1. Riccardo DeSalvo (Cal State LA)
Implications of the millions of scattering points found in LIGO coatings
Riccardo DeSalvo (Cal State LA)
Lamar Glover (Cal State LA)
Innocenzo Pinto (UniSannio)
LVC meeting 03/15/16 LIGO-G
Related doc: LIGO-G

2. Scatterer light intensity distribution
Hundreds of thousand scatterers identified within the beam spot
Only the ones of the surface layers visible, due to the double whammy of reflectivity,
Only the surface layers are well illuminated,
Most scattered light from deeper layers is reflected back into the mirror and is invisible
There are millions of scatterers
distributed within the coating layers
Distribution of scatterers is fairly uniform
Where do they come from?
LIGO-G 70 40
5000
20000
LVC meeting 03/15/16 LIGO-G

3. Number of Scatterers vs. Exposure Time
Number increases almost linearly with exposure
Larger exposure explores smaller scatterers
But also deeper in the dielectric coating layers ! !
LVC meeting 03/15/16 LIGO-G

4. Scatterer light intensity distribution
Only a thermodynamic origin can possibly explain such ubiquitous items
The classical nucleation theory offers an explanation 70 40
Scatterer distribution in adLIGO mirror
5000
20000
LVC meeting 03/15/16 LIGO-G

5. Crystallite Formation in a single evaporated layer
Deposition starts as a glassy layer
As layer grows, crystallites form in the glassy matrix. Slowly grow.
Columnar growth ensues
Dielectric mirror layers are thin: ~ 0.1 µm. They remain well inside glassy-matrix region
in which there “appear” to be NO crystallites
Deposition
Growth Direction
Chapter 4: Thin Film Deposition
Web:
LVC meeting 03/15/16 LIGO-G

6. Why do we believe that crystallites are the likely culprits
Because depositing thick layers produces columnar growth of polycrystalline films:
Crystallites must pre-exist
Starting at all depths !
The surprise is that nobody expected them in such large number !
LVC meeting 03/15/16 LIGO-G

7. Crystallite growth within an evaporated layer
Growth direction
Side View
Top View
Deposition
Growth Direction
Crystallites start at all depths.
Those that appear earlier in the depositing process grow larger
LVC meeting 03/15/16 LIGO-G

8. Why do we believe that crystallites could be the source of mechanical dissipation?
Experimentally:
Doping with other oxides is expected to reduce the frequency of nucleation AND was associated with reduced mechanical dissipation
Nanolayering interrupts the growth of crystallites into columns AND there is evidence that nano-layering reduces mechanical dissipation and increases crystallization temperature
LVC meeting 03/15/16 LIGO-G

9. Why do we believe that they could be the source of mechanical dissipation
Theoretically:
Classical theory of nucleation
LVC meeting 03/15/16 LIGO-G

10. There are competing forces in nucleation
Ordered volume inside crystal
Disordered outside
Inside crystallite ordered bonds are formed
There is energy gain:
LVC meeting 03/15/16 LIGO-G

11. Competing forces in nucleation
Glasses are disordered, but are free to rotate bonds; annealing allows a maximally relaxed, most energetically advantageous state (short of forming a crystal)
Few frustrated bonds
LVC meeting 03/15/16 LIGO-G

12. Competing forces in nucleation
The surface between crystallites and glass is much more disordered
Many more bonds are strained and frustrated than in the glassy matrix
=> Large Energy cost
LVC meeting 03/15/16 LIGO-G

13. Competing forces in nucleation
This costs energy
Here energy is gained
Ordered volume
Disordered surface
Gain (+)
Loss (-)
LVC meeting 03/15/16 LIGO-G

14. Grain Critical radius As a result of the competition
between area ΔGs and volume ΔGv
ΔG has a maximum Nucleation Energy ΔG*
Crystallites have a critical size r* r2 r3
LVC meeting 03/15/16 LIGO-G

15. Is the surface of Crystallites the source of mechanical losses?
Surface layer is the most disordered state
More frustrated bonds exist on the surface than inside the glassy matrix ! !
Frustrated bonds flip between energy states during vibrations
This is a typical energy loss mechanism !
LVC meeting 03/15/16 LIGO-G

16. How can we depress losses from crystallites
An activation energy means that the nucleation probability depends on the deposition temperature
Deposition temperature has influence
But “Forced” parameters need to be respected during deposition
LVC meeting 03/15/16 LIGO-G

17. How can we depress losses from crystallites
The critical size means”
Below r* there is energetic advantage to disappear
Above r* there is energetic advantage to grow
Nanolayering depresses both number and growth of crystallites
Nanolayering below r* may
inhibit crystallite generation
LVC meeting 03/15/16 LIGO-G

18. How can we depress losses from crystallites
Note: there often appears to be a “grace” layer where columns do not start
Two possible reasons:
Surface stress, due to change in atomic spacing, may inhibit nucleation until a more relaxed glass is achieved.
Initially there may not be sufficient thickness for nucleation
Nanolayering may depress nucleation, scattering and thermal noise
Growth Direction
LVC meeting 03/15/16 LIGO-G

19. How else can we depress losses from crystallites
Work on chemistry and entropy to design better glass-formers
Learn from glassy metal development
Mix different size metal dopants (Beryllia, Silica)
Work on compactification of layers during deposition (ion assisted deposition)
Other ideas ?
LVC meeting 03/15/16 LIGO-G

20. Conclusions
An unsuspected large population of scatterers was found on aLIGO mirrors under high-power illumination.
Besides the optical scattering, if the observed points are the locus of mechanical dissipation, they could explain the anomalous mechanical dissipation observed in all sputtered coatings.
Nucleation theory indicates that these scatterers may be a property (of thermodynamic origin) of deposited films.
The discovery may indicate possible ways to improve the Sensitivity limitation of Gravitational Wave detectors
LVC meeting 03/15/16 LIGO-G