Two-beam Interference Research Articles

Phase Analysis Method of Interferometric Pattern Generated for Optical Axis Direction Using Three Laser Beam Interferometry

In this paper, the authors propose a phase analysis method of an interferometric pattern generated for the optical axis direction using three-beam interferometry. Authors perform shape measurement using the proposed method. In three-beam interferometry, the fringe pattern along the optical axis direction appears by superimposing each two-beam interference fringe. Therefore, the power distribution of the fringe pattern appears along the optical axis direction. This method enables us to directly obtain shape and displacement using the interferometric pattern. Our method intended for wide application with 3D shape measurements and out-of-plane displacement measurements for the inspection and maintenance of large constructions. As the first step of developing a device for 3D shape and displacement measurement, in this paper, simulation and an experiment were used to confirm that interferometric pattern for optical axis directional appeared. Further, slope of object was measured from the phase shift of optical axis directional pattern.

Fabrication of inclined non-symmetrical periodic micro-structures using Direct Laser Interference Patterning

The direct fabrication of microstructures, having a non-symmetrical morphology with controllable inclination, presents nowadays a challenging task. Natural examples of surfaces with inclined topographies have shown to provide anisotropic functionalities, which have attracted the interest of several researchers in the last years. This work presents a microfabrication technique for producing microstructures with a determined and controllable inclination angle using two-beam Direct Laser Interference Patterning. Polyimide foils are irradiated with a 4 ns UV (266 nm) laser source producing line-like structures with a period varying from 4.6 µm to 16.5 µm. The inclinations, retrieved by tilting the sample with respect to the optical axis of the setup, are changed from 0° to 75°, introducing a well controllable and defined inclination of the structure walls. The structuring parameters (laser fluence, number of laser pulses and interference period) as well as the inclination of the microstructures are correlated with the global tilting of the sample. As a result, a determined laser fluence and number of pulses are necessary to observe a remarkable non-symmetrical morphology of the structures. In addition, the presence of structural undercuts is reported, which opens the possibility for developing new direction-dependent properties on polymeric materials. As an example, preliminary results on light diffraction are presented, showing a similar behavior as blazed diffraction gratings.

A spatial phase modulation laser interferometer

We propose and realize a spatial phase modulation laser interferometer (SPMLI), which mainly consists of a laser source, an optical fiber, a transparent plate and a linear charge coupled device (CCD). The reflected beams from the upper and lower surfaces of the transparent plate produce two-beam interference. The generated spatial phase modulation interference image is taken by the CCD. The spatial frequency of the image is proportional to the wave number of the laser light and the thickness of the transparent plate. Experimental results show that the wavelength and thickness can be measured in picometer-level and nanometer-level resolution, respectively. Compared with other interferometers, the proposed interferometer has the advantages of simple structure, low cost and no need for mechanical scanning components.

One-step fabrication of asymmetric saw-tooth-like surface structures on stainless steel using Direct Laser Interference Patterning

We report about the fabrication of asymmetrical periodic arrays on stainless steel by means of Direct Laser Interference Patterning. The fabrication strategy consists in irradiating the surface of the material under a certain inclination angle using a two-beam interference setup equipped with a pulsed picosecond laser system (70 ps) operating at a wavelength of 1064 nm. In particular, for tilting angles higher than 30°, well-defined saw-tooth like geometries could be produced. Due to the ultra-short duration of the laser pulses, also Laser Induced Periodic Surfaces Structures are fabricated, which create hierarchical morphologies similar to the one present in nature.

Holographic stability and storage capacity on bulk green-light sensitive TI/PMMA materials

An emerging cationic photo-initiator titanocene (TI) dispersed poly (methyl methacrylate) (PMMA) photopolymer was fabricated by an optimized three-step thermo-polymerization method with excellent holographic performances. Materials with different thicknesses (1-3 mm) were prepared and characterized experimentally. The influences of material thickness changes on holographic properties have been investigated in detail. We achieved the response time of 4.98s in 1mm TI/PMMAs, while the cumulative gratings strength of 6.88 and single grating diffraction efficiency of 74% in 3 mm ones. Furthermore, gratings recorded in materials were examined with controlling experimental conditions in a green-light two-beam coupling interference system, under different polarization directions, ambient temperatures and intersection angles, respectively. A better holographic recording condition was proposed. Meanwhile, the influence on recording surroundings for TI/PMMA were analyzed. This work can provide a basis to depict the holographic storage capacity and stability in TI/PMMA polymers.

