Terahertz Imaging

There are numerous processes that occur in the far-infrared (FIR) region of the spectrum that have not been studied directly due to the lack of availability of ultrashort FIR pulses. Recent developments have yielded ultrashort FIR pulses, referred to as THz pulses (0.1 to about 6 THz), and the full capability of this technique is just starting to take off. Techniques once limited to the UV, visible and IR region can now begin to be applied to the FIR region as well. Attention has been focused on generating these THz pulses and understanding the physics of generation and propagation; now the actual THz pulses can be used as a spectroscopic tool. THz spectroscopy has applications in semiconductors, liquids, gases and 2-D imaging. Imaging is rapidly emerging as an exciting THz application, and images can be taken using transmission or reflection geometry. By analyzing the THz waveform in either the time domain (material homogeneity or thickness variations) or the frequency domain (frequency-dependent absorption) as well as by other methods, images identifying material properties can be constructed (J.V. Rudd, D. Zimdars, and M. Warmuth, Picometrix, Inc., "Compact, fiber-pigtailed, terahertz imaging system"). Polar liquids and gases are highly absorptive in the THz regime; therefore, these type of samples are readily suitable for THz imaging. Such imaging serves as a complement to existing imaging methods or allows substances that haven't been studied previously to be imaged. Recent examples of published THz imaging applications include: identifying raisins in a box of cereal by water content; studying water uptake and evaporation in leaves; examining circuit interconnects in packaged ICs; reading text in envelopes or beneath paint; identifying tooth decay; locating water marks in currency (also from J.V. Rudd et al, "Compact, fiber-pigtailed, terahertz imaging system"). The number of commercially available terahertz imaging systems is extremely few even though many applications are emerging for THz imaging. New techniques have been developed for the generation and detection of THz radiation based on frequency conversion using nonlinear optics. THz techniques combine pulsed ultrafast laser technology with optoelectronics to generate terahertz radiation with sub-picosecond pulse duration. A typical set-up includes a modelocked solid-state laser that produces pulses with 100 femtosecond pulsewidths.