THz Wave Near-Field Imaging

Abstract

THz waves offer innovative imaging and sensing capabilities for applications in material characterization, microelectronics, medical diagnosis, environmental control, and chemical and biological identification. However, the spatial resolution of conventional THz imaging technique is limited by diffraction of THz waves to be in the same order as THz wavelength (1 THz = 300 μm). This diffraction limit is an obstacle for using THz technology in probing the electronic and optical properties of semiconductor and bimolecular nanostructures. Several approaches have been used to obtain a sub-wavelength spatial resolution based on near-field techniques. One way to overcome diffraction is to use a sub-wavelength size aperture to limit the detection or generation area. This technique is known as apertured THz wave near-field microscopy. The aperture could be a static aperture made on a metallic screen or a dynamic one excited by an optical beam. Localized THz wave emitter or sensor based on real or virtual instant photocurrent excited by a highly focused optical beam can also provide spatial resolution much finer than THz wavelength. Another way, called apertureless THz near-field microscopy, use a sharp tip as local field enhancer which scatters the evanescent light in the near-field region of the target to make it detectable in the far field, and provide a spatial resolution well below the diffraction limit. Last but not least, THz wave emission microscope based on STM technique can achieve a nanometer resolution. A pulsed laser is used to generate photo-carriers on the semiconductor surface and a biased scanning-tunneling-microscope (STM) needle is used to modulate the localized electric field in the Schottky barrier under the tip. The transient photo-carriers driven by the modulated field emit THz waves, which can be detected at the modulated frequency in the far field. THz wave near-filed microscopy described above represents a milestone toward THz wave spectroscopic imaging of materials and devices at nanometer, sub-nanometer, and even atomic scales.