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The contrast images based on the ratios of ν2PO4 3−/Amide III (a, e), ν4PO4 3−/Amide III (b, f), ν1CO3 2−/ν2PO4 3− (c, g) and ν1PO4 3−/Amide I (d, h). Arrows indicate polarization of laser light, and the color bar displays the maximum ratio for each image
