Terahertz Transmission and Reflection Time-Domain Spectroscopy
Quantum Cascade Laser Applications
We are currently working on novel experimental techniques and reconstruction algorithms to produce high quality tomographic images using quantum cascade lasers (QCL) as a terahertz source. A custom algebraic reconstruction technique is being used that allows accurate tomographic reconstruction from non-refractive index matched samples. This allows a wider range of samples, particularly those of industrial, pharmaceutical, and medical interest to be studied. By using multicolour QCLs chemical maps of the samples can be obtained.
Work has also been conducted using a 2D bolometric array to detect the terahertz radiation produced by a QCL. By using a wide collimated beam to illuminate a sample it is possible to obtain real-time reflection or transmission mode images of the sample. This is useful when studying rapidly changing systems.
Together in collaboration with SP group and THz group in Southampton we currently develop novel THz source based on amplification of terahertz field in quantum cascade structure. Instead of coupling terahertz radiation into the QC structure from an external source, the THz radiation is generated in the QC structure substrate using Photo-Dember effect. The source therefore combines advantages of broadband THz spectroscopy with strong electric fields coming out of the quantum cascade lasers.
Simulations: Understanding of Vibrational Modes at Terahertz Frequencies
We work towards a better understanding of the spectral features which are observed in terahertz spectra. Using a state-of-the-art low temperature transmission THz-TDS setup we acquire spectra at 4 K. These spectra serve as the reference set for the calculations in Dr Day's group which cover a range of computational methods, including both atomistic and solid state periodic Density Functional Theory (DFT) simulations in the harmonic and quasi-harmonic approximation.
Our work aims to advance the application of molecular simulations in understanding far infrared/THz spectra of molecular crystals. We are currently developing a critical analysis of the various computational methods, their ability to model the range of interactions in organic molecular crystals and the validity of harmonic calculations for such systems at low and ambient temperatures.