Two-beam Interference Research Articles

Formation of multibeam dynamic gradient interference light fields with refractive optical elements

This paper presents methods for the formation and properties of light fields that controllably vary in time and space, obtained as a result of the interference of three or four coherent light beams using refractive optical elements. The formation of three-beam and four-beam interference fields is carried out using trihedral and tetrahedral glass pyramids respectively. The possibility of the interference field displacement in the transverse plane is ensured by devices for controlled phase changes of at least two of the interfering beams. The directions of propagation of these beams do not lie in the same plane with the optical axis of the original beam incident on the pyramid. In threeand four-beam dynamic interference fields the peak intensity values are higher than in the original laser beams and two-beam interference fields, so it is advisable to use them for processing flat objects with laser radiation, moving the interference maxima along the surface of the object. With a pairwise azimuthal displacement of the propagation directions of four interfering beams around the longitudinal axis, a dynamic interference field is formed, the periodically structured maxima of which cyclically smoothly change their shape from cells to band and back. At different speeds of pairs of directions the interference structure of the maxima rotates around the longitudinal axis. Therefore, this field can be used for therapeutic effects on biological tissues and for mixing microparticles in suspensions and emulsions. Since the local maxima of the intensity of all these interference fields have dimensions of the order of several micrometers while exceeding in value the maximum intensity of the initial light beam, these fields in the cross section are gradient and therefore can be used not only for laser exposure, but also for moving ensembles of microparticles including for sorting and changing concentration.

Optimizing broadband antireflection with Au micropatterns: a combined FDTD simulation andtwo-beam LIL approach.

Broadband antireflection (AR) is highly significant in a wide range of optical applications, and using a gold (Au) micropattern presents a viable method for controlling the behavior of light propagation. This study investigates a novel, to the best of our knowledge, methodology to achieve broadband AR properties in Au micropatterns. It employed the three-dimensional finite-difference time-domain (FDTD) method to simulate and optimize the design of micropatterns. In contrast, the fabrication of Au micropatterns was carried out using two-beam laser interference lithography (LIL). The fabricated Au micropatterns were characterized by a scanning electron microscope (SEM) and spectroscope to validate their antireflection and transmission properties and evaluate their performance at various wavelengths. The optimized Au micropatterns had a high transmittance rating of 96.2%. In addition, the device exhibits a broad-spectrum antireflective property, covering wavelengths ranging from 400 to 1100nm. The simulation data and experimentally derived results show comparable patterns. These structures can potentially be employed in many optical devices, such as solar cells and photodetectors, whereby achieving optimal device performance reduced reflection and enhanced light absorption.

Transversely graded polarization volume gratings fabricated by freeform holographic photoalignment.

Polarization volume gratings (PVGs) based on chiral nematic liquid crystals offer a great potential as polarization-dependent holographic optical elements, but it is not easy to fabricate PVGs with varying pattern periods in the transverse plane. Here, we fabricate a PVG with an in-plane gradient of the pattern period by performing two-beam interference photoalignment on a flexible polyimide substrate. The pattern period varies depending on the local interference angle, which is controlled by the bent shape of the flexible substrate. We demonstrate fabrication of a PVG with a linearly graded sub-micrometer period, showing the potential of the proposed method to fabricate designer PVGs.

Real-time spectral interferometry enables ultrafast acoustic detection

Optical interferometry is a promising alternative for acoustic detection as it records the changes of interference patterns. Apart from interferometric sensor heads, readout systems also play a crucial role in sensing performance. Here, inspired by the soliton molecule vibrations in ultrafast lasers, we introduce an efficient real-time spectral interferometry (RSI) approach to read out the Fabry–Pérot interferometer (FPI) for acoustic detection. Broadband pulses, emitted from an ultrafast fiber laser, are launched into the FPI sensor. Pseudo dual-pulse molecule is constructed by virtue of the equivalent two-beam interference of the FPI and modulated by the diaphragm transducer. The acoustic driven “molecular vibration” conforms to the sound applied on the metal diaphragm. Hence, the acoustic signals can be directly recorded by the successive dual-pulse spectral interferograms, imaged as a spectral encoded “soundtrack.” We experimentally achieve the real-time characterization of both the audible and ultra sounds by retrieving the relative phase evolutions with a phase resolution of 37.6 mrad and preliminarily verifying the feasibility of the RSI in acoustic detection. This approach to wideband acoustic detection highlights an advanced application of ultrafast laser sources and paves an efficient way for interrogating the interferometric fiber sensors.

