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We report a micropolarizer array technology exploiting “guest-host” interactions in liquid crystals for visible imaging polarimetry. We demonstrate high resolution thin micropolarizer arrays with a 5 μm × 5 μm pixel pitch and a thickness of 0.95 μm. With the “host” nematic liquid crystal molecules photo-aligned by sulfonic azo-dye SD1, we report averaged major principal transmittance, polarization efficiency and order parameter of 80.3%, 0.863 and 0.848, respectively across the 400 nm – 700 nm visible spectrum range. The proposed fabrication technology completely removes the need for any selective etching during the fabrication/integration process of the micropolarizer array. Fully CMOS compatible, it is simple and cost-effective, requiring only spin-coating followed by a single ultraviolet-exposure through a “photoalignment master”. This makes it well suited to low cost polarization imaging applications.
© 2011 Optical Society of America
1. Introduction
The integration of a micropolarizer array (i.e. a mosaic of micron-scale polarizer elements of different orientations) over the pixel array of an image sensor enables the concept of a low cost single-chip polarization camera capable of capturing, in a single frame, the polarimetric information of a scene [1, 2]. A number of micropolarizer array implementations have been demonstrated for image sensors. Examples include patterned dichroic films such as polyvinyl alcohol (PVA) [3–5], birefringent YVO4 crystal covered by patterned aluminum films [6], multiple-domain liquid crystal (LC) with micro-patterned alignment layers [7], and nanometer-scale metal wire-grids [8, 10, 11]. In each case, selective etching was used to pattern micropolarizer elements at the pixel pitch. The complexity of the selective etching process grows substantially with the number of patterned microdomains (i.e. micropolarizer element orientations) in the micropolarizer array. Furthermore, to operate in the visible range, commercially available wire-grid polarizers need to exhibit a grid pitch much smaller than the incident wavelength (i.e. less than 100 nm) [12]. Such requirements are not compatible with standard ultraviolet (UV) lithography. As a result, highly specialized equipment is required to enable advanced lithography. In addition, because wire-grid polarizers are made of delicate thin nano-wires, they are very fragile and can be easily damaged by standard assembly processes. The cost associated to the above complex processes is too prohibitive for the envisioned low cost complementary metal-oxide-semiconductor (CMOS) polarization camera applications.
To completely remove the need for complex/expensive processes, we proposed to use LC photoalignment techniques to fabricate micropolarizer arrays capable of extracting the first two Stokes parameters [9] or all four Stokes parameters [13]. These LC-based micropolariz-ing devices need to exhibit relatively large thicknesses (5 μm – 10 μm) in order to satisfy the Mauguin condition [9] or achieve enough phase retardation [13]. This leads to not only reduced light collection angles but also increased cross-talk between adjacent photo-sensing pixels, especially when their size is scaled down to 10 μm and below [3].
These issues can be addressed by fabricating thin micropolarizer arrays using photosensitive dichroic dyes [14,15]. In [14], an LC photoalignment technique was adopted to selectively align a lyotropic liquid crystal (LLC) dye. This implementation can only achieve a spatial resolution of 70 μm with a polyamide having a dimethylaminoazobenzene substituent (MNC10-PAM) as the photoalignment material. In addition, it suffers from severe spindle-like craters (2 μm in width, 3 μm – 4 μm in length) and careful selection of the surfactants is necessary to suppress these defects, whose size can be comparable to the pixel pitch. In [15], the micropolarizer array was fabricated with a spin-coated photosensitive azo-dye-1 (AD1) as the polarizing film. This material exhibits a strong dichroism after sufficient exposure to linearly polarized ultraviolet (UV) light. The rod-like molecules of a spin-coated AD1 film can be oriented with their long molecular axes perpendicular to the orientation of the projected polarized UV light. However, the orientation degree of AD1 molecules is a function of the exposure energy and saturates after prolonged UV exposure. As a result, the UV-patterned polymer AD1 film exhibits limited major principal transmittance, polarization efficiency and order parameter [15].
