Optical grating

According to one aspect of the invention, there is provided an optical grating comprising a substrate comprising a plurality of protrusions with a space in between any two adjacent protrusions; and a cap provided on at least one of the plurality of protrusions at an end that is furthest from the substrate, wherein the cap has a higher degree of optical attenuation compared to the substrate material and wherein the combination of each protrusion and the respective cap thereon has a generally symmetric cross-sectional profile.

PRIORITY CLAIM TO RELATED APPLICATIONS

This application is a U.S. national stage application filed under 35 U.S.C. § 371 from International Application Serial No. PCT/SG2013/000340, which was filed Aug. 12, 2013, and published as WO 2014/025318 on Feb. 13, 2014, and which claims priority to Singapore Application No. 201205942-4, filed Aug. 10, 2012, which applications and publication are incorporated by reference as if reproduced herein and made a part hereof in their entirety, and the benefit of priority of each of which is claimed herein.

FIELD OF INVENTION

The invention relates generally to an optical grating.

BACKGROUND

It has been demonstrated that nanowire grid polarizers comprising a metal grating, traditionally fabricated on glass substrates, can also be fabricated on plastic films. Compared to glass, plastic is cheap, allows fabrication over a large area with high throughput and finds use in applications which need to be lightweight and flexible (e.g. printed electronics, wearable electronics).

For example, the publications “Large flexible nanowire grid visible polarizer made by nanoimprint lithography” (by Chen, L., Wang, J. J., Walters, F., Deng, X. G., Buonanno, M., Tai, S. & Liu, X. M. (2007) Applied Physics Letters, 90) and “Direct imprinting on a polycarbonate substrate with a compressed air press for polarizer applications” (by Lin, C. H., Lin, H. H., Chen, W. Y. & Cheng, T. C. (2011), Microelectronic Engineering, 88, 2026-2029) disclose optical element structures that have non-flat spectral response of light over a visible wavelength range. This non-flat spectral response results in an undesirable reddish artifact for plastic nanowire grid polarizer operating in transmission mode. A further discussion on the operating principle of a nanowire grid polarizer is described further below, with reference toFIG. 10A.

A need therefore exists to provide an optical element and its method of fabrication that can address the undesirable attributes, such as the associated reddish artifact, of optical element structure fabricated from existing plastic nanowire grid polarizers.

SUMMARY

According to one aspect of the invention, there is provided an optical grating comprising a substrate comprising a plurality of protrusions with a space in between any two adjacent protrusions; and a cap provided on at least one of the plurality of protrusions at an end that is furthest from the substrate, wherein the cap has a higher degree of optical attenuation compared to the substrate material and wherein the combination of each protrusion and the respective cap thereon has a generally symmetric cross-sectional profile.

According to another aspect of the invention, there is provided a method of forming an optical grating, the method comprising providing a substrate; forming a plurality of protrusions on the substrate with a space in between any two adjacent protrusions, providing a cap on at least one of the plurality of protrusions at an end that is furthest from the substrate, wherein the cap has a higher degree of optical attenuation compared to the substrate material and wherein the combination of each protrusion and the respective cap thereon has a generally symmetric cross-sectional profile.

DEFINITIONS

The following provides sample, but not exhaustive, definitions for expressions used throughout various embodiments disclosed herein.

The phrase “optical grating” may mean an optical element which acts upon light passing through the optical element. Examples of optical elements include lenses, mirrors, prisms, nanowire grid polarizers and parallax barriers, whereby various embodiments of the optical grating disclosed herein function either as a nanowire grid polarizer or a parallax barrier. Accordingly, the phrases “optical grating” and “optical element” may be used interchangeably throughout the entire specification.

The operating principle of a nanowire grid polarizer and a parallax barrier is described with reference toFIGS. 10A and 10Brespectively.FIG. 10Ashows a nanowire grid polarizer1000. The nanowire grid polarizer1000has sub-wavelength metal gratings1004on a transparent substrate1002(e.g. glass, plastics). The submicron size metal gratings1004provide a series of alternate opaque and transparent slits. In principle, the optical characteristics of the polarizer1000enables incident light with polarization parallel to the metal grating (TE-polarized light, denoted TE) to be reflected (denoted RTE) with little amount of this incident TE-polarized light transmitted through the polarizer (the transmitted TE-polarized light is denoted TTE), while enabling the incident light with perpendicular polarization (TM-polarized light, denoted TM) to be transmitted through the polarizer (the transmitted TM-polarized light is denoted TTM). The spectra of TTMand TTEof the polarizer1000can be measured using a UV-vis Microspectrophotometer (such as “CRAIC QDI 2010”) across the visible wavelength range from 400 nm to 800 nm. A contrast ratio is calculated from the ratio of (TTM/TTE). Transmission spectrum (TTM) and the contrast ratio (TTM/TTE) are the two main optical performance parameters of polarizers.FIG. 10Bshows a system1050for creating a parallax barrier. The parallax barrier is created by placing an optical element1040in front of an image source (such as a liquid crystal display1054), so that each eye1042sees a different sets of pixels through parallax brought about by the optical element1040, thereby creating a sense of depth required for 3D imaging. The optical element1040provides a series of alternate opaque and transparent slits of micron-size gratings.

