Wright E M, Koch S W, Kolesik M, Moloney J V
College of Optical Sciences, University of Arizona, Tucson, AZ 85721, United States of America.
Rep Prog Phys. 2019 Jun;82(6):064401. doi: 10.1088/1361-6633/ab1a07. Epub 2019 Apr 16.
There are currently intense efforts being directed towards extending the range and energy of long distance nonlinear pulse propagation in the atmosphere by moving to longer infrared wavelengths, with the purpose of mitigating the effects of turbulence. In addition, picosecond and longer pulse durations are being used to increase the pulse energy. While both of these tacks promise improvements in applications, such as remote sensing and directed energy, they open up fundamental issues regarding the standard model used to calculate the nonlinear optical properties of dilute gases. Amongst these issues is that for longer wavelengths and longer pulse durations, exponential growth of the laser-generated electron density, the so-called avalanche ionization, can limit the propagation range via nonlinear absorption and plasma defocusing. It is therefore important for the continued development of the field to assess the theory and role of avalanche ionization in gases for longer wavelengths. Here, after an overview of the standard model, we present a microscopically motivated approach for the analysis of avalanche ionization in gases that extends beyond the standard model and we contend is key for deepening our understanding of long distance propagation at long infrared wavelengths. Our new approach involves the mean electron kinetic energy, the plasma temperature, and the free electron density as dynamic variables. The rate of avalanche ionization is shown to depend on the full time history of the pulsed excitation, as opposed to the standard model in which the rate is proportional to the instantaneous intensity. Furthermore, the new approach has the added benefit that it is no more computationally intensive than the standard one. The resulting memory effects and some of their measurable physical consequences are demonstrated for the example of long-wavelength infrared avalanche ionization and long distance high-intensity pulse propagation in air. Our hope is that this report in progress will stimulate further discussion that will elucidate the physics and simulation of avalanche ionization at long infrared wavelengths and advance the field.
目前，人们正致力于通过转向更长的红外波长来扩展大气中长距离非线性脉冲传播的范围和能量，以减轻湍流的影响。此外，皮秒及更长的脉冲持续时间正被用于增加脉冲能量。虽然这两种方法都有望改进诸如遥感和定向能等应用，但它们也引发了一些关于用于计算稀薄气体非线性光学特性的标准模型的基本问题。其中一个问题是，对于更长的波长和更长的脉冲持续时间，激光产生的电子密度呈指数增长，即所谓的雪崩电离，会通过非线性吸收和等离子体散焦限制传播范围。因此，评估雪崩电离在更长波长气体中的理论和作用对于该领域的持续发展至关重要。在此，在对标准模型进行概述之后，我们提出一种基于微观动机的方法来分析气体中的雪崩电离，该方法超越了标准模型，并且我们认为这对于深化我们对长红外波长下长距离传播的理解至关重要。我们的新方法将平均电子动能、等离子体温度和自由电子密度作为动态变量。结果表明，雪崩电离速率取决于脉冲激发的完整时间历程，这与标准模型中速率与瞬时强度成正比的情况不同。此外，新方法还有一个额外的好处，即它在计算上并不比标准方法更密集。通过长波长红外雪崩电离以及空气中长距离高强度脉冲传播的例子，展示了由此产生的记忆效应及其一些可测量的物理后果。我们希望这份正在撰写的报告能够激发进一步的讨论，从而阐明长红外波长下雪崩电离的物理原理和模拟，并推动该领域的发展。
