H. Park, J. Kim, M. Kim, and H. Han
Department of Electrical and Computer Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea
Department of Electrical and Computer Engineering, Ajou University, Suwon 443-749, Korea
Abstract- We report a terahertz pulse apertureless nearfield microscope (THz NFM) which combines THz time domain spectroscopy (THz-TDS) and atomic force microscopy (AFM)techniques. By scanning the probe over a GaAs/Au edge, it is found that the THz NFM has a lateral resolution of ～80 nm.
Recently THz science and technology has been attracting a lot of attention for various applications such as THz imaging and characterization of materials by THz time-domain spectroscopy (THz TDS) . THz TDS provides powerful means to probe the electronic and optical properties of both semiconductor and biomolecular nanostructures. However,the spatial resolutions of conventional THz TDS systems are limited by diffraction.
In this study, we report a THz pulse apertureless near-field microscope (THz NFM), which combines THz TDS with atomic force microscopy (AFM) technique. THz NFM provides THz spectral imaging as well as topography of the sample surface [2-5]. THz near-field is scattered by a probe tip, and the large background THz signal is eliminated by a lock-in detection based on the modulation of the tip-sample distance. By scanning the probe over a GaAs/Au edge, it is found that the THz microscope has a lateral resolution of ～80 nm.
II. EXPERIMENTS AND RESULTS
The samples were prepared by evaporating a 35 nm thick gold (Au) film on the half surface of a GaAs substrate. A 1mm long probe tip was fabricated by electrochemically etching a 50 um diameter tungsten wire and the etched tip radius was ～25 nm. The probe tip was mounted parallel to one prong of a quartz crystal tuning fork so that the tip oscillates perpendicularly to the sample surface. As the tip approaches the sample surface, the amplitude, phase, and resonant frequency of the tuning fork’s oscillation change. In our experiment, the amplitude parameter was used as a source of the feedback signal for tip-to-sample distance regulation, and maintained constant by controlling the piezo actuator in order to track the sample surface. The THz tip signal was detected by a THz photoconductive antenna, and simultaneously the AFM image of the sample surface was obtained from the feedback signal for the piezo actuator.
The experimental setup of THz NFM is similar to that of the conventional THz TDS except the AFM setup. A femtosecond (fs) Ti:Sapphire laser is used, which operates at 790 nm center wavelength with 80 fs pulse width to generate THz wave from InAs wafer. The generated THz wave is illuminated on the tip-sample system with the incidence angle of 60° and the focused beam diameter of 500 um. The incident THz wave generates the tip signal by scattering THz near-field in the vicinity of the tip apex. The scattered tip signal and the specularly reflected THz signal were measured by photoconductive antenna. The tip signal modulated by dithering the probe tip is recorded by a lock-in amplifier and the background THz signal is eliminated.
The measured time-domain tip signal from the Au surface was larger than that from the GaAs surface since the tip signal is dependent on the magnitude of the dipole induced in the material. The spatial resolution of THz NFM was estimated by scanning cross the GaAs/Au edge. The AFM and THz near-field images of the one-dimensional (1D) step were measured by keeping constantly the tip-sample distance.From the AFM image, the thickness of the Au film was confirmed in ~ 35 nm. A 10 % - 90 % lateral resolution of ～80 nm for THz NFM was achieved.
We developed the THz NFM system by combining AFM and THz TDS techniques, and directly measured the tip scattering signal of THz near-field. It was found that THz NFM can have the lateral resolution less than 100 nm.
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