In this paper, we describe the design, modeling, fabrication, and optical characterization of the first micropolarimeter array enabling full Stokes polarization imaging in visible spectrum. The proposed micropolarimeter is fabricated by patterning a liquid-crystal (LC) layer on top of a visible-regime metal-wire-grid polarizer (MWGP) using ultraviolet sensitive sulfonic-dye-1 as the LC photoalignment material. This arrangement enables the formation of either micrometer-scale LC polarization rotators, neutral density filters or quarter wavelength retarders. These elements are in turn exploited to acquire all components of the Stokes vector, which describes all possible polarization states of light. Reported major principal transmittance of 75% and extinction ratio of 1100 demonstrate that the MWGP’s superior optical characteristics are retained. The proposed liquid-crystal micropolarimeter array can be integrated on top of a complementary metal-oxide-semiconductor (CMOS) image sensor for real-time full Stokes polarization imaging.
© 2010 OSA
The polarization state of light can be described by one vector, known as the Stokes vector S. Through its four components (S 0, S 1, S 2, S 3), this vector provides valuable information about reflecting objects that traditional intensity-based cameras ignore. Concretely, geometrical, chemical, physical, physiological and metabolic properties of the target such as surface smoothness, shape, size, color, orientation, molecular structure can be extracted if the Stokes vector components are captured [1, 2]. Polarimetric imaging systems are typically not capable of full Stokes polarization imaging [3–6], relying instead on only a subset of the Stokes vector components: (S 0, S 1, S 2) for linear polarization imaging [3, 4] or just S 3 for circular polarization imaging [5, 6]. For instance, in [3, 4], Rowe et al. describe how the difference of orthogonal linearly polarized components (also known as polarization-difference imaging) can be used to improve the object’s visibility in scattering media. It is attributed to the fact that light reflected by the targeted object is linearly polarized and the effects of background scattering can be removed by examining the polarization difference . In [5, 6], Gilbert et al. demonstrate a circular polarization imaging system to improve the contrast of the underwater image, which is based on the principle that the handedness of circularly polarized light changes with each reflection. Therefore, the light reflected from the targeted object can be easily distinguished from that reflected from the medium by simply examining the difference in handedness through a set of circular polarization analyzers.
Full Stokes polarization imaging systems have been proposed to enable the simultaneous capture of both linear and circular components of the Stokes vector [2,7–10]. These implementations consist typically of a combination of image sensors, electro/mechanically controlled linear polarizers, retarders and DSPs/CPUs . To sense all possible polarization states of light, these systems would typically require the capture of multiple images of the same scene, each of which is acquired by means of looking through different polarization elements . In [9, 10], electronically-controlled liquid crystal variable retarders (LCVRs) are used in combination with an image sensor, to capture different polarization images in successive frames. The obvious drawback of this approach is that both the scene and the camera must be stationary across multiple frames, to avoid introducing interframe motion. To eliminate these errors, some systems acquire multiple images at the same time, but then the problem becomes spatial registration since each camera will image the scene from a slightly different perspective [2, 7, 8]. Spatial registration of multiples images is complicated by the need to compensate for both mechanical misalignment and aberrations due to separate optical paths .
