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The photonic Dirac cone is a type of degenerate state with linear dispersion that is found in many two-dimensional photonic crystals. The photonic Dirac cone will transition to valley states when the spatial inversion symmetry is broken. Valley photonic crystals are a kind of photonic crystal. There are valley-protected edge states in the band gap of the valley photonic crystal, which has been proven in silicon photonic crystal slabs.

Many micro-nano photonic integrated devices, including sharp bend waveguides and microcavity lasing, have been created using these edge states. Photonic nodal ring, on the other hand, is a type of energy band degeneracy that occurs in three dimensions and appears as a closed ring in the energy band. The nodal ring can transition to ridge states by inducing symmetry breakdown.

The ridge state features more optical characteristics than the valley state, including negative refraction and surface-dependent Goos–Hänchen shift. The complicated three-dimensional structure, on the other hand, makes it difficult to realize the above behaviors in the optical area, and unique functional photonic devices are even more difficult to make. As a result, figuring out how to achieve the ridge state using a basic optical structure has become a critical concern in this field.

A team led by Professor Jianwen Dong of Sun Yat-sen University’s School of Physics and State Key Laboratory of Optoelectronic Materials and Technologies has realized ideal nodal ring and ridge states in the visible region using simple 1D photonic crystals, according to a new paper published in Light Science & Applications.

Researchers also detected the surface states in the ridge photonic crystal’s bandgap experimentally. The following three parts of this work’s research are highlighted below.

One-dimensional photonic crystals can only achieve topological states in one-dimensional momentum space. In three-dimensional momentum space, the nodal ring is a degenerate state. One-dimensional photonic crystals appear to be incapable of producing a nodal ring.

A one-dimensional photonic crystal made of silicon dioxide and silicon nitride materials was explored by the researchers. Researchers eventually accomplished the ideal nodal ring by imaginatively considering momenta along with aperiodic directions and applying rotational symmetry.

The researchers used inductance coupled plasma-enhanced chemical vapor deposition (ICP-CVD) to produce the samples. Scientists validated the existence of the nodal ring by examining angle-resolved reflectance spectra in the range of wavelength of 500—1100 nm.

The researchers also broke the spatial inversion symmetry of the structure by replacing a layer of silicon nitride (n = 2) in the main cell of the nodal ring photonic crystal with silicon-rich nitride (n = 3). The nodal ring degeneracy in this scenario transitions to a ridge state with a bandgap. Ridge photonic crystal is the name given to this type of photonic crystal.

The calculation findings demonstrate that a toroidal-shaped Berry flux forms around the site of the original nodal ring, suggesting the existence of topologically protected interface states in this bandgap, similar to the valley photonic crystal.

The study team constructed a one-dimensional ridge photonic crystal using the low-loss silicon-rich nitride film growing approach reported earlier and studied the interface states by analyzing the angle-resolved reflectance spectrum in the range of wavelength of 600–1100 nm.

People found that there may be surface states at the interface of a one-dimensional photonic crystal and a metal at the start of the century, known as the optical Tamm state. The researchers discovered that the nodal ring simply locates the singularity of the photonic crystal’s reflection phase, ensuring that the optical Tamm state’s existence requirement is always fulfilled.

As a result, optical Tamm states must be safeguarded by phase singularities at the metal-nodal ring photonic crystal interface, which gives theoretical direction for deterministic optical Tamm state design.

Using electron beam evaporation, the researchers created a silver film on the surface of the one-dimensional nodal ring photonic crystal. Angle-resolved reflectance spectra in the range of wavelength of 600–1000 nm were used to confirm the presence of optical Tamm states.

Researchers predict, “Our work paves the way for realizing optical phenomena such as negative refraction of surface states and surface-dependent Goos–Hänchen shift in the optical region. Furthermore, nodal ring can also transit to Weyl point degeneracy by introducing other types of symmetry breaking.”

“Therefore, the one-dimensional nodal ring photonic crystal proposed in this work provides the possibility to explore the applications of Weyl point and its associated topological surface states in micro-nano optics,” the scientists conclude.

Deng, W-M., et al. (2022) Ideal nodal rings of one-dimensional photonic crystals in the visible region. Light: Science & Applications. doi.org/10.1038/s41377-022-00821-9.

Source: http://english.ciomp.cas.cn/

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