Periodic Dot Arrays Research Articles

Bragg gratings with novel waveguide models fabricated in bulk glass via fs-laser writing and their slow-light effects.

We present the experimental realization of an innovative parallel partially overlapping waveguides (PO-WGs) model grounded in the thermal accumulated regime and fabricated using femtosecond (fs) laser direct-writing within low-iron bulk glass. The 75mm long novel PO-WGs model was made by partially overlapping the shell parts of two core-shell types of waveguides via a back-and-forth single pass fs-laser inscription. The detailed evolution of the PO-WGs model from inception to completion was offered, accompanying by a thorough characterization, which unveils a substantial refractive index (RI) change, on the order of 10-3, alongside low propagation loss (0.2 dB/cm) and distinctive features associated with the single mode and shell-guided light. Notably, the unsaturated performance of PO-WGs model after the primary inscription paves the way for potential applications in the successful creation of two distinctive types of Bragg gratings: first-order dot-Bragg grating and second-order line-Bragg grating. The 75 mm long dot-Bragg grating was written by a periodic dot array with a height of 6 µm atop the PO-WGs, and the birefringence was measured of 1.5 × 10-5 with a 16 pm birefringence-induced wavelength difference. The line-Bragg grating, which was inscribed with dual PO-WGs extending the line grating part to 40 mm in length along its period for increasing the transmission dip, exhibits a pronounced polarization dependence showcasing an effective birefringence of 4.2 × 10-4 at the birefringence-induced wavelength difference of 0.45 nm. We delved into the slow-light effects of the two Bragg gratings thoroughly, which the theoretical analysis revealed an effective group delay of 0.58 ns (group index 2.3) for the dot-Bragg grating. Similarly, the line-Bragg grating exhibited an effective group delay of 0.3 ns (group index 2.3), in good agreement with experimental measurements. These findings underscore the exciting potential of our gratings for creating optical slow-wave structures, particularly for future on-chip applications.

2D compound structures with deep subwavelength period on silicon fabricated by double time delayed femtosecond laser beams

In this work, two orthogonally polarized femtosecond laser beams are employed to irradiate a p-doped silicon wafer with an electrical resistivity of 0.008 Ω·cm. It is interesting to find that 2D compound structures composed of sub-wavelength periodic ripples and deep sub-wavelength nanodot array can be produced when proper laser fluence and time delay between the dual laser beams are used. The formation of the periodic ripples can be explained by the interference between the preceding incident laser and it induced surface plasmon polaritons (SPPs). The periodic nanodot array has a period down to ∼200 nm and the radius of the nanodot is ∼30 nm, most of which appear at the boundary between the ditch and ridge of the ripple. During the ripples’ formation, the residual melting silicon is most probably located at the boundary between the ditch and ridge of the ripple. Furthermore, the period of the nanodot array is roughly equal to the perimeter of the nanodot. Therefore, it is considered that the dot array may be generated due to the Rayleigh-Taylor instability of the melting silicon. It is also noted that these nanodots are all uniformly arranged along vertical lines, indicating that the subsequent incident laser may break the stochastic characteristic of the Rayleigh-Taylor instability and produce the 2D periodic dot array. The thermo-hydrodynamical process combined with the interference effect between SPPs and the incident laser can benefit the formation of complex surface structures with versatile functions.

Fabrication of moth-eye structures on silicon by direct six-beam laser interference lithography

This paper presents a new method for the generation of cross-scale laser interference patterns and the fabrication of moth-eye structures on silicon. In the method, moth-eye structures were produced on a surface of silicon wafer using direct six-beam laser interference lithography to improve the antireflection performance of the material surface. The periodic dot arrays of the moth-eye structures were formed due to the ablation of the irradiance distribution of interference patterns on the wafer surface. The shape, size, and distribution of the moth-eye structures can be adjusted by controlling the wavelength, incidence angles, and exposure doses in a direct six-beam laser interference lithography setup. The theoretical and experimental results have shown that direct six-beam laser interference lithography can provide a way to fabricate cross-scale moth-eye structures for antireflection applications.

