Ultrafast Coherent Terahertz Spectroscopy in High Magnetic Fields and Directed Energy Flows in Quantum Dot Assemblies

We describe the design, construction, and use of fiber-coupled terahertz antennas for performing ultrafast coherent THz spectroscopy directly in the cryogenic bore of high-field magnets. With an aim towards measuring the high-frequency (100 GHz to 2000 GHz) complex conductivity of correlated electron materials in the regime of low temperatures and high magnetic fields, these miniature THz emitters and receivers are demonstrated to work down to 1.5 K and up to 18 T, for eventual use in higher-field magnets. Results from a variety of semiconducting and superconducting samples are presented. This paper also describes a separate effort towards achieving coupling between colloidal semiconductor nanocrystal quantum dots, wherein we realize and study inter-dot communication via resonant (Förster) energy transfer. We present studies of the dynamics of resonant energy transfer in monodisperse and energy gradient (layered) assemblies of CdSe nanocrystal quantum dots. Time- and spectrally-resolved photoluminescence data directly reveal the energy-dependent transfer rate of excitons from smaller to larger dots. Results from layered nanocrystal quantum dot assemblies demonstrate unidirectional energy flows, a first step towards artificial light-harvesting structures. Lastly, time-resolved studies at millikelvin temperatures elucidate the nature of ground-state "dark" excitons in these quantum dots.