[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Apr 2009 · IEEE Transactions on Microwave Theory and Techniques
[Show abstract][Hide abstract] ABSTRACT: We study terahertz emission due to charge transport in inhomogeneous fields of semiconductor devices. The dependence on the applied field unveils individual constituents such as intervalley transfer of electrons and the contribution of hole transport.
[Show abstract][Hide abstract] ABSTRACT: We present a novel terahertz technique for detecting concealed objects by sensing their individual acoustic phase lags when they perform a minute oscillation. The technique is sensitive to virtually all materials.
[Show abstract][Hide abstract] ABSTRACT: We present a novel technique for detecting objects that are concealed by textiles. Items of virtually all materials can be sensed, which includes metals as well as dielectrics. Our technique detects the acoustic phase lag of objects when they follow a driven oscillation. The acoustic phase is measured interferometrically with continuous wave radiation at about 77 GHz. Typical oscillation amplitudes are about 1 mum, which is close to the human perception level. The technique provides no insight into the concealed item's material properties. It exclusively shows whether or not an object is hidden, which is the most relevant information for many sensing applications.
Full-text · Article · Apr 2008 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: We present an experimental study that provides insight into the charge carrier dynamics that lead to terahertz (THz) emission. We perform time-resolved THz experiments on semiinsulating GaAs structures with periodically poled surface electrodes. The individual charge carrier dynamics that contribute to the THz emission process are identified by their dependence on the applied field. Electron transport in inhomogeneous fields is found to dominate the emission of THz radiation. Other dynamics that become visible by their specific THz emission are the Dember effect, the intervalley transfer of electrons, and hole currents at high field strengths.
No preview · Article · Mar 2008 · IEEE Journal of Selected Topics in Quantum Electronics
[Show abstract][Hide abstract] ABSTRACT: A large aperture terahertz (THz) emitter is presented. The planar photoconducting GaAs device consists of a periodical interdigitated Schottky structure. Additional trenches in the GaAs have double periodicity. They are required for far-field THz emission. Terahertz field strengths of 15 V/cm were achieved.
Full-text · Article · Feb 2008 · Electronics Letters
[Show abstract][Hide abstract] ABSTRACT: We report on apertureless terahertz (THz) microscopy and its application for semiconductor characterization. Extreme subwavelength
resolutions down to 150nm are achieved with few-cycle THz pulses having a bandwidth of 3THz. The imaging mechanism is characterize
by time-resolved THz techniques. We find that apertureless THz microscopy can be well described by the electronic resonance
of the scanning-tip interacting with the sample’s surface. The capacitance between tip and surface is a key parameter, which
provides insight into the local high frequency permittivity of the semiconductor structure. Applying electromodulation techniques
allows for imaging electronic charge distributions in microstructured semiconductors. The sensitivity of THz microscopy suffices
to detect as few as about 5000 electrons.
[Show abstract][Hide abstract] ABSTRACT: 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 dielectric permittivity of the surface.
Full-text · Article · Dec 2007 · Review of Scientific Instruments
[Show abstract][Hide abstract] ABSTRACT: 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 data demonstrate that virtually all objects can be sensed, which includes metals as well as dielectric materials. (c) 2007 Optical Society of America.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We report on THz microscopy for sensing charge carrier distributions in semiconductors. The contactless technique gives insight into the Drude response of electrons with a bandwidth of about 2.5 THz. We have achieved a spatial resolution of about 1.8 mum, which corresponds to a sensitivity sufficient to detect as few as 5000 electrons.
[Show abstract][Hide abstract] ABSTRACT: In this contribution we present a novel far infrared technique that is capable to detect concealed objects of virtually all materials. In our approach, we detect the acoustic phase of the objects when they follow a driven oscillation. This acoustic phase can be measured inferferometrically with continuous wave gigahertz radiation or by time resolved terahertz techniques. In both cases objects can be located behind several layers of textiles (Buersgens et al., 2007; and Chen and Kaushik, 2007). The presented technique gives no insight into the item's material properties. It exclusively shows whether or not an object is concealed, which is the most relevant information for many sensing applications.
[Show abstract][Hide abstract] ABSTRACT: We report on the application of apertureless THz near-field microscopy for sensing charge carriers in semiconductors. This technique allows for contactless probing of electron concentrations on a micrometer scale. Experimental data and model calculations indicate that as few as about 5000 electrons can be detected in a GaAs structure.
Preview · Article · Mar 2006 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: The recent development of the apertureless terahertz scanning near-field optical microscope (THz-SNOM) allows for submicron spatial resolution  and enables a broad variety of novel applications in material characterization. The basic mechanism is that a metallic probe allows for mapping of the THz permittivity of a surface. In this contribution, we report on measurements of microscopic scale charge carrier distributions and dielectric contrast with sub-micrometer resolution in various material systems and structures using THz-SNOM. We have identified a novel imaging mechanism in terms of a configurational resonance , which contrasts the widely accepted scattering model at visible and near-infrared frequencies.  H.-T. Chen, et al., Appl. Phys. Lett. 83, 3009 (2003).  H.-T. Chen, et al., Phys. Rev. Lett. 93, 267401 (2004).
[Show abstract][Hide abstract] ABSTRACT: We report on the application of apertureless THz microscopy for sensing charge carrier distributions in semiconductors on submicron scales. A spatial resolution of less than 1 micron allows for the detection of about 1000 electrons.