Abstract
Hafnium oxide (HfO2) is the most frequently used high-index material in multilayer thin-film coatings for high-power laser applications ranging from near-infrared to near-ultraviolet (UV). Absorption in this high-index material is also known to be responsible for nanosecond-pulse laser-damage initiation in multilayers. In this work, modification of the near-UV absorption of HfO2 monolayer films subjected to irradiation by continuous-wave (cw), 355-nm or 351-nm laser light focused to produce power densities of the order of ∼100 kW/cm2 is studied. Up to a 70% reduction in absorption is found in the areas subjected to irradiation. Temporal behavior of absorption is characterized by a rapid initial drop on the few-tens-of-seconds time scale, followed by a longer-term decline to a steady-state level. Absorption maps generated by photothermal heterodyne imaging confirm the permanent character of the observed effect. Nanosecond-pulse, 351-nm and 600-fs, 1053-nm laser-damage tests performed on these cw laser–irradiated areas confirm a reduction of absorption by measuring up to 25% higher damage thresholds. We discuss possible mechanisms responsible for near-UV absorption annealing and damage-threshold improvement resulting from irradiation by near-UV cw laser light.
Highlights
Hafnium oxide (HfO2) is used in numerous applications as a high-index component in coatings for high-power lasers ranging from a near-infrared to near-ultraviolet (UV) light spectrum
A cross-sectional profile of the photothermal heterodyne imaging (PHI) signal [Fig. 1(b)] shows up to 70% reduction in absorption within the irradiated spot, which appeared to be permanent when confirmed by PHI scans performed after one week and one month later
To evaluate the effect of absorption annealing on HfO2 film-damage resistance, a series of 20 × 20-μm cw laser– exposed areas were produced on a sample and irradiated by a pulsed laser at fluences exceeding the damage threshold
Summary
Hafnium oxide (HfO2) is used in numerous applications as a high-index component in coatings for high-power lasers ranging from a near-infrared to near-ultraviolet (UV) light spectrum. We explore the possibility of using continuous-wave (cw) laser radiation with power densities in the range of 50 kW∕cm[2] to >1 MW∕cm[2] to anneal absorption in HfO2 monolayer films in the near-UV spectral range. Near-UV cw laser–absorption annealing was studied using either a 351-nm, 1-W Arþ laser or a 355-nm, diode-pumped semiconductor laser that works as a pump laser for PHI and delivers up to 6 mW on the sample The latter, after being focused into a ≤0.7-μm [full width at half maximum (FWHM)] spot by a 40 × ∕0.95 numerical-aperture objective, produced power densities of up to 1 MW∕cm[2], and the former was focused into a 50-μm spot (1∕e2), resulting in a 46-kW∕cm[2] power density. Laserdamage morphology was investigated by means of atomic force microscopy (AFM)
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