Ultra Broadband Multilayer  Dielectric Beamsplitter Coating

Coatings for optical devices, such as beamsplitters, are provided. The coatings include at least one bilayer of a layer of a material having an index of refraction n1 in contact with a layer of a material having an index of refraction n2 and an uppermost layer of a material having an index of refraction n3 over the bilayer, wherein n3>n2>n1. The bilayer(s) can be composed of BaF2 and KRS5. The uppermost layer can be composed of Ge. Certain coatings provide beamsplitters which exhibit highly efficient emission over broad spectral ranges.

DETAILED DESCRIPTION

Provided herein are optical coatings and optical devices using the coatings, including beamsplitters. Also provided are related methods.

Coatings

In one aspect, a coating for an optical device, such as a beamsplitter, is provided. The coating includes a bilayer of a layer of a material having an index of refraction n1in contact with a layer of a material having an index of refraction n2. By “in contact” it is meant that no intervening layer is between the layers of the bilayer. In some embodiments, the layer of the material having an index of refraction n1is under the layer of the material having an index of refraction n2. In other words, in such embodiments, the layer of the material having an index of refraction n1is the lower layer of the bilayer and the layer of the material having an index of refraction n2is the upper layer of the bilayer. The coating further includes an uppermost layer of a material having an index of refraction n3over the bilayer. The indices of refraction of the layers in the coating are such that n3>n2>n1. In some embodiments, the uppermost layer is in contact with the bilayer. In some embodiments, the uppermost layer is in contact with the layer of the material having an index of refraction n2in the bilayer.

In some embodiments, the coating includes two or more bilayers, each bilayer a layer of a material having an index of refraction n1in contact with a layer of a material having an index of refraction n2, and an uppermost layer of a material having an index of refraction n3over the two or more bilayers. The indices of refraction of the layers in the coating are such that n3>n2>n1. In some embodiments, the coating includes two bilayers, three bilayers, or more. In some embodiments, the two or more bilayers form a stack of bilayers in which each bilayer is in contact with an adjacent bilayer, without intervening layers between adjacent bilayers. As noted above, the layers of the bilayers may be arranged such that the layer of the material having an index of refraction n1is under the layer of the material having an index of refraction n2. Similarly, the uppermost layer may be in contact with a bilayer and the uppermost layer may be in contact with the layer of the material having an index of refraction n2within the bilayer.

In other embodiments, the coating consists essentially of, or consists of, a stack of one, two, or three bilayers, each bilayer a layer of a material having an index of refraction n1in contact with a layer of a material having an index of refraction n2, and an uppermost layer of a material having an index of refraction n3over the stack. The indices of refraction of the layers in the coating are also such that n3>n2>n1. As noted above, the layers of the bilayers may be arranged such that the layer of the material having an index of refraction n1is under the layer of the material having an index of refraction n2. Similarly, the uppermost layer may be in contact with a bilayer and the uppermost layer may be in contact with the layer of the material having an index of refraction n2within the bilayer

The materials for the layers of the bilayer and the uppermost layer may vary. A variety of dielectric materials may be used for the layers of the bilayers. In some embodiments, the material having an index of refraction n1is KBr, BaF2, PbF2, or Na3AlF6. In some embodiments, the material having an index of refraction n2is Thallium Bromo-Iodide (also known as KRS5 or TlBr—TlI) or ZnSe. In some embodiments, the material having an index of refraction n3is Ge.

The coating may be further characterized by specifying certain materials that are not included in certain layers of the coating. In some embodiments, the layers of the bilayer(s) do not include Ge. In other embodiments, the layers of the bilayer(s) and/or the uppermost layer do not include a metal oxide, e.g., silica, a carbide or a nitride. In still other embodiments, the layers of the bilayer(s) and/or the uppermost layer do not include a polymer or a substituted or unsubstituted organic molecule. In each of these embodiments, it is meant that the coating or layers of the coating do not intentionally include these materials. One or more of these materials may be present in the coating at a level (e.g., as an impurity) that is typical for standard techniques for forming optical coatings.

In some embodiments, the materials for the coating are selected such that the bilayer(s) are composed of a layer of BaF2and a layer of KRS5. This particular combination provides surprisingly beneficial results as the inventors have found that this is a particularly good combination of materials for the disclosed optical devices. Specifically, the inventors have found that the inter-diffusion and/or chemical interaction of BaF2and KRS5 is minimal or nonexistent during the deposition process. Thus, during the deposition process itself, because the inter-diffusion and/or chemical interaction of BaF2with the Ge (which is often a weather resistant coating) is surprisingly minimal or nonexistent there is no degradation of the Reflectance/Transmittance (R/T) properties of the final coating/substrate combination because the designed thicknesses of such layers are maintained with a degree of precision.

