characteristics of the HOE and the wavelength sensitivity of the observer to optimize the color gamut. Consequently, an expanded color gamut was achieved. The results of the theoretical and experimental analyses substantiate the effectiveness and practicality of this method in enhancing the color gamut of HOE-based WG displays. Thus, the proposed method can facilitate the implementation of more immersive AR displays.

Untrained neural network enabling fast and universal structured-illumination microscopy.

Structured-illumination microscopy (SIM) offers a twofold resolution enhancement beyond the optical diffraction limit. At present, SIM requires several raw structured-illumination (SI) frames to reconstruct a super-resolution (SR) image, especially the time-consuming reconstruction of speckle SIM, which requires hundreds of SI frames. Considering this, we herein propose an untrained structured-illumination reconstruction neural network (USRNN) with known illumination patterns to reduce the amount of raw data that is required for speckle SIM reconstruction by 20 times and thus improve its temporal resolution. Benefiting from the unsupervised optimizing strategy and CNNs' structure priors, the high-frequency information is obtained from the network without the requirement of datasets; as a result, a high-fidelity SR image with approximately twofold resolution enhancement can be reconstructed using five frames or less. Experiments on reconstructing non-biological and biological samples demonstrate the high-speed and high-universality capabilities of our method.

A Review of Microsphere Super-Resolution Imaging Techniques.

Conventional optical microscopes are only able to resolve objects down to a size of approximately 200 nm due to optical diffraction limits. The rapid development of nanotechnology has increased the demand for greater imaging resolution, with a need to break through those diffraction limits. Among super-resolution techniques, microsphere imaging has emerged as a strong contender, offering low cost, simple operation, and high resolution, especially in the fields of nanodevices, biomedicine, and semiconductors. However, this technology is still in its infancy, with an inadequate understanding of the underlying principles and the technology's limited field of view. This paper comprehensively summarizes the status of current research, the advantages and disadvantages of the basic principles and methods of microsphere imaging, the materials and preparation processes, microsphere manipulation methods, and applications. The paper also summarizes future development trends.

Generation of needle-shaped beam with super-resolution spot and large depth of focus using diffractive optics

The finite depth of focus (DOF) of a focusing lens gives rise to the degradation of lateral resolution at defocused positions. Consequently, the generation of a needle-shaped beam with sub-diffraction size and extended DOF can break through this limitation, but few studies have effectively balanced these two characteristics. In this paper, we propose a method based on the stochastic parallel gradient descent (SPGD) algorithm for designing multi-ring diffractive optical elements (DOEs) to produce the needle-shaped beam, which maintains a super-resolution and uniform spot within a large extended DOF range. Numerical simulations and experimental results have validated that our method can extend the DOF by 14 times longitudinally and reduce the lateral resolution to 73% of Airy spot, while maintaining up to 92% uniformity of the beam diameter. Furthermore, we demonstrate that the DOF of the needle-shaped beam extends with the increase of the number of rings and phase levels within the DOE. Our work provides an effective strategy for the design of DOF-extended DOE and it is anticipated to find potential applications in microscopy and three-dimensional imaging, photolithography, and optical manipulation.

Ultraprecision machining of aberration-corrected diffractive optics with phase correcting groove trajectories

Ultraprecision machining of aberration-corrected diffractive optics with phase correcting groove trajectories

Subwavelength Photorefractive Grating in a Thin‐Film Lithium Niobate Microcavity

AbstractSubwavelength gratings play a fundamental and pivotal role in numerous science and applications for wave manipulation, exhibiting distinctive features such as filtering, phase manipulation, and anti‐reflection. However, conventional fabrication methods for ultrasmall periodic structures are constrained by the fundamental optical diffraction limit, making it challenging to produce subwavelength gratings for optics. Here, a novel technique is demonstrated to build a reconfigurable subwavelength photorefractive grating (SPG) in a thin‐film lithium niobate on the platform of an optical microcavity. Such SPGs are optically induced through the photorefractive effect and the subwavelength features originate from the spatial phase modulations of the pump's standing wave. The resulting SPGs lead to the mode splitting of two counter‐propagating modes inside the microcavity, exhibiting an Electromagnetically Induced Transparency (EIT)‐like transmission spectrum. Moreover, the unique subwavelength characteristic of SPGs enables first‐order quasi‐phase‐matching for backward second‐harmonic generation, a long‐standing problem in nonlinear optics. Also, free‐space‐to‐chip vertical nonlinear frequency conversion can be achieved in a similar manner. These results provide a flexible approach toward fabricating subwavelength gratings, which holds significant potential in various applications such as nonlinear frequency conversion, optical communication, sensing, and quantum technologies.

