Patent Description:
Highly reflective optical interference coatings are indispensable tools for modern scientific and industrial efforts. Systems with ultralow optical losses, namely parts-per-million, ppm, levels of scatter and absorption, were originally developed for the construction of ring-laser gyroscopes in the late <NUM>, cf. As an outcome of this, ion-beam sputtering, IBS, has been established as the gold standard process technology for generating ultralow-loss reflectors in the visible and near infrared, NIR. Typically, such multilayers consist of alternating layers of amorphous metal-oxides, most commonly high index Ta<NUM>O<NUM>, tantala, and low index SiO<NUM>, silica, thin films, finding application in narrow-linewidth laser systems for optical atomic clocks, gravitational wave detectors, cavity QED, and tests of fundamental physics. Limitations of these amorphous coatings include excess Brownian noise, negatively impacting the limiting performance of precision optical interferometers, poor thermal conductivity, typically below <NUM> Wm-<NUM>K-<NUM>, as well as significant levels of optical absorption for wavelengths beyond <NUM>, excluding the operation of such low-loss reflectors in the mid-infrared, MIR. The latter limitation means that the highest performing metal oxide structures, while exhibiting phenomenal performance in the visible and NIR, cannot operate with low losses in this important long-wavelength region (for wavelengths significantly longer than <NUM>) and thus requires a switch to amorphous II-VI, group IV, or IV-VI compounds which are less well developed.

<CIT> discloses a mirror assembly based on a monocrystalline Bragg mirror bonded to a curved carrier substrate and a process of manufacturing the mirror assembly. Additionally, <CIT> describes an optical resonator system including a pair of such mirror assemblies forming an optical cavity for application in optical precision measurement systems. Processes disclosed therein proved very robust from a manufacturing point of view and have been proven to yield a number of improved performance metrics when compared with IBS-deposited amorphous metal oxide coatings. The proven advantages of these crystalline coatings, which are based on substrate-transferred GaAs/AlGaAs multilayers, include a significant reduction in Brownian noise when compared with typical dielectric mirror systems, with demonstrated loss angles <<NUM>×<NUM>-<NUM> at room temperature and the potential for ~<NUM>×<NUM>-<NUM> at cryogenic temperatures near <NUM>, a superior thermal conductivity of at least <NUM> Wm-<NUM>K-<NUM> compared with <NUM> Wm-<NUM>K-<NUM> for low-optical-loss Ta<NUM>O<NUM>/SiO<NUM> multilayers, and finally the ability to realize ppm-level optical absorption losses for wavelengths in the <NUM> to <NUM> range.

These monocrystalline coatings are typically grown via molecular beam epitaxy, where the total thickness is effectively limited to ~<NUM> - <NUM> due to technological restrictions including significant drift of the growth rate during such long (><NUM> hours) crystal growth runs, inherent build-up of strain due to lattice mismatch, as well as the accumulation of surface defects within such a thick structure. As a consequence of these issues, the quality and ultimate optical performance typically degrades for very thick single-crystal coatings. However, thicker coatings having a thickness at or beyond <NUM> are necessary for ultra-high reflectivity mirrors, in particular for the mid-infrared spectral region for mirror center wavelengths in excess of <NUM>.

Given the rapidly expanding interest in such low-noise and low optical loss end mirrors at these long operating wavelengths, primarily in the region from <NUM> to <NUM>, further improvements of the optical performance of these substrate-transferred crystalline coatings, particularly the position dependence of the optical scatter losses, is now in high demand from the ultimate end users.

<NPL>), presents high-reflectivity substrate-transferred single-crystal GaAs/AlGaAs interference coatings at a center wavelength of <NUM> with record-low excess optical loss below <NUM> parts per million. These high-performance mirrors are realized via a novel microfabrication process that differs significantly from the production of amorphous multilayers generated via physical vapor deposition processes. This new process enables reduced scatter loss due to the low surface and interfacial roughness, while low background doping in epitaxial growth ensures strongly reduced absorption.

The present disclosure provides an alternative solution to overcome the above-mentioned limitations, namely a means to reduce the overall optical losses and improve the position dependence of the coating optical properties. With the use of crystalline multilayers of high mechanical quality, it also serves to significantly reduce the Brownian noise of the mirror materials while simultaneously exhibiting optical performance on par with IBS deposited multilayer mirrors, with the benefit of extending these performance metrics into the mid-infrared spectral range.

