Patent Description:
Major limiters of laser systems peak power and energy are laser-induced damage processes. One of these processes is the formation of filamentation bulk damage.

It has been shown that propagating the light from a laser with an elliptical/ circular polarization increases the permissible intensities with respect to linear polarization, which results from a reduced effective Kerr coefficient. The reduction in the effective Kerr coefficient from linear to circular polarization is <NUM>% (both from theoretical and experimental demonstrations), and thus also within permissible intensities.

Designing the laser system chain for a linearly polarized light beam, however, is simpler. For example, amplifiers are designed at the Brewster incidence angle. Mirrors and frequency converters are of simpler design as well, usually at one linear polarization. Therefore, the desirable solution is for the light to propagate at a linear polarization through most of the system and to be converted to a circular polarization only at the sections of lowest laser damage threshold, which are usually found at the final optics assembly, and in the ultraviolet (UV) spectrum.

The optical component that converts linear to elliptical polarization is a wave retardation plate, which is typically referred to as a "waveplate". The waveplate introduces a delay between the two linear polarization components. Specifically, to convert between linear and circular polarizations, the required delay between the two linear polarization components is λ/<NUM> (quarter of a wavelength), and thus the waveplate construction that achieves this conversion is referred to as a quarter wave plate or "QWP".

Traditional waveplate implementations include crystalline (spatially homogeneous) birefringent materials. Birefringent materials have different propagation indices/speeds at two principal axes. Other, more recent solutions include glancing angle deposition ("GLAD") of columnar nano-structure, which is not necessarily made of birefringent material. However, the GLAD columnar nano-structure produces asymmetry between the two polarizations which results in the sub-wavelength structure (i.e., also known as a "meta-surface") making the effective material of the surface layer anisotropic.

Since the main optical components of the laser system are made of low interaction cross-section with light (by design), deposition of a new material usually results in reduced laser damage threshold, which defies the purpose of adding the waveplate. This has been a major limitation so far for using QWP made of birefringent materials, or for using same glass material (or other oxides) in a columnar structure meta-surface implementations (e.g., GLAD), which usually has more traps/dislocation for light interaction with material. An additional drawback of the GLAD methodology is that it produces scatter, which has a negative impact on performance, especially critical for energetic lasers (i.e., because of the loss of power, and stray/scattered light that might cause further damage).

From experience, it is generally understood that when it comes to maintaining or increasing the laser induced damage threshold (LIDT), a subtractive process via etching of optical grade optics results in a higher LIDT. Structuring the surface of the optic by etching a sub-wavelength periodic grating will also result in a birefringence. For a prototypical case of a high power laser system, the incidence wavelength is <NUM> in fused silica glass. Thus, to avoid diffracting higher orders, the grating period has to be at least smaller than twice the wavelength. However, obtaining such a grating over large apertures with present day lithography technology is extremely challenging.

D1 (<NPL> A), D2 (<NPL> A) and D3 (<NPL> A) propose to create waveplates by a deposition process of nanoscale features onto a substrate.

According to the claimed invention the present disclosure relates to a waveplate which includes a substrate forming an optic, with the the substrate including an integral portion forming a plurality of angled columnar features on an exposed surface thereof as a result of removal of a portion of a surface of the substrate, leaving the angled columnar features forming a metasurface. The plurality of angled columnar features are further. aligned parallel with a directional plane formed non-parallel to a reference plane, the reference plane being normal to a surface of the substrate. The metasurface forms a birefringent metasurface which forms a portion of the substrate.

In another aspect the present disclosure relates to a waveplate for receiving an optical signal. The waveplate may comprise a substrate forming an optic, with the substrate including an integral portion forming a metasurface formed on an exposed surface thereof, and extending partially into the substrate. The metasurface including a plurality of angled, columnar features formed using a portion of the substrate, the angled columnar features being in a generally uniform grid-like pattern and each having a length of between <NUM>. 5λ-4λ of a wavelength of the optical signal passing through the waveplate. The plurality of angled, columnar features further being aligned parallel with a directional plane defined by an angle θ, where θ is between <NUM>°-<NUM>° relative to a reference plane, the reference plane being normal to a surface of the substrate. The metasurface forms a birefringent metasurface.

