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M. De Rosa INOA, LENS, INFN F. Marin University of Florence, LENS, INFN F. Marino INFN

New Results on Photothermal Effect: Size and Coating Effect. M. De Rosa INOA, LENS, INFN F. Marin University of Florence, LENS, INFN F. Marino INFN O. Arcizet, M. Pinard, A. Heidmann Laboratoire Kastler Brossel, Paris. Cascina 9/06/05. Photothermal effect. Photon absorption 

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M. De Rosa INOA, LENS, INFN F. Marin University of Florence, LENS, INFN F. Marino INFN

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  1. New Results on Photothermal Effect: Size and Coating Effect M. De Rosa INOA, LENS, INFN F. Marin University of Florence, LENS, INFN F. Marino INFN O. Arcizet, M. Pinard, A. Heidmann Laboratoire Kastler Brossel, Paris Cascina 9/06/05

  2. Photothermal effect Photon absorption  Local heating  Thermal expansion In GW detectors: Intensity noise turns into position fluctuations A theoretical model: beam waist mirror size materials (substrates and coatings)

  3. Half-infinite mirror Cerdonio et al., Phys. Rev. D 63, 082003 (2001) Braginsky et al., Phys. Lett. A 264, 1 (1999) Logarithmic divergence ! Size effects? Coatings ?

  4. First experimental test Power modulation, P=Po + Pm sin ωmt Phase-sensitive detection Fitting curve: l = Lo K(/c)½ c = 2.8 ± 0.6 Hz (calculated: 1.8 — 2.7 Hz) absorption coefficient: ~ 5 • 10-7 De Rosa et al., Phys. Rev. Lett. 89,237402 (2002)

  5. Scheme of the measurements • The laser frequency is locked to the reference cavity • The intensity of the light impinging on the probed cavity is modulated at a frequency Ωmod • Phase sensitive detection of the induced mirror displacements by measuring the frequency detuning of the cavity resonance (Amplitude and phase information)

  6. Old setup (AURIGA laser system) • Modulation on both cavities: differential effect • Small modulation depth (1% of total power) • No relative locking of the cavities: large errors on long time series, drift... • Small relative tuning range

  7. New setup • Modulation on one cavity • High modulation depth (> 20% of total power) • AOM allows large and fast tunability of the cavities and relative locking of the two cavities

  8. s xe xc GPDH Gloop GAOM xe xc PD3: Power monitor (Wmod) PD2: error/correction signal Numerical lock-in: amplitude and phase of syncronous detection at Wmod

  9. Probed Cavities Mirrors substrate: Fused Silica Coatings: SiO2/Ta2O5

  10. Long cavity a) half-infinite mirror b) finite size effects c) coating effect

  11. Short cavity we cannot investigate very low frequency (long time PT drift) coating effect

  12. Conclusions • beam waist dependence of cut-off frequency • finite size effects at low frequency • coating effects • improvement of the half-infinite mirror model including finite size and coating effect (material properties) • Future • behaviour at low temperature • different substrates (Sapphire, Silicon,…)

  13. FP transmission by sweeping nL: hysteresis

  14. Optical cavities showed bistability and stochastic resonance due to photo-thermal effect Effects very important for ultra-sensitive displacement detection New questions: Stochastic-driven nonlinear dynamics will prevent from observing signals? Stochastic Resonance can improve sensitivity?

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