Views
4 years ago

Characterization of the laser induced damage threshold of mirrors in ...

Characterization of the laser induced damage threshold of mirrors in ...

technique for a

technique for a damage threshold measurement, but the majority of publications does not stick to this standard. In this text we will use the terms “damage” and “dielectric breakdown” as reference to catastrophic mechanisms such as laser-induced ablation or crack formation. There are other phenomena that don’t cause macroscopic modifications of the structure like coloration of a region or a spatially altered third order dielectric susceptibility. We will distinguish these modifications by using the term “incubation”. This nomenclature is used in most of the related literature. 2.2 Femtosecond laser interaction with dielectrics Without going into too much detail, this section will provide an overview about the theoretical understanding of fs damage and its mathematical modeling. The presented theory applies to electrical isolators with a bad gap larger then 3 eV, further called wide bandgap materials. An overview to fs ablation of metals and its theory can be found elsewhere.[37, 38] 2.2.1 Theoretical understanding of fs damage For pulses longer than a few tens of picoseconds, the generally accepted picture of damage in wide bandgap dielectrics is that electrons get excited into the CB by the incident light and transfer their energy to the lattice by creating phonons. Damage occurs if this deposited heat is sufficient to alter the dielectric material. The critical energy density at which damage occurs corresponds to a critical electron density in the conduction band of Ncr = 10 16 − 10 18 cm −3 .[39] For pulse durations in the femtosecond regime there are now several proposed damage processes. Among the suggested mechanisms is coulomb explosion [40], thermal melting [40, 41], plasma formation [41] and material cracking caused by thermoelastic stress.[42] While the underlying physics may be totally different, all cases have a critical energy density in common, at which damage occurs. The energy coupling to the lattice is said to happen on a timescale of 100 fs to 1 ps [5, 43]. Now, there are impressive femtosecond resolved microscopic images displaying this process in narrow bandgap semiconductors [44], but it remains unclear whether that behavior can be extrapolated to wide bandgap materials. The critical energy density for the short pulse duration regime still corresponds to a critical electron density Ncr in the CB of the material. This time the density is 6

Figure 2.3: Schematic diagram of the photo excitation of an electron in an atomic potential for different values of the Keldysh parameter. The tilting of the potential is due to an external electric field in the indicated direction, caused by the incident light. From ref [3] considered critical when the electron-hole plasma frequency equals the laser frequency. At such electron-hole densities the energy transfer from the incident light to the free-carrier gas is claimed to be best [2] and thus strong energy deposition occurs. A simple approach based on the Drude-Lorentz model is shown in the appendix, it leads to equation 2.1. Ncr = ɛ0 · ɛel · m ∗ e 2 · ω 2 laser (2.1) For a wavelength of 800nm and typical values for the reduced mass m∗ 2 and the dielectric susceptibility of the bound electrons ɛel 3 the value for Ncr is about 10 21 cm −3 . [46, 23, 28, 2, 47, 31, 4] As far as we know, this critical electron density can be created by different mechanisms. First, electrons can be excited into the CB by photo ionization. 4 Due to the large bandgap energy 5 , direct single photon excitation is impossible for the common fs-lasers in the visible and the infrared, without inducing intermediate states like self trapped excitons before. Instead multi photon ionization (MPI) and tunnel ionization are the primary photo ionization pathways. A still widely used theory of photo ionization was developed by Keldysh in 1965.[48] The Keldysh parameter γ is an indicator which excitation pathway dominates, see figure 2.3. Keldysh theory has been derived for the continuous wave case and it has been questioned in the past whether it yields correct results for the ultra short pulse regime.[47] An electron already in the CB can sequentially absorb photons through a process called 2about m∗ = 0.5·mel for hot electrons in fused silica[45] 3ɛel ≈ 1 4ionization is referring in this case to an excitation of an electron into the CB leaving an ion behind 5EGap(SiO2) ≈ 9eV , EP hoton(800nm) = 1.5eV 7

femtosecond laser induced breakdown spectroscopy (LIBS) - CSIR
Laser induced damage - Fusion Energy Research Program - UC ...
Microstructure and laser-induced damage threshold of ZrO2 ...
Effect of laser pulse duration on damage to metal mirrors for laser IFE
Laser Damage Threshold Measurements of Micro-Structure Based High Reflectors
Femtosecond laser-induced damage in dielectrics
Deterministic Nanosecond Laser-Induced Breakdown Thresholds In ...
Laser-induced damage in deuterated potassium ... - mfeit.net
Progress on Laser Induced Damage Studies of Grazing Incidence ...
Characterization techniques for high brightness laser diodes
Short and ultrashort laser pulse induced bubbles on - University of ...
Spectroscopic characterization of laser-induced tin plasma
Single-shot spatially resolved characterization of laser-induced ...
759 ablation threshold, laser wavelength influences on ... - Wiley-VCH
Modeling of Charge Trapping Induced Threshold ... - IEEE Xplore
Molecular dynamics simulations of laser-induced damage of ...
Growth of Laser Initiated Damage in Fused Silica at 1053 ... - mfeit.net
Time-Resolved Studies of Laser-Induced Phase ... - Mazur Group
Modeling of Laser Induced Damage in NIF UV Optics
Growth of Laser Damage in SiO2 under Multiple ... - mfeit.net
Extracting the distribution of laser damage precursors on ... - mfeit.net
Femtosecond Laser Induced Micro-welding of Silver and Copper
Fabrication of Microoptical Elements in Quartz by Laser Induced ...
Invited paper Theory and design of chirped dielectric laser mirrors
Structural modifications in fused silica due to laser damage induced ...
Laser induced microexplosions and applications in laser ...
Influence of bandwidth and phase shaping on laser induced ...
Effects of pulse duration on the ns-laser pulse induced removal of ...
Laser induced ignition of gasoline direct injection engines