Grating Lines Research Articles

Nanoimprinted ultrafine line and space nanogratings for liquid crystal alignment

Ultrafine 50 nm line and space nanogratings were fabricated using nanoimprint lithography, and were further used as an alignment layer for liquid crystals. The surface morphologies of the nanogratings were characterized and their surface energies were estimated through the measurement of the contact angles for two different liquids. Experimental results show that the surface energies of the nanogratings are anisotropic: the surface free energy towards the direction parallel to the grating lines is higher than that in the direction perpendicular to the grating lines. Electro-optical characteristics were tested from a twisted nematic liquid crystal cell, which was assembled using two identical nanogratings. Experimental results show that such a kind of nanograting is promising as an alternative to the conventional rubbing process for liquid crystal alignment.

Directed Self-Assembly of Lamellar Copolymers: Effects of Interfacial Interactions on Domain Shape.

The depth-dependent structure of a poly(styrene-b-methylmethacrylate) (PS-PMMA) line grating (46 nm pitch) was calculated from quantitative analysis of small-angle X-ray scattering profiles. These data demonstrate that domain shapes are significantly deformed near the substrate interface, where the local PS domain shape resembles an hourglass. The bulk equilibrium dimension is recovered near the center of a 64 nm thick film. Simulations based on self-consistent field theory suggest that deformations near the substrate are caused by extensive penetration of the copolymer domains into the underlying substrate coating (a PS-brush). These findings suggest that new coatings for block copolymer directed self-assembly should consider copolymer penetration lengths in addition to tailoring surface energetics. Furthermore, given the resolution and ensemble-averaging features of synchrotron X-ray scattering, we argue that it has the potential to emerge as a "gold-standard" or "benchmark" dimensional metrology and library validation tool for high density, sub-10 nm features.

Waveguide Grating Radial Polarizer for the Photolithography of Circularly Symmetrical Optical Elements

A radially periodic slab waveguide grating of concentric circular lines composed of a corrugated quartz substrate and a ZnS waveguiding layer under the condition of normal incidence <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ \hbox{TE}_{0}$</tex></formula> mode coupling at the 442-nm wavelength of a circularly polarized HeCd laser exposure beam causes a close to 100% reflection of the local TE polarization and a high transmission of the local TM polarization, thus transmitting a radially polarized beam. The functionality demonstration is first made with a grating written by e-beam; then, the same element is obtained by means of a phase-mask projection with radial spatial frequency doubling with the first element used as a radial polarizer.

Direct structural characterisation of line gratings with grazing incidence small-angle x-ray scattering

Grazing incidence small-angle x-ray scattering (GISAXS) and x-ray reflectometry (XRR) have been used to investigate structural parameters, especially period length, line width, groove width, and line height, of grating test structures in the sub-micron range. The gratings are e-beam written structures on a quartz substrate with a fixed period length, but different line and groove widths, covered by a layer of ruthenium. A Ru layer thickness of 9.4 nm has been determined with XRR. GISAXS was performed in two orientations, with an incident beam alignment perpendicular and parallel to the grating lines. The scattering patterns in parallel orientation have been analysed without numerical simulation by Fourier transformation. The obtained results for line and groove width are in good agreement with nominal values. The analysis method has been validated by analysing simulated scattering data. A superposition of scattering intensities measured for different azimuthal rotation angles close to parallel alignment was used to determine the line height of a grating of 27.3 nm, which is also close to the nominal value. The Fourier analysis procedure opens up the possibility of traceable structure determination with GISAXS in the nanometre range.

