Super-resolution coherent anti-Stokes Raman scattering microscopy with photonic nanojets

We demonstrate far-field super-resolution coherent anti-Stokes Raman scattering (CARS) microscopy by exciting the sample with photonic nanojets. The sub-diffraction photonic nanojets were formed on the surface of the sample by microspheres illuminated by laser beams, and images were acquired by a standard laser-scanning CARS microscope. When the laser beams were focused on the microspheres, the photonic nanojets determined the excitation volume instead of the diffraction-limited laser foci, leading to super-resolution. We imaged the sub-diffraction features of a Blu-ray disc using glass spheres with a refractive index of 1.46 and diameters in the 1-6 µm range. The microspheres provided a lateral magnification factor up to 5.0X and a lateral resolution of at least 200 nm at 796 nm laser wavelength, allowing us to resolve the features on the disc which were invisible under normal CARS imaging. The magnification factor depended on both the microsphere size and the focal plane position of the incident beams. To explain the magnification factor we performed theoretical simulations which showed excellent agreement with experimental results. This super-resolution technique could be very useful for the vibrational imaging of nano-scale objects on films and surfaces.