Application Note: THz Time-Domain Spectroscopy

By IMRA America, Inc.

Terahertz (or THz) radiation lies in a frequency range of approximately 1012 Hz in the electromagnetic spectrum. This region falls between microwaves (on the long side) and far-infrared (on the short side). Up until recently, this region of the EM spectrum has been under-utilized due to a lack of convenient technology. However, this situation is changing with the advent of THz emitter and sensor technology, and also with the commercial availability of compact, femtosecond lasers such as the Femtolite.

Terahertz spectroscopy allows the inspection, detection, or sensing of a variety of materials (solid, liquid, or gas). For certain classes of materials, especially dielectrics such as paper and plastic, THz rays can penetrate the material and be detected on the other side. Thus, THz rays can reveal hidden structures or properties inside of an object in a manner reminiscent of X-rays.

In THz time-domain spectroscopy, a femtosecond laser pulse activates a THz emitter device which generates an electromagnetic pulse. (See Figure 1.) This very short, single-cycle THz pulse has a broad spectrum spanning a range of 0.1 to 2 THz. The generated pulses are reflected by (or transmitted through) the sample. This alters the THz waveform, depending on the dielectric properties of the sample. The change in the electric field intensity of the THz pulses is measured by using gated detection of the THz pulse which is activated by a delayed probe light pulse from the femtosecond laser.