A two-dimensional lamellar crystal composed of atomically thin layers of lead iodide (PbI₂) could be used to manufacture a new generation of circuits that use light and mechanical vibrations (rather than electrons) to transmit information in the terahertz frequency range.
This promising technology was studied by researchers at the Brazilian Center for Research in Energy and Materials (CNPEM), in partnership with colleagues from the University of Lille (France) and other international institutions, and published in Nature Communications.
The terahertz band corresponds to a low-energy region of the electromagnetic spectrum situated between infrared and microwaves. Despite this, it is considered crucial for developing high-speed communication technologies. "Today, Wi-Fi and 5G operate at frequencies of a few gigahertz (GHz, 10⁹ hertz). But there is interest in moving toward hundreds of gigahertz, or even terahertz (10¹² hertz), because the higher the frequency, the greater the bandwidth and data transmission capacity," says Raul de Oliveira Freitas , head of the "Imbuia" beamline at the Brazilian Synchrotron Light Laboratory (LNLS-CNPEM) and coordinator of the study.
The study investigated how to produce a high-quality layered crystal capable of acting as a waveguide for radiation in this frequency range using lead iodide, an inexpensive material. This platform could function as a resonator, which confines light and selects specific frequencies by amplifying certain modes of oscillation. It could also function as a beam splitter, which splits a beam of light into two or more paths to allow the optical signal to be distributed, or as a modulator, which alters the properties of light, such as intensity, phase, or frequency, to encode information.
The most innovative aspect of the work is the ability to confine light within volumes much smaller than its wavelength. "In the terahertz range, light has wavelengths of hundreds of micrometers. What we do is confine this light within submicrometer regions," Freitas explains.
This is made possible by the formation of phonon-polaritons, which are hybrid quasiparticles that combine the vibrations of the atoms in the crystal lattice (phonons) with light. "It's as if the phonon were dressed in light, forming a quasiparticle with unique properties. The propagation characteristics and interaction with matter of these quasiparticles differ from both isolated light and isolated phonons," the researcher comments.
The extreme confinement of light involves operating beyond the diffraction limit, which restricts the resolution of conventional optical systems. "In classical optics, it isn't possible to observe or manipulate structures much smaller than the wavelength of light. With polaritons, we've managed to overcome that limit," says Freitas.
To achieve this, the researchers used scattering-type near-field optical scanning microscopy (s-SNOM), a technique that employs nanoscale metal tips to compress electromagnetic fields extremely. "The tip acts as an antenna and creates an electric field hotspot with dimensions on the order of tens of nanometers, regardless of the original wavelength. This allows for a drastic reduction in the spatial scale of light. Furthermore, the electric field density in s-SNOM probes is up to 10⁵ times higher than in free waves, which explains the superiority of the technique for nanophotonic research. We were able to confine a 200-micrometer wave into a volume smaller than 50 nanometers," Freitas says.
Another key finding of the study was the high quality factor of the phonon polytons in PbI₂. The quality factor is a measure of how long the oscillation persists before dissipating. "The longer the system oscillates, the higher the quality factor. PbI₂ performed comparably to hexagonal boron nitride (hBN), which is the reference material in the infrared range," says Freitas.
A simple and sustainable substitute
Unlike lead iodide, hexagonal boron nitride (hBN) is an extremely difficult material to synthesize, requiring extreme pressure and temperature conditions. Even after more than two decades of research, few groups worldwide have mastered producing this material at a high quality. Furthermore, its properties make it suitable for the mid-infrared range but not the terahertz range.
Lead iodide, on the other hand, has two inexpensive, naturally occurring precursors: iodine and lead. It can also be crystallized in an extremely simple way. "Simply dissolve the salt in water until a supersaturated solution is obtained and heat it to about 80 °C – something that can be done on a household stove. During cooling, the material crystallizes, forming structures that can be collected," says the researcher.
The ability to manipulate light on a nanoscale paves the way for integrated photonic circuits capable of replacing or complementing electronic circuits. "Currently, information is transmitted within devices via electrons. Using light can drastically increase speed and reduce losses. It's analogous to what happened in the field of telecommunications. Before, we used electrical cables; today, we use optical fibers, which allow for much higher speeds. The same principle can be applied inside chips. And, in addition to higher speeds, there are energy savings: light suffers far fewer losses than electrical currents. That can result in more efficient and sustainable solutions," Freitas argues.
Lead iodide is also relevant in another strategic area: perovskite-based technologies. Perovskites are materials with a specific crystalline structure of the ABX₃ type. In this structure, A is a larger cation (either organic or inorganic), B is a smaller metallic cation, and X is an anion (usually a halogen, such as I⁻, Br⁻, or Cl⁻). Due to their high efficiency in light absorption and conversion, this class of materials is widely used in solar cells and optoelectronic devices. Consequently, there is currently an explosion of research related to perovskites.
Since PbI₂ is a common perovskite precursor, studying its properties can help explain perovskite degradation mechanisms, a topic that has perplexed many researchers.
The outcomes of this work include implementing new experimental infrastructure at the CNPEM. "We already operate an infrared nanospectroscopy station called Imbuia at Sirius. We're now setting up the Tatu line, dedicated to terahertz. The new line will enable us to explore a broad class of materials with properties similar to those of lead iodide. It'll be a one-of-a-kind facility, enabling the study of how these materials behave across various frequencies. FAPESP's strong support has been essential to this," Freitas emphasizes.
Although the study is still in the basic science stage, it points to a broad technological horizon related to transmission and, eventually, information processing. "The expectation of the scientific community is to make light circuits increasingly present in everyday devices," Freitas summarizes.
About FAPESP
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the state of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration.
