< (From left) Ph.D. candidate Jiwon Kang from KAIST, Dr. Nina Hong from J.A. Woollam Co., Professor Harry A. Atwater from Caltech, and Professor Min Seok Jang from KAIST >
Faster, more efficient optical communications require precise control of light and minimal optical loss. A KAIST-led research team has characterized how light propagates through and is absorbed by barium titanate (BTO), a promising material for next-generation optical communication devices. By clarifying optical properties that had been difficult to determine because of the material's complex internal structure, the study provides a foundation for designing these devices.
< Intrinsic optical properties of barium titanate revealed by Mueller matrix ellipsometry. >
KAIST (President Choongsik Bae) announced on November 8 that a research team led by Professor Min Seok Jang of its School of Electrical Engineering had precisely characterized BTO's refractive index and absorption properties across wavelengths ranging from the ultraviolet to the far-infrared. The study was conducted in collaboration with Professor Harry A. Atwater's team at the California Institute of Technology (Caltech), the Korea Research Institute of Standards and Science (KRISS), and U.S.-based J.A. Woollam Co.
BTO's refractive index changes when an electric field is applied. Its strong electro-optic response, known as the Pockels effect, makes it a promising material for optical modulators, which encode information from electrical signals onto light.
< Figure 2. Structural characteristics of single-crystal barium titanate and a schematic of the optical constant measurements. >
Designing these devices requires accurate values for the refractive index and extinction coefficient, which describe how light propagates through a material and how strongly it is absorbed, respectively. In a waveguide—a structure that guides light—these values help designers predict how light will travel and how much of the signal will be lost to absorption.
However, BTO can contain small regions, known as domains, with different crystallographic orientations. These regions can respond differently to light, making it difficult to determine the material's intrinsic optical properties from their combined response. Reported values have varied widely for decades.
The research team used Mueller-matrix spectroscopic ellipsometry, a technique that measures changes in the polarization of light reflected from or transmitted through a material. By analyzing two types of single-crystal samples with relatively uniform domain orientations, the researchers reduced the effects of mixed responses from differently oriented domains and determined the material's intrinsic optical properties.
The analysis covered wavelengths from 190 nanometers (nm) to 40 micrometers (μm), spanning the ultraviolet to the far-infrared. Because light interacts with a material differently depending on its wavelength, the researchers divided this range into three spectral regions and used an analytical approach suited to each region. This allowed them to determine how the refractive index and absorption properties vary with wavelength and crystal orientation.
< Figure 3. Intrinsic complex refractive index of tetragonal BTO determined over a broad spectral range. >
The researchers also quantified very weak absorption in the wavelength range where BTO is nearly transparent. By combining ellipsometry with separate transmittance measurements, they determined extinction coefficients as low as approximately 10⁻⁵ in the 2–10 μm wavelength range. These data can help predict absorption losses in components that transmit or confine light.
The team also presented a method for calculating the overall optical response of a multidomain crystal from the orientations and relative proportions of its domains.
These optical data provide a basis for designing BTO-based photonic devices. Before fabrication, designers can use the data to more accurately predict how light will propagate and how much will be absorbed in devices of different sizes and shapes. This could reduce the time and cost associated with repeated fabrication and redesign.
Potential applications include optical modulators for communications, devices that generate light at new wavelengths, and circuits that process quantum information using light.
"This study reveals the optical response of barium titanate, which had long remained unclear, and provides a reliable reference for how the material interacts with light," said Professor Min Seok Jang. He added that the findings are expected to contribute to the development of optical communication devices that reduce light loss and enable faster data transmission, as well as quantum photonic devices that require precise control of light.
Jiwon Kang, a Ph.D. candidate in KAIST's School of Electrical Engineering, and Dr. Nina Hong of J.A. Woollam Co. are co-first authors of the paper. Professors Min Seok Jang of KAIST and Harry A. Atwater of Caltech are the corresponding authors. The findings were published online on October 20, 2026, in the optics journal ACS Photonics.
Paper title: Broadband Optical Characterization of Tetragonal Barium Titanate by Mueller-Matrix Spectroscopic Ellipsometry
DOI: 10.1021/acsphotonics.6c01514
This research was supported by the Culture, Sports and Tourism R&D Program through the Korea Creative Content Agency (KOCCA), with funding from the Ministry of Culture, Sports and Tourism; the Basic Science Research Program through the National Research Foundation of Korea (NRF), with funding from the Ministry of Education; the Ministry of Science and ICT; and the U.S. Air Force Office of Scientific Research (AFOSR).