Disintegration of multiple-beam Fizeau fringes in transmission using FFT analysis

An investigation of fast Fourier transformation (FFT) spectrum appears from multiple-beam Fizeau fringes is presented. It is proven theoretically and demonstrated experimentally that the number of the appeared peaks is related to the interfered rays’ number. In addition, a detailed interpretation of (FFT) yields from multiple-beam Fizeau fringes analyses is illustrated. This interpretation proved that every peak of the FFT spectrum represents a set of two-beam interference. Therefore, the higher order FFT peaks are not an error. It is found that the frequencies of the appeared peaks are separated by a constant increment. Therefore, when we apply the inverse fast Fourier transformation (IFFT) on a selected peak we can get a two-beam intensity distribution image with a fringe frequency depending on the peak order number. This study removes confusion and answers some important questions concerning the multiple-beam interference. The presented analysis leads to disintegrate multiple-beam interferogram to its components of two-beam interferograms. This could facilitate recovering the phase map and provides more information from one multiple-beam interferogram.

Fabrication and characterization of diffraction gratings in ophthalmic polymers by using UV direct laser interference patterning

The fabrication of diffractive elements in ophthalmic polymers to induce refractive index changes which may be applied for refractive correction is of great interest in the fields of Optics and Ophthalmology. In this work, poly-hydroxyethyl-methacrylate and silicone hydrogel polymers used as soft contact lenses were structured with linear periodic patterns by means of Direct Laser Interference Patterning (DLIP). As the laser source, a Q-switched laser system emitting 10 ns pulses at a wavelength of 266 nm was used to generate the periodic modulation on the surface of the polymer materials. The experiments were carried out employing a two-beam interference setup, studying the features of the laser processed areas as a function of both the laser fluence and the interference period. The topography of the structured areas was investigated using optical confocal microscopy. Compositional and structural modifications on the materials were studied by means of micro-Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Finally, periodic patterns were characterized through diffractive techniques under illumination of a continuous-wave 632.8 nm He-Ne laser to determine the diffractive properties of the DLIP periodic patterns and the refractive index change induced by the laser processing.

Direct quantitative imaging of the writing stage in a photosensitive azopolymer by digital holography.

In this paper, we demonstrated that the gradual formation of a surface relief grating (SRG) in azopolymer thin films under continuous light exposure could be directly observed in situ and in real-time, allowing full-field characterization with high spatial resolution. We reported here for the first time, to the best of our knowledge, that digital holography (DH) can be adopted for investigating and monitoring an inscribed holographic surface relief grating (SRG) of azopolymers by two-beam laser interference lithography over a wide area. The writing process could be assessed through quantitative phase imaging (QPI). The reported results show that the proposed method is a truly valuable diagnostic tool that can be useful for investigating the spatial distribution of the writing process, which can eventually contribute to shedding light on the still unclear origin and related mechanism of SRG formation in azopolymers.

Line-field confocal time-domain optical coherence tomography with dynamic focusing.

A time-domain optical coherence tomography technique is introduced for high-resolution B-scan imaging in real-time. The technique is based on a two-beam interference microscope with line illumination and line detection using a broadband spatially coherent light source and a line-scan camera. Multiple (2048) A-scans are acquired in parallel by scanning the sample depth while adjusting the focus. Quasi-isotropic spatial resolution of 1.3 µm × 1.1 µm (lateral × axial) is achieved. In vivo cellular-level resolution imaging of human skin is demonstrated at 10 frames per second with a penetration depth of ∼500 µm.

Modeling the Diffraction Efficiency of Reflective-Type PQ-PMMA VBG Using Simplified Rate Equations

A simulation based on the rate equations was performed to analyze the concentration distribution of molecules in phenanthrenequinone (PQ) doped polymethyl methacrylate (PMMA) polymers (PQ-PMMAs) exposed using a two-beam interference configuration. The concentration distributions were used to predict the diffraction efficiency of PQ-PMMA-based volume Bragg gratings (VBGs) by using the transfer matrix method. The simulation results suggest that an intermittent exposure method can promote the diffusion of PQ molecules and increase the final diffraction efficiency. A series of experiments validated our hypothesis that the diffraction efficiency of reflective-type PQ-PMMA VBG can be predicted.

