Development of Terahertz Time Resolved Spectroscopy

Terahertz frequencies are between optical and electronic frequencies. There are certain unique specifications of Terahertz waves that make the scientific community to continue working in this field and there has been an extensive advancement in the past two decades. Terahertz waves have vast applications in fields of medical imaging, security screening, pharmaceutical industry, industrial quality control, just to name a few. Scientists are continuously working towards improving the available mechanisms to generate Terahertz.
My work focusses on generating Terahertz radiation through the process of optical rectification in a non-linear crystal such as Zinc Telluride (ZnTe). A femtosecond laser is incident on the crystal creating a time varying polarisation which leads to the emission of radiation in the Terahertz regime. This radiation is then detected using a process called electro-optic sampling in a second ZnTe crystal. Inside the crystal, we try to achieve a spatial and temporal overlap between the generated Terahertz and a femtosecond pulse known as the probe pulse and observe the change in polarisation of the probe as it senses the field of the Terahertz pulse inside the crystal. The main idea behind this detection mechanism is that, in absence of the Terahertz pulse, the probe passes through the crystal unaffected and then passes through a quarter wave plate becoming circularly polarised and the orthogonal components of this circularly polarised pulse are separated by a Wollaston prism. The two components impinge on two photodiodes which detect equal intensity. However, in presence of a Terahertz pulse, the output after the quarter wave plate is elliptically polarised and thus the two orthogonal components are unequal giving different intensities on the photodiodes.