Optical absorption measurements in sapphire

Transcription

1 Optical absorption measurements in sapphire Alexei Alexandrovski Martin Fejer Roger Route Ginzton Laboratory, Stanford University LIGO-G D

2 Optical absorption measurements in sapphire OUTLINE ¾Background ¾Photothermal technique ¾As-grown sapphire ¾Annealed sapphire ¾How to go below 40 ppm/cm ¾Prospects

3 Space resolution sample 2w 0 2w 1 probe L eff L β pump L eff = π 2 w 0 sin β α = κ P L eff

4 Space resolution: surface-to-surface scan Translation Probe Pump Sample ABSORPTION SIGNAL (arb.un) DISTANCE (mm) Example: 3 mm-thick neutral filter, 15%-absorbing Leff = 0.25 mm

5 Photothermal Common-path Interferometer (PCI) Probe Pump Projecting lens Chopper Sample PD Dump Pump waist 50 µ Chopping frequency 380 Hz (10Hz - 2 khz) Probe waist 120 µ Crossing angle 1 o - 20 o (in air) Pump power 5 W Probe power 0.5 mw ac-component of probe distortion is detected by photodiode + lock-in absorption coefficient of 10-7 cm -1 can be detected with a 5 W pump crossed beams help to avoid false signals from optics and surfaces of the sample

6 Data on sapphire crystals (1998) Crystal α (ppm/cm) 532nm 1064nm Scattering Fluorescenc e Window 3mmthick 1400* 81 No 2 x 10-3 F, Ti 3+ CS White #0 415* (bulk, anomaly near the surface) 41 (bulk, anomaly near the surface) Large near the surface 1 x 10-3 F, Ti 3+ (bulk) CS White # No 3 x 10-4 F CS White # Weak band in the 1 x 10-3 F bulk CS White Yes, broad band 3 x 10-3 F (Perth) near one face CS Hemex No 1 x 10-4 F Ultra 0.1% Ti-doped (reference #2) 0.68/cm (total) 0.145/cm 6400 Yes, macrodefects F, Ti % Ti-doped laser rod (reference #1) (thermal part) ** - 0.7F, Ti 3+ * 514 nm ** Absorption measured directly Relative fluorescence brightness estimated with calibrated neutral filters, Ti-doped reference #2 brightness denoted as F

7 Data on sapphire crystals (1999) Crystal CS White, H 2 - annealed CS White, O 2 - annealed α (ppm/cm) 514nm 1064nm Scattering Fluorescence No 2 x 10-4 F 600 (bulk, anomaly near the surface) 47 (bulk, anomaly near the surface) Large near the surface 2 x 10-4 F (bulk) Substrate (TRW) - 66 No - Window 3mmthick 1400* 81 No 2 x 10-3 F, Ti % Ti-doped (reference #2) 6400 Yes, macrodefects F, Ti /cm (total) 0.145/cm (thermal part) Relative fluorescence brightness estimated with calibrated neutral filters, Ti-doped reference #2 brightness denoted as F

8 Data on sapphire crystals (2000) Crystal Systems, Inc. Crystal α (ppm/cm) 514nm 1064nm Scattering Fluorescence 1T No 10 x10-5 F 1M No 5 x 10-5 F 1B No 2.5 x 10-5 F 2T No 4 x 10-4 F 2M No 10 x 10-4 F 2B No 40 x 10-4 F 3T No 10 x10-5 F 3M No 5 x 10-5 F 3B No 5 x 10-5 F 4T No 10 x 10-4 F 4M No 2 x 10-4 F 4B No 1 x 10-4 F Nuclear Research Center Negev, ISRAEL Crystal α (ppm/cm) 514nm 1064nm Scattering Fluorescence No 2 x 10-3 F No 2 x 10-3 F No 2 x 10-3 F

9 Annealed sapphire data 20 mm-long, H 2 -annealed sample Absorption at 1064 nm, scan from surface to surface absorption (ppm/cm) ppm/cm distance (a.u.) Reference sample: Ti-doped sapphire with the absorption of 6400 ppm/cm at 1064 nm

10 Annealed sapphire data 20 mm-long, H 2 -annealed sample Absorption at 514 nm, scan from surface to surface 800 absorption (ppm/cm) ppm/cm distance (a.u.)

11 Annealed sapphire data 20 mm-long, O 2 -annealed sample Absorption at 1064 nm, scan from surface to surface 250 absorption (ppm/cm) ppm/cm distance (a.u.)

12 Annealed sapphire data 20 mm-long, O 2 -annealed sample Absorption at 1064 nm, scan from surface to surface 250 absorption (ppm/cm) ppm/cm distance (a.u.)

13 Annealed sapphire data 20 mm-long, O 2 -annealed sample Absorption at 514 nm, scan from surface to surface absorption (ppm/cm) ppm/cm distance (a.u.)

14 Annealed sapphire data 20 mm-long, O 2 -annealed sample Absorption at 514 nm, scan from surface to surface absorption (ppm/cm) distance (a.u.)

15 Model O 2 -annealed sample Wrap: no fluorescence, scattering, enhanced absorption Line of scan Core: red fluorescence, no scattering, normal absorption Transition layer: low absorption

16 Conclusions The best as-grown sapphire shows 40 ppm/cm of absorption at 1064 nm H 2 -annealed sapphire shows no change in absorption, fluorescence or scattering O 2 -annealed sapphire shows a complex response to oxidation with local decrease of both IR and green absorption Defects responsible for current IR and green absorption levels are yet to be identified Proper annealing may offer means to reach the ppm/cm level. Further decreases will depend on the ability to identify and eliminate specific defects