Ultrafast carrier dynamics in Al2O3/SiO2 high reflectors has been investigated by UV femtosecond laser. It is identified by laser spectroscopy that, the carrier dynamics contributed from the front few layers of Al2O3 play a dominating role in the initial laser-induced damage of the UV reflector. Time-resolved reflection decrease after the UV excitation is observed, and conduction electrons is found to relaxed to a mid-gap defect state locating about one photon below the conduction band . To interpret the laser induced carrier dynamics further, a theoretical model including electrons relaxation to a mid-gap state is built, and agrees very well with the experimental results.. To the best of our knowledge, this is the first study on the pre-damage dynamics in UV high reflector induced by femtosecond UV laser.
© 2015 Optical Society of America
Femtosecond laser-induced damage of dielectric coatings seriously restricts the development of laser systems with ever higher average powers and peak intensities [1, 2 ]. Compared with the excitation at 800 nm which corresponding to higher multiphoton absorption order, the ionization rate by 400 nm excitation will increase by several orders of magnitude and laser-induced damage of dielectric coatings is easily to take place. In the UV spectral range, only limited coating material can be selected for the manufacture of interference coatings in the UV range, because of the high absorption at wavelengths near to the material electronic band-gap [3,4 ]. Especially, alumina (Al2O3) is one of the most important oxide thin film materials in this spectral region.
In the ultrashort pulse regime, the damage of optical materials displays highly deterministic damage performance, which is closely related to the nonlinear ionization processes, such as multiphoton ionization (MPI), avalanche ionization (AI), and decays of electrons associated with the diffusion and recombination [5–10 ]. Therefore, it is of great significance to study the instinctive response dynamics inside the dielectric coating materials after ultrashort intense laser excitation  and have the understanding of the origin of LIDT in UV HR coatings. Even though there are already plenty of studies on femtosecond pulse-matter interaction, most of them were investigated by 800 nm laser [12–14 ]. The experimental study on the direct interaction between UV dielectric coating materials and femtosecond UV laser pulses has been rarely studied and only few works has been reported . In the present study, to gain a better understanding of the ultrafast dynamic process, UV femtosecond laser pulses are used to excite the ultrafast carrier dynamics inside the Al2O3/SiO2 UV reflector. Laser induced reflectance decrease and reflection spectral shift has been observed and explained by free carrier dynamics.
2.1. Sample fabrication
The Al2O3/SiO2 reflectors are designed to reflect highly at 400 nm (HR = 99.2%) with the incidence angle of 30°, as shown in Fig. 1(a) . The coating design is G|(LH)30 4L|A, where H and L denote the high (Al2O3) and the low (SiO2) refractive index material, respectively, with one quarter wavelength optical thickness, G represents the K9 substrate (Ø 50mm × 5mm), and A is the incident medium (air).
2.2. Femtosecond UV laser setup
A commercial Ti:sapphire laser system (Coherent, Legend-USP) operating at 800nm, 1 kHz, 35 fs is utilized as the laser source. The UV laser pulse is generated by frequency doubled in a 200-μm-thick BBO (type I, θ = 29.2°) crystal. Here, the thin BBO crystal is used to maintain the broad bandwidth. After dispersion compensated by chirped mirrors, laser pulses with pulse duration of 70 fs at 400 nm are focused into Al2O3/SiO2 reflectors, as shown in Fig. 1(b). They are used both in the test of laser damage threshold and in pump-probe experiment. In pump-probe experiment, the pulse energies of the pump and probe beams are adjusted to be about 8600 and 150 nJ, respectively. The pump-probe signal is collected using a bundle fiber, and then dispersed by a polychromator and guided to the photodetectors connected with 128-channel lock-in amplifiers. The spectral resolution of the total system is about 0.75 nm.
3. Results and discussion
3.1. Femtosecond laser damage threshold
TEM00 mode 70 fs laser pulse operated at 400 nm, S-polarization is used for damage testing. The effective area of laser focal spot on the specimen surface is 3.05 × 104 μm2. 1-on-1 test is made according to ISO21254 . The occurrence of damage is judged by the on-line intensity change of scattered light, and then ascertained by off-line Leica DMR polarizing optical microscope. The LIDT of Al2O3/SiO2 reflectors are determined to be 0.59 J/cm2.
3.2. Damage morphology
The fine morphologies and structures of damage craters are characterized by Auriga SEM and Dektak XT surface profiler. For the single-pulse damage experiment, the typical damage craters of the Al2O3/SiO2 reflector present flat bottom morphologies with delaminating feature of the outer layers flake off from inner materials. Figure 2 shows the crater depth is ~325 nm, which is related to the first Al2O3 layer from 286 nm to 351 nm. That means the laser ablation first occurs in the Al2O3 layer.
3.3. Ultrafast laser spectroscopy
In dielectric materials, electrons are initially excited to the conduction band, then further excited by free-carrier absorption. After that, thermalization of the electron system will take place due to electron–electron and electron–phonon (e–ph) scattering, and self-trapped excitons (STEs) will be generated by interaction of the excited electron–hole pairs with the lattice . In the present study, the probe intensity is only ~1/57 of the pump, which is too weak to induce multiphoton ionization, so that free carriers dynamics purely generated by pump laser can be observed. The pump-probe signal we measured is the reflectance change of the Al2O3/SiO2 mirror with and without the excitation of pump light. Because the intensity of the probe light is as weak as nearly 1/300 of the LIDT, the Al2O3/SiO2 mirror will reflect it by the nascent reflection if the pump light does not exist. However, when the reflector is excited by the pump laser, multiphoton ionization will occur and the conduction electrons will be generated. If the probe laser pulse comes to the mirror before the full relaxation of conduction electrons, it will be absorbed by the conduction electrons. So the reflected probe light is weaker than the one without pump excitation, and represented as the decrease of reflectance. Therefore, the reflectance reduction of the reflector is related to the relaxation dynamics of conduction electrons.