Direct laser fabrication of blaze gratings in fused silica

Results of our investigations on direct laser fabrication of blaze gratings in fused silica using a special fluorine-laser mask projection technology allowing to produce plane and smooth reflecting areas and optimum blaze geometry will be presented. In particular, it will be shown that gratings with nearly optimum blaze geometry and relatively low surface roughness of the reflecting areas can be produced. The technique is a one-step method and has a high variability with regard to grating geometry and also substrate materials. We use masks made of tantalum foil, projected onto a fused silica substrate surface with a demagnification ratio of 26:1 and also novel scattering masks made of calcium fluoride. In these masks, up to 25 times smaller transmission apertures can be manufactured allowing thus grating constants in the range of a few μm. The average surface roughness of the reflecting areas along a blaze grating line was measured to be Ra=38 nm without any surface smoothing. The maximum variance of the blaze angle within one grating is about 0.5 degree.

Angled cavity broad area quantum cascade lasers

Angled cavity broad area quantum cascade lasers (QCLs) are investigated with surface grating-based distributed feedback (DFB) mechanisms. It is found that an angled cavity incorporating a one dimensional DFB with grating lines parallel to the laser facet offers the simplest solution for single mode and diffraction limited emission in the facet normal direction. A room temperature single mode QCL with the highest output power for wavelengths longer than 10 μm is demonstrated. This structure could be applied to a wide range of laser structures for power scaling along with spectral and spatial beam control.

Single-step 3D nanolithography using plasma polymerized hexane films

► Fabrication of nanostructures in 3D is an area of increased interest. ► Using plasma polymerized resists allows for conformal coating. ► Enables high resolution nanostructures to be defined on existing topography. ► Demonstrator shown: line grating at top and bottom of 20 μm trench. The fabrication of nanostructured textures on top of a microstructured topography is a topic of considerable interest, with a number of different approaches having been proposed, most of which are either cumbersome, or represent a radical departure from standard fabrication processes. In this work, we present a simple fabrication scheme for obtaining nanopatterned structures in 3D. The scheme relies on the use of plasma polymerized hexane as an electron beam resist. We utilize the unique property of plasma polymerization where the films are deposited with conformal coverage across pre-structured surfaces. This enables high resolution electron beam lithography to be performed on a pre-patterned/etched substrate. As a demonstrator, we show that nanometric line gratings can be produced at both the top and bottom surfaces of preetcheded, 15–20 μm deep, box arrays. The final structure can be replicated in PDMS, to provide a soft working stamp for subsequent 3D patterning.

FIB Moiré Gratings and Their Application in the Measurement of Optical Fibers' Mechanical Properties

Focused ion beam (FIB) milling technique is used to fabricate specimen grating with frequency of 1000 lines/mm on the surface of optical fiber, and in-situ tensile tests are carried out by using a scanning electron microscope (SEM). The SEM scanning Moiré is formed by the superposition of the specimen grating and SEM scanning lines. The displacement field and strain field are obtained by taking advantage of the SEM Moiré method combining with random phase-shifting technique. The tensile properties of the optical fibers are measured according to the stress-strain curve. The experimental results of the SEM Moiré method based on FIB grating demonstrate a good agreement with the parameters obtained from commercial tensile testing machine.

Surface Plasmon Antenna with Gold Line and Space Grating for Enhanced Visible Light Detection by a Silicon-on-Insulator Metal–Oxide–Semiconductor Photodiode

A gold (Au) line and space (L/S) grating antenna for the silicon-on-insulator MOS (SOI-MOS) photodiode is investigated. The SOI-MOS photodiode has the advantages such as high-speed operation, large voltage gain per electric charge, and small dark current. However, its light sensitivity is usually very low due to the small volume of light absorption. In order to enhance the light sensitivity, a surface plasmon (SP) antenna is adopted. The SP antenna is constructed with a periodic metallic structure, and can convert the incident light to optical near-field around the surfaces of the antenna. By replacing the MOS gate with the SP antenna, it becomes possible to enhance the light absorption while keeping the advantages of SOI-MOS photodiode. This paper focuses on the visible light absorption in the SOI-MOS photodiode with the SP antenna. The structural conditions for high absorption efficiency and single-peaked spectroscopic characteristics, polarized light response, and resonance mechanisms are clarified by electromagnetic simulations using the finite-difference time-domain (FDTD) method.

