Measurement of quadratic terahertz nonlinearities in time-domain spectroscopy (Conference Presentation)

Optical response of materials to intense terahertz electric fields has become a new frontier in optics in the recent decade. We focus on the nonlinear optical responses that are quadratic in terahertz electric field, which can arise in second and third orders of nonlinearity. The second order nonlinear polarizability can lead to terahertz second harmonic generation, a phenomenon that has not been experimentally observed yet. The difficulty in detecting terahertz second harmonic generation stems from vanishingly small conversion efficiencies in this region of spectrum. However, an additional experimental difficulty results from a significant overlap of the fundamental and second-harmonic terahertz pulses both in the time and frequency domain. This makes it hard to distinguish the second harmonic from the fundamental terahertz wave. The third order nonlinear polarizability results in terahertz Kerr effect, which has been observed as an induced gating-beam birefringence that is quadratic in terahertz electric field. In noncentrosymmetric materials, terahertz Kerr effect may coexist with terahertz Pockels effect that is commonly used for time-domain terahertz detection via electro-optic sampling. Distinguishing the terahertz Kerr effect from the Pockels effect can also be difficult if the latter is significantly stronger. In this paper, we will present a method for measuring quadratic terahertz nonlinearities based on the second-harmonic lock-in detection in a time-domain electro-optic sampling experiment. We illustrate our method using a measurement of terahertz Kerr effect in a zinc blende semiconductor in geometry where both terahertz Kerr and Pockels effects are present. We will also discuss the possibility of measuring terahertz second harmonic generation in metamaterials.