Jason A. Deibel

Imaging with terahertz radiation

Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

Enhanced coupling of terahertz radiation to cylindrical wire waveguides

Wire waveguides have recently been shown to be valuable for transporting pulsed terahertz radiation. This technique relies on the use of a scattering mechanism for input coupling. A radially polarized surface wave is excited when a linearly polarized terahertz pulse is focused on the gap between the wire waveguide and another metal structure. We calculate the input coupling efficiency using a simulation based on the Finite Element Method (FEM). Additional FEM results indicate that enhanced coupling efficiency can be achieved through the use of a radially symmetric photoconductive antenna. Experimental results confirm that such an antenna can generate terahertz radiation which couples to the radial waveguide mode with greatly improved efficiency.

Finite-Element Method Simulations of Guided Wave Phenomena at Terahertz Frequencies

As the science and engineering associated with terahertz time-domain spectroscopy and imaging evolves past the use of conventional free-space optics, the continued development of waveguides for terahertz pulses is increasingly relevant. The ability to model and simulate terahertz wave propagation aids in the development, visualization, and understanding of novel terahertz devices and phenomena. We discuss the use of the finite-element method, a powerful computational tool for the modeling of guided wave phenomena and devices at terahertz frequencies.

Frequency-dependent radiation patterns emitted by THz plasmons on finite length cylindrical metal wires

We report on the emission patterns from THz plasmons propagating towards the end of cylindrical metal waveguides. Such waveguides exhibit low loss and dispersion, but little is known about the dynamics of the terahertz radiation at the end of the waveguide, specifically in the near- and intermediate-field. Our experimental results and numerical simulations show that the near- and intermediate-field terahertz spectra, measured at the end of the waveguide, vary with the position relative to the waveguide. This is explained by the frequency-dependent diffraction occurring at the end of the cylindrical waveguide. Our results show that near-field changes in the frequency content of THz pulses for increasing wire-detector distances must be taken into account when studying surface waves on cylindrical waveguides.

The metal-insulator transition in VO2 studied using terahertz apertureless near-field microscopy

We have studied the metal-insulator transition in a vanadium dioxide (VO2) thin film using terahertz apertureless near-field optical microscopy. We observe a variation of the terahertz amplitude due to the phase transition induced by an applied voltage across the sample. The change of the terahertz signal is related to the abrupt change of the conductivity of the VO2 film at the metal-insulator transition. The subwavelength spatial resolution of this near-field microscopy makes it possible to detect signatures of micron-scale metallic domains in inhomogeneous VO2 thin films.

Nondestructive terahertz imaging for aerospace applications

The full potential of terahertz imaging systems for nondestructive aerospace imaging applications has not been realized due to the lack of data linking damage and defects to terahertz signatures coupled with the complexity of modeling the signatures. Terahertz systems (0.1 - 2.0 THz) may be ideally suited for NDI applications because of the ability of THz radiation to penetrate through substances commonly found on the surfaces of aircraft structures while maintaining the optical resolution required to detect defects. We will discuss several systems that we have used to study the signatures of a set of target samples with known defects.

A terahertz dual wire waveguide

We numerically model the propagation of terahertz radiation along a double wire waveguide using finite element analysis. This is a promising alternative configuration for terahertz waveguiding.

Temperature dependence of terahertz emission from InMnAs

We have studied the terahertz radiation emitted by the dilute magnetic semiconductor InMnAs under femtosecond laser illumination, as a function of temperature. We observe a reversal of the polarity of the emitted terahertz field as the temperature is decreased, in both p-type and n-type materials. This effect is not observed in p-InAs. A competition between two oppositely directed currents, the photo-Dember current and the surface-field-induced current, can explain the observed polarity reversal. In contrast to nonmagnetic materials, these two currents are of comparable magnitude because the Mn impurity substantially reduces the carrier mobility in dilute magnetic semiconductors.

A technique to measure optical properties of brownout clouds for modeling terahertz propagation

Brownout, the loss of visibility caused by dust resultant of helicopter downwash, is a factor in the large majority of military helicopter accidents. As terahertz radiation readily propagates through the associated dust aerosols and is attenuated by atmospheric water vapor within short distances, it can provide low-profile imaging that improves effective pilot visibility. In order to model this application of terahertz imaging, it is necessary to determine the optical properties of obscurants at these frequencies. We present here a method of empirical calculation and experimental measurement of the complex refractive index of the obscuring aerosols. Results derived from terahertz time-domain spectral measurements are incorporated into the AFIT CDE Laser Environmental Effects Definition and Reference (LEEDR) software.

Non-Destructive Evaluation of Aerospace Materials using Terahertz Time-Domain Imaging

THz time-domain imaging was utilized in the non-destructive evaluation of various composite materials used in aerospace structures and components for the characterization of defects and changes in material properties due to mechanical and thermal strain.