Abstracts of the 17th International Symposium on Bioluminescence and Chemiluminescence ‐ (ISBC 2012)

WARNING : The light-emitting molecular structures responsible for the chemiluminescence and fluorescence phenomena are not necessarily the same!

Geometric Control of Rippling in Supported Polymer Nanolines

We study the swelling behavior of finlike polymer line gratings supported on a rigid substrate and show that the edge-supported polymer laminae undergo a rippling instability with a well-defined ripple wavelength λ transverse to the plane of the solid supporting substrate and a ripple amplitude that monotonically decreases from its maximum at the free-edge. These ripple patterns develop due to inhomogeneous compressive strains that arise from the geometric constraints that progressively suppress swelling near the supporting substrate where the laminae are clamped. By experimentally examining the influence of swelling strain and pattern geometry on the observed rippling instability, we find that the ripple wavelength λ scales with line width w for sufficiently long gratings, which is consistent with a simple theory. These trends were validated for polymer nanoline test patterns having w between (50 to 250) nm and a height-to-width aspect-ratio in the range 0.5 to 5. Our results suggest that line geometry, rather than material properties, governs the onset of rippling and suggest simple rules for their control.

Multifunctional Lipid Multilayer Stamping

Nanostructured lipid multilayers on surfaces are a promising biofunctional nanomaterial. For example, surface-supported lipid multilayer diffraction gratings with optical properties that depend on the microscale spacing of the grating lines and the nanometer thickness of the lipid multilayers have been fabricated previously by dip-pen nanolithography (DPN), with immediate applications as label-free biosensors. The innate biocompatibility of such gratings makes them promising as biological sensor elements, model cellular systems, and construction materials for nanotechnology. Here a method is described that combines the lateral patterning capabilities and scalability of microcontact printing with the topographical control of nanoimprint lithography and the multimaterial integration aspects of dip-pen nanolithography in order to create nanostructured lipid multilayer arrays. This approach is denoted multilayer stamping. The distinguishing characteristic of this method is that it allows control of the lipid multilayer thickness, which is a crucial nanoscale dimension that determines the optical properties of lipid multilayer nanostructures. The ability to integrate multiple lipid materials on the same surface is also demonstrated by multi-ink spotting onto a polydimethoxysilane stamp, as well as higher-throughput patterning (on the order of 2 cm(2) s(-1) for grating fabrication) and the ability to pattern lipid materials that could not previously be patterned with high resolution by lipid DPN, for example, the gel-phase phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or the steroid cholesterol.

Estimating the standard uncertainty contribution of the straight-line fit algorithm used to determine the position and the width of a graduation line

We propose an implementation of the straight-line fit algorithm to evaluate data obtained as part of measurements of line structures, e.g. a grating line, which allows us to determine, at a conventional threshold value, the coordinates of the edges, the position and the width of its photometric profile as well as the related standard uncertainty contribution. These are derived for the (1 − α)100% confidence intervals with α ∊ [0, 1]. The implementation is demonstrated using data obtained by the PTB Nanometer Comparator (Fluegge and Köning 2001 Proc. SPIE 4401 275–83). The investigation revealed that large parts of the uncertainty of the position and the width of the line are due to the choice of the fit interval and the uncertainty of the determination of the minimum and maximum level of the photoelectric signal of the line.