Liquid-bridging in particle self-assemblies toward constructing periodic nano-mesh structures and nano-dot arrays

We demonstrate a simple generic principle of constructing periodic nano-mesh structures and nano-dot arrays by liquid-bridging in particle self-assemblies. A self-assembly of nano- to micro-sized spheres of polystyrene, produced in a close-packed monolayer on a substrate by a convective method, is used as a template for liquid-bridging of a functional fluid in the pores among the neighboring particles. After the solidification of the fluid, followed by the detachment of the particles from the substrate, a periodic mesh structure is directly obtained. Moreover, deposition of a desired substance onto such mesh structure made of a sacrificial material, which is subsequently removable through a lift-off process, enables to produce a periodic dot array of the substance. Depending on the size of the particle and the nature of liquid bridging in the particle assembly, a variety of nano-mesh structures of silver-ink and nano-dot arrays of gold are constructed. This simple liquid bridging-based methodology is easily applicable for different classes of functional fluids such as soluble organic semiconductors.

Dewetting-Induced Formation of Periodic Dot Arrays of Polymer/Au Composites by Capillary Force Lithography

This paper demonstrates that simple cross-stamping in capillary force lithography (CFL) enables the fabrication of periodic dots, rings, and line patterns of the metal/polymer composites. The key to the success was the compatibility of polymer/inorganic precursor composite liquids on substrates. As a model study, the composites consisting of poly(ethylene oxide) (PEO) and the gold precursor (HAuCl4) were investigated. The contact angle of the composite liquid on the Si substrate significantly increased when the fractional composition of the precursor was over a critical value, while the contact angle on poly(dimethylsiloxane) (PDMS) molds decreased. The reduced wettability on the substrate resulted in isolated dot patterns upon cross-stamping on the line patterns fabricated by CFL. The shape, size, and number of the dots could be controlled by adjusting the thickness of the composite films. Small coupled dots were obtained from dual-line patterns and large single dots were obtained from single-line patter...

Fabrication of TiO2‐Organic Hybrid Dot Arrays Using Nanosecond Laser Interference Lithography

Laser interference lithography is an established fabrication technique for periodic arrays such as photonic crystals (PCs). In this paper, we report the fabrication of a TiO2‐organic hybrid periodic array by nanosecond laser interference lithography. TiO2‐organic hybrid films were prepared from titanium tetra‐n‐butoxide and 2‐(methacryloyloxy) ethyl acetoacetate (MEAcAc) by the sol–gel method. The films were irradiated by using 10 ns pulses of 355 nm diffracted light from a Nd:yttrium–aluminum–garnet laser. The complexes formed by the Ti alkoxide and the MEAcAc were decomposed by laser irradiation, and the irradiated parts remained as periodic dot arrays on the substrate after the development. The two‐dimensional array of this TiO2‐organic hybrid material was calcined at 450°C and formed an anatase TiO2 dot array. The refractive index measured was 2.00. These dots were crack free and adherent to the substrate.

Lateral Assembly of Metal Nanoparticles Directed by Nanodomain Control in Block Copolymer Thin Films

We have produced and assembled palladium (Pd) nanoparticles in two-dimensional (2-D) periodic arrays in nanoscales by using block copolymer (BC) thin films as templates. A symmetric diblock copolymer of poly(methyl methacrylate)-block-poly(2-hydroxyethyl methacrylate) (PMMA-b-PHEMA) was coated on a Si-wafer by dip-coating from three different solvents having different solubility against each component. Two of them are selective solvents for PMMA and PHEMA, respectively, and the third one is a common solvent for the two components. A monolayer of the micelles was formed from the solutions of the selective solvents and the micelles are arranged in a hexagonal 2-D lattice, whereas a lamellar pattern was obtained from a homogeneous solution of the common solvent. The monolayer films thus-prepared were exposed to the vapor of palladium (II) bis(acetylacetonato), and Pd nanoparticles were selectively produced in the PHEMA phase due to its stronger reducing power than the PMMA phase. The patterns thus-obtained were successfully transferred to a Si-wafer by reactive dry etching technique because of the high etching resistivity of the metal nanoparticles against gas plasmas. By using this process, three lateral periodic arrays of dots, holes, and lines were created with 20-nm repeating distances.

