Nano-fabricated pixelated micropolarizer array for ...

Nano-fabricated pixelated micropolarizer array for visible imaging polarimetry Zhigang Zhang, Fengliang Dong, Teng Cheng, Kang Qiu, Qingchuan Zhang, Weiguo Chu, and Xiaoping Wu Citation: Review of Scientific Instruments 85, 105002 (2014); doi: 10.1063/1.4897270 View online: http://dx.doi.org/10.1063/1.4897270 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhanced reflection from inverse tapered nanocone arrays Appl. Phys. Lett. 105, 053108 (2014); 10.1063/1.4892580 Fabrication of nanoscale, high throughput, high aspect ratio freestanding gratings J. Vac. Sci. Technol. B 30, 06FF03 (2012); 10.1116/1.4755815 Subwavelength grating structures with magnetic resonances at visible frequencies fabricated by nanoimprint lithography for large area applications J. Vac. Sci. Technol. B 27, 3175 (2009); 10.1116/1.3243228 Large flexible nanowire grid visible polarizer made by nanoimprint lithography Appl. Phys. Lett. 90, 063111 (2007); 10.1063/1.2472532 Monolithically integrated circular polarizers with two-layer nano-gratings fabricated by imprint lithography J. Vac. Sci. Technol. B 23, 3164 (2005); 10.1116/1.2127948

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 105002 (2014)

Nano-fabricated pixelated micropolarizer array for visible imaging polarimetry Zhigang Zhang,1 Fengliang Dong,2,a) Teng Cheng,1,a) Kang Qiu,1 Qingchuan Zhang,1,b) Weiguo Chu,2,b) and Xiaoping Wu1 1 CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Modern Mechanics, University of Science and Technology of China, Hefei 230027, China 2 Nanofabrication Laboratory, National Center for Nanoscience and Technology, Beijing 100190, China

(Received 13 June 2014; accepted 23 September 2014; published online 8 October 2014) Pixelated micropolarizer array (PMA) is a novel concept for real-time visible imaging polarimetry. A 320 × 240 aluminum PMA fabricated by electron beam lithography is described in this paper. The period, duty ratio, and depth of the grating are 140 nm, 0.5, and 100 nm, respectively. The units are standard square structures and the metal nanowires of the grating are collimating and uniformly thick. The extinction ratio of 75 and the maximum polarization transmittance of 78.8% demonstrate that the PMA is suitable for polarization imaging. When the PMA is applied to real-time polarization imaging, the degree of linear polarization image and the angle of linear polarization image are calculated from a single frame image. The polarized target object is highlighted from the unpolarized background, and the surface contour of the target object can be reflected by the polarization angle. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4897270] I. INTRODUCTION

Polarization imaging has developed rapidly over the past few decades.1–3 Traditional stealth technology for optical detection is by disguising the target object in a background of the same color, which makes it difficult to distinguish the target. Since polarization information of the target object and background are somewhat dissimilar, the target object can be distinguished by the acquisition of polarization information. Polarization imaging can be used to measure the polarization information of the target object, such as Stokes parameters, degree of linear polarization (DoLP), and angle of linear polarization (AoLP).4–6 In traditional polarization imaging technology, a polarizer is placed in front of the camera, and multiple images are captured while rotating the polarizer. Stokes parameters and other polarization information can be calculated from these images, providing polarization imaging. Since these polarization images are captured at different times, this method requires very precise vibration control, and only can be used to measure static or quasi-static objects, which greatly limits the range of application. A pixelated micropolarizer array (PMA) can overcome the shortcomings of traditional polarization imaging, and may greatly promote the development of this technology. When the PMA is used, polarization information, such as DoLP and AoLP, can be obtained from a single frame image,7 precise vibration control is not necessary and dynamic measurements can be achieved. Depending on the polarization material, PMAs are mainly divided into three types: PMA based on iodinedoped polyvinyl alcohol (PVA) layers,8 PMA based on liquid crystals,9–11 and PMA based on metal nano-gratings.12 The a) F. Dong and T. Cheng contributed equally to this work. b) Authors to whom correspondence should be addressed. Electronic

addresses: [email protected] and [email protected].