The term “protrusion” may mean a structure that extends from a surface of a substrate. In various embodiments, where the optical grating is referred to as an optical element, the term “grating” may be used to refer to the “protrusion”. Accordingly, the terms “grating” and “protrusion” may be used interchangeably throughout the entire specification.

The phrase “cross-sectional arc profile” is to be understood in the context of a fabrication process performed at microscopic levels, whereby such a “cross-sectional arc profile” may refer to a profile with a deliberately fabricated curvature due to the process of fabrication.

DETAILED DESCRIPTION

In the following description, various embodiments are described with reference to the drawings, where like reference characters generally refer to the same parts throughout the different views.

FIG. 1Ashows the cross-sectional structure of an optical grating100according to a first embodiment.

The optical grating100includes a substrate102comprising a plurality of protrusions104. A space106exists in between any two adjacent protrusions104. A cap118is provided on at least one of the plurality of protrusions104at an end103that is furthest from the substrate102. The cap118has a higher degree of optical attenuation compared to the substrate102material. The combination of each protrusion104and the respective cap118thereon has a generally symmetric cross-sectional profile.

The combination of each protrusion104and the respective cap118thereon is generally symmetric about a longitudinal axis114extending along a centre and intersecting a base116of the respective protrusion104. This symmetry provides the optical grating100with a flat optical spectral response. Accordingly, an identical degree of symmetry is not required, but rather a degree of symmetry that allows the optical grating100to provide a flat optical spectral response.

It has been found that the symmetric profile such as those presented by the cross-sectional arc profile112of each protrusion104provides a flatter spectral response compared to the case where each protrusion104were to have non-symmetric structure (e.g. straight sidewalls with a cross-sectional linear profile, seeFIG. 1B). While an arced cross-sectional profile is shown inFIG. 1A, other profiles such as square shaped or round shaped are possible, as long as the overall cross-sectional profile of the cap118and the respective protrusion104has symmetry.

The cap118is preferably opaque. Such an opaque overcap can be made using metals or dielectric with light-absorbent additive molecules. The degree of opacity (i.e. degree of optical attenuation) in metals is controlled by the thickness of the metals. Using metals to realise the cap118provides the cap118with reflective properties. The degree of opacity (i.e. degree of optical attenuation) in a dielectric is controlled by the amount of light-absorbent additive molecules (e.g. pigments, dyes, colorants and photosensitive emulsions, etc.) present in the dielectric. Using a dielectric to realise the cap118provides the cap118with non-reflective properties.

The physical properties of the cap118imparted by the material used to fabricate the cap118determines whether the optical grating100functions as a parallax barrier or a nanowire grid polarizer. Given that a non-reflective overcap does not reflect TE polarized light, an opaque and reflective overcap allows the optical grating100to function as a nanowire grid polarizer. An opaque and reflective overcap; or an opaque and non-reflective overcap allows the optical grating100to function as a parallax barrier. In the case where the cap118is opaque and reflective, the optical grating100will function as a parallax barrier when the optical grating100is used under conditions allowing the optical grating100to function as a parallax barrier; or as a nanowire grid polarizer when the optical grating100is used under conditions allowing the optical grating100to function as a parallax barrier. The optical grating100will function as a parallax barrier when it is placed in front of an image source, such as a liquid crystal display (LCD). The optical grating100will function as a nanowire grid polarizer when used in any linear polarization required equipment or techniques such as a flexible LCD or an optical filter.

The cap118has a shape such that both sidewalls108,110of each of the plurality of protrusions104have a cross-sectional arc profile112along at least a portion of both of these sidewalls108,100.

