Emission characteristics of proton implanted vertical cavity surface emitting lasers have been studied by spectrally-resolved near-field scanning optical microscopy (NSOM). Measured spatial intensity distributions of individual transverse modes show a fundamental mode at threshold, and two double-lobed first-order modes excited around aperture edge above1.3×threshold injection current. The use of high spectral and spatial resolution NSOM enabled the detection of a local resonance wavelength dependence on position across the laser aperture; from these data, a lateral refractive index variation is calculated. As the injection current is increased, these index distributions are found to be consistent with the observed reduction of the spatial width of the intensity distribution and the increase in the spectral mode spacing. ©1998 American Institute of Physics.