Deep correlated speckles: suppressing correlation fluctuation and optical diffraction

The generation of speckle patterns via random matrices, statistical definitions, or apertures may not always result in optimal outcomes. Issues such as correlation fluctuations in low ensemble numbers and diffraction in long-distance propagation can arise. Instead of improving results of specific applications, our solution is catching deep correlations of patterns with the framework, Speckle-Net, which is fundamental and universally applicable to various systems. We demonstrate this in computational ghost imaging (CGI) and structured illumination microscopy (SIM). In CGI with extremely low ensemble number, it customizes correlation width and minimizes correlation fluctuations in illuminating patterns to achieve higher-quality images. It also creates non-Rayleigh nondiffracting speckle patterns only through a phase mask modulation, which overcomes the power loss in the traditional ring-aperture method. Our approach provides new insights into the nontrivial speckle patterns and has great potential for a variety of applications including dynamic SIM, X-ray and photo-acoustic imaging, and disorder physics.

Characterizing glare effects associated with diffractive optics in presbyopia-correcting intraocular lenses.

To objectively quantify glare of intraocular lenses (IOLs) utilizing a diffractive principle to extend visual range. To identify models with increased susceptibility to inducing glare. David J Apple Laboratory, Heidelberg, Germany. Laboratory investigation. Glare was assessed by means of straylight parameter with a standard C-Quant (Oculus GmbH) intended for 7°. In addition, two C-Quant modifications were used to test lower angles (i.e., 2.5° and 3.5°). The following IOL models were assessed: PanOptix (Alcon Inc.) and AT Lisa Tri (Zeiss Meditec), Synergy (J&J Vision), and Triumf (BVI Medical), the latter two with chromatic-aberration correction at distance. Straylight from trifocals was compared against a monofocal W-60R lens (Santen). The C-Quant test was performed through the studied IOLs via additional optical components attached to its ocular. Straylight (deg2sr-1) of the control was <1 at all tested angles, with the trifocal models showing comparable straylight at 7°. At 3.5°, Triumf's straylight increased to 15.5 ±0.6, followed by Synergy (6.2 ±1.1), PanOptix (4.1 ±0.3), and AT Lisa Tri (2.0 ±0.8). The chromatic-aberration correcting models demonstrated correspondingly high straylight (Synergy: 18.8 ±1.3; Triumf: 17.3 ±0.5) at 2.5° compared to PanOptix (4.3 ±0.4), AT Lisa Tri (2.1 ±0.1) and monofocal IOLs yielding minimal or no increase. Trifocal IOLs induced increased straylight, but it was limited to lower angles, which may cause difficulties detecting these effects using a standard clinical approach. The latest IOL designs featuring chromatic aberration correction at far-focus appear more susceptible than the established trifocals to inducing glare phenomena.

Terminal-substituted benzoxazole based Schiff-base Mesogens and their Cu(II) complexes: Synthesis and structural characterization

Three homologous series of 3‑hydroxy-4-(((-4-benzoxazol-2-yl)phenyl)imino)methyl)phenyl-4-alkoxybenzoate (with ester linkage) and 5-(alkoxy)-2-(((4-(5-benzoxazol-2-yl)phenyl)imino)methylphenol (without ester linkage) Schiff based derivatives, bearing different substituents at 5 position i.e., 5-methylbenzoxazole, benzoxazole and 5-chlorobenzoxazole units at the end of the molecule and their Copper complexes were synthesised. The molecular structure of all the compounds were elucidated by different spectroscopic techniques such as FT-IR, 1H & 13C NMR and mass spectrometry together with elemental analysis. Their phase transition behaviour was investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. The structure-property relationships, compounds bearing a variety of polar substituents and changing the spacer from ester linked to without ester-linked has been studied. Substituents with stronger electron-withdrawing properties gave a wider mesomorphic temperature domain, in the order Cl > CH3>H.

