Subnanosecond Electrical Pulses Research Articles

Dynamic structure factor of a lipid bilayer in the presence of a high electric field.

The influence of a high average electric field (∼1 V/nm) in the hydrophobic interior of a bilayer lipid membrane on short-wavelength in-plane phononic motions of lipid chains is considered. The average electric field is assumed to be nearly constant on a picosecond time scale and a nanometer length scale. This field may be induced, for instance, by externally applied subnanosecond electric pulses or the membrane dipole potential. Using a generalized hydrodynamic approach, we derive a corresponding electrohydrodynamic model generalized to high wave numbers. In the considered approximation, all electric field effects are reduced only to a constant contribution to the generalized isothermal compressibility modulus. The corresponding dynamic structure factor for a lipid bilayer is derived. We show that due to polarization effects, the high field can critically impact the dynamics of longitudinal acousticlike modes at wave numbers near the major peak of the static structure factor. We estimate quantitatively that for typical lipid bilayers, transverse high electric fields can cause strong phonon energy softening, enhancement of phonon population, and formation of a gap in the dispersion of excitation frequency. The results obtained agree with simulations of the initiation of lipid bilayer electropores, suggesting that the proposed model reproduces the essential features of the field's impact on atomic density fluctuations. The proposed mechanism may have significant implications for the understanding of electroporation, passive molecular transport, and spontaneous pore formation in lipid bilayers.

Picosecond and Terahertz Perturbation of Interfacial Water and Electropermeabilization of Biological Membranes.

Non-thermal probing and stimulation with subnanosecond electric pulses and terahertz electromagnetic radiation may lead to new, minimally invasive diagnostic and therapeutic procedures and to methods for remote monitoring and analysis of biological systems, including plants, animals, and humans. To effectively engineer these still-emerging tools, we need an understanding of the biophysical mechanisms underlying the responses that have been reported to these novel stimuli. We show here that subnanosecond (≤500 ps) electric pulses induce action potentials in neurons and cause calcium transients in neuroblastoma-glioma hybrid cells, and we report complementary molecular dynamics simulations of phospholipid bilayers in electric fields in which membrane permeabilization occurs in less than 1 ns. Water dipoles in the interior of these model membranes respond in less than 1 ps to permeabilizing electric potentials by aligning in the direction of the field, and they re-orient at terahertz frequencies to field reversals. The mechanism for subnanosecond lipid electropore formation is similar to that observed on longer time scales-energy-minimizing intrusions of interfacial water into the membrane interior and subsequent reorganization of the bilayer into hydrophilic, conductive structures.

Antennas for Transcranial Stimulation with Subnanosecond Electric Pulses

Antennas for Transcranial Stimulation with Subnanosecond Electric Pulses

Electropore Dynamics in Time-Dependent Electric Fields

Experimental studies have shown that cell membranes can be permeabilized by exposure to nanosecond and subnanosecond electric pulses with megavolt-per-meter amplitudes [1,2]. For pulses in the low nanosecond and subnanosecond range, the rise time of the actual waveform delivered to a biological load is a significant percentage of the entire pulse duration, but experimental and modeling studies of the effects of varying the rise time are rare. Molecular dynamics (MD) simulations have contributed to our understanding of lipid bilayer electropermeabilization, and we show here that when time-varying external fields are introduced into MD, it becomes possible to modulate pore properties dynamically by sampling a large range of frequencies, fields, pulse shapes, and polarities with various charged and uncharged bilayer systems. This capability also enables the refinement of models of nanosecond pore formation by loosening the constraints of models with constant and idealized external perturbations. Results are compared to and, to the extent possible at this time, reconciled with existing mathematical models of electroporation, presenting a more unified and complete framework for future studies.1. Vernier, P. T., Y. Sun, and M. A. Gundersen. 2006. Nanoelectropulse-driven membrane perturbation and small molecule permeabilization. BMC Cell Biol. 7:37.2. S. Xiao, S. Guo, V. Nesin, R. Heller, and K. H. Schoenbach. 2011. Subnanosecond electric pulses cause membrane permeabilization and cell death. IEEE Trans. Biomed. Eng. 58:1239-1245.

Subnanosecond Electric Pulses Cause Membrane Permeabilization and Cell Death

Subnanosecond electric pulses (200 ps) at electric field intensities on the order of 20 kV/cm cause the death of B16.F10 murine melanoma cells when applied for minutes with a pulse repetition rate of 10 kHz. The lethal effect of the ultrashort pulses is found to be caused by a combination of thermal effects and electrical effects. Studies on the cellular level show increased transport across the membrane at much lower exposure times or number of pulses. Exposed to 2000 pulses, NG108 cells exhibit an increase in membrane conductance, but only allow transmembrane currents to flow, if the medium is positively biased with respect to the cell interior. This means that the cell membrane behaves like a rectifying diode. This increase in membrane conductance is a nonthermal process, since the temperature rise due to the pulsing is negligible.

