DTU Fotonik scientists directly observe the single-cycle optical nonlinearity
The researchers at DTU Fotonik, Max Planck Institute for Polymer Research - MPIP (Mainz, Germany), and SLAC Linear Accelerator Laboratory (California, USA) have recently reported on the direct observation of a nonlinear-optical effect, occurring in the regime of single-cycle pulse of light at terahertz (THz) frequencies. In their paper, published in Physical Review B  , they used a doped semiconductor as an efficient nonlinear medium, where the THz-range optical nonlinearity arises from the response of free-electron plasma to THz electric fields.
The single-cycle pulses of THz light, propagating through such a medium, experienced an effect called the self-phase modulation (SPM), which led to the nonlinear reshaping and time delay of the THz pulses, as shown in the Figure above and in this movie. The SPM is one of the most important effects in nonlinear optics. It is used in numerous applications, ranging from supercontinuum generation for biophotonics imaging to ultra-high speed optical signal processing in telecom systems.
Single-cycle pulses, irrespective of the frequency range to which they belong, inherently have an extremely broad spectral bandwidth covering many octaves of frequencies. Unlike the single-cycle pulses at optical frequencies, the THz pulses can be easily generated and detected in the time-domain using conventional femtosecond lasers. One of the discoveries in this work was the coexistence of both positive and negative refractive index nonlinearity within the broad spectrum of a single-cycle THz pulse. This is quite a unique observation for nonlinear optics in general, which demonstrates a great potential of using THz pulses as accessible model tools for study of single-cycle nonlinear optical effects.
The authors of this work are Dmitry Turchinovich, associate professor at DTU Fotonik and a leader of Ultrafast dynamics & THz spectroscopy group at MPIP, Jørn M. Hvam, professor at DTU Fotonik, and Matthias C. Hoffmann, staff scientist at SLAC.
 D. Turchinovich, J. M. Hvam, and M. C. Hoffmann, “Self-phase modulation of a single-cycle terahertz pulse by nonlinear free-carrier response in a semiconductor” Phys. Rev. B 85, 201304(R) (2012)