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A near-field scanning optical microscope was used in collection mode to generate spatially resolved maps of the first derivative with respect to operating current of power emission from vertical-cavity surface-emitting lasers (VCSELs). This technique is highly sensitive to the formation of new modes and can be used to identify mode cutoff points. An example of the usefulness of this technique is demonstrated as we estimate the index of refraction profile of the VCSEL under study at the single mode cutoff point. This profile is a primary feature of the waveguiding characteristics of such lasers. © 2002 American Institute of Physics.