Fabrication of biomimetic graphene films on fabric base by two-beam laser interference

Through millions of years of evolution and natural selection, various biological species own unique physiological structure and amazing functionality. Biomimetic fabrication demonstrate remarkable trend of development, artificial biomimetic materials and intelligent functional surfaces have been extensively studied by scientists both theoretically and experimentally. In general, traditional biomimetic materials consisting of metal, metal oxides, carbon materials and some other polymer materials, which make outstanding contribution to the generation and development of engineering bionics science and interface chemistry analysis. However, simpleminded traditional materials are not capable to deal with the presence of intelligent driving control and micro nano device applications in artificial biomimetic science. It is well established that the combination of novel materials and novel biomimetic technology may lead to promote progress and development of fabrication with smart integration of multifunctional biomimetic devices. More recently, a kind of carbon nanomaterials with an atomic layer thickness which named graphene has attracted most widely attention due to its excellent properties, for instance ultrahigh carrier mobility, good electrical conductivity, high transmittance and biocompatibility since it has been founded by mechanical stripping adhesive tape, it becomes a new star of materials scientific fields. Its stable physical/chemical properties make it widely applied in electronic devices, optical devices, biochips, and even intelligent robots. Based on graphenes distinctive material properties, micronano-structured graphene surfaces with special wettability may be fabricated in a biomimetic manner that not only contribute to the experimental research but also bring about a new revolution of graphene-devices. A method of fabrication of biomimetic graphene surface with multilevel-structures by two-beam laser interference system has been proposed. Graphene oxide films on different fabric bases have been reduced and ablated by Nd:YAG laser interference system. Periodic graphene micronano-structures with grating-like micro structures and additional nanoscale structures have been produced during laser processing process with appropriate laser power (0.4 W), which resulted in deoxidization of abundant hydrophilic oxygen containing groups (OCGs) on GO sheets. To original GO film, the reduced GO (RGO) surface not only exhibited more rough geometry, but also owned lower surface energy. Raman spectroscopy has been characteristiced, the result of the D and G band peaks of RGO show neglectable changes contrast GO. The reason is that the laser cutting and ablation can bring additional new defects on the edges of graphene sheets, which leads to the increase of D band peak. As a result, the spectra show unobvious changes. Roughness of the fabric surface (silk, cotton, nylon) further increased the surface roughness of biomimetic graphene surface. Graphene biomimetic surface with multi-structures has been formed by the double action of rough fabric base and graphene micronano-structure. The graphene bionic surface with nylon substrate has the largest hydrophobic angle, ~156°. The biomimetic graphene surface not only possesses superhydrophobic wettability, but also exhibits rainbow structural color due to the existence of periodic graphene micronano-structures. We believe that the biomimetic graphene surface on fabric substrate will not only have important application prospects in the field of bionics, but also be a major exploration and breakthrough in the future research of optoelectronic and wearable devices.

Nature-Inspired Quasicrystal SRG Using Fibonacci Sequences in Photo-Reconfiguration on Azo Polymer Films

In this study, we present the fabrication route of quasicrystal surface relief grating (SRG) of azopolymer. Based on photo-reconfigurable azobenzene property, various morphologies of grating patterns were inscribed on azopolymer thin-films by two-beam coupling light interference lithography (LIL) process. Multiplexing the LIL with different rotation sequence on azopolymers resulted in unusual surface structures including quasicrystal-like SRGs with Fibonacci sequences in rotational LIL. We investigated the effect of rotation sequence on the grating patterns by atomic force microscope, revealing that rotation sequence is a critical parameter determining surface structures of azopolymer films.