Single-shot digital holography of multiple-beam Fizeau holograms for optical phase reconstruction through the focal depth of optical fibers

Multiple-beam Fizeau interference (MBFI) is characterized by its ability to represent the fine features of phase objects. Single-shot digital holographic interferometry (SSDHI) is used for numerical focusing of the recorded holograms. In this work, the two methods are gathered and employed for solving the problem of studying the thick phase objects. The problem of thick phase objects, such as optical fibers, in MBFI is the full focusing of the fiber's optical phase in one-time single shot phase map. The proposed method is designed to collect the optical phase map due to the optical fiber through the focal depth. The main idea is based on the analysis of the MBFI to several two-beam interference sets. Each two-beam interference is treated by SSDHI method to get the optical phase at different planes of focus inside the object. Then, the resultant optical phase due to the optical fiber is collected from these planes of focus. Retrieving the optical phase maps of the multiple-beam holograms through the focal depth, for the first time, is possible using the angular spectrum method. In this case, a laser source with quite long coherence length is used as a light source.The proposed method is applied on two thick samples of, single mode and graded index, optical fibers to reconstruct the resultant optical phase maps through the focal depth. The reconstructed optical phase maps are determined for the harmonics of MBFI holograms’ angular spectra. The method proves its ability to provide more accurate, numerical focusing and phase maps reconstruction. Where, MBFI analyses indicate the magnification of optical phase differences.

Gradient polarization volume grating with wide angular bandwidth for augmented reality.

Angular bandwidth, which is critical to field-of-view, plays important role in diffractive optical waveguide augmented reality display. However, design and fabrication of large angular bandwidth is still a challenge. Herein, we demonstrate a liquid crystal reflective gradient polarization volume grating with three-dimensional gradient periodic structure for waveguide near-eye display. Two-beam polarization interference with special designed periodic gradient photomask are applied to chiral-dopant reactive mesogens doped with ultraviolet dye for generating gradient three-dimensional configuration of liquid crystals, resulting in gradient polarization volume grating with extended angle bandwidth of 61° while keeping 80% diffraction efficiency, with peak efficiency near 100%. The proposed gradient polarization volume grating provides an effective method to broaden the angular bandwidth in waveguide for wide field-of-view augmented reality display.

Optical phase retrieving of a projected object by employing a differentiation of a single pattern of two-beam interference

In this work, we present a new approach to retrieve the optical phase map of an object which is projected by a single differentiated two-beam interference pattern. This approach is based on the differentiation of the intensity equation of the two-beam interference with respect to the carrier’s phase angle. Therefore, two interference patterns which are shifted by a very small phase angle can be obtained. Then, these two patterns are projected on the object. By exploiting the definition of the mathematical differentiation, the optical phase object’s variations are retrieved from the recorded intensity distributions of both projected patterns. According to this method, the extracted optical phase angles are raised as an inverse “sin” function. This means that the unwrapping process of this function limits the recovered phase angles between − π/2 and π/2. So, the unwrapping process of these unusual wrapped phase angles is explained. The proposed method is applied on (a) two objects which are simulated by combinations of multiple Gaussian functions and (b) a 3D real object. It is found that the inclination of the projected interference pattern on the object redistributes the intensity distribution due to the Lamber’s “cos” aw of illumination. This effect is considered in the retrieving process of the object’s phase map. The limitations of the presented method are discussed and the obtained results are found promising.

Optical phase retrieving employing differentiation of a couple of two-beam π/2 phase-shifted projected interference patterns

An approach to retrieve the 3D surface map of an estimated (i.e., digitally generated) object by projection of a couple of differentiated two-beam interference patterns is presented. A couple of two-beam interference patterns of a certain spatial frequency, shifted by a phase angle (π/2), are produced. Each one of these patterns is differentiated with respect to the carrier’s phase angle. The four obtained interference patterns are projected onto two estimated objects. Each object’s phase variations are retrieved utilizing the definition of the mathematical differentiation of the intensity distribution of the projected patterns. The inclination of the projected interference pattern on the object’s topography caused a redistribution of the intensity according to Lambert’s “cos” law of illumination. This effect is considered when the object’s phase map is retrieved. The limitations of the presented approach are discussed.

