Simultaneous Measurement of Thermal Expansion and Thermo‐Optic Effect in Solids Using Terahertz Spectroscopy

Mechanical and optical properties of solids are temperature‐dependent and closely coupled. Temperature changes alter mass density through thermal expansion and intrinsic polarizability through electron–phonon coupling, together producing the thermo‐optic effect. Terahertz (THz) radiation, due to its relatively low frequency, is ideally suited to probe minuscule changes in polarizability and thus in optical constants. However, simultaneous THz measurements of thermal expansion remain challenging because its magnitude is typically deep‐subwavelength compared to the sub‐millimeter THz wavelength. Here, using echo‐resolved THz spectroscopy, we simultaneously and accurately determine sub‐micrometer thermal expansion and the complex refractive index of two crystalline solids, MgO and Al 2 O 3 , in the temperature range of 10 – 325 K. The obtained results establish THz spectroscopy as a promising method for investigations of coupled thermal expansion and thermo‐optic effects across many materials, thereby advancing optical metrology.