Within the present disclosure, the term crystalline, single crystal or monocrystalline refers to a low defect density single-crystal film as can be produced via epitaxial growth techniques, such as molecular beam epitaxy, MBE; metalorganic vapor phase epitaxy, MOVPE; liquid phase epitaxy, LPE; etc. In this application the terms crystalline and monocrystalline may be used interchangeably. It is important to note that a single crystal or monocrystalline material structure will still exhibit a finite number of defects or dislocations. However, a monocrystalline material does not contain grain boundaries and defects associated with said boundaries, separating neighboring crystallites of varying orientation in a polycrystalline sample.

Within the present disclosure the term low absorption should be understood to indicate an optical absorption level with a maximum upper limit of <NUM> ppm. Preferably, this may be reduced to <<NUM> ppm or even into the range below <NUM> ppm.

Within the present disclosure, the term "dielectric multilayer coating" corresponds to a "thin film coating" which may also be referred to as a "multilayer mirror. " The term mirror assembly refers to the multilayer coating together with the substrate.

Within the present disclosure the term host substrate should be understood as a synonym for donor substrate as well as growth substrate.

The present invention is disclosed in accordance with the independent method claims.

The above and other aspects, features and advantages of the present disclosure will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

An example of the present disclosure not forming part of the invention as claimed provides a method for manufacturing hybrid optical coatings or hybrid mirror assemblies, including: a) providing a first optical coating having layers of alternating high and low refractive indices of crystalline materials on a first host substrate via an epitaxial growth technique; b) providing a second optical coating having layers of alternating high and low refractive indices of dielectric materials on a second host substrate via a physical vapor deposition (PVD) technique; c) directly bonding the first optical coating to the second optical coating; and d) removing the first host substrate.

Many aspects of the present disclosure can be better understood with reference to the following drawings. Moreover, in the drawings, like reference numerals designate corresponding parts throughout several views.

<FIG> illustrates a side view of an optical coating <NUM> provided on a host substrate <NUM> which form a base coating structure <NUM>. The optical coating <NUM> may be denoted as a first optical coating and the host substrate <NUM> may be denoted as a first host substrate. The optical coating <NUM> may include layers <NUM> and <NUM>. In an oversimplified schematic drawing the optical coating <NUM> is illustrated having only four layers <NUM>, <NUM>, respectively, provided in an alternating way. It should be understood, however, that the coating <NUM> typically includes many more layers. The maximum reflectivity of the coating may be determined by the refractive index contrast of the individual layers, the total number of layers, as well as the refractive index of the substrate-asymptotically approaching a reflectivity of <NUM>%. The number of layers for the present example may be about <NUM> pairs of layers, i.e., <NUM> total layers, but other numbers of layers such as <NUM>-<NUM> total layers may be used for such a structure. The layers <NUM> and <NUM> as shown in <FIG> are monocrystalline semiconductor layers alternating with respect to having a high and a low index of refraction, respectively. In <FIG>, it may be assumed that layers <NUM> correspond to the layers having a low index of refraction whereas layers <NUM> correspond to the layers having a high index of refraction. Typically, the difference in refractive index should be as large as possible; for example, with AlGaAs at a wavelength of <NUM> index values of <NUM> and <NUM> may be used for an implementation made from GaAs and Al<NUM>Ga<NUM>As layers respectively. The stack of layers <NUM> and <NUM> form a coating <NUM> that in combination with the host substrate <NUM> form the base coating structure denoted by reference sign <NUM>.

The coating <NUM> of <FIG> is provided onto the host substrate <NUM> via a suitable deposition technique. For example, the optical coating <NUM> may be a single-crystal multilayer as can be produced via epitaxial growth techniques, MBE, MOVPE, LPE, etc. The coating <NUM> may include a monocrystalline Bragg mirror. It should be understood that the term monocrystalline refers to a low defect density single-crystal film. Throughout this text, the terms crystalline and monocrystalline may be used interchangeably.

The host substrate <NUM> may be a semiconductor wafer. Said semiconductor wafer may be a standard wafer having a standard wafer size. Additionally, or alternatively, the host substrate <NUM> may include monocrystalline GaAs, germanium, Ge, or silicon, Si, InP, InSb, or BaF<NUM>, although other materials may also be possible, depending on the desired operating wavelength for the application. Such materials may additionally include InP, or GaN/AlN. The thickness of the growth substrate is typically around <NUM>-<NUM> though values between <NUM> and <NUM> are possible.

