Our lab is primarily involved with the scientific development of the Laser Interferometer Gravitational Wave Observatory (LIGO). The current instrument (very) basically converts mirror motion due to gravitational waves into a phase change measurement from interfering laser beams that are reflected from the instrument mirrors.

For next upgrade of LIGO, design sensitivity is expected to be limited by thermal noise. To give this level of sensitivity some context the instrument will essentially be sensitive to the motion of the atoms that make up the mirrors (among other things) and this will compromise the measurement.

One strategy that we are testing at ANU to limit thermal noise is the introduction of Silicon as a material for the mirrors. Silicon has the interesting property that at a temperature of 123 K the coefficient of thermal expansion goes to zero, and this minimizes substrate thermoelastic noise and thermal lensing. Additionally silicon has higher thermal conductivity at 123K and has low mechanical loss, thereby reducing Brownian noise at cryogenic temperatures.

Left: Sensitivity   to   the   strain   in   spacetime   due   to gravitational wave sources over a bandwidth of 10 Hz to several kHz. Mirror coating thermal noise is clearly one   of   the   largest   limiting   factors   to   improved sensitivity. Right: The silicon test cantilever that forms part of an   optical   cavity   to   increase   measurement sensitivity.

The pink section reduces laser frequency noise by stabilizing the laser to an external reference cavity (RC). The blue section uses the stabilized laser to probe the motion of the cantilever that forms part of a test cavity (TC) that is suspended on a torsion pendulum inside a five cubic meter vacuum tank. This enables measurements on the order of 10-14 m/Hz. This can be thought of as a length measurement to a precision of 10-14 m if we were able to average the measurement over a second.

To perform measurements of atomic motion the silicon test material is isolated by hanging it from a large torsion pendulum to reduce degradation of the measurement due to environmental noise. The silicon test material is a cantilever structure to enhance mechanical motion due to thermal noise. The measurement of the cantilever motion is performed by adding a mirror to the end of the cantilever and forming an optical cavity with a second, relatively stable, mirror. This optical cavity is used to enhance the phase measurement when a laser beam is used to probe the mirror motion [1]. Measurements have been made at room temperature [2]. And this experiment is currently being upgraded to repeat the measurement at 123 K.

[1] Nguyen T. T-H., et. al. Frequency dependence of thermal noise in gram-scale cantilever flexures Physical Review D 92, 112004 (2015).

[2] Ward R. et. al. Direct observation of thermal noise spectrum of a silicon flexure membrane, in preparation