Terahertz Photoacoustic Generation Using Ultrathin Nickel Nanofilms

Picosecond ultrasonic laser sonar (PULSE), one of the noninvasive technologies with a high thickness sensitivity, is capable of probing multilayer films directly on the production wafers. To reach high spatial resolution in the current PULSE system, generation of terahertz photoacoustic pulses through an ultrathin metal film has been the focus of recent studies. In this study, we propose and demonstrate the high potential of ultrathin nickel (Ni) films for THz photoacoustic generation. After femtosecond laser excitation, the photoacoustic frequency launched by the ultrathin Ni nanofilms could be up to 1.4 THz, corresponding to a record-breaking spatial resolution in the single metallic layer system. We discuss the detailed thickness-dependent mechanism with the two-temperature model, which indicates that the ultrahigh-frequency acoustic signals were generated due to the significant contribution of hot electron pressure in Ni, very different from the case for gold. By looking into the ultrafast hot electron relaxation mechanism, our results provide a guideline to further break the technical limit of the metallic terahertz photoacoustic frequency generation in the near future.