<FIG> further discloses a side view of another optical coating <NUM> provided on a host substrate <NUM> which form another or second coating structure <NUM>. The optical coating <NUM> may be denoted as a second optical coating and the host substrate <NUM> may be denoted as a second host substrate. The optical coating <NUM> may include layers <NUM> and <NUM>. As indicated in <FIG> the number of layers <NUM> and <NUM> may be the same as for the base coating structure <NUM>. Also, the sequence of layers <NUM> and <NUM> as well as their parameters may be the same as for the first optical coating <NUM>, such that the coating structure <NUM> is similar or even equal to the coating structure <NUM>. This then provides a starting point for accumulating arbitrary coating thickness as will be described, below.

In another example the coating structure <NUM> may differ from the first coating structure <NUM>. The difference between the coating structure <NUM> and <NUM> may then include different materials and/or different thicknesses of the host substrate <NUM> as compared to the host substrate <NUM>. Additionally, or alternatively the host substrates <NUM> and <NUM> may be similar or equal and instead the layers <NUM> and <NUM> of the optical coating <NUM> may be different from the layers <NUM> and <NUM> of the optical coating <NUM>. This then describes a starting point for using different source wafers as host substrates, such as amorphous and/or crystalline structures, electro-active and/or passive structures etc. Thus, for this example, in a stacked structure, the individual components of the coating may consist of monocrystalline materials with different lattice constants, e.g., GaAs-based, InP-based, GaN-based materials etc. or a combination of these, as well as fully amorphous materials, polycrystalline materials, or mixtures of each. This additional degree of freedom enables the design of advanced passive and active features of structures as well as optical coatings that cannot be realized with a single material platform.

<FIG> further indicates a double arrow P1 which should indicate that the base coating structure <NUM> is to be bonded to the second coating structure <NUM>. The double arrow P1 indicates that it is an arbitrary choice whether to bond the structure <NUM> to the structure <NUM> or vice versa. The first coating structure <NUM> has a top surface-or free surface-<NUM> facing away from the first host substrate <NUM> and the second coating structure <NUM> similarly has a top surface <NUM> facing away from the respective host substrate <NUM>. Bonding the base coating structure <NUM> to the second coating structure <NUM> thus means bonding the first optical coating <NUM> to the second optical coating <NUM>. This means that the top surface <NUM> of the first optical coating <NUM> is bonded to the top surface <NUM>S of the second optical coating <NUM>.

<FIG> further indicates a double arrow P1 which should indicate that the base coating structure <NUM> is to be bonded to the second coating structure <NUM>. The double arrow P1 indicates that it is an arbitrary choice whether to bond the structure <NUM> to the structure <NUM> or vice versa. The first coating structure <NUM> has a top surface-or free surface-<NUM> facing away from the first host substrate <NUM> and the second coating structure <NUM> similarly has a top surface <NUM> facing away from the respective host substrate <NUM>. Bonding the base coating structure <NUM> to the second coating structure <NUM> thus means bonding the first optical coating <NUM> to the second optical coating <NUM>. This means that the top surface <NUM> of the first optical coating <NUM> is bonded to the top surface <NUM> of the second optical coating <NUM>.

<FIG> illustrate schematically process steps according to the present disclosure.

By combining the first coating structure <NUM> and the second coating structure, it is also possible that a majority of growth defects may become buried at the bonding interface between the two structures instead of the top layers facing outward after the combining step. Growth defects present at the surface have a negative influence on the optical scatter as well as the wavefront error. Buried growth defects have less influence on optical losses, including scatter, and may also have a reduced impact on the wavefront error in stacked optical coatings as a higher quality bond interfaces can be achieved with planar samples. Thus, by burying these defects it may be possible to have a reduction in the coating scatter loss to levels below <NUM> ppm, which is an improvement of a factor of <NUM> - <NUM> compared to previously applied processes.

<FIG> illustrates as subsequent processing step. <FIG> illustrates an intermediate result which may become final after evaluating a determining step. As illustrated in <FIG>, one of the first and second host substrates <NUM> and <NUM> is removed from the combined coating structure. For illustrational purposes, <FIG> illustrates that the second host substrate <NUM> has been removed from the combined coating structure. That is, a removal or detaching step with regard to detaching one of the first and the second host substrates <NUM> and <NUM> is performed.

The removal of the host substrate <NUM> may be achieved by a removal process <NUM> as indicated in <FIG>. The removal process <NUM> may include at least one of wet-etching, grinding, lapping, etc. such that the host substrate <NUM> is detached from the respective optical coating <NUM>. Previously described substrate removal processes such as epitaxial lift-off, ELO, or the Smart Cut process involving ion implantation and subsequent annealing may not be applicable for the production of such low loss coatings as depicted in <FIG>. ELO is not compatible with low index ternary AlxGa<NUM>-xAs alloys, i.e., for x><NUM>%, while the ion implantation step required in the Smart Cut process may prove damaging to highly sensitive multilayers. Thus, the donor substrate removal process may include a first step of mechanically thinning the host substrate <NUM> by, for example, a grinding process. Then, the host substrate <NUM> material may be chemically removed from the combined optical structure, i.e., from the optical coating <NUM>, thereby obtaining a resulting combined optical structure <NUM>' of which one of the two host substrates, <NUM> or <NUM>, has been detached. As pointed out above, for illustrational purposes, <FIG> choose to illustrate that host substrate <NUM> is selectively detached.

