Laser-terahertz emission from the chemical sensing plate

A new type of sensing system to measure the concentration of the protons in the solutions by means of the laser-terahertz emission and detection was proposed and developed. The simple layered structures of SiO₂/Si thin films on a sapphire substrate were prepared as a chemical sensing plate. The system measure the amplitude of the terahertz emission from the depletion layer of SiO₂/Si interface by irradiating the femtosecond laser pulses with the center wavelength of 790 nm. The solution was contacted with the SiO₂ surface and the Si thin film layer was electrically grounded. An Ag/AgCl reference electrode was used to control the potential of the solution. When the proton was adsorbed on the surface of the plate, the surface electric dipole was formed at the surface, which changes the local field of the depletion layer. Thus the peak amplitude of terahertz emission was changed depending on the number of the proton adsorbed on the surface of the plate. Actually, we demonstrated that the peak amplitude of terahertz emission from the sensing plate increased by increasing the pH values of the test solution when the bias voltage of the solution was 0 V. (pH values indicate the concentration of the proton in the solution) and the sensitivity was about 31 muV/pH.