The rate of Q(x)→Q(y) relaxation in bacteriochlorophylls of reaction centers from Rhodobacter sphaeroides determined by kinetics of the ultrafast carotenoid bandshift.

Transient absorption changes induced by excitation of isolated reaction centers (RCs) from Rhodobacter sphaeroides with 600nm laser pulses of 20fs (full width at half maximum) were monitored in the wavelength region of 420-560nm. The spectral features of the spectrum obtained are characteristic for an electrochromic band shift of the single carotenoid (Car) molecule spheroidene, which is an integral constituent of these RCs. This effect is assigned to an electrochromic bandshift of Car due to the local electric field of the dipole moment formed by electronic excitation of bacteriochlorophyll (BChl) molecule(s) in the neighborhood of Car. Based on the known distances between the pigments, the monomeric BChl (B(B)) in the inactive B-branch is inferred to dominate this effect. The excitation of B(B) at 600nm leads to a transition into the S(2) state (Q(x) band), which is followed by rapid internal conversion to the S(1) state (Q(y) band), thus leading to a change of strength and orientation of the dipole moment, i.e., of the electric field acting on the Car molecule. Therefore, the time course of the electrochromic bandshift reflects the rate of the internal conversion from S(2) to S(1) of B(B). The evaluation of the kinetics leads to a value of 30fs for this relaxation process. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Matching p-i-n-junctions and optical modes enables fast and ultra-small silicon modulators.

In this article a new method is presented that allows for low loss implementation of fast carrier transport structures in diffraction limited photonic crystal resonators. We utilize a 'node-matched doping' process in which precise silicon doping results in comb-like shaped, highly-doped diode areas that are matched to the spatial field distribution of the optical modes of a Fabry-Pérot resonator. While the doping is only applied to areas with low optical field strength, the intrinsic diode region overlaps with an optical field maximum. The presented node-matched diode-modulators, combining small size, high-speed, thermal stability and energy-efficient switching could become the centerpiece for monolithically integrated transceivers.

Pyrometry with flexible infrared fibers for temperature-controlled laser surgery.

Pyrometry is widely used in science, medicine, and industry to measure the surface temperature of objects in a non-contact way. IR fibers are an ideal solution for the flexible delivery of thermal radiation emitted from objects inside a complex structure like internal organs inside the human body. Silver halide polycrystalline infrared fibers (PIR) are transparent in a spectral range of 3 - 18 µm, matching perfectly with the spectra of black body radiation for temperatures ranging from 20°C to 200°C. These fibers are non-toxic and allow small bending radii. They could become critical components in pyrometric systems for temperature-controlled laser surgeries. Here we discuss the ability of the PIR fibers for simultaneous laser power delivery and real-time temperature monitoring in laser surgery applications and demonstrate two different setups for this purpose.

Femtosecond spectroscopy of native and carotenoidless purple-bacterial LH2 clarifies functions of carotenoids.

EET between the two circular bacteriochlorophyll compartments B800 and B850 in native (containing the carotenoid rhodopin) and carotenoidless LH2 isolated from the photosynthetic purple sulfur bacterium Allochromatium minutissimum was investigated by femtosecond time-resolved transient absorption spectroscopy. Both samples were excited with 120-fs laser pulses at 800 nm, and spectral evolution was followed in the 720-955 nm range at different delay times. No dependence of transient absorption in the B800 band on the presence of the carotenoid rhodopin was found. Together with the likewise virtually unchanged absorption spectra in the bacteriochlorophyll Q(y) region, these observations suggest that absence of rhodopin does not significantly alter the structure of the pigment-protein complex including interactions between bacteriochlorophylls. Apparently, rhodopin does also not accelerate B800 to B850 EET in LH2, contrary to what has been suggested previously. Moreover, "carotenoid-catalyzed internal conversion" can also be excluded for the bacteriochlorophylls in LH2 of A. minutissimum. Together with previous results obtained with two-photon fluorescence excitation spectroscopy, it can also be concluded that there is neither EET from rhodopin to B800 nor (back-)EET from B800 to rhodopin.

Highly efficient 1.3-microm second-stokes PbWO4 Raman laser.

A second-Stokes Raman laser based on PbWO4 with a 1.316-microm wavelength pumped by an approximately 120-ps Nd:YAG laser at 1.06415 microm scattered at 901 cm(-1) phonon was developed. High-reflection mirrors for the Stokes wavelength were used. The maximum output energy was 0.85 mJ. The conversion efficiency increased with crystal length of 40-100 mm up to 30%. The spatial beam profile was smooth, and the Raman laser emission lasted for approximately 18 ns. New stimulated Raman scattering lines from a phonon at 323 cm(-1) were observed.