Effect of Al-Si and Al-Mg filler wires on microstructure and mechanical properties of bead-on-plate weld on AA6061-T6 using CMT welding

In recent times, cold metal transfer (CMT) welding has become a popular choice to weld large structures of aluminium and its alloys as it provides good quality weld beads with lesser heat input. The welded joint of AA6061-T6 achieved through CMT welding provides the desired characteristics and better mechanical performance than other fusion welding methods. In the present work, bead-on-plate experimentation has been performed on an AA6061-T6 plate with two different filler wires ER4043 and ER5356 using two combinations of welding speed and current, keeping the gas flow rate constant throughout the experiment in CMT welding. Optical micrograph and X-ray diffraction analysis have been studied from which it has been observed that bead welded with ER5356 exhibits more fine equiaxed grain structure in the fusion zone and columnar-dendritic fragmentation in HAZ than beads welded with ER4043 due to the presence of higher amount of Mg2Si. X-ray diffraction reveals the presence of different phases with silicon found exclusively in specimens welded with ER4043. Porosity, dilution, microhardness and tensile strength have been examined over the weld bead and a comparison has been done between the filler wires at the same welding parameters. Porosity levels have notably elevated, measuring 53.69% and 55.71% higher in specimens welded with ER4043 filler than ER5356. Similarly, the dilution level is 33.33% and 17.51% higher in specimens welded with ER4043 filler than ER5356. Conversely, specimens welded with ER5356 filler exhibited a 4.02% and 20.37% increase in microhardness and a 3.42% and 8.82% improvement in tensile strength as compared to ER4043.

Broadband high-efficiency plasmonic metalens with negative dispersion characteristic

Controlling the dispersion characteristic of metasurfaces (or metalenses) along a broad bandwidth is of great importance to develop high-performance broadband metadevices. Different from traditional lenses that rely on the material refractive index along the light trajectory, metasurfaces or metalenses provide a new regime of dispersion control via a sub-wavelength metastructure, which is known as negative chromatic dispersion. However, broadband metalenses design with high-performance focusing especially with a reduced device dimension is a significant challenge in society. Here, we design, fabricate, and demonstrate a broadband high-performance diffractive-type plasmonic metalens based on a circular split-ring resonator metasurface with a relative working bandwidth of 28.6%. The metalens thickness is only 0.09λ0 ( λ0 is at the central wavelength), which is much thinner than previous broadband all-dielectric metalenses. The full-wave simulation results show that both high transmissive efficiency above 80% (the maximum is even above 90%) and high average focusing efficiency above 45% (the maximum is 56%) are achieved within the entire working bandwidth of 9–12 GHz. Moreover, an average high numerical aperture of 0.7 ( NA=0.7 ) of high-efficiency microwave metalens is obtained in the simulations. The broadband high-performance metalens is also fabricated and experimental measurements verify its much higher average focusing efficiency of 55% (the maximum is above 65% within the broad bandwidth) and a moderate high NA of 0.6. The proposed plasmonic metalens can facilitate the development of wavelength-dependent broadband diffractive devices and is also meaningful to further studies on arbitrary dispersion control in diffractive optics based on plasmonic metasurfaces.

Ultrafast Kapitza-Dirac effect.

Similar to the optical diffraction of light passing through a material grating, the Kapitza-Dirac effect occurs when an electron is diffracted by a standing light wave. In its original description, the effect is time independent. Here, we extended the Kapitza-Dirac effect to the time domain. By tracking the spatiotemporal evolution of a pulsed electron wave packet diffracted by a 60-femtosecond (where one femtosecond = 10-15 seconds) standing wave pulse in a pump-probe scheme, we observed time-dependent diffraction patterns. The fringe spacing in the observed pattern differs from that generated by the conventional Kapitza-Dirac effect. By exploiting this time-resolved diffraction scheme, we can access the time evolution of the phase properties of a free electron and potentially image ionic potentials and electronic decoherences.

