Ultra-compact optical modulator developed based on ultrathin 2D semiconductors
New possibilities presented for ultra-compact optical circuits
Prof. Gong Su-hyun’s team publishes paper in Nature Communications

▲ (From left) Dr. Lee Seong-Won (first author), Lee Jong-Seok (first author, master’s), Prof. Gong Su-Hyun (corresponding author, Department of Physics)

The team led by Professor Gong Su-Hyun of the Department of Physics developed an ultra-compact optical modulator of length 10 μm and thickness 20 nm using 2D thin-film semiconductors.

This study was published online in the international journal Nature Communications (IF=17.694) on March 14.

* Title of paper: Ultra-compact exciton polariton modulator based on van der Waals semiconductors
* URL: ttps://www.nature.com/articles/s41467-024-46701-1

An optical modulator is a crucial component that controls the intensity of light, similar to how an electronic transistor controls the on/off state of a current. Processing data with optical modulators enables the rapid transmission of data at the speed of light with a wide bandwidth allowing for the quick processing of large amounts of data. Furthermore, unlike electronic devices, optical modulators generate minimal heat, resulting in excellent energy efficiency.

To control the intensity of the light propagated in an optical waveguide, it is essential to control the refractive index. However, in most optical modulators, changes in the refractive index of the material have been very subtle, limiting the modulation possible per unit length of the optical waveguide. As such, devices ranging in size from hundreds of micrometers to millimeters have been required for light intensity control.

For these reasons, the miniaturization of optical modulators has been inherently limited, and conventional optical modulators have mainly been used for optical communications. However, recently, the demand for ultra-compact optical modulators has increased significantly for various applications such as VR/AR systems, quantum photonic circuits, and AI photonics circuits.

In this study, the team succeeded in reducing the size of the optical modulator more than 50 times by using 2D semiconductors, which are van der Waals materials. 2D semiconductors have a very high refractive index, allowing the implementation of optical waveguides approximately 10 nm thick. The high refractive index of 2D semiconductors is due to the strong resonance with light near the semiconductor's bandgap. Controlling the bandgap of 2D semiconductors enables very effective manipulation of the refractive index.

The team led by Professor Gong Su-Hyun of the Department of Physics observed the refractive index changes of the optical waveguide using an external laser, and, based on such observations, experimentally implemented the performance of the optical modulator. Due to the significant refractive index change of the 2D semiconductor induced by the external laser, they managed to effectively control the intensity of light even for a very short modulation length of 2 μm.

The results of this study demonstrate the potential for implementing not only optical modulators but also photonic devices constituting photonic integrated circuits based on thin-film 2D semiconductors. This implementation of ultra-compact photonic integrated circuits is expected to greatly impact advancements in related fields.

Professor Gong Su-Hyun said, "This research began by exploring the potential of 2D semiconductors, and the performance was more impressive than expected. By utilizing the high nonlinearity of the optical modes existing in these 2D semiconductors, we hope to expand our research into photonic devices for AI computing."

This study was supported by the Mid-career Researcher Support Program and the Next-Generation Intelligent Semiconductor Technology Development Program by the Ministry of Science and ICT and National Research Foundation of Korea, as well as the Samsung Technology Incubation Program.

<Fig. 1>

[Caption] Development of ultra-compact optical modulators based on 2D semiconductors

In this study, the team induced optical modulation by controlling the refractive index of a waveguide with the output of a modulation laser, utilizing the ability to modulate the energy of excitons formed in 2D semiconductors. This method has been confirmed to induce significant modulation even over very short lengths, enabling the development of ultra-compact optical modulators based on 2D semiconductors, smaller than those based on conventional dielectric modulators currently being researched for commercialization.

<Fig. 2>

[Caption] Control of refractive index of 2D semiconductor based on modulation laser output

This experimental result confirms the ability to control exciton energy depending on the output of the modulation laser. As the output of the modulation laser increases, the energy of the excitons decreases. The internal graph shows how the real and imaginary parts of the refractive index change as the exciton energy decreases.

<Fig. 3>

[Caption] Degree of modulation control using the output of the modulation laser

This experimental result confirms the ability to control the induced modulation in the optical modulator depending on the output of the modulation laser: (a) Degree of induced modulation at each wavelength depending on the output of the modulation laser; (b) Maximum degree of modulation induced per unit length depending on the output of the modulation laser; (c) Stable operation of the ultrathin 2D semiconductor-based optical modulator while varying the state of the modulation laser.