Abstract:
Method for fabrication of vector vortex waveplates of improved quality due to reduced singularity size and widened spectral band, the method comprising creating a boundary condition for vortex orientation pattern of a liquid crystal polymer on a substrate using materials with reversible photoalignment, equalizing exposure energy over the area of the waveplate by redistributing the energy of radiation used for photoalignment from the center of the beam to its peripheries, and using vector vortex waveplate as a linear-to-axial polarization converter. Fabrication of spectrally broadband vector vortex waveplates further comprises two or more liquid crystal polymer layers with opposite sign of twist.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to fabrication of liquid crystal polymer vector vortex waveplates (VVWs) with improved optical properties due to decreased defect size, widened spectral bandwidth, and large area. VVWs are used in astronomy, imaging, optical communication, photonics and display technologies. 
       BACKGROUND OF THE INVENTION 
       [0002]    The challenge in imaging faint objects near bright stars is in reducing the starlight and the noise by many orders of magnitude while efficiently transmitting the planet light. Due to scattered starlight, the planets near bright stars could be observed earlier only at large angular separations and with large telescopes. Using a small aperture telescope system for detecting exoplanets at nearly diffraction limit of their separation from the star can have a significant impact on astronomy as well as other imaging and space communication systems. Vector vortex coronagraphs prove to provide such an opportunity. When used with larger aperture telescopes, a vortex coronagraph would allow detecting planets even closer to the stars, hence brighter and in early stages of formation. 
         [0003]    In this new generation of smaller, lighter and more affordable coronagraph systems, the star light is rejected with the aid of phase-based transparent “masks” capable of transmitting planetary light at small angular separation from the star. These so-called vortex vector waveplates (VVW) are complex optical components wherein the optical axis orientation is rotating in space in an axially symmetric manner. Liquid crystals (LCs), particularly, LC polymers (LCPs) are the only material systems that allow fabrication of VVWs with continuous rotation of the optical axis orientation at a high spatial frequency required for obtaining a high topological charge and high contrast. LCs are transparent in visible, near IR and even for longer wavelengths and, due to their high optical anisotropy, the half-wave phase retardation condition is achieved in thin material layers (˜1 micrometer). 
         [0004]    Thus, there is a need for a technique that would allow fabricating large diameter VVWs with small singularity size, and broadband at different spectral ranges, including visible and infrared. Particularly important is reducing the defect size to subwavelength sizes. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The objective of the present invention is providing LCP VVWs with improved optical properties due to defect size reduced to micrometers and wide spectral range of operation, said improvements achieved by using materials with reversible photoalignment, redistributing light energy from the axis of the beam to its peripheries, and combining multiple LCP layers into a single WW. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0006]      FIG. 1A  schematically shows a VVW with radially aligned optical axis orientation deposited on a substrate. 
           [0007]      FIG. 1B  schematically shows radial alignment of optical axis orientation in VVW. 
           [0008]      FIG. 1C  schematically shows the defect area at the axis of a VVW. 
           [0009]      FIG. 2  schematically shows the setup for fabricating a VVW with light intensity redistribution. 
           [0010]      FIG. 3  shows a polarizing microscopy photo of a WW with 3 micrometer defect size. 
           [0011]      FIG. 4  shows a photos of WW achromatic in visible spectral range VVW (450-650 nm). Photos are taken between crossed polarizers. 
           [0012]      FIG. 5  shows photos of doughnut beams of different wavelengths produced by an achromatic WW. Photos were taken for Ar +  laser lines (458 nm, 488 nm, 514 nm), for a diode pumped solid state laser (532 nm), and a He—Ne (633 nm) laser beam. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not limitation. 
         [0014]    An example of a VVW is shown in  FIG. 1A . It comprises a substrate  102  and the LCP layer  101  deposited on said substrate. The optical axis orientation in this example of a preferred embodiment is radially aligned as shown in  FIG. 1B . The optical axis orientation is not well defined in the center of a VVW. Typically, a defect of not oriented LCP is present within the axial region of an average size indicated as d in  FIG. 1C . Defect sizes as large as d˜100 micrometer were known in the earlier art. Even if the diameter of the VVW indicated as I in  FIG. 1B  may be much larger than the defect size d, light scattering in this region may make the functionality of the VVW inadequate, particularly, for demanding astronomy, optical communications and high energy laser applications. 
