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Description
Fused silica (amorphous SiO₂) and quartz crystal both feature ultra-low thermal expansion and exceptional transparency from the visible to the deep-ultraviolet. Quartz additionally offers strong piezoelectricity and pronounced birefringence, making both materials cornerstones of modern optics. Yet their optical properties in the extreme- and vacuum-ultraviolet (35–140 nm) remain largely unexplored because metrology in this spectral region is highly demanding; consequently, reliable optical constants of these two SiO₂ polymorphs are still scarce.
To fill this gap, we carried out the angle-resolved reflectometry of thermally grown amorphous SiO₂ and Y-cut α-quartz from 36 nm to 140 nm at a synchrotron beamline. A transfer-matrix model, solved with Markov-chain Monte Carlo sampling, yielded n and k for the ordinary and extraordinary crystal axes together with their relative uncertainties. Two sharp resonances were resolved in quartz on each optic axis whereas amorphous SiO₂ displays only a single, broader resonance matching the feature on quartz’s ordinary axis. The crystalline phase shows noticeably higher absorption at this wavelength, consistent with its greater oscillator strength. Although the materials are bulk-opaque below ≈130 nm, their complex refractive indices still govern reflection, scattering, and absorption in emerging EUV/VUV technologies.
