Photonic Spin-Multiplexing Metasurface for Switchable Spiral Phase Contrast Imaging

Abstract

As the two most representative operation modes in an optical imaging system, bright-field imaging and phase contrast imaging can extract different morphological information on an object. Developing a miniature and low-cost system capable of switching between these two imaging modes is thus very attractive for a number of applications, such as biomedical imaging. Here, we propose and demonstrate that a Fourier transform setup incorporating an all-dielectric metasurface can perform a two-dimensional spatial differentiation operation and thus achieve isotropic edge detection. In addition, the metasurface can provide two spin-dependent, uncorrelated phase profiles across the entire visible spectrum. Therefore, based on the spin-state of incident light, the system can be used for either diffraction-limited bright-field imaging or isotropic edge-enhanced phase contrast imaging. Combined with the advantages of planar architecture and ultrathin thickness of the metasurface, we envision this approach may open new vistas in the very interdisciplinary field of imaging and microscopy.

Keywords: Metasurface; edge detection; orbital angular momentum; phase contrast imaging.

Figure 1.

(a) Schematic of the Fourier transform setup for spatial filtering. An object (b) illuminated with a plane wave will diffract the light to outer regions in the Fourier plane, where the metasurface is placed. With a constant phase (c) or an ideal spiral phase (d) implemented in the metasurface, the calculated results of the image plane show a traditional bright image and an edge enhanced image. (e) and (f) Schematic of the concept for spin-dependent function control. (e) For light incident on the device with LCP, the metasurface imprints a masking function (M₁) the output beam resulting in a constant phase profile and a Gaussian intensity distribution and flips the handedness of the incident polarization. For light incident on the same device with RCP, the metasurface imprints another masking function (M₂), resulting in a spiral phase profile and a donut-shaped intensity distribution, and again flips the handedness of the polarization. (g) Schematic of the designed all-dielectric metasurface spatial filter to implement the function in (e) and (f). Inset: perspective and top view of the metasurface unit-cell formed by amorphous TiO2 nanopillar sitting on a silica substrate. (h) Optical photograph of the fabricated metasurface device. Scale bar: 500 μm. (i) Top view and oblique view of the SEM images of TiO2 nanopillar array. Scale bar: 1 μm.

Figure 2.

Simulated and experimental demonstration of the two independent masking functions imposed on the spin-dependent metasurface spatial filter. Upper: simulated output states of the metasurface spatial filter at the wavelength of 530 nm. An intensity distribution (a) and a phase profile (b) shows a Gaussian beam for LCP incident light, the donut intensity distribution (c) and the spiral phase profile (d) shows a 1ħ OAM beam for RCP incident light. Below shows measured intensity distributions (e), (g) and non-paraxial interference patterns with a plane wavefront (f), (h) of output states corresponding to LCP and RCP incident light at the wavelength of 530 nm, respectively. Insets show the handedness of incident light.

Figure 3.

(a) Sketch of the experimental setup for spin-dependent spiral phase contrast imaging. LP and QWP can make the incident light with circular polarization. CP can be used to eliminate background light in the optical path. (b)-(e) The bright-field image of the resolution test chart under the LCP incident light at the wavelength of 480 nm, 530 nm, 580 nm, 630 nm, respectively. (f)-(i) Spiral phase contrast images of the resolution test chart under the RCP incident light at the same wavelengths. Insets show the handedness of incident light. Scale bars: 100 μm.

Figure 4.

Images of the undyed onion epidermal cells with a 20 × objective lens. (a)-(d) Traditional bright-field images captured with LCP incident light at the wavelength of 480 nm, 530 nm, 580 nm, 630 nm, respectively. (e)-(h) Spiral phase contrast images captured with RCP incident light at the corresponding wavelengths. Scale bar: 100 μm.