According to the claimed invention the present disclosure relates to a method for forming a birefringent waveplate. The method comprises providing a substrate, creating a mask on an outer surface of the substrate, and using a material removal process, together with the mask, to remove select material portions from the substrate. The material removal process forms a plurality of angled, columnar features which collectively form a birefringent metasurface using a portion of the substrate. The birefringent metasurface forms an integral portion of the substrate.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings, wherein:.

The present disclosure involves a method and system that enables implementing waveplates to withstand energetic laser beams by directional etching through a nano-particles etch mask at a high incidence angle to obtain a tilted, columnar, birefringent meta-surface formed on the optical substrate.

Referring to <FIG>, one embodiment of a waveplate <NUM> in accordance with the present disclosure is shown. In this embodiment the waveplate <NUM> includes a generally uniform pattern of tilted or angled columnar features, which in this embodiment may be termed "columnar projections" 12a. The columnar projections 12a collectively form a birefringent metasurface <NUM> that enables the waveplate <NUM> to form a subwavelength structure, in this example a quarter wave plate (QWP). However, it will be appreciated that the methodology described herein may be applied to any form of waveplate, and in fact to any structure or substrate, where one wishes to create a carefully controlled surface pattern on the structure or substrate involving highly controlled material removal with tilted features.

To create the birefringent metasurface <NUM>, reference is made to <FIG>. Initially, an etch mask <NUM> is formed on a surface 16a of a substrate <NUM>. The etch mask <NUM>, in one embodiment, may be comprised of subwavelength mask nano-particles 14a deposited on the surface 16a. Alternatively, as shown in <FIG>, nano-voids 14b may be formed in a material layer (e.g., film) <NUM>' placed, deposited or otherwise formed on the surface 16a. However, for the purpose of the following discussion, it will be assumed that nanoparticles 14a are being used to form the etch mask <NUM>.

In one preferred methodology a directional etching method, for example, but not limited to, reactive ion etching ("RIE") or reactive ion beam etching ("RIBE"), may then be used to etch the substrate <NUM> to form the pattern of angled columnar features 12a that collectively form the birefringent metasurface <NUM>. It will be appreciated, however, that the present system and method is agnostic as to how the nano-particle etch mask <NUM> is applied/formed on the surface 16a of the substrate <NUM>. For example, the nano-particle etch mask <NUM> may also be achieved using thermal annealing (dewetting) of a thin film, or alternatively by other well known methods such as spin/dip coating.

To obtain a QWP the effective index difference of the product of the two axis index difference (Δn) times the effective thickness of the modified layer (L) should be a quarter of a wave: L·Δn =λ/<NUM>.

Referring to Figures 3a-3c, a suitable etching operation, for example RIE or RIBE as mentioned above, may be used to etch the surface 16a to form the pattern of columnar projections 12a that make up the birefringent metasurface <NUM>. For example, <FIG> shows arrows <NUM> to indicate one example of the angle θ of the trajectory of the reactive ions impinging onto the surface 16a of the substrate <NUM>. As shown in <FIG>, the angle θ of the columnar projections 12a is an angle that is non-parallel to a reference plane "R", where the reference plane R extends normal to the surface 16a. <FIG> illustrates a plurality of the columnar projections 12a formed in the substrate16 at the angle θ, which collectively form the birefringent metasurface <NUM>. The tops of the columnar projections 12a represent what is left of the upper surface 16a, and a new upper surface 16b is formed around a base of each of the columnar projections 12a. Since the columnar projections 12a are angled relative to the outer surface 16a, a depth "L" of each columnar projection 12a will be understood as meaning the overall depth (i.e., or height) of the columnar projection (i.e., the distance that the columnar projection 12a extends down from its upper end (upper surface 16a) to the new upper surface portion 16b (normal to the outer surface 16a), which will be slightly less than the linear length of the columnar projection 12a ("D"). It will also be noted that the ions performing the etching converge slightly as they extend into the substrate <NUM>, which is a known phenomenon when using reactive ion etching, and which results in the columnar projections 12a being formed such that each has a slightly outwardly tapering shape as they reach the new upper surface portion 16b (<FIG>). The depth L of each of the columnar projections 12a, in one example, may be about <NUM>.

