Apertureless Scanning Terahertz Near Field Microscopy

Abstract

A terahertz reflection spectrometer was set up and combined with a scanning tunneling microscope into an apertureless scanning near field microscope in order to achieve a lateral resolution better than the diffraction limit.The experimental basics and the setup of the scanning near field microscope are described. The THz microscope is based on THz time domain spectroscopy using ultrashort laser pulses. For generation and detection of the THz pulses a large area microstructured photoconductive switch and an electro-optic crystal was employed, respectively.Two different methods for generation of the time delay necessary for time domain spectroscopy were realized. In the ‘classical’ setup, a mechanical delay stage was employed. In the second setup, the pump–probe technique was realized using “asynchronous optical sampling” (ASOPS), enabling faster data acquisition.Reflection measurements using the THz time domain spectrometer were presented. The reflectivity data of caesium iodide were compared to literature data in order to test the spectrometer setup. In addition, low temperature reflectivity measurements on blue bronze were performed, showing the presence of the charge density wave state below the critical temperature.Measurements performed with the scanning near field microscope were presented. The samples investigated were gold structures deposited on glass or semiconductor substrates. Lateral resolutions better than the diffraction limit were achieved during scanning in the tunneling mode as well as in the constant height mode: The obtained resolution corresponds to 1/300 of the central wavelength of the THz pulse and was limited by the accuracy of the motors of the sample translation stages. A test measurement on a doped semiconductor sample, where the tip was moved instead of the sample, showed that higher resolutions are possible.Lateral scans displayed additional modulations when the THz field was recorded at different time delays after the main peak of the THz transient. From the comparison of experimental data and simulations it can be concluded that these modulations cannot be explained by a dipole-like excitation of the metallic structure alone. They were attributed to the influence of the metallic STM tip and explained by a simplified model of the interaction between THz pulse, sample and tip.