0034-6748/2014/85(10)/105002/6/$30.00

thickness of a PVA layer is approximately 10 μm, significantly larger than the pitch of the CCD sensor device unit.8 Hence, the cross-talk between adjacent units reduces the precision and accuracy. The extinction ratios of liquid crystal based PMAs are low in the red wave band, which limits the range of applications.10 The metal layer thickness of nanograting based PMAs is approximately 100 nm, and the theoretical extinction ratio and transmittance are excellent in the visible wave band, which makes this type of PMA suitable for more practical applications. The current method to fabricate nano-grating based PMAs is multiple holography exposure (or interferometry exposure) and hierarchical etching of the metal layer for different polarization direction units.12 This distributes the units of different polarization directions on different layers and generates some issues: (a) field uniformity and field distortion of exposure light source are common problems for optical lithography, (b) the fabrication processes are complex and difficult, (c) hierarchical etching increases the metal layer thickness, (d) it is difficult to ensure the consistency of the polarization performance of different layers, and (e) the fabrication failure of one layer will disable the whole chip, thus the yield is low. Electron beam lithography (EBL) has better resolution than optical lithography and solves the problems of field uniformity and field distortion of exposure light source. EBL technology has advanced the beam power and greatly improved the exposure speed, making the fabrication cost of PMAs by EBL acceptable. Thus PMAs can be fabricated by EBL technology, which also overcomes the issues with interference exposure. Gruev et al. fabricated 2 × 2 PMAs by EBL, and discussed the effect of different grating period on the polarization performance.13 We fabricated a 320 × 240 PMA by EBL, and the PMA was integrated to the focal plane of the

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FIG. 1. Fabrication processes of PMA by EBL. (a) Double polished and cleaned glass substrate. (b) 100 nm thick aluminum layer is deposited on the substrate. (c) A layer of positive electron beam photoresist is spin-coated on the aluminum surface. (d) The photoresist layer is exposed to the electron beam to form a mask. (e) The aluminum layer is etched by inductively coupled plasma-reactive ion etcher (ICP-RIE). (f) The photoresist is removed.

CCD sensor device to achieve real-time polarization imaging and phase measurement. In Sec. II, the fabrication processes is described and we analyze the polarization performance. In Sec. III, we discuss the applications of PMA in polarization imaging.

II. PMA FABRICATION PROCESS AND PERFORMANCE ANALYSIS A. Fabrication process of PMA by EBL

We chose aluminum as the base metal material to produce our PMA by EBL. The cross section of the grating is rectangular and the period, duty ratio, and depth are 140 nm, 0.5, and 100 nm, respectively. The pitch of PMA units is 7.4 μm, which is a commonly used size for CCD sensor devices. Surrounding the units is a 1-μm wide opaque area to reduce cross-talk of adjacent units. The fabrication process is as follows (see Fig. 1): (1) High transmittance glass is double-side polished and cleaned (Fig. 1(a));

(2) A 100-nm thick aluminum layer is deposited on the substrate (Fig. 1(b)); (3) A layer of positive electron beam photoresist (ZEP 520) is spin-coated on the aluminum surface, and the assembly baked at 180 ◦ C (Fig. 1(c)); (4) The photoresist layer is exposed to the electron beam (Vistec EBPG 5000+ ES), and the nano-grating structure (in four directions) is obtained after development of the photoresist layer (Fig. 1(d)); (5) The nano-grating structure in the photoresist layer forms a mask and the nano-grating structure is transferred from the photoresist layer to the aluminum layer by inductively coupled plasma-reactive ion etching (ICP-RIE) (Sentech PTSA ICP-RIE Etcher SI 500) (Fig. 1(e)); (6) The remaining photoresist layer is removed (Fig. 1(f)). B. Performance analysis of PMA

An scanning electron microscope (SEM) was used to observe the microstructure of PMA and Figs. 2(a)–2(c) show the PMA at different magnifications. The pitch of the units is 7.4 μm, and the units are standard square structures

FIG. 2. Microscopic observation of the PMA. (a)–(c) are SEM images of the PMA under different magnifications and (d)–(g) are optical microscopic images of the PMA when the polarization directions of the illuminating light are 0◦ , 45◦ , 90◦ , and 135◦ , respectively.

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(Fig. 2(a)). Fig. 2(b) shows adjacent 2 × 2 units. The normal directions of the gratings are 0◦ , 135◦ , 90◦ , and 45◦ . Fig. 2(c) shows the details of the grating. The grating period and duty ratio are 140 nm and 0.5, respectively. The grating lines are straight and uniformly thick, with 1-μm wide aluminum borders surrounding each unit. An optical microscope was used to analyze the polarization performance of the PMA. The PMA was placed on the microscope stage, and the transmission light path is chosen for the observation. Color filters were added in the light source path to generate the desired wave bands. A hollow electric precision rotating platform containing a polarizer was placed between the color filters and the microscope stage, and the polarization direction of the light was adjusted (Figs. 2(d)–2(g)) to 0◦ , 45◦ , 90◦ , and 135◦ , respectively. It is clear that the units of the PMA are standard square structures. The image intensity is largest when the polarization direction of the PMA units is parallel to the illumination light polarization, lowest when the polarization is perpendicular, and intermediate when the angle is 45◦ or 135◦ . Surrounding the units is a 1-μm wide opaque area to reduce the cross-talk of adjacent units. The maximum polarization transmittance and extinction ratio were calculated to evaluate the polarization performance

