Fabrication and simulation of diffractive optical elements with superimposed antireflection subwavelength gratings

Optimal design method on diffractive optical elements with antireflection coatings

The effect of antireflection coatings on diffraction efficiency of diffractive optical elements (DOEs) was studied and the mathematical model of diffraction efficiency affected by antireflection coatings for DOEs is presented. We found antireflection coatings can cause a significant reduction on diffraction efficiency at the designed, or the central wavelength. In order to solve this problem, we proposed a method to keep 100% diffraction efficiency at the designed wavelength by ensuring the 2π phase induced by DOEs and the antireflection coatings. Diffraction efficiency affected by antireflection coatings for DOEs with consideration of antireflection coatings are simulated. Analysis results can be utilized for refractive-diffractive hybrid imaging optical system optimal design and image quality evaluation.

Dielectric V-ridge gratings: transition from antireflection to retroreflection

We present a systematic study of the optical properties of dielectric gratings with symmetric V-shaped ridges having a 90 degree apex angle and refractive index n. Such structures exhibit completely different optical properties if the dimensions are scaled with respect to the vacuum wavelength λ₀ of light. In the subwavelength domain, where the grating period d is less than λ₀/n, the grating behaves as an antireflection layer. In the large-period domain d≫λ₀ (with normal incidence from the dielectric side), the grating turns into a micro-retroreflector array. The transition between these well-known domains is studied using rigorous diffraction theory. The results are verified experimentally by fabricating and characterizing V-profile gratings with the aid of wet etching of silicon using a process that defines a 90 degree apex angle and replication into a polymer.

Theoretical and experimental research on laser-induced damage of cylindrical subwavelength grating

We designed and fabricated two-dimensional cylindrical subwavelength gratings (SWGs) on fused silica for use in 1064 nm laser system. The transmittance and laser-induced damage threshold (LIDT) under the irradiation of 1064 nm pulses were performed, and a lower LIDT compared to blank fused silica was obtained. To understand damage mechanism quantitatively, macro-temperature on the SWGs integrated fused silica and micro-Electric field, micro-thermal-stress distribution on SWGs during the laser irradiation process were investigated by Finite Element Analysis, and the comparison between theoretical and experimental research indicated the presence of absorption centers and the non-uniform thermal mechanical distribution of SWGs contributed to the LIDT reduction on SWGs integrated fused silica.

Design of a high-efficiency seven-port beam splitter using a dual duty cycle grating structure

In this paper, we propose a compact seven-port beam splitter which is constructed using only a single-layer high-density grating with a dual duty cycle structure. The properties of this grating are investigated by a simplified modal method. The diffraction efficiency can be achieved around 10% more than conventional Dammann gratings while the uniformity can still be maintained at less than 1%. The effect of deviations from the design parameters on the performance of the grating is also presented.

Biomimetic antireflective hierarchical arrays

We report a simple method for the fabrication of biomimetic antireflective hierarchical arrays based on the combination of self-assembled polymer spheres and nanoimprint lithography (NIL). The hierarchical structures are fabricated by creating nanopillars on the microscale round protrusion arrays, which are similar to natural mosquito eyes consisting of combined micro- and nanostructures. The hierarchical arrays dramatically suppress the surface reflection from visible to near-infrared regions with an angle of incidence of up to 70°.

Reduction of light reflection at silicon-plate surfaces by means of subwavelength gratings in terahertz region

We investigate triangular surface-relief gratings for reducing reflection at a planar silicon surface for light in the terahertz frequency region of 0.3-3.0THz. Structural parameters of the one- and two-dimensional (1D 2D) subwavelength gratings required for the antireflection (AR) layer were obtained numerically. Experimental results for the AR effects agreed well with those obtained numerically, except for gratings fabricated with laser ablation, which causes structural fluctuations of the grating. In the 1D grating, a high transmittance exceeding 90% for the TM wave was obtained. 2D gratings comprised of arrayed micropyramids were experimentally confirmed to be polarization insensitive.

Fabrication of antireflection and antifogging polymer sheet by partial photopolymerization and dry etching

We present a simple method to fabricate a polymer optical sheet with antireflection and antifogging properties. The method consists of two consecutive steps: photocross-linking of UV-curable polyurethane acrylate (PUA) resin and reactive ion etching (RIE). During photopolymerization, the cured PUA film is divided into two domains of randomly distributed macromers and oligomers due to a relatively short exposure time of 20 s at ambient conditions. Using the macromer domain as an etch-mask, dry etching was subsequently carried out to remove the oligomer domain, leaving behind a nanoturf surface with tunable roughness. UV-vis spectroscopy measurements demonstrate that transmittance of a nanoturf surface is enhanced up to 92.5% as compared to a flat PUA surface (89.5%). In addition, measurements of contact angle (CA) reveal that the etched surface shows superhydrophilicity with a CA as small as 5 degrees. To seek potential applications, I-V characteristics of a thin film organic solar cell were measured under various testing conditions. It is shown that the efficiency can be increased to 2.9% when a nanoturf film with the surface roughness of 34.73 nm is attached to indium tin oxide (ITO) glass. More importantly, the performance is maintained even in the presence of water owing to superhydrophilic nature of the film.