Fig. 1. Schematic for time-domain THz-OA measurement. (a) Schematic of the setup. MT, metal tape; PM1,2, parabolic mirrors; PE, Peltier element; THz-OA, terahertz optoacoustic; TS, temperature sensor; UST, ultrasonic transducer. The light blue region represents the interaction area between the terahertz radiation and sample. (b) Amplitude of THz-OA signal from water measured by the UST as a function of the energy of the terahertz radiation measured by a terahertz pyroelectric detector. The best-fit line (dotted blue) is also shown.

Fig. 2. THz-OA responses from an agar-in-water phantom, water flowing through a microfluidic chip, and a fresh beef brisket slice. (a) Schematic of the production of THz-OA signal by an agar phantom of thickness L after terahertz irradiation to a penetration depth l. I, II, and … denote the primary THz-OA signal and its echoes. (b) THz-OA signals in the time domain for three L values. (c) First time-domain THz-OA signals from (b) were transformed to the frequency domain. (d) Schematic of the production of THz-OA signal by water circulating through a channel of depth d on a microfluidic chip. A peristaltic pump drives water into the sample holder at the inlet, and the water exits at the outlet. (e) THz-OA signals in the time domain for three d values. (f) THz-OA signals from (e) were transformed to the frequency domain. (g) Schematic of the production of THz-OA signal by ex vivo tissue, with the fatty and lean areas of interest marked. (h) Time-domain THz-OA signals from each area of interest. (i) THz-OA signals from (h) were transformed to the frequency domain.

Fig. 3. Temperature dependence of the THz-OA signal of water. (a) Time-domain THz-OA signal of water at different temperatures. (b) The THz-OA signals from (a) were transformed to the frequency domain. (c) THz-OA amplitudes of water over the temperature range from 0°C to 5°C. The insets show the raw THz-OA signal at 0°C (dark blue) or 4°C (light blue).

Fig. 4. Concentration dependence of the time-domain THz-OA signal of NaCl solutions with light concentrations at 24°C and 5°C. (a) THz-TDS profiles of pure water and increasingly concentrated NaCl solutions. Measurements were taken at 24°C. (b) THz-OA response of pure water and increasingly concentrated NaCl solutions at 24°C. (c) The same measurements were performed as in (b) but at 5°C. (d) Normalized amplitudes obtained for different NaCl concentrations in aqueous solution using the proposed THz-OA effect at 24°C (purple) and 5°C (black) and a commercially available THz-TDS (brown).

Fig. 5. Concentration dependence of THz-OA amplitude of CaCl2 and NaCl solutions with high concentrations at 24°C and 5°C. THz-OA amplitude of aqueous solutions of (a) CaCl2 and (b) NaCl before normalization (gray) and after normalization (blue). Measurements were taken at 24°C (solid bar and line) or 5°C (dotted bar and line).