Defects in (La 0.7Sr 0.3Mno 3/SrTiO 3) 15 superlattices grown by pulsed injection MOCVD

(La 0.7Sr 0.3MnO 3/SrTiO 3) 15 superlattices were grown in situ by liquid-injection metal-organic chemical vapour deposition on (0 0 1)LaAlO 3 and (0 0 1)SrTiO 3 substrates. Defects in these superlattices were studied by X-ray texture diffractometry and by transmission electron microscopy. Two types of defects were identified: (i) defects with feather-like and inverse pyramidal shapes and with particular crystallographic orientational relationships with respect to the epitaxial (0 0 1) superlattice, and (ii) defects with an inverse pyramidal shape but of the same crystallographic orientation as the epitaxial superlattice. Both these defects originate from nucleus observed either on La 0.7Sr 0.3MnO 3 or SrTiO 3 layers. They propagate across the following deposited layers and create dots at the surface of the multilayer structure. Whether a nucleation and growth of these defects could be controlled then a new process for producing 2D periodic arrays of dots could be envisaged for photonic applications.

Characterization of the nanostructures of a lithographically patterned dot array by x-ray pseudo-Kossel lines

Grazing x-ray scattering from a nanofabricated periodic dot array exhibits an interesting diffraction pattern, resembling x-ray Kossel lines, due to the anisotropic x-ray resolution function. We demonstrate that the unique diffraction pattern can be used for precise characterization of the deep nanostructures, which cannot be obtained accurately by microscopy techniques.

Crystal of Semiconducting Quantum Dots Built on Covalently Bonded T5 [In28Cd6S54]‐12. The Largest Supertetrahedral Cluster in Solid State.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Crystal of semiconducting quantum dots built on covalently bonded t5 [in(28)cd(6)s(54)](-12): the largest supertetrahedral cluster in solid state.

Periodic array of nanoparticles is essential for practical applications in optical devices. Periodic dot arrays often exhibit very interesting collective phenomenon. We report a periodic crystal of InCdS pseudo-T5 nanocluster, the largest supertetrahedral cluster found thus far in solid state. Each InCdS cluster behaves like a nanoparticle with the same size. Unlike the array of colloidal dots in which the dot-dot separation is large ( approximately 5 nm), the neighboring T5 clusters in [In28Cd6S54].[(CH3)4N]12[(HSCH2COOH)2]3.5 crystal form a natural point contact by sharing covalently bonded S atoms. Both experimental and theoretical studies show that this crystal is a semiconductor with a band gap of 3.0 eV.

Fabrication of pseudo-spin-valves and 100 nm sized periodic elements for magnetic memory application

Co–Cu–Co pseudo spin-valves (PSV’s) and Co–Cu multilayers have been fabricated and studied for the giant magnetoresistance (GMR) effect. Attention is focused on optimizing the low field response. They are deposited on Si (001) wafers without removing the natural silicon-dioxide layer, using S-research sputtering guns in high vacuum. A buffer layer of Cr–Cu was used before depositing the PSVs. Realization magnetic switching at low field for the soft layer and high field for the hard-layer makes the structure suited for MRAM application. The low and high switching fields are typically 100 and 2200 Oe, respectively. The magnetic measurement using SQUID magnetometry and VSM confirms the switching fields in the hysteresis loops of the magnetization. The GMR multilayers have a low switching field of 60 Oe at low temperatures. They are patterned into two-dimensional 100 nm sized periodic dot-array using ultraviolet interferometric lithography. The purpose is to develop processes of fabricating GMR structures over large areas with nanometer feature sizes for high-density memory devices. The coercive field for the patterned structures is enhanced by a factor of four at low temperatures and by a small fraction at higher temperatures. The magnetization remains to be in plane as in un-patterned case.

Magnetization reversal of nanostructured Co/Pt multilayer dots and films studied with magnetic force microscopy and MOKE

Large scale periodic arrays of magnetic nanostructures consisting of e.g. Co/Pt multilayer dots with perpendicular magnetic anisotropy are possible candidates for future high-density magnetic storage media. We have fabricated periodic dot-arrays by optical interference lithography with Ar/sup +/ ion lasers operating at wavelengths of 457 nm and 244 nm, respectively. Periodicities range from 125 nm to 1100 nm with dot diameters between 70 nm and 740 nm covering a total area of up to 20 cm/sup 2/ with a maximum dot density of about 4.1/spl times/10/sup 10/ dots/in/sup 2/. The global magnetic properties of these Co/Pt dot-arrays and of corresponding homogeneous films are investigated by the magneto-optical Kerr effect (MOKE) in polar geometry. Results of the magnetization reversal are compared to magnetic force microscopy (MFM) investigations in external magnetic fields of up to H=1 kOe perpendicular to the sample. A simple MFM-data analysis is described which allows to determine hysteresis loops from a series of MFM images obtained in various external magnetic fields. This then allows to measure the coercivity H/sub c/ of a given sample on small length scales. A comparison with MOKE hysteresis loops shows good agreement, although the unknown hysteretic behavior of the tip cannot be separated. The MFM-image analysis described therefore stands as a semi-quantitative method, which is however useful to measure relative changes of the coercivity on small length scales.

