Liquid crystals enable tunability to bound states in the continuum

When light is inside a structure, it travels off into open space through a radiative process. This range of outgoing light is continuous, meaning it includes all frequencies within it. All frequencies, but the ones corresponding to the so-called bound states in the continuum (BICs). BICs are the exceptions that prove the rule. At these specific frequencies, the structure surprisingly does not leak energy by radiation; instead, it keeps the light trapped inside it. This makes BICs potentially interesting for settings where extremely narrow bandwidths as well as efficient energy storage and transmission capabilities are needed, such as certain types of sensors, laser cavities, and filters.

BICs typically exist at a specific frequency and when light propagates in a particular direction, with both aspects being determined by the material’s refractive index and geometry. These structural features become fixed during the fabrication process, and so do BICs. If BICs are fixed, it gets hard to adapt to different operational conditions or to correct errors occurring during fabrication, which might lead to an undesired frequency and direction.

ICFO researchers, Dr. Marlin Baral, Dr. Smyobrata Mukherjee, Pilar Pujol-Closa, ICFO and UPC Prof. Lluís Torner, and Dr. David Artigas, have explored a new platform that allows for more dynamic control over these states. In a recent publication in Optica, they reported, for the first time, the observation of a particular type of BIC (anisotropy-induced interference BIC), whose existence was elucidated eight years ago by themselves. Moreover, through a specific set-up based on specially designed waveguides and a liquid crystal core, the team managed to change at will the frequency of the detected BICs.  

To achieve this, the team used liquid crystals – matter that can flow like a liquid but also have some molecular order, similar to a crystal. The researchers took advantage of the fact that the crystal’s optical properties can change in response to external stimuli. They then demonstrated that, depending on these properties, the frequency of the BIC also changes.

Specifically, they applied a voltage to reorient the liquid crystal molecules. This realignment modified the conditions under which light propagates without altering the geometry or the material media at all. This way, the team was able to select one BIC or another after the fabrication process, effectively choosing which frequency was captured within the structure.

The successful results did not come out of the blue, though. “We already had the idea in mind since the publication of the theoretical article, back in 2017,” shares Dr. David Artigas, senior author of the paper. “We started working on it that same year, and more intensively in the last three years, when Marlin brought her experience in liquid crystals to the group.” He admits that the process was no easy feat. They had to face new challenges every day, from the fabrication of samples to the interpretation of the results, which, as David recalls, did not initially fit with the theory. “It has been a long-distance race, a whole marathon,” he adds.

In the end, the concerted efforts paid off, and the team achieved the long-standing goal of dynamically modifying the frequency of anisotropy-induced BICs. ICFO researchers are now seeking ways to extend and further develop the proposed method; for instance, to include control over the direction of light propagation, something they have already demonstrated theoretically.

References:

1. Gomis-Bresco, D. Artigas, and L. Torner, “Anisotropy-induced photonic bound states in the continuum,” Nat. Photon. 11, 232–236 (2017).
2. Mukherjee, J. Gomis-Bresco, P. Pujol-Closa, et al., “Topological properties of bound states in the continuum in geometries with broken anisotropy symmetry,” Phys. Rev. A 98, 063826 (2018).

Marlin Baral, Samyobrata Mukherjee, Pilar Pujol-Closa, Lluis Torner, and David Artigas, "Dynamic tuning of anisotropy-induced bound states in the continuum in liquid crystal waveguides," Optica 12, 1061-1067 (2025). DOI: https://doi.org/10.1364/OPTICA.558590

Acknowledgements:

Agencia Estatal de Investigación, (PID2022-138280NB-I00), Fundación Carmen y Severo 296 Ochoa (CEX2019-000910-S) Agència de Gestió d’Ajuts Universitaris i de Recerca (2021 SGR 01448).

This work was partially supported by the Ministerio de Economía y Competitividad, 298 funded by MCIN/AEI/10.13039/501100011033/FEDER, by and Generalitat de Catalunya (CERCA).