Plane photoacoustic wave generation in liquid water using irradiation of terahertz pulses

We demonstrate photoacoustic wave propagation with a plane wavefront in liquid water using a terahertz (THz) laser pulse. The THz light can effectively generate the photoacoustic wave in water because of strong absorption via a stretching vibration mode of the hydrogen bonding network. The excitation of a large-area water surface irradiated by loosely focused THz light produces a plane photoacoustic wave. This is in contrast with conventional methods using absorbers or plasma generation using near-infrared laser light. The photoacoustic wave generation and plane wave propagation are observed using a system with a THz free-electron laser and shadowgraph imaging. The plane photoacoustic wave is generated by incident THz light with a small radiant exposure of < 1 mJ/cm2 and delivered 600 times deeper than the penetration depth of THz light for water. The THz-light-induced plane photoacoustic wave offers great advantages to non-invasive operations for industrial and biological applications as demonstrated in our previous report (Yamazaki et al. in Sci Rep 10:9008, 2020).


This file includes:
Pulse width measurement of the THz micropulse Single THz-pulse pick-up Figure S1. THz-FEL spectra and absorption spectrum of liquid water.   Figure S2 shows a temporal intensity profiles of the single THz micropulse. We evaluated the typical value of the pulse width of the single micropulse as 1.7 ps.
Single THz-pulse pick-up Figure 2(B) shows the single THz-micropulse pick-up from the pulse train by the plasma mirror with nanosecond gating. 3,4 We employed a GaAs wafer irradiated by an intense femtosecond Ti:sapphire laser pulse as the nanosecond plasma mirror. The GaAs is transparent to THz light and has a Brewster angle of 75° for the p-polarized THz light. When the GaAs wafer is irradiated by the near-IR light, a dense electron plasma is generated on the surface with a minority carrier lifetime of less than 10 ns. 5 Then, the GaAs plasma mirror can pick up only a single micropulse from the pulse train with a time interval of 36.9 ns.
In our experiment, the optical pump light was provided by the Ti:sapphire regenerative amplifier with the wavelength of 800 nm, the pulse duration of 100 fs, the pulse energy of 500 μJ. This pump light irradiated the GaAs surface with a spot size of 1 cm 2 , and generated the thin dense carrier layer with the density of 10 19 cm -3 which was estimated by the penetration depth of the optical light into the GaAs, 1 μm, corresponding to the absorption coefficient of 10 4 cm -1 at the wavelength of 800 nm. In this estimation, we also included the Fresnel loss of the pump light at the GaAs surface. Figure S3 shows the calculated power reflectivity of the THz light from the GaAs wafer. In this calculation, we assumed the THz light with the incident angle of 75° and ppolarization. On the surface of GaAs, the carrier was formed with the density of 10 16 ~ 10 19 cm -3 and with the thickness of 1 μm. The calculation was performed with the parameters obtained in the previous work by one of the authors. 6 With the carrier density larger than 10 18 cm -3 , the THz reflectivity drastically increases to 90%. In our present experiment, the carrier density on the GaAs surface was estimated to be 10 19 cm -3 . Therefore, the high reflectivity for the p-polarized THz light and the effective single micropulse pick-up were expected. However, in the actual experiment, due to the imperfect Brewster condition, the small reflectivity for the p-polarized THz light is still remained without the carrier formation on the GaAs. This is the reason why the residual micropulses appear in Fig. 2(B). To minimize the residual micropulses, the electron beam injection was stopped just after the pick-up pulse.
We also have to consider how the optical light absorption on the GaAs surface changes the reflectivity and absorbance of the THz light. In our present experiment, the lowdensity residual carriers with long lifetime (> 1 ms) is remained before and after the pump pulse irradiation because we use the optical pump light with the 1 kHz repetition rate. But Fig. S3 indicates that the residual carrier with the density lower than 10 17 cm -3 does not enhance the reflectivity with the p-polarized THz light at the frequency from 3 to 7 THz. The absorption spectrum of liquid water at a room temperature is also shown as the red solid line.

Supplementary Movie Captions
Video S1. Time evolution of the photoacoustic waves induced by the THz pulse train during 1 μs. This is the original sequential images to obtain the amplitude map shown in Fig. 3(b).
The images were captured with a time gate of 10 ns at a time interval of 10 ns.
Video S2. Time evolution of the photoacoustic waves induced by the pick-up single THz pulse during 4.5 μs. The images were captured with a time gate of 10 ns at a time interval of 15 ns.