Continuum model accounts for glass losing transparency at terahertz frequencies

Electromagnetic waves in the gigahertz (GHz) range, such as those used in mobile communications, pass readily through many types of glass. Transmission falls significantly, however, at terahertz frequencies above a characteristic threshold. The effect has been observed experimentally for some time, but a quantitative model connecting it to the microscopic structure and dynamics of glass has been lacking.

A research team led by the University of Tsukuba has now developed a continuum model that incorporates elastic heterogeneity in glass alongside microscopic charge fluctuations at atomic and molecular scales. The model describes how terahertz electromagnetic waves interact with vibrational dynamics in glass, linking the material's terahertz dielectric response to its internal mechanical properties.

The researchers applied the model to glycerol glass, a representative molecular glass, and found that the calculations accurately reproduced the experimentally measured real and imaginary parts of the complex dielectric function across 0.3 to 2.5 THz. The model also captured the transition from a resonance-like response below the boson-peak frequency to a broad relaxation-like response above it.

The analysis further showed that around the boson peak, the transverse contribution dominates both the real and imaginary parts of the terahertz dielectric response, while the longitudinal contribution is comparatively small. This suggests that transverse shear dynamics play a central role in the response. The model also reproduced the nearly linear frequency dependence of the infrared light-vibration coupling coefficient observed near the boson peak.

The findings provide a framework for quantitatively linking terahertz absorption to charge fluctuations on the basis of material-specific mechanical properties. The approach could help in the design and evaluation of glass materials with low permittivity and low dielectric loss, characteristics that are increasingly important for terahertz communications and photonic technologies.

The corresponding researchers are Assistant Professor Tatsuya Mori of the University of Tsukuba, Assistant Professor Hideyuki Mizuno of The University of Tokyo, Specially Appointed Associate Professor Yasuhiro Fujii of The University of Osaka, and Professor Akitoshi Koreeda of Ritsumeikan University. The work was supported by JSPS KAKENHI grants, the Asahi Glass Foundation, and GIC and NGF.

CONTACT: Naoko Yamashina, University of Tsukuba, [email protected]