Polaritons are coupled excitations of electromagnetic waves with either charged particles or vibrations in the atomic lattice of a given material. One of their most attractive properties is the capacity to confine light at the nanoscale, which is even more extreme in two-dimensional (2D) materials. 2D polaritons have been investigated by optical measurements using an external photodetector. However, their effective spectrally resolved electrical detection via far-field excitation remains unexplored. This hinders their exploitation in crucial applications such as sensing, hyperspectral imaging, and optical spectrometry, banking on their potential for integration with silicon technologies.
Recently, researchers from Spain's ICFO, the University of Ioannina, Universidade do Minho, the International Iberian Nanotechnology Laboratory, Kansas State University, the National Institute for Materials Science (Tsukba, Japan), POLIMA (University of Southern Denmark) and URCI (Institute of Materials Science and Computing, have reported on the electrical spectroscopy of polaritonic nanoresonators based on a high-quality 2D-material heterostructure, which serves at the same time as the photodetector and the polaritonic platform. Subsequently, the team electrically detected these mid-infrared resonators by near-field coupling to a graphene pn-junction. The nanoresonators simultaneously exhibited extreme lateral confinement and high-quality factors.
In their work, the researchers demonstrated the integration of 2D polaritons with a detection system into the same 2D material. The integrated device enables, for the first time, spectrally resolved electrical detection of 2D polaritonic nanoresonators and marks a significant step towards device miniaturization. The team applied electrical spectroscopy to a stack of three layers of 2D materials. Specifically, an hBN (hexagonal boron-nitrate) layer was placed on top of graphene, which was layered on another hBN sheet.
During the experiments, the researchers identified several advantages of electrical spectroscopy compared to commercial optical techniques. With the former, the spectral range covered is significantly broader (that is, it spans a wider range of frequencies, including the infrared and terahertz ranges), the required equipment is significantly smaller, and the measurements present higher signal-to-noise ratios.
This electro-polaritonic platform represents a breakthrough in the field due to two main features. First, an external detector for spectroscopy, required by most optical techniques, is no longer needed. A single device serves at the same time as a photodetector and a polaritonic platform, therefore enabling further miniaturization of the system.
Second, while in general higher light confinement is detrimental to the quality of this confinement (for instance, shortening durations of light trapping), the integrated device successfully 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," explains Dr. Castilla, first co-author of the article.
Moreover, the electrical spectroscopy approach enables the probing of extremely small 2D polaritons (with lateral sizes of around 30 nanometers). "That was highly challenging to detect with conventional techniques due to the imposed resolution limitations," he adds.
Castilla reflects on what future discoveries could be unlocked by their new approach. "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," he suggests. "I believe that our work will open the door to many applications that the bulky nature of standard commercial platforms has been inhibiting."
This work opens a venue for investigating this tunable and complex hybrid system and its use in compact sensing and imaging platforms.
