Compact Zernike phase contrast x-ray microscopy using a single-element optic

We demonstrate Zernike phase contrast in a compact soft x-ray microscope using a single-element optic. The optic is a combined imaging zone plate and a Zernike phase plate and does not require any additional alignment or components. Contrast is increased and inversed in an image of a test object using the Zernike zone plate. This type of optic may be implemented into any existing x-ray microscope where phase contrast is of interest. © 2008 Optical Society of America OCIS codes: 340.0340, 340.7460. High-resolution x-ray microscopy is a wellestablished imaging technique. Since the refractive index at x-ray wavelengths is close to one, radial diffractive gratings, also known as Fresnel zone plates, are commonly used as microscope objectives, and up to 15 nm resolution can be achieved [1]. When using soft x-rays in the so-called water window 2.3–4.4 nm, wet samples up to 10 m in thickness can be imaged owing to a natural absorption contrast between carbon and water, suiting applications in, e.g., biology [2] and soil science [3]. Beside synchrotron storage rings, which are most often used as sources, other compact laboratory-based sources exist for x-ray microscopy. Here, laser-produced plasmas generate the x-ray radiation that makes the arrangement fit on an optical table [4]. In this Letter we present the implementation of Zernike phase contrast in a compact soft x-ray microscope using a single optical element. Absorption in the water window yields good image contrast between carbon and water. However, the primary result when x-rays interact with matter at this wavelength range is the phase shift of the radiation. This is clear when studying the complex index of refraction for x-ray wavelengths, given by n=1�+i. The describes the phase shift introduced by the object, and describes any absorption. The ratio of and is larger than 1 in the entire water window, indicating that contrast in soft x-ray microscopy would benefit from using this phase shift in phase contrast imaging. In fact, calculations have shown that the dose can be reduced by nearly a factor of 10 [5,6] when using phase contrast.