Institute for Terahertz Science and Technology

Wave propagation equations in the stationary-phase approximation have been used to identify the theoretical bounds of a miniature holographic Fourier-transform spectrometer ͑HFTS͒. It is demonstrated that the HFTS throughput can be larger than for a scanning Fourier-transform spectrometer. Given room-or a higher-temperature constraint, a small HFTS has the potential to outperform a small multichannel dispersive spectrograph with the same resolving power because of the size dependence of the signal-to-noise ratio. These predictions are used to analyze the performance of a miniature HFTS made from simple optical components covering a broad spectral range from the UV to the near IR. The importance of specific primary aberrations in limiting the HFTS performance has been both identified and verified.

Temperature dependent mm-wave (0.7-3 mm) absorption in mesoscopic amorphous silicate grains synthesized by sol-gel reaction has been measured between 1.2 and 15 K. These amorphous powders are precursors to forsterite (Mg2SiO4), enstatite (MgSiO 3) and MgO. 2SiO 2. The measured bleaching of the absorption with increasing temperature characteristic of two level system behavior is observed in all cases and the results are compared with those obtained from bulk aerogel and soda-lime-silica glass. The weighted spectral density of two level states throughout this wavelength region is shown to be larger for all the silicate particles when compared with bulk soda-lime-silica glass.

Because of the development of detector arrays stationary interferometric spectrometers now have many applications in the visible and IR; however, these same array sizes make it impractical to design a single Fourier optics system with the necessary large field angles required for the THz region. By dividing the Fourier optics into independent components in each arm of an interferometer we show that the aberrations are dramatically reduced, while maintaining the same theoretical throughput as the scanning Michelson interferometer.

One way to obtain new information about the ubiquitous low-lying two-level systems found in glasses is to identify them in appropriately modified crystals and then vary the crystal properties. Here our low-temperature mm-wave absorption and Raman scattering measurements on a number of mixed fluorite crystals are reviewed. It is shown that these single-crystal materials possess a broad distribution of two-level systems similar to what is found in the amorphous state.

THz spectroscopy is used to identify a broad distribution of two-level systems, characteristic of glasses, in the substitutional monatomic mixed crystal systems, Ba 1ÿx Ca x F 2 and Pb 1ÿx Ca x F 2 . In these minimally disordered systems, two-level behavior, which was not previously known to occur, begins at a specific CaF 2 concentration. The concentration dependence, successfully modeled using the statistics of the impurity distribution in the lattice, points to a collective dopant tunneling mechanism.

High-density amorphous (HDA) ice has a two-level system (TLS) optical density of states with a strength comparable to that found in many conventional glasses, whereas for the low-density amorphous (LDA) phase the TLS strength is at least 30 times smaller. Isotopic substitution resulting in 50% of the molecules having lower symmetry fails to change this strength ratio suggesting that the observed difference is not due to a breaking of the tetrahedral selection rule in the HDA phase. By doping the samples with different concentrations of LiCl, methanol, HF, NaCl, LiOH, and NaOH we have explored both the defect-induced TLS optical density of states and the glass transition characteristics in the LDA ice phase. r

Intrinsic localized modes (ILMs) -also known as discrete breathers -are localized excitations that form without structural defects in discrete nonlinear lattices. For crystals in thermal equilibrium ILMs were proposed to form randomly, an idea used to interpret temperature activated signatures of ILMs in a-U and NaI. Here, however, we report neutron scattering measurements of lattice vibrations in NaI that provide evidence of an underlying organization: (i) with small temperature changes ILMs move as a unit back-and-forth between [111] and [011] orientations, and (ii) when [011] ILMs lock in at 636 K the transverse optic (TO) mode splits into three modes with symmetry-breaking dynamical structure resembling that of a superlattice, but there are no superlattice Bragg reflections and the pattern itself has crystal momentum. We conclude that this dynamical pattern is not derived from the rearrangement of atoms but from a coherent arrangement of ILMs decorating the crystal lattice in equilibrium.