The plurality of protrusions104are fabricated simultaneously with the substrate102and therefore are integral to and made of the same material as the substrate102. The plurality of protrusions104may be arranged in an array. To achieve such an array, one embodiment has any two adjacent protrusions104spaced apart about equally (i.e. the space106between any two adjacent protrusions104is around the same). Each protrusion104may have a width from a range 10 nm to 300 nm, with a suitable width being for example 70 nm. The distance for the space106may be from a range 10 nm to 300 nm, with a suitable distance being for example 70 nm. In another embodiment, the array may be realised by the optical grating100having a repetitive pattern, whereby two adjacent protrusions104are spaced106a first distance apart and another two adjacent protrusions104are spaced106a second distance apart, the size of the first distance being different from that of the second distance.

As shown inFIG. 1A, the cross-sectional arc profile112is present along, a portion of the entire sidewalls108,110. However, although not shown, the cross-sectional arc profile112can also extend over the entire surface of the sidewalls108,110. Also, although not shown, the cross-sectional arc profile112can commence from the portions of both sidewalls108,110that are adjacent to their respective base116. Rather, the portion of both sidewalls108,110, having the cross-sectional arc profile112, is proximate to the end103of each of the plurality of protrusions104that is furthest from the substrate102.

FIG. 2shows the cross-sectional structure of an optical grating200according to a preferred embodiment.

The optical grating200ofFIG. 2includes a substrate202comprising a plurality of protrusions204. A space206exists in between any two adjacent protrusions204. The cross-sectional profile212of each of the sidewalls208,210has a convex shape relative to a longitudinal axis214extending along the centre and intersecting the base216of the respective protrusion204.

Like the optical grating100ofFIG. 1A, the overall structure of each of the plurality of protrusions204and its respective cap218has a combined cross-sectional profile that is generally symmetrical about a longitudinal axis214extending along a centre and intersecting a base216of each respective protrusion104. The cap218also has a higher degree of optical attenuation compared to the substrate202material. However, while the optical grating100ofFIG. 1Ahas the cap118only provided on a selected number of the plurality of protrusions104, the optical grating200ofFIG. 2has the cap218provided on each of the plurality of protrusions204at an end203that is furthest from the substrate202.

Similar to the optical grating100ofFIG. 1A, the plurality of protrusions204may be arranged in an array. The plurality of protrusions204are also fabricated simultaneously with the substrate202and therefore are integral to and made of the same material as the substrate202. The substrate202may be made from direct imprinting onto a plastic sheet (which includes all types of thermoplastic films, e.g. polycarbonate (PC), polymethylmethacrylate (PMMA) and polyethylene so that the substrate202is flexible.

The cap218may comprise material that has a higher degree of optical attenuation, compared to the substrate202material. In various embodiments, the cap218may be opaque. The cap218may be made of the same materials as the cap118ofFIG. 1A. Thus, the cap218may be made of material that is both opaque and reflective (e.g. metals) whereby the cap218may comprise one or more of the following materials: aluminum, gold and chromium. Alternatively, the cap218may be made of material that is both opaque and non-reflective, whereby the cap218may comprise of dielectric with light-absorbent additive molecules (e.g pigments, dyes, colorants and photosensitive emulsions etc.)

FIG. 3shows a flowchart300of a method to fabricate an optical grating according to the first embodiment shown inFIG. 2.

In step302, a substrate is provided.

In step304, a plurality of protrusions on the substrate is formed with a space in between any two adjacent protrusions.

In step306, a cap is provided on at least one of the plurality of protrusions at an end that is furthest from the substrate, wherein the cap has a higher degree of optical attenuation compared to the substrate material and wherein the combination of each protrusion and the respective cap thereon has a generally symmetric cross-sectional profile.

In one implementation of the method ofFIG. 3to fabricate an optical grating in accordance to the preferred embodiment (seeFIG. 2), the cap provides each of the plurality of protrusions with sidewalls having a cross-sectional arc profile along at least a portion of the sidewalls. The cap may comprise different material from the substrate. The cap is preferably opaque. The cap may be made of material that is both opaque and reflective. It is also possible for the cap to be made of material that is opaque and non-reflective. The cap may be provided by a process which forms a portion of the cap on one sidewall of each protrusion and forms a remainder of the cap on the other sidewall, i.e. an opposite sidewall of the same protrusion. This process is described in greater detail below with reference toFIGS. 4A to 4C.

FIGS. 4A to 4Cshow an exemplary process of this implementation to fabricate the preferred embodiment of an optical grating400(seeFIG. 4D) directly onto a substrate402, such as commercially available free-standing polycarbonate (PC) (Innox, PC2151, thickness 0.25 mm) sheet, through a two-step process. Other materials such as plastic, polycarbonate and polymethylmethacrylate are also usable.