In addition, for those embodiments in which the uppermost layer is in contact with the layer of KRS5 within a bilayer, the KRS5 inhibits or prevents the diffusion of BaF2and/or its constituents into the uppermost layer, thereby maintaining the index of refraction and integrity of the uppermost layer. In some embodiments, the materials for the coating are selected such that the bilayer(s) are composed of a layer of BaF2and a layer of KRS5 and the uppermost layer is composed of Ge.

The thicknesses of each of the layers in the coatings may vary. In some embodiments, the thickness of the layer of the material having an index of refraction n1is in the range from about 800 Å to 1500 Å and the thickness of the layer of material having an index of refraction n2is in the range from about 500 Å to 2800 Å. In other embodiments in which the coating includes a first bilayer, a second bilayer and an uppermost layer in contact with the first bilayer, the thicknesses may vary as follows. The thickness of the layer of the material having an index of refraction n1in the first bilayer is in the range from about 1250 Å to 1500 Å; the thickness of the layer of the material having an index of refraction n2in the first bilayer is in the range from about 2550 Å to 2800 Å; the thickness of the layer of the material having an index of refraction n1in the second bilayer is in the range from about 800 Å to 1000 Å; and the thickness of the layer of the material having an index of refraction n2in the second bilayer is in the range from about 500 Å to 700 Å. In some embodiments, the thickness of the layer of the material having an index of refraction n3is in the range from 1250 Å to 1500 Å.

The coatings may be further characterized by the optical properties they provide. In some embodiments, the coating is characterized in that it provides a beamsplitter when coated over a substrate. In some embodiments, the coating is characterized in that it provides a beamsplitter including the coating with a transmission of about 50% at about 2 μm. In some embodiments, the coating is characterized in that it provides a beamsplitter including the coating with a transmission percentage of about 50%+/−5% over the spectral range from about 1000 cm−1to about 10000 cm−1(i.e., about 9.5 μm down to about 1. μm) with a high transmission percentage of up to 90% at about the 9000 cm−1(˜1.1 μm) energy range. The coatings may be distinguished from antireflective coatings and coatings for optical filters. Thus, in some embodiments, the coating is characterized in that it does not provide an antireflective coating and/or an optical filter when coated onto a substrate.

Optical Devices

In another aspect, optical devices including the disclosed coatings are provided. The optical devices include a substrate and any of the coatings disclosed above coated over the substrate. In some embodiments, the optical device consists essentially of, or consists of, the substrate and any one of the disclosed coatings coated over the substrate. As example beneficial substrate materials to be utilized herein, the substrate can be selected from KBr, Silicon, Quartz, Calcium Fluoride, and Zinc Selenide. In some embodiments, the layer of the material having an index of refraction n1of a bilayer is in contact with the substrate. In some embodiments, the optical device is a beamsplitter. An illustrative beamsplitter100is shown inFIG. 1. The beamsplitter includes a substrate104and a coating102. The coating102includes a first bilayer106, a second bilayer108and an uppermost layer110(i.e., fifth layer) over the bilayers. The first bilayer106includes an example third layer112of a material having an index of refraction n1under a fourth layer114of a material having an index of refraction n2. The second bilayer108ofFIG. 1is shown to often include a first layer116of a material having an index of refraction n1under a second layer118of a material having an index of refraction n2. In some embodiments, the optical device is not an antireflective optical device and/or an optical filter.

The optical devices may be used in a variety of spectroscopic applications, such as Fourier Transform Infrared (FTIR) Spectroscopy. Thus, also provided are FTIR instruments including any of the disclosed optical devices.

Methods

Also provided are methods of forming the coatings and optical devices. The methods involve sequential deposition of the layers of any of the disclosed coatings. Standard techniques and deposition parameters may be used for depositing layers of dielectric material, including electron beam evaporation, thermal evaporation, sputtering, chemical vapor deposition and plasma enhanced chemical vapor deposition. By way of example only, a beamsplitter may be formed by depositing a layer of a material having an index of refraction n1on a substrate, depositing a layer of a material having an index of refraction n2over the layer of the material having an index of refraction n1to form a lower bilayer; depositing a layer of a material having an index of refraction n1on the lower bilayer, depositing a layer of a material having an index of refraction n2on the layer of the material having an index of refraction n1to form an upper bilayer; and depositing an uppermost layer of a material having an index of refraction n3on the upper bilayer. Additional details of this embodiment of the method are provided in the Examples, below.

