University of Delaware - College of Engineering


Photonics & Electromagnetics

Devices and Imaging in the High-Terahertz Band

Keith Goossen

Penetrating imaging, that is taking an image through an intervening material, can take on many forms depending upon what the intervening material. In its simplest sense, penetrating imaging applies a portion of the electromagnetic spectrum for which the intervening material is transparent. Most materials have an absorption band in the visible-ultraviolet band due to excitation of electrons to higher atomic energy levels, and in the infrared due to vibration of molecules. Water, for example, is opaque in the ultraviolet and in the infrared, which is probably why we see in the visible, since our eyeballs are filled with water. Water also absorbs high frequency radio waves, by increasing its rotational energy, (which is how a microwave oven works), and in fact is opaque from several GHz into the infrared. Since most materials contain water, imaging for them is limited to low-frequency radar.

However, for dry materials, such as drywall, cardboard, and paper, there are extensive transmission bands throughout the THz, that portion of the spectrum between high-frequency radar and the infrared. Here we are exploring the highest range of the THz band, just below the vibrational absorption bands of most materials. By doing so we use the shortest wavelength of light possible, and thus obtain the sharpest resolution. Furthermore we can employ high sensitivity thermal imagers (specially designed to extend their minimum frequency) as well as silicon-based emitters, recently discovered here at the University of Delaware. One of our first applications is hoped to be imaging through envelopes in high resolution. Additionally, if filtering is introduced, we can also detect chemical signatures by varying the wavelength, and so provide not only an image but also identify where in a package or suitcase dangerous chemicals are placed.

Recent publications

P.-C. Lv, R.T. Troeger, S. Kim, S.K. Ray, K.W. Goossen, J. Kolodzey, I.N. Yassievich and M.S. Kagan, "Terahertz Emission from Electrically Pumped Gallium Doped Silicon Devices," Appl. Phys. Lett., vol. 85, pp. 366-368 (2004).

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