Figure 3 shows the two-dimensionally plotted transient reflection change normalized to the nascent one (ΔR/R). The ΔR/R is negative indicates the reflection is decreased after the laser excitation. Despite both the real part and imaginary part of the complex dielectric function can contribute to the reflection data; after the initially energetic carrier distribution relaxed towards the band edge as a result of e–ph scattering in the first 100~200 fs, the induced carrier absorption contributed from the imaginary part is much more significant than the real part . Therefore, the decrease of the reflected probe light could be mainly assigned to the absorption of the probe light by the carriers generated by the pump laser. In addition, Al2O3 has much smaller band gap than SiO2, and the 2.8 ps formation time of mid-gap state, which will be discussed later, is also quite different from the STEs of ~300 fs in SiO2 film [19–21 ]. Consequently, the reflection decrease was mostly due to the carrier absorption dynamics in Al2O3 layers.
As shown in Fig. 3(c), the initial reflection change appears around 407 nm, which indicates that the conduction electrons have an absorption peak at ~407 nm. To characterize the behavior of conduction carriers, we use an exponential function expressed by to simulate the experimental results, where t is the probe delay time; ω is the probe frequency; a(ω), b(ω), c(ω) are amplitude parameters, and τ 1, τ 2 are decay lifetime constants. The real-time ΔR/R around 407 nm decrease along withτ 1 = 2.6 ± 0.3 ps and τ 2 = 15 ± 1 ps, respectively, as shown in Fig. 3(b). Considering the formation of self-trapped excitons in other oxide films (TiO2, Ta2O5, and HfO2) was on a 1-ps time scale , it is possible to assign the shortest lifetime of ~2.6 ps to the population decay of the conduction electrons to some defect state, and the lifetime of ~15 ps to the free electron relaxation from conduction band to valance band.
The peak of reflection decrease band shifts from 407 nm to 394 nm at ~3 ps as indicated by the arrow in Fig. 3(a). Different from the decay process around 407 nm, clear growth with time constant of 2.8 ± 0.2 ps appears around 394 nm, and then decays with lifetime much longer than our longest experimental measurement time of 20 ps. Therefore, it cannot be assigned to the conduction electrons absorption. Considering the decay time of 2.6 ± 0.3 ps at 407 nm and arising time of 2.8 ± 0.2 ps at 394 nm are consistent with each other, it is reasonable to interpret that the conduction electrons are trapped into some mid-gap electronic defect state with an absorption maximum at 394 nm at a time scale of ~2.8 ps. It should be noticed that, the band gap energy is 6.5 eV for Al2O3, which leads to three-photon transition close to the two-photon transition for 400 nm laser pulses. So that the shallow traps and deep traps are not clearly separated in energy level, it is different from those excited by 800 nm . In one word, when operating under UV excitation, only the mid-gap electronic defect state will be involved in the damage of Al2O3/SiO2 reflector.
4. Theoretical analyses
In order to expound the damage mechanism, a theoretical model including MPI, AI, and the mid-gap defect state is built to simulate the evolution process of the electron density in the conduction band; a simplified energy diagram is shown in Fig. 4(a) . When the electron density in the conduction band reaches the critical plasma density, generally considered as the damage criterion, the respective plasma wave resonates with the incident laser wavelength, after that, the material absorbs laser radiation strongly through the process of inverse bremsstrahlung resulting in permanent structural changes and material damage.
The corresponding kinetic rate equations for the CB electron density n and the defect state electron density nd were as follows:22,23 ], denotes the AI rate calculated according to Drude’s ionization model [24–26 ], q is a correction factor that takes into account interference effects in the multilayer, σ is the absorption cross section of the native defect state, nd, max is the maximum density of the defect state electrons. The relaxation time of electrons from conduction band to valence band (Tcv) and the one to the defect state (Tcd) have been determined in pump-probe experiment. The one from the defect state to valence band (Tdv) is estimated to be 100 ps. In view of the time scale of Tdv is much longer than Tcv and Tcd, it can be neglected in the simulation. The initial density of the defect state electrons nd0 is set to be 1 × 1017 cm−3.
As shown in Fig. 4(b), when σ is smaller than 6.3 × 10−20 m2, LIDT is not sensitive and the value higher than 0.579 J/cm2 could be maintained, which agrees very well with the experimental value. Once the value of σ is larger than 6.3 × 10−20 m2, the LIDT would be dropped sharply, probably because the amount of initial seed electrons provided by the native defect could not be neglected anymore. Similar effect will take place for the laser-induced defect state in the case of multiple-pulse damage threshold. If the subsequent laser pulse arrived before the laser-induced defects relaxed to the valence band, the defect states are expected to be an extra contributor of seed electrons to the conduction band, and hence responsible for the decrease of the LIDT.
With the help of UV femtosecond laser spectroscopy, pre-damage carrier dynamics in Al2O3/SiO2 UV high reflector has been investigated. Time-resolved reflection decrease due to laser-induced absorption of conduction electrons has been observed. The carrier dynamic is mainly contributed from Al2O3 layers. In addition, the fast relaxation of free electrons interacted with lattice results in the population of a mid-gap defect state, which is located about one photon below the conduction band, which is found to play important role in the UV laser induced damage. To interpret the laser induced carrier dynamics further, a theoretical model including a mid-gap state is built to simulate the evolution of electron density in the conduction band.
This work is partly financially supported by 100 Talents Program of CAS, the National Basic Research Program of China (Grant No. 2011CB808101), and National Natural Science Foundation of China (Grant No. 61475169, 61221064).
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