Revolutionize light control with a new class of crystals


July 21, 2023

(Nanowerk Highlights) Manipulating light at dimensions smaller than its wavelength, a scale known as the subdiffraction scale, is a critical capability in fields such as imaging, optical communications, integrated optical circuits, and molecular sensing. A promising way to control light on these small scales is through the use of polaritonic (PoC) crystals.

Photonic crystals are periodic optical nanostructures that affect the motion of photons in the same way that ionic lattices affect electrons in a solid. They can control and manipulate the flow of light. A polaritonic crystal is a type of photonic crystal made from a polaritonic medium, which can support polariton – quasi-particles resulting from the strong interaction between photons and electric dipole oscillations.

Based on the permittivity of the medium in which it is produced, PoC can be categorized into three types: isotropic, elliptical and hyperbolic PoC. Permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. It is a property that tells us how much electric charge a material can store in an electric field, thereby affecting the speed of light in that medium.

Isotropic PoC has the same permittivity in all directions, which means that the behavior of the polariton is the same no matter which way they move in the crystal. Elliptic PoC has permittivity that varies in different directions, but in a certain way that forms an ellipse if you want to graph the corresponding direction and permittivity. This means that the behavior of the polariton varies depending on the direction, but in a predictable elliptical pattern.

Hyperbolic PoC, the most complex type, has permittivity that varies in different directions, but in a way that forms a hyperbola if you graph the corresponding directions and permittivities. This means that the behavior of the polariton varies depending on the direction, but in a more complex hyperbolic pattern.

Conventional photonic crystals made from bulk materials such as silicon and metals, and more recently polariton-based photonic crystals made from van der Waals materials such as graphene and hBN, all have isotropic properties. This symmetry causes a very symmetrical pattern of trapped light. In addition, these light-trapping resonances in conventionally symmetric polaritonic crystals are very sensitive to defects or irregularities in the pattern of the engineered nanostructures.

A recent research study on Nature Communications (“Hyperbolic polaritonic crystal with configurable low-symmetry Bloch mode”), led by Dr Yingjie Wu of Zhejiang University, Prof Cheng-Wei Qiu of National University of Singapore, and Dr Qingdong Ou of Macau University of Science and Technology, presented a type of asymmetric photonic crystal based on in-plane hyperbolic polariton phonons in α-MoO hollow van der Waals crystals.3features a configurable deep wavelength and low symmetry Bloch mode that is resistant to lattice rearrangement in a given direction. Low symmetry Bloch mode in hyperbolic PoC based on anisotropic 2D α-MoO3 crystals Low symmetry Bloch mode in hyperbolic PoC based on anisotropic 2D α-MoO3 crystal. (Image courtesy of the researchers)

“By periodically punching nanoscale holes in the MoO3 crystal plate, we created a hyperbolic polaritonic crystal that can trap patterns of infrared light propagating mainly along one direction of the crystal,” Dr. Ou, co-author of the corresponding paper, told Nanowerk. “Unlike conventional symmetric crystal structures, rotating hole patterns produce skewed light modes and low symmetry in this hyperbolic MoO.3 crystal. Remarkably, the defects introduced in the pattern of holes along the forbidden light propagation direction do not affect the primary resonance wavelength.”

By tuning the size and placement of the holes, the researchers can control the pattern of trapped light and its resonant frequency over a wide infrared bandwidth. Their findings introduce hyperbolic polaritonic crystals as a new member of the photonic crystal family, with unique asymmetric optical properties. Further research into these engineered asymmetric crystals could enable new applications in microscopy, sensing, on-chip photonic circuits, and more.

The researchers also created a diamond-type hyperbolic PoC with an arrangement similar to a square-type PoC rotated 45 degrees. This was done to analyze the polaritonic behavior of different lattice structures. Interestingly, the strongest absorption peaks of the diamond- and square-type PoC were found to be located at nearly the same frequency.

“Our work shows that specially engineered asymmetric photonic crystals can provide a new approach to tightly manipulating light for advanced technologies,” concludes Ou. “It also highlights the potential of layered materials like MoO3 as a tunable platform for controlling light at the nanoscale using polaritonic crystals.”

This study expands the toolkit for nanophotonics research by uncovering the unique physics of anisotropic polaritonic crystal structures. In conclusion, this research represents a significant step forward in the field of light manipulation. Hyperbolic PoC development based on α-MoO3 opens up new possibilities for controlling light at the wavelength scale, with potential applications in areas such as optical communications and molecular sensing.

Michael Berger

– Michael is the author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: A Small Future And
Nanoengineering: Skills and Tools for Making Technology Invisible
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