         [0015]    The preferred embodiment of the setup for fabricating large area WWs with reduced defect size,  FIG. 2A , comprises a substrate  206  chosen for high transparency in desired wavelength range that may including visible and infrared. The substrate is coated with a photoalignment layer  205 . In the preferred embodiment of the present invention the photoalignment layer comprises an azobenzene dye based on chromocentranine R structures. An example of such a dye is sulfonic bisazodye SD1 
         [0000]    
       
                 
         
             
             
         
       
     
         [0016]    Typically, azobenzene dyes are in the form of a powder at room temperature, and they can be used for coating by dissolving them in a variety of solvents, including water. The film thickness is determined by the concentration of the azo dye in the solvent and the coating technique. Variety of coating techniques are applicable, including dip coating, printing, stamping, and spin coating. In the latter case, approximately 1 wt. % of said azo dye can be used in a DMF as solvent. Spinning at 3000 rpm for 60 s provides thus a photoalignment layer of a few tens of nm thickness. 
         [0017]    The photoalignment layer  205  is coated on a substrate  206  that is chemically resistive to the solvents used in the process (glass, for example). The photoalignment layer  205  is exposed to a polarized light comprising a wavelength in the absorption band of said photoalignment material. Said absorption band can be in the UV or visible part of the spectrum. The exposure dose depends on the specific azobenzene photoalignment material and the radiation wavelength. PAAD-72 (available at www.beamco.com), for example, produces high quality alignment conditions for Merck&#39;s RMS series reactive mesogens within 5 min exposure time to a UV radiation of 325 nm wavelength and 10 mW/cm 2  power density. 
         [0018]    In the preferred embodiment of the photoalignment setup shown in  FIG. 2A , UV radiation  200  (325 nm wavelength, as an example) is expanded to uniformly cover the whole area coated by the photoalignment layer. Radiation intensity can be approximately 10 mW/cm 2  for the photoalignment materials used in the preferred embodiment. A linear aperture  203  selects then a linear beam stripe, which is focused onto the photoalignment layer  205  by a cylindrical lens  204 . Thus, a linear beam of a micrometer width can be obtained. A linear polarizer  202  is then used for setting the polarization of the laser beam along (or perpendicular to) the transmitted beam stripe in order to obtain radial (or azimuthal) alignment boundary conditions in the photoalignment layer. 
         [0019]    The substrate  206  carrying the photoalignment layer  205  is attached to a high precision rotational stage such as the rotation axis is aligned with the central part of the beam stripe. The optimum values of the rotation speed and duration are determined experimentally for given exposure conditions and the photoalignment layer. For example, rotation speed 250°/s and duration˜30 min may be used for the azobenzene photoalignment materials subject to 10 mW/cm 2  beam intensity. Note that the exposure time for producing orientation patterns modulated at high spatial frequencies need to be longer than the time required for uniform alignment since longer exposure typically results in stronger surface alignment energy. 
         [0020]    For recording VVWs of higher topological charge, the polarizer may be rotated as well synchronized with the rotation of the substrate. 
         [0021]    Light energy received by the central part of the substrate is higher than at the periphery by a factor ˜π/ld&gt;&gt;1, where d is the beam size and l is the size of the waveplate under fabrication. Thus, the dose of energy received by the peripheral areas is orders of magnitude smaller compared to that of the central area. As a result, if the exposure is sufficient for obtaining good photoalignment at the periphery of the layer, the central part becomes highly overexposed degrading the alignment in a large area around the rotation axis. Thus, reducing the exposure energy density results in smaller defect sizes, but at the expense of degraded alignment in the underexposed periphery. Reducing the exposure energy by a factor of nearly 20, for example, by reducing the light intensity from 10 mW/cm 2  to 0.6 mW/cm 2 , may result in a factor of 4-6 decrease in defect size, from 20 μm to ˜5 μm for VVWs within a given setup and materials. However, the overall diameter of the WW with good alignment quality may be reduced then by a similar factor, i.e., from 20 mm to ˜4 mm. 
         [0022]    To obtain a VVW with both a small central defect and a large overall diameter, the entire substrate must receive comparable net exposure energy. Variable density optical filters such as bullseye filters are hard to customize for producing the strong attenuation gradients required for light redistribution, and they achieve such a redistribution by absorbing or reflecting light, hence causing overall attenuation of light energy. Such variable density filters thus reduce the efficiency of the photoalignment process. 