To improve both the spatial resolution of the micropolarizer and its optical performance, this paper proposes to exploit “guest-host” interactions [16, 17] in nematic liquid crystals (NLCs). The latter consist of rod-like organic molecules, whose regional ordering is characterized by the parallel alignment of molecules, along their long molecular axes. A well-known property of NLCs is that the orientation of the molecules can be controlled with an external electric field. Heilmeier et al. discovered that by controlling the molecular orientation of a nematic “host” material, the properties of the “guest” materials mixed with the nematic “host” can be controlled [16]. This property was exploited in [17], with LC molecules used as the “host” and polarization dichroic dye molecules dissolved in the LC used as the “guest”. When an external tunable electric field is added to the LC cell with its direction perpendicular to the LC substrates, the “host” LC molecules are reoriented with their molecular axes having an angle ranging from 0° to 90° with respect to the LC substrates. The “guest” dichroic dye molecules, mixed with the LC molecules, can thus be cooperatively aligned and the transmittance of input linearly polarized light passing the LC cell can be tuned by changing the electric field magnitude. This interaction enables electrically-tunable brightness and was first introduced for liquid crystal display (LCD) applications [17]. Subsequently, it was extended to the fabrication of coatable high quality thin-film patterned linear polarizers used for binocular disparity 3D stereoscopic displays [18, 19]. In the latter, instead of applying an electric field [16, 17], linearly polarized UV light [18] or LC alignment layers [19] were exploited to provide planar alignment (i.e. parallel to the LC substrates) of the “host” LC molecules. With the “guest” dichroic dye molecules cooperatively aligned by their LC “host”, the device is optically equivalent to a linear polarizer [18, 19]. In [18], reported peak polarization efficiency and order parameter are limited to 0.98 and 0.82, at a wavelength of 584 nm. In addition, the “guest” dichroic dye (N256 from Hayashibara Biochem. Labs. Inc.) exhibits poor absorbance in the blue and red regions of the visible spectrum. In [19], the patterned micropolarizer array is implemented with selective polymerization of the LC “host” on top of a rubbed LC alignment layer. The latter can inherently provide micropolarizers with only one local polarization direction. Moreover, the fabricated micropolarizer array in [19] exhibits a relatively large thickness (5 μm) and the reported spatial resolution is limited to 100 μm.
In this paper, a high-resolution submicron thin “guest-host” micropolarizer array is presented with a sulfonic azo-dye SD1 as the LC photoalignment material to photo-align the polymerizable NLC “host” material. A pixel pitch of 5 μm × 5 μm is achieved with improved major principal transmittance, polarization efficiency and order parameter. In addition, a patterned UV-regime metal-wire-grid polarizer is exploited as a “photoalignment master” to enable a one-step photolithography of the NLC layer. The proposed high resolution micropolarizer array technology is simple, cost-effective and removes alignment errors associated with multi-mask UV-exposure steps. This paper is organized as follows. Section 2 describes the design and fabrication process flow of the “guest-host” micropolarizer array. Experimental characterization results are reported and discussed in Section 3. Finally, a conclusion is drawn in Section 4.
2. “Guest-host” micropolarizer array
In this paper, we propose a UV-sensitive sulfonic azo-dye SD1 film-based non-contact photoalignment technique to fabricate high-resolution thin “guest-host” micropolarizer arrays. As depicted in Fig. 1 (A), a substrate, representing the image sensor, is first spin-coated with this SD1 film. After subsequent irradiation by linearly polarized UV light, photoalignment of SD1 molecules occurs [Fig. 1 (B)] with the SD1’s long molecular axes perpendicular to the polarization direction of projected linearly polarized UV light. The “guest-host” mixture of dichroic dye and NLC is then spin-coated on top of the SD1 film, which will act as an alignment layer for the NLC “host” molecules and the dichroic dye “guest” molecules. Fig. 1 (C) depicts this reorientation process with “host” and “guest” molecules aligned with SD1 molecules.
To demonstrate the proposed “guest-host” technology, we fabricated micropolarizer arrays capable of extracting full partial linear polarization information [20]. Each fabricated micropolarizer array exhibits a 2×2 pattern comprising 0°, 90°, 45° and −45° micropolarizers (Fig. 2). Here, each pixel will look through either a micropolarizer of 0°, 90°, 45° or −45° (Fig. 2). As a result, a single intensity value is available per pixel. The three other missing intensity values are recovered by examining the intensity values of neighboring pixels. A 2×2 (or larger) convolution kernel can be applied to estimate the first three Stokes parameters at each point. This approach trades off spatial resolution to allow for polarization measurements to be made simultaneously during a single image capture. In essence, this process is similar to color filter array interpolation or demosaicing. Ratliff et al. and Tyo et al. have recently reviewed and analyzed possible interpolation strategies for polarimetry [21–23].
To enable the optical characterization of the fabricated micropolarizer array, a transparent glass substrate was used instead of the silicon-based opaque CMOS image sensor substrate. The detailed fabrication steps of the “guest-host” micropolarizer array, shown in Fig. 2, can be summarized as follows:
- Organic contaminants were removed from the surface of the transparent glass substrate, using an ultraviolet-ozone (UVO) cleaning machine (Jelight 144AX).
