Third harmonic (TH) microscopy with circularly polarized illumination directly reveals material anisotropy owing to suppression of background optical signals from isotropic media. Because optical thin films and their substrates are expected to be highly isotropic, TH microscopy presents a path to study induced and intrinsic anisotropy in films, providing both insight into laser-induced material modification that precedes damage and feedback about the deposition process. Because nanoscale defects and material strain influence the damage behavior of films, we examined TH sensitivity to similar sources of contrast. We demonstrate imaging of individual 10 nm colloidal gold nanoparticles and 100 mN nanoindentations in fused silica both with signal-to-noise ratio (SNR) ≥ 100∶1. We present TH images (SNR ≥ 210∶1) of sites exposed to femtosecond laser pulses below damage in 100 nm HfO2 films that are barely visible (SNR ≤ 2.3∶1) with Nomarski and polarization imaging, traditional microscopic techniques known to display contrast for material anisotropy. At our detection limit (320 mW, 50 fs, 790 nm, ≈ 106 photomultiplier tube gain), we examined root mean square in the TH image of nascent films that correlated to the film's macrostrain. TH microscopy presents a relatively simple all-optical method to monitor nanoscale anisotropy in thin films during exposure to high-intensity radiation and during deposition.
Experimental and theoretical progress on subpicosecond laser pulse breakdown in dielectric films is reviewed. The
single pulse threshold fluences can be related to fundamental material properties and scaling laws with respect to pulse
duration and material bandgap. Multiple pulse thresholds are controlled by native and laser-induced defects. A
phenomenological model is introduced which describes the accumulation and relaxation of such defects. The model is
able to explain the experiments and can be used to assess relevant defect parameters. Experimental results are presented
that exemplify how the ambient atmosphere affects the multiple-pulse laser damage thresholds.
Avalanche ionization plays a crucial role in the photoionization of dielectric materials and as such is important for
optical damage. Although it has been investigated closely during the last years, there is little experimental evidence
of how the impact ionization parameter changes with electron density and/or incident pulse intensity. One reason is
that in most dielectric materials there are several competing ionization and relaxation processes. Here we present an
UV-pump IR-probe experiment that allowed us to isolate the avalanche ionization from other major ionization
processes, especially multiphoton ionization, electron tunneling, and relaxation into traps and their re-excitation. We
have measured the intensity dependence of a transmitted IR pulse, propagating through a thin sample of UV-grade
sapphire (α-Al2O3), after seeding electrons in the conduction band with a UV pulse. We show that the assumption of
an intensity independent impact ionization factor α cannot explain the results. Application of a simple avalanche
ionization model within the flux-doubling approximation requires an intensity dependent coefficient a(I) to explain
the data. We also determined the two photon absorption coefficient of sapphire at 266 nm (β(2) = (2.7 ± 0.1) • 10-11cm/W) as well as the "free" electron absorption cross section for 800 nm of conduction band electrons in sapphire
(δ0 = (12.5 ± 0.2) •10-18 cm2).
Third harmonic (TH) imaging is inherently suited for optical material characterization. Under linearly polarized
illumination the total TH signal is dominated by the signal resulting from material interfaces. For symmetry reasons,
circularly polarized illumination of a medium with isotropic or cubic symmetry yields zero TH, and prevailing signals
originate from localized anisotropic sample sites. Such anisotropy may result from laser induced stress, crystallinity, or
birefringence. Pairing THG with complementary imaging techniques proves to be a useful diagnostic for investigating
additional material characteristics. We report TH imaging of 10 nm colloidal gold nanoparticles, 100 mN
nanoindentations, nascent film anisotropy, and laser induced material modification of HfO2 films both pre- and post-laser
damage.
Scandium oxide is an attractive candidate for the engineering of interference coatings, although not widely explored.
This paper describes the ion beam sputtering of Sc2O3. It is shown that the structural properties of the material are
affected by the deposition conditions. Laser damage in different regimes of pulsewidths is investigated. These results
show that the 1-on-1 laser damage fluence, in both the thermal and deterministic regimes, varies with deposition
conditions but this is not the case for S-on-1, indicating that laser-induced defects are important.
In the ultra short laser pulse regime, the damage process is driven by the interaction of the laser pulse with the electronic
structure of the material. The way of excitations in dielectric materials is dominated by multi photon and avalanche
ionization processes. Often, the complete theoretical description is limited by the lack of knowledge of the precise
material properties. Usually, LIDT measurement data are only available for pure materials (e.g. TiO2, Ta2O5 or SiO2).
The development of composite materials opens the way to vary material properties, continuously. Additionally, all
material changes are based on the same chemical elements in different compositions.
The paper compares measurement results of the University of New Mexico and Vilnius University performed on the
same set of TixSi1-xO2-mixtures to calculations based on Keldysh theory. When applying simple approximations for the
physical properties of the mixture, the theoretical description agrees well with the measurement results.
At ~10-7 Torr, the multiple femtosecond pulse LIDT, F(∞), is about 10% of the single pulse damage fluence for hafnia
and silica films compared to ~75% at 630 Torr. The 1-on-1 LIDT is pressure independent. The decrease of F(∞) is
related to the water vapor and oxygen content of the ambient gas with the former having the largest effect. The decrease
of F(∞) is associated with a change in damage morphology. In air, the damage "crater" starts at the center of the beam
and grow in diameter as the fluence increases. In contrast, the damage starts at random "sites" within the exposed area
under vacuum. Absorbing centers are created by the removal of oxygen from predisposed sites (for example grain
boundaries) on the film surfaces, producing absorbing states.
