Lateral and axial sub-diffraction imaging using photonic crystal in MID-IR

Imaging with conventional optical systems has the limitation of the diffraction limit of the point spread function (PSF) where the maximum attainable resolution reaches down to half of the incident wavelength in lateral direction and down to the wavelength in axial direction. In fact, the tradeoff between the lateral and axial confinement is inevitable resulting in a deterioration of the 3D optical image. In this work, we report a metalens design attaining high resolution imaging in lateral and axial directions at lambda=3 mu m. The proposed design employs a 2D square photonic crystal lattice of silicon nanorods with an engineered optical cavity and a silver slit on top of the structure. Our design exhibits a simultaneous sub-diffraction limit in lateral and axial directions reaching down to 0.38 lambda and to 0.8 lambda respectively. The ideal PSF requires the equality of the full width at half maximum (FWHM) for both lateral and axial directions. Hence, the figure of merit (FOM) can be expressed as the ratio between the (FWHM) in lateral and axial directions approaching unity for ideal PSF. The proposed design demonstrates enhanced FOM, up to 0.47, compared to the confocal microscopy that shows FOM of 0.36. Our proposed design provides a great potential usage in the field of biomedical imaging and molecular dynamics.