For more than four decades band-pass filters are important components of microscopes used for the fluorescence spectroscopy. During all the time this special field of application has been one of the main drivers for research and development in thin-film optics, particularly for the thin-film design software and the coating technology. With a shortwave pass filter, a multi-notch filter, and a classical band-pass filter as examples of such filters provided for the latest generation of fluorescence microscopes we present the state-of-the-art in coating design and technology. Manufacturing these filters is a great challenge because the required spectral characteristics need necessarily multilayers with up to 300 layers and overall thicknesses up to 30 μm. In addition, the designs require also 3 to 5 nm as thinnest layers and all the layers are completely of non-quarterwave type. The filters were manufactured in a rapid-prototyping regime by a Leybold Helios plant using plasma-assisted reactive magnetron sputtering of thin films of different metal oxides. Designed and real spectra are compared and differences are discussed. Measurement results of other optical and non-optical characteristics as film stress, total integrated scattering, and micro roughness are presented.

Besides the typical channels in the visible and near infrared spectrum, optical remote sensing of the earth from air and space utilizes also several channels in the short-wave infrared spectrum from 1000 nm to 3000 nm. Thin-film optical filters are applied to select these channels, but the application of classical multiple-cavity band-pass filters is impossible. Because of their additional blocking elements they are disallowed due to geometrical or other non-optical reasons. Within the sensitivity region of an MCT detector as typical detector device, the selection and blocking of radiation by the filter has to be provided by a single multilayer system. The spectral region of the SWIR as well as blocking width and depth require necessarily designs with overall thicknesses of more than 20 μm, with layer numbers up to 100. SiO 2 and TiO 2 were used as thin-film materials deposited with reactive e-beam evaporation under ion assistance in a Leybold SyrusPro box coater. A special challenge was the thickness measurement of the thin films by an optical broadband monitoring device in the visible range. The results of manufacturing and characterizing of such filters are presented by three examples for the center wavelengths of 1375 nm, 1610 nm, and 2190 nm.

It is shown how the discussion about antireflection coatings for the visible and near infrared region has been changed dramatically with recent experimental applications of nanostructures that realize media with effective refractive indices less than the 'magic border' of 1.34. Using the so-called binary optics as an example, a glass-like nanostructure similar to the moth-eye structure is theoretically designed as antireflection coating for the visible and near infrared region. With the aim of this example and considering only known design principles of thin-film optics, a connection between nanostructures and thin films regarding their alternative or combined application as antireflection coatings is presented. As summary regarding the nanostructures vs. thin film discussion, a reference list is presented that cited different types of antireflection coatings presented in the past 70 years with respect to their applications, designs, and deposition technologies.

Rugate designs for the realization of notch filters are well known in the literature. The required deposition of gradient index layers is difficult to manufacture. In our approach we apply the equivalent index theory to replace the gradient index profile of a notch filter design. We produce single and multiple notch filters with plasma ion-assisted deposition and broad-band optical monitoring. As examples, a 500nm notch filter for the GREGOR telescope and a 589nm notch filter for the GALACSI instrument of the VLT are discussed. Additionally, a 4-line multiple notch filter and a 218nm notch filter made for fluorescence spectroscopy applications are presented.

For single layers of SiO2, Nb2O5, and Ta2O5 that are deposited by plasma-assisted reactive magnetron sputtering (PARMS), we present measurement results for basic optical and mechanical properties, in particular, optical index, intrinsic film stress, thermal shift of spectral transmittance, and microroughness. We find high refractive indices combined with low intrinsic film roughness, moderate compressive stress, and almost a vanishing shift, indicate high potential for the production of high-performance optical coatings. The high thickness accuracy and process stability are exemplified by the measured spectral performance of multilayer stacks with about 200 single layers.

A precise identification of the dielectric materials dispersion data, involved forming the layer stack, is crucial for obtaining satisfactory results utilizing optical broadband monitoring for high precision control of thin-film optical filters.

For Notch Filters, Rugate designs with a small index contrast and apodisation are well known in the literature. The required deposition of gradient index layers or so called flip flop structures is very complicated and difficult to manufacture. Higher order H/L stacks of coating materials with high index contrast result in very thick layer stacks. In our approach we replace the second refractive index by equivalent layers consisting of H/L materials with high index contrast. This leads to a combination of thick (>100nm) and very thin layers. Stable coating processes with dense layers are strict requirements. Another challenge is the accurate thickness control of very thin layers in the nanometer range. Single notch filters were produced with PIAD and broad-band optical monitoring. The most challenging filters were demonstrated with magnetron sputtering and monochromatic optical monitoring. Some Outstanding results of single and multiple notch filter coatings will be presented.