Precise Measurement of Moving Object by Moir&amp;#233;-Based Image Processing Technique

Moiré images that are generally termed as moiré fringes have been generated due to the interference of two repetitive gratings. These patterns can be applied to many uses in metrology such as the measurements of surface profilometry of aerofoil, stress-strain effects, thermal deformation and so on. Moreover, 3D surface reconstruction as well as movement characterization of linearly and rotary moving objects can be visualized and identified by the moiré imaging technique. Recently it is approached as an emerging tool in the fields of biotechnology-particularly in biomechanics, nanotechnology, broadband communication and optoelectronics as well. Conventional Moiré interferometry evaluates the interference of two light waves being reflected on a reference surface and the object to be profiled. However, satisfactions in the requirements for the current significant issues in obtaining accurate measurements regarding the information of the movements as well as the dimensional deformations of objects dealing with the online inspection in micro-level and nano-level are still challenging. Particularly, for the demand of the present real-time auto-inspection of different precise information of movements objects statistically and dynamically. In that case single light wave system makes the moiré sensing system easier and applicable in real-time imaging. Furthermore, avoiding the employment of expensive conventional imaging facilities in 3-D measurement in mechanical as well as bio-mechanical systems has become a critical problem to be tackled. Therefore, research has been conducted focusing on the objective of developing a simple but precise measuring tool based on a single wave moiré imaging technique for multidimensional motion sensing by employing simple image processing approaches. An experimental set-up with a small CMOS camera has been developed capable of measuring the motion of an object by using a simply ink-printed straight optical grating lines attached to the moving objects. Several model experiments have been conducted for getting the information of movements of an object by adopting several mouse click options only on the moiré image visible at the computer screen. After getting information of the moiré image by the proposed technique, the movements of the object have been accurately identified. The system has been found simple and faster compared to the other conventional methods as well.

Application of unfolding technique to dispersion calculating of reflective grating dispersion delay lines in the chirped pulses amplifier

Dispersion delay lines (such as the compressor and the stretcher) consisting of reflective optical elements (mirrors and reflection diffraction gratings), are now widely used in chirped-pulse amplification technique. It is shown that for their calculations the unfolding technique can be applied. According to this technique, the dispersion of the compressor and the stretcher can be characterized by the "zero-distant pulse fronts" in their spatial chirps.

Noninvasive Intratissue Refractive Index Shaping (IRIS) of the Cornea with Blue Femtosecond Laser Light

To test the feasibility of intratissue refractive index shaping (IRIS) in living corneas by using 400-nm femtosecond (fs) laser pulses (blue-IRIS). To test the hypothesis that the intrinsic two-photon absorption of the cornea allows blue-IRIS to be performed with greater efficacy than when using 800-nm femtosecond laser pulses. Fresh cat corneas were obtained postmortem and cut into six wedges. Blue laser pulses at 400 nm, with 100-fs pulse duration at 80 MHz were used to micromachine phase gratings into each corneal wedge at scanning speeds from 1 to 15 mm/s. Grating lines were 1 μm wide, 5 μm apart, and 150 μm below the anterior corneal surface. Refractive index (RI) changes in micromachined regions were measured immediately by recording the diffraction efficiency of inscribed gratings. Six hours later, the corneas were processed for histology, and TUNEL staining was performed to assess whether blue-IRIS causes cell death. Scanning at 1 and 2 mm/s caused overt corneal damage in the form of bubbles and burns. At faster scanning speeds (5, 10, and 15 mm/s), phase gratings were created in the corneal stroma, which were shown to be pure RI changes ranging from 0.037 to 0.021 in magnitude. The magnitude of RI change was inversely related to scanning speed. TUNEL staining showed cell death only around bubbles and burns. Blue-IRIS can be performed safely and effectively in living cornea. Compared with near-infrared laser pulses, blue-IRIS enhances both achievable RI change and scanning speed without the need to dope the tissue with two-photon sensitizers, increasing the clinical applicability of this technique.