- An SD1 solution was then spin-coated onto the glass at 800 rpm for 10 s then 3000 rmp for 40 s. In order to eliminate particle impurities, the solution of SD1 in dimethyl-formamide (DMF) with a concentration of 1% by weight was filtered before the spin-coating.
- The glass substrate was then baked at 110 ºC for 20 min to remove the remaining solvent and strengthen the adhesion of the SD1 material to the substrate.
- The spin-coated SD1 layer on the glass substrate was subsequently photo-aligned with the customized “photoalignment master” applied (Fig. 2), which is actually a patterned UV-regime metal-wire-grid polarizer from Moxtek Inc. This “photoalignment master”, featuring 5 μm × 5 μm pixel pitch, enables one-step UV-photoalignment of the SD1 layer making the fabrication process simple, cost-effective and high resolution with no misalignment errors. The UV-exposure duration was 15 min and the UV light intensity at 365 nm was around 5.6 mW/cm2. As a result, SD1 molecules were photo-aligned in different microdomains along 0°, 90°, 45° and −45°, respectively.
- After patterning the NLC photoalignment SD1 layer, a mixture of the dichroic dye solution and the NLC solution (with a mass ratio of 1:1) was spin-coated on top of the patterned SD1 layer at a speed of 800 rpm for 5 s then 3000 rpm for 30 s.
- Next, the substrate with the spin-coated “guest-host” mixture was baked at 50 ºC for 3 min to eliminate the solvents.
- Finally, a UV light with an intensity of 2 mW/cm2 and a wavelength of 254 nm, which is not within SD1’s sensitive spectrum and cannot thus reorient SD1 molecules, is applied for 3 min to polymerize the NLC “host” material. This last step ensures stability and protection against changing environmental conditions.
3. Experimental results and discussion
In order to examine each micropolarizer domain, the fabricated “guest-host” micropolarizer array sample was back-illuminated by the white light source of a microscope from Olympus Corp. (BH3-MJL). A broadband linear polarizer (from Moxtek Inc.) was inserted and rotated between the white light source and the fabricated sample to provide four different polarized inputs: 0° linearly polarized, 90° linearly polarized, 45° linearly polarized and −45° linearly polarized. According to [20], the normalized Stokes parameters (S1/S0, S2/S0) of the four different polarized inputs are (1, 0), (−1, 0), (0, 1) and (0, −1), respectively. Fig. 3 presents the sample’s microphotographs examined by a linear polarization analyzer and recorded by the microscope’s camera system (MotionBLITZ© Cube2). With the microscope’s embedded polarization analyzer and lens system, the camera can record a 1280×1024 image of the fabricated micropolarizer array with a resolution as small as 1 μm. Note that 0°, 90°, 45° and −45° micropolarizers appear dark as expected when the input is 90°, 0°, −45° and 45° linearly polarized, respectively.
Fig. 3 Microphotographs of the fabricated “guest-host” micropolarizer array, inspected by a linear polarization analyzer along the following orientations: (A) 0 degree; (B) 90 degree; (C) 45 degree; (D) −45 degree.
In addition, the micropolarizer pitch is shown to be 5 μm × 5 μm, which matches state-of-the-art commercially available wire-grid polarizers [11]. It is important to note that this resolution is not an indication of the resolution limit of the proposed micropolarizer fabrication technology but only the result of our choice of readily available general masks. The resolution limit of the proposed micropolarizer fabrication technology is set by the resolution of the adopted photolithography process. This performance enables the integration of the fabricated micropolarizer array with mainstream commercial CMOS image sensors, whose photo-sensing pixel size is usually less than 10 μm. The overall “guest-host” micropolarizer array thickness including the SD1 photoalignment layer was measured by a surface profiler (Tencor P-10) and found to be 0.95 μm. Compared to PVA [3–5] and crystal [6] based micropolarizer arrays, the proposed technology enables a reduction of the micropolarizer thickness by a factor of 20 to 887, respectively. Wire-grid polarizers provide the thinnest solution with less than 100 nm reported [11]. However, the latter rely on a far more complex and costly process, with multiple lithography and etching steps required. Because wire-grid polarizers are parallel arrangements of delicate thin aluminium nano-wires, they are fragile and can thus be easily damaged by standard assembly processes. This adds significant complexity/cost in the handling of the wire-grid polarizers during assembly.
