55-nm Ultrathin Film Converts Light Wavelength, Boosts Intensity 120,000-Fold [Reading Science]

Korea University Team Develops Optical Resonator Using Metal Mirrors
Confines Light Without Complex Microfabrication... Potential Applications in Next-Generation Optical Devices

A technology has been developed that increases the intensity of light generated during wavelength conversion by about 120,000 times using an ultrathin material just one-thousandth the thickness of a human hair. The technology creates a structure that confines light by stacking thin films, without complex microfabrication, and is expected to be used in next-generation optical devices and quantum optics.

Korea University said a research team led by Professors Park Gyuhwan and Gong Suhyeon of its Department of Physics developed an optical resonator that improves light wavelength-conversion performance using molybdenum disulfide (MoS₂) about 55 nm thick. The results were published on October 24 in the international journal Nature Communications.

Operating principle of metal-based ultrathin resonators and design map for critical coupling. The study demonstrates the principle behind “zero-order resonance,” which confines light in a much thinner film than conventional designs by using phase shifts in light reflected from a metal surface (a, b). It also presents design criteria for selecting the optimal resonator structure based on the material’s refractive index and absorption characteristics (c). Provided by the research team.

The research team focused on the phenomenon of "second-harmonic generation." It occurs when light interacts with a specific material to produce new light with twice the frequency and half the wavelength of the original light. In this study, near-infrared light with a wavelength of 910 nm, invisible to the human eye, was converted into blue visible light with a wavelength of 455 nm.

The challenge is that the thinner the material, the fewer opportunities light has to interact with it. This also reduces the efficiency of converting light to the desired wavelength. Optical resonators, which confine light inside them, have been used to address this issue. However, conventional methods require precise microfabrication or have the limitation of requiring a newly designed structure for each material.

The research team harnessed the phase change that occurs when light reflects off a metal mirror. Phase refers to the position of a wave within its cycle. By appropriately controlling the phase of light, it is possible to use the phenomenon in which different light waves reinforce or cancel one another.

Using this principle, the team created a resonator structure that confines light even in an extremely thin film. They also applied the "critical-coupling" condition, which minimizes the reflection of incoming light, to strengthen the interaction between the light and the material.

Research team photo. (From left) Professor Park Gyuhwan of the Department of Physics at Korea University (corresponding author), Professor Gong Suhyeon (corresponding author), Dr. Lee Sunjae (co-first author), and Jeong Haeseok, a student in the integrated master's and doctoral program (co-first author). Provided by Korea University.

In the experiment, a semitransparent metal mirror was combined with molybdenum disulfide about 55 nm thick. As a result, reflection of the incoming light was suppressed under specific conditions, and the intensity of the generated second harmonic was about 120,000 times higher than that produced by a single-layer molybdenum disulfide material.

The significance of this study goes beyond simply increasing light intensity. The research team also presented design criteria for determining an appropriate resonator structure based on a material's refractive index and light absorption characteristics. The findings demonstrate the potential to design optical resonators suited to a variety of ultrathin materials without fabricating complex nanostructures.

The research team expects the technology to be used in optical devices that convert light wavelengths, optical sensors, and quantum optics.

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