Silicon, the test mass substrate of tomorrow? Jerome Degallaix The Next Detectors for Gravitational Wave Astronomy Beijing

Transcription

1 Silicon, the test mass substrate of tomorrow? Jerome Degallaix The Next Detectors for Gravitational Wave Astronomy Beijing

2 Program of the talk... What we have now What we know about silicon What we are still looking for... 2

3 I What we have now and what we want...

4 The king: fused silica THE test mass substrate for the room temperature first and second generation of gravitational wave detectors A well justified choice: extremely good optical properties bulk absorption < nm birefringence < 1nm / cm excellent homogeneity Δn < D isotropic available in large size polishing and coating well mastered 4

5 Fused silica plate of 550 mm diameter 5

6 The king: fused silica Some more properties particularly relevant to GW detectors: very low bulk thermal noise possibility of monolithic suspension Reduction of the undesired displacement of the surface mirror due to thermal noise 6

7 What has been achieved: the polishing Outstanding polishing capabilities for fused silica! Low spatial frequency surface figure: High spatial frequency figure: RMS < 0.2 nm over 150 mm diameter RMS < 0.1 nm 7

8 What has been achieved: the coating State of the art Ion Beam Sputtering (IBS) coating: possibility to coat large substrates (up to Ø 80 cm) loss mechanical loss (HR coating φ ~ ) very low absorption (< nm) Low scattering, 4 ppm (on average over 20 mirrors) RMS of ~ 0.7 nm for HR ETM (coating thickness: 6 μm) Surface over 160 mm of the 6 μm HR coating (T = 3 ppm) 8

9 The cool outsider: sapphire crystalline material not available in large size (Ø<220 mm) Sapphire substrate relatively high absorption (~ 50 ppm/cm) high thermal conductivity polishing used to be difficult ~ 0.5 RMS over Ø 180 mm 9

10 Comparison of bulk mechanical loss The lower, The better 10

11 For third generation detectors... Longer arm length larger beam size looking for larger substrates For room temperature: keep the fused silica For low temperature: the size and optical properties of fused silica the mechanical / thermal properties of sapphire Is silicon the answer? 11

12 II What do we know about silicon?

13 Transmission spectra Not transparent at 1064 nm new laser, likely 1550 nm High refractive index, n=3.45 uncoated: reflectivity of 30% 13

14 Absorption below the band gap The energy of the photon absorbed brings one electron to the conduction band Absorption tail possible in the near infrared but negligible at 1550 nm 14 ET wavelength

15 Absorption above band gap For near infrared wavelength Free carrier absorbed light (intra-band absorption) Absorption proportional to the free carrier density Well characterized for moderately doped silicon 15

16 Example p type silicon Free carrier = free holes (phosphorus doped) Free carrier absorption proportional to λ2 16

17 Example n type silicon Free carrier = free electron (boron doped) Intraband transition in the conduction band Absorption more complex, with a bump around 2.3 μm 17

18 Absorption as a function of the doping Absorption is due to the free carrier Free carrier concentration is equal to dopant concentration Absorption as low as 5 ppm/cm has been measured 18

19 And the absorption at low temperature? Optical Absorption critical at low temperature (due to the heat generated) Free carrier 'frozen' at low temperature (i.e. impurities not ionized), so no absorption? 19

20 Absorption vs temperature With different dopant: Phosphorus Boron Gallium Same magnitude of the optical absorption at low temperature! 20

21 The mechanism... How, so far, it is understood... At room temperature Impurites ionized Free carrier absorption At low temperature Impurites neutral Photoionization trail 21

22 Bulk mechanical loss Best (lowest) mechanical loss at low temperature with sapphire 22

23 Polishing and coating Polishing: Companies have extensive experience in polishing silicon Could achieve same surface figure as for fused silica RMS micro-roughness slightly higher than for FS Silicon more brittle, easier to damage during cleaning / handling Coating: IBS coating compatible Coating routinely done on wafer or cantilever Crystalline coating (GaP/AlGaP)? 23

24 Laser source (and squeezing) Since working at low temperature, no need for high power (few Watt of input laser) 1550 nm, the 'telecom' wavelength, so stable lasers/amplifiers are available Squeezing also demonstrated at 1550 nm (-12.3 db max) 24

25 III The open issues

26 With new material comes new noise... The carrier density noise: Refractive index changes with the free carrier concentration Local variation of free carrier concentration will create phase noise for transmissive optics Increasing with the doping level 26

27 The size matters... Large diameter mirror is essential. Silicon industry move toward 450 mm wafer (already available in small quantities) HOWEVER, Very difficult to achieve high purity, large silicon substrate Diameter [mm] Absorption [/cm] ppm ppm 450 5% 27

28 Suspension for silicon Try to reproduce the success of the fused silica monolithic suspension (essential to guarantee low suspension thermal noise) Research is focusing on Silicon fiber Silicon bonding New essential function for the suspension fibers: Cool (and extract the heat) from the optics, important constraint on the diameter and length of the fiber. 28

29 Conclusion Silicon is a promising material for cryogenic interferometer But the high optical absorption of large substrate could be a serious drawback 29

30 Sources and credit Title slide photos left: middle: right: 150 mm diameter silicon sample from Jena Slide 5 Picture taken at LMA during the cleaning of the Advanced Virgo beamsplitter Slide 6 Found at : From the Virgo logbook post Slide 7 Data taken from the polishing reports for the Advanced Virgo arm cavity end mirrors Slide 8 L. Pinard, mirrors talk at the LVC meeting of March 2015 Slide 9 Sapphire picture from KAGRA Status by S. Miyoki 11/09/2012 Development of a cryogenic mirror system for the KAGRA GW detector E. Hirose 2014 Slide 10 Silicon and Sapphire - mechanical, thermal and optical properties by R. Nawrodt, P8 (ET-0002A-15 ) 30

31 Sources and credit Slide 13 Measurement done at LMA Slide 14 Optical measurements at cryogenic temperatures - J. Komma GWADW 2013 Slides 16, 17 & 18 Measurement done at LMA Slide 22 Silicon and Sapphire - mechanical, thermal and optical properties by R. Nawrodt, P8 (ET-0002A-15 ) Slide 23 Coating done at LMA on a wafer Slide 24 Photo: Keopsys CEFL TERA 1550 nm: Slide 26 Free carrier driven noise in transmissive semiconductor optics - D. Heinert

32 Sources and credit Slide 27 Photo: picture taken from Internet My data after talking with the industry Slide 28 Informations taken from:low Thermal Noise Suspensions for Future Detectors G. Hammond (ET-0035A-14) 32