Demand for high-quality optics for ultrafast lasers and their various applications has grown over the last years, and in contrast to optics applied for continuous-wave (CW) or longer-pulse lasers, additional dispersion specifications must be fulfilled when optics need to handle pulses with durations in the femtosecond (fs) range.1 Especially below 100 fs, the required spectral bandwidth, which makes coating design and manufacturing more challenging.
Optimizing the laser damage threshold (LDT) of these coatings usually requires a compromise of the selected coating materials and the electric field distribution within the multilayer stack to achieve all optical specifications. A promising three-material approach is discussed, based on an example of a low group delay dispersion broadband mirror centered at a wavelength of 920 nm.
Damage mechanisms in dielectric thin films
Laser optics typically consist of coatings, interfaces, and a substrate. When a laser beam encounters an optic, damage can occur to each of these parts. If we concentrate on the thin-film coatings, three different competing damage mechanisms are observed.2
The first is thermal damage, which is usually dominant for CW lasers and is driven by absorption and heat conductance of the layer materials.
The second mechanism is the defect-induced damage, which is most relevant for non-ultrafast pulsed lasers and is related to particles and contamination.
The third mechanism is electronic damage, which is dominant for ultrafast lasers and is dependent on the electric field distribution within the multilayer stack and the ultraviolet (UV) absorption edge of the layer materials. The threshold fluence Fth of a thin film can be derived for the pulse duration τp and the coating material’s UV absorption edge Eg applying this equation:
Figure 1 shows the threshold fluence of common pure materials and of hafnium dioxide/silicon dioxide (HfO2/SiO2) mixtures for several pulse durations measured at an 800 nm wavelength.4 At a pulse duration of 35 fs, tantalum pentoxide (Ta2O5) has a threshold fluence of 0.5 J/cm2, whereas HfO2 shows a value of 0.8 J/cm2 (with a small content of SiO2, a mixture can even reach 1.0 J/cm2). The threshold fluence of the typical low refractive index material SiO2 is with 1.6J/cm2 at least twice as high as the results for the two high refractive index materials Ta2O5 and HfO2.
These threshold fluence values can be used to estimate the LDT of multilayer coatings. The calculation is based on electric field distribution through the stack, which can be determined with commercial thin-film design software like OptiLayer or LZH Spektrum. To optimize the damage threshold, the refractive index profile of a quarter-wave thin-film stack is modified by adding a third pure material or mixture with a higher threshold fluence and a lower refractive index, as shown in red in Figure 2.
This optimization approach is applied below for an example of a low group delay dispersion broadband mirror based on the requirements for the 2023 SPIE damage competition. Table 1 summarizes the optic specifications and LDT measurement parameters.5
