Researchers have achieved a significant milestone by combining a detection system with a polaritonic platform within the same two-dimensional (2D) material, allowing for the first time the detection of 2D polaritonic nanoresonators with detailed spectral resolution. This new device is compact and demonstrates exceptional levels of lateral confinement along with high-quality factors simultaneously.
Polaritons are formed when electromagnetic waves couple with charged particles or vibrations of atoms in a material’s lattice. These entities are extensively utilized in nanophotonics due to their capability to confine light within extremely tiny spaces, measured in nanometers, crucial for enhancing interactions between light and matter. Typically, two-dimensional materials (essentially one-atom-thick) are favored in this context, because the polaritons they support provide even tighter confinement, lower energy losses (leading to longer lifetimes), and greater tunability compared to bulk materials. To further refine light confinement and bolster polaritonic characteristics, researchers can use nanoscale structures known as nanoresonators. Furthermore, when light engages with a nanoresonator, it triggers polaritons that oscillate and resonate at specific frequencies set by the resonator’s structure and material, facilitating precise manipulation of light at the nanoscale.
Although the usage of polaritons for light confinement is well-established, techniques to probe them require advancement. In recent years, optical measurements have gained popularity, but these rely on large detectors needing external setups, which hinders both the miniaturization of the detection system and the clarity of the signals (the signal-to-noise ratio) obtained from measurements. This limitation restricts the use of polaritonic properties in applications where compactness and signal clarity are crucial, such as in molecular sensing.
A team of researchers from ICFO—Dr. Sebastián Castilla, Dr. Hitesh Agarwal, Dr. David Alcaraz, Dr. Adrià Grabulosa, Matteo Ceccanti, Dr. Roshan Krishna Kumar, and led by ICREA Prof. Frank Koppens—along with collaborators from the University of Ioannina, Universidade do Minho, the International Iberian Nanotechnology Laboratory, Kansas State University, the National Institute for Materials Science (Tsukuba, Japan), POLIMA (University of Southern Denmark), and URCI (Institute of Materials Science and Computing, Ioannina), showcased their findings in a Nature Communications article. They demonstrated for the first time the seamless integration of 2D polaritons with a detection system within the same 2D material, achieving spectrally resolved electrical detection of 2D polaritonic nanoresonators, marking a significant advance toward miniaturized devices.
The research team implemented electrical spectroscopy on a tri-layer stack of 2D materials, where a layer of hexagonal boron nitride (hBN) was placed atop graphene, which was, in turn, situated on another hBN layer. The experiments highlighted several benefits of electrical spectroscopy over traditional optical methods. Notably, the spectral coverage of the electrical approach is much broader (extending over a wide range, including infrared and terahertz frequencies), the necessary apparatus is significantly smaller, and the measurements yield higher signal-to-noise ratios.
This innovative electro-polaritonic platform stands out due to two key advantages. First, it eliminates the need for an external detector for spectroscopy, often required by typical optical techniques. Instead, a single device functions as both a photodetector and a polaritonic platform, paving the way for further miniaturization of the system. Second, although typically, increased light confinement can degrade this confinement quality (like reducing light trapping times), the integrated device effectively navigates this issue. “Our platforms boast remarkable quality, achieving unprecedented levels of lateral optical confinement and high-quality factors nearing 200. This combination of exceptional confinement and quality in graphene greatly improves photodetection efficiency,” explains Dr. Sebastián Castilla, the article’s first co-author.
Additionally, the electrical spectroscopy method allows researchers to examine incredibly tiny 2D polaritons (around 30 nanometers in lateral size), an endeavor previously hindered by resolution constraints using conventional techniques, as Dr. Castilla comments.
Reflecting on future possibilities, Castilla envisions various applications that could be realized through this novel approach. “Sensing, hyperspectral imaging, and optical spectrometry may benefit from this integrated electro-polaritonic platform. For example, electrical detection of molecules and gases on-chip could soon become feasible,” he proposes. “I believe our research will pave the way for numerous applications that have been restrained by the cumbersome nature of conventional commercial platforms.”
