Breakthrough in Light Manipulation: Revealing New Finite Barrier Bound States

Exploring the propagation and localization of waves in various media has always been a core focus of optics and acoustics. Specifically, in photonics and phononics, scientists have been dedicated to understanding and controlling the behavior of light and sound waves in periodic media.

Photonic crystals have unique bandgap characteristics, providing an excellent platform for studying wave propagation and localization. These bandgaps caused by the periodic structure of crystals can control the propagation of waves and even completely suppress waves in certain frequency ranges.

Traditionally, it is believed that the boundary modes in photonic crystals are strongly influenced by the size of the crystal. It is widely believed that these patterns are more likely to be limited to large systems, as the probability of tunnels significantly decreases with increasing system size. This phenomenon is crucial for the design and implementation of high-performance photonic devices, especially in the pursuit of high integration and miniaturization of devices.

In addition, in the study of photonic crystals, the bound states in the continuous spectrum have attracted people's attention because they reveal that certain unique modes can be confined to specific regions, even in the continuous spectrum. This phenomenon provides a new perspective for understanding and controlling the positioning of light waves. It has shown great potential in practical applications, such as improving the performance and efficiency of optical devices.

A new study published in the journal Light: Science and Applications proposes and confirms the existence of finite barrier bound states. The spectrum of a system typically consists of continuous and discrete spectra. The traditional view holds that the eigenvalue spectrum of bound states is discrete, while non bound states form a continuous spectrum.

For example, in an electronic system, if the energy of a particle is lower than the infinite potential energy, then the state is bound to a discrete spectrum; And particles with energy higher than the potential energy scatter, forming a continuous spectrum.
For light and sound waves, discrete states are formed due to the boundary conditions imposed by potential barriers. These discrete states can be completely limited to ideal conditions. However, when the barrier width is limited, the state may cross the barrier and become a resonant state.
It is worth noting that the bound states in the continuous spectrum are spatially bound within the energy/frequency range of the continuous spectrum. This study introduces a counterintuitive concept parallel to BIC: certain states can be completely bound in very thin bandgap materials, making it impossible for them to pass through the bandgap material.

This study first demonstrated a special mirror symmetric photonic band structure, in which the transition of boundary modes can be finely controlled. When the width of a photonic crystal is very small, the boundary modes on both sides interact and split into odd and even modes.
Under a specific wave vector, the coupling strength of boundary modes is zero. Even if the width of a photonic crystal is very small, the boundary mode cannot jump from one side of the photonic crystal to the other. Usually, it is believed that many lattice sites are needed to suppress the coupling of boundary modes. However, this study still challenges this viewpoint and opens up a new method for manipulating photon behavior at the microscopic scale.

According to the previous configuration, researchers removed one PEC boundary of the photonic crystal and revealed a new configuration. They found that the remaining boundary modes at specific node wave vectors were completely captured, forming a finite barrier enabled bound state in the continuum.

Due to the decoupling of the two boundary modes, these FBICs exhibit non radiative characteristics. On nodes with zero coupling strength in the boundary mode, when one side of the PEC is removed, there exists a state where the radiation coefficient is zero, and its frequency matches the frequency of the nodes in the dual PEC scenario, which is recognized as FBIC.

In addition, by changing the circular dielectric to an elliptical shape to break the original mirror symmetry and introduce new geometric parameters η， This study was conducted in kx- η  A number of windings is defined in the parameter space, revealing the topological characteristics of FBIC and confirming that these patterns are BIC.

Considering the inevitable dielectric loss at microwave frequencies, this study experimentally verified the attenuation of boundary modes by measuring them, demonstrating their complete localization within a very small number of lattice sites, providing a novel method for achieving BIC.

This groundbreaking study explores new physical phenomena in photonic crystals and achieves precise control of boundary modes. This work not only provides new insights into the tunneling and boundary of boundary modes in photonic crystals in theory, but also confirms the complete localization of boundary modes at specific wave vectors through microwave experiments, bringing a new perspective to the field of photonics.

This study reveals new methods for manipulating photon behavior, which is of great significance for the development of highly integrated photonic devices. It also provides a new strategy for enhancing light matter interactions using photonic crystals, which may lead to breakthroughs in nonlinear optics and the interaction between light and two-dimensional materials. These findings may inspire future research, such as applying these principles to other wave systems, such as phononic crystals.

Source: Laser Net