Optical dipole model for photodetection in the near field

Near-field photodetection optical microscopy (NPOM) is a scanning probe tec hnique that has been developed to perform nanometer-scale optical intensity mapping and spectroscopy. In NPOM a nanometer-scale photodiode detector ab sorbs power directly as it is scanned in the near field of an illuminated s ample surface. A model of photodetection in the near and intermediate field s is presented. A brief review of far-field absorption is given for compari son. Far-field absorption measurements measure the imaginary part of the po larizability to first order. In contrast, photodetection in the near field measures the real part of the polarizability. Other aspects of near-field p hotodetection are also examined, including contrast mechanisms and lateral resolution. NPOM measurements performed on isolated 300-nm spheres show goo d agreement with the theory. (C) 2001 Optical Society of America.