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0High-power laser interaction with transparent solid-solid interface: applications to laser damage of optical coatings
V. Gruzdev1
1-Department of Physics and Astronomy, University of New Mexico, 210 Yale BoulevardNE, Albuquerque, NM, 87106, USA
vgruzdev@unm. edu
Interactions of high-power laser pulses with interfaces between two transparent solids involves some effects that are characteristic of the system and are not met in case of bulk solids or surfaces (i.e., solid-gas interfaces). The special features arise from a fundamental fact [1] that nonlinear absorption and associated excitation of electronic sub-systems of the two solids happen at different rates and result in different levels of energy deposition and free-carrier density. Gradients of the free-carrier density across the interface can reach huge values and stimulate ultrafast diffusion through the interface. The diffusion results in charge separation within a layer of few-nanometers thick and build-up of a quasi-static electric field around the interface. The quasi-static field can stimulate numerous effects, e.g., dynamic Shottky effects, Stark shift of energy levels of interface defects, or band-gap reduction due to the Franz-Keldysh effect. Localization of those effects in space substantially depends on screening capabilities of electron sub-systems of the two materials that make the interface. Formation of large temperature gradients can also take place at the interface due to different values of heat-transfer coefficients of the two materials. Relevant effects include thermo-mechanical stresses and possible thermo-voltaic effects at the interface if heating is combined with separation of laser-generated free carriers.
In this talk, we overview those laser-interface interactions and estimate their contributions in case of an interface between two transparent materials characteristic of multilayer optical coatings. To be specific, we consider material parameters of fused silica SiO2 and hafnia HfO2 as those materials are the most frequently utilized in optical coatings [2,3]. Of particular interest is application of the laserinterface interaction models to interpret some experimental data from the field of laser damage of high-quality optical coatings. Optical coatings suffer from reduced thresholds of laser damage in spite of tremendous improvement of their quality [2,3]. Traditional approaches attribute the reduced damage thresholds to various defects that inevitably appear during deposition process [2-5]. However, formation of flat-bottom damage sites [2,5] and anomalously high damage thresholds of Rugate high reflectors with cosine variations of refractive index [2,4] cannot be interpreted based on the concept of defect-induced local damage. In particular, the Rugate multilayer reflectors demonstrated increase of absorption by 200%-300% accompanied by increase of damage threshold almost by one order of magnitude compared to similar multilayers of the traditional design [2,4]. Considering 5.6-eV band gap of hafnia and 9-eV band gap of fused silica, we first simulated nonlinear absorption and free-carrier generation. Difference between the free-carrier densities delivered a huge gradient of free-electron density of the order of 1028 1/cm4 at the interface. Free-electron dynamics was treated based on equation for ambipolar diffusion that incorporated generation of quasi-static electric field due to charge separation. Estimations of the quasi-dc field magnitude delivered values of the order of 106 - 107 V/m. The field was sufficient enough to stimulate band-gap reduction by as much as 1 -2 eV. Estimations of diffusion time for the free-electron transfer across the interface delivered values between 1 and 10 fs depending on material and laser-pulse parameters. In this connection, we consider applications of this model to the case of ultrashort laser pulses bearing in mind that flat-bottom damage sites were also observed for ultrashort pulses [5].
Based on obtained results, we discuss approaches to control the laser-interface interactions and some of the recently proposed modifications of coating design [6] to address the interface effects.
[1] V. Gruzdev, JOSA B, under review (2024).
[2] C. J. Stolz, in Laser-Induced Damage in Optical Materials, D. Ristau, Ed. (CRC Press, New York), Chapter 14, pp. 385-409 (2014).
[3] J. Zhang, et al, Opt. Eng. 57 (12) (2018), 127909.
[4] S. Dong, et al, Prog. Surf. Sci. 97, 100663 (2022).
[5] N. Talisa, et al, Opt. Lett. vol. 45, p. 2672 (2020).
[6] M. Zhu, N. Xu, B. Roshanzadeh, et al, Nanolaminate-based design for UV laser mirror coatings, Light Sci Appl 9, 20 (2020).
