Two-beam Interference Pattern Research Articles

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.

In Sight on the Enhancement of Optical and Structural Properties of Polyamide-6 Fibers

The work aims to modify the physical properties and structural parameters of Polyamide-6 fiber to be available for different applications through the annealing process. Polyamide-6 samples were annealed at every 10 degrees of temperatures ranging from 60 to 130 ±1°C when the time is constant (2 hours). The computer-aided two-beam polarizing interference microscope attached with variable wavelength interference filter (VAWI- technique) was used to achieve the optical measurements. The device was installed and calibrated using a software program to obtain a sharp two-beam interference pattern and sharp fringes inside the fiber. A CCD camera was used to transfer the microinterferograms to the image analysis screen to automatically measure the index of refraction and optical anisotropy of polyamide-6 fibers before and after the annealing process. To detect the improvements of the fiber structure, the oscillation and dispersion energies, dielectric constant, orientation factor, density, and crystallinity degree were calculated. The results shed light on the structural improvements of Polyamide-6 fibers on the molecular level.

Laser interference induced dielectrophoresis for cell manipulation

In this paper, laser interference induced dielectrophoresis (LIIDEP) was presented for cell manipulation and three-dimensional array assembly. Two-beam and three-beam interference systems were built to manipulate yeast cells in deionized water. When the laser power was 60 μW and the sinusoidal AC signal was 6 V at 10 kHz, the two-beam interference pattern manipulated the cells into a single-row array distribution. For three-beam interference, when the laser power was 70 μW and the frequency was 4.5 kHz, the voltage of 2.5 V enabled each spot to control one cell, and 7.5 V enabled many spots to control multiple cells. When the cell concentration was 2 × 107 cells/mL and the electrical signal was 8 V and 4.5 kHz, the cells were assembled into a three-dimensional columnar array. The feature size of laser interference pattern was adjusted to microscale and maintained sharp boundaries, which effectively improved the manipulation accuracy. Compared to the projector or laser as the ODEP light source, LIIDEP has the advantages of high manipulation resolution, good contrast and large dielectrophoretic force. This work provides an effective way for high resolution three-dimensional array manipulation and assembly of a large number of living cells on the microscale in liquid environments.

Fabrication of four-level hierarchical topographies through the combination of LIPSS and direct laser interference pattering on near-beta titanium alloy

Complex repetitive periodic surface patterns were produced on a near-beta Ti-13Nb-13Zr alloy, using two-beam Direct Laser Interference Patterning (DLIP) employing a picosecond-pulsed laser source with wavelengths of 355 nm, 532 nm and 1064 nm. Different types of Laser-induced periodic surface structures (LIPSS) are produced, including low and high spatial frequency LIPSS, which are observed frequently on top of the line-like DLIP microstructures, as well as quasi-periodic microstructures with periods greater than the laser wavelength. The feature size of the fabricated LIPSS features could be tuned as function of the utilized laser process parameters.

Fabrication of functional superhydrophobic surfaces on carbon fibre reinforced plastics by IR and UV direct laser interference patterning

The fabrication of functional microstructures on surfaces by laser enables unique material properties and is presently a leading research topic. This work addresses the production of functional hierarchical microstructures on carbon fibre reinforced polymer composites in order to control the wettability properties of the material. Two-beam Direct Laser Interference Patterning using either ultraviolet (263 nm) or infrared (1053 nm) nanosecond laser source is employed to produce melt-free and well-defined hierarchical microstructures on carbon fibre reinforced plastics. The resulting water contact angles after thin film deposition of 1H,1H,2H,2H-Perflorodecyl-triethoxysilane were analysed with respect to structure depth and quality. The maximum static contact angle of 171° is demonstrated for dual hierarchical microstructures composed of 11 µm deep large-scale pillars, covered by 1.7 µm pillars, both fabricated in a single step.

Direct laser interference patterning of stainless steel by ultrashort pulses for antibacterial surfaces

Direct Laser Interference Patterning (DLIP) with ultrashort pulses was exploited to produce tailored periodic sub-micrometer structures on stainless steel surfaces to reduce bacterial attachment and retention. Laser pulses with wavelength 1030 nm and duration 8 ps were employed to form a two-beam line interference pattern that was applied in a two-pass strategy to produce fine cross-wise surface structures with a period of ~ 850 nm and a depth of ~ 500 nm. The laser setup and process parameters were selected based on a simple theoretical model of the resulting interference pattern and ablation depth to limit the number of contact points available for bacterial cells with dimensions 500–2000 nm. Periodic ‘cones’ and ‘holes’ were produced covering areas of 250 mm2 with the same interference pattern by exploiting the dependence of laser-induced periodic surface structures on polarization. Cones and holes yielded reductions in E. coli retention of 99.8% and 99.4%, respectively, and S. aureus retention of 70.6% and 79.1%, respectively, after two hours immersion in bacterial solution compared to reference samples. Such reductions achieved over large surface areas suggests that this approach is appropriate for upscaling and high throughput production of antibacterial metallic surfaces in the food and healthcare industries.

