Technical Report: Final project report for Terahertz Spectroscopy of Complex Matter

This project designed characterization techniques for thin films of complex matter and other materials in the terahertz spectral region extending from approximately 100 GHz to 4000 GHz (4 THz) midway between radio waves and light. THz has traditionally been a difficult region of the spectrum in which to conduct spectroscopic measurements. The “THz gap” arises from the nature of the sources and detectors used in spectroscopy both at the optical (high frequency) side and electronic (low frequency) side of the gap. To deal with the extremely rapid oscillations of the electric field in this frequency region this research project adapted techniques from both the electronics and optics technologies by fabricating microscopic antennas and driving them with short optical pulses. This research technique creates nearly single cycle pulses with extremely broad spectral bandwidth that are able to cover the THz spectral range with a single measurement. The technique of THz time domain spectroscopy (THz-TDS) has seen increasing use and acceptance in laboratories over the past fifteen years. However significant technical challenges remain in order to allow THz-TDS to be applied to measurement of solid materials, particularly thin films and complex matter. This project focused on the development and adaptation of time domain THz measurement techniques to investigate the electronic properties of complex matter in the terahertz frequency region from 25 GHz to beyond 5 THz (165 inv. cm). This project pursued multiple tracks in adapting THz Time Domain Spectroscopy (THz-TDS) to measurement of complex matter. The first, and most important, is development of a reliable methods to characterize the complex dielectric constant of thin films with high accuracy when the wavelength of the THz radiation is much longer than the thickness of the film. We have pursued several techniques for measurement of thin films. The most promising of these are waveguide spectroscopy and THz interferometry. Since THz spectroscopy measures the changes of the transmitted spectra, any noise on the THz signal contributes to measurement errors. The dynamic range—defined as the RMS noise of the THz detector compared to the peak THz signal—of THz spectroscopy using photoconductive antennas is extremely high, typically over 10,000. However the precision with which spectroscopic data can be measured is limited by the noise on the laser source which is typically 0.1% to 1%. For low values of the sample absorbance and for values of optical thickness less than approximately 0.01, the change in transmission approaches the measurement accuracy. The sample refractive index can be measured with better accuracy since the index causes a temporal shift of the THz pulse by an amount time shift of nd/c where n is the refractive index, d the sample thickness, and c the speed of light. Time shifts of tens of femtoseconds can generally be resolved so that index-thickness values of nd > ten microns can be accurately measured. Waveguide spectroscopy is a way to increase the path length in thin film by several orders of magnitude, and thus have a large interaction length even when the film is much less than a wavelength in thickness. Film thicknesses of 10’s of nm have been measured. THz interferometry cancels out many of the noise sources of THz spectroscopy and can thus result in measurements of films of several hundred nm in thickness and is additionally suitable for optical pump, THz probe spectroscopic techniques. A large amount of additional work was performed in support of the main project direction or to explore promising alternative avenues for research. This report discussed work on the the confinement of low density species for measurement of nanogram or picogram quantities of material. Whispering gallery mode resonators to achieve long path lengths were also investigated as were imaging techniques for sub-wavelength imaging of thin films. The report concludes with a report on investigations of fundamental issues in THz beam propagation and coupling that impact THz spectroscopy and the other spectroscopic techniques developed during this project