Subwavelength Structured Surface Research Articles

Spectrally selective antireflection of nanoimprint lithography-formed 3D spherical structures on film coated with a silver layer

We fabricate moth-eye antireflection (AR) coatings using high-resolution and low-cost UV nanoimprint lithography with polyethylene terphthalate (PET) molds. Several various thicknesses of silver films placed on the moth-eye structure were analyzed for reflectance and transmission. On PET, the conical nanostructured surface arrays had a spatial period length of approximately 250 nm, a diameter of approximately 200 nm, and a height of approximately 160 nm. After them, a silver (Ag) layer of 18 nm is deposited satisfactorily on the PET substrate surface. The never-ending moth-eye formations of the imprinted mold were fabricated by Ni mold electroplating, interference lithography, and replication. We found that an Ag layer of suitable thickness on AR film in the spectrum range that can be seen has high transmittance (Highest value is 72%) while in the infrared spectrum it has high reflectance (At least 60%). For an optical film with a silver coating has been placed on an anti-reflection subwavelength-structured (ASS) surface, such properties, including heat insulation, have obvious applications in windows for homes and vehicles.

Structured plasmonic beam: in-plane manipulation of light at the nanoscale

The brief review on recent approaches on the formation of a new class of subwavelength scale localized structured surface plasmon polaritons (SPP) beams is discussed. For the Janus-like particle (including the geometrically symmetric particles with different dielectrics) the morphology of the field localization area and its properties depends on the particle shape and material. Plasmonic hook (PH) beam does not propagate along straight line but instead follow curved self-bending trajectory. Wavefront analysis behind of such symmetric and asymmetric mesoscale rectangle structure reveals that the unequal phase of the transmitted plane wave results in the irregularly concave deformation of the wavefront inside the dielectric which later leads to creation of the PH. Such dielectric structures placed on metal film enable the realization of new ultracompact wavelength-selective and wavelength-scaled in-plane nanophotonic components. SPP have potential to overcome the constrains on the speed of modern digital integrated devices limitation due to the metallic interconnects and increase the operating speed of future digital circuits.

Fabrication of micro hole array on the surface of CVD ZnS by scanning ultrafast pulse laser for antireflection

Chemical vapor deposited (CVD) ZnS is a promising long-wave infrared (8–12 μm) window material. Yet antireflection is necessary since Fresnel reflection from its surface is high due to the high refractive index of ZnS. Sub-wavelength structured surface of micro hole array was fabricated on CVD ZnS by scanning ultrafast pulse laser ablation. The effects of beam profile, pulse width and beam power on the radius and morphology of the holes were studied. Gaussian beam can cause severe melted-resolidified layers around the hole, yet Bessel beam only resulted in thin ribbon around the hole. The picosecond Bessel laser is more suitable than femtosecond laser for ablating holes on ZnS. The radius of the holes increases with increasing the Bessel beam pulse width and the beam power. But larger power may cause circle grooves around the central holes. Ordered hole array was fabricated on single side of CVD ZnS and antireflection was realized.

Two-spherical-wave ultraviolet interferometer for making an antireflective subwavelength periodic pattern on a curved surface

An ultraviolet two-spherical-wave interferometer was developed in order to make a subwavelength structured surface on a curved surface. The change in fringe period on the curved surface was significantly suppressed compared with the two-plane-wave interferometer. The optical setup method for suppressing the change in fringe period is described. The effect of the two-spherical-wave interferometer was investigated by numerical simulations. In an experimental demonstration for a concave spherical surface with 11.1 mm radius of curvature and 10 mm diameter, the change in period of the photoresist pattern was reduced to 12 nm for the target period of 250 nm.

Enhanced surface acceleration of fast electrons by using subwavelength grating targets

Surface acceleration of fast electrons in intense laser-plasma interaction is improved by using sub-wavelength grating targets. The fast electron beam emitted along the target surface was enhanced by more than three times relative to that by using planar target. The total number of the fast electrons ejected from the front side of target was also increased by about one time. The method to enhance the surface acceleration of fast electron is effective for various targets with sub-wavelength structured surface, and can be applied widely in the cone-guided fast ignition, energetic ion acceleration, plasma device, and other high energy density physics experiments.