To alleviate the above obstacles, a promising avenue is to directly pattern micro-optical polarization elements on top of each pixel of the image sensor to form a micropolarimeter array [2, 11]. In this way, each pixel will image through a micropolarizer element of a given orientation with a single polarization component sensed by each pixel. The three other missing components of the Stokes vector are recovered by examining the intensity values of neighboring pixels. In essence, this process is similar to color filter array interpolation or demosaicing. The adopted approach tolerates a 1-pixel registration error to allow for all polarization measurements to be made simultaneously at each pixel . Because it uses semiconductor industry standard complementary metal-oxide-semiconductor (CMOS) fabrication process, this approach offers other significant advantages in terms of manufacturing cost, system volume, weight, power dissipation and system integration on a single silicon chip. A number of micropolarimeter array implementations have been reported in the literature [12–18]. In [12–15], reactive-ion-etching (RIE) is used to pattern dichroic polymer films and form a micropolarimeter array capable of extracting (S 0, S 1, S 2) for partial-linear polarization imaging. In , Momeni et al. demonstrate a micropolarimeter array made of refractive YVO 4 crystal for linear polarization imaging. An aluminum film is evaporated on top of the YVO 4 crystal then patterned by liftoff to form the birefringent micropolarizer array. In , Harnett et al. present a liquid-crystal micropolarizer array for linear polarization imaging with evaporated gold films used as orientation layers. Liftoff is subsequently used to pattern the gold film. More recently, we reported a superior high resolution liquid-crystal micropolarimeter array fabrication technology, removing the need for complex selective etching. A 2µm pitch was achieved using ultraviolet (UV) light to define the micropolarimeter elements . However, the latter micropolarimeter array is only capable of linear polarization imaging.
In fact, none of the reported micropolarimeter implementations can extract the complete polarization information. In this paper, we report the first micropolarimeter array capable of full Stokes polarization imaging. The proposed micropolarimeter is fabricated by patterning a liquid-crystal (LC) layer on top of a visible-regime metal-wire-grid polarizer (MWGP) using UV sensitive sulfonic-dye-1 (SD1) as the LC photoalignment material. This arrangement enables the formation of either micrometer-scale LC polarization rotators, neutral density filters or quarter wavelength retarders. These elements are in turn exploited to acquire all components of the Stokes vector. This paper is organized as follows. The design and implementation of the proposed micropolarimeter array are described in Section 2. Details of its fabrication process are provided in Section 3. Its optical characterization is described in Section 4 together with a discussion of experimental results. Finally, a conclusion is drawn in Section 5.
2. System design and implementation
An integrated polarization image sensing system consists of three fundamental building blocks: optics, light polarizing elements and underlying photodetectors. By redirecting the light reflected from an illuminated scene through the optical lens to its focal plane, the different polarized components of light can be filtered by the polarizing elements covering individual photodetectors. For example, linearly polarized components with different polarization orientations can be examined by a combination of a polarization rotator and a linear polarizer . On the other hand, circularly polarized components can be examined by a combination of a polarization retarder and a linear polarizer . In this paper, we propose to combine a visible-regime MWGP, used as a linear polarizer, together with a patterned LC layer, used here as either polarization rotator, polarization retarder or neutral density filter depending on the LC orientation layers. Figure 1 illustrates how the proposed LCMP array can be integrated on top of a CMOS image sensor to extract the full Stokes components for every pixel of the captured image.
Suppose the initial Stokes parameters for the incident light are (S 0, S 1, S 2, S 3), they become (S′0, S′1, S′2, S′3) after passing through the LC cell and (S″0, S″1, S″2, S″3) after passing through the MWGP. As a unitary intensity-lossless system, the Mueller matrix of the LC cell can be expressed as :
When an electric field is applied and exceeds a given threshold value, the LC cell can be represented by a neutral density filter. When there is no electric field applied, the LC cell can be represented by the combination of a polarization phase retarder and a polarization rotator. Its Mueller matrix is given as follows :
where ϕ is the LC twist angle, d is the LC layer thickness, Δn(λ) is the LC birefringence and λ is the wavelength of the incident light. The twist angle ϕ and the phase retardation 2δ are the two main design parameters in the design and fabrication of the LC cell. The Mueller matrix of the MWGP with its polarization axis oriented at an angle of θ is :
The Stokes parameters of the emerging light, depicted in Fig. 1, can be thus expressed by multiplying the above-mentioned initial Stokes parameters (in vector form) by the Mueller matrices of the LC cell and the MWGP successively as follows:
The total intensity, S″0, of the emerging light (Fig. 1) is given:
Mathematically, at least four intensity measurements taken through four different LCMPs, each with its unique set of parameters (J, L, N), are needed to determine the four Stokes parameters S 0, S 1, S 2, S 3. The four intensity measurements can be quantitatively expressed as:
To determine all four Stokes parameters simultaneously, we propose to adopt an approach similar to that used in a color filter array, with each pixel making only one of the four necessary intensity measurements (I 1 – I 4). The three other intensities are recovered by examining the intensity values of neighboring pixels. This approach trades-off spatial resolution to allow for the acquisition of all Stokes components in a single image capture. The basic element of the LCMP array, referred to as superpixel, consists of four micropolarimeters. Equation (11) suggests that one can choose from a number of different possible combinations of four micropolarimeters, with parameters (Ji, Li, Ni) for i = [1,2,3,4]. However, a judicious choice of the design parameters (θ, ϕ and δ) and coefficients (J, L, N) of each micropolarimeter can significantly reduce the complexity of the silicon implementation of the Stokes parameters computation circuitry.