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of the PMA. Maximum transmittance occurs when the polarization direction of the PMA units is parallel to the illumination light polarization and minimum when the polarization is perpendicular. The ratio of the maximum to minimum polarization transmittance is the extinction ratio. We conducted the experiments in a darkroom and used red, green, and cyan filters to generate our desired spectral range. Our recording CCD camera provided 12-bit data and we set the recording frame rate to 20 frame images per second. The precision rotating platform was uniformly rotated at 20◦ per second, providing a 1◦ rotation angle between adjacent frame images. Figs. 3(a)–3(c) show the transmittance curves of the 0◦ , ◦ 45 , 90◦ , and 135◦ for polarizer angle and the different color filters. The transmittance spectra of the filters are also shown in Fig. 3, as determined by spectrophotometer (SOLID 3700). The maximum polarization transmittances for our PMA for the red, green, and cyan wave bands are 75.2% ± 0.5%, 78.8% ± 0.1%, and 77.6% ± 0.4%, the minimum polarization transmittances are 1.0% ± 0.1%, 1.7% ± 0.2%, and 2.6% ± 0.1%, and the extinction ratios are 75, 46, and 30, respectively. The extinction ratio is smaller at shorter wavelength, which will be improved by reducing the period of the grating and optimizing the fabrication process in the future.

FIG. 3. Transmittances of the 0◦ , 45◦ , 90◦ , and 135◦ units changed with the polarization direction of the incident light. (a) Red light, (b) green light, and (c) cyan light. The curves on the right side of the figure are the transmittance spectra of the color filters.

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C. Integrating the PMA in the focal plane of the CCD sensor device

The fabricated PMA (approximately 3 × 2 mm) was integrated in the focal plane of the CCD sensor device (SONY 202CZYK-ICX424AL, black and white, 640 × 480, 7.4 μm pitch, 10-bits). The integrating procedures were as follows: (a) The dust-proof glass cover was removed from the CCD sensor device. (b) Ultraviolet curing adhesive (Kafuter 100021A) was coated in the focal plane of the CCD sensor device. This adhesive is transparent in the visible range and has no imaging effect. (c) The PMA was placed on the CCD sensor device, with the aluminum layer of the PMA towards the CCD sensor. (d) The position and angle of the PMA were adjusted to ensure the PMA units were aligned with the CCD sensor device units as precisely as possible. (e) Ultraviolet light was used to cure the adhesive. (f) The dust-proof glass cover was replaced back over the CCD sensor device. In step (d), the CCD sensor is a component of a CCD camera which was in a working state, and the illumination light was adjustable linearly-polarized parallel light. The

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position and angle of the PMA were precisely adjusted to obtain the optimum extinction ratio of the image as captured by the CCD camera itself. When the integrated CCD camera with PMA is used to capture images, the adjacent 2 × 2 units gray values represent the intensities when the polarization directions are 0◦ , 45◦ , 90◦ , and 135◦ , respectively. To obtain four images of different polarization directions from a single frame, linear interpolation is used (Fig. 4). Fig. 4(a) shows a single frame image captured by CCD camera, and then the gray values from pixels of the same polarization directions are copied to the same position of blank images with the same resolution, generating four images (Fig. 4(b)). Each of the four images has a quarter of original data and three quarters of null values. Linear interpolation is used to fill the null values based on the original data (Fig. 4(c)), providing the four images of different polarizations from a single frame image.

III. APPLICATIONS OF PMA ON POLARIZATION IMAGING

Four images can be obtained from a single frame image when the CCD camera is integrated with the PMA, representing polarization directions 0◦ , 45◦ , 90◦ , and 135◦ . The

FIG. 4. Schematic diagram of the linear interpolation to obtain four images of different polarization directions from a single frame. (a) A single frame image captured by the integrated CCD with PMA. (b) The gray values from pixels of the same polarization directions are copied to the same position of blank images with the same resolution. (c) Linear interpolation method is used to fill the null values.