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.

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.

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.

Two-beam interference patterning of biodegradable magnesium alloy: Influence of number of passes and spots overlap

Laser based texturing methods provide enhanced surface properties exploitable, especially in biomedical applications. Direct writing methods allow for processing features in tens of micrometers in size due to the use of diffraction limited beams. Feature size can be further reduced exploiting the light interference combined with the pulsed laser ablation. In this work, an industrial grade single mode nanosecond-pulsed green fiber laser was used to realize two-beam direct laser interference patterning system. The system was employed on a biodegradable Mg alloy to test the feasibility of the approach for submicrometric patterning. The combination of low melting point and high thermal conductivity of Mg alloy with the use nanosecond pulses generates difficulties in terms of the machining quality. The influence of number of pulses and number of passes was evaluated on the patterned area diameter as well as the pattern periodicity. Finally, patterned regions were overlapped on a scanned line to assess the feasibility of the process on larger areas.

Fabrication of diffraction based security elements using direct laser interference patterning.

Product piracy impacts wide areas of the global economy resulting in multi-billion dollar losses per year. Therefore, product protection technologies are required to produce security elements with high flexibility and complexity. In this work, the fabrication of diffraction-based security elements, so called grating cell arrays, using direct laser interference patterning is presented. Ultraviolet two-beam interference patterning is used to structure line-like gratings into 80 µm in diameter spots with spatial periods ranging from 0.7 µm to 2.2 µm on PET substrates. The developed grating cell arrays are generated by the combination of several holographic surface gratings with variable spatial period and pattern orientation. The stored security information can be visualized by illumination with a coherent light source such as a laser pointer.

Sub-micrometric surface texturing of AZ31 Mg-alloy through two-beam direct laser interference patterning with a ns-pulsed green fiber laser

Two-beam direct laser interference patterning (DLIP) is the method that employs two beams and provides control over the pattern geometry by regulating the angle between the beams and the wavelength of the beam. Despite the simplistic optical arrangement required for the method, the feasibility of sub-micrometric patterning of a surface depends on the correct manipulation of the process parameters, especially in the case of metallic materials. Magnesium alloys, from this point of view, exhibit further difficulty in processability due to low melting point and high reactivity. With biocompatibility and biodegradability features, Mg-alloy implants can take further advantage of surface structuring for tailoring the biological behaviour.In this work, a two-beam DLIP setup has been developed employing an industrial grade nanosecond-pulsed fiber laser emitting at 532nm. The high repetition rate and ramped pulse profile provided by the laser were exploited for a more flexible control over the energy content deposited over the heat-sensitive Mg-alloy. The paper describes the strategies developed for controlling ramped laser emission at 20kHz repetition rate. The process feasibility window was assessed within a large range of parameters. Within the feasibility window, a complete experimental plan was applied to investigate the effect of main laser process parameters on the pattern dimensions. Periodic surface structures with good definition down to 580nm±20nm spacing were successfully produced.

Photoresistless fabrication of periodic patterns on GaAs by laser interference photochemical lithography

SEM images of (a) two-beam and (b) three-beam interference patterns formed on GaAs wafers by photochemical etching with an incident angle of 17°.

Periodic surface pattern fabrication via biprism interference micro-machining

A novel surface micro-texturing process is proposed that is capable of generating extremely scalable periodic patterns on a workpiece surface. The process, henceforth named as ‘biprism interference micro-machining’ utilizes a two-beam interference pattern generated by a Fresnel biprism placed coaxially in the path of a laser pulse to fabricate periodic micro-channels on aluminum surfaces. The channels were fabricated over an area of approximately 8 mm × 6 mm and with a periodicity of 9 and 21 μm, by using custom-built two-faceted biprisms with side angles of 4° and 1.5°, respectively. A beam propagation simulation was carried out to predict the intensity distribution and contrast of the intensity pattern of laser pulse at the workpiece surface. The entire process takes 1–8 laser pulses, thereby demonstrating ultra-fast speed and scalability. Also, the efficiency, precision and resolution of the process are higher than that of conventional mask-based and interference-based micro-machining.