Direct observation of a crescent-shape chromosome in expanded Bacillus subtilis cells

Bacterial chromosomes are folded into tightly regulated three-dimensional structures to ensure proper transcription, replication, and segregation of the genetic information. Direct visualization of chromosomal shape within bacterial cells is hampered by cell-wall confinement and the optical diffraction limit. Here, we combine cell-shape manipulation strategies, high-resolution fluorescence microscopy techniques, and genetic engineering to visualize the shape of unconfined bacterial chromosome in real-time in live Bacillus subtilis cells that are expanded in volume. We show that the chromosomes predominantly exhibit crescent shapes with a non-uniform DNA density that is increased near the origin of replication (oriC). Additionally, we localized ParB and BsSMC proteins – the key drivers of chromosomal organization – along the contour of the crescent chromosome, showing the highest density near oriC. Opening of the BsSMC ring complex disrupted the crescent chromosome shape and instead yielded a torus shape. These findings help to understand the threedimensional organization of the chromosome and the main protein complexes that underlie its structure.

Spin–orbit torque reconfigurable diffraction gratings

Spin–orbit torque (SOT) provides a convenient, fast, and efficient way to switch the magnetization and has been applied in memory, logic, and microwave devices. Here, we report the application of SOT in the spatial modulation of the optical field. We construct a diffraction grating composed of perpendicularly magnetized current channels that can be switched using the SOT current pulses, and the resulting periodic magnetization patterns are transferred into the polarization modulation of light through the magneto-optical effect. We demonstrate that the periodic structures and diffraction peaks of the grating can be reconfigured electrically. These results would open up an avenue to the application of the state-of-the-art spintronic technologies in fast tunable diffractive optics.

Cross-examination of photoinitiated carrier and structural dynamics of black phosphorus at elevated fluences.

Revived attention in black phosphorus (bP) has been tremendous in the past decade. While many photoinitiated experiments have been conducted, a cross-examination of bP's photocarrier and structural dynamics is still lacking. In this article, we provide such analysis by examining time-resolved data acquired using optical transient reflectivity and reflection ultrafast electron diffraction, two complementary methods under the same experimental conditions. At elevated excitation fluences, we find that more than 90% of the photoinjected carriers are annihilated within the first picosecond (ps) and transfer their energy to phonons in a nonthermal, anisotropic fashion. Electronically, the remaining carrier density around the band edges induces a significant interaction that leads to an interlayer lattice contraction in a few ps but soon diminishes as a result of the continuing loss of carriers. Structurally, phonon-phonon scattering redistributes the energy in the lattice and results in the generation of out-of-plane coherent acoustic phonons and thermal lattice expansion. Their onset times at ∼6ps are found to be in good agreement. Later, a thermalized quasi-equilibrium state is reached following a period of about 40-50ps. Hence, we propose a picture with five temporal regimes for bP's photodynamics.

Non-steady state thermometry with optical diffraction tomography.

Label-free thermometry is a pivotal tool for many disciplines. However, most current approaches are only suitable for planar heat sources in steady state, thereby restricting the range of systems that can be reliably studied. Here, we introduce pump probe-based optical diffraction tomography (ODT) as a method to map temperature precisely and accurately in three dimensions (3D) at the single-particle level. To do so, we first systematically characterize the thermal landscape in a model system consisting of gold nanorods in a microchamber and then benchmark the results against simulations and quantitative phase imaging thermometry. We then apply ODT thermometry to resolve thermal landscapes inaccessible to other label-free approaches in the form of nonplanar heat sources embedded in complex environments and freely diffusing gold nanorods in a microchamber. Last, we foresee that our approach will find many applications where routine thermal characterization of heterogeneous nanoparticles samples in 3D or in non-steady state is required.