Water-Based Coherent Detection of Broadband Terahertz Pulses

Both solids and gases have been demonstrated as the materials for terahertz (THz) coherent detection. The gas-based coherent detection methods require a high-energy probe laser beam and the detection bandwidth is limited in the solid-based methods. Whether liquids can be used for THz detection and relax these problems has not yet been reported, which becomes a timely and interesting topic due to the recent observation of efficient THz wave generation in liquids. Here, we propose a THz coherent detection scheme based on liquid water. When a THz pulse and a fundamental laser beam are mixed on a free-flowing water film, a second harmonic (SH) beam is generated as the plasma is formed. Combining this THz-induced SH beam with a control SH beam, we successfully achieve the time-resolved waveform of the THz field with the frequency range of 0.1–18 THz. The required probe laser energy is as low as a few microjoules. The sensitivity of our scheme is 1 order of magnitude higher than that of the air-based method under comparable detection conditions. The scheme is sensitive to the THz polarization and the phase difference between the fundamental and control SH beams, which brings direct routes for optimization and polarization sensitive detection. Energy scaling and polarization properties of the THz-induced beam indicate that its generation can be attributed to a four-wave mixing process. This generation mechanism makes simple relationships among the probe laser, THz-induced SH, and THz field, favorable for robustness and flexibility of the detection device.

Figure 1

Coherent detection of THz field by water film. (a) Schematic of the experimental setup. An 800 nm laser beam passes through a BBO crystal to generate a CSH of 400 nm and a THz pulse is focused on a water film to generate the other SH (i.e., TISH). Combining the TISH with CSH, the signal is collected by the PMT. (b),(c) Measured THz waveforms and the corresponding spectra, where the blue solid and red broken lines in each plot correspond to the results of water-based scheme and EOS with a GaP crystal, respectively.

Figure 2

TISH energy signal and interference signals from the water film. (a) Measured (blue solid line) and calculated (red broken line) TISH energy signals without the CSH imposed in the detection. Inset shows the TISH energy as a function of the THz field strength, where a quadratic fit is presented by the red solid line. The black broken line in the inset marks the CSH energy used in the experiments of Figs. 2 and 2. (b), (c) Interference signals of the TISH and CSH, where the THz field strength is taken as $1\mathrm{MV}/\mathrm{cm}$ and $14.5\mathrm{MV}/\mathrm{cm}$, respectively. The blue solid line shows the measured result, and the red and green broken lines correspond to the calculation results of the first term (coherent component) and second term (incoherent component) in Eq. (2), respectively.

Figure 3

Dependence of the measured signal on THz polarization. (a) TISH energy as a function of the relative polarization angle $\theta$ between the probe laser and THz fields, where the crosses and dots correspond to the vertical and horizontal components, respectively. (b) Measured signal as a function of $\theta$ in the case of the coherent detection.

Figure 4

Comparison of the water- and air-based detections. (a) THz time-domain waveforms and the corresponding spectra (inset). (b) Required probe beam energy to generate the TISH energy for the given THz field of $1\mathrm{MV}/\mathrm{cm}$. In each plot, the blue solid and red broken lines correspond to the results with the water- and air-based schemes, respectively.