Excellent antireflection properties of vertical silicon nanowire arrays

We report a simple approach to prepare cost effective antireflective surface directly on silicon wafers, which consists of arrays of vertically aligned silicon nanowires (VA-SiNWA). Large area VA-SiNWA were realized by silver induced wet chemical etching of p-silicon (1 0 0) substrates in aqueous HF and AgNO 3 solution at room temperature. Length of Si wires (diameter in 50–300 nm range) was found to increase linearly with etching time (0–120 min). A remarkable reduction in reflectivity ( R λ ) for surfaces with Si wires was observed. The value of R λ less than 2% was realized in the 300–600 nm wavelength range in the case of ∼12 μm long Si wires, a value better than the best R λ reported in anisotropically textured surface or single layer antireflection coatings. The VA-SiNWA behaves as a subwavelength structured surface that could suppress the reflectivity to a great extent. Such surfaces may have potential applications as antireflection surface for silicon solar cells.

Heat-insulation film with gold layer deposited on antireflection subwavelength-structured surface

The optical properties of gold layer deposited on antireflection subwavelength-structured surface are first exposed. The experimental results of the reflectances and transmittances for several different thicknesses of gold-deposited layers on the subwavelength structures are carried out. The nanostructured surface with spatial period and a diameter of about 230 nm and height of about 150 nm on polyethylene terphthalate (PET) film is fabricated by micro-replication process of UV imprinting. Comparing these with the bare gold-deposited layer and bare nanostructure, the results show that the optical films with the suitable gold layer deposited on antireflection subwavelength-structured surface has high transmittance and low glare in the visible spectral range and high reflectance in the infrared range. That is to say, when the antireflection subwavelength-structured surface is coated with a gold layer of several tens of nm thickness, it will have some unique optical characters.

Subwavelength structures for broadband antireflection application

The subwavelength structures are designed and fabricated for broadband antireflection application. Under target of zero reflectivity, the parameters of periodic 2-D continuous conical structures are analyzed by the finite-difference time-domain (FDTD) method. The corresponding conical structures are obtained with spatial period of 350 nm and structure height of 300 nm, respectively. The 2-D continuous conical structured surface is fabricated by micro-replication process combining with the originated structure fabrication realized by interference lithography, Ni mold electroplation and replication by using UV imprinting into plastics. The average reflectances of the simulation and replicated polymer prototype are about 0.50% and 0.54% within the spectral ranges of 400–650 nm, respectively. In a word, the subwavelength structured surface with low reflection is developed and proved to be highly consistent with the simulation results.

Experimental Study of a Silver Layer on an Antireflection Subwavelength-Structured Surface

The optical characteristics of a silver layer deposited on an antireflection subwavelength-structured surface are investigated. The experimental results of the reflectance and the transmittance of several different thicknesses of silver layer on the subwavelength-structured surface are carried out. A subwavelength structure with the spatial period and diameter of about 230 nm and height of about 150 nm on polyethylene terphthalate film is fabricated by microreplication process. It is shown that such an element with the suitable silver layer deposited on an antireflection subwavelength-structured surface has high transmittance and low glare in the visible spectral range and high reflectance in the infrared range. Obviously, the optical film with silver layer can not only obtain the high performance of heat insulation but also give an application in automobile and house windows.

Antireflective subwavelength structures on crystalline Si fabricated using directly formed anodic porous alumina masks

A simple fabrication technique for subwavelength structured (SWS) surfaces by means of anodic porous alumina masks directly formed on Si substrates was proposed and demonstrated. By this technique, SWS surfaces were fabricated on polished single-crystalline Si and chemically etched as-cut multicrystalline Si wafers. Smoothly tapered SWS surfaces with a periodicity of 100nm and a height of 300–400nm were obtained. A low reflectivity below 1% was observed from 300to1000nm for both of the wafers, in agreement with numerical simulation. After thermal annealing at 800°C, the reflectivity of the SWS surface increased to 3%.

Broadband antireflection gratings fabricated upon silicon substrates

We fabricated a two-dimensional subwavelength structured (SWS) surface upon a crystal silicon substrate. The SWS surface was patterned by electron beam lithography and etched by an SF(6) fast atom beam. The SWS grating had a conical profile, the period was 150 nm, and the groove was approximately 350 nm deep. The reflectivity was examined at 2002500-nm wavelengths. At 400 nm the reflectivity decreased to 0.5% from the 54.7% of the silicon substrate. We also used HeNe laser light to examine the reflectivity as a function of the incident angle.