The process step illustrated in <FIG> is followed by a determining step. The determining step refers to the parameters of the resulting combined optical structure, here <NUM>', of the previous step. The determining step determines whether or not the resulting combined optical structure <NUM>' fulfills a predetermined condition. The predetermined or predefined condition may include whether a thickness of the combined coating <NUM>' is larger, i.e., thicker, than a predefined thickness or else the predefined condition may include whether a predefined number n of repetitions of the previous steps has been performed, where n is a positive integer larger than or equal to <NUM>. In particular, having a known thickness of the first optical coating <NUM> and the second optical coating <NUM>, repeating the above steps n times will accumulate a combined coating having a corresponding thickness which adds up from the individual thicknesses of the optical coatings <NUM> and <NUM>, respectively.

In case the result of the determining step is negative, meaning that the predetermined condition has not been fulfilled, the process flow continues with the following steps.

<FIG> illustrates an immediate next step in case the determining step results to be negative. In <FIG>, the resulting combined coating structure <NUM>' as shown in <FIG> is taken to be the next, effective base coating structure. Likewise, as illustrated in <FIG> it is provided another optical coating <NUM>' provided on another host substrate <NUM>' which together form another or coating structure <NUM>'. The optical coating <NUM>' may include layers <NUM>' and <NUM>'. As indicated already in <FIG> the number of layers <NUM>' and <NUM>' may be the same as for the first coating structure <NUM> of <FIG>, but now the coating structure <NUM>' typically has more layers than the coating structure <NUM>'. Also, as already indicated above the sequence of layers <NUM>' and <NUM>' as well as their parameters may be the same as for the first optical coating <NUM>, such that the coating structure <NUM>' is similar or even equal to the first coating structure <NUM> of <FIG>. Alternatively, as described above, the coating structure <NUM>' may differ from the coating structure <NUM> of <FIG>. Thus, the effective base coating structure <NUM>' as well as another coating structure <NUM>' of <FIG> provide a pair of effective coating structures to be combined as indicated by the double arrow P2 of <FIG>. In other words, the step as illustrated in <FIG> resembles the step of <FIG> but with a different base coating structure.

<FIG> illustrates a subsequent step of the process flow following the step illustrated in <FIG>, i.e., under the condition that the result of the determining step was negative. <FIG> resembles <FIG> by illustrating a combination of coating structure <NUM>' and <NUM>'. As described with respect to <FIG> and <FIG>, combining coating structures <NUM>' and <NUM>' is achieved by a direct bonding step. The direct bonding step may typically be the same as described for <FIG> and <FIG> such that its description will not be repeated here. Further, and also similar as in <FIG>, the bonding step is to be followed by a removal step <NUM> so as to remove the host substrate <NUM>' from the structure shown in <FIG>. The removal step <NUM> typically may be of the same kind as described with respect to <FIG> and thus will not be described here, again.

As indicated above, by combining the current base coating structure <NUM> and another second coating structure, it is possible that a majority of growth defects may become buried at the bonding interface between the two structures instead of the top layers facing outward after the combining step. This again may lead to a reduction in the coating scatter loss to levels below <NUM> ppm, which is an improvement of a factor of <NUM> - <NUM> compared to previously applied processes.

<FIG> illustrates a subsequent step following the step illustrated in <FIG>. After the removal step <NUM> of <FIG>, it is obtained a current resulting combined optical structure <NUM>'. The current resulting combined optical structure <NUM>' thus has a larger thickness than the corresponding resulting combined optical structure obtained after having performed the step illustrated in <FIG>.