Also provided are methods of using any of the disclosed optical devices. In some embodiments, the optical device is a beamsplitter and the methods include splitting a light beam into a transmitted beam and a reflected beam with the beamsplitter. The methods can further include directing a light beam at the beampslitter.

The coatings, optical devices and related methods will be understood more readily by reference to the following example, which is provided by way of illustration and is not intended to be limiting.

Example

In particular, the reader is directed again to the example configuration of the beamsplitter100arrangement shown inFIG. 1. Specifically, the preferred design is configured In Table 1 as follows:

FIG. 2shows the theoretical transmission properties of a single layer Ge coating202(about 1388 Angstroms (denoted as a solid line)) and a novel 5 layer206design (denoted as a dashed line) of the present application having the recipe of Table 1, as shown above. The theoretical calculations assume the material layers have well defined boundaries (no diffusion layer) and the index of refraction of the materials maintains known measured values.

Specifically,FIG. 2shows that a designed 5 layer beamsplitter206configuration results in a beneficial transmission percentage of about 50%+/−10% to provide a broader range of efficiency across the spectral range from about 1000 cm−1to about 10000 cm−1(i.e., about 9.5 μm down to about 1. μm) with a noted notch high transmission (˜90%) at about the 9000 cm−1(˜1.1 μm) energy range. It is also to be appreciated thatFIG. 2shows some information about the Reflection*Transmission product value of the coatings. In particular, at 1000 cm−1it is clear that the 5 layer coating206has a value of 60% T. By contrast, the Ge coating202has a value of 75% T. Accordingly, the 5 layer206R*T product=0.4*0.6=0.24 wherein the Ge202coating R*T product=0.75*0.25=0.188. This makes the 5 layer coating 28% better than the Ge coating (0.24/0.188=1.28). This translates into 28% more signal (i.e., increased efficiency) when used as a beamsplitter in a desired spectrometer for a desired application.

It is to be appreciated that the design parameters of the example optical device beamsplitter shown above was with the aid of TFCalc (Thin Film Design Software) from: Software Spectra, Inc., 14025 N.W. Harvest Lane, Portland, Oreg. 97229, although any thin film design software capable of aiding in the construction of the present embodiments may also be used when desired.

To further appreciate the novel aspects of the embodiments herein, the reader is directed toFIG. 3A, which illustrates a single beam (non-ratio) intensity spectra comparison between a beamsplitter coating of a known design302(a 2 layer design as denoted by a dashed line) and a 5-layer design306(denoted as a solid line).FIG. 3Bshows an expanded view of the spectral region between 6000 cm−1and 9000 cm-1 illustrating beneficially the extended transmission performance beyond 9000 of the present example application. In particular, using the novel beamsplitter recipe shown above, it is to appreciated when specifically reviewingFIG. 3Bthat at the high energy end, i.e., beyond 7000 cm−1, the energy throughput of the known formulation302(again note dashed line) rapidly drops to zero starting around 6000 cm−1while the 5-layer structure306disclosed herein continues to transmit energy up to 8000 cm−1and beyond (e.g., up to at least 10000 cm−1), resulting in a significantly expanded spectral range for measurement.

In addition, it is to be noted that a surprising additional aspect of the 5 layer design306is that the configuration also leaves intact (i.e., substantially non-shifted in spectral location) the location of a maximum spectral transmission308(˜1000 cm−1-1500 cm−1) region, which is desirably situated over the infrared fingerprint region with no loss of energy, as generally shown in the dashed elliptical region ofFIG. 3A. This is an important aspect because previous designs that have provided for an expanded spectral coverage into the high energy region(s) suffer from a significant shift in the maximum transmission away from the fingerprint region as well as a drop in throughput. This combination is the novelty provided herein.

The word “illustrative” or “exemplary” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “illustrative” or “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, “a” or “an” means “one or more.” Still further, the use of “and” or “or” is intended to include “and/or” unless specifically indicated otherwise.

All patents, applications, references, and publications cited herein are incorporated by reference in their entirety to the same extent as if they were individually incorporated by reference.