Sensor relies on enhanced Raman scattering architecture

Princeton researchers have invented a sensor that opens up new ways to detect a wide range of substances, from tell-tale signs of cancer to hidden explosives.

The sensor relies on a completely new architecture developed by the Princeton researchers. The device boosts faint signals generated by the scattering of laser light from a material placed on it, enabling the identification of various substances based on the colour of light they reflect.

The technology is an advance in Raman scattering, a phenomena discovered in the 1920s by an Indian physicist, Chandrasekhara Raman, where light reflecting off an object carries a signature of its molecular composition and structure.

’Raman scattering has enormous potential in biological and chemical sensing and could have many applications in industry, medicine, the military and other fields,’ said Prof Stephen Chou, the professor of electrical engineering who led the research teams that developed the sensor.

’But current Raman sensors are so weak that their use has been very limited outside of research,’ he added. ’We’ve developed a way to significantly enhance the signal over the entire sensor, and that could change the landscape of how Raman scattering can be used.’

Researchers discovered in the 1970s that the Raman signals were much stronger if the substance to be identified is placed on a rough metal surface or tiny particles of gold or silver. The technique, known as surface-enhanced Raman scattering (SERS), showed great promise, but even after four decades of research it has proven difficult to put to practical use. The strong signals appeared only at a few random points on the sensor surface, making it difficult to predict where to measure the signal.

Abandoning the previous methods, Chou and his colleagues developed a new SERS architecture — a chip studded with uniform rows of tiny pillars made of metals and insulators.

The pillar arrays have two key components: a cavity formed by metal on the top and at the base of each pillar; and metal particles of about 20nm in diameter, known as plasmonic nanodots, on the pillar wall, with small gaps of about 2nm between the metal components.

The small particles and gaps significantly boost the Raman signal. The cavities serve as antennas, trapping light from a laser that it shone onto them so it passes the plasmonic nanodots multiple times to generate the Raman signal rather than only once. The cavities also enhance the outgoing Raman signal.

Chou’s team named the new sensor disk-coupled dots-on-pillar antenna-array, or D2PA for short.

So far, the chip has been shown to be a billion times more sensitive than devices that do not boost Raman signals, and the sensor is uniformly sensitive, making it more reliable for use in sensing devices.

Already, researchers at the US Naval Research Laboratory are experimenting with a less sensitive chip to explore whether the military could use the technology to detect chemicals, biological agents and explosives.