Therefore, the determining step as was performed after obtaining the result illustrated in <FIG> may now be performed, again. This determining step then may refer, again, to the parameters of the resulting combined optical structure, here <NUM>', of the previous step. The determining step determines whether or not the resulting combined optical structure <NUM>' fulfills the predetermined condition, i.e., the same predetermined condition as was posed with respect to <FIG>. The predetermined or predefined condition thus again may include whether a thickness of the combined coating <NUM>' is larger, i.e., thicker, than a predefined thickness or else the predefined condition may include whether a predefined number n of repetitions of the previous steps has been performed, where n is a positive integer larger than or equal to <NUM>. For the latter, a respective counter counting the number of iterations has to be increased by <NUM>. In particular, having a known thickness of the first optical coating <NUM> and the second optical coating <NUM>, and thereby having a known thickness of the effective optical coating <NUM>' and <NUM>', respectively will accumulate a combined coating having a corresponding thickness which adds up from the individual thicknesses. Thus, by iterating the above described steps, monocrystalline coatings with essentially arbitrary thickness can be achieved. This includes thicker coatings which are necessary for ultra-high reflectivity mirrors, in particular for the mid-infrared spectral region, for mirror center wavelengths in excess of <NUM>.

In case the result of the determining step performed after <FIG> is still negative, the procedure will continue by taking the optical structure <NUM>' as the current, effective base structure and adding another coating structure as described with respect to <FIG> and <FIG> or likewise <NUM> and <NUM>, respectively.

In case the result of the determining step performed after any of the previous steps is positive, the predetermined condition has been fulfilled. This then means that the desired thickness of the combined coating has been achieved and/or the predefined number of envisaged repetitions/iterations of the above steps has been reached. Then <FIG> indicates a subsequent step provided that the determining step yielded a positive result.

In <FIG> it is illustrated that the resulting combined optical structure <NUM>' of <FIG>, or likewise of <FIG>, is provided alongside an optical substrate <NUM>.

The optical substrate <NUM> of <FIG> has a top surface or working surface <NUM> of the optical substrate <NUM>. This surface <NUM> may be polished. Likewise, the optical structure <NUM>' has an outmost or free surface which here is denoted <NUM>'. The surface <NUM>' may also be polished. The optical structure <NUM>' may then be combined with the optical substrate <NUM> by directly bonding the respective surfaces <NUM>' and <NUM>, respectively. This is indicated by the double arrow P3.

Similar to the above illustrated coating-relevant bonding process, the bonding process between the combined optical structure <NUM>' and the optical substrate <NUM> may involve direct bonding, i.e., with no intermediate adhesive layers. Again, growth defects which may be present at the surface <NUM>' will be buried when bonding against the surface <NUM> of the optical substrate <NUM>. To achieve proper bonding a press may be used. Thus, a defect-free bonding interface for the final substrate-transfer process onto the final optical substrate is advantageous for increasing manufacturing yield and also for suppressing wavefront errors caused by defect-induced voids at the coating-substrate interface.

Further, the entire structure shown in <FIG> can be annealed in order to generate a stronger bond between the structure <NUM>' and the optical substrate <NUM>. After pressing and annealing the structure <NUM>' onto the optical substrate <NUM>, the structure <NUM>' has been firmly bonded to the optical substrate <NUM> thereby forming a mirror assembly <NUM>. As is illustrated in <FIG> and <FIG>, the host substrate <NUM> is still attached to said mirror assembly <NUM>.

<FIG> illustrates a further step in which a removal process <NUM> is applied to remove the remaining host substrate <NUM> from the optical structure <NUM>'. The removal process <NUM> may be similar as the removal process <NUM>, and <NUM> described above.

<FIG> illustrates the resulting transferred combined optical stack <NUM> on the optical substrate <NUM>.

Whereas the above Figures have been shown with planar substrates, it should be understood that at least the optical substrate <NUM> may be also be chosen to be curved and may have a pre-determined radius of curvature between <NUM> and <NUM>, with a typical value of <NUM>, or a radius of curvature between <NUM> and <NUM>.

If the final application requires an extremely stable mirror structure with low optical losses and low Brownian noise, the coating should consist of a monocrystalline semiconductor multilayer. One potential example is AlGaAs-based coatings, which typically exhibit a limiting loss angle, i.e., the inverse of the mechanical quality factor, of a maximum of <NUM> × <NUM>-<NUM> to a value below <NUM>-<NUM> depending on the system operating temperature. In addition, such coatings can typically provide a reflectivity ><NUM>%, with a total absorption <<NUM> ppm for center wavelengths covering the near infrared spectral region, i.e., <NUM> - <NUM>. Typical values for center wavelengths are <NUM> and <NUM>, though the range of ~<NUM> to ~<NUM> is possible with GaAs/AlGaAs multilayers.

In summary, this disclosure covers the production of separately stacked coatings for a subsequent substrate-transfer step in order to transfer the previously stacked coating onto arbitrary substrates. The stacking procedure allows for various technological barriers to be overcome, including limitations of the total thickness for various deposition and/or crystal growth techniques as employed for the production of ultralow-loss optical coatings, as well as reductions in defect densities that may degrade the final performance of the optic. Additionally, the stacking process may enhance the optical quality and surface quality necessary for the substrate-transfer coating process, while also allowing for the combination of two different coating materials or structures including monocrystalline materials with different lattice constants, various amorphous and polycrystalline materials, electro-optically passive and active structures, or combinations therein.

