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43269-AC7
The Size-Dependent Dynamical Properties of Nanometer-Sized Liquid Water Pools
The goal of this research project was a thorough study of the mid- and far-infrared properties of confined water pools, using a variety of spectroscopic techniques. The use of inverse micelles as a model system for studying nanometer-scale confinement of liquid water had been validated by our earlier work, in which we were able to observe a size-dependent absorption resonance which was attributed to acoustic-like vibrational modes. Unfortunately, our follow-up experiments have been far less illuminating, in part because the interpretation of the data at higher frequencies has been more complicated than we originally anticipated.
Subsequently, we have undertaken several new projects. One of these has involved a study of the electronic dynamics of magnetically doped semiconductor materials, using terahertz emission spectroscopy. Dilute magnetic semiconductors (DMS) based on III-V compounds, which were first produced in 1989, have inspired much interest in recent years. III-V DMS are attractive candidates for studying dynamical processes in magnetically ordered systems, due to the clear distinction between mobile carriers and localized spins in the III-V semiconductors. They also show great potential for use in spintronic applications. Such applications will require a detailed understanding of the carrier dynamics, spin magnetism, and in particular their interactions in these materials. We have measured the first THz emission from InAs-based DMS samples. We have observed a temperature-induced polarity reversal of the THz radiation from both p-InMnAs and n-InMnAs, in contrast to what has been previously observed in p-InAs. We attribute the observed polarity reversal of the THz radiation to the competition between two oppositely-directed sources of photo-induced current: the surface field-induced current and the photo-Dember effect. Due to the presence of the Mn dopant, the photo-Dember effect in InMnAs is much weaker than in InAs. As a result, the dominant THz radiation mechanism in InMnAs switches from the surface field current at high temperature to the photo-Dember effect at low temperature, leading to a change in the polarity of the emitted THz field. This result highlights an important new aspect of the influence of the magnetic dopant on carrier dynamics. We have very recently obtained a set of new samples of both n-doped and p-doped materials, including a few with much higher Curie temperatures. We are now beginning the process of studying these new samples.
In addition to ongoing studies of magnetically doped semiconductors, we have recently begun two other projects. The first involves a study of terahertz radiation propagation in a photonic crystal slab. We have fabricated nearly perfect photonic crystals using deep reactive ion etching, and studied the superprism effect for the first time at terahertz frequencies. We find that none of the conventional theoretical tools used to compute superprism phenomena are adequate to explain our results. Indeed, the recent proposal that such systems can be modeled as ideally two-dimensional is extremely inaccurate in our case. This highlights the need for further theoretical work in the field of metal-clad photonic crystal slabs.
The second project involves a collaboration with researchers at Shell International Exploration and Production. We are beginning a study of the far infrared spectroscopy of liquid alkanes and mixtures, in order to explore the possibility of using terahertz technologies for monitoring impurities in hydrocarbon streams. Impurities such as H2O and various sulphur compounds are of significant interest and relevance in the processing of petrochemicals. A real-time non-invasive technique for monitoring impurity concentrations would be of considerable value. As a first step, we will measure the broadband response of a series of neat alkanes, ranging from C6 up to C18. Our initial observations indicate that relatively sharp vibrational absorption bands can be observed in solid alkanes (specifically octadecane, C18). These are only present if the material is frozen slowly – flash freezing does not produce the long-range order necessary to support a phonon mode. The nature of these low frequency vibrations is still to be determined.
