Frequency-Resolved THz Thermometry of Nanoparticle Solutions

Terahertz (THz) time domain spectroscopy (TDS) is a non-invasive technique with high sensitivity, allowing for the extraction of the complex refractive index of materials in the commonly defined frequency range between 0.1 THz and 10 THz. Water, in particular, interacts strongly with electromagnetic radiation at these frequencies, making it a subject of active research [1]. Metal nanoparticles (NP) in solution act as selective, nanoscopic absorbers when excited by laser radiation resonant with the surface plasmon excitation, typically in the visible range. This triggers a process of local and ultra-fast energy transfer that is of interest to diverse fields spanning medicine and photovoltaics [2]. For example, the characterization of the heating mechanism is of relevance to the design of photothermal therapies [3]. In [2] it was proposed to perform this heating characterization by measuring the complex refractive index of the liquid water surrounding the nanoparticles at THz frequencies, which depends strongly on temperature. However, in this early work, in spite of the rather broadband nature of the THz-TDS it was not possible to obtain a complete frequency-resolved picture, due to limitations of the setup in repetition rate and speed. In this study, we present the full temperature- and frequency-dependent complex THz refractive index of a concentrated colloidal solution of ~50 nm gold nanospheres in water, capped with Polyvinylpyrrolidone (Nanocomposix GEPE50-10M). This was enabled by using a Yb-based driving laser system with a repetition rate of 400 kHz, allowing for fast measurements of subtle, temperature-dependent amplitude changes and phase shifts exhibited by the THz pulses through the 210-μm-thick liquid cell. The THz-TDS is based on the setup described in [4] and offering more than 10mW of average power at a bandwidth of 0.1 THz to 1.5 THz.