The present disclosure provides a novel concept and process for manufacturing high-performance (i.e., low optical loss) hybrid infrared thin film optical interference coatings. An embodiment not forming part of the present invention provides a single-crystal multilayer stack directly bonded to the surface of an amorphous multilayer stack deposited by physical vapor deposition (PVD). Such a combination of materials enables mirrors to be produced with excess optical losses (optical scatter + absorption) at the part-per-million level, enabling mirror reflectivity >> <NUM>% over the wavelength range of roughly <NUM> to approximately <NUM>, while maintaining a commercially viable manufacturing method. This is a completely novel approach combining crystalline and amorphous materials to form a hybrid coating enabling completely new metrics in IR coatings at a reasonable cost.

Traditionally, optical interference coatings are based on stacks of thin dielectric layers directly deposited onto an appropriate optical substrate (for example, in the mid-IR range including highly polished CaF<NUM> among other materials) to form a highly reflective mirror assembly. These films are typically deposited using various evaporation or sputter technologies, generally referred to as PVD techniques. Such processes yield mirrors with modest reflectivity (up to <NUM>%), ultimately limited by optical absorption and scatter at the few hundred part-per-million (ppm) (>><NUM> ppm) level in the mid- and long-wave infrared spectral region, which is defined here as <NUM> to approximately <NUM>.

As an alternative solution, previous efforts have focused on drastically reducing these limiting excess optical losses by employing substrate-transferred crystalline coatings. This unique class of optical interference coatings relies on a separate crystal or "epitaxial" growth process on a seed wafer (most commonly via molecular beam epitaxy, MBE), followed by patterning and etching of individual coating discs or coupons, and ultimately direct bonding (using no adhesive layers) the crystalline stack onto the final optical substrate to form the final mirror assembly. In addition to the present disclosure, this technology and manufacturing process is covered in detail in <CIT>.

These so-called crystalline coatings or semiconductor supermirrors exhibit exceptional optical and thermomechanical properties owing to their high purity and near structural perfection as a consequence of their single-crystal nature.

However, challenges are encountered when extending this technology to long wavelengths, particularly in the mid- and long-wave infrared spectral region (<NUM> to approximately <NUM>). For this intended wavelength range, the crystalline stack ends up extremely thick, since to obtain a maximized reflectivity for a given number of layers in the stack, each of the alternating layers ideally has an optical layer thickness corresponding to a quarter of the desired center wavelength of the mirror. In addition, the reflectivity may be further increased by increasing the number of layers in the stack. Hence, the longer the mirror center wavelength, the thicker such an optical interference coating with maximized reflectivity will become. As an example, for a roughly <NUM> center wavelength, a target reflectivity ><NUM>% would entail a crystalline coating thickness approaching <NUM>. High quality epitaxy is only possible for film stacks on the order of <NUM> - <NUM> in thickness. To get around this, a method not forming part of the present invention describes the stacking of two crystalline multilayers (followed by the substrate transfer process to create the mirror assembly) in the previous sections above. As each sub-stack is within the thickness limit for high quality material, it is possible to generate ultralow-loss mirrors (with scatter and absorption < <NUM> ppm). For example, <NPL>, discloses optical coatings capable of achieving a reflectivity >><NUM>% in the mid-infrared spectral range from <NUM> to <NUM>.

However, this process suffers from poor yield (as two bonding steps are needed) and associated high costs, making it impractical for commercial production. Going to even longer wavelengths (for example beyond <NUM> as in the Winkler et al. paper) would require further sub-stack bonding processes, making these mirrors even more difficult to manufacture. Moreover, these mirrors exhibit a relatively narrow optical bandwidth owing to the limited refractive index contrast in the all-crystalline system. (See, for example, Wikipedia for definitions of "Refractive index contrast").

As a solution to these major technological and production challenges, an embodiment of the present disclosure provides a novel "stacked" coating solution, building off of that in <CIT>. Rather than the repeated stacking of crystalline multilayers as in those embodiments, this new approach combines a high index contrast but optically lossy, with nominally high absorption, amorphous thin film PVD base layer combined with a bonded crystalline coating cap. Although the lossy amorphous films may at first glance appear to limit the overall performance, it turns out that the crystalline surface layers predominantly provide the desired optical effects and thus absorption levels < <NUM> ppm, can be maintained in a mirror with drastically simplified manufacturing, owing to the need for just a thin epitaxial growth process and a single bonding step.