Prior to performing a nanoimprint process shown inFIG. 4A, a silicon mould420with topography (such as 70 nm line, 70 nm space, 300 nm height grating), designed to provide a protrusion pattern on the substrate402, was fabricated. The mould420was cleaned using oxygen plasma and silanized with an anti-stiction monolayer (FDTS, (1H,1H,2H,2H)-Perfluorodecyltrichlorosilane). The silanization treatment was used to reduce the surface energy of the mould420to facilitate easy demoulding of the mould420from the substrate402.

In a first step470shown inFIG. 4A, thermal nanoimprinting using a nanoimprinter (such as an “Obducat AB” Nanoimprinter) was employed to directly pattern the substrate402with the desired grating feature from the silicon mould420. A batch or roll-to-roll processing can be used for the thermal nanoimprinting. The silicon mould420was placed in direct contact with the substrate402at an imprinting temperature of 180° C. and at a pressure of 60 bar for a duration of 10 minutes. Following this, the temperature of the system was cooled down to 25° C. and demoulding was performed at this temperature. The grating pattern feature from the silicon mold was thus imprinted onto the substrate402.

In a second step472shown inFIGS. 4B and 4C, the protrusions404formed from the imprinted grating on the substrate402undergo a “dual-side coating” method to form a cap418on each of the protrusions404. Equipment that can be used to coat the protrusions404with the cap418include a metal evaporator/coater/sputterer (such as an “Edwards Auto306 Ebeam Evaporation System”). The metal evaporator/coater/sputterer can be integrated with the nanoimprinter, described above, into a single system. Accordingly, the formation of the plurality of protrusions404and the provision of the cap418may be performed in an integrated system where imprinting and metal evaporation/coating/sputtering occur.

A portion of the cap418is formed on one of the both sidewalls of each protrusion404by tilting θ the substrate402in a first direction422and forming the portion of the cap418on the one sidewall. The remainder of the cap418is formed on the other of the both sidewalls by tilting θ the substrate402in a second direction424that is opposite to the first direction and forming the remainder of the cap418on the other sidewall.

The angle θ at which the substrate402is tilted in the first direction422may be approximately the same as angle θ at which the substrate is tilted in the second direction424. The angle θ may range from 1° to 89°, with an exemplary tilt angle θ being around 5 to 30°. The angle θ of tilting during cap formation will affect the light transmittance percentage of the resulting optical grating400(seeFIG. 4D). For cap418formation at a smaller tilt angle (such as θ=1°), the optical grating400has higher light transmittance and less light blockage as compared to cap418formation done at a higher tilt angle (such as θ=10°). If aluminum is evaporated onto the substrate402inFIGS. 4B and 4C, aluminum metal will be deposited onto the top of the grating400, i.e. onto the two sidewalls of each protrusion404of the grating400.FIG. 4Dshows a schematic illustration of the resulting optical grating400following this “dual-side coating”.

The two-step process and the two pieces of equipment used to implement this two-step process (both described above) provide a simple way to fabricate an optical grating in accordance to an embodiment of the invention. In comparison to other optical gratings (such as: 1) “Fabrication of a 50 nm half-pitch wire grid polarizer using nanoimprint lithography” by Ahn, S. W., Lee, K. D., Kim, J. S., Kim, S. H., Park, J. D., Lee, S. H. and Yoon, P. W. (2005). Nanotechnology, 16, 1874-1877; and 2) “Wire Grid Polarizer and Manufacturing Method Thereof” by Ahn, S. W., Lee, K. D., Kim, J. S., Kim, S. H., Park, J. D., Lee, S. H. and Yoon, P. W. (2006)) that require at least five to six processing steps performed using four processing equipment, it is easier and cheaper to fabricate an optical gratings in accordance to an embodiment of the invention. The four processing equipment to manufacture such other optical gratings include a metal evaporator, a spin-coater, a nanoimprint system and a reactive ion etcher for metal etching/reactive ion etcher for resist etching. An exemplary six step process includes: (1) metal deposition onto a glass substrate; (2) spin-coating a resist onto the metal over the glass substrate; (3) nanoimprinting to transfer the pattern from a mould onto the resist; (4) reactive ion etching to remove residual layer; (5) reactive ion etching to etch metal; and (6) resist stripping by reactive ion etching. An exemplary five step process includes: (1) spin-coating a resist onto a glass substrate; (2) nanoimprinting to transfer the pattern from a mould onto the resist; (3) reactive ion etching to etch residual layer resist; (4) metal evaporation; and (5) lift-off process using acetone solution.

FIG. 5Ashows the cross-section of a structure500of an optical grating fabricated using the “dual-side coating” method described with respect toFIGS. 4A to 4C. The cross-section view is obtained using transmission emission microscopy, such as by way of a “Phillips CM300”.

