Nanoscale Ultrasonic Transducers

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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

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model of mechanical motion
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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

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Experiment system
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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.

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Experimental result
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Transducer Attached to hair

Experimental results on 10 micron patch agree very well with modelled response

Weighting of harmonics differ due to slight variations in material properties and created layer sizes

Nanoparticle Transducers

Structure is gold shell around silica core Mie theory optical modelling shows repeating regions of high sensitivity 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).