Fabrication of periodic hierarchical structures with anti-icing performance by direct laser interference lithography and hydrothermal treatment

Ice accumulation could result in a massive economic burden as well as major life safety issues. Because of their immense potential to ease icing problems, superhydrophobic structures have captured the curiosity of many researchers. Herein, the periodic hierarchical porous structures that have outstanding anti-icing performance were created using two-beam direct laser interference lithography and hydrothermal treatment. The periodic hierarchical porous structures were composed of periodic micro ridge arrays covered with interlaced grown standing nanoflakes. The high-water repellency of the periodic hierarchical porous structures was characterized by the high contact angle and the low rolling angle of 163.7° ± 1.5° and 3.4° ± 0.8°, respectively. The high-water repellency of the periodic hierarchical porous structures exhibits a long-term stability in air and durability in contact with water. The experimental results indicated that the anti-icing performance of the periodic hierarchical porous structures was achieved through the long-term inhibition of ice nucleation and low ice adhesion strength. The ice nucleation inhibition time could reach up to 378 s, which was more than 11.8 times longer than the untreated surface. The ice adhesion strength could be reduced to 48.3 kPa, which was more than 6.3 times lower than the untreated surface. The proposed manufacturing strategy and designed structures could be useful for designing anti-icing structures.

A refined method for investigating the morphological and optical properties of a biodegradable monocryl suture

An interferometric computed tomography (ICT) technique with a refined method is proposed and analysed for investigating the morphology of the Monocryl surgical sutures during the degradation process. This method includes four integrated stages. In the first stage, the two-beam Pluta polarizing interference microscope is equipped with the rotational device for repeatedly recording the 2D interference patterns of the degraded sample at various angular rotations. The second stage in which an artificial intelligence model is designed to detect and remove the noise classes in the interference patterns. In the third stage, the computational Phase-Shifting interferometric approach (CPSI) is employed for demodulating the phase maps of the tested sample. Finally, autocorrelation and the filtered back projection (FBP) algorithms are utilized to explore the 3D phase morphology of the tested sample. The presented method is useful for determining the refractive indices and the birefringence during the whole degradation duration. Collectively, the results of the proposed method reveal that ICT technology has the potential to investigate the degradation performances of this kind of biodegradable polymer effectively.

Multi-Scale Structuring of CoCrMo and AZ91D Magnesium Alloys Using Direct Laser Interference Patterning

In this work, the technique of Direct Laser Interference Patterning (DLIP) was used to fabricate micrometric structures at the surface of Cobalt-Chromium-Molybdenum and AZ91D magnesium alloys. Line-like patterns with spatial periods of 5 μm were textured using an ultra-short pulsed laser (10 ps pulse duration and 1064 nm wavelength) with a two-beam interference setup. The surface topography, morphology, and chemical modifications were analysed using Confocal Microscopy, Scanning Electron Microscopy, and Energy Dispersive Spectroscopy (EDS), respectively. Laser fluence and pulse overlap were varied to evaluate their influence on the final structure. Homogeneous structures were achieved for the CoCrMo alloy for every condition tested, with deeper structures (up to 0.85 μm) being achieved for higher energy levels (higher overlap and/or fluence). For high energy, sub-micrometric secondary structures, so-called LIPSS, could also be observed on the CoCrMo. The EDS analysis showed some oxidation after the laser texturing. Regarding the AZ91D alloy, deeper structures could be achieved (up to 2.5 μm), but more melting and oxidation was observed, forming spherical oxide particles. Nonetheless, these results bring new perspectives on the fabrication of microtextures on the surface of CoCrMo and AZ91D using DLIP.

Realization of high depth resolution using two-beam self-mixing interferometry with a terahertz quantum cascade laser

We study a two-beam self-mixing interference in a terahertz quantum cascade laser. The behavior of the self-mixing interference is simulated theoretically, and is demonstrated by designing experiments. The information of terahertz waves reflected from targets can be read out from the amplitude and phase of the self-mixing signals. Using the terahertz two-beam self-mixing interference method, a pattern on the back surface of a GaAs sample is imaged in both amplitude and phase modes, with the terahertz laser incident on the front surface. The sample thickness variation is accurately measured with a depth resolution in micron scale. Our study provides a way for achieving high precision terahertz far-field thickness measurement and imaging.

Fabrication of Cu grating guided-mode resonance filter by electroplating

A polarization wavelength filter was fabricated using two-beam interference and electroplating methods. The device consists of a Cu grating with a pitch of 400 nm on an ITO film. For normal incidence, the transverse magnetic (TM) transmission spectrum had a sharp spectrum at a wavelength of 648 nm and a significant dip at 735 nm. Furthermore, a sharp reflection spectrum in TM-light was obtained at a 685 nm wavelength and incident angle of 40°, and the peak wavelength shifted to a longer wavelength as the incident angle increased. This fabrication method is less complex and inexpensive than conventional methods.

