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We characterize terahertz metamaterials by applying apertureless near-field microscopy with a bandwidth that covers the entire spectral response of the structures. The observations agree with the interpretation of the fundamental mode of the metamaterial. But the high frequency resonance shows properties that deviate from the common interpretation. We show… (More)
We report on apertureless near-field microscopy in the far infrared. We identify a configurational resonance of the scanning tip-surface system to be the dominating mechanism that forms the image. Experimental data such as the high imaging contrast and its spectral properties can be well explained and make the framework of a mesoscopic resonance an… (More)
We report on acoustic phase imaging of objects using terahertz radiation. The sensitivity of the technique is sufficient to detect objects at oscillation amplitudes down to about 300 nm. Such acoustic amplitudes are comparable to the human physiological perception level, which offers novel opportunities in security imaging.
We developed a far infrared technique for detecting objects that are concealed behind a barrier, which is opaque for visible light. The technique makes use of the fact that mechanically coupled objects can be distinguished by their acoustic phase during a driven vibration. The pattern of the acoustic phase is imaged using 70 GHz radiation. The experimental… (More)
We report on 2-D acoustic phase imaging with millimeter-wave radiation for locating concealed objects. Active adaptation of the imaging interferometer provides enhanced images. We demonstrate that the method allows for finding metallic as well as dielectric materials. Even objects having similar optical properties as the background can be located.
We report on the advancement of apertureless terahertz microscopy by active shear force control of the scanning probe. Extreme subwavelength spatial resolution and a maximized image contrast are achieved by maintaining a tip-surface distance of about 20 nm. The constant distance between scanning tip and surface results in terahertz images that mirror the… (More)
We present a time-resolved technique to measure optical excitation processes with a time resolution shorter than the oscillation period of the exciting light. Our terahertz (THz) experiments fully resolve the polarization dynamics of electrons in semiconductor heterostructures when they are excited by a THz pulse. The time resolution of the polarization… (More)
We report on the time-domain differentiation of light waves by metallic transmission gratings. Time-resolved terahertz experiments show that the first time derivative of an arbitrary waveform can be achieved by use of gratings of subwavelength period. The results are in accord with classical diffraction theory and may permit novel applications for tailoring… (More)
We report on the development of a terahertz time-domain technique for measuring the momentum relaxation time of charge carriers in ultrathin semiconductor layers. Making use of the Drude model, our phase sensitive modulation technique directly provides the relaxation time. Time-resolved THz experiments were performed on n-doped GaAs and show precise… (More)