What is a Terahertz Wave?
Terahertz (THz) waves, electromagnetic radiation in the frequency interval from 0.1 to 10 THz, is the next frontier in imaging science and technology. THz radiation, or THz waves, occupy a large portion of the electromagnetic spectrum between the infrared and microwave bands (in different units: 1 THz = 1 ps = 300 µm = 33 cm^-1 = 4.1 meV = 47.6^oK.). However, compared to relatively well-developed medical imaging at microwave and optical frequencies, basic research, new initiatives and advanced technology developments in the THz band are very limited. The THz band is a scientifically rich but technologically limited frequency band, since THz wave emitters and receivers are less developed, compared to its neighboring bands (microwave and optical). During the past decade, THz waves have been used to characterize the electronic, vibrational and compositional properties of solid, liquid and gas phase materials. It is a common belief that the future ‘killer-application’ of THz waves will be in biomedicine.

What is the Significance of  a  THz Wave?
Unlike common optical spectroscopes, which only measure the intensity of light at specific frequencies, the THz time-domain spectroscopic technique directly measures the THz wave’s temporal electric field. Fourier transformations of this time-domain data gives the amplitude and phase of the THz wave pulse, therefore providing the real and imaginary parts of the dielectric constant without the use of the Kramers-Kronig relations. This allows precise measurements of the refractive index and absorption coefficient of samples that interact with the THz waves. Many rotational and vibrational spectra of various liquid and gas molecules lie within the THz frequency band, and their unique resonance lines in the THz wave spectrum allow us to identify their molecular structures. Raman spectroscopy directly uses the frequency domain to fingerprint the lattice vibrations. Similarly, THz wave spectroscopy describes molecular rotational and vibrational spectra from 10 GHz to 10 THz using the real and imaginary parts of the dielectric function, obtained by measuring the THz wave in the time-domain. This is not possible by any current optical or microwave technique.

Due to the diffraction-limit, the standard imaging resolution at 1 THz cannot be much smaller than 300 mm. This limits the spatial sensing or imaging of biological objects at the cell level. To overcome this limit, we propose to develop a near-field THz imaging technology by focusing the optical beams into an electro-optic crystal to generate (by optical rectification) and detect (by the electro-optic effect) the THz wave with sub-micron resolution. The imaging area of the biomedical tissue is comparable to the optical focal spot, and it is independent of the THz beam wavelength, l. This near-field microscope will have a sub-wavelength spatial resolution better than 1/1000 l (or 0.5 mm @ 0.5 THz).

Our Previous Activities
The research developments of THz wave science and technology at Rensselaer have been supported by the National Science Foundation, the Army Research Office, the Air Force Department of Scientific Research, the Department of Energy, the Research Corporation and Zomega Technology Corporation.