Generating arbitrary waveforms of electric field vectors
in the terahertz frequency range

When light interacts with matter, its constituent electrons, ions and molecules move by following the electric field vector of light. These light-driven motions provide unique opportunities for the manipulation of matter within an ultrashort time scale. To control these motions, it is essential to engineer the direction and amplitude of the electric field vector of the light as functions of time in a prescribed manner. Until now, such control of the electric field has been quite challenging, in any frequency range, even at visible frequencies where various mature technologies are available.

Researchers overcame this difficulty within the terahertz frequency range, which has a frequency 1/100th of that for visible light. The terahertz light waves can be generated through frequency conversion from visible light pulses. Since the oscillation period of the terahertz waves is longer than the duration of the light pulses, we can control the timing of terahertz oscillations with great accuracy. Despite this advantage, the conventional polarization state control methods have not been suitable to achieve arbitrary field control. A key finding by Sato and Higuchi et al. is that the temporal profiles of the direction of the terahertz electric field vector can be determined by the polarization states of the visible light pulse that are to be converted to terahertz waves. Therefore, well-developed techniques to control the polarization states of the visible light waves can be employed, to achieve what would otherwise be difficult for the control of terahertz polarization states. In addition to the polarization state, the intensity of the light wave must be varied as a function of time, to determine the shape of the terahertz waveform. For this purpose, researchers had to develop a ‘Vector pulse shaper’, whichcan independently control these two required parameters for the light waves. They also provided an algorithm to determine the required input parameters for the vector pulse shaper, for a given target terahertz waveform.

The accomplishment of arbitrary electric field vector control in the terahertz frequency range provides a variety of potential applications. For example, terahertz waves are known to efficiently excite heavy particles, such as atoms or molecules, from which the structure of materials can be determined. Therefore, the optical control of a material’s structure, for example the preparation of a selected enantiomer of a chiral molecule, is particularly attractive.