Laser induced nano-patterning with atomic-scale thickness on an InAs/GaAs surface

Nano-patterned substrates fabricated by different techniques have been widely applied in high performance semiconductor devices. For the influence of surface morphology on semiconductor electronic properties and epitaxy growth, there is an increasing interest to realize atomic-scale surface modification by a simple and effective method. In this study, laser induced nano-patterning with atomic-scale thickness on an InAs/GaAs surface was studied in both atmosphere and vacuum environments. Periodic nano-grooves with an average depth of 1 to 2 atomic layers were fabricated by focused laser spot scanning in the air. The depth and width of nano-grooves increase with the laser energy intensity. The result of nano-patterning is attributed to atom desorption stimulated by photon-induced electronic excitation. To explore the situation in the vacuum, in situ two-beam laser interference was applied during InAs/GaAs sample MBE growth. Periodic InGaAs nano-islands were fabricated, which show significant influence on subsequent crystal growth. The formation of ordered InAs quantum dots agreeing with the atomic layer nano-pattern demonstrates the potential application of the technique in site-controlled quantum dot growth.

Cationic photo-initiator titanocene dispersed PMMA photopolymers for holographic memories

Bulk polymer composites have been investigated in a two-beam interference exposure system of titanocene (Irgacure 784, BASF) (TI) molecules dispersed in poly(methyl methacrylate) (PMMA) substrates, which are created through an optimized three-step thermo-polymerization method, at a wavelength of 532 nm. As a property of PMMA photopolymers, the dark enhancement of grating formation is observed, which mainly corresponds to the diffusion of TI molecules. We also examine the influences of dark enhancement on Bragg angular selectivity that shifts after a long-time dark reaction. Single grating under continuous exposure was recorded in TI/PMMA photopolymers with a diffraction efficiency of 74% and response time of 20 s. An enhanced multiplexed grating was also obtained after a completely dark reaction with the cumulative grating strength of 6.88. Compared to bulk PQ/PMMA photopolymers in millimeter magnitude, TI/PMMA photopolymers are more competitive for further research on volume holographic memory storage.

Determination of the phase in the center of a circular two-beam interference pattern to determine the displacement of a rough surface

We propose a fast algorithm to extract the phase in the center of an interferogram with circular fringes obtained in a simple Michelson interferometer setup with one rough surface. The phase in the center is a linear function of the displacement of an object, which is very advantageous. It avoids the uncertainty in displacement associated with intensity measurements near a maximum or minimum. Also the direction of movement is always clear. The method used also overcomes the problems associated with speckle, unavoidable with a rough surface, common in industrial applications, when using laser as a light source. Using a large number of points in the interferogram to obtain one value largely reduces noise and also increases the accuracy. Displacement is measured with an accuracy <10 nm.

Optical modeling of the emission zone profile and optimal emitter position based on the internal field profile of the air mode in organic light-emitting diodes.

We propose a theoretical formulation to calculate the internal profile of the air mode in the organic light-emitting diode (OLED) on the combination of the transfer matrix method and source-term method. The spatial distributions of the air mode are calculated in a top-emitting OLED with respect to the light polarization, extraction angle, dipole orientation, and dipole position. Air modes are also calculated on the basis of the previously used external source model, where the input optical wave is injected from the air into the OLED multilayer. Comparison of the calculated air modes between two models checks the validity of the external source model. In addition, we propose an improved formula to determine the optimal emitter positions that maximize the two-beam interference of the micro-cavity effect. In the improved formula, a non-ideal reflection phase shift at a reflective metal anode is treated as the skin depth of the air mode. Finally, the effect of the dipole orientation on the air mode is investigated. Compared with the air mode emitted by the horizontally oriented dipole, the air mode generated by the vertically oriented dipole has relatively small intensity and shows the opposite dependence of the emitter position variation. The calculation results of the internal profile of the air mode within the emission layer are matched with the profile of the emission zone obtained by output radiant flux on the basis of the currently used point dipole model.

Fabrication of Flexible Hydrogel Sheets Featuring Periodically Spaced Circular Holes with Continuously Adjustable Size in Real Time.

