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Transition from linear- to nonlinear-focusing regime in filamentation.

Lim K, Durand M, Baudelet M, Richardson M - Sci Rep (2014)

Bottom Line: Nevertheless, this transition point has not been identified.In high-NA conditions, filamentation is primarily governed by geometrical focusing and plasma effects, while the Kerr nonlinearity plays a more significant role as NA decreases.We find the transition between the two regimes to be relatively insensitive to the intrinsic laser parameters, and our analysis agrees well with a wide range of parameters found in published literature.

View Article: PubMed Central - PubMed

Affiliation: Laser Plasma Laboratory, Townes Laser Institute, College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA.

ABSTRACT
Laser filamentation in gases is often carried out in the laboratory with focusing optics to better stabilize the filament, whereas real-world applications of filaments frequently involve collimated or near-collimated beams. It is well documented that geometrical focusing can alter the properties of laser filaments and, consequently, a transition between a collimated and a strongly focused filament is expected. Nevertheless, this transition point has not been identified. Here, we propose an analytical method to determine the transition, and show that it corresponds to an actual shift in the balance of physical mechanisms governing filamentation. In high-NA conditions, filamentation is primarily governed by geometrical focusing and plasma effects, while the Kerr nonlinearity plays a more significant role as NA decreases. We find the transition between the two regimes to be relatively insensitive to the intrinsic laser parameters, and our analysis agrees well with a wide range of parameters found in published literature.

No MeSH data available.


Related in: MedlinePlus

Simulation of a filamenting beam with different initial focusing conditions.Left column (a,d,g,j) shows the beam sizes (fluence FWHM) derived from different versions of the simulation, as well as the positions of zK, zp and the theoretical collapse position of the beams as predicted by Marburger's formula. Center column (b,e,h,k) shows the on-axis temporal profiles of the pulse derived from the full NLSE, while the right column (c,f,i,l) shows the temporal profiles when the simulation is performed without the terms describing the Kerr effect.
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f2: Simulation of a filamenting beam with different initial focusing conditions.Left column (a,d,g,j) shows the beam sizes (fluence FWHM) derived from different versions of the simulation, as well as the positions of zK, zp and the theoretical collapse position of the beams as predicted by Marburger's formula. Center column (b,e,h,k) shows the on-axis temporal profiles of the pulse derived from the full NLSE, while the right column (c,f,i,l) shows the temporal profiles when the simulation is performed without the terms describing the Kerr effect.

Mentions: To verify that the proposed analytical method accurately determines the transition between focusing regimes, a series of numerical simulations were carried out. The same initial beam parameters as those in Figure 1 were used, and the beam was initially focused at different conditions varying from NA = 0.85 × 10−3 (f = 5 m) to 11×10−3 (f = 0.4 m). To evaluate the relative importance of KSF and plasma defocusing, the same simulations were repeated, but with the terms describing Kerr effect or plasma effects removed. Some of the results are shown in Figure 2. In the left column, the beam sizes from the different simulations are plotted together for comparison. The positions of zK, zp and the theoretical collapse position predicted by Marburger's formula25 are also indicated in the plots. By comparing the beam sizes for linear focusing (black dotted lines) against the results without Kerr effect (red dash-dot lines), the location where plasma defocusing becomes important, assuming the absence of Kerr effect, can be easily determined. This position corresponds well to zp. This substantiates our definition of zp. The definition of zK is also consistent with the simulations, as it occurs before the theoretical collapse of the beam; KSF must become prominent before beam collapse takes place.


Transition from linear- to nonlinear-focusing regime in filamentation.

Lim K, Durand M, Baudelet M, Richardson M - Sci Rep (2014)

Simulation of a filamenting beam with different initial focusing conditions.Left column (a,d,g,j) shows the beam sizes (fluence FWHM) derived from different versions of the simulation, as well as the positions of zK, zp and the theoretical collapse position of the beams as predicted by Marburger's formula. Center column (b,e,h,k) shows the on-axis temporal profiles of the pulse derived from the full NLSE, while the right column (c,f,i,l) shows the temporal profiles when the simulation is performed without the terms describing the Kerr effect.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4248277&req=5

f2: Simulation of a filamenting beam with different initial focusing conditions.Left column (a,d,g,j) shows the beam sizes (fluence FWHM) derived from different versions of the simulation, as well as the positions of zK, zp and the theoretical collapse position of the beams as predicted by Marburger's formula. Center column (b,e,h,k) shows the on-axis temporal profiles of the pulse derived from the full NLSE, while the right column (c,f,i,l) shows the temporal profiles when the simulation is performed without the terms describing the Kerr effect.
Mentions: To verify that the proposed analytical method accurately determines the transition between focusing regimes, a series of numerical simulations were carried out. The same initial beam parameters as those in Figure 1 were used, and the beam was initially focused at different conditions varying from NA = 0.85 × 10−3 (f = 5 m) to 11×10−3 (f = 0.4 m). To evaluate the relative importance of KSF and plasma defocusing, the same simulations were repeated, but with the terms describing Kerr effect or plasma effects removed. Some of the results are shown in Figure 2. In the left column, the beam sizes from the different simulations are plotted together for comparison. The positions of zK, zp and the theoretical collapse position predicted by Marburger's formula25 are also indicated in the plots. By comparing the beam sizes for linear focusing (black dotted lines) against the results without Kerr effect (red dash-dot lines), the location where plasma defocusing becomes important, assuming the absence of Kerr effect, can be easily determined. This position corresponds well to zp. This substantiates our definition of zp. The definition of zK is also consistent with the simulations, as it occurs before the theoretical collapse of the beam; KSF must become prominent before beam collapse takes place.

Bottom Line: Nevertheless, this transition point has not been identified.In high-NA conditions, filamentation is primarily governed by geometrical focusing and plasma effects, while the Kerr nonlinearity plays a more significant role as NA decreases.We find the transition between the two regimes to be relatively insensitive to the intrinsic laser parameters, and our analysis agrees well with a wide range of parameters found in published literature.

View Article: PubMed Central - PubMed

Affiliation: Laser Plasma Laboratory, Townes Laser Institute, College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA.

ABSTRACT
Laser filamentation in gases is often carried out in the laboratory with focusing optics to better stabilize the filament, whereas real-world applications of filaments frequently involve collimated or near-collimated beams. It is well documented that geometrical focusing can alter the properties of laser filaments and, consequently, a transition between a collimated and a strongly focused filament is expected. Nevertheless, this transition point has not been identified. Here, we propose an analytical method to determine the transition, and show that it corresponds to an actual shift in the balance of physical mechanisms governing filamentation. In high-NA conditions, filamentation is primarily governed by geometrical focusing and plasma effects, while the Kerr nonlinearity plays a more significant role as NA decreases. We find the transition between the two regimes to be relatively insensitive to the intrinsic laser parameters, and our analysis agrees well with a wide range of parameters found in published literature.

No MeSH data available.


Related in: MedlinePlus