         [0023]    In the preferred embodiment, redistribution of the illumination from the vicinity of the beam&#39;s axis to its peripheries is accomplished by making use of a VVW  201  as such a waveplate produces a doughnut beam by itself. To efficiently redistribute the light energy from the center of the beam to its peripheries, a radial VVW designed for the wavelength of the photoaligning radiation, 325 nm wavelength, for example, can be used. 
         [0024]    The photoalignment process can be reduced to 5-7 min when using VVW for beam energy redistribution due to full utilization of all the available light. Precise setting of the half-wave retardation condition at 325 nm wavelength is essential for efficient reshaping of the beam. The obtained VVW with reduced defect size can further be used as a beam shaping element to fabricate VVWs with further reduced defect size. Such consecutive improvements allow obtaining a high quality, large area (˜1″) VVWs with defect sizes less than 5 μm. Combined with fine adjustment of the position of the beam shaping VVW with respect to the beam axis, this approach ultimately can yield VVWs with defect sizes of &lt;5 μm when using, for example, a cylindrical lens of 75 mm focal length, and below 3 μm for a lens of 25 mm focal length as shown in  FIG. 3 . The actual photo is reduced to black and white with 100% contrast to avoid grey scale in  FIG. 3 . 
         [0025]    In the final stage of fabrication, polymerizable liquid crystal (PLC) is spin coated on a substrate photoaligned in a rotation process and polymerized in unpolarized UV light (for example, of 365 nm wavelength and 10 mW/cm 2  power density) for 5 min in nitrogen atmosphere. Subsequent PLC layers can be deposited for adjusting the half-wave retardation condition. 
         [0026]    The principle of fabrication of a broadband VVW of the present invention consists in combining VVWs twisted along their layer thickness at opposite signs. The twist can be obtained, for example, by doping the polymerizable liquid crystals with chiral molecules of opposite sign of chirality. Half-wave retardation condition shall be maintained for each type of the set of layers around the wavelength in the middle of the desired spectral bandwidth. 
         [0027]    Chiral polymerizable liquid crystals can be obtained, for example, by doping PLC RMS03-001c with chiral dopants CB15 and ZLI 811 (both from Merck). CB15 and ZLI 811 are characterized by so-called helical twisting power (HTP) equal to 7.3 μm −1  (right-handed) and −11 μm −1  (left-handed), correspondingly. HTP determines the pitch h of a LC as a function of dopant concentration, HTP=(hc) −1 , where c is the dopant concentration normalized to the concentration of the molecules in the material. Small concentrations of dopants can be used, 0.34% for CB 15 and 0.25% for ZLI 811. 
         [0028]    The deposition conditions for the right- and left-twisted layers can be similar. For example, the 1 st  and the 2 nd  layers (0.34 wt. % CB15) are spin coated at 6500 and 2000 rpm, respectively. The 3 rd  and 4 th  layers (0.25% ZLI 811) are spin coated with 2000 and 6500 rpm, respectively. A broadband VVW thus fabricated can exhibit over 300 nm diffraction bandgap around 700 nm wavelength. 200 nm bandgap around 550 nm wavelength is obtained for thicker 1 st  and the 4 th  layers coated at 3000 rpm. The measured defect size was 12-15 μm for the broadband IR VVW and 5 μm for the broadband visible WW. 
         [0029]      FIG. 4  shows a photo of VVW between crossed polarizers achromatic in visible spectral range VVW (450-650 nm). The actual photo is reduced to black and white with 100% contrast to avoid grey scale in  FIG. 4 .  FIG. 5  shows photos of doughnut beams of different wavelengths produced by a broadband VVW. Photos were taken for Ar +  laser lines (458 nm, 488 nm, 514 nm), for a diode pumped solid state laser (532 nm), and a He—Ne (633 nm) laser beam. Photos were taken from the screen at 1 meter distance from the VVW, expanding the beams at the output of the WW by a concave lens of −20 mm focal length. The actual photos are reduced to black and white with 100% contrast to avoid grey scale. 
         [0030]    Although the present invention has been described above by way of a preferred embodiment, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.