GLAD waveplates with Δn of up to <NUM> have been previously created with oxide materials. Using this Δn value typically requires roughly about L=<NUM>. 0λ depth layer to obtain a QWP. The deposition angle, θ, for the above-mentioned GLAD waveplate was between about <NUM>° and <NUM>°. The deposition angle range gives a cosθ value of <NUM>-<NUM>, and therefore, with this estimation, a required etch length (D) of <NUM>-4λ. At the typical λ=<NUM> for a final optics assembly in high power lasers, this means etched columnar recesses 12a which, in this specific example, will have a length of roughly about <NUM>. In applying this understanding to the present disclosure, one will appreciate that the exact angle θ will depend in part of the thickness of the substrate <NUM>, and θ may represent an angle from roughly about <NUM>-<NUM> degrees, and more preferably about <NUM> degrees to roughly about <NUM> degrees, and still more preferably an angle approximately between about <NUM> degrees to <NUM> degrees. As noted above, these angle values will change depending on the thickness of the substrate <NUM> that one is working with, and possibly other factors as well.

<FIG> illustrates the waveplate <NUM> after the etch mask <NUM> has been completely removed, leaving only the substrate <NUM> with the birefringent metasurface <NUM>, formed by the angled, columnar projections 12a, projecting from the new upper surface 16b of the substrate <NUM>. <FIG> show the columnar projections 12a from two different views rotated <NUM> degrees from one another.

<FIG> illustrates a birefringent waveplate <NUM>' in accordance with another embodiment of the present disclosure which may be formed using the nano-voids 14b of the nano-voids mask <NUM>' of <FIG>. In this example the columnar features form a pattern of angled, columnar recesses 12a' in the substrate <NUM>', rather than the angled columnar projections 12a.

<FIG> is a flowchart <NUM> illustrating one example of major operations which may be performed in creating the quarter waveplate <NUM>. Initially at operation <NUM> the etch mask <NUM> is formed on the upper surface of the substrate <NUM>. At operation <NUM> the selected material removal process (e.g., RIE, RIBE, etc.) may be used to removal selected material portions from the substrate <NUM> to create the columnar projections 12a which collectively form the birefringent metastructure <NUM>. At operation <NUM> the nano-particles 14a forming the etch mask <NUM> may then be removed from the outer surface 16a of the substrate <NUM> to leave the finished quarter waveplate <NUM>. Removal of the nano-particles 14a forming the etch mask <NUM> will be material dependent, as one example, via a wet process selective etchant.

A typical etching selectivity aspect ratio between mask metal nano-particles 14a and the etched substrate <NUM> material (e.g., fused silica glass) is between about <NUM>:<NUM> and <NUM>:<NUM>, and could be higher depending on the material system and the process. By "etching selectivity aspect ratio" it is meant the ratio of the rate at which the mask and the substrate are being etched away. For example, for a <NUM>:<NUM> etch ratio, a D~3λ channel length requires at least <NUM>. 2λ nano-particle 14a thickness, and about the same nano-particle diameter (or less), which is within the parameter space for mask fabrication methods, such as thin metal film dewetting.

A few methods to address cases where a larger retardation layer thickness than the resulting retardation layer thickness L is required (for example, if the etch ratio of the mask/substrate sets a limit), may be summarized as follows. One method is to split the retardation between several independent surfaces. The principal axis of each independent surface should then be aligned, so the retardation layer formed on two or more plates may be easier to manufacture, and then the alignment of the plates may be performed by a suitable calibration procedure. However, a more robust solution, at the system level, may be to have the retardation layer formed on two surfaces of the same element. This may be accomplished by splitting a retardation layer into two (e. g, split a QWP into two <NUM>/<NUM> wave plates) although the principal axis would need to be aligned, which creates an additional complexity that would need to be addressed.