The novelty of this proposal lies in the fact that, in a high-reflectivity optical interference coating, the vast majority of the optical intensity lies in the surface layers (see <NPL>), thus these are most important for controlling the optical performance (in terms of losses, driving the absorption and scatter). The remaining layers, i.e., the base layers, while necessary for contributing to the interference effects and pushing the total reflectivity towards a limiting value, here targeting ><NUM>%, only weakly contribute to the overall losses, given the low optical field strength at the surface layers. In this way, the much higher losses can be tolerated in the base layers, while still yielding coatings and mirror assemblies with <<<NUM> ppm of scatter and absorption at these long wavelengths.

To manufacture such coatings and mirror assemblies, referred to here as "hybrid coatings" or "hybrid mirrors" when joined to a final optical substrate, an embodiment of the present disclosure not forming part of the present invention provides a target optical substrate, deposits a dielectric coating on the target host substrate, then directly bonds a crystalline coating to the surface of the dielectric coating, generating an optical interference coating which is referred to as a "hybrid optical coating", with a comparably lossy base layer (dielectric coating, with high index contrast, thus being only a few micrometers thick) capped with a very low loss crystalline coating. The crystalline coating has a thickness well below <NUM>, maintaining high structural quality and low background absorption. Therefore, an embodiment of the present disclosure can generate high-performance mirrors in a much more manageable fashion, well beyond the optical performance of amorphous (PVD) dielectrics alone and approaching that of all-crystalline structures which have unwieldy production requirements in the wavelength range from <NUM> to <NUM>.

In one embodiment not forming part of the present invention, the process involves pre-coating the target host substrate (e.g., including or consisting of silicon, Si or calcium fluoride, CaF<NUM>) with the second optical coating via PVD and then transferring, via direct bonding, the separately grown crystalline coating, i.e. the first optical coating to the surface of the second optical coating. The crystalline coating direct bonding process is described in detail in "Direct-bonded optical coatings," <CIT> and "Substrate transferred monocrystalline Bragg mirrors," <CIT>. To complete the hybrid optical mirror assembly, the first host substrate, which preferably is a growth wafer (e.g., including or consisting of at least one of: gallium arsenide, GaAs, germanium, Ge, silicon, Si, and indium phosphide, InP) on which the crystalline coating has been grown, is removed from the crystalline coating. <FIG> illustrates a side view of a first substrate <NUM> and a second substrate <NUM>. A first optical coating <NUM> having multiple layers of alternating high and low refractive indices is provided on the first host substrate <NUM> via an epitaxial growth technique, such as molecular beam epitaxy or organometallic vapor-phase epitaxy. A second optical coating <NUM> having multiple layers of alternating high and low refractive indices is provided on the second host substrate <NUM> via a physical vapor deposition (PVD) technique. e.g., ion-beam sputtering, or evaporation. In an oversimplified schematic drawing the optical coatings <NUM>, <NUM> are illustrated having only four and two layers respectively, provided in an alternating way. It should be understood, however, that an optical coating typically includes many more layers, and that the first optical coating may have more, fewer or the same number of layers as the second optical coating.

The arrow shown in in <FIG> further indicates that the first optical coating <NUM> is to be directly bonded to the second optical coating <NUM> to form a first-type intermediate hybrid structure. The first optical coating <NUM> has a top surface <NUM>-or free surface-facing away from the first substrate <NUM> and the second optical coating <NUM> similarly has a top surface <NUM>-or free surface-facing away from the second substrate <NUM>. Bonding the first optical coating <NUM> to the second optical coating <NUM> means that the top surface <NUM> of the first optical coating <NUM> is bonded to the top surface <NUM> of the second optical coating <NUM>. The various bonding techniques have already been discussed above.

<FIG> illustrates the first-type intermediate hybrid structure after the first optical coating <NUM> is bonded to the second optical coating <NUM>. As illustrated in <FIG>, the first host substrate is to be removed from the first-type intermediate hybrid structure. The removal of the first host substrate <NUM> may be achieved by a removal process <NUM> as indicated in <FIG>. The various removal processes have already been discussed above.

<FIG> illustrates the hybrid optical mirror assembly after the removal process. This hybrid optical mirror assembly includes the second substrate <NUM>, the second optical coating <NUM> provided on the second substrate, and the first optical coating <NUM> directly bonded on top of the second coating <NUM>. The first and second optical coatings together form a hybrid optical coating.

