Abstract
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 μm 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.
Highlights
Ultrafast photoexcitation of semiconductor materials is a commonly used technique to generate broadband terahertz (THz) pulses for imaging and spectroscopy applications [1,2,3]
Surface by the Fabry-Perot cavity resonance, leading to increased optical absorption by the film [33]. This leads to enhanced THz emission because this forces most of the light to be absorbed in a region of the semiconductor, i.e. in the Schottky depletion region, where this counts for the THz emission
Significant THz emission from extremely thin layers becomes possible because practically all the charge carriers are generated in the depletion region
Summary
Ultrafast photoexcitation of semiconductor materials is a commonly used technique to generate broadband terahertz (THz) pulses for imaging and spectroscopy applications [1,2,3]. For crystalline Si, the relatively weak absorption leads to absorption depths which are fairly large, larger than the surface depletion depth This means that most of the light gets absorbed in a region of the material where there is no depletion field present and which does not contribute to the emission of THz pulses, in essence wasting optical pump photons. We find that the THz emission amplitude is related to the optical absorption, which is strongly enhanced by the so called cavity-resonance effect, which causes the semiconductor layer to act as its own antireflection coating. Our results provide a way to turn bad THz emitters into good ones, and as an application we show that we can prepare curved surfaces, such as a paraboloidal mirror, that both emit and concentrate THz light
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