Adrian Lucero

The importance of corona generation and leader formation during laser filament guided discharges in air

Images taken with an intensified CCD camera show the dynamics during filament guided discharge events. The images reveal that filament initiated corona plays a role in the presented results. Furthermore, the images show the formation of leaders, propagating and eventually bridging the gap between the high voltage (HV) electrodes. Analysis of the images and comparison to oscilloscope traces of voltage and current dynamics reveal the origin of the delay between the filament and HV discharge and allows for a probability of discharge analysis.

Third-order optical nonlinearity effect of DNA- and polyvinylpyrrolidone-functionalized carbon nanotubes

We have used single-stranded DNA (ssDNA) and polyvinylpyrrolidone (PVP) to disperse multiwalled carbon nanotubes (MWNTs) in water solution through sonication and centrifuge procedures. The advantage of these two polymers is that they do not need toxic organic solvents to distribute the carbon nanotubes. The scanning electron microscope (SEM) technique has been used to investigate the interaction between polymer molecules and MWNTs. The images show that MWNTs can be distributed effectively into the two polymer solutions. The third-order optical susceptibility, nonlinear optical absorption coefficient and optical power limiting of these dispersions have been characterized experimentally using a femtosecond laser system with a tunable range of 750–850 nm. The imaginary part of the third-order optical susceptibility has also been computed.

Electric field measurements during filament-guided discharge

Abstract. One application of ultrashort pulse filamentation is the coupling of external electric fields to filament plasmas and guiding of high-voltage discharges. However, the full physics of the guiding mechanism is still in question. Several models have been presented and explanations have been suggested to capture the full physics of the discharge event. For the first time, measurements of the electric field dynamics between two electrodes during filament-guided discharges are presented here, to the best of our knowledge. The electric field dynamics show an exponential growth region, a plateau, followed by a sharp drop off coinciding with the discharge event. We believe these results will ultimately answer the questions regarding the guiding mechanism.

Plasma Generation by Laser Filamentation in Air

Streak cameras, with high-temporal resolution, are a powerful tool to image light propagation and plasma dynamics. Here, we present the image of a 2.2-TW peak power, 50-fs laser pulse, propagating in a nonlinear fashion over a distance of 1.3 m, leaving behind plasma. A direct measurement of the Rayleigh scattering of the pulse and the plasma emission are presented here.

Generation of multiterawatt vortex laser beams

We report the fabrication of large-area phase masks on thin fused-silica substrates that are suitable for shaping multiterawatt femtosecond laser beams. We apply these phase masks for the generation of intense femtosecond optical vortices. We further quantify distortions of the vortex beam patterns that result from several common types of mask defects.

Dependence of single-shot pulse durations on near-infrared filamentation-guided breakdown in air

We present results of an experimental investigation of laser pulsewidth dependence of filamentation-guided high voltage breakdown in air. The experiments are conducted at laser peak power levels of 1 TW and pulse durations of 0.7 to 10 ps with a discharge gap separation of 10 cm. Synchronized electrical and optical diagnostic techniques were used to determine the pulsewidth dependence on the breakdown mechanism, threshold levels, time delays and associated jitter. The results indicate that longer pulses provide greater than 30% reduction in breakdown threshold voltage.

Radiofrequency electromagnetic pulses generated by ultrafast laser filaments

Summary form only given. The weakly ionized plasma channel formed in the wake of a self-focused femtosecond laser pulse (i.e. a filament) emits a broadband electromagnetic pulse (EMP) having frequencies from several THz to below 1 GHz. While the THz emissions have been fairly well-studied, little attention has been paid to the RF emissions from the filaments themselves, or from interactions of the filaments with solid materials. The EMP components in the THz regime have been attributed to electron ponderomotive motion driven by the laser pulse1. While RF components have been observed in several experiments2,3 attempts to identify their source have been inconclusive. We report on progress toward determining the source of the RF components of the EMP from filaments propagating in air, as well as filaments impinging on conducting and dielectric materials. Spectrally-resolved maps of the near-field EMP power density from filaments in air at a laser power of about 2.5 TW show that the structure of the radiation pattern is frequency-dependent over frequencies of 1-1000 GHz. The EMP generated by filaments interacting with solids is fundamentally different. In addition to having much larger field amplitude than filaments in air alone, the emission pattern over the low-frequency range (1-14 GHz) has the same shape as that of a dipole antenna. Evaluating the spectral power density and the coherence properties of the RF emissions will give insight as to whether thermal or collective plasma motions are the dominant generating mechanism.

