Engineers develop new sensor to detect tiny individual nanoparticles

The team led by Lan Yang, PhD, the Das Family Career Development Associate Professor in Electrical & Systems Engineering, and their collaborators at Tsinghua University in China say the sensor could potentially detect much smaller particles, viruses and small molecules.                                                                                                            

Nanoparticles, engineered materials about a billionth of a meter in size, can be found in cosmetics, sunscreens and electronics.

Yang and her colleagues have created the Raman microlaser sensor in a silicon dioxide chip to find individual nanoparticles without the need to "dope" the chip with chemicals called rare-earth ions to provide optical gain for the microlaser.

Powerful sensor makes it easier to detect nanoparticles​

Incorporating additions to the microresonator creates the need for more processing steps and increased costs and invites biocompatibility risks.

"This gives us the advantage of using the same dopant-free sensor at different sensing environments by tailoring the lasing frequency for the specific environment, for example, at the band where the environment has minimum absorption, and for the properties of the targeted nanoparticles by just changing the wavelength of the pump laser​," says Sahin Kaya Ozdemir, PhD,first author of the paper.

According to Yang, using the Raman process loosens the requirement of specific wavelength bands for pump lasers because Raman gain can be obtained using pump at any wavelength band.

The team integrated Raman lasing in a silica microcavity with the mode splitting technique pioneered by her group to develop a new, powerful sensor that more readily detects nanoparticles.

Differences between early resonators and the novel resonator​

One of the main differences between early resonators and the novel resonator, known as a morphology dependent resonator, was they didn't use mirrors to reflect light. Yang's WGMR is an actual mini-laser that supports "frequency degenerate modes," patterns of excitation inside the mini-laser's doughnut-shaped ring that are of the same frequency.

When a particle lands on the ring and scatters energy between these modes, the single Raman lasing line splits into two lasing lines with different frequencies.

When a Raman laser beam is generated in the resonator, it likely will encounter a particle, such as a virus nanoparticle, on the circle. When the beam initially sees the particle, the beam splits into two, generating two lasing lines that serve as reference to the other to form a self-referenced sensing technique.

"Our new sensor differs from the earlier whispering gallery sensors in that it relies on Raman gain, which is inherent in silica, thereby eliminating the need for doping the microcavity with gain media, such as rare-earth ions or optical dyes, to boost detection capability​," Ozdemir says. "This new sensor retains the biocompatibility of silica and could find widespread use for sensing in biological media​." 

Ozdemir S, Zhu J, Yan X, Peng B, Yilmaz H, He L, Monifi F, Huang S, Long G, Yang L. Highly sensitive detection of nanoparticles with a self-referenced and self-heterodyned whispering-gallery Raman microlaser. Proceedings of the National Academy of Sciences​, online Early Edition, Sept. 1, 2014.