Decreased depth of field in near-field microscopy

Summary form only given. Our work combining the equal pulse correlation (EPC) technique with the near-field scanning optical microscopy (NSOM) is, to our knowledge, the only femtosecond near-field measurement made by sending both "pump" and "probe" pulses through the NSOM tip. This arrangement may be advantageous when applying NSOM to "thick" samples, such as semiconductor materials and devices, because it appears to have a shallow depth of field built in to it. We present measurements that put an upper bound on the depth of field in this type of experiment and show the main features of the data can be qualitatively reproduced by a simple dipole model.