For example, if we orient the MWGP’s polarizing axis at θ = 45°, (J, L, N) are simplified to (D, E, F) and Eq. (11) becomes:
Let’s now constrain the phase retardation 2δ of each cell to quarter wavelength:
for m = 1,2,3…, where m is referred to as the phase retardation factor. According to Eq. (2)–(8), when there is no electric field, the coefficients (D, E, F) in Eq. (14) become functions of the LC twist angle ϕ and the phase retardation factor m, as shown in Fig. 2. The following observations can be exploited to simplify the expressions of the elements (D, E, F) of the LC Mueller matrix:
- When the LC twist angle ϕ = 45°, F and D become 0 and 1 respectively.
- When the LC twist angle ϕ = −45°, F and D become 0 and −1 respectively.
- When the LC cell is untwisted (ϕ = 0°), elements (D, E, F) are equal to (0, 0, 1) for all values of m. According to Eq. (14), only the right-handed circularly polarized component represented by S 3 can be transmitted for the LCMP with untwisted LC cell. In other words, it is optically equivalent to a right-handed circular polarization analyzer.
The above observations have motivated our choice of 45° twisted, −45° twisted and untwisted LC cells for three out of the four micropolarimeters covering a superpixel (Fig. 3). For the fourth LCMP, we chose to operate the LC cell in electrically controlled birefringence (ECB) mode. If the electric field exceeds a threshold value, this fourth LC cell is optically equivalent to a neutral density filter, which has no influence on the polarization state of incident light. It follows that the Mueller matrix elements (D, E, F) are equal to (0, 1, 0). Note that in conjunction with the 45° oriented visible-regime MWGP, the LCMP with the neutral density filter LC cell is optically equivalent to a 45° linear polarization analyzer.
Figure 3 shows cross-sections for the four micropolarimeters. Note the different LC molecules arrangements are achieved by using LC photoalignment layers. By selectively photo-patterning the inner surfaces of the LC cells, we can make an LC cell act as a retarder, neutral density filter or rotator (Fig. 3). The latter occurs wherever the orientation of the bottom inner layer is not identical to the top inner layer, in which case incoming light follows the rotation of the molecules. In E-field mode, the orientation of the LC molecules is that of the applied electric field.
3. Liquid-crystal micropolarimeter array fabrication
To demonstrate the proposed micropolarimeter array technology, we have fabricated LCMP arrays comprising 40000 superpixels (200 × 200). The size of each superpixel is 20µm × 20µm. Each LCMP was fabricated on a transparent glass substrate, referred to as “second substrate” in Fig. 4. The use of this dummy CMOS imager substrate enables the optical characterization of each fabricated LCMP, since a CMOS imager substrate is itself opaque. Future work will focus on the actual integration of the proposed LCMP on top of custom made CMOS imager. Figure 4 shows a cross-section of the fabricated LCMP array. The two transparent thin glass slides, used to encapsulate the LC layer, are referred to as “first substrate” and “second substrate”. Note that the visible-regime 150nm-thick MWGP (from Moxtek Inc.) is placed on the inner surface of the second substrate. Azo-dye SD1 was synthesized and used as photoalignment layer to orient the LC molecules . This SD1 material is sensitive to UV light with its peak absorbance at a wavelength of 360nm. This material exhibits a molecular photo-reorientation mechanism characterized by SD1 long molecular axis perpendicular to the polarization direction of projected polarized UV light . Depending on the orientation of the photo-patterned inner surfaces of the LC cell, the latter can act as a retarder, neutral density filter, or a rotator . The fabrication of the proposed liquid-crystal micropolarimeter array can be summarized as follows:
- An indium tin oxide (ITO) layer with 70nm thickness is deposited on top of the inner surfaces of both the first and the second substrates.
