Amorphous Silicon with Extremely Low Absorption: Beating Thermal Noise in Gravitational Astronomy

Amorphous silicon has ideal properties for many applications in fundamental research and industry. However, the optical absorption is often unacceptably high, particularly for gravitational-wave detection. We report a novel ion-beam deposition method for fabricating amorphous silicon with unprecedentedly low unpaired electron-spin density and optical absorption, the spin limit on absorption being surpassed for the first time. At low unpaired electron density, the absorption is no longer correlated with electron spins, but with the electronic mobility gap. Compared to standard ion-beam deposition, the absorption at 1550 nm is lower by a factor of $\approx 100$. This breakthrough shows that amorphous silicon could be exploited as an extreme performance optical coating in near-infrared applications, and it represents an important proof of concept for future gravitational-wave detectors.

Figure 1

Schematic of the deposition setup for producing ultralow absorbing $a$-Si.

Figure 2

Extinction coefficient $k$ at 1550 nm as a function of heat treatment temperature for our coating and, for comparison, of a commercial coating (data from Ref. [15]).

Figure 3

Extinction coefficient $k$ at 1550 nm as a function of deposition temperature. At each temperature, different coatings are indicated by different shapes. (Crosses represent our coating from Fig. 2; stars indicate coatings deposited on Corning 7979 substrates; all other coatings were deposited on JGS-1 substrates.)

Figure 4

Extinction coefficient $k$ at 1064 nm (red) and 1550 nm (blue) of $a$-Si coatings as a function of electron-spin density. Stars (*) indicate coatings on Corning 7979 substrates; all other coatings were on JGS-1.

Figure 5

Extinction coefficient $k$ as a function of the calculated mobility gap energy from the OJL model for absorption results in the plateau region of Fig. 4, with linear fit. Stars indicate coatings on Corning 7979 substrates; all other coatings were on JGS-1.

Figure 6

Coating mechanical loss as a function of deposition temperature: Each point represents the average loss of several resonant modes (purple squares: coating as deposited, green circles: heat treatment for 1 hour at $400°\mathrm{C}$).