Optical absorption measurements in sapphire

Transcription

1 Optical absorption measurements in sapphire Alexei Alexandrovski Martin Fejer Eric Gustafson Roger Route Ginzton Laboratory, Stanford University

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 (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

7 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

8 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.)

9 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.)

10 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.)

11 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.)

12 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.)

13 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

14 Crystal 1T Data on CSI sapphire crystals (2000) Absorption in sapphire cubes for both polarizations, ordinary (o) and extraordinary (e). Fluorescence brightness relative to sapphire window* o e 1/10 1/ M /40 1/5 1B /80 1/20 2T /5 1 2M /2 2 2B T M 90 3B T 60 4M 130 4B IR absorption, ppm/cm o e Green absorption, ppm/cm o e /20 1/ /40 1/ /40 1/ / / /20 1/2 *Sapphie window showed brightness of 2 x 10-3 relative to 0.1%-doped Tisapphire crystal

15 Transmission of CSI sapphire in UV-VIS transmission H2 cylinder 3M 1T Examples of transmission spectra for crystals with low absorption at 255 nm (cylinder), average one (3M) and strong one (1T) wavelength (nm)

16 Fluorescence vs 255 nm absorption band in CSI sapphire 10 Fluorescence relative to sapphire window nm absorption (ppm/cm) Open circle: hydrogen-annealed cylinder Squares: crystal #1

17 Fluorescence vs absorption at 1064 nm in CSI sapphire Fluorescence relative to sapphire window absorption (ppm/cm)

18 Correlation of absorption at 255 and 1064 nm nm absorption (ppm/cm) nm absorption (ppm/cm)

19 Correlation of absorption at 255 nm and 514 nm nm absorption (ppm/cm) nm absorption (ppm/cm)

20 Conclusions The best as-grown sapphire shows 40 ppm/cm of absorption at 1064 nm 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. Ti seems now to be an unlikely source of residual IR absorption Proper annealing in oxygen may offer means to reach the ppm/cm level. Further decreases will depend on the ability to identify and eliminate specific defects