Abstract
Two-color terahertz (THz) generation is a field-matter process combining an optical pulse and its second harmonic. Its application in condensed matter is challenged by the lack of phase matching among multiple interacting fields. Here, we demonstrate phase-matching-free two-color THz conversion in condensed matter by introducing a highly resonant absorptive system. The generation is driven by a third-order nonlinear interaction localized at the surface of a narrow-band-gap semiconductor, and depends directly on the relative phase between the two colors. We show how to isolate the third-order effect among other competitive THz-emitting surface mechanisms, exposing the general features of the two-color process.
Highlights
The nonlinear generation of broadband terahertz (THz) fields from ultrafast optical pulses is a subject of great interest for fundamental research and disruptive applications in imaging, spectroscopy, and the design of materials and devices [1,2,3,4,5,6,7,8,9,10,11,12]
To overcome the limitations imposed by phase matching, a promising alternative is provided by new types of emitters capable of high-conversion efficiency over short propagation distances [13], as in the case of ultrathin spintronic substrates [15,19,20]
Narrowband-gap semiconductor surfaces have emerged as remarkably efficient surface THz sources, under ultrafast illumination
Summary
Absorption at 2ω and two-photon absorption at ω, leads to the ultrafast injection of directional currents acting as THz sources [41,44,45]. In the presence of highly above-band-gap illumination in narrow-band-gap semiconductors, (Eg < ħω), the direct one-photon absorption is the dominant mechanism, effectively reducing the number of photons available for multiphoton nonlinear current injection processes. The dimensionally reduced interaction length allows the relaxation of typical phasematching constraints found in bulks It affects the dynamics of competitive surface mechanisms, e.g., the photo-carrier-driven screening, on the emission. The THz signal was collected along the specular reflection direction (green beam) through a standard timedomain spectroscopy (TDS) setup, implemented via a nonlinear h110i ZnTe crystal [48]. In this configuration, the generated THz field consists of three major contributions:
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