Details of Grant 

EPSRC Reference:	EP/C010469/1
Title:	Sound-light interactions in spatially periodic nanoporous silicon structures
Principal Investigator:	Snow, Dr PA
Other Investigators:	Russell, Professor PS Birks, Professor TA
Researcher Co-Investigators:
Project Partners:
Department:	Physics
Organisation:	University of Bath
Scheme:	Standard Research (Pre-FEC)
Starts:	05 September 2005	Ends:	04 February 2010	Value (£):	380,119
EPSRC Research Topic Classifications:	Acoustics Materials Processing Optical Devices & Subsystems
EPSRC Industrial Sector Classifications:	Electronics
Related Grants:
Panel History:
Summary on Grant Application Form
We want to explore the interaction between sound and light in porous silicon, which could lead to a new generation of devices exploiting this effect that could be more than a hundred times smaller than the existing versions. No one has used porous silicon like this before to trap both sound and light.Nanoporous silicon is made by passing electric current through silicon that is in contact with a strong acid solution. An acid front nibbles away into the silicon, and as it does so it leaves behind a sponge-like structure that is full of linked passages or pores . The pores are only about ten nanometers across; that is, they are only about twenty silicon atoms wide and so small that not even a 'flu virus could fit in them. The current used controls the fraction of silicon that is dissolved where the front of acid has reached, so by varying the current as the front moves we can vary the porosity from place to place. Thus, we can design and control the exact porosity profile of a sample.The porosity of a sample affects how light and sound travel through it. It is known that alternating layers of two different characteristics can dramatically affect how both types of waves are transmitted or reflected. In the case of light, this is used to make reflective coatings on glasses and binoculars. Since we can make such multilayer structures in porous silicon, we plan to use our technique to make cavities sandwiched between two sets of such structures that reflect light and sound at the same time, and which therefore trap both waves in a very small space. This intensifies the amplitude of the waves, which we can use to make devices. We will use the fact that sound waves in the porous silicon also affect how light moves through it, because the compression effect of the sound changes the refractive index from place to place. This is the acousto-optic effect. In order to build devices we need to know exactly how strong the effect is. We can measure it by sending a sound wave into the cavity and measuring what it does to a beam of light reflected by it.This information will allow us to make excellent acousto-optic devices because we can then design the multilayer mirrors and the cavity itself to optimise the interaction between sound and light. An example of such a device is an acousto-optic switch, where the transmission of light is controlled by the sound wave. There are other aspects of the behaviour of sound in porous silicon to be explored. Sound can be trapped in discs or boxes as well as just layers, and it is a great feature of porous silicon is that it can be formed into many such different shapes. Researchers are rapidly finding new ways to improve the usefulness of porous silcon devices; not just the silicon left in a sample but its pores and its surface too. For example, the pores can be filled with liquid. We will explore the effects of vibrations on liquids trapped in the tiny pores. Also, we can link the porous silicon vibration to pressure waves in a volume of liquid. We will use this to pull very small objects suspended in the liquid to the surface of the sample where they can then be moved about on the surface of the sample: an effect that could be used to grade, sort and arrange nanoparticles.
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Organisation Website:	http://www.bath.ac.uk