After several investigations in laser induced damage behavior of oxide mixtures of different compositions, also
HfO2 could be steplessly mixed with SiO2. A study of SiO2/HfO2 IBS single layers and high reflectors is presented.
Damage testing has been performed at 800nm and 355nm on an extensive set of single layers employing different
mixture ratios of silica and hafnia. The analysis of the response of optical single layer coatings to femtosecond
and nanosecond pulse exposure provides input for further coating designs, in particular for the optimization in
respect to the damage threshold properties. A deeper understanding of the damage mechanisms is gained by
comparing the ns and fs pulse results as a function of the mixing ratio.
The multiple-pulse laser-induced breakdown behavior of dielectrics is modeled. The model is based on a critical conduction
band (CB) electron density leading to dielectric breakdown. The evolution of the CB electron density during the
pulse train is calculated using rate equations for the occupation and ionization of band and midgap states (native and laser
induced). Using realistic estimations for the trap density and ionization cross-section, the model is able to reproduce
the experimentally observed drop in the multiple-pulse damage threshold relative to the single-pulse value, as long as the
CB electron density is controlled primarily by avalanche ionization seeded by multiphoton ionization of the traps and the
valence band. The model shows that at long pulse duration, the breakdown threshold becomes more sensitive to presence
of traps close (within one photon energy) to the conduction band. The effect of native and laser-induced defects can be
distinguished by their saturation behavior. The model explains the principal behavior of the LIDT of a pair of pulses as a
function of the temporal separation. Using the model, the observed transients can be related to rate constants of electrons
leaving the CB and midgap states.
A film of hafnium oxide, doped with 5 atomic % nitrogen, was prepared by dual-ion-beam-assisted deposition. The
properties were compared to a pure hafnium oxide film. The damage threshold is lower for the nitrogen-doped film.
However, the multiple-pulse damage threshold for a 1 kHz train of 800 fs pulses shows no drop relative to the singlepulse
value. These results are discussed within the context of a
multiple-damage model, based on midgap trapping states.
The single pulse femtosecond laser induced damage threshold (LIDT) of hafnia and silica films is not affected by the
ambient gas pressure. In vacuum, the multiple pulse LIDT drops to ~10% (~10%) of its atmospheric value for hafnia
(silica). The water vapor content of the ambient gas was found to control the change in the LIDT. The LIDT of bulk
fused silica surfaces did now show any dependence on the ambient gas pressure. Hydrocarbons (toluene) did not change
the multiple pulse LIDT for Hafnia films
We discuss the physical and optical properties of Sc2O3 single layers deposited by the dual ion beam sputtering technique
at oxygen partial pressures ranging from 1.7×10-5 to 5.1×10-5 Torr. The films are amorphous with crystallite size ~10
nm and have surface roughness RMS values of 1.2±0.3 nm. The refractive index at 1 μm is 1.95. Absorption loss is shown
to be sensitive to the oxygen partial pressure during growth. Multiple-pulse damage experiments suggest that the scandia
film deposited at the higher oxygen partial pressure accumulates laser-induced trap defects more slowly than the scandia
film deposited in a lower oxygen partial pressure atmosphere.
In this contribution we will summarize the fundamental mechanisms that lead to subpicosecond laser
damage in dielectric films, discuss the resulting scaling laws of single pulse (1-on-1) damage with respect
to pulse duration and bandgap, of the multiple pulse (S-on-1) damage threshold as a function of pulse
number, and compare these findings to recent experimental results.
Subpicosecond laser induced breakdown of dielectric films has gained a great deal of attention in laser nano- and
micromachining and in the development of optical coatings for the next generation of high-power ultrafast laser system.
The understanding of the fundamental processes affecting the breakdown behavior and how they depend on the material
properties and the film deposition is highly desirable for improving the coating performance. In the present work we
compare the single and multiple pulse damage behavior of as-grown and annealed HfO2 films. Annealing can reduce the
film absorption near the band edge but its impact on the single and multiple femtosecond pulse damage behavior
remained open. Damage measurements with pairs of pulses of variable subpicosecond delay in bulk fused silica revealed
a partial recovery toward single pulse behavior on a few hundred fs time scale. We investigate if such behavior also
occurs in hafnia films and identify the time scale for a full recovery. Our experimental results are compared with
existing theoretical models[1], which allows us to suggest microscopic changes that occur during the annealing process.
Laser induced breakdown of single-layer, ion-beam sputtered TixSi1-xO2 composite films was studied using single and
multiple pulses from a femtosecond Ti:sapphire laser. The bandgap of this coating material can be gradually adjusted
with the composition parameter x. A scaling law with respect to the bandgap energy and pulse duration dependence of
the single-pulse damage threshold that was observed previously for pure oxide films was found to apply to the composite
films as well. The dependence of the damage threshold as a function of pulse number F(N) was similar to the behavior
observed for pure oxide films. It was possible to explain the dependence as a function of pulse number using a
theoretical model based on the formation and accumulation of defects. The shape of F(N) can be used to estimate the role of shallow traps and deep traps on the multiple-pulse breakdown behavior.
The scaling law of subpicosecond laser induced damage (LID) with respect to pulse duration and band gap for TixSi1-xO2 composite films is studied. The band gap in these materials can be changed gradually by varying the composition pa-rameter x. Damage is very deterministic and scaling laws with respect to pulse duration and band gap energy derived previously for pure materials are found to apply to composite films. The scaling can be explained theoretically by using a modified Keldysh theory. The composite materials also show a dependence of the damage threshold as a function of pulse number F(N) (incubation) that is similar to observations in pure dielectric oxides. The measured F(N) is explained with a theoretical model that assumes the formation of an intermediate sample state that increases the absorption of sub-sequent pulses in the train.
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