Extreme sub-wavelength resolution with a scanning radio-frequency transmission microscope

By scanning a metal grid sample between two coaxial transmission line tips, we resolve Delta x approximate to 10 mu m features using up to lambda = 20 m wavelengths, achieving wavelength-relative resolution Delta x/lambda < 5 X 10(-7) in both magnitude and phase. As an example of the technique's quite general application to conductivity-contrast microscopy, we measure the local transmittance of a GaAs/AlGaAs heterostructure versus magnetic field. We view this work as a proof-of-principle experiment which justifies scaling down the coaxial tip geometry to the sub-micrometer regime.