Abstract
We present mechanical Q-factors (quality factors) of a crystalline quartz test mass with a nano-structured surface, measured in the temperature regime from 5 to 300 K. The nano-structure was a grating with a period of 2 μm and a depth of about 0.1 μm. Comparative measurements were performed on the plain substrate and on the structured test mass with different numbers of SiO2/Ta2O5 coating layers. The measurements at different stages of the test mass fabrication process show that the surface distortion induced by the nanostructure does not severely lower the mechanical Q-factor of the substrate. Damping due to a multi-layer coating stack was found to be orders of magnitude higher. The results provide vital information concerning the potential usage of low-thermal noise nano-structured test masses in future generations of high-precision laser interferometers and in current attempts to measure quantum effects of macroscopic mirror oscillators.
Highlights
In this paper, we present the first Q-factor measurements of a test mass with a nano-structured surface
The size of the test mass, as well as the material chosen, enable the measurement of high Q values and are suitable to reveal the influence of a nanostructured surface in a dedicated experiment which we describe .All mechanical Q-values were measured on the same test mass sample, but at different stages of preparation
We decided to use laser-directwriting for the grating pattern generation, which results in patterns of much lower optical quality, but with feature sizes comparable to those used in [10, 11]
Summary
We chose a cylindrical test mass of 76.2 mm diameter and 12 mm thickness made from crystalline quartz. The second measurement series was done with a grating structure etched into one of the test mass surfaces. The third measurement series was done with a 200 nm silica coating layer on top of the grating structure, and the fourth with an additional high reflection coating stack from silica and tantala. For laser-direct-writing the substrate was coated with a thin layer (300 nm) of a UV-sensitive polymer In this layer, a grating structure with a period of 2 μm was first written by a laser-beam. Left: atomic force microscope scan of the structured test mass surface without coating. Right: atomic force microscope scan of the final test mass with coating. The HR coating was the final step of the test mass preparation and was composed of 36 alternating layers of 190 nm SiO2 and 140 nm Ta2O5
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