Laser plasmas from picosecond laser filamentation in the atmosphere and its application on guided high voltage discharges

Summary form only given. Femtosecond laser filamentation in the atmosphere produces plasmas with densities on the order of 1016 cm-3 or less than 1% of air ionized [1]. Filamentation is a nonlinear process where a short laser pulse is propagating through a nonlinear medium while a dynamic balance between non-linear focusing, diffraction and plasma defocusing is established. The short lifetime and relative low temperature of the laser-generated plasma results in a high resistivity, making femtosecond filaments difficult to utilize for some applications including guiding high voltage electric discharges [2]. To achieve guided discharges with femtosecond filaments over several meters requires voltages larger than 100 kV between the electrodes. Alternatively nanosecond lasers have been used to create plasma sparks and trigger short discharges. The nanosecond laser plasma is denser and hotter in comparison to the femtosecond filament plasma. These properties result in a high conductivity and a fast trigger of the high voltage discharge [3]. However, the spatial extent of these plasmas is limited and only short discharges can be initiated. A recent experiment on picosecond laser filamentation [4] indicates the possibility of combining the properties of the two plasmas described above: an extended plasma channel with high conductivity. Here, we present experimental results that show the effect of picosecond laser filamentation on guiding high voltage discharges. The results indicate that higher conductivity plasma is produced in comparison to a femtosecond pulse. We achieved a reduction of threshold break down voltage by 20% with our experimental setup. The experiments are conducted using the COMET laser, at Jupiter Laser facility, Lawrence Livermore Laboratory.

The role of corona and space charges during femtosecond laser pulse filament guided high voltage discharges in air

Summary form only given. The plasma column left behind by ultrashort laser pulse filamentation is utilized to guide high voltage (HV) discharges in air. Many experiments have been carried out where the filament plasma is placed between two HV electrodes and a discharge is guided across the large gap (for example Ref [1]). Researchers still believe that one application could be potentially guiding lightning in the atmosphere, like a lightning rod, due to the longitudinal extent of the plasma ranging over several 10s of meters [2]. However lab experiments can only demonstrate discharges over a few meters. The reason for this is still not very clear due to the overall physics of the process not being fully understood. Here, we present experimental results in air on the role of space charges and corona during the filament guided HV discharge. Our conclusion is that the main driver of the breakdown is an enhancement of the corona between the electrodes and that the electric fields available limit the discharge distance achievable.Filamentation is a non-linear process where non-linear selffocusing (due to the Kerr effect), diffraction, and plasma defocusing create a dynamic balance [3]. The laser pulse propagates in a small beam diameter longer than the usual Rayleigh length. The plasma left behind the filament has a density on the order of 1016 cm-3 and a low conductivity. Initially it was believed that the plasma could directly guide HV discharges. However, a delay between the actual discharge and the filament placement is observed, ruling out the direct wire-like guiding mechanism. Previously, the reason for this delay was not completely understood. Our experimental results show that an enhancement of the formation of corona at the HV probes can be observed. From there, the dynamics rely on the electric field as a driver for developing leaders across the gap until a conducting connection is formed between the electrodes. These results show that the discharge distance is limited by the electric field and therefore limits the usefulness for the applications originally intended.

Plasma kinetics in ultrashort pulse laser filament: Time resolved spectral measurement

Summary form only given. Filamentation of ultrafast laser pulses in air has been studied extensively. The very intense light channels, small in diameter for a long distance (widely believed to be an equilibrium of Kerr self-focusing, linear diffraction and plasma defocusing) leave a low density plasma in its wake [1]. Over the years, researchers have tried to characterize these plasma channels for applications. Such applications range from filament guided discharges to remote sensing. In order to use the filament plasma for applications, the plasma properties and dynamics need to be understood in great detail. In the air, the plasma densities are on the order of 1016 cm-3 [2] Until now, there are only few experimental reports that have measured the temperature of the plasma, yielding a temperature between 3000 and 5000 K [3,4]. One approach focused on the absorption and diffraction of a probe beam propagating perpendicularly to the filament [3], a second approach studied spectral line broadening and emission intensities [4]. The first approach is a pump probe experiment with good time resolution, but is not doable in single shot. Whereas the second approach integrates over the lifetime of the filament and therefore does not capture the time dynamics of the plasma. In our approach, we are looking at certain spectral lines of molecules to study the time dependent kinetics of the filament plasma produced by a 800nm, 1.5 mJ, 35 fs laser pulse by using a spectrometer coupled to a streak camera. The time resolution of the streak camera is 25 ps, and can capture a streak of more than 1 ns. Investigating the spectral lines in molecular nitrogen (337.1 nm, 357.6 nm and 353.6 nm) of the second positive system C3Πu-B3Πg, we observed the buildup of the light emission. The maximum of the emission is reached in the order of hundred picosecond. These results are expected since the excitation of these levels is driven by inelastic electron-molecule collisions and therefore closely correlated to the plasma kinetics. A computer model for the plasma kinetics is currently being developed to explain the observed experimental data and to extract the plasma temperature and density.