Titania Mixed with Silica: A Low Thermal-Noise Coating Material for Gravitational-Wave Detectors

Coating thermal noise is one of the dominant noise sources in current gravitational wave detectors and ultimately limits their ability to observe weaker or more distant astronomical sources. This Letter presents investigations of ${\mathrm{TiO}}_2$ mixed with ${\mathrm{SiO}}_2$ (${\mathrm{TiO}}_2:{\mathrm{SiO}}_2$) as a coating material. We find that, after heat treatment for 100 h at $850°\mathrm{C}$, thermal noise of a highly reflective coating comprising of ${\mathrm{TiO}}_2:{\mathrm{SiO}}_2$ and ${\mathrm{SiO}}_2$ reduces to 76% of the current levels in the Advanced LIGO and Advanced Virgo detectors—with potential for reaching 45%, if we assume the mechanical loss of state-of-the-art ${\mathrm{SiO}}_2$ layers. Furthermore, those coatings show low optical absorption of $<1\mathrm{ppm}$ and optical scattering of $\lesssim 5\mathrm{ppm}$. Notably, we still observe excellent optical and thermal noise performance following crystallization in the coatings. These results show the potential to meet the parameters required for the next upgrades of the Advanced LIGO and Advanced Virgo detectors.

Figure 1

Mechanical loss of the 58.5% and 62.6% single layers and the HR coatings as a function of heat treatment temperature. Each point represents the average loss of six–eight modes, with the standard deviation used as the error bars. Inset: mechanical loss of our ${\mathrm{SiO}}_2$ estimated by comparing the single layers and HR losses [33], and ${\mathrm{SiO}}_2$ loss from the literature $A$ [34] and $B$ [18]. Axis labels are identical to the main plot.

Figure 2

Directly measured CTN at 100 Hz of the 69.5% HR coating, scaled to the aLIGO end test mass beam size (purple, solid points), and CTN calculated from the HR coating mechanical loss shown by yellow squares, corresponding to the losses in Fig. 1. The gray circles show CTN calculated from individual ${\mathrm{TiO}}_2:{\mathrm{SiO}}_2$ and ${\mathrm{SiO}}_2$ losses—see Fig. 1. The red, dashed line represents a reduction compared to aLIGO (green line), which is the CTN goal for the $\mathrm{aLIGO}+\text{upgrade}$.

Figure 3

Optical absorption at 1064 nm of the 69.5% HR stack as a function of heat treatment temperature. The measurement of sample 2 after heat treatment at $850°\mathrm{C}$ for 100 h is indicated with arrows. All other measurements on both systems were from samples heat treated for 10 h.

Figure 4

Raman spectra of the 69.5% HR coating (main plot), and for the 63.2% (inset—axis range identical to main plot). Peak magnitude increases with heat treatment temperature. The data for the 69.5% $850°\mathrm{C}$ and 63.2% $800°\mathrm{C}$ heat treated coatings are scaled down with respect to the other spectra.