Study of scattering points on LIGO mirrors Lamar Glover (Cal State LA) Riccardo DeSalvo (Cal State LA) Innocenzo Pinto (Unisannio) LVC meeting 03/15/16 LIGO-G1600430 Related doc: LIGO-G1600431

Brief Summary At Cal State LA, we are analyzing scattering points detected in beam illuminated photos of Advanced LIGO mirrors In LIGO mirrors, A large number of scatterers were observed where none would be expected, through the depths of the coating layer stack Perhaps the problem of scatterers can now be better understood LVC meeting 03/15/16 LIGO-G1600430

Method Individual scatterers were identified using astronomical algorithms for stars in galaxies Extract apparent amplitude and position of each scatterer Fit the stored beam position and profile to determine the illumination power on each scatterer. LVC meeting 03/15/16 LIGO-G1600430

Daophot mechanism: Point Spread Templates “daophot” creates a point spread template from selected sources within the image. The template is used to search scatterers Magnitude data is derived for each identified source. Point Spread Template for an exposure time of 1.25 x 10-4 s LVC meeting 03/15/16 LIGO-G1600430

Identifying/subtracting scatterers Original Photo Subtracted Photo ignored 256 100 Dark pixels pixel scale is expanded 256 Black dots mark the place where bright scatterers have been excised “dirt” does not fit template and is ignored by daophot and is less bright LVC meeting 03/15/16 LIGO-G1600430

Daophot effectiveness and resolution Pixel amplitude histogram of original Image 100 200 In a good image Daophot is very effective in extracting all scatterers Width of residuals (2-3 pixels FWHM) illustrates Daophot’s good resolution Residual pixel amplitude histogram after scatterer extraction Note for you, the central pixel does not count because it is just the black background that daophot ignores, but this is log scale, ignoring the central pixel you get 2-3 pixel FWHM -50 50 LVC meeting 03/15/16 LIGO-G1600430

Scatterer light intensity distribution @ Exposure Time = 0.0125 sec 70 The number of reconstructed scatterers grows rapidly at low amplitude 40 5000 20000 LVC meeting 03/15/16 LIGO-G1600430

Number of Scatterers vs. Exposure Time Exposure scatterers @ 0.125 ms 134 @ 1 ms exposure 11,806 @ 400 ms exposure 363,789 Number increases almost linearly with exposure Exploring smaller scatterers But also deeper in the dielectric coating layers ! ! LVC meeting 03/15/16 LIGO-G1600430

The apparent size of scatterer correctable in photos: Local illumination level not correctable: Physical scatterer size inside host layer Depth of host layer inside dielectric coating stack Beam profile Just put the image there and state that it is correctable, do not tell if you corrected or not. One image is worth a thousand words (Note: all scatterers are nanometer scale, diffraction limited) LVC meeting 03/15/16 LIGO-G1600430

1: Crystallite Formation in a single evaporated layer Growth direction Side View Top View Growth Direction As mirror layers are formed, crystallites appear at different depths. Those that start earlier in the depositing process grow more and create larger scatterers LVC meeting 03/15/16 LIGO-G1600430

Scattered light detected by CCD 2: Depth in stack effect There are two depth attenuation components: Attenuation of impinging light due to reflections through depth of coating layers Attenuation of Scattered light from back-reflection on outer layers Scattered light detected by CCD Complex effect to simulate LVC meeting 03/15/16 LIGO-G1600430

Discussion of crystallite number and distribution intensity How can so many scatterers exist, and be so uniformly distributed? Cannot be only “dirt” ! Only a thermodynamical source can explain the observed large number Probable dirt smear Unresolved by daophot LVC meeting 03/15/16 LIGO-G1600430

What’s Next Two steps proposed: Take more images at Hanford/Livingston and study more photos Study spare ad-LIGO mirrors with Caltech’s setup to determine scatterer’s depth distribution through layers and size distribution How to? LVC meeting 03/15/16 LIGO-G1600430

AdLIGO Multi-Layered Mirror X, Y Axis micropositioning CCD Camera 1 3 2 White light LED 50% Reflector @ 45° Z Axis piezo focusing Microscope lens AdLIGO Multi-Layered Mirror LVC meeting 03/15/16 LIGO-G1600430

X,Y Grid scan of AdLIGO Mirror … Xn Raster scan X-Y of microscope frames on mirror surface to identify rough scatterer position Y1 Y2 Defocused Scatterers … Yn LVC meeting 03/15/16 LIGO-G1600430

Depth determination by Z- focussing No scatterer present: No reflected light Scatterer present out of f-plane Low illumination density Defocused scatterer image Z-zoom with piezo-movement to bring focal plane on scatterer: Brighter illumination Focused airy disk image Defocused Scatterer Focused Scatterer Defocused Scatterer Maximum Luminosity +Z -Z LVC meeting 03/15/16 LIGO-G1600430

Depth determination by Z- focussing fits Depth of field 0.6 µm / n Sub-wavelength Z-position determination from Z-feedback fit For your information, n is the refraction index of the material, in Ta2O5 or TiO2 it is between 2.5 and 3 Defocused Scatterer Focused Scatterer Defocused Scatterer Maximum Luminosity Nikon lens calculator http://www.microscopyu.com/tutorials/java/depthoffield/ +Z -Z LVC meeting 03/15/16 LIGO-G1600430

X-Y determination by profile fitting Sub-wavelength X-Y position determination from Airy disk profile fitting D = 0.61λ/NA LVC meeting 03/15/16 LIGO-G1600430

Scatterer strength Within Mirror Layers In photo images, the signal amplitude depends on scatterer depth, due to reflectivity on layers White light and large angle focusing mitigate the effect Complex but probably soluble or mappable function 1 2 LVC meeting 03/15/16 LIGO-G1600430

Scatterers strength Within Mirror Layer 1 Independent of illumination/trans-reflection function: at same depth, information of scatterer’s size is extracted from amount of light collected 2 LVC meeting 03/15/16 LIGO-G1600430

Conclusions Further measurements on actual mirrors should nail down the observation that millions of scatterers are present throughout the layers of the Ad-LIGO dielectric coatings Such large number of scatterers require a thermodynamic origin (see next presentation) LVC meeting 03/15/16 LIGO-G1600430