We report on the formation of stimuli-responsive structured hydrogel thin films whose pattern geometry can be adjusted on demand and tuned reversibly by varying solvent quality or by changing temperature. The hydrogel films, ∼100 nm in thickness, were prepared by depositing layers of random copolymers comprising N-isopropylacrylamide and ultraviolet (UV)-active methacryloyloxybenzophenone units onto solid substrates. A two-beam interference pattern technique was used to cross-link the selected areas of the film; any unreacted material was extracted using ethanol after UV light-assisted cross-linking. In this way, we produced nanoholes, perfectly ordered structures with a narrow size distribution, negligible tortuosity, adjustable periodicity, and a high density. The diameter of the circular holes ranged from a few micrometers down to several tens of nanometers; the hole periodicity could be adjusted readily by changing the optical period of the UV interference pattern. The holes were reversibly closed and opened by swelling/deswelling the polymer networks in the presence of ethanol and water, respectively, at various temperatures. The reversible regulation of the hole diameter can be repeated many times within a few seconds. The hydrogel sheet with circular holes periodically arranged may also be transferred onto different substrates and be employed as tunable templates for the deposition of desired substances.

Phase-sensitivity-doubled surface plasmon resonance sensing via self-mixing interference.

Conventional phase-based surface plasmon resonance (SPR) sensing can achieve 10-8 RIU, but commonly requires two-beam interference. It therefore leads to complexity in terms of utilized devices, poor anti-noise ability, and demand for fine working conditions. With these requirements imposed, conventional SPR sensing has difficulties in commercial use. In this Letter, we report a simple, compact, and phase-sensitivity-doubled self-mixing interference (SMI)-based SPR sensing approach. The reported approach employs SMI and, therefore, needs only one optical path, enabling the advantages of compactness and simplicity in experimental setup, and strong anti-vibration property. With the proposed setup, the phase of light from the prism to the sample changes twice. Consequently, the sensitivity of phase is doubled. For experimental demonstration, we monitor the refractive index change of NaCl solution by using the proposed technique. The experimental results coincide with the theoretical analysis and simulation results.

(Invited) Molybdenum Disulfide Biosensors

The two-dimensional layered material MoS2 is the most common transition metal dichalcogenide. The bulk MoS2 is a semiconductor material with an indirect energy gap (about 1.3 eV) with strong sulfur and molybdenum metal in the plane of the monolayer valence bond function, and there is a very weak Van der Waals force between layers. The monolayer MoS2 has a direct energy gap (about 1.9 eV) and an N-type semiconductor material with strong light emission characteristics, high planar electron mobility and tough mechanical properties, making it applicable applies to transistors, gas sensing detector, photodetector, battery, optoelectronic components. In the past, our laboratory successfully used CVD method to grow graphene and molybdenum disulfide films, combined with the hyper-spectral imaging for the optical properties detection of a few layers of thin film. On the other hand, we also have been successfully fabricated some nanostructures of cuprous oxide and zinc oxide. These nanostructures are made of anodized aluminum and two-beam interference technology. We expect to combine two-dimensional materials and semiconductor device fabrication into biochip through the use of biochips. Using biosensors based on semiconductor material synthesis and micro-nanostructure technology, we hope to develop a low-cost and fast response time, simultaneous detection of biosensors simple program. Recent studies have found that the change of surface stress of transition metal dichalcogenide will change its optical properties such as energy gap, absorption spectrum, even the electron mobility and induced magnetic force. The changes of these properties will enhance the optoelectronic performances of the transition metal dichalcogenide device and material properties. In this study, we will design the best transition metal dichalcogenide devices based on the previous research results. We will apply these devices to biological, gas, and photoelectric sensors and discuss the relationship between the photoelectric characteristics and stress distribution of various devices, and through the deep learning technology to enhance the photoelectric efficiency of devices.

Three-layer polarization-selective dielectric transmission grating with the enhanced angular bandwidth

We design a three-layer rectangular-groove dielectric transmission grating that can be used as a polarization-selective beam splitter. For TE polarization, the grating is equivalent to a high-efficiency grating. For TM polarization, a good splitting ratio can be exhibited with 48.4%/48.8% output. Each thickness of the three grating layers can be approximately calculated by using modal method based on the physical theory of two-beam interference with the chosen duty cycle of 0.41 and period of 690 nm for an operating wavelength of 800 nm. In order to obtain accurate grating layer thickness, the rigorous coupled-wave analysis based on Maxwell's theory is used to calculate the diffraction efficiency. The polarization-selective beam splitter grating reported previously can work only at the special prescribed incident angle or narrow angular width. The three-layer dielectric grating is aimed to obtain the wide incident angular bandwidth.