R.H.M. Groeneveld

The capacitive origin of the picosecond electrical transients detected by a photoconductively gated scanning tunneling microscope

We present experimental results and numerical calculations on the detection of picosecond electrical transients by a photoconductively gated scanning tunneling microscope. We show that the transient signal detected in the tunneling regime is coupled by the 5 fF geometric capacitance between tip and sample leading to a correlation current that is linearly proportional to the tunnel conductance.

Near‐field magneto‐optical imaging in scanning tunneling microscopy

Images of magnetic bits written in a Pt/Co multilayer are presented. Using photosensitive semiconducting tips in a scanning tunneling microscope, both surface topography as well as polarization‐dependent optical transmission are measured. Magnetic contrast is achieved by detection of the Faraday effect. Magneto‐optical lateral resolution of 250 nm is demonstrated.

Wavelength dependence of the magnetic resolution of the magneto-optical near-field scanning tunneling microscope

A magneto-optical near-field scanning tunneling microscope is used to image the prewritten magnetic domain structure of a Pt/Co multilayer. A semiconducting tip acts as a local photodetector to measure the magnetic circular dichroism signal coming from the magnetic sample. The resolution of the magnetic imaging is given by the photoelectrically active volume of the tip. Reduction of the laser light wavelength resulted in a factor of 4 improvement of the magnetic resolution. Based on a sound and applicable definition we estimate the resolution to be (60±35) nm for a wavelength of 532 nm.

Scanning tunneling microscope for magneto-optical imaging

Images of magnetic bits written in a Pt/Co multilayer are presented. Using photosensitive semiconducting tips in a scanning tunneling microscope the surface topography as well as the polarization‐dependent optical transmission are measured. Magnetic contrast is achieved by detection of the Faraday effect. Magneto‐optical lateral resolution of 250 nm is demonstrated.

Detection of picosecond electrical transients in a scanning tunneling microscope

We have developed a scanning tunneling microscope using an optoelectronic switch which gates the tunneling tip current. The switch is fabricated within several tens of microns from the tip by photolithography and an accurate cleavage method. We demonstrate this approach by detecting picosecond electrical transients on a coplanar stripline. We have investigated the signal dependence on contact resistance and found significant differences when the tip is brought from low-ohmic contact into the tunneling regime.

Modulated Photodetection with Semiconductor Tips in a Scanning Tunneling Microscope

We report on the detection of modulated light power irradiated into the tunnel junction of a scanning tunneling microscope. When semiconductor tips are used we can distinguish three contributions to the measured current: photocurrent due to electron-hole pair generation at the apex of the tip, a contribution of thermal tip-sample distance modulation, and a displacement current due to the surface photovoltage that develops on the tip surface. Using a simple model we can understand the phases of the detected signals with respect to the photoexcitation, and extract values for several capacitances and conductances involved in the tunnel junction. This is relevant for applications of high-frequency photoexcitation in scanned probe microscopies.

Photoconductive sampling with a scanning tunneling microscope

An experimental equivalent of the lumped circuit model of a photoconductively gated scanning tunneling microscope (PG-STM) is studied. This model can be used as long as the distance between the tunnel-junction and the pc-switch is small. For suitable parameters it implies a response-time below the carrier lifetime in the pc-switch.