Research
Terahertz (THz) Generation
Optical rectification in nonlinear optical crystals (NLO) is commonly used to generate high-field THz light. We have collaborated with Dr. David Michaelis’ group here at BYU to discover, grow, and characterize new organic NLO crystals.
To discover completely novel THz generators, we use a combination of data-mining and first-principles calculations. As depicted in the figure below, we search the CSD database for known crystal structures with favorable characteristics (for example, noncentrosymmetric crystals are a necessity), then we calculate values such as the molecular hyperpolarizability and crystal packing order-parameter for each material identified in a database search. We can then predict the material’s ability to generate THz light.
In recent data mining efforts, we discovered four THz generators with pictures, structures, and spectra shown in the figure below:
Another material we have discovered is MNA, which has a similar structure to the common NLO THz generation crystal, BNA. Although many years ago it was predicted that MNA would work well as a nonlinear optical crystall, we were the first to successfully grow large high-quality MNA single crystals. Below, we show the THz generation output of MNA compared to the current industry standard yellow crystal, BNA. MNA outperforms BNA when pumped with 1250 nm and 800 nm light.
The most impressive crystal we discovered through this data mining process was PNPA. PNPA generates stronger THz electric fields than DAST and OH1 when pumped with ~100 fs 1450 nm pump pulses, making it a new industry standard of NLO THz generators. A comparison of the time and frequency spectra of PNPA to DAST and OH1 is shown below, highlighting the substantial increase in THz generation ability that PNPA has over the other two crystals.
Nonlinear THz Spectroscopy
We use high-field THz radiation to nonlinearly excite vibrational, magnetic, and electronic transitions. Impulsive stimulated Raman scattering (ISRS) excitation can be used to coherently excite vibrations, but we found that by directly exciting the same vibrational modes with THz frequency light, we could dramatically increase the strength of excitation, driving the modes into anharmonic regions of the potential energy surface. We were the first to experimentally extract the shape of the anharmonic potential energy surface of a solid material, like that for LiNbO3 as shown in the figure below.
Even though we can drive nonlinear excitations with a single strong THz pulse, we cannot always distinguish between different types of nonlinear excitations, in particular anharmonic coupling (coupling between vibrational modes) and Raman excitation (multiple phonons combining to excite a vibration). Two-dimensional (2D) THz spectroscopy has enabled us to distinguish electronic Raman scattering excitation (two-photon absorption) in the crystal CdWO4. Our 2D spectra of CdWO4 are shown in the figure below.
Since identifying two-photon absorption in CdWO4, we have rebuilt our experimental 2D spectroscopy setup to allow us to isolate nonlinear signal and achieve higher THz field strengths to more strongly drive nonlinear excitations. We can now identify anharmonic coupling between phonons in CdWO4 and other collective excitations in other materials, and to quantify the amplitude of these couplings. We also use 2D spectroscopy in measuring and understanding how conduction-band dynamics influence conductivity in semiconductors on ultrafast time scales. Using THz pulses, we can probe the dynamics, and with intense pulses, we can control carriers, impart large amounts of momentum, and induce carrier multiplication.
THz Hyperspectral Imaging
Terahertz time-domain spectroscopy (THz-TDS) is of particular interest for imaging applications due to the non-ionizing, non-destructive nature of THz radiation, as well as the unique THz-range spectral signatures of many molecules. We are seeking to improve analysis and applications of THz imaging. Current applications target the development of a diagnostic system for idiopathic pulmonary fibrosis (IPF), comparative tissue studies, and determination of organic and inorganic sample structures.
In our recent work, “Combining spectral amplitude and phase improves terahertz hyperspectral imaging” (publication pending), we demonstrated significant improvements in sample analysis that can be achieved by incorporating full-field THz data into hyperspectral analysis. THz-TDS has the unique capability to easily capture both the amplitude and phase components of transmission data. While standard measurements treat these two components separately in analyses, we show that using the full transmission data allows for a much more accurate breakdown of THz images. Using this data, we demonstrate the ability to differentiate unique components within a sample. This allows for deeper studies on localization of chemical and structural variation within a sample of interest.