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gray-scale values can be expressed as I(0), I(45), I(90), and I(135), and so the Stokes parameters are S0 =I (0) + I(90),

(1)

S1 = I (0) − I(90),

(2)

S2 = I (45) − I(135),

(3)

and

where S0 is the gray-scale values without the polarizer in front of the CCD sensor device. Hence the S0 image can be considered as the image captured by an ordinary camera. The DoLP of the image is  S12 + S22 , (4) DoLP = S0 and DoLP ranges from 0 to 1. When DoLP = 1, the light is completely linearly polarized, whereas when DoLP = 0, the light is completely nonlinearly polarized. For area of high DoLP, AoLP is   S 1 (5) AoLP = arctan 2 , 2 S1 and AoLP ranges from 0◦ to 180◦ . As the AoLP of the light is affected by the surface contour of the object, the AoLP image can reflect surface contour information of the object. Fig. 5 is a polarization imaging example of the PMA. A single frame image was captured (Fig. 5(a)). The object is a rough disc with 12 triquetrous polarizers, and the background is a rough wall. The polarization angles of the polarizers are in the central axes of the triangles. Four images were obtained from Fig. 5(a) by linear interpolation (Figs. 5(b)–5(e)), which represent polarization directions 0◦ , 45◦ , 90◦ , and 135◦ . The S0 image was calculated from Eq. (1) (Fig. 5(f). The DoLP image was calculated from Eq. (4) and the values range from 0 to 1 (Fig. 5(g)). The DoLP value of the wall and frame is

FIG. 5. A polarization imaging example of the PMA. (a) A single frame image is captured by the CCD camera integrated with PMA, (b)–(e) are 0◦ , 45◦ , 90◦ , and 135◦ images obtained from (a) by linear interpolation method, (f) S0 image, (g) DoLP image, and (h) AoLP image.

FIG. 6. Polarization imaging examples of PMA in daily life. (a) A partial photo of a car taken from the left anterior of the car, (b) a photo of three cars taken from the left rear of the car, and (c) a photo of a moped. For the AoLP maps, the horizontal direction is defined as the reference direction 0◦ .

very low, while the DoLP value of the polarizers is very high. For DoLP > 0.1, the AoLP values are calculated, which range from 0◦ to 180◦ and are shown in pseudo color in Fig. 5(h). The measured polarization angles of the polarizers are along the central axes of the triangles, which is consistent with the actual setting. For DoLP < 0.1, AoLP values are not necessary, and these regions are shown as black in Fig. 5(h). Polarization imaging using the PMA can be applied to objects in daily life. The reflected light from the object often has polarization information, and the DoLP of reflected light from a smooth surface is higher than that from a rough surface. The AoLP of reflected light is usually perpendicular to incident plane. Therefore, the acquisition of polarization information provides more information about the object. Fig. 6 shows applications of the PMA in the polarization imaging of surrounding objects. A partial photo was taken from the left anterior of a car (Fig. 6(a)). The car body is black and smooth and the wheel and fender are rough, thus the DoLP values of the car body are high while the values of the wheel and fender are low. For the AoLP maps, the horizontal direction is defined as the reference direction 0◦ . As the car surface angle changes, the AoLP of the reflected light from the car rotates correspondingly. Due to that the polarization of reflect light from a smooth surface is perpendicular to incident plane, the polarization angle of reflected light is approximately 90◦ (represented by cyan color) on the side of the car, whereas the polarization angle approximately 180◦ (represented by red color) on the top of the car. From the side to the top following the contour of the car, the polarization angles change from 90◦ to 180◦ correspondingly, as shown in the AoLP image. Thus the AoLP image represents the surface contour of the car, which cannot be expressed with an ordinary camera image. Fig. 6(b) shows three cars, taken from the left rear. The smooth cars are quite distinct from the rough ground in the DoLP image. From the top to the side of the cars, the polarization angles change from 0◦ to 90◦ , correspondingly, as shown in the AoLP image. Fig. 6(c) shows a moped. The smooth moped stands out from the rough ground in the DoLP image. The contour of the

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moped changes somewhat more rapidly than that of the cars. From right to left of the moped, the corresponding polarization angles change from 90◦ to 180◦ , then 0◦ to 90◦ , as shown in the AoLP image. IV. DISCUSSIONS AND CONCLUSIONS

A 320 × 240 PMA fabricated by EBL and ICP-RIE is described in this paper. Comparing to the PMAs fabricated by interference exposure, the four polarization direction units are placed in one layer and the ICP-IRE is conducted only one time, resulting in the uniform grating structure and consistent polarization performance for the four different grating direction units. The complexity of the fabrication process is significantly reduced, leading to potential high yield production of PMAs. The PMA is applied to real-time polarization imaging. The ordinary light intensity camera image as well as the DoLP and AoLP images is obtained from a single frame image. The polarized target objects strand out from the unpolarized background. The AoLP image reflects the contour change of the polarized target object, which helps envisioning the three dimensional shape of the object from a two-dimensional image. ACKNOWLEDGMENTS

Project supported by the State Key Development Program for Basic Research of China (Grant No. 2011CB302105), the National Natural Science Foundation of China (Grant Nos. 11332010, 11102201, 11472266,

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and 11372300), and the Instrument Developing Project of the Chinese Academy of Science (Grant No. YZ201265). 1 G.

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