Design and validation of an accelerator for an ultracold electron source

We describe here a specially designed accelerator structure and a pulsed power supply that are essential parts of a high brightness cold atoms-based electron source. The accelerator structure allows a magneto-optical atom trap to be operated inside of it, and also transmits subnanosecond electric field pulses. The power supply produces high voltage pulses up to 30 kV, with a rise time of up to 30 ns. The resulting electric field inside the structure is characterized with an electro-optic measurement and with an ion time-of-flight experiment. Simulations predict that 100 fC electron bunches, generated from trapped atoms inside the structure, reach an emittance of 0.04 mm mrad and a bunch length of 80 ps.

The Effect of Intense Subnanosecond Electrical Pulses on Biological Cells

Nanosecond electrical pulses have been successfully used to treat melanoma tumors by using needle arrays as pulse delivery systems. Reducing the pulse duration of intense electric field pulses from nanoseconds into the subnanosecond range will allow us to use wideband antennas to deliver the electromagnetic fields into tissue with a spatial resolution in the centimeter range. To explore the biological effect of intense subnanosecond pulses, we have developed a generator that provides voltage pulses of 160 kV amplitude, 200 ps rise time, and 800 ps pulse width. The pulses are delivered to a cylindrical Teflon chamber with polished flat electrodes at either end. The distance between the electrodes is variable and allows us to generate electric fields of up to 1 MV/cm in cell suspensions. The pulses have been applied to B16 (murine melanoma) cells, and the plasma membrane integrity was studied by means of trypan blue exclusion. For pulse amplitudes of 550 kV/cm, approximately 50% of the cells took up trypan blue right after pulsing, whereas only 20% were taking it up after 1 h. This indicates that the plasma membrane in a majority of the cells affected by the pulses recovers with a time constant of about 1 h. The cells that show trypan blue uptake after this time suffer cell death through apoptosis. Evaluation of the experimental results and molecular dynamics modeling results indicate that with a pulse duration of 800 ps, membrane charging and nanopore formation are the dominant bioelectric effects on B16 cells. This information has been used in a continuum model to estimate the increase in membrane permeability and, consequently, the increase in pore size caused by repetitive pulsing.

Origin of Dark Counts in Nanostructured NbN Single-Photon Detectors

We present our study of dark counts in ultrathin (3.5 to 10 nm thick), narrow (120 to 170 nm wide) NbN superconducting stripes of different lengths. In experiments, where the stripe was completely isolated from the outside world and kept at temperature below the critical temperature T/sub c/, we detected subnanosecond electrical pulses associated with the spontaneous appearance of the temporal resistive state. The resistive state manifested itself as generation of phase-slip centers (PSCs) in our two-dimensional superconducting stripes. Our analysis shows that not far from T/sub c/, PSCs have a thermally activated nature. At lowest temperatures, far below T/sub c/, they are created by quantum fluctuations.

Generation of high-energy (0.3 μJ) short pulses (>400 ps) from a gain-switched laser diode stack with subnanosecond electrical pump pulses

Record pulse energy (E/spl sim/0.3 /spl mu/J) is reported for a gain-switched laser diode stack with subnanosecond electrical pump pulses. Pulsewidths near 100 ps are seen to be feasible. A detailed time-resolved analysis of emitted near-field patterns is presented, which provides guidelines for further improvements in time performances. A stack design for pulsed laser diodes of high energy is discussed.

Subnanosecond photovoltaic response in 6H–SiC

We report observation of a subnanosecond photovoltaic response in 6H–SiC illuminated with an ultraviolet laser. Using different incident laser energies, the transient photovoltaic response in 6H–SiC was investigated and photovoltaic electrical pulses with various durations and amplitudes observed. Subnanosecond electrical pulses, with amplitudes of hundreds of millivolts, were generated from 6H–SiC lateral photoconductive switches excited by a nanosecond laser pulse without external bias. Modeling of this fast photovoltaic response has been performed and good qualitative agreement obtained.

Generation of KrF laser pulses on a picosecond time scale using electro-optic modulation

A new technique has been developed for the generation and shaping of KrF laser pulses on a picosecond time scale using a fast electro-optic modulator. Subnanosecond electrical pulses with an amplitude much greater than the half-wave voltage are applied to the Pockels cell, which generates a train of short ultraviolet pulses. The first pulse in this train is then isolated and shortened to 30 psec by the combined effect of propagation through a saturated amplifier and optical breakdown in xenon.

Spectral bistability in coupled cavity semiconductor lasers

Bistability in the spectral output of a two-section cleaved-coupled-cavity (C3) semiconductor laser is observed. A wide (10 mA) and temperature insensitive hysteresis loop is obtained at laser currents up to at least 3×Ith and the laser can be switched with subnanosecond electrical pulses between its two spectraly pure states. The switching is characterized by a power-delay product of 4×10−12 J and a switching time of less than 1 ns.