Interference filters for spectroscopic purposes or sensor applications are characterized by strictly specified spectral blocking and transmitting regions with intermediate steep edges. These steep edges must be positioned within nanometer accuracy while the coating may consist of more than one hundred non-quarterwave layers. Though modern ion assisted deposition processes in conjunction with quartz crystal control are well suited for the production of complex filters, an optical monitoring device seems to be necessary to fulfill the demanding spectral requirements. Broad band optical monitoring (BBM) directly on the calotte has been employed to control the production of this type of band stop filters. For a large number of also different types of these coatings the BBM-technique demonstrated its capability to improve the reliability and flexibility in industrial production. Within a stable well-characterized deposition process error self-compensation effects allow for a fast realization of various designs within specified tolerances. Nevertheless, optical broad band monitoring could not be applied to all types of these steep edge filters because error propagation leads to unreachable solutions of the thickness tracing algorithm for specific cases. The given examples of complex steep filters and the corresponding post analysis of stored online spectra as well as the simulation of the monitoring process reveal the influence of the design itself to this occurrence. A suggestion for an identification of critical thickness values within the layer sequence is discussed and solutions to the problems are presented.

It is shown that most of the known AR coatings that start with the famous quarter-half-quarter coating including the multi-cycle designs display simply alternative solutions to replace unavailable refractive indices of a standard solution. On the basis of this viewpoint some design principles are presented that help to derive an approach to solve a given AR problem. Principal objective of the presented approach is the application of previously defined terms 'equivalent substrate index' and 'equivalent stack index' that characterise a quarter-wave optical thickness multilayer system. It is shown that the presented design principles are also suitable if, instead of a thin-film system, a modification of the interface is used to reduce the surface reflection.

Some novel concepts of designing antireflection (AR) coatings with equivalent layers are presented. As an introduction, essential papers concerning thin-film optics and AR designs are cited, and the AR problem and a previously introduced AR-hard design type are discussed. Based on the known matrix formalism, a potential AR region, an equivalent stack index, and an equivalent substrate index are defined to use the theory of stop-band suppression as a starting point for the design of broadband AR coatings. The known multicycle AR design type is identified as a typical solution to the AR problem if the presented approach is used.

Based on a special definition of the edge region of a thin-film interference filter, an approach to design steep-edge filters using the theory of equivalent layers is presented. Some features are discussed using the known theory to meet the topical filter requirements. An example is theoretically outlined and the spectral performance of a manufactured filter is presented.

Rugate structures, as well as gradient refractive index films in general, attract a lot of interest. The gradient index systems may provide advantages in both, optical performance and mechanical properties of the optical coatings. Rugates have shown to be especially interesting for design of notch filters. A lot of theoretical work on design of rugate filters has been done in the last decades. However, only few of the designs could be deposited, which is often caused by practical problems, e.g. preparing materials with the desired refractive index values. In this paper two different gradient refractive index designs are compared to a classical high-low stack. One gradient design is synthesized by an apodized sinusoidal structure that is approximated by homogeneous sublayers. The other one is based on an apodized sinusoidal structure as well, but it is approximated by a hybrid structure, i.e. a combination of linear gradient index ramps between the lowest and the highest refractive index applicable and homogeneous layers of high index values. The two gradient designs take into account the constraints posed by limitations of the real deposition systems. Both designs are compared to a classical high-low stack and the advantages and drawbacks of each approach are commented.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text U. Schallenberg, "New AR design concepts with equivalent layers," in Optical Interference Coatings, OSA Technical Digest Series (Optica Publishing Group, 2004), paper TuB1. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text U. Schulz, N. Kaiser, and U. B. Schallenberg, "AR-hard broadband antireflective coatings generated by a controlled needle-optimization technique," in Optical Interference Coatings, OSA Technical Digest Series (Optica Publishing Group, 2004), paper TuB2. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Plastic optical parts require antireflective as well as hard coatings. A novel design concept for coating plastics combines both functions. Symmetrical three-layer periods with a phase thickness of 3/2pi are arranged in a multilayer to achieve a step-down refractive-index profile. It is shown mathematically that the equivalent index of symmetrical periods can be lower than the lowest refractive index of a material used in the design, if the phase thickness of the symmetrical period is set equal to 3/2pi instead of the usual pi/2. The straightforward application of the concept to the design of antireflection coatings in general is demonstrated by example.

A theoretical approach to design multicycle AR coatings with predetermined residual reflectance, bandwidth, and cycle number is presented. The approach uses a novel step-index concept involving symmetric or asymmetric quarter-wave layer sequences to substitute refractive indices which are less than a given low refractive index. These substitutions result in a chosen step-down index profile matching the refractive index of the substrate to air. Each step of the index profile is the origin of a cycle of the synthesized AR design. Multicycle AR designs for the visible region with bandwidths of 1.6, 2.0, and 3.0 are presented as examples.

Arrays of longpass filter coatings for high order suppression in miniature spectrometers were produced by plasma-ion assisted deposition and photolithogaphy. The filter edges were imaged by optical microscopy, scanning electron microscopy, and scanning force microscopy. Whereas a positioning accuracy of about 2 microns was achieved, the width of the filter edges varied between 2 and 10 microns.

The coating of plastics for optical applications is intended to improve the mechanical durability of soft polymers and to serve an antireflection function. Usually a classic four-layer antireflection system is added on top of a single-layer hard coating. With needle optimization, an alternative coating design has been developed. The design is characterized by thin high-refractive-index layers that are almost evenly distributed over the whole stack. Plasma ion-assisted deposition was used to deposit coatings upon poly(methyl methacrylate), polycarbonate, and cyclo-olefin copolymer. Uniform antireflection and high scratch resistance have been achieved.