Another option is simply having less than the target λ/<NUM> retardation. This will still translate linear polarization of an optical beam to elliptical polarization, however, it is expected that this construction will lead to an increased LIDT (i.e., a higher damage threshold).

Still another option is using a multi-step etching method. For example, after the mask has eroded by etching, additional deposition of mask will build mainly at the non-etched regions. This is assuming that that the deposition height of the deposited material is substantially smaller than the columnar projection 12a diameter formed between portions of deposited material, so as to not block the deposition of additional material, and since the columnar projections are tilted at an angle relative to the vertical, and the deposition is at normal incidence (or could be further optimized at close to a normal angle opposing the etching direction). After each additional deposition of mask material, further angled etching is enabled.

One example for a potential mask formation method is forming nano-particles with controlled size by depositing energy to thin metal (i.e., thermal annealing and deweting of thin films). Another method may involve nano-particle self-assembly methods (e.g., using block co-polymer construction). However, as noted above, the present disclosure is agnostic as to how the etch mask <NUM> is formed, and therefore not limited to any one specific way in which to form the etch mask.

The ability to spatially control the distribution of the mask nano-particles 14a, as was previously shown using laser-induced local heating by spatially patterned laser-raster scan, enables one to spatially pattern the wave retardation, which is advantageous for other methods extending beyond the laser damage resistivity. Controlling the spatial patterning of the mask nano-particles <NUM> can also be used to reduce the focal spot contrast via polarization smoothing.

It will also be appreciated that sub-wavelength structuring of the waveplate <NUM> affects not only the index difference between the two principal polarizations, but also their refractive index value. Therefore, with proper design, the design of the waveplate <NUM> layer may also be used to reduce the reflection from the interface.

The present disclosure thus presents a waveplate <NUM> and a method for forming the waveplate. The method effectively patterns a portion of a layer of a substrate with a metasurface that has a designed birefringence feature. The layer is a result of angled directional etching through a nano-particle mask or through a mask having nano-voids, which enables one to create either a pattern of the angled, columnar projections 12a or the angled, columnar recesses 12a'. In either case, the resulting meta-surface is monolithic with the substrate and a result of the etching process, and has a relatively high laser-induced damage resistivity and structural stability with respect to other previously utilized methods based on material deposition. The birefringence of the metasurface layer results from the geometry of the metasurface, and thus applies also to non-birefringent substrates. The use of previously proposed methods that spatially control the nano-particle mask characteristics, combined with the present method, is expected to enable spatial control of birefringence of a substrate material. The present method allows for simultaneously tailoring the refractive index of the meta-surface layer as well as its birefringence, thus enabling the combination of desired anti-reflective and birefringence properties to the resulting meta-surface layer. These factors are highly important considerations for optics being used with high power lasers.

While the angled columnar features 12a and the angled columnar recesses 12a' have been illustrated as being arranged in a uniform, grid-like pattern, it will be appreciated that any arrangement or pattern (uniform or non-uniform) of the columnar projections or columnar recesses may be formed using the teachings of the present disclosure to meet a specific optical application. Accordingly, the present disclosure is not limited to only creating uniform patterns of angled columnar features.

Claim 1:
A waveplate (<NUM>, <NUM>') comprising:
a substrate (<NUM>, <NUM>') forming an optic;
the substrate (<NUM>, <NUM>') including an integral portion forming a plurality of angled columnar features (12a) on an exposed surface (16a) thereof as a result of removal of a portion of a surface (16a) of the substrate (<NUM>, <NUM>'), leaving the angled columnar features (12a) forming a metasurface (<NUM>);
the plurality of angled columnar features (12a) further being aligned parallel with a directional plane formed non-parallel to a reference plane (R), the reference plane (R) being normal to a surface (16a) of the substrate (<NUM>, <NUM>'); and
the metasurface (<NUM>) forming a birefringent metasurface (<NUM>) which forms a portion of the substrate (<NUM>, <NUM>').