RESUMO
We present a numerical study of the structured light produced by a laser beam transmitted by a symmetric pyramidal prism. From the Fresnel diffraction formulation, we obtain expressions for the amplitudes valid for an arbitrary number of prism faces, and for both acute and flat-topped prisms. The expressions are readily evaluated numerically and are a significant advancement over the restrictive plane wave models used in prior work. We consider applications in optical trapping and give examples in which the intensity distributions contain a number of bright spots having similar power, as is suitable for the simultaneous trapping of several particles. We also consider applications in lithography and, under other conditions, obtain results in which uniform periodic intensity patterns are produced. Advantages of employing pyramidal prisms in such applications are their excellent stability and their efficiency in the production of structured light.
RESUMO
We report on the observation of the detachment in situ and in vivo of Dunaliella tertiolecta microalgae cells from a glass surface using a 1064 nm wavelength trapping laser beam. The principal bends of both flagella of Dunaliella were seen self-adhered to either the top or bottom coverslip surfaces of a 50 µm thick chamber. When a selected attached Dunaliella was placed in the trapping site, it photoresponded to the laser beam by moving its body and flagellar tips, which eventually resulted in its detachment. The dependence of the time required for detachment on the trapping power was measured. No significant difference was found in the detachment time for cells detached from the top or bottom coverslip, indicating that the induced detachment was not due solely to the optical forces applied to the cells. After detachment, the cells remained within the optical trap. Dunaliella detached from the bottom were seen rotating about their long axis in a counterclockwise direction, while those detached from the top did not rotate. The rotation frequency and the minimal force required to escape from the trap were also measured. The average rotation frequency was found to be independent of the trapping power, and the swimming force of a cell escaping the laser trap ranged from 4 to 10 picoNewtons. Our observations provide insight into the photostimulus produced when a near-infrared trapping beam encounters a Dunaliella. The microalgae frequently absorb more light than they can actually use in photosynthesis, which could cause genetic and molecular changes. Our findings may open new research directions into the study of photomovement in species of Dunaliella and other swimming microorganisms that could eventually help to solve technological problems currently confronting biomass production. In future work, studies of the response to excess light may uncover unrecognized mechanisms of photoprotection and photoacclimation.
Assuntos
Clorofíceas/fisiologia , Microalgas/fisiologia , Pinças Ópticas , Vidro , Lasers , Luz , FotossínteseRESUMO
We present a method to provide absolute intensity calibration of a common low-resolution, wideband optical spectrometer. Terrestrial solar radiation is employed as the source of illumination, and the spectrometer output is compared to a solar spectral standard to determine its calibration. Over 3 days of observation, the procedure is demonstrated to yield reproducible results. Other evidence of calibration accuracy is presented.
RESUMO
An experimental study of the dispersion cancellation occurring in frequency-entangled photon pairs is presented. The approach uses time-resolved up-conversion of the pairs, which has temporal resolution at the femtosecond level, and group-delay dispersion sensitivity of ≈ 20 fs2 under experimental conditions. The cancellation is demonstrated with dispersion stronger than ± 10(3) fs2 in the signal (-) and idler (+) modes.
RESUMO
In the process of spontaneous parametric down-conversion, photons from a pump field are converted to signal and idler photon pairs in a nonlinear crystal. The reversed process, or up-conversion of these pairs back to single photons in a second crystal, is also possible. Here, we present experimental measurements of the up-conversion rate with a controlled time delay introduced between the signal and idler photons. As a function of delay, this rate presents a full width at half maximum of 27.9 fs under our experimental conditions, and we further demonstrate that group delay dispersion of the photon pairs broadens this width. These observations are in close agreement with our calculations, thus demonstrating an ultrafast, nonclassical correlation between the signal and idler waves.
RESUMO
We report spontaneous parametric downconversion having an unusually wide spectral bandwidth. A collinear type 1 phase-matching configuration is employed with degeneracy near the zero group-velocity dispersion frequency. With a spectral width of 1080 nm and degenerate wavelength of 1885 nm, the source also emits a high flux of 3.4 x 10(11) s(-1)W(-1) photon pairs constrained to a cone of only approximately 2 degrees half-angle. A rigorous theoretical approach is developed that confirms the experimental observations. The source properties are consistent with an ultrashort photon-pair correlation time and, for a narrowband pump, extremely high-dimensional spectral entanglement.