- The deposited ITO layer of the second substrate is selectively etched by a solution composed of hydrochloric acid (HCl), nitric acid (HNO 3) and water (4:1:2 by volume). The remaining ITO regions form the electrodes for LCMPE−field (Fig. 3).
- The inner surfaces of the two substrates are processed with an ultraviolet-ozone (UVO) cleaner (Model 144AX from Jelight Inc.) for 20min to remove organic contaminants and improve the uniformity of the spin coated LC photoalignment layer.
- An SD1 solution is spin-coated onto the inner surfaces of the two substrates at 800rpm for 10s then 3000rpm for 40s. In order to eliminate particle impurities, an SD1 solution in dimethylformamide (DMF), with a concentration of 1% by weight, is filtered before the spin coating.
- The substrates are then baked at 110°C for 20 min to remove the remaining solvent and strengthen the adhesion of SD1 material to the substrates.
- The inner surfaces of the two substrates with the SD1 coating are exposed to 90° linearly polarized UV light for 15min without using any photolithography mask. This results in a 0° photoalignment of the SD1 molecules throughout the entire photoalignment layer.
- Subsequently, the inner surface of the second substrate with the SD1 coating is exposed to −45° linearly polarized UV light for 15min, with a photolithography mask exposing the regions of LCMP 45°twisted (Fig. 3), resulting in a 45° photo-reorientation of the SD1 molecules within the exposed regions.
- The inner surface of the second substrate with the SD1 coating is then exposed to 45° linearly polarized UV light for 15min with a photolithography mask exposing the regions of LCMP −45°twisted (Fig. 3), resulting in a −45° photo-reorientation of the SD1 molecules within the exposed regions.
- Glass fiber rod spacers with 5µm diameter are sprayed on the inner surface of the first substrate. The two substrates are then assembled together with their inner surfaces facing each other and a 5µm cell gap between the inner surfaces. Thermal epoxy is used for this assembly and the attached substrates are placed into a 120°C oven for one hour to cure the epoxy.
- The resulting empty LC cell is then filled with liquid crystal E7 (from Merck Inc.) before being end-sealed with a thermal epoxy, cured as outlined in the previous step.
4. Experimental results
In order to characterize the optical performance of the fabricated micropolarimeters, a polarization state generator (PSG) composed of a commercial linear polarizer (from Moxtek Inc.) and a commercial broadband quarter wavelength retarder (from Nitto Denko Corp.) was used to generate six different polarized inputs: 0° linearly polarized, 90° linearly polarized, 45° linearly polarized, −45° linearly polarized, right-handed circularly polarized and left-handed circularly polarized. Their corresponding normalized Stokes parameters (S 1/S 0, S 2/S 0, S 3/S 0) are (1, 0, 0), (−1, 0, 0), (0, 1, 0), (0, −1, 0), (0, 0, 1) and (0, 0, −1), respectively . The fabricated LCMP array was placed under a microscope with back illumination collimated by a 500nm monochromatic filter and the above-mentioned PSG. Figure 5 presents recorded microphotographs. Note that LCMP 45°twisted, LCMP −45°twisted and LCMP E−field should appear dark when the polarization state of the input light is 90°, 0° and −45° linearly polarized, respectively. LCMPUntwisted should be insensitive to linearly polarized input and should theoretically attenuate half of the input light intensity . This is why it appears grey in Fig. 5(A)–5(C). In addition, LCMPUntwisted appears bright when the input is right-handed circularly polarized [Fig. 5(D)] and dark when the input is left-handed circularly polarized [Fig. 5(E)]. It follows that LCMPUntwisted behaves as a right-handed circular polarization analyzer.
