We present the generation of THz radiation by focusing ultrafast laser pulses with three incommensurate wavelengths to form a plasma. The three colors include 800 nm and the variable IR signal and idler outputs from an optical parametric amplifier. Stable THz is generated when all three colors are present, with a peak-to-peak field strength of ~200 kV/cm and a relatively broad, smooth spectrum extending out to 6 THz, without any strong dependence on the selection of signal and idler IR wavelengths (in the range from 1300-2000 nm). We confirm that 3 colors are indeed needed, and comment on the polarization characteristics of the generated THz, some of which are challenging to explain with plasma current models that have had success in describing two-color plasma THz generation. a Contributed equally to this work.

We show experimentally that the terahertz (THz) emission of a plasma, generated in air by a two-color laser pulse (containing a near IR frequency and its second harmonic), can be enhanced by the addition of an 800-nm pulse. We observed enhancements of the THz electric field by a factor of up to 30. This provides a widely accessible means for researchers using optical parametric amplifiers (OPA) to increase their THz yields by simply adding the residual pump beam of the OPA to the plasma generating beam. We investigate the dependence of the THz electric field enhancement factor on the powers of the two-color beam as well as the 800-nm enhancement beam. Numerical calculations using the well-known photocurrent model are in excellent agreement with the experimental observations.

With circularly polarized terahertz pulses, we excite a pair of degenerate perpendicularly polarized phonons in LiNbO3. The resulting circular ionic motion induces a magnetic field in LiNbO3, which is measured through the Faraday effect.

We investigate the breakdown of the well-known electric-field-squared dependence of the terahertz Kerr effect in diamond. Using two-dimensional (2D) terahertz spectroscopy we show that higher order effects occur at high fields.

Using two-dimensional (2D) terahertz spectroscopy we investigate fundamental electronic, lattice, and spin excitations in solids, and the couplings between them.