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#### Optical Absorption Measurement at 1550 nm on a Highly-Reflective Si/SiO2 Coating Stack

##### MPS-Authors

Steinlechner,  Jessica
Laser Interferometry & Gravitational Wave Astronomy, AEI-Hannover, MPI for Gravitational Physics, Max Planck Society;

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Khalaidovski,  Alexander
Laser Interferometry & Gravitational Wave Astronomy, AEI-Hannover, MPI for Gravitational Physics, Max Planck Society;

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Schnabel,  Roman
Laser Interferometry & Gravitational Wave Astronomy, AEI-Hannover, MPI for Gravitational Physics, Max Planck Society;

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##### Fulltext (public)

1401.4916.pdf
(Preprint), 487KB

CQG_31_10_105005.pdf
(Any fulltext), 470KB

##### Supplementary Material (public)
There is no public supplementary material available
##### Citation

Steinlechner, J., Khalaidovski, A., & Schnabel, R. (2014). Optical Absorption Measurement at 1550 nm on a Highly-Reflective Si/SiO2 Coating Stack. Classical and quantum gravity, 31(10): 105005. doi:10.1088/0264-9381/31/10/105005.

Cite as: http://hdl.handle.net/11858/00-001M-0000-001A-0E67-0
##### Abstract
Future laser-interferometric gravitational wave detectors (GWDs) will potentially employ test mass mirrors from crystalline silicon and a laser wavelength of $1550\,\rm{nm}$, which corresponds to a photon energy below the silicon bandgap. Silicon might also be an attractive high-refractive index material for the dielectric mirror coatings. Films of amorphous silicon (a-Si), however, have been found to be significantly more absorptive at $1550\,\rm{nm}$ than crystalline silicon (c-Si). Here, we investigate the optical absorption of a Si/SiO$_2$ dielectric coating produced with the ion plating technique. The ion plating technique is distinct from the standard state-of-the-art ion beam sputtering technique since it uses a higher processing temperature of about 250$^\circ$C, higher particle energies, and generally results in higher refractive indices of the deposited films. Our coating stack was fabricated for a reflectivity of $R=99.95\,\%$ for s-polarized light at $1550\,\rm{nm}$ and for an angle of incidence of 44$^\circ$. We used the photothermal self-phase modulation technique to measure the coating absorption in s-polarization and p-polarization. We obtained $\alpha^{\rm coat}_{s}=(1035 \pm 42)\,\rm{ppm}$ and $\alpha^{\rm coat}_{p}=(1428 \pm 97)\,\rm{ppm}$. These results correspond to an absorption coefficient which is lower than literature values for a-Si which vary from $100\,\rm{/cm}$ up to $2000\,\rm{/cm}$. It is, however, still orders of magnitude higher than expected for c-Si and thus still too high for GWD applications.