Carrier envelope phase control of monocycle THz pulses using an artificial dispersive medium

Abstract

To control the carrier envelope phase (CEP) is very important when we employ a few cycle electro-magnetic pulses for nonlinear phenomena such as higher-order harmonic generation [1], strong-field ionization and dissociation[2], and population transfer between two bound states [3]. Hence it should also be essential for nonlinear spectroscopy in the terahertz (THz) frequency region. Recently, the generation of extremely intense monocycle THz pulses has been established with the nonresonant optical rectification process and various THz nonlinear spectroscopies have been demonstrated [4]. The CEP of such THz pulses is originally locked in principle. However, to change the CEP arbitrarily has been impossible so far. To overcome the task, we focused our attention on the novel property of a series of parallel plate waveguides. For the TE mode in the waveguide [5], the phase velocity v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> is larger than the light velocity c in the vacuum while the group velocity v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> is smaller. If the chirp caused by the group velocity dispersion is negligible, the CEP of the THz pulse is changed during its propagation in the dispersive medium.The schematic of the dispersive medium, which we used for the CEP control is shown in Fig. 1 (a). It consists of tens of 50×10×0.1mm3 stainless plates aligned with an equal spacing of 3, 2, and 1mm. The cut-off frequencies of the media are c/2g =0.05, 0.07, and 0.15 THz, respectively. Figure 1(b) shows the temporal electric-field profile of the transmitted THz pulse with the polarization parallel (TE mode; bold) and perpendicular (TEM mode; thin) to the steel plates. In the case of g=1mm, the profile for the TE mode pulse is chirped due to the group velocity dispersion. The chirp is negligible for g≥2mm and we can clearly see the carrier phase of the pulse slightly shifts towards earlier time keeping the envelope phase stable. This means that the CEP of the THz pulse is obviously modulated; the shifted value is ~π/2 CEP for g=2mm. Figure 1 (c) shows the complex transmission coefficient of these medium, and the transmissivity of this optics is above 50%. Such arbitrary-CEP-controlled THz pulses will give us a new field of phase-sensitive THz nonlinear spectroscopy.