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-G1600431 Related doc: LIGO-G1600430

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-G1600430 70 40 5000 20000 LVC meeting 03/15/16 LIGO-G1600431

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-G1600431

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-G1600431

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 Mannan.Ali@physics.org Chapter 4: Thin Film Deposition Web: http://members.xoom.com/MannansZone/thesis.html LVC meeting 03/15/16 LIGO-G1600431

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-G1600431

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-G1600431

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-G1600431

Why do we believe that they could be the source of mechanical dissipation Theoretically: Classical theory of nucleation LVC meeting 03/15/16 LIGO-G1600431

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-G1600431

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-G1600431

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-G1600431

Competing forces in nucleation This costs energy Here energy is gained Ordered volume Disordered surface Gain (+) Loss (-) LVC meeting 03/15/16 LIGO-G1600431

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-G1600431

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-G1600431

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-G1600431

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-G1600431

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-G1600431

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-G1600431

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-G1600431