W. Krieger

Decay Length of Surface Plasmons Determined with a Tunnelling Microscope

A tunnelling microscope is used to detect surface plasmons (SP) on a silver-vacuum interface excited by a He-Ne laser. The decay length of the SP is determined experimentally, and the detection mechanism of the SP in the tunnelling microscope is discussed.

Optical mixing of CO2‐laser radiation in a scanning tunneling microscope

Two infrared laser beams coupled into the tunneling junction of a scanning tunneling microscope lead to the generation of a signal at the difference frequency. In this article it is described that two different frequency mixing mechanisms are responsible for this process. One part of the signal is generated through a mixing process owing to the nonlinearity in the static current‐voltage characteristic. Another part has its origin in a nonlinear susceptibility at the surface; it therefore corresponds to frequency mixing in nonlinear optics. It will be shown that the difference‐frequency signals generated by the two processes can be separated owing to their different dependence on the tip‐sample distance.

Laser-frequency mixing in a scanning tunneling microscope at 1.3 μm

The radiation of two single-mode diode lasers at 1.3 μm is focused into the tunneling junction of a scanning tunneling microscope, and gigahertz difference-frequency signals radiated from the tip are detected. Simultaneous measurements of the bias-voltage dependence of the mixing signal and the tunneling current for different surface samples show that the mixing process is due to the nonlinearity of the static current–voltage characteristic of the tunneling junction. The coupling of the laser radiation into the junction conforms to antenna theory. The experimental results are compared with previous measurements at a laser wavelength of 9.3 μm. Surface images produced by means of the difference-frequency signal show the chemical contrast between micron-sized Au islands and a graphite substrate.

Detection of local conductivity by laser‐frequency mixing in a scanning force microscope

Coupling of laser radiation into a conducting tip of a scanning force microscope allows one to distinguish between conducting and nonconducting parts of a sample. This is demonstrated for a pattern of small metal islands on a nonconducting BaF2 substrate. In the experiment two infrared laser beams are coupled into the tip. The difference frequency is generated in the tip‐sample junction, emitted, and detected by means of an open waveguide. Images with this signal are recorded simultaneously with the topography. Difference‐frequency generation is observed only on conducting parts of the surface and at islands larger than about 1 μm in diameter. The size of the conducting island as well as the tunneling distance between the tip and conducting surface determine the magnitude of the difference‐frequency signal. Frequency mixing of visible laser light, using the excitation of localized plasmons for field enhancement, may lead to the detection of local conductivity on much smaller structures.

The generation of laser difference frequencies using the scanning tunneling microscope

Infrared laser radiation and radio frequency signals were coupled into the tunneling junction of a scanning tunneling microscope (STM), and the difference frequencies and their harmonics generated in the nonlinear junction were studied as a function of the tunneling parameters. Difference frequencies up to 9 GHz were observed. The importance of the results for the difference frequency mixing in metal-insulator-metal (MIM) diodes and for the STM is discussed. The experiments demonstrate that the tunneling junction of the STM can be used to generate difference frequencies of injected electromagnetic radiation in a similar way and with a similar efficiency to those of the MIM diodes. Due to the precise controllability of the tunneling parameters in the STM the frequency mixing process in its tunneling junction is highly reproducible. >

DETERMINATION OF THE PROPAGATION LENGTH OF SURFACE PLASMONS WITH THE SCANNING TUNNELING MICROSCOPE

Abstract A scanning tunneling microscope is used to measure the propagation length of surface plasmons generated on thin silver and gold films by laser radiation with wavelengths of 633 nm and between 2.4 and 2.8 μm, respectively. From the values obtained the optical constants of the metal films at the corresponding wavelengths are determined. A decrease of the propagation length observed at wavelengths larger than 2.6 μm is explained by the presence of a water layer adsorbed on the gold film at ambient air pressure.

A Laser‐Driven Scanning Tunneling Microscope

New modes of operation of a scanning tunneling microscope (STM) with infrared laser radiation coupled into the tip of the tunneling junction are demonstrated. Difference frequency generation of two laser beams is observed to be caused by the nonlinearity in the current‐voltage characteristic of the junction. The dc current produced by laser‐light rectification and the difference frequency signal are used to obtain atomic resolution surface images of graphite, as well as to control the tip‐sample distance. Such a laser‐driven STM can generate surface images without external bias voltage, and‐when the difference frequency signal is used for the distance control‐even without any dc current between tip and sample. This mode of operation may also allow to study surfaces of insulators.

Ferrimagnetic resonance excitation by light-wave mixing in a scanning tunneling microscope

Ferrimagnetic resonance is measured in a scanning tunneling microscope. The infrared light of two lasers is focused into the tunneling junction and a difference-frequency signal in the microwave region is generated. This microwave signal is used to excite spin waves in an yttrium–iron–garnet film with a thin Au capping. The coupling of the light to the tunneling junction is explained by an antenna mechanism. Characteristic antenna patterns of the angle-dependent receiving efficiency are obtained. The mixing of the two laser frequencies is due to the nonlinearity of the tunneling junction. The microwave signal obtained is absorbed in the ferromagnetic sample if the resonance condition is fulfilled. This method might allow the measurement of magnetic properties with a lateral resolution down to the nm scale.

A New Mechanism for Laser-Frequency Mixing in a Scanning Tunneling Microscope

Two infrared laser frequencies coupled into the tunneling junction of a scanning tunneling microscope (STM) lead to the generation of a difference-frequency signal. In this article two different mechanisms of difference-frequency generation (DFG) in the tunneling junction are presented. Besides DFG due to the nonlinearity of the current-voltage characteristic known from previous experiments, nonlinear optical DFG arising from the breakdown of the inversion symmetry at the metal-vacuum interface is demonstrated. This nonlinear optical DFG may be a step further towards laser spectroscopy with an STM.