Surface engineering of lead-free two-dimensional organic-inorganic hybrid perovskite by laser interference lithography

Laser interference patterning of environment-friendly material with micro/nanoscale surface structures is in high demand for applications. We demonstrated the fabrication of periodic structure in copper-based organic-inorganic hybrid perovskite thin film by 532 nm laser two-beam interference. The extinction coefficient and index of refraction of the copper-based perovskite thin film were characterized by spectroscopic ellipsometry. The morphology of laser-induced structure was investigated by a combination of optical microscopy, scanning electron microscopy, and atomic force microscopy. In addition, the mechanism of the interaction between laser and copper-based perovskite thin film was discussed. This investigation expands the materials selection in laser fabrication and paves the way for optimization towards obtaining micro/nano features for applications in diffractive optics.

Photo-Induced Relief in Rheology of Liquid Crystals

In this paper, we report the first experimental results on capillary shear flows of a nematic liquid crystal 5CB (4-cyano-4′-pentylbiphenyl), arising due to interaction of the anisotropic liquid, correspondent to the continuous rotational symmetry, with photo-profiled polymer surfaces. The regular surface relief was obtained due to opto-mechanical deformation of azobenzene containing potoresponsive polymer film (PAZO) during irradiation with two-beam interference. Such surface treatment makes it possible to obtain a regular submicron profile with well-defined characteristics (direction, period, and height). The polarizing microscopy (PM) and dynamic light scattering (DLS) techniques were used to determine the direction of the surface orientation of LC and anchoring strength, which characterize the interaction of LC with the photo-profiled polymer surface. Two types of shear flows—spreading of LC droplets and capillary flow in a plane capillary, induced by the interaction of LC with one or two photo-profiled surfaces—were investigated for different directions of the flow relative to the direction of the relief. Strong anisotropy in the dynamics of the precursor film and contact line motion, as well as in the dynamical contact angle, was established. The experimental results were analyzed and compared with those previously obtained at the investigation of the spreading of LC droplets over a mechanically stamped submicron profile and capillary flows in plane capillaries with photo-aligned surfaces.

Estimation of hazardous and noxious substance (toluene) thickness using hyperspectral remote sensing

A hazardous noxious substance (HNS) spill accident is one of the most devastating maritime disasters as it is accompanied by toxicity, fire, and explosions in the ocean. To monitor an HNS spill, it is necessary to develop a remote sensing–based HNS monitoring technique that can observe a wide area with high resolution. We designed and performed a ground HNS spill experiment using a hyperspectral sensor to detect HNS spill areas and estimate the spill volume. HNS images were obtained by pouring 1 L of toluene into an outdoor marine pool and observing it with a hyperspectral sensor capable of measuring the shortwave infrared channel installed at a height of approximately 12 m. The pure endmember spectra of toluene and seawater were extracted using principal component analysis and N-FINDR, and a Gaussian mixture model was applied to the toluene abundance fraction. Consequently, a toluene spill area of approximately 2.4317 m2 was detected according to the 36% criteria suitable for HNS detection. The HNS thickness estimation was based on a three-layer two-beam interference theory model. Because toluene has a maximum extinction coefficient of 1.3055 mm at a wavelength of 1,678 nm, the closest 1,676.5 nm toluene reflectance image was used for thickness estimation. Considering the detection area and ground resolution, the amount of leaked toluene was estimated to be 0.9336 L. As the amount of toluene used in the actual ground experiment was 1 L, the accuracy of our estimation is approximately 93.36%. Previous studies on HNS monitoring based on remote sensing are lacking in comparison to those on oil spills. This study is expected to contribute to the establishment of maritime HNS spill response strategies in the near future based on the novel hyperspectral HNS experiment.

Multi-scale textured PEEK surfaces obtained by direct laser interference patterning using IR ultra-short pulses

In this work, we aimed at getting the first insight into the influence of the laser processing parameters on the production of functional multi-scale PEEK surfaces using the Direct Laser Interference Patterning technology. An infrared ultra-short pulsed laser source (1064 nm, 10 ps) and a two-beam interference setup were used. The fabricated structures morphology was analyzed by Scanning Electron Microscopy and Confocal Microscopy. As a result, line-like structures with periodic spatial repetition (p) at two levels were produced: main structures with p = 5 µm and mean depth values up to ∼2.5 µm (aspect ratio of ∼0.5) and secondary structures, perpendicularly oriented relative to the main structures, formed at DLIP maxima position (top part of ridges) with p≈1 µm (LIPSS).

