Terahertz-field-induced second harmonic generation for nonlinear optical detection of interfaces buried in transparent materials

Abstract

We propose and demonstrate experimentally a nonlinear optical technique that allows for detection and characterization of invisible (or low-contrast) microscale objects buried in the bulk of materials transparent in the optical and terahertz frequency ranges. The technique is based on the effect of terahertz-field-induced second harmonic generation and uses collinearly propagating femtosecond optical and picosecond terahertz pulses to probe a sample. Due to a difference between the optical and terahertz velocities, the pulses can be overlapped in different regions of the sample by varying the time delay between them. Overlapping in the bulk of the material does not produce optical second harmonic emission, whereas overlapping at the microobject does produce the emission. The technique was verified experimentally for two plates of fused quartz glued by a thin (15–35 μm thick) layer of optical adhesive. The presence of the adhesive was detected, and its third-order nonlinear susceptibility was measured.