In another embodiment, the process involves first depositing the dielectric coating, i.e. the second optical coating via PVD on the crystalline coating, i.e., on the first optical coating, which is separately grown on the first host substrate (e.g., including or consisting of at least one of: GaAs, Ge, Si, and InP), leading to a second-type intermediate hybrid structure, then flipping and directly bonding said second-type intermediate hybrid structure to a second host substrate (e.g., including or consisting of Si or CaF<NUM>). Finally, as with above, the first host substrate is removed leaving a hybrid optical mirror consisting of the second host substrate, the PVD coating and the crystalline coating. <FIG> illustrates a first substrate <NUM> coated with a first optical coating <NUM> having layers of alternating high and low refractive indices onto which a second optical coating <NUM> is to be deposited, the second optical coating <NUM> having layers of alternating high and low refractive indices to form a second-type intermediate hybrid structure. In this case alternating high and low refractive indices layers <NUM>, <NUM> are deposited directly on the first optical coating <NUM> via PVD. Again, the optical coatings <NUM>, <NUM> are illustrated in an oversimplified schematic drawing.

<FIG> illustrates a side view of the second-type intermediate hybrid structure and a second host substrate <NUM>. <FIG> further indicates that the second-type intermediate optical hybrid structure is to be bonded to the second host substrate <NUM> to form a first-type intermediate hybrid structure. In this case the top surface <NUM>-or free surface of the second optical coating <NUM> is directly bonded to the surface <NUM> of the second substrate <NUM>. The first-type intermediate hybrid structure is already shown in <FIG>. After the removal of the first host substrate <NUM> by the process <NUM>, a hybrid optical mirror as shown in <FIG> is obtained.

The above embodiments disclose that a hybrid optical mirror is generated by combining a PVD (sputtered or evaporated) bottom coating on a base/target optical substrate, and capped with a thin crystalline coating, wherein the crystalline coating interacts with the vast majority of the optical intensity when the hybrid optical mirror interacts with optical radiation at a wavelength for which the hybrid optical coating was designed. Such a hybrid optical coating largely preserves the beneficial properties of all-crystalline coatings, while remedying the above noted shortcomings; challenges in stacking, and inferior bandwidth. These unique advantages are illustrated by the following examples.

This design would be valid for a hybrid optical mirror assembly operating in the <NUM> - <NUM> spectral region; for example, <NUM> center wavelength mirror:.

For a <NUM> center wavelength mirror, layer thicknesses are preferably chosen to be:.

This hybrid optical coating yields a mirror with a transmission of <NUM> ppm and absorption of <NUM> ppm. With optical scatter of ~<NUM> ppm given the long operating wavelength and low micro-roughness of the GaAs, the reflectivity (R) is <NUM> - <NUM>×<NUM>-<NUM> - <NUM>×<NUM><NUM> = <NUM> (or <NUM>%). <FIG> shows a graph of the transmissivity (T) of this mirror.

This design would be valid for a hybrid optical coating acting as a mirror in in the <NUM> - <NUM> spectral region; for example, <NUM> center wavelength mirror:.

For a <NUM> center wavelength mirror, each of the multiple layers has a thickness which is preferably chosen to be:.

This hybrid optical coating yields a mirror with a transmission of <NUM> ppm and absorption of <NUM> ppm. With optical scatter of ~<NUM> ppm, again given the long operating wavelength and low micro-roughness of the GaAs, the reflectivity (R) is <NUM> - <NUM>×<NUM><NUM> - <NUM>×<NUM>-<NUM> = <NUM> (or <NUM> %). <FIG> shows a graph of the transmissivity (T) of this mirror.

These examples show that hybrid optical mirror with a reflectivity that is greater than <NUM>% may be achieved, while the thickness of the crystalline coating stack is maintained to be less than <NUM>, ensuring excellent optical quality.

Claim 1:
A method for manufacturing a hybrid optical mirror assembly, comprising:
a) providing a first optical coating (<NUM>) having multiple layers of alternating high and low refractive indices of crystalline materials on a first host substrate (<NUM>) via an epitaxial growth technique;
b) providing a second optical coating (<NUM>) having multiple layers of alternating high and low refractive indices of dielectric materials on the first optical coating (<NUM>) via a physical vapor deposition (PVD) technique;
c) directly bonding the second optical coating (<NUM>) to a second host substrate (<NUM>); and
d) removing the first host substrate (<NUM>);
wherein the multiple layers of alternating high and low refractive indices of crystalline materials comprise GaAs/AlGaAs layers,
characterised in that
the multiple layers of alternating high and low refractive indices of dielectric materials comprise Si/SiO<NUM> layers.