Ball lens-assisted smartphone microscopy with diffraction-limited resolution

It is shown that the resolution of portable and lightweight smartphone-based microscopy can be increased up to the diffraction limit by using imaging through ball lenses with refractive index sufficiently close to n=2 under contact conditions with nanoplasmonic and biomedical samples. It is demonstrated that the millimeter-scale ball lenses made from LASFN35 glass with n=2.02 at lambda = 600 nm allow achieving extraordinarily high image magnification up to 50x. Under these conditions, it is possible to move away from the conventional resolution limitations of cellphone microscopy determined by the pixilation of the images and to reach the diffraction-limited resolution similar to 600 nm. It is shown that the dispersion of n allows tuning of magnification in a very broad range. The magnification values are explained by the exact numerical solution of the Maxwell equations and a good agreement with the experiment is demonstrated. We performed smartphone imaging of different biomedical samples such as melanoma and human aorta and demonstrated that the quality of imaging is comparable to that in conventional microscopy with the 10x objectives. The in-principle possibility is suggested of melanoma diagnostics based on observation of distribution of lymphocyte cells by application of cellphone microscope to the patient's skin without a need to make histological samples.