Abstract-Enhanced terahertz emission from Schottky junctions using plasmonic nanostructures

Gopika K. P. Ramanandan , Aurèle J. L. Adam , and Paul C. M. Planken

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/ph500251a

Publication Date (Web): October 30, 2014

Copyright © 2014 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/ph500251a

We present measurements of the enhanced emission of terahertz pulses after the optical excitation of grating-coupled near-IR surface plasmons at the interface of gold and cuprous oxide, using femtosecond laser pulses. Terahertz emission is the result of the acceleration of charge carriers optically excited in the Schottky depletion field of the metal/semiconductor interface. The enhancement is a direct consequence of the localized nature of the surface plasmon field which is strongest near the nanostructured metal surface where the Schottky electric field is strongest too. Surface plasmon excitation is confirmed by reflection spectroscopy of gratings with different periods, by varying the azimuthal angle of the grating, and by calculations of the plasmon frequencies and fields. We observe a terahertz field enhancement factor of ~5.8 when compared to the emission from a flat sample. This corresponds to a THz power-enhancement factor of ~34. Our results show that for THz emission from these metal/semiconductor interfaces it matters more where the pump light is absorbed, than how much pump light is absorbed.

Abstract-Optical characterization of gold-cuprous oxide interfaces for terahertz emission applications

Gopika K. P. Ramanandan, Aurèle J. L. Adam, Gopakumar Ramakrishnan, Peter Petrik, Ruud Hendrikx, and Paul C. M. Planken  »View Author Affiliations

Applied Optics, Vol. 53, Issue 10, pp. 1994-2000 (2014)
http://dx.doi.org/10.1364/AO.53.001994

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We show that the interface between gold and thermally formed cuprous oxide, which emits terahertz radiation when illuminated with ultrafast femtosecond lasers, is in fact an AuCu/Cu2O interface due to the formation of the thermal diffusion alloy AuCu. The alloy enables the formation of a Schottky-barrier-like electric field near the interface which is essential to explain the THz emission from these samples. We confirm the formation of this AuCu layer by x-ray diffraction measurements, ellipsometry, and visual inspection. We determined the frequency-dependent complex refractive indices of the Cu2O and AuCu layer and verified them using reflection spectroscopy measurements. These refractive indices can be used for optimizing the thickness of Cu2O for maximum THz emission from these interfaces.

© 2014 Optical Society of America