Strain-Driven Terahertz Phase Modulation in Piezoelectric and Nonlinear Optical Crystals: Mechanisms, Material Developments, and Modulation Strategies

Abstract

Terahertz (THz) technology has witnessed significant advance in wireless communication, imaging, spectroscopy, and sensing which have increased the demand for high-performance THz modulators. Among the various approaches for controlling the THz waves, external stimuli such as electric, optical pumping, thermal and magnetic fields on modulating materials are widely employed as promising strategy for modulation. This review focuses on recent advances in THz modulators based on different materials including two-dimensional, semiconductors, and metamaterials with attention to their response under external stimuli. In addition, strain-driven THz modulation in non-centrosymmetric piezoelectrics including organic and semi-organic crystals is examined with emphasis on their ability to establish strong coupling between propagating THz waves and lattice vibrations under piezoelectric resonance conditions. We further highlight earlier reports on strain-driven THz modulation in lithium niobate (LiNbO₃) and critically assessed the potential of several nonlinear optical crystals exhibiting piezoelectric resonance peaks suitable for strain-driven THz modulation. An overview of THz modulators integrated with tunable materials is presented with highlighting their operating principles and key performance metrics including modulation depth, bandwidth, modulation speed and insertion loss.