Hybrid metallic photonic crystals with higher-order coupling processes

Hybrid metallic photonic crystals (MPCs) combine the orderly arranged grating structures and the disordered gold nanostructures, where the gratings are fabricated by interference lithography on a layer of waveguide and the isolated gold nanoislands arranged randomly in the grating grooves are produced using solution-processible gold nanoparticles. Strong coupling between multiple higher-order resonance modes of the waveguide grating structures and particle plasmon resonance of the isolated gold nanoislands has been observed in the visible spectral range. This technique actually realizes fabrication of the waveguide grating structures and the plasmonic gold nanostructures separately. In this way, the plasmonic resonance of the gold nanostructures is not determined by the dimensions of the grating lines anymore. Thus, the grating structures can be fabricated easily with a period of microns, whereas the size of the gold nanostructures can be controlled flexibly by changing the colloidal concentration and the annealing temperature. Large grating period leads to higher-order coupled modes in the visible. This contributes to the “family” of the MPC fabrication techniques and lowers significantly the rigorous requirements on fabrication method of the large-area nanoscale MPCs.

Towards a resolution of conflicting models of illusory contour processing in humans

Despite myriad studies, neurophysiologic mechanisms mediating illusory contour (IC) sensitivity remain controversial. Among the competing models one favors feed-forward effects within lower-tier cortices (V1/V2). Another situates IC sensitivity first within higher-tier cortices, principally lateral-occipital cortices (LOC), with later feedback effects in V1/V2. Still others postulate that LOC are sensitive to salient regions demarcated by the inducing stimuli, whereas V1/V2 effects specifically support IC sensitivity. We resolved these discordances by using misaligned line gratings, oriented either horizontally or vertically, to induce ICs. Line orientation provides an established assay of V1/V2 modulations independently of IC presence, and gratings lack salient regions. Electrical neuroimaging analyses of visual evoked potentials (VEPs) disambiguated the relative timing and localization of IC sensitivity with respect to that for grating orientation. Millisecond-by-millisecond analyses of VEPs and distributed source estimations revealed a main effect of grating orientation beginning at 65 ms post-stimulus onset within the calcarine sulcus that was followed by a main effect of IC presence beginning at 85 ms post-stimulus onset within the LOC. There was no evidence for differential processing of ICs as a function of the orientation of the grating. These results support models wherein IC sensitivity occurs first within the LOC.

Polarization Control and Dynamic Properties of VCSELs for MEMS Atomic Clock Applications

We report the polarization stability and small-signal characteristics of single-polarization single-mode vertical-cavity surface-emitting lasers (VCSELs) emitting at 894.6-nm wavelength for Cs-based miniature atomic clock applications. Polarization control is achieved by integration of a semiconducting surface grating in the top Bragg mirror. The VCSELs are polarized orthogonal to the grating lines with a peak-to-peak difference between the dominant and the suppressed polarization modes reaching 29 dB even at substrate temperatures up to 65 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}\hbox{C}$</tex> </formula> . A modulation bandwidth of more than 5 GHz is reached at only 0.5-mA bias. Modulation current efficiency factors larger than 12 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\hbox{GHz}/\sqrt{\rm mA}$</tex></formula> are achieved. Moreover, the intrinsic modulation characteristics of the VCSELs are investigated. A <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$K$</tex></formula> -factor of less than 0.30 ns and a maximum 3-dB bandwidth exceeding 30 GHz are obtained.

Application of “parallel” moiré deflectometry and the single beam Z-scan technique in the measurement of the nonlinear refractive index

In this paper, the application of "parallel" moiré deflectometry in measuring the nonlinear refractive index of materials is reported. In "parallel" moiré deflectometry the grating vectors are parallel, and the resulting moiré fringes are also parallel to the grating lines. Compared to "rotational" moiré deflectometry and the Z-scan technique, which cannot easily determine the moiré fringe's angle of rotation and is sensitive to power fluctuations, respectively, "parallel" moiré deflectometry is more reliable, which allows one to measure the radius of curvature of the light beam by measuring the moiré fringe spacing. The nonlinear refractive index of the sample, including the sense of the change, is obtained from the moiré fringe spacing curve. The method is applied for measuring the nonlinear refractive index of ferrofluids.