Multi-Spiral Laser Patterning of Azopolymer Thin Films for Generation of Orbital Angular Momentum Light.

Recently, the realization of the spiral mass transfer of matter has attracted the attention of many researchers. Nano- and microstructures fabricated with such mass transfer can be used for the generation of light with non-zero orbital angular momentum (OAM) or the sensing of chiral molecules. In the case of metals and semiconductors, the chirality of formed spiral-shaped microstructures depends on the topological charge (TC) of the illuminating optical vortex (OV) beam. The situation is quite different with polarization-sensitive materials such as azopolymers, azobenzene-containing polymers. Azopolymers show polarization-sensitive mass transfer both at the meso and macro levels and have huge potential in diffractive optics and photonics. Previously, only one-spiral patterns formed in thin azopolymer films using circularly polarized OV beams and double-spiral patterns formed using linearly polarized OV beams have been demonstrated. In these cases, the TC of the used OV beams did not affect the number of formed spirals. In this study, we propose to use two-beam (an OV and a Gaussian beam with a spherical wavefront) interference lithography for realization spiral mass transfer with the desired number of formed spirals. The TC of the OV beam allows for controlling the number of formed spirals. We show the microstructures fabricated by the laser processing of thin azopolymer films can be used for the generation of OAM light at the microscale with the desired TC. The experimentally obtained results are in good agreement with the numerically obtained results and demonstrate the potential of the use of such techniques for the laser material processing of polarization-sensitive materials.

ЧУТЛИВИЙ ЕЛЕМЕНТ СЕНСОРА ПОВЕРХНЕВИХ ПЛАЗМОНІВ З ТЕРМІЧНОЮ МОДИФІКАЦІЄЮ СТРУКТУРНИХ ВЛАСТИВОСТЕЙ ПОЛІМЕРНОЇ ПІДКЛАДКИ

The development of an effective sensing element (ChE) sensor with a prism type of excitation (Kretchman configuration) of surface plasmon resonance (SPR) in a gold film and a mechanical survey of the angle of incidence of monochromatic light based on a polymer substrate is based on a combination of radiation binding schemes using a prism and a lattice to prevent losses in the active metal film. Replacing the glass substrate with a polymer one has reduced the cost of ChE, led to increased sensitivity and simplification of its manufacturing technology, which does not require the use of intermediate adhesion layers. Additionally, we applied a thermal method of modifying the optical and structural properties of the substrate – hot pressing of the matrix, which contained a periodically nanostructured surface relief in the form of a diffraction two-dimensional (2D) lattice. The original 2D lattice was recorded on photoresist-covered (Shepley 1805) glass plates using the method of two-beam interference (He-Cd laser, = 440 nm) at double exposure (time 2040 s, power 20 mW/cm2) of the sample with a rotation of 90. Characteristics of ChE on an optical polycarbonate substrate (d = 2.25 mm, n = 1.58 at ( = 650 nm) after hot pressing, which contained a flat and periodically nanostructured surface in the form of a diffraction 2D lattice, followed by the deposition of a thin (d 40 nm) layer of gold on them was examined using an atomic force microscope (AFM) and the device "Plasmon". AFM studies have shown that the technological technique used made it possible to obtain lateral-ordered structures in the form of pyramids with calculated parameters – a period of 422435 nm (spatial frequency of about 2350 lin/mm) and a relief depth of 7090 nm. However, the preservation of a wave-like relief (about 300 nm) on a flat part of the surface and double ray refraction of the polymer substrate, encourage optimization of the hot pressing process using the resulting matrix on polymer substrates of smaller thickness.

Improved spectral reflectance model of oil films on seawater based on two-beam interference theory.

The theoretical properties of electromagnetic transmission and radiation of light are combined with the propagation law of light in a uniform medium and the Fresnel formula to provide an improved two-beam interference model for simulating oil spills. This model enables the study of spectral reflectance under different thicknesses and different illumination angles. Through observation and analysis of simulation results, it is found that light waves reflected at the interface of water and oil films and of oil films and air oscillate due to interference. In order to overcome this problem, solar irradiance is simulated and multiplied with the reflection model to obtain the outgoing illuminance values at different wavelengths, which are convoluted with the slit function of the hyperspectral camera. The processed results are compared with incoming illuminance values to obtain new simulation results. In addition, based on portable hyperspectral imagery and imported xenon lamp light sources and other experimental equipment, indoor simulated oil spill experiments were conducted. By comparing and analyzing the results of these simulations, the accuracy of the improved model was further improved, demonstrating the feasibility and utility of the model in studying oil spills at water surfaces.