New Methods for Terahertz and Ultrafast Infrared Spectroscopy of Biological Materials and Graphene Buffer Layer

Abstract

The development of advanced optical spectroscopy has always been a frontier in optics and material science. This dissertation develops several new methods for terahertz (THz) and infrared spectroscopy and presents their applications for the better understanding of biological materials and novel two-dimensional (2D) materials.

One limitation of current THz spectroscopy has been its inability to reveal the chiral properties, e.g. circular dichroism (CD), of complex materials. Our first new spectroscopic method aims to solve this issue using existing experimental techniques. Based on the conventional THz time-domain spectroscopy and with three additional wire-grid linear polarizers, a complete THz time-domain polarimetric (THz-TDP) methodology is developed to extract CD in any generalized complex materials through a sequence of linear polarization measurements. The accuracy and effectiveness of the new method are demonstrated and justified by the measurements of several test samples including anisotropic polymer sheet and chiral DNA solutions.

Next, THz-TDP is applied for the study of several bacterial biofilms, including S. aureus and B. subtilis biofilms grown in PNG and TSBg media, both in freeze-dried and pristine wet states. An absorption band at 0.6 – 0.8 THz and negative circular dichroism at around 1 THz are consistently found in all the samples and match the predictions from molecular dynamics simulations. These results are not only essential for the understanding of the low-energy vibrations of the extracellular matrix (ECM) components in these biofilms, but also could provide insight into the further investigation of the potential electromagnetic communication between biological cells.

Another long-standing problem in THz spectroscopy is the lack of THz polarization modulators and related optical devices. To tackle this problem, we develop an entirely new approach to the real-time polarization modulation of THz beams using tunable and reconfigurable kirigami plasmonic metasurfaces, achieving polarization rotation as large as 80° and ellipticity up to 40°. Based on these kirigami modulators, another new spectroscopic method – THz circular dichroism (TCD) spectroscopy – is developed and demonstrated on several chemical and biological materials.

Lastly, ultrafast time-resolved infrared spectroscopy for both room temperature and high temperature above 1000 °C is developed to study the optical properties of epitaxial graphene buffer layer on SiC(0001). This novel 2D material exhibits very different optical responses from conventional graphene layers, with a proposed bandgap opening between 2 eV to 2.4 eV due to the interaction with the substrate. The preliminary results also indicate the possibility of its temperature-dependent transition from semiconducting to semimetal form, which is hypothesized to be caused by the decoupling of the buffer layer from the substrate.