The structure500comprises a grating having a plurality of protrusions504, each having thickness of around 70 nm, patterned on a plastic substrate502. Adjacent protrusions504are spaced around 70 nm apart, for a pitch distance506of around 140 nm. Each protrusion504has a metal over-cap518of non-uniform thickness, such as a top thickness520of around 50 nm (measured from the top of each protrusion504) and a side thickness522of around 20 nm thickness (measured from the sidewalls of each protrusion504). The metal over-cap520provides at least a portion of both sidewalls of each of the plurality of protrusions504with a cross-sectional arc profile. Ideally, the metal deposition is desired to be symmetrical on both sides of each protrusion504. However, this is not achieved in the fabricated structure500due to a slight tilt of the grating because of its high aspect ratio, and thus a slight shadow deposition effect on the fabricated structure500.

FIG. 6Ashows a plot of the measured transmission spectrum of TM polarized light (TTM) against the visible light wavelength range from 400 nm to 800 nm.FIG. 6Bshows a plot of the contrast ratio between transmitted TE polarized light and transmitted TM polarized light (TTM/TTE) against the same visible light wavelength range from 400 nm to 800 nm.

In bothFIGS. 6A and 6B, each curve600represents the result obtained for the structure500ofFIG. 5A, while each curve650represents the result obtained for a conventional nanowire grid polarizer540, schematically shown inFIG. 5B, with a transmission emission microscopic image shown inFIG. 5C. The results show that a flatter spectral response is achieved by the structure500(i.e. an optical grating according to the preferred embodiment) compared to the conventional structure. The contrast ratios for both the structure500and the conventional nanowire grid polarizer are comparable.

FIG. 7Ashows a picture of a document730viewed through an optical film700fabricated from the structure500ofFIG. 5A.FIG. 7Bshows a picture of the document730viewed through an optical film750fabricated from the nanowire grid polarizer ofFIG. 5C. Thus, in bothFIGS. 7A and 7B, the optical films700and750are operated under transmission mode, i.e. viewing of the document730through the respective optical film700,750.

FIG. 7Bshows a reddish artifact for the document730viewed through the optical film750. The reddish artifact is caused by the non-constant transmission intensity spectrum across the visible wavelength range (graphically represented by the curve650inFIG. 6A), which is not desirable to the user. On the other hand, no such reddish artifact is present for the document730viewed through the optical film700. The flatter spectral response (graphically represented by the curve600inFIG. 6A) across the visible wavelength (400-800 nm) eliminates the reddish artifact observed in the conventional nanowire grid polarizer operating in the transmission mode, allowing the structure500to be used in applications that require a nanowire grid polarizer.

The structure500ofFIG. 5Aprovides a series of alternate opaque and transparent slits of micron-size gratings. Due to its high resolution (brought about by each protrusion504having a width of 70 nm) and sharing a similar structure to that of a parallax barrier (compare against the optical element1040ofFIG. 10), the structure500also functions as a high resolution plastic parallax barrier film, finding applications for auto-stereoscopic display with sub-pixel resolution below 1 μm. Although present liquid crystal display (LCD) technology is still using sub-pixel resolution in the micrometer scale, the trend (seeFIG. 8) between 2003 and 2012 to improve LCD sub-pixel resolution is to reduce sub-pixel size below the micrometer scale. Thus, the optical film700shown inFIG. 7Acan thus function as a parallax barrier for auto-stereoscopic display for future displays with sub-pixel resolution below 1 μm.

From the above, an optical grating according to various embodiments allow for an optical element with a structure fabricated on a substrate to function either as a nanowire grid polarizer or as a high resolution parallax barrier (PB) film for auto-stereoscopic display. Applications of such an optical grating include placement into an auto-stereoscopic display and optical films.

FIG. 9Ashows a plot of the simulated transmission spectrum of TM polarized light (TTM) against the light wavelength range from 450 nm to 650 nm.FIG. 9Bshows a plot of the simulated curve of the contrast ratio between the transmitted TE polarized light and transmitted TM polarized light (TTM/TTE) against the same light wavelength range from 450 nm to 650 nm.

In bothFIGS. 9A and 9B, each curve900represents simulated results obtained using the optical grating200ofFIG. 2. The simulated results were generated using software from FDTD Solutions™. The results obtained from the fabricated structure500ofFIG. 5Averify the simulation results, as seen from comparing the curves600shown inFIGS. 6A and 6Bagainst the curves900shown inFIGS. 9A and 9B.