THz parametric amplification in a candidate excitonic insulator

Coherent terahertz pulses are a powerful probe of low-energy electrodynamics in quantum materials. This includes materials such as superconductors and charge density wave materials since ~0.1-10 THz is the appropriate energy scale to access electrodynamic signatures associated with condensate responses. Advances in terahertz techniques including broadband and high-field generation enable dynamic measurements that provide new insights into these materials. For example, terahertz drive can induce nonlinear optical responses arising from coherent order parameter dynamics. This, in turn, provides a route to identify subtle effects not available from linear response measurements. In this contribution, we focus on the coherent response in the excitonic insulator Ta₂NiSe (TNS). We observe a reflectivity enhancement from 0.5 - 7.5 THz following excitation with 0.5 eV pulses. The broad reflectivity enhancement is punctuated by peaks corresponding to infrared active phonons in TNS, with the largest enhancement occurring at 4.7 THz. DFT calculations show strong electron phonon coupling for this mode. This leads to phonon-squeezing, which serves as a coherent many-body response that drives stimulated parametric emission in the presence of a terahertz probe beam. The amplitude of this enhanced reflectivity rapidly decreases with increasing temperature and serves as a reporter of a condensate-like behavior in TNS which is difficult to otherwise detect. These results highlight that nonlinear terahertz techniques enable characterization of subtle interactions in quantum materials. In particular, condensates exhibit strong nonlinear and parametric light-matter interactions that encode information about these contemporary materials.