Discrimination of simulated texture patterns on the human hand

1. Textures formed by periodic dot arrays are defined by the dot density, spacing, and angular orientation with respect to the direction of motion. In this report we evaluate the effects of the dot density (intensive cues) and arrangement (spatial cues) on the ability of human subjects to discriminate texture patterns scanned across an OPTACON tactile stimulator that selective stimulates rapidly adapting cutaneous mechanoreceptors. We compared dot arrays arranged on the index finger in specific patterns (horizontal, vertical, diamond, up diagonal, or down diagonal orientation) and spaced 4.8, 7.2, or 9.6 mm apart (high, medium, and low density) with the use of a two-alternative forced-choice protocol. 2. Textures are well discriminated when their elements are tightly spaced along one axis and widely spaced on all other axes. Humans distinguish textures that differ only in orientation with mean accuracy of 75% at low density and 65% at medium density, but discriminate high-density textures poorly (mean accuracy = 48%). Accuracy is related to the angular disparity between patterns, and to similarity of spacing and orientation along major and minor axes of the arrays. Vertical and horizontal patterns are more accurately distinguished than the oblique ones, and diamond arrays are the least well discriminated. Diagonal and diamond textures are often confounded, and the up and down diagonal patterns are confused with each other particularly as the texture density rises. The preference for the vertical and horizontal patterns may relate to an interaction between the orientation axis of the texture and its direction of motion across the skin. 3. Intensive cues provided by the total number of applied stimuli supplement the spatial cues inherent to the pattern orientation, because textures that differ in both spacing and orientation are discriminated better than those that differ only in orientation or spacing. Mean accuracy ranges from 96% for comparisons of high- and low-density textures, which differ in the total number of dots by a factor of 2, to 80% when medium-density patterns are compared with high- or low-density textures. 4. Textures that differ in density but not in orientation are less well discriminated than those of different orientation. For example, 82% of patterns that differ in both density and orientation are distinguished correctly in pairings of low- and medium-density textures, whereas those that differ only in density are discriminated correctly on 45% of trials. Subjects seem to use spatial rather than intensive cues when discriminating patterns of similar density, suggesting that the similarity of form (the spatial arrangement of the closely spaced dots) is more readily apparent than small differences in spacing along the axis of motion. 5. Subjects are most most successful in differentiating texture patterns when they are able to mentally picture the orientation and spacing of the pattern. We found a strong correlation between the subjects' ability to discriminate textures of a given spacing and their ability to identify the specific texture by matching it to the appropriate visual representation. Subjects are able to identify correctly all five orientations at low and medium densities, with mean accuracy of 76%, but recognize only the vertical arrays when high-density patterns are presented. The ability to image the textures is noteworthy, because subjects received no feedback about performance. 6. Spatial imaging of textures appears limited by the diameter of cutaneous receptive fields on the hand. We propose that the structural axis of a regular texture array results from perceptual linkage of adjacent elements along one principal axis by continuous bands of neural activity when their spacing is smaller than the receptive field diameter.(ABSTRACT TRUNCATED)

Magnetostatic forward volume wave reflective dot arrays

Experiments are described for oblique incidence of magnetostatic forward volume wave (MSFVW) on metal dot arrays (DA). The MSFVW propagate in epitaxial YIG films at microwave frequencies. Different configurations of periodic dot arrays have been tested and their characteristics are reported. The study of propagation of MSVFW through dot arrays had led to evaluation of the velocity variation and the excess of loss induced by a dot array. Empirical relationships are proposed to predict these effects. Preliminary results about oblique reflection from an ion-implanted bar array has been carried out and compared to those obtained with metal dot array.