Difference between revisions of "Nanoscale Ultrasonic Transducers"

Revision as of 13:09, 20 April 2011

Nanoscale Ultrasonic Transducers

two approaches to making nanoscale transducers

Ultrasound is a powerful tool for diagnosing medical and mechanical problems. Conventional ultrasonics work at megahertz frequencies and with wavelengths of 1-2mms to 10’s mm. This means it cannot "see" very small objects at the nanoscale. Our new transducers are so small it is impractical to communicate with them electrically. Instead we have devised a non contact method of talking to them using short pulses of laser light.

We have adopted two approaches for producing these transducers, one method builds plates devices the other uses self assembled nanoparticles. The transducers are made from alternating metal and soft transparent layers. They have optical and mechanical resonances and so the devices have to be made such that they work well both mechanically and optically

Plate Transducers

model of mechanical motion

optical model

The transducer is made up of alternating layers of gold and ITO to produce a sandwich structure of the reflective metal layers and the soft transparent core.

We have modeled the optical behaviour of these devices as the gold and ITO layer thicknesses are altered . The model shows that the devices have a wide operating region where there is good sensitivity.With the optimal device being made from 40 nm gold layers sandwiching a 160nm ITO layer.

We have also constructed an FE Mechanical model of the devices to show how the devices behaviour when they are excited by an optical pulse. the model shows the difference in displacement between the the two gold layers, this motion has a main frequency component of ~9GHz with harmonics up to ~70GHz

Experiment system

240nm x 5 micron transducers on substrate

These devices can be made using standard photolithography techniques where an array of transducers is made on a glass substrate by exposing photo resist to UV through an optical mask. once developed the gold and ITO layers are sputter coated and lift off reveals the structures on the substrate. This approach is very useful as we have an ordered array of devices which we can test in situ. Once tested we can release the transducers from substrate into solution and then reattached them for measurements.

These transducers are 240nm high with 5-25 micron patch sizes. The lateral dimensions as limited by our in house photo lithography equipment. Other techniques could be employed to reduce the lateral dimensions further. We test the devices with a picosecond laser ultrasound system, which utilises locked femtosecond lasers with control electronics to provide the pulse delay. The blue pump pulse is absorbed by the transducer and set it in motion. The probe pulse then measures the interaction of device with the generated acoustic waves.

Experimental result

Transducer Attached to hair

We have measured the response on a 240nm x10 micron patch and they agree very well with modelled response. The main frequency is around 9 GHz and there are harmonics up to ~70GHz. The weighting of harmonics do differ from those in the model, this could be due to slight variations in actual material properties and final created layer sizes being different from those modelled.

We hope to employ different dicing techniques in the future to reduce the lateral size of the transducers to below 1 micron.

Nanoparticle Transducers

model of mechanical motion

optical model

The transducers are made with a similar structure to the plate devices. There is a soft transparent core made from silica nanoparticle. This core is then coated with small gold nanoparticles to form a shell.

The optical response of the transducers has been investigated via Mie theory. The particles have repeating regions of high sensitivity with changes in the layer thicknesses. 3D FEM mechanical modelling shows main breathing mode oscillation and higher order harmonics

We make gold nanoparticles of the chosen shell size (10nm) Cores are made to size and functionalised with amine groups Mixing the particles together creates the final transducers The gold particles bind to the functional groups with electrostatic attraction forming a gold shell

We have made transducers with 180nm cores and 10nm shells Each batch produces trillions of transducers We need to improve the fabrication process to control the size variance to keep particles in optimal operating region Testing of these devices is ongoing

The Future

This success of this project requires the input from many different disciplines; engineering, electronics, optics, physics, pharmacy and chemistry. The devices we have made will have many potential applications e.g. chemical sensors, embedded in paints to detect the presence of specific compounds (e.g. explosives) or in the petrochemical industry for pollution and process monitoring. Biological, the transducers will be used for performing elasticity measurements inside cells to aid understanding of cell function (formation of cancers, stem cell differentiation).

Related Publications

Journal of Physics: Conference Series 278:012035,2011

http://iopscience.iop.org/1742-6596/278/1/012035
Bibtex<div>Author : R Smith, A Arca, X Chen, L Marques, M Clark, J Aylott, M Somekh
Title : Design and fabrication of ultrasonic transducers with nanoscale dimensions
In : Journal of Physics: Conference Series -
Address :
Date : 2011
</div>

Technical Report - Poster, The University of Nottingham ,2011

Bibtex

Author : Richard Smith
Title : Set for Britain 2011 London
In : Technical Report - Poster, The University of Nottingham -
Address :
Date : 2011

Technical Report - Talk, The University of Nottingham ,2011

Bibtex

Author : Richard Smith
Title : AFPAC 2011 Frejus
In : Technical Report - Talk, The University of Nottingham -
Address :
Date : 2011

<bibtex> @techreport{noref, title = {LU2010 2010 Bordeaux}, author = {Richard Smith}, year = {2010}, institution = {The University of Nottingham}, type = {Talk}, pdf = {http://optics.eee.nottingham.ac.uk/w/images/c/c4/Talk_2010_LU2010_Bordaeux_RJS_nano_transducers.pdf} }