Dynamical rate equation model for femtosecond laser-induced breakdown in dielectrics (original) (raw)
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Nanosecond-to-femtosecond laser-induced breakdown in dielectrics
Physical Review B, 1996
We report extensive laser-induced damage threshold measurements on dielectric materials at wavelengths of 1053 and 526 nm for pulse durations ranging from 140 fs to 1 ns. Qualitative differences in the morphology of damage and a departure from the diffusion-dominated 1/2 scaling of the damage fluence indicate that damage occurs from ablation for р10 ps and from conventional melting, boiling, and fracture for Ͼ50 ps. We find a decreasing threshold fluence associated with a gradual transition from the long-pulse, thermally dominated regime to an ablative regime dominated by collisional and multiphoton ionization, and plasma formation. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional ͑avalanche͒ ionization is in quantitative agreement with the experimental results.
Optics & Laser Technology, 2012
In this paper dynamics of optical breakdown process in dielectrics induced by femtosecond laser pulses have been simulated numerically. Using rate equations, the dynamics of free electron density in the focusing region during a few laser pulse duration time was studied numerically. Sources of free electron production as well as sinks of electron reduction in the interacting region were considered in the calculations. In the simulation, the propagation of the laser beam through the focusing volume was also taken into account and the footprint of the breakdown process (as an estimation for medium bulk damage) was simulated 3-dimensionally. The temporal and spatial evolution of free electron density has been used for simulating the 3-dimensional footprint of optical breakdown region. The results show that the dynamics of the breakdown footprint (and volume) strongly depends on the laser and medium characteristics.
Radiation produced by femtosecond laser-plasma interaction during dielectric breakdown
Optics Letters, 2005
Optical breakdown by femtosecond and nanosecond laser pulses in transparent dielectrics produces an ionized region of dense plasma confined within the bulk of the material. This ionized region is responsible for broadband radiation that accompanies the breakdown process. Spectroscopic measurements of the accompanying light have been used to show that, depending on the laser parameters, the spectra may originate from plasma-induced second-harmonic generation, supercontinuum generation, or thermal emission by the plasma. By monitoring the emission from the ionized region, one can ascertain the predominant breakdown mechanism and the morphology of the damage region.
Femtosecond Optical Breakdown in Dielectrics
Physical Review Letters, 1998
We report measurements of the optical breakdown threshold and ablation depth in dielectrics with different band gaps for laser pulse durations ranging from 5 ps to 5 fs at a carrier wavelength of 780 nm. For t , 100 fs, the dominant channel for free electron generation is found to be either impact or multiphoton ionization (MPI) depending on the size of the band gap. The observed MPI rates are substantially lower than those predicted by the Keldysh theory. We demonstrate that sub-10-fs laser pulses open up the way to reversible nonperturbative nonlinear optics (at intensities greater than 10 14 W͞cm 2 slightly below damage threshold) and to nanometer-precision laser ablation (slightly above threshold) in dielectric materials. [S0031-9007(98)05969-9] 4076 0031-9007͞98͞80(18)͞4076(4)$15.00
Transient response of dielectric materials exposed to ultrafast laser radiation
Applied Physics A, 2006
We present results describing several characteristics of energy coupling into dielectric materials (fused silica) irradiated by ultrashort laser pulses in a regime close to the surface optical breakdown threshold. The results intend to illustrate the energy balance in the interaction process by observing the spatio-temporal variations of a laser pulse transversing a dielectric slab as a function of its energy. The measurements are based on real-time observations of the self-action of the laser pulse and associated effects on its temporal envelope, as well as on ex-situ phase-contrast microscopy of induced permanent material reactions. The experimental results are accompanied by numerical simulations of the pulse traces inside the dielectric material at different energetic conditions. The optical observations allow insights into the development and the dynamics of the laser-induced free carrier population, emphasizing the role of the bulk effects related to the nonlinear wave propagation into the transparent material during laser exposure. PACS 79.20.Ds; 52.50.Jm
Comment on “Ultrafast Electron Dynamics in Femtosecond Optical Breakdown of Dielectrics”
Physical Review Letters, 1999
We measured the optical breakdown threshold (OBT) in dielectrics with different band gaps for single and double 25-fs 800-nm transform-limited laser pulses. Our pump-probe double pulse measurements indicate that the plasma energy in dielectrics experiences ultrafast decay which lasts only ϳ100 fs and does not follow an exponential decay curve. Therefore, a decay term must be included in the electron density rate equation. Our double pulse measurements also demonstrate that the OBT is temperature dependent. The OBT in dielectrics was determined using a novel technique, which eliminates the ambiguity in its definition and also allows real-time data acquisition. [S0031-9007(99)08660-3] PACS numbers: 78.47. + p, 42.50.Ct, 42.62.Cf Many studies have been conducted on the ultrafast breakdown of dielectrics in order to understand the different processes involved . Such investigations are important to such diverse fields as micromachining, medical physics, and solid state physics. Generally, laser-induced breakdown in dielectrics is described in terms of three major processes: (i) multiphoton ionization (MPI) and/or tunneling causing the excitation of electrons to the conduction band, (ii) electron-electron collisional ionization (avalanche process) due to Joule heating, and (iii) plasma energy transfer to the lattice [3,4,6]. While the first two processes deposit energy in the plasma, the third process releases the deposited energy to the lattice, thereby inducing the actual damage. This transfer of energy to the lattice is expected to occur after the laser pulse . Until recently, the above processes were studied by measuring the pulse duration dependence of optical breakdown threshold (OBT). Although single pulse OBT measurements have been extended to the 5-fs range, it is very difficult to extract information regarding electron dynamics from such measurements, especially the time scale for the plasma energy decay.
2008
We present a first-principles calculation for an optical dielectric breakdown in a diamond, which is induced by an intense laser field. We employ the time-dependent density-functional theory by solving the timedependent Kohn-Sham equation in real time and real space. For low intensities, the ionization agrees well with the Keldysh formula. The calculation shows a qualitative change of electron dynamics as the laser intensity increases, from dielectric screening at low intensities to optical breakdown at and above 7 ϫ 10 14 W / cm 2. Following the pulse, the electrons excited into the conduction band exhibit a coherent plasma oscillation that persists for tens of femtoseconds.