Development of Terahertz Waves for Materials Evaluation, 14-9405

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Principal Investigator
Todd Goyen

Inclusive Dates:  07/01/03 – 02/28/05

Background - Terahertz, or submillimeter, waves are electromagnetic radiation with wavelengths below 300 micrometers, inhabiting a window in the electromagnetic spectrum between infrared light and microwaves. They may find use in surveillance and nondestructive inspection because of their unique combination of low photon energy (non-ionizing) and penetration through most insulating materials. The goal of this project has been to develop terahertz expertise at the Institute to address potential needs of clients. Terahertz waves have been little studied until recently, in large measure because of technical difficulties in their generation and detection. Laboratory terahertz systems typically use ultrashort (femtosecond) laser pulses to trigger wide-band photoconductive transmitters. Similar antennas are used for coherent detection. Pulsed terahertz sources typically have wide bandwidth (~1THz), offering the possibility to performtime-domain spectroscopy of materials. 

Approach - The research team have adopted the "canonical" approach to terahertz generation and detection, pioneered by groups at other laboratories. Short pulses from a home-made titanium:sapphire femtosecond laser are incident on photoconductive switch antennas (used for generation and collection of terahertz radiation). We use metal mirrors to collimate and focus terahertz radiation to eliminate potential chromatic distortion of the beam. Our terahertz beamline includes a collimated beam path for material absorption studies and a focusing stage for confocal imaging through transparent specimens. 

Accomplishments - We built a time-domain, photoconductive switch antenna terahertz system and collected transmission images from test samples assembled in the lab. The system operates under computer control and includes a home-made "rapid dither" delay line to facilitate alignment of the laser and terahertz beams. This proved to be a critical element both for aligning the system and for facilitating imaging. The terahertz experiment is enclosed in a vinyl tent that can be flushed with nitrogen gas to reduce water vapor. Using near-field apertures, we obtained images of samples such as integrated circuit (IC) packages with spatial resolution below the diffraction limit. Although spectroscopy per se was not an expected accomplishment of the project, we did measure time-domain absorption spectra of materials, including plastics, carbon composite tiles from the space shuttle, and construction materials such as wood, gypsum sheetrock, concrete, and fiberglass-filled insulating foam. Characteristic spectral features in condensed matter typically appear at frequencies above 1THz up to the infrared region of the electromagnetic spectrum, while our terahertz pulses have bandwidth slightly greater than 1 THz limited by phonon losses in the antenna substrate and silicon lenses. 

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