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 In GW detectors:
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 Logarithmic divergence ! Size effects? Coatings ?
Cerdonio et al., Phys. Rev. D 63, (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, (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. xe xc
PD3: Power monitor (Wmod)
PD2: error/correction signal
Numerical lock-in:
amplitude and phase of syncronous detection at Wmod
GPDH
Gloop s
GAOM xe xc

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 Future 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?