Ultra-low Absorption Research Articles

Configuration and Dynamics of Xanthophylls in Light-harvesting Antennae of Higher Plants: SPECTROSCOPIC ANALYSIS OF ISOLATED LIGHT-HARVESTING COMPLEX OF PHOTOSYSTEM II AND THYLAKOID MEMBRANES

Resonance Raman excitation spectroscopy combined with ultra low temperature absorption spectral analysis of the major xanthophylls of higher plants in isolated antenna and intact thylakoid membranes was used to identify carotenoid absorption regions and study their molecular configuration. The major electronic transitions of the light-harvesting complex of photosystem II (LHCIIb) xanthophylls have been identified for both the monomeric and trimeric states of the complex. One long wavelength state of lutein with a 0-0 transition at 510 nm was detected in LHCIIb trimers. The short wavelength 0-0 transitions of lutein and neoxanthin were located at 495 and 486 nm, respectively. In monomeric LHCIIb, both luteins absorb around 495 nm, but slight differences in their protein environments give rise to a broadening of this band. The resonance Raman spectra of violaxanthin and zeaxanthin in intact thylakoid membranes was determined. The broad 0-0 absorption transition for zeaxanthin was found to be located in the 503-511 nm region. Violaxanthin exhibited heterogeneity, having two populations with one absorbing at 497 nm (0-0), 460 nm (0-1), and 429 nm (0-2), and the other major pool absorbing at 488 nm (0-0), 452 nm (0-1), and 423 nm (0-2). The origin of this heterogeneity is discussed. The configuration of zeaxanthin and violaxanthin in thylakoid membranes was different from that of free pigments, and both xanthophylls (notably, zeaxanthin) were found to be well coordinated within the antenna proteins in vivo, arguing against the possibility of their free diffusion in the membrane and supporting our recent biochemical evidence of their association with intact oligomeric light-harvesting complexes (Ruban, A. V., Lee, P. J., Wentworth, M., Young, A. J., and Horton, P. (1999) J. Biol. Chem. 274, 10458-10465).

Optical fringes reduction in ultrasensitive diode laser absorption spectroscopy

The simple and effective interference fringe suppression technique for ultralow absorption measurements is described. The angular beam modulation can reduce fringe effects by a factor of 100 and more. The shot noise limit in absorption experiments can be achieved using this technique in combination with a dual-beam logarithmic registration of wavelength modulated radiation from the diode laser (DL).

Ultra-low absorption measurement in dielectrics in millimeter- and submillimeter-wave range

A measurement of ultra-low absorption of microwave radiation in dielectrics is reported. Two Fabry-Perot resonators with /spl ges/600000 quality factors, fully general-purpose interface bus programmable millimeter-wave frequency synthesizer with 10-15-mW continuous wave (CW) power level, 100-Hz frequency resolution from 78 to 118 GHz, and corresponding hardware and software for signal processing were used. The /spl plusmn/500-Hz accuracy of resonance curve width measurements was reached. This high accuracy allowed loss tangent measurement as small as 10/sup -6/-10/sup -7/ in dielectric samples with a thickness of /spl sim/0.5 mm. A convenient method of measurements of almost arbitrary plane parallel samples has been developed and described. Practical applications such as development and control of thin low-loss resonant windows of powerful (/spl sim/1-MW CW) gyrotrons used in thermonuclear experiments, precise reflection coefficient of metals measurements, as well as other applications are discussed. The existence of such technique up to frequencies exceeding 1 THz makes measurements described at the whole millimeter- and submillimeter-wave bands affordable.

Intracavity photothermal measurements of ultralow absorption

A new method of ultralow optical absorption measurement based on the photothermal deflection effect is reported. An increase in sensitivity up to 4 orders of magnitude in comparison to a conventional transverse photothermal deflection method is achieved by placing the sample inside a high quality laser resonator. The method is realized for a diode laser pump emitting at 687 nm and an amorphous silicon (a-Si:H) sample of 1 μm thickness, using the air as a deflecting medium. The advantages of the method are discussed.

Low OH excess loss PCVD fibres prepared by fluorine doping

Single-mode and multimode fibres have been prepared with PCVD using C2F6 as dopant. With increasing concentrations of C2F6 an exponential decrease of the OH excess loss was found. Ultra-low OH absorption losses of 0.2 dR/km at 1.38 μm have been achieved.