Rapid Fabrication of Periodic Patterns on Poly(styrene-co-acrylonitrile) Surfaces Using Direct Laser Interference Patterning

Periodic microstructures in styrene-acrylonitrile (SAN) copolymers are fabricated by two-beam direct laser interference patterning using a nanosecond pulsed laser operating at a wavelength of 266 nm. The SAN copolymers are synthesized using different molar ratios (styrene to acrylonitrile) by a free radical polymerization process. The chemical composition of the copolymers and their properties are determined using Fourier transformed infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Depending on the composition of the irradiated copolymer films, with weight ratios ranging from 58 to 96.5% of styrene to acrylonitrile, different ablation behaviors are observed. The laser fluence necessary to locally ablate the copolymer is found to be dependent on the copolymer composition. Unlike other dielectric polymers, the laser irradiation produced both direct ablation of the irradiated material and collapse of the surface. It is shown that, by varying the laser fluence and the copolymer composition, the surface structure can be changed from a periodic pattern with a swelled topography to an ablated-like structure. The number of holes does not depend monotonically on the amount of PS or PAN units but shows a more complex behavior which depends on the copolymer composition and the laser fluence.

A novel simultaneous photoelastic and two‐beam interferometric system: I. Dynamic full‐field evaluation of the elasticity modulus profile of polymeric fibres

A novel optical setup for simultaneous capturing of photoelastic and two-beam interference patterns was designed. The designed optical setup was used to simultaneously record two types of patterns. The first pattern is two-beam interference pattern, and the second one is photoelastic interference pattern produced by objects under stress. This simultaneous capturing of the two patterns allowed us to calculate the full-field distribution of the elasticity modulus profile of fibres. A mathematical expression of the profile of the elasticity modulus was derived. This was applied to evaluate the elasticity modulus of anisotropic isotactic polypropylene fibres during stretching processes. The profile of the elasticity modulus was determined for both static and dynamic in situ cases where the propagation of different structural deformations was observed and studied using the designed optical setup. Patterns were given for illustration.

Fabrication of holographic blazed grating with Fourier synthesis exposure

A simple, direct, and universal fabrication method for holographic blazed gratings is proposed and a 79 lines/mm holographic echelle grating with 4 μm depth quasi-triangular grooves experimentally demonstrated as an example of a blazed grating fabricated directly by Fourier synthesis exposure. With the method, we converted a periodic grating groove profile into the exposure energy spatial distribution on the surface of a photoresist with the response characteristics of the particular photoresist. Then the exposure energy could be decomposed into a series of sinusoidal functions using Fourier series, and realized by superposing a series of two-beam interference patterns. In contrast with mechanically ruled gratings, the fabrication process of holographic gratings is quite quick and low cost, and especially no Rowland ghosts appear in their dispersion spectrum. The reported work will offer a new approach for blazed grating fabrication with a holographic process.

Photo- and thermal-processing of azobenzene-containing star-shaped liquid crystals

A new class of star-shaped, liquid crystalline, low-molecular weight compounds functionalized with photochromic azobenzene and mesogenic groups was investigated in terms of light-induced anisotropy. The behaviour of the materials under the action of light with simultaneous or subsequent thermal treatment was examined with respect to the induction of anisotropy. The unconventional UV light treatment prior to the irradiation with linearly polarized light allowed induction of very high values of anisotropy (D = 0.77) at room temperature. Moreover, the simultaneous action of light and temperature led to the induction of higher values of dichroism in comparison with anisotropy generated by the standard procedure. Subsequent thermal treatment led to dewetting and the formation of 3D macroscopic stripe- and dome-like structures for one of the investigated compounds. Despite photoinduction of anisotropy by a single beam, the formation of polarization and surface relief gratings by two-beam interference pattern was also investigated.

Strong Bragg Gratings in Highly Photosensitive Photo-Thermo-Refractive-Glass Optical Fiber

A new type of photosensitive fiber is demonstrated. Long lengths <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$({&gt;}{\rm 100}~{\rm m})$</tex></formula> of coreless optical fiber are fabricated from highly photosensitive photo-thermo-refractive glass. A minimum loss of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${&lt;}{\rm 2}~{\rm dB}/{\rm m}$</tex></formula> is measured. A holographic technique using low power near-UV two-beam interference patterns is applied to record strong and robust Bragg gratings inside the fiber. The gratings show no degradation when heated up to 425 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula> for several hours.