2D Electro-Polariton Platform for Miniaturized Spectrometers

In a recent study in Nature Communications, researchers from ICFO and an international team introduced a breakthrough in integrating a detector system with a polaritonic platform within the same 2D material.

This advancement enables, for the first time, the detection of 2D polaritonic nanoresonators with spectral resolution. The device is highly suitable for miniaturization and simultaneously demonstrates unprecedented levels of lateral confinement and high-quality factors.

Polaritons, coupled excitations of electromagnetic waves with either charged particles or vibrations in a material's atomic lattice, play a key role in nanophotonics due to their ability to confine light to extremely small volumes, typically at the nanometer scale, which is crucial for enhancing light-matter interactions.

2D materials of only one atom thick are often used for this purpose, as the polaritons they support exhibit even stronger confinement, reduced energy losses leading to longer lifetimes, and greater tunability compared to bulk materials. To further improve light confinement and enhance polaritonic properties, nanoscale structures known as nanoresonators are employed.

Light interacting with a nanoresonator excites polaritons that oscillate and resonate at specific frequencies determined by the resonator's geometry and material characteristics. This allows for precise control of light at the nanoscale.

Although the use of polaritons for light confinement is well-established, there remains potential for enhancing the techniques used to probe them. In recent years, optical measurements have gained popularity, but they rely on bulky detectors that require external equipment.

This restricts the detection system's miniaturization and affects the signal-to-noise ratio obtained from the measurements. These limitations hinder the application of polaritonic properties in fields where miniaturization and signal precision are critical, such as molecular sensing.

Researchers from ICFO, including Dr. Sebastián Castilla, Dr. Hitesh Agarwal, Dr. David Alcaraz, Dr. Adrià Grabulosa, Matteo Ceccanti, and Dr. Roshan Krishna Kumar, 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), have demonstrated the integration of 2D polaritons with a detection system within the same 2D material.

For the first time, this integrated device facilitates spectrally resolved electrical detection of 2D polaritonic nanoresonators and represents a significant advancement toward device miniaturization.

The team utilized electrical spectroscopy on a stack of three layers of 2D materials, consisting of a hexagonal boron nitride (hBN) layer positioned above graphene layered on another hBN sheet.

Researchers observed several advantages of electrical spectroscopy over commercial optical techniques throughout the experiments. Electrical spectroscopy offers a significantly broader spectral range, encompassing a more comprehensive array of frequencies, including infrared and terahertz regions.

The equipment required for electrical spectroscopy is much more compact, and the measurements yield higher signal-to-noise ratios.

This electro-polaritonic platform signifies a breakthrough in the field due to two key features. First, it eliminates the need for an external detector for spectroscopy, which is typically required by most optical techniques. Instead, a single device functions simultaneously as a photodetector and a polaritonic platform, facilitating further system miniaturization. Second, while greater light confinement generally compromises the quality of that confinement—such as reducing the duration of light trapping—the integrated device effectively overcomes this limitation.

Our platforms have exceptional quality, achieving record-breaking optical lateral confinement and high-quality factors of up to 200, approximately. This exceptional level of both confinement and quality of graphene significantly enhances the photodetection efficiency.

Dr. Sebastián Castilla, Study Co-First Author, ICFO

Additionally, the electrical spectroscopy method allows for probing very small 2D polaritons with lateral sizes of approximately 30 nm.

That was highly challenging to detect with conventional techniques due to the imposed resolution limitations. 

Dr. Sebastián Castilla, Study Co-First Author, ICFO

Castilla now contemplates the potential future discoveries that their innovative approach could enable.

"Sensing, hyperspectral imaging, and optical spectrometry applications could benefit from this electro-polaritonic integrated platform. For instance, in the case of sensing, on-chip electrical detection of molecules and gases could become possible. I believe that our work will open the door to many applications that the bulky nature of standard commercial platforms has been inhibiting," continued Castilla.

Journal Reference:

Castilla, S., et al. (2024) Electrical spectroscopy of polaritonic nanoresonators. Nature Communications. doi.org/10.1038/s41467-024-52838-w.