Laser damage comparisons of broad-bandwidth, high-reflection optical coatings containing TiO₂, Nb₂O₅, or Ta₂O₅high-index layers

Abstract

Broad bandwidth coatings allow angle of incidence flexibility and accommodate spectral shifts due to aging and water absorption. Higher refractive index materials in optical coatings, such as TiO 2 , Nb 2 O 5 , and Ta 2 O 5 , can be used to achieve broader bandwidths compared to coatings that contain HfO 2 high index layers. We have identified the deposition settings that lead to the highest index, lowest absorption layers of TiO 2 , Nb 2 O 5 , and Ta 2 O 5 , via e-beam evaporation using ion-assisted deposition. We paired these high index materials with SiO 2 as the low index material to create broad bandwidth high reflection coatings centered at 1054 nm for 45 deg angle of incidence and P polarization. High reflection bandwidths as large as 231 nm were realized. Laser damage tests of these coatings using the ISO 11254 and NIF-MEL protocols are presented, which revealed that the Ta 2 O 5 ∕SiO 2 coating exhibits the highest resistance to laser damage, at the expense of lower bandwidth compared to the TiO 2 ∕SiO 2 and Nb 2 O 5 ∕SiO 2 coatings.

Low Group Delay Dispersion Optical Coating for Broad Bandwidth High Reflection at 45° Incidence, P Polarization of Femtosecond Pulses with 900 nm Center Wavelength

“…For the H index layer material, we have studied and developed processes for depositing TiO 2 , Ta 2 O 5 and Nb 2 O 5 using Sandia's large optics coater, and have produced 42-layer HR coatings for 45˝AOI, Ppol at a central wavelength of 1054 nm that have bandwidths greater than 200 nm and relatively high LIDTs [18,19]. We choose TiO 2 for this BBHR coating design because it offers the highest refractive indices of the layers that we have deposited.…”

“…We have produced TiO 2 layers by means of e-beam evaporation of either Ti metal [19] or Ti 3 O 5 [18] under reactive, IAD conditions using oxygen back pressure of~10´4 Torr in the coating chamber. Our L index layers are produced by means of e-beam evaporation of SiO 2 under IAD conditions.…”

“…Details of this deposition process can be found in [19]. In our experience, this process is very well behaved without the occurrence of spit from the evaporation melt, and it affords double the TiO 2 deposition rate compared to evaporation from Ti 3 O 5 as the starting material [18]. For such TiO 2 layers, we find, using the OptiChar analysis, that A 0 = 2.224101, A 1 = 0.044657, and A 2 = 8.2192065ˆ10´4 for the Cauchy parameters, and absorption that is close to negligible and well characterized in terms of an exponential decrease with increasing wavelength.…”

Sandia's Z-Backlighter Laser Facility

“…The chamber uses e-beam deposition with the option of ion-assisted deposition to produce dielectric coating layers. Most optics use coatings consisting of HfO 2 and SiO 2 layers due to their ability to meet our optical and laser damage requirements, although other oxides such as [34][35][36] have also been investigated. Most of the coatings are for anti-reflection (AR) or high reflection (HR) at 1054 nm and 1064 nm or at the second harmonics of 527 nm and 532 nm, with specifications covering a variety of use angles and use environments (typically vacuum or dry air).…”

Impact of contamination and aging effects on the long-term laser damage resistance of SiO2/HfO2/TiO2 high reflection coatings for 1054 nm

“…In previous studies, 3,4 we experimented with replacing HfO 2 layers with TiO 2 layers in mirror coatings to increase high reflection bandwidth and angle-of-incidence (AOI) flexibility at 1054 nm. However, TiO 2 has a lower bandgap and exhibits lower LIDTs compared to HfO 2 .…”