We present a dual-frequency electromagnetic scanning probe and apply it for quantitative mapping of the sheet resistance of conducting films. The high-frequency (82 GHz) mode is used for image acquisition, while the low-frequency (5 MHz) mode is used for distance control. We measure magnitude and phase of the near-field microwave reflectivity from conducting films of varying thickness and develop a model which accounts fairly well for our results. This brings us to a quantitative understanding of the contrast in the microwave near-field imaging using an aperture probe, and allows us to achieve quantitative contactless characterization of conducting layers with sheet resistance even below 2 Ω.

We report a near-field microwave scanning probe which allows local phase-sensitive polarimetric measurements with a subwavelength spatial resolution (at least λ/16). The probe is a symmetrical transmitting/receiving antenna formed by two very narrow orthogonal slits fabricated at the end plate of a circular waveguide. A linearly polarized incident wave passes through one slit and is reflected from the sample mounted in the near field of the probe. A reflected wave with the incident polarization is received by the same slit, while the orthogonal polarization component is received by another, orthogonal slit. Our probe is very sensitive to local inhomogeneities and may be also useful for mapping Hall mobilities in semiconductors.

We report the phase locking of an oscillating GaAs/AlGaAs resonant tunneling diode (RTD) to a train of optical pulses. Locking was achieved by direct illumination of a specially designed RTD with a diode laser pulse train. Locking occurs at all rational frequency ratios and with excitation energies as low as 0.1 pJ/pulse for the principle lock.

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An inexpensive and easy experiment to measure the electrical resistance of high-T c superconductors as a function of temperature Am.We report a vacuum cryogenic ͑80 KϽTϽ350 K͒, near-field microwave scanning system based on a 90 GHz transmitting/receiving resonant slit antenna with a capacitive measurement of the probe-sample separation. The probe allows local measurement of resistance as a function of temperature with the spatial resolution of 20-50 m. The mm-wave probe is integrated with the eddy-current probe which allows global measurement of resistance of conducting and superconducting samples. This integrated probe is used for local study of the superconducting transition in high-T c superconducting thin films. The sensitivity of our present mm-wave probe is sufficient for probing conductivity in the normal state and in the superconducting state close to T c however not yet sufficient for probing conductivity variations far below superconducting transition temperature.