Mechanical loss of a hydroxide catalysis bond between sapphire substrates and its effect on the sensitivity of future gravitational wave detectors

Hydroxide catalysis bonds are low mechanical loss joints which are used in the fused silica mirror suspensions of current room temperature interferometric gravitational wave detectors, one of the techniques which was essential to allow the recent detection of gravitational radiation by LIGO. More sensitive detectors may require cryogenic techniques with sapphire as a candidate mirror and suspension material, and thus hydroxide catalysis bonds are under consideration for jointing sapphire. This paper presents the first measurements of the mechanical loss of such a bond created between sapphire substrates and measured down to cryogenic temperatures. The mechanical loss is found to be $0.03\pm 0.01$ at room temperature, decreasing to $(3\pm 1)\times 10^{-4}$ at 20 K. The resulting thermal noise of the bonds on several possible mirror suspensions is presented.

Figure 1

Schematic diagram of the sapphire samples studied before and after hydroxide catalysis bonding. All the samples have a diameter of 30 mm, and the cylindrical axis of the sample is the c axis of the sapphire crystal. The reference sample is 120 mm long, and the two samples that were bonded together were 50 and 70 mm long, respectively.

Figure 2

Bonded sapphire rod suspended with $0.35\mu \mathrm{m}$ tungsten wire for mechanical loss measurements. The sample is excited by an electrostatic drive plate. As the sample rings down, the decaying motion of the front face of the sample is recorded using a SIOS interferometer. The second sapphire mass which is used for temperature calibration is suspended behind the electrostatic drive plate and so cannot be observed in this photograph.

Figure 3

Mode shapes of the four resonant frequencies at which the mechanical loss was measured. (a) 17 kHz, (b) 44 kHz, (c) 89 kHz, and (d) 132 kHz. The color indicates the relative magnitude of the motion, blue showing minimal motion and red showing maximum motion.

Figure 4

Mechanical loss of sapphire samples. Top: A reference sample (30 mm diameter, 120 mm long, and a cylindrical axis cut along the c axis of the crystal). Bottom: A hydroxide catalysis bonded sapphire sample (50 mm long sample bonded to a 70 mm long sample, both having a 30 mm diameter, and the cylindrical axis cut along the c axis of the crystal). The mechanical losses of both samples were measured at four resonant frequencies.

Figure 5

Extracted mechanical loss of a hydroxide catalysis bond created between sapphire substrates assuming a 74.5 nm thick bond (top) and a 149 nm thick bond (bottom). The bond loss has been measured for four resonant frequencies. The data shown assume the Young’s modulus of the bond to be 7.9 GPa.

Figure 6

Finite element model showing the deformation of a test mass and two hydroxide catalysis bonded ears caused by a Gaussian pressure wave. The color indicates the amplitude of the deformation, blue showing the minimum and red showing maximum. (a) Design A, (b) design B showing the finite element mesh. The deformation of the bonds themselves (not visible here) is used to extract the strain energy and therefore calculate the bond thermal noise.