Background-free imaging of luminescent nanodiamonds using external magnetic field for contrast enhancement.

We demonstrate a way of contrast increasing in wide-field optical images of diamond nanocrystals containing nitrogen-vacancy (NV) centers surrounded by an autofluorescing material, as in the case of intercellular imaging. Subtraction of an image taken in the presence of an external magnetic field of about 25 mT from an image of the same area measured when the field was absent eliminates the background and enhances the contrast because luminescence of the NV centers decreases in the presence of the field while the auto luminescence is not affected.