News release

From: ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS)

Researchers have developed a smart nanomaterial capable of doing the work of two devices at once, forming the basis for new compact point-of-care medical imaging devices.

Researchers from the Australian Research Council Centre of Excellence for Transformative Meta-Optical Systems (TMOS) have invented a new metasurface capable of imaging microscopic objects that are otherwise nearly invisible. Led by Professor Ann Roberts and TMOS PhD student Haiwei Wang, the research brought together teams from the Centre's RMIT University and The University of Melbourne nodes and was recently published in Nanophotonics.

In medicine, biology, and materials science, being able to capture images of microscopic objects that are nearly transparent provides an important source of information for disease diagnosis and pure research.

The principal technique researchers use to do this is called ‘phase microscopy’. Here an image is created of the tiny shifts in the relative position of peaks and troughs in light waves instead of using the intensity of the light field as is the case with a common your eye.

This allows the sample to be seen without using techniques, like staining, which are time consuming, bespoke, and can alter the sample.

However, all current phase microscopy setups face a trade-offs between fundamental capabilities, compactness, and operation time.

But now, Ann and Wang’s new metasurface may finally offer a means around these trade-offs by bringing together multiple advances in nanophotonics onto a single surface.

Metasurfaces are a key technology for designing compact optical systems due to their microscopic size and novel properties. They are made from structures whose sizes are comparable to the wavelength of light arranged to form an ultrathin surface. Because of this, these surfaces interact with light in ways not achievable with natural materials.

Using this technology, the researchers’ aim was to design a system that could perform a technique called ‘quantitative phase microscopy’ (QPM).

QPM provides not only the qualitative shape of a sample, but the exact phase shift of light transmitted through each region of the image.

This information is critical because it can be used to measure objective properties such as refractive index, and the dry weights of tissues, quantities important for accurate diagnosis and research.

With this goal in mind, the challenge was to design a system that could perform QPM in a compact, fast, and field deployable form factor, and this meant bring together multiple metasurface technologies.

To achieve compactness, the researchers relied upon designing their metasurface to be ‘nonlocal’.

Instead of changing the trajectories of individual light rays like a traditional lens, a nonlocal metasurface processes an image as a whole.

This makes nonlocal metasurfaces invaluable for saving space because they can be placed directly in front of a sample without large gaps between optical elements.

Prior to this work, nonlocal metasurfaces had been used for phase microscopy but with only qualitative results.

Full QPM requires measuring how the phase changes along perpendicular directions of an image. This means taking two separate images, one that shows how the phase changes left-to-right, and another showing its shift up-and-down across the image.

However, measuring the phase gradient across both directions poses a time and convenience cost.

In between capturing both images, the metasurface would need to be rotated or a new surface inserted, requiring precise alignment, and/or a lengthy characterisation process.

The researchers realised they could avoid this if they could incorporate the capability of making these two measurements into a single surface without them interfering.

By engineering the nanoscopic structures of the metasurface to respond to only a very narrow band of light, they could construct two independent sets of structures each sensitive to a different wavelength of light. This removed the need to physically alter any setup, instead selecting the function of the metasurface using a particular wavelength of light.

This principle of ‘multiplexing’ functionalities enabled the researchers to incorporate another technique, called differential phase contrast, which reduced the noise in the final QPM images.

By combining multiple functionalities into a single nonlocal metasurface, the researchers had developed a method for achieving QPM in a highly compact form factor without the need for physically moving components.

For Wang, this was a big step forward, not only for the applications they hope for, but for metasurface research in general, stating:

“[These techniques] have been used quite a lot, but not in the same way we have done it… here we are applying them to access different […] functionalities, which hasn’t been done before.”

The potential for smaller devices and a streamlined imaging process could prove invaluable in the diagnosis of diseases that rely on observing cell shape and growth patterns, such as sickle cell disease, various cancers, and neurodegenerative diseases.

When asked about the potential applications of this technology Wang replied, “I think it could go into small devices you could take into point-of-care settings… Instead of having to lug around a giant microscope system, we can try to slot this into a smaller device, like a microscope on a smartphone…”

However, in the short term, there are still hurdles to overcome in the costs of supporting hardware, and manufacturing, but for Wang these new challenges are simply a matter of time and effort to overcome.

“Right now, we use electron beam which scans the entire surface, whereas the commercial [nanoimprint lithography] takes one shot”.

And when asked about the next step on the road to a true point-of-care system, Wang noted that “There are also cost associated with generating the illumination… [and] it would still require a laser that can be tuned between two wavelengths very precisely.

“[All the] supporting components, right now, are implemented with traditional optics still.

“…[Now] I’m integrating [our design] into a commercial microscope… This one does not require a laser … It still needs an infrared LED, but this one is quite simple to insert into the system.”

When asked where this technology was ultimately headed, Wang stated “Miniaturised sensors is the main direction.” Suggesting that as these sensors inevitably become smaller, they will only see an expanding array of applications from microscopic internal medicine to astronomy.

“People have been trying to put this on the head of an endoscope, which can be a bundle of fibres, and then you can move it inside the body. Then you can do imaging in vivo… that is really showing how something can be really miniaturised; The end of an endoscope is extremely small.

“…another application is wavefront sensing. So, these are in a lot of telescopes to correct the effects of turbulence… Wavefront sensing is sort of similar to phase imaging.”

Wang was confident that this marked a new step for metasurface research in general, as their paper also concluded:

“To our knowledge, this is the first time such a multiplexing scheme has been demonstrated, and this work opens the path to exploring additional functionalities that can be multiplexed onto a single layer metasurface.”