By: Ki-Yong Kim
Sandwiched between the traditional optical and microwave regimes, far infrared or terahertz (THz) frequency (1 THz = 1012 Hz) has recently drawn special attention due to its potential for molecular sensing, biomedical imaging and spectroscopy, security scanners, and plasma diagnostics. These applications provide strong motivation to advance the state of the art in THz source development. In particular, large-scale electron accelerators such as synchrotrons and free electron lasers are currently available to produce THz radiation energy in excess of several microjoule per pulse. However, due to its large cost to build those facilities and thereby limited access, there is a present and growing need to realize such strong THz generation at the tabletop scale. In this effort, we have recently demonstrated a high-energy (>5 microjoule), super-broadband (>75 THz), tabletop THz source via ultrafast photoionization in gases .
In this scheme, an ultrafast pulsed laser’s fundamental and second harmonic fields are mixed in a gas of atoms or molecules, causing them to ionize. Microscopically, the laser fields act to suppress the atom’s or molecule’s Coulomb potential barrier, and, via rapid tunneling ionization, bound electrons are freed. The electrons, once liberated, oscillate at the laser frequencies, and also drift away from their parent ions at velocities determined by the laser field amplitudes and the relative phase between the two laser fields. Depending on the relative phase, symmetry can be broken to produce a net directional electron current. As this current occurs on the timescale of photoionization, for sub-picosecond lasers, it can generate electromagnetic radiation at THz frequencies.
This THz generation mechanism turns out to be closely related to the mechanism used to explain high harmonic generation (HHG) in gases, as both processes originate from a common source, that is, a nonlinear electron current. The electrons re-colliding with the parent ions are responsible for HHG, whereas the electrons drifting away from the ions without experiencing re-scattering ions account for THz generation. As demonstrated experimentally , the generated THz and third-harmonic are strongly correlated in such a way that changing the relative phase can effectively switch the emission between THz and harmonics. This provides the basis to coherently control electromagnetic radiation in a broad spectral range, from THz to extreme ultraviolet.
Now, the next step is to scale up the laser power to produce even more powerful THz radiation. Using the Maryland’s 30 terawatt (TW) laser, we anticipate producing an unprecedented millijoule level of THz radiation. Such radiation may allow us to observe extreme nonlinear THz phenomena in a university laboratory.
 K. Y. Kim et al., Nature Photon. 2, 605 (2008).