Clausius-Mossotti approximation for a family of nonlinear composites

Photon echoes using atomic frequency combs in Pr:YSO - experiment and semiclassical theory

Photon echoes in rare-earth-doped crystals are studied to understand the challenges of making broadband quantum memories using the atomic frequency comb (AFC) protocol in systems with hyperfine structure. The hyperfine structure of Pr³⁺ poses an obstacle to this goal because frequencies associated with the hyperfine transitions change the simple picture of modulation at an externally imposed frequency. The current work focuses on the intermediate case where the hyperfine spacing is comparable to the comb spacing, a challenging regime that has recently been considered. Operating in this regime may facilitate storing quantum information over a larger spectral range in such systems. In this work, we prepare broadband AFCs using optical combs with tooth spacings ranging from 1 MHz to 16 MHz in fine steps, and measure transmission spectra and photon echoes for each. We predict the spectra and echoes theoretically using the optical combs as input to either a rate equation code or a density matrix code, which calculates the redistribution of populations. We then use the redistributed populations as input to a semiclassical theory using the frequency-dependent dielectric function. The two sets of predictions each give a good, but different account of the photon echoes.

Solute-induced retardation of water dynamics probed directly by terahertz spectroscopy

The dynamics of water surrounding a solute is of fundamental importance in chemistry and biology. The properties of water molecules near the surface of a bio-molecule have been the subject of numerous, sometimes controversial experimental and theoretical studies, with some suggesting the existence of rather rigid water structures around carbohydrates and proteins [Pal, S. K., Peon, J., Bagchi, B. and Zewail A. H. (2002) J. Phys. Chem. B 106, 12376-12395]. Hydrogen bond rearrangement in water occurs on the picosecond time scale, so relevant experiments must access these times. Here, we show that terahertz spectroscopy can directly investigate hydration layers. By a precise measurement of absorption coefficients between 2.3 THz and 2.9 THz we could determine the size and the characteristics of the hydration shell. The hydration layer around a carbohydrate (lactose) is determined to extend to 5.13 +/- 0.24 A from the surface corresponding to approximately 123 water molecules beyond the first solvation shell. Accompanying molecular modeling calculations support this result and provide a microscopic visualization. Terahertz spectroscopy is shown to probe the collective modes in the water network. The observed increase of the terahertz absorption of the water in the hydration layer is explained in terms of coherent oscillations of the hydration water and solute. Simulations also reveal a slowing down of the hydrogen bond rearrangement dynamics for water molecules near lactose, which occur on the picosecond time scale. The present study demonstrates that terahertz spectroscopy is a sensitive tool to detect solute-induced changes in the water network.