Xiaojin Zhao

Liquid-crystal micropolarimeter array for full Stokes polarization imaging in visible spectrum

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 MWGPs superior optical characteristics are retained. The proposed liquidcrystal micropolarimeter array can be integrated on top of a complementary metal-oxide-semiconductor (CMOS) image sensor for real-time full Stokes polarization imaging.

Thin Photo-Patterned Micropolarizer Array for CMOS Image Sensors

We fabricated and characterized a thin photo-patterned micropolarizer array for complementary metal-oxide-semiconductor (CMOS) image sensors. The proposed micropolarizer fabrication technology completely removes the need for complex selective etching. Instead, it uses the well-controlled process of ultraviolet photolithography to define micropolarizer orientation patterns on a spin-coated azo-dye-1 film. The patterned polymer film micropolarizer (10 mum x 10 mum) exhibits submicron thickness (0.3 mum) and has an extinction ratio of ~ 100. Reported experimental results validate the concept of a thin, high spatial resolution, low-cost photo-patterned micropolarizer array for CMOS image sensors.

High-resolution photoaligned liquid-crystal micropolarizer array for polarization imaging in visible spectrum

We report a superior high-resolution micropolarizer array fabrication technology that combines a linear polarizer with a micropatterned liquid-crystal (LC) cell. A 2 microm pitch is achieved by using UV light to define the orientation of the micropolarizer elements. Reported experimental results validate the concept of high-performance photoaligned LC micropolarizer arrays with major principal transmittance of approximately 80% and extinction ratio as high as approximately 3200(35 dB).

Patterned dual-layer achromatic micro-quarter-wave-retarder array for active polarization imaging

In this paper, we present a liquid-crystal-polymer (LCP)-based dual-layer micro-quarter-wave-retarder (MQWR) array for active polarization image sensors. The proposed MQWRs, for the first time, enable the extraction of the incident lights circularly polarized components in the whole visible regime, which correspond to the fourth parameter of Stokes vector. Compared with the previous implementations, our proposed MQWRs feature high achromaticity, making their applications no longer limited to monochromatic illumination. In addition, the presented thin structure exhibits an overall thickness of 2.43μm, leading to greatly alleviated optical cross-talk between adjacent photo-sensing pixels. Moreover, the reported superior optical performance (e.g. minor transmittance, extinction ratio) validates our optical design and optimization of the proposed MQWRs. Furthermore, the demonstrated simple fabrication recipe offers a cost-effective solution for the monolithic integration between the proposed MQWR array and the commercial solid-state image sensors, which makes the multi-spectral full Stokes polarization imaging system on a single chip feasible.

Characterization of a 0.18

In this paper, a 0.18 mum triple-well CMOS color processing scheme is presented and characterized. The proposed on-chip sensor processing enables real-time skin detection, while removing the need for complex on-chip demosaicing circuitry. Reported experimental results validate the concept of a compact, low cost, CMOS fully compatible microsensor for skin detection.

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This paper presents a compact Address Event Representation AER-based CMOS image sensor for real-time focal-plane polarization imaging. The image sensor integrates a unique micropolarizer array, patterned using the well-controlled process of UV photolithography. Real-time Stokes parameters extraction is achieved using a novel time-to-first-spike (TFS) pixel architecture. The proposed implementation enables low power operation and efficient readout of polarization information. Moreover, it is scalable and well suited to the next generation of deep submicron CMOS technologies owing to decreased supply voltage and increased noise level.

m CMOS Color Processing Scheme for Skin Detection

In this paper, a CMOS digital pixel sensor integrating a novel photo-patterned micropolarizer array is proposed for real-time polarization imaging. The sensor is based on a compact Pulse Frequency Modulation (PFM) spiking pixel, which provides a relative immunity to the continuing scaling of power supply voltage in CMOS technologies. In contrast to previously reported implementations, the proposed micropolarizer array fabrication technology is not only simple but also completely removes the need for complex selective etching. Instead, we propose to use light to pattern micropolarizer elements at the pixel pitch. Reported results in 0.18 mum CMOS technology validate the concept of a compact, low cost, polarization image sensor integrated on a single chip.

An AER based CMOS polarization image sensor with photo-aligned micropolarizer array

In this paper, we report an on-chip gas sensor based on novel zinc oxide (ZnO) nanocombs for carbon monoxide (CO) sensing. With ZnO gas sensing nanocombs fully integrated on a single silicon chip, the concept of low cost complementary-metal-oxide-semiconductor (CMOS) microsensor capable of on-chip gas sensing and processing is enabled. Compared with all previous implementations, the proposed ZnO nanocombs feature much larger effective sensing area and exhibit ultra-high sensitivity even at the room temperature. Specifically, at room temperature, we demonstrate peak sensitivities as high as 7.22 and 8.93 for CO concentrations of 250 ppm and 500 ppm, respectively. As a result, by operating the proposed ZnO-nanocomb-based gas sensor at the room temperature, the widely adopted power consuming heating components are completely removed. This leads to not only great power saving, but also full compatibility between the gas sensor and the on-chip circuitry in term of acceptable operating temperature. In addition, the reported fast response/recovery time of ~200 s/~50 s (250 ppm CO) makes it well suited to real-life applications.

A CMOS digital pixel sensor with photo-patterned micropolarizer array for real-time focal-plane polarization imaging

Conventional two-step ADC for CMOS image sensor requires full resolution noise performance in the first stage single slope ADC, leading to high power consumption and large chip area. This paper presents an 11-bit two-step single slope/successive approximation register (SAR) ADC scheme for CMOS image sensor applications. The first stage single slope ADC generates a 3-bit data and 1 redundant bit. The redundant bit is combined with the following 8-bit SAR ADC output code using a proposed error correction algorithm. Instead of requiring full resolution noise performance, the first stage single slope circuit of the proposed ADC can tolerate up to 3.125% quantization noise. With the proposed error correction mechanism, the power consumption and chip area of the single slope ADC are significantly reduced. The prototype ADC is fabricated using 0.18 μm CMOS technology. The chip area of the proposed ADC is 7 μm × 500 μm. The measurement results show that the energy efficiency figure-of-merit (FOM) of the proposed ADC core is only 125 pJ/sample under 1.4 V power supply and the chip area efficiency is 84 k μm2 ·cycles/sample.

Ultra-High Sensitivity Zinc Oxide Nanocombs for On-Chip Room Temperature Carbon Monoxide Sensing

In this paper, a high-resolution micro-circular-polarization-analyzer (MCPA) array is presented and characterized. MCPAs for extracting right-handed and left-handed circularly polarized components of incident light are implemented by fabricating polymerizable-liquid-crystal (PLC) micro-retarders with perpendicular fast axes on top of a 45° laminated linear polarizing film. With sulfonic-dye-1 (SD1) as the PLC photoalignment material, an MCPA feature size as small as 5/Ltm is achieved. In addition, the concept of real-time active circular polarization image sensing and processing with targets illuminated by collimated circularly polarized light is enabled by the MCPA arrays on-chip integration with complementary metal-oxide-semiconductor (CMOS) image sensors.