Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios

In the widely studied two-color laser scheme for terahertz (THz) radiation from a gas, the frequency ratio of the two lasers is usually fixed at ${\omega }_2/{\omega }_1=1:2$. We investigate THz generation with uncommon frequency ratios. Our experiments show, for the first time, efficient THz generation with new ratios of ${\omega }_2/{\omega }_1=1:4$ and $2:3$. We observe that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization and the polarization adjustment becomes inefficient by rotating the other laser polarization; the THz energy shows similar scaling laws with different frequency ratios. These observations are inconsistent with multiwave mixing theory, but support the gas-ionization or plasma-current model. This study pushes the development of the two-color scheme and provides a new dimension to explore the long-standing problem of the THz generation mechanism.

Figure 1

(a) Experimental setup. L, lens; DM, dichromatic mirror; PM, parabolic mirrors. (b) THz waveforms with ${\omega }_2/{\omega }_1=1:4$ and $2:3$, respectively, obtained from the autocorrelation measurements, in which the THz powers are normalized by the one with the 800- and 1600-nm pulses. (c), (d) THz energy as a function of the second pulse wavelength ${\lambda }_2$, where the first pulse wavelength ${\lambda }_1$ is fixed as 800 nm in (c) and 400 nm in (d). Powers of the two pulses are taken as $P_1=120\mathrm{mW}$ and $P_2=400\mathrm{mW}$ in (c) and $P_1=180\mathrm{mW}$ and $P_2=250\mathrm{mW}$ in (d).

Figure 2

THz energies of the horizontal and vertical components as a function of the rotation angle $\theta$ of the field polarization of (a) the 1600-nm pulse, (b) 400-nm pulse, (c) 1200-nm pulse, and (d) 800-nm pulse, respectively, where when polarization of one pulse is rotated, polarization of the other pulse is fixed at the horizontal. Experimental results are shown by crosses and circles and PIC results by lines. The left-hand column corresponds to the case with the 400-nm (with 180 mW) and 1600-nm (250 mW) pulses and the right-hand column to the case with the 800-nm (120 mW) and 1200-nm (400 mW) pulses.

Figure 3

THz energy as a function of the power of (a) the 1600-nm pulse, (b) 400-nm pulse, (c) 1200-nm pulse, and (d) 800-nm pulse, respectively, where when the power of one pulse is changed, the power of the other pulse is fixed. The left-hand column corresponds to the case with the 400- and 1600-nm pulses and the right-hand column to the case with the 800- and 1200-nm pulses.