STREGA WP1/M1 mirror substrates GEO LIGO ISA Scientific motivation: Mechanical dissipation from dielectric mirror coatings is predicted to be a significant source of thermal noise for advanced detectors. Coatings must also be of low optical loss. Main workpackage outcomes/long term aim: –Measurement of the thermal expansion, thermal conduction and mechanical losses of CaF2 (Calcium Fluorite) and Si (Silicon), varying the temperature from 300K down to 4K. –Investigation of the alteration of thermo-mechanical properties of silicon as a function of quantity and nature of dopants –Realisation and test of prototypes in connection with the tasks M4 and M5

Status at last meeting To high voltage Excitation plate (behind mass) Silicon samples cut along different crystal axes, [111] and [100]. The [111] sample was boron- doped. Preliminary room T measurements of mechanical dissipation of two silicon samples of identical geometry, supplied by Stanford, was measured over a range of frequencies. Clamp Suspension thread/wire Schematic diagram of front view of suspended test mass. Test mass

Status at last meeting Lowest loss obtained so far = (9.6±0.3)  Comparable with the lowest loss factors measured at room temperature Sample [b] typically showed lower dissipation Sample [a]: [100] cut, nominally undoped Sample [b]: [111] cut, boron doped Reason for difference seen in measured loss factors (eg crystalline orientation, dopant, other?) is under investigation Some evidence to suggest may be due to crystalline orientation Measured loss factors for two samples of bulk silicon [a] [b]

Consider the [100] sample Q of two modes of the sample studied as a function of orientation of sample in suspension loop FE model of relative displacement of surface for 2 modes of sample Friction at suspension points is clearly significant and dependent on position of crystal Mode shapes are dependent on crystal axis

Future plans Constructing a prototype ‘nodal support’ system to attempt to reduce suspension losses for these samples Several sets of samples purchased of various aspect ratios and different crystal cuts for further study Measurements of bulk silicon at cryogenic temperature –Second cryostat being commissioned in Glasgow –Sample suspended – initial cooling taking place this week

STREGA WP1/M5 suspension substrates Suspension technology status: –To achieve the desired sensitivities of future long- baseline gravitational wave detectors will require a reduction in thermal noise associated with test masses and their suspensions –Requirement to develop ultra-low thermal noise suspensions for 3rd generation detections (cryogenic temperatures) Current eg. GEO600 Advanced eg. AdLIGO Future eg. EGO Silicon suspension technology

Silicon cantilever fabrication Initial samples have been fabricated by etching from silicon wafers at Stanford (Stefan Zappe) First sample studied: – P-type doping (Boron), resistivity = Ohm-cm – ~ 92 microns thick – Resonant modes of samples excited using an electrostatic drive – Sample displacement monitored using shadow sensor – Measure rate of decay of the mode amplitudes, from which mechanical dissipation,  (  0 ) can be determined. Set of samples fabricated with varying properties and dimensions: 1 x Ohm-cm to >100 Ohm-cm ~40 microns to ~100’s  m thick

Experimental setup GEO LIGO ISA

The two cantilevers tested GEO LIGO ISA Identical material properties. Thermoelastic curves for tested cantilevers with marked cantilever resonant frequencies

Experimental measurements Measured dissipation is the sum of dissipation arising from a number of sources: calculate from silicon material properties measurements of samples of varying surface to volume ratios should allow estimates measurements in vacuum <10 -5 Torr rigid clamp holding thick end of sample

Temperature dependence of (a) measured loss, and (b) calculated thermoelastic loss for bending mode at 670Hz (cant length 57mm) 4.7x10 -7

Temperature dependence of (a) measured loss, and (b) calculated thermoelastic loss for bending mode at 2185Hz. (cant length 57mm)

Temperature dependence of (a) measured loss, and (b) calculated thermoelastic loss for bending mode at 1935Hz. (cant length 34mm)

Temperature dependence of (a) measured loss, and (b) calculated thermoelastic loss for bending modes at 3785 Hz and 6265 Hz. (cant l = 34mm)

Results and interpretation The intermittent dissipation peaks observed appear related to changes in temperature distribution in system Suggests they are due to energy coupling to resonances in the clamping structure Also need to consider surface effects (sample is 92 microns thick)

Surface losses Yasumura et al. measured the loss factors of single-crystal cantilevers with thickness 0.06  0.24  m and found they could be represented by:  surface is the limit to the measurable loss of a cantilever of thickness t and Young’s modulus E 1 set by the presence of a surface layer of: Surface layer thickness , Young’s modulus E 1 S, and loss  s For simplicity assume, E 1  E 1 S Can use the above to estimate limit to measurable loss for our sample set by surface loss

Surface effects are significant but do not account for total loss measured - next source to investigate is ‘stick-slip’ damping of end of sample in clamp (a)Ave. loss of third bending mode (670Hz) compared to (b) sum of scaled surface loss + calculated th-elas loss.

Results and interpretation Studies have been carried out to investigate energy coupling to the clamping structure

Normalised piezo response showing energy coupling from resonating cantilever into steel clamp at 1.3, 2.2 and 3.1 kHz

Response of the clamping structure to a constant driving signal from 2-3 kHz at 240, 270 and 285 K from 2–3 kHz.

The measured dissipation Q-1 in silicon oscillators (kHz frequency band) 1 – Calculated from “phonon- phonon” mechanism (f = 10 kHz) 2 – MSU – 1980, unpublished (t  10 cm, f = 10 kHz) 3 – D.F. McGuigan et al., J.Low Temp.Phys. 30 (1978), 621 (t  10 cm, f = 19.5 kHz) 4 – B.H.Houston et al., Appl.Phys. Lett. 80 (2002), 1300 (t  100  m, f = 5.5 kHz) 5 – U.Gysin et al., Phys.Rev. B69 (2004), (t  2  m, f = 10.8 kHz) Slide courtesy of V. Mitrofanov, Moscow State University

Summarise So far we do not see evidence of dissipation peaks intrinsic to our sample Evidence of coupling to resonances of clamping structure under investigation In short term: We will study both thinner and thicker samples to further quantify surface loss effects Reduce surface loss effects to allow studies of other dissipation mechanisms presents In medium term: Continue studies of effects of doping on intrinsic dissipation and thermoelastic loss