Terahertz (THz) time-domain spectroscopy and optical pump-THz probe spectroscopy are today widely used within the THz community to study material properties and carrier dynamics in semiconductors and 2D materials. Along with these methods, Laser THz Emission Microscopy (LTEM) is an alternative and somewhat simpler technique where THz pulses are collected from a sample after it is directly photo-excited by a femtosecond (fs) laser. Since most semiconductors intrinsically are able to emit THz pulses when carriers are accelerated on a fs time scale, this method is directly able to provide information about carrier mobility. At the same time, LTEM can image a sample with a much higher spatial resolution than conventional THz systems due to the much lower diffraction-limit of the fs laser pulses.
Recently, it has been demonstrated that spintronic THz emitters have the potential of out-competing conventional THz sources showing an impressive efficiency and a bandwidth up to 10s of THz, which is normally only achieved in much more sophisticated THz sources such as air-plasmas generated with amplified fs laser systems. While some work has already been done in optimizing the spintronic structures to generate maximum THz signal and bandwidth, understanding the carrier dynamics from the measured THz pulses is still a challenge. In this presentation we discuss the ability to understand the carrier dynamics spintronic samples with LTEM and THz time-domain spectroscopy, and present an experimental platform where samples can be studied with these methods simultaneously.
In this paper we present a numerical study of terahertz pulses interacting with crystals of cesium iodide. We model the molecular dynamics of the cesium iodide crystals with the Density Functional Theory software CASTEP, where ultrafast terahertz pulses are implemented to the CASTEP software to interact with molecular crystals. We investigate the molecular dynamics of cesium iodide crystals when interacting with realistic terahertz pulses of field strengths from 0 to 50 MV/cm. We find nonlinearities in the response of the CsI crystals at field strengths higher than 10 MV/cm.
We present a spatial characterization of terahertz (THz) beams generated from a two-color air plasma under different
conditions by measuring full 3D beam profiles using a commercial THz camera. We compare two THz beam profiles
emitted from plasmas generated by 35 fs and 100 fs laser pulses, and show that the spatial properties of the two THz
beams do not change significantly. For the THz beam profile generated by the 35 fs pulse, the spatial effect of
eliminating the lower frequencies is investigated by implementing two crossed polarizers working as a high-pass filter.
We show that this reduces the beam waist, and that the beam spot shape changes from Lorentzian to Gaussian. Finally,
we observe a forward-propagating Gaussian THz beam by spatially filtering away the conical off-axis radiation with a
1 cm aperture.
We present a characterization of THz beams generated in both a two-color air plasma and in a LiNbO<sub>3</sub> crystal. Using a commercial THz camera, we record intensity images as a function of distance through the beam waist, from which we
extract 2D beam profiles and visualize our measurements into 3D beam profiles. For the two-color air-plasma, we
measure a conical beam profile that is focused to a bell-shape at the beam waist, whereas we observe a Gaussian beam
profile for the THz beam generated from the LiNbO<sub>3</sub> crystal.