Biomedical subjects
Publications and source records attributed to J Nees.
Using femtosecond microscopy, we observe subpicosecond transport of thermal energy radially outward from a micrometer-sized spot of an aluminum target following P-polarized excitation at >10(18) W/cm2 with a 24 fs pulse. The rapid expansion coincides with the onset of nonlocal energy transport dominated by radiation and hot electrons.
Electron bunches of attosecond duration may coherently interact with laser beams. We show how p-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches. Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer. In the plasma, due to the self-intersection of electron trajectories, electron concentration is abruptly peaked. A group of counterstream electrons is pushed away from the plasma through nulls in the electromagnetic field, having inherited a peaked electron density distribution and forming relativistic ultrashort bunches in vacuum.
BACKGROUND: Percutaneous dilational tracheostomy (PDT) is considered to be an accepted method in intensive care patients. In 2002 Frova and Quintel described a method of dilation that employed controlled rotation of the PercuTwist dilational device. The goal of the present study was to evaluate the new technique employed by an experienced team. PATIENTS AND METHODS: Prospective, observational clinical study in 54 intensive care patients who required PDT. All tracheostomies were accompanied by bronchoscopic control. Vital parameters and perioperative complications were registered. RESULTS: In all 54 consecutive PercuTwist tracheostomies no severe complications were noted. Accidental tracheal ring fracture was noted in 7 patients while bleeding that needed surgical care occurred in 1 patient. CONCLUSION: The PercuTwist tracheostomy is a safe procedure for intensive care patients. More prospective studies that would compare the PercuTwist tracheostomy with the other PDT methods are necessary.
Generation of relativistic electrons from the interaction of a laser pulse with a high density plasma foil, accompanied by an underdense preplasma in front of it, has been studied with two-dimensional particle-in-cell (PIC) simulations for pulse durations comparable to a single cycle and for single-wavelength spot size. The electrons are accelerated predominantly in forward direction for a preplasma longer than the pulse length. Otherwise, both forward and backward electron accelerations occur. The primary mechanism responsible for electron acceleration is identified. Simulations show that the energy of the accelerated electrons has a maximum versus the pulse duration for relativistic laser intensities. The most effective electron acceleration takes place when the preplasma scale length is comparable to the pulse duration. Electron distribution functions have been found from PIC simulations. Their tails are well approximated by Maxwellian distributions with a hot temperature in the MeV range.
The photovoltaic signal associated with the primary photochemical event in an oriented bacteriorhodopsin film is measured by directly probing the electric field in the bacteriorhodopsin film using an ultrafast electro-optic sampling technique. The inherent response time is limited only by the laser pulse width of 500 fs, and permits a measurement of the photovoltage with a bandwidth of better than 350 GHz. All previous published studies have been carried out with bandwidths of 50 GHz or lower. We observe a charge buildup with an exponential formation time of 1.68 +/- 0.05 ps and an initial decay time of 31.7 ps. Deconvolution with a 500-fs Gaussian excitation pulse reduces the exponential formation time to 1.61 +/- 0.04 ps. The photovoltaic signal continues to rise for 4.5 ps after excitation, and the voltage profile corresponds well with the population dynamics of the K state. The origin of the fast photovoltage is assigned to the partial isomerization of the chromophore and the coupled motion of the Arg-82 residue during the primary event.