Oxidation kinetics of nanoscale copper films studied by terahertz transmission spectroscopy

Terahertz (THz) transmission spectroscopy is used to measure the oxidation kinetics of copper thin films evaporated on silicon substrates. The transmission of broadband THz pulses from 1 to 7 THz through the copper film is measured while it gets oxidized at an elevated temperature in ambient air. The change in the transmitted THz electric field is correlated with the growth of the cuprous oxide layer and the decrease in thickness of the copper layer. Oxidation curves were obtained for heating temperatures of 120–150?°C and were found to follow a parabolic rate law. Using the Arrhenius equation, we calculate an activation energy for diffusion of 0.55?eV. By measuring the THz transmission through unoxidized copper layers of several thicknesses, we also measured the optical properties of thin copper films around the percolation threshold thickness of 7?nm. Around the percolation transition, the optical properties of freshly deposited copper thin films are very different from that of copper layers of the same thickness remaining after partial oxidation of thick copper films.IST/Imaging Science and TechnologyApplied Science

Enhanced Terahertz Emission from Schottky Junctions Using Plasmonic Nanostructures

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

Terahertz emission from surface-immobilized gold nanospheres

Electromagnetic wave emission based on optical rectification at terahertz (THz) wavelengths was observed from surface-immobilized gold nanospheres (SIGNs) above a gold surface. Although the excitation wavelength is off-resonant with the localized surface plasmons, the THz emission field was observed to be approximately 4.8 times greater than that from a percolated gold thin film of 10 nm thickness. A theoretical calculation predicts that the light electric field is enhanced in the SIGN system, even at off-resonance wavelengths. The observed THz field amplitude was quadratic with the illumination light field, suggesting that the THz generation is due to a second-order nonlinear optical process.IST/Imaging Science and TechnologyApplied Science

Enhanced terahertz emission by coherent optical absorption in ultrathin semiconductor films on metals

We report on the surprisingly strong, broadband emission of coherent terahertz pulses from ultrathin layers of semiconductors such as amorphous silicon, germanium and polycrystalline cuprous oxide deposited on gold, upon illumination with femtosecond laser pulses. The strength of the emission is surprising because the materials are considered to be bad (amorphous silicon and polycrystalline cuprous oxide) or fair (amorphous germanium) terahertz emitters at best. We show that the strength of the emission is partly explained by cavity-enhanced optical absorption. This forces most of the light to be absorbed in the depletion region of the semiconductor/metal interface where terahertz generation occurs. For an excitation wavelength of 800 nm, the strongest terahertz emission is found for a 25 nm thick layer of amorphous germanium, a 40 nm thick layer of amorphous silicon and a 420 nm thick layer of cuprous oxide, all on gold. The emission from cuprous oxide is similar in strength to that obtained with optical rectification from a 300 ?m thick gallium phosphide crystal. As an application of our findings we demonstrate how such thin films can be used to turn standard optical components, such as paraboloidal mirrors, into self-focusing terahertz emitters.IST/Imaging Science and TechnologyApplied Science