Furthermore, the fabricated micropolarizer array was characterized by measuring the four important figures of merit: transmittances (T||, T⊥) and absorbances (A||, A⊥). Measurements were performed using a polarization state generator (PSG) comprising a mini deuterium halogen light source (DT-Mini-2-GS from Mikropack GmbH) and a broadband linear polarizer (from Moxtek Inc.). This PSG can provide linearly polarized input light with wavelengths ranging from 400 nm to 700 nm. Since the micropolarizer pitch is in the micrometer scale, which is much smaller than the PSG’s laser beam, we fabricated unpatterned “guest-host” linear polarizer samples (2.5 cm × 2.0 cm) together with the micropolarizer arrays to cover the PSG’s laser beam and enable the characterization of the fabricated micropolarizers. Fig. 4 (A) shows the spectral measurement results of both the major and the minor principal transmittances T||, T⊥. The major principal transmittance is seen to range from 71.9% (551 nm) to 96.8% (699 nm) with an average of 80.3% across the whole visible spectrum (i.e. from 400 nm to 700 nm). Another important figure of merit is the polarization efficiency (PE), defined as [19]:
The average PE across the whole visible spectrum was found to be 0.863 with the maximum PE equal to 0.996 at a wavelength of 545 nm and the minimum PE equal to 0.114 at a wavelength of 699 nm [Fig. 4 (B)]. Moreover, the two absorbances A⊥, A|| are reported in Fig. 5 (A). The order parameter S, defined as the ratio of (A|| – A⊥) and (A|| + 2A⊥) [19], is calculated and plotted in Fig. 5 (B) with an average of 0.848 across the whole visible spectrum. The maximum and minimum order parameters are 0.872 (623 nm) and 0.699 (699 nm), respectively.Fig. 4 Spectral measurement results: (A) the major and minor principal transmittances T||, T⊥; (B) polarization efficiency PE.
Fig. 5 Spectral measurement results: (A) absorbances A⊥, A|| corresponding to the major and minor principal transmittances T||, T⊥; (B) order parameter S.
Comparing this optical performance against previously reported micropolarizer arrays is difficult because figures of merit (e.g. PE, S, transmittance) are a function of the wavelength and only their peak values are typically reported in the literature. As a result, it is very difficult to directly compare some of our results with prior art, without access to the actual raw measurements and without considering the targeted application, for which the micropolarizer arrays were optimized [18,19]. Nevertheless, we can make the following observations in regard to previously reported photosensitive dichroic dyes. In [18], the N256 dichroic dye exhibits poorer absorbance in the blue and red regions of the visible spectrum, with a peak polarization efficiency and order parameter limited to 0.98 and 0.82, respectively, at a wavelength of 584 nm. In [19], only an averaged order parameter of 0.94 is reported. However, this relatively high order parameter is shown to decrease dramatically down to 0.83 with the polymerization of the “host” LC material [19]. In addition, the covered spectrum ranges from 450 nm to 550 nm, which corresponds to only about one third of the whole visible spectrum [19]. In this paper, the spectrum band, for which PE exceeds 0.90, ranges from 417 nm to 635 nm. This corresponds to 73% of the whole visible spectrum. This enables high-quality monochromatic or achromatic polarization image sensing applications. Ongoing efforts are focusing on the development and synthesis of dichroic dyes, which are more sensitive to the visible spectrum towards the UV and infrared (IR) ends. Compared to state-of-the art visible range wire-grid polarizers [11], the proposed micropolarizer technology boasts similar performance in terms of pixel size or transmittance. However, it does not perform as well in terms of polarization efficiency (99.6% versus 99.8%) or layer thickness (950 nm versus 100 nm). Nevertheless, given the proposed fabrication technique drastically simplifies the fabrication/integration process (e.g. spin-coating followed by a one step-UV lithography with no etching) of the micropolarizer array, it constitutes an attractive trade-off between performance and cost, making it well suited for low cost polarization imaging applications.
4. Conclusion
We have fabricated and characterized a high-resolution “guest-host” micropolarizer array with dichroic dye as the “guest” and polymerizable NLC as the “host”. Experimental results demonstrate that micropolarizer arrays exploiting “guest-host” interactions can offer higher resolution (5 μm × 5 μm pixel pitch), 0.95 μm thickness but also superior optical performance across the whole visible spectrum, with averaged major principal transmittance, polarization efficiency and order parameter of 80.3%, 0.863 and 0.848, respectively. This is achieved by controlling the “guest” molecular orientation through the photoalignment of “host” molecules. The proposed non-contact micropolarizer array fabrication technology prevents mechanical damage, electronic charge or contamination to the substrate. Furthermore, it is simple and cost-effective, requiring only a single UV-exposure through a “photoalignment master”. It is also fully compatible with standard CMOS process, enabling the integration of a “guest-host” micropolarizer array over a CMOS image sensor to realize the concept of a low cost single-chip polarization camera.
Acknowledgments
This work was supported by the Research Grant Council of Hong Kong SAR, P. R. China (Ref. GRF610608).
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