Physical Society Colloquium

Liquid Optics

Ermanno Borra

Université Laval

Liquids give essentially perfect surfaces that follow equipotentials so that their optical quality is assured `for free'. Costs are hence substantially lower than those of existing optical elements. Originally, I began exploring the technology to make large rotating mercury liquid mirrors (LMs) to be used in Astronomy. Several LM Telescopes have now been built and employed as research instruments for use in Astronomy, Atmospheric Sciences and Optics.

The technology has evolved and we are now working on Nanoengineered LMs (NLMs). NLMs are an entirely new class of optical elements consisting of highly reflective metallic films deposited on liquids. The technology promises to deliver on an ambitious goal: To allow an incoming wavefront of arbitrary shape to be transformed upon reflection to an outgoing wavefront of any desired shape. This capability is at the very heart of optics. Consequently, this project has the potential to produce a revolutionary technology. For example, by coating ferromagnetic liquids we create reflective surfaces that can be shaped with magnetic fields, allowing us to make complex surfaces that can vary rapidly in time. It must be noted that it should be possible to miniaturize the components and that it is possible to tilt a magnetic liquid mirror (we have done it) since a ferrofluid sticks to a magnet. The technology is far more versatile than conventional adaptive optics for it allows one to obtain very large strokes (>>1 millimeter) as well as strokes of the order of a fraction of a wave. In Astronomy, they can be used to make inexpensive extreme adaptive optics needed for the next generation of large telescopes. The large strokes of magnetic LMs are useful for ophthalmologic applications. By coating viscous liquids we can also make rotating liquid mirrors tiltable by more than 10 degrees. This will be a spectacular development that will greatly increase the versatility of LMTs with a significant impact on optical Astronomy.

I will briefly describe a project, funded by NASA and CSA, involving a 100-m diameter moon-based LM Telescope.