Long-Lived Nanolight in a 2D Material

An inter­national team led by researchers from Monash Univer­sity in Melbourne, Univer­sity of Oviedo in Asturias, Spain, CIC nanoGUNE in San Sebastián, and Soochow Univer­sity in Suzhou, China, discover squeezed light in the nanoscale that propa­gates only in specific directions along thin slabs of molyb­denum trioxide. Besides its unique directional character, this nanolight lives for an excep­tionally long time, and thus could find appli­cations in signal proces­sing, sensing or heat management at the nanoscale.

Illustration of directional nanolight propagating along a thin layer of molybdenum trioxide. (Source: S. Li)

Future infor­mation and communi­cation techno­logies will rely on the mani­pulation of not only electrons but also of light at the nanometer-scale. Squeezing light to such a small size has been a major goal in nano­photonics for many years. A successful strategy is the use of polaritons, which are electro­magnetic waves resulting from the coupling of light and matter. Parti­cularly strong light squeezing can be achieved with polari­tons at infrared frequencies in 2D materials, such as graphene and hexa­gonal boron nitride. However, although extra­ordinary polari­tonic properties such as electrical tuning of graphene polari­tons have been recently achieved with these materials, the polari­tons have always been found to propa­gate along all directions of the material surface, thereby losing energy quite fast, which limits their appli­cation potential.

Recently, it was predicted that polari­tons can propagate aniso­tropically along the surface of 2D materials, in which the electronic or structural properties are different along different direc­tions. In this case, the velocity and wave­length of the polari­tons strongly depend on the direction in which they propagate. This property can lead to highly direc­tional polariton propa­gation in the form of nanoscale confined rays, which could find future appli­cations in the fields of sensing, heat manage­ment or maybe even quantum computing.

Now, an inter­national team have discovered ultra-confined infrared polari­tons that propagate only in specific directions along thin slabs of the natural 2D material molybdenum trioxide (α-MoO3). “Our findings promise α-MoO3 to become a unique platform for infrared nano­photonics”, says Qiaoliang Bao. “It was amazing to discover polaritons on our α-MoO3 thin flakes travelling only along certain directions”, says post­graduate-student Weiliang Ma. “Until now, the direc­tional propa­gation of polaritons has been observed experi­mentally only in arti­ficially structured materials, where the ultimate polariton confine­ment is much more difficult to achieve than in natural materials”, adds Shaojuan Li.

Apart of direc­tional propa­gation, the study also revealed that the polaritons on α-MoO3 can have an extra­ordinarily long lifetime. “Light seems to take a nanoscale highway on α-MoO3; it travels along certain direc­tions with almost no obstacles”, says Pablo Alonso-González. He adds: “Our measure­ments show that polaritons on α-MoO3 live up to 20 pico­seconds, which is 40 times larger than the best-possible polari­ton lifetime in high-quality graphene at room tempera­ture”.

Because the wave­length of the polari­tons is much smaller than that of light, the researchers had to use a near-field optical micro­scope, to image them. “The establish­ment of this technique coincided perfectly with the emergence of novel van der Waals materials, enabling the imaging of a variety of unique and even unex­pected polari­tons during the past years”, adds Rainer Hillen­brand.

For a better under­standing of the experi­mental results, the researchers developed a theory that allowed them to extract the relation between the momentum of polari­tons in α-MoO3 with their energy. “We have realized that light squeezed in α-MoO3 can become hyper­bolic making the energy and wave-fronts to propagate in different directions along the surface, which can lead to interesting exotic effects in optics”, says Alexey Nikitin, Iker­basque Research Associate at Donostia Inter­national Physics Center (DIPC), who developed the theory in colla­boration with Javier Taboada-Gutiérrez, and Javier Martín-Sánchez, PhD and post­doctoral researchers, respectively at Alonso-Gonzalez´s group.

The current work is just the beginning of a series of studies focused on directional control and mani­pulation of light with the help of ultra-low-loss polari­tons at the nanoscale, which could benefit the develop­ment of more efficient nano­photonic devices for optical sensing and signal processing or heat manage­ment. (Source: Elhuyar)

Reference: W. Ma et al.: In-plane anisotropic and ultra-low-loss polaritons in a natural van der Waals crystal, Nature 562, 557 (2018); DOI: 10.1038/s41586-018-0618-9

Link: Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, China • CIC nanoGUNE, Donostia-San Sebastián, Spain

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