Terahertz Images of Biological Molecules: Frequency Dependence of Spatial Resolution Using a Tunable Terahertz Laser Source

The spatial resolution of transmitted terahertz (THz) images of metal slits and protein [myoglobin (Mb)] thin films was analyzed at 1.40, 1.63, and 2.55 THz by using continuous THz waves from a tunable gas laser. The resolution of the THz images of the metal slit was 0.40, 0.35, and 0.45 mm at 2.55, 1.63, and 1.40 THz, respectively. The spatial resolution is determined not only by the diffraction limit of the wavelength of the THz wave but also by other experimental conditions such as laser power. The resolution of the THz images of Mb thin films was 0.55 and 0.60 mm at 2.55 and 1.63 THz, respectively. The approximately twofold stronger THz wave absorption of Mb film at 2.55 THz than at 1.63 THz provides the better resolution. By using a THz wave with a frequency at which the target sample has strong absorption we can obtain a THz image with good resolution.