Reducing the reflected light in optical systems represents one of the basic aims of photonics. Reflected light causes losses to the intensity of transmitted light and can generate ghost images and stray light. To reduce these aberrations, antireflection (AR) coatings that consist of alternating high-index and low-index oxide layers (interference multilayers) are typically used. The application of sub-wavelength structures—inspired by the AR nanostructures observed on the eyes of nocturnal flying insects—represents an alternative approach that enables decreased reflectance in a broader spectral range and for a wider range of incident angles. These nanostructures can be produced simply via plasma etching on surfaces consisting of organic materials (e.g., polymers).1, 2
Due to the requirement for uniform performance over the entire lens area, the effective application of AR coatings on the surface of strongly curved lenses represents a challenge. Layers deposited via physical vapor deposition are generally thinner on inclined areas, leading to non-uniform coating. Figure 1 shows a 50% thickness loss at a vapor incidence angle of 60° on a spherical lens (n=1.53). As a consequence of this decreased thickness, the reflectance spectrum in the inclined regions is shifted to shorter wavelengths. This can lead to a significant increase in the overall reflectance of incident light in the visible spectral range. To ensure sufficient performance in this range on inclined surfaces, the spectral range of an AR coating can be extended to include the longer-wavelength NIR (near-IR) region, thereby enabling coverage of the visible range over the entire lens.
Figure 1. The reflectivity spectra of theoretical common and wideband coatings (a) D1 and (b) D2, respectively. (c) Illustration of layer thickness behavior on extremely curved lenses. Both coatings appear 50% thinner at lens position B. The wideband coating (D2) maintains its antireflection (AR) property in the visible spectral range.
A sufficiently deep nanostructure should provide excellent performance for wideband AR applications.3 However, most of the available techniques (e.g., plasma etching, wet-chemical phase separation, glancing incidence deposition, and nano-imprinting4) deliver nanostructures with limited thicknesses, resulting in limited spectral bandwidth. Wideband AR performance can also be achieved by using complex interference stacks. However, the refractive index of the last low-index layer has a dominant influence on the reflectance obtained for a given bandwidth.
To achieve broadband AR performance on glass, we have fabricated coatings comprising inorganic layers and organic nanostructures: see Figure 2.5 We use plasma etching, which can be applied to a number of polymeric materials,1 to produce AR nanostructures on melamine. When applied to glass, these organic films operate as a transfer medium, thereby generating a low-index single layer.2 Figure 2(a) shows homogeneous inorganic layers in a step-down arrangement with a low-index organic nanostructure as a top layer. An interference stack with a low-index organic nanostructure as the top layer is shown in Figure 2(b).
Figure 2. Schemes of advanced broadband AR coatings. (a) Step-down arrangement of inorganic layers with an organic nanostructure (melamine) as the top layer. MgF2: Magnesium fluoride. SiO2: Silicon dioxide. (b) An illustrative interference stack with nanostructured melamine as the top layer. H: High-index layer (e.g., titanium dioxide or tantalum pentoxide, Ta2O5). L: Low-index layer (e.g., SiO2).
The reflectance spectra and scanning electron micrographs of two coatings are shown in Figure 3. To fabricate the D3 coating, we deposited layers of silicon dioxide (SiO2, n=1.46) and magnesium fluoride (MgF2, n=1.38) on a B270 glass substrate. These materials decrease the substrate index in steps, making them useful intermediate layers. We then deposited a nanostructured-melamine layer (n=1.15) as a top layer. This D3 coating decreases the average residual reflectance to below approximately 0.8% in the spectral range between 400 and 1200nm: see Figures 3 and 4.
Figure 3. Reflectance spectra (top) and scanning electron microscope images (bottom) of coatings D3 (SiO2/MgF2layers topped with nanostructured melamine and deposited on B270 glass) and D4 (an interference stack made up of alternating layers of Ta2O5/SiO2, covered with an MgF2 layer and a nanostructured-melamine layer).
Figure 4. A strongly curved glass lens. The right half of the lens is coated using D3.
Our interference system (D4) consists of alternating high-index and low-index layers of tantalum pentoxide (n=2.1) and SiO2, covered with an MgF2 layer and a nanostructured-melamine layer. Using this coating, the residual average reflectance fell to below 0.3% in the 400–1500nm spectral range. This is a significantly lower reflectance than is achievable using classical interference systems.
We have produced wideband AR coatings by combining organic nanostructured layers and inorganic layers. The step-down design of our three-layer coatings can be employed to reduce the refractive index of a lens. Additionally, by employing sufficient thicknesses, these coatings should exhibit enough latitude to be effective on a curved lens in the visible spectral range. Moreover, combinations of nanostructured layers with multilayer interference stacks achieve a very low residual reflectance in the spectral range between 400 and 1500nm. Organic nanostructured layers also have the advantage of being easy to fabricate on glass via vapor deposition and etching. This method therefore represents a realistic means to achieve AR properties on strongly curved lenses. In our future work, we will investigate the use of other organic materials for this purpose and work to improve the mechanical stability of the coatings by adding protective layers.
This research was supported by the Bundesministerium für Bildung und Forschung (BMBF, FKZ 13N12160).
Fraunhofer Institute for Applied Optics and Precision Engineering (IOF)
Ulrike Schulz received her PhD in physical chemistry from the University of Jena, Germany, in 1993. Since 1997, she has been head of the coating on plastics research group at IOF.
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