Cavity-enhanced direct frequency comb spectroscopy

Cavity-enhanced direct frequency comb spectroscopy combines broad spectral bandwidth, high spectral resolution, precise frequency calibration, and ultrahigh detection sensitivity, all in one experimental platform based on an optical frequency comb interacting with a high-finesse optical cavity. Precise control of the optical frequency comb allows highly efficient, coherent coupling of individual comb components with corresponding resonant modes of the high-finesse cavity. The long cavity lifetime dramatically enhances the effective interaction between the light field and intracavity matter, increasing the sensitivity for measurement of optical losses by a factor that is on the order of the cavity finesse. The use of low-dispersion mirrors permits almost the entire spectral bandwidth of the frequency comb to be employed for detection, covering a range of ~10% of the actual optical frequency. The light transmitted from the cavity is spectrally resolved to provide a multitude of detection channels with spectral resolutions ranging from a several gigahertz to hundreds of kilohertz. In this review we will discuss the principle of cavity-enhanced direct frequency comb spectroscopy and the various implementations of such systems. In particular, we discuss several types of UV, optical, and IR frequency comb sources and optical cavity designs that can be used for specific spectroscopic applications. We present several cavity-comb coupling methods to take advantage of the broad spectral bandwidth and narrow spectral components of a frequency comb. Finally, we present a series of experimental measurements on trace gas detections, human breath analysis, and characterization of cold molecular beams.Comment: 36 pages, 27 figure

Interactions of Neisseria meningitidis with cells of the human meninges

The interaction of Neisseria meningitidis with the meninges that surround and protect the brain is a pivotal event in the progression of bacterial meningitis. Two models of the human meninges were established in vitro, using (i) sections of fresh human brain and (ii) cultures of viable cells grown from human meningiomas. Neisseria meningitidis showed a specific predilection for binding to the leptomeninges and meningeal blood vessels in human brain and not to the cerebral cortex. There was a close correlation between the adherence of different Neisseria species to leptomeninges and cultured cells. The major ligand that mediated adherence was the pilus, and pilin variation modulated the interactions. The presence of Opa protein increased the association of Cap+ meningococci that expressed low-adhesive pili, but did not influence the association of high-adhesive pili. In contrast, Opc did not influence the adherence of Cap+ meningococci, whereas loss of capsule was associated with a more intimate interaction between the bacteria and the meningioma cell that was not apparent with Cap+ meningococci. There was no evidence of internalization of meningococci by meningioma cells in vitro, an observation that is consistent with the barrier properties of the leptomeninges to N. meningitidis observed in vivo