Terahertz Waves For Materials Evaluation, 14-9405

Principal Investigator
Todd Goyen

Inclusive Dates: 04/01/02 - 09/30/03

Background - Terahertz, or submillimeter, waves are electromagnetic radiation with wavelengths below approximately 300 µm, inhabiting a "window" in the electromagnetic spectrum between infrared light and microwaves. They are useful 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 a terahertz imaging and spectroscopy facility at the Institute, to serve as a demonstration site to address potential client needs. 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, offering the possibility to perform "time-domain" spectroscopy of materials.

Approach - The research team adopted the "canonical" approach to terahertz generation and detection, pioneered by groups at other well-known 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). The team used metal mirrors to collimate and focus terahertz radiation, to eliminate potential chromatic distortion of the beam. The terahertz beamline includes a collimated beam path for material absorption studies, and a focusing stage for confocal imaging through transparent specimens.

Accomplishments - The team has a working time-domain terahertz system (one of only a handful outside research universities) built and operating and has collected time-domain absorption spectra of materials, including plastics, carbon composite tiles from the Space Shuttle, wood, gypsum panels, concrete, and fiberglass-filled insulating foam. The terahertz pulses have approximately 2 THz bandwidth; higher frequencies are attenuated by phonon losses in the antenna substrate and silicon lenses. 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 delay line proved to be a critical element for setting up the experiment; the coherent detection scheme requires careful spatial overlap of pulses less than 300 µm long, propagating over several meters of optical path. The terahertz experiment is enclosed in a vinyl tent that can be flushed with nitrogen gas to reduce water vapor.

Pulsed Terahertz Spectroscopy System in SwRI Applied Physics Division

Terahertz pulse intensity as a function of time

Fast Fourier transform of Terahertz waves, illustrating spectral bandwidth of the pulses

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