Malus measurements were conducted for LCMP 45°twisted, LCMP −45°twisted and LCMPE−field to extract the two important figures of merit that are major principal transmittance and extinction ratio . They are defined as the maximum transmittance and the ratio between the maximum transmittance and the minimum transmittance in the Malus measurement, respectively . Single LCMP samples with a size of 2.5cm × 2.0cm were fabricated together with the LCMP array to cover the laser beam and enable the LCMP optical characterization. The above-mentioned PSG was inserted between a tunable mini deuterium halogen light source (Model DT-Mini-2-GS from Mikropack GmbH) and the LCMP samples to provide linearly polarized input light with wavelength varied from 400nm to 700nm. A high resolution spectrometer (Model HR2000 from Ocean Optics Inc.) and a computer control were used to calculate and record the light transmitted through the samples. Figure 6(A)–6(C) present the Malus measurement results for three different wavelengths: 450nm (blue light), 550nm (green light) and 650nm (red light). Note that Malus’ law is well satisfied for LCMP 45°twisted, LCMP −45°twisted and LCMPE−field for different wavelengths. Right-handed and left-handed circularly polarized inputs were also provided to illuminate the corresponding LCMP sample and measure its spectral responses. Figure 6(D) shows the spectral measurement results, with the transmittance maximum for a wavelength of 500nm. This means that the LC cell of LCMPUntwisted behaves as a quarter wavelength retarder for a wavelength of 500nm. Table 1 reports the measured extinction ratios for the four LCMPs at a wavelength of 500nm. Note that the values are around 1100, indicating that the MWGP’s original extinction ratio is well retained .
The PSG-collimated monochromatic light source (500nm) was used to evaluate the performance of the proposed LCMP optical model [Eq. (14)]. The recorded transmittances were compensated by taking into account the transmission loss due to non-polarizing effects, such as surface reflection of glass substrate, non-polarizing absorption/scattering of both LC and MWGP layers. A collimated unpolarized light beam was projected onto the samples to measure this transmission loss. The compensated transmittances through the single LCMP samples were fed into the proposed LCMP model [Eq. (14)], with the model parameters evaluated using Eq. (2)–(8):
The extracted Stokes parameters, normalized to light intensity (i.e. S 0), are reported in Table 2. The fabricated LCMPs are found to behave as expected within 2.3%. Errors can be attributed to measurement noise, round off errors in calculations but also to slight deviations of:
- ±0.5° for the LC photoalignment direction, which was manually controlled with a protractor.
- ±1° for the LC twist angle ϕ, which was defined by assembling the two glass substrates.
- ±0.1µm for the LC layer thickness d, which was controlled by glass fiber rod spacers.
- ±5nm for the applied monochromatic filter and ±2 ~ 3°C for the room temperature control. These variations can affect the LC birefringence.
Future work will focus on integrating the proposed LCMP array on top of a custom made CMOS imager to evaluate the polarimetric imaging capabilities of a single-chip polarization CMOS imager.
In this paper, we have demonstrated a liquid-crystal based micropolarimeter array for full Stokes polarization imaging in visible spectrum. In contrast to previously reported micropolarizer arrays, the proposed implementation enables, for the first time, the extraction of circularly polarized components of light for real-time full Stokes polarimetry. To accommodate applications with large oblique incidence angle, a 150nm ultra-thin visible-regime MWGP is used to reduce the overall thickness of the micropolarimeter array layer to 5µm. This provides a relatively good aspect ratio for pixel size larger than 10µm. Experimental results indicate that the LC-MWGP structure retains MWGP’s superior optical characteristics with a major principal transmittance of 75% and extinction ratios of around 1100.
This work was supported by the Research Grant Council of Hong Kong SAR, P. R. China (Ref. GRF610608).
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