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

Abstract

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.

Investigation of Terahertz Emission from BiVO4/Au Thin Film Interface

“…5a, the mobility of electrons is greater than the mobility of holes). When the optical thickness of the beam path within BiVO 4 approaches λ/4, the layer begins to act like an antireflection coating, trapping more light inside the layer and giving rise to more absorption, as we recently observed for several semiconductors on gold substrates [5]. Assuming that the metal behaves like a perfect metal, we calculate that the BiVO 4 thickness for which the coating acts like an antireflection coating at 800 nm is about 71 nm, assuming a refractive-index of 2.9 for the BiVO 4 .…”

“…When a femtosecond laser pulse is incident on such a semiconductor-metal junction, a transient current is formed in the Schottky depletion layer of the metal/semiconductor interface, which gives rise to the emission of an electromagnetic transient in the THz range. In the past, THz emission from, mostly, conventional semiconductors like gallium arsenide, silicon, germanium, and some unconventional ones, such as cuprous oxide, has been studied [5].…”

Manipulation of terahertz waves by work function engineering in metal-graphite structures

“…48 The excitation of these surface plasmons could also contribute to the enhancement of THz radiation through depletion field induced second order non-resonant optical rectification. 49 Moreover, internal photoelectric emission of charge carriers from metal into the graphite results in additional transient currents. 50 The photon drag effect in graphene layers near interface can also lead to enhancement of the THz emission.…”

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

“…This poor THz generation efficiency results from the power loss of the pump laser pulses and of the generated THz pulses in the metal film and from the fact that light is also absorbed in a region of the semiconductor where there is no depletion field. Recently, we showed that the THz emission of inverted structures, ultrathin semiconductor layers deposited on gold, is much stronger [3]. When an optimum thickness of the semiconductor layer is chosen, multiple reflections occur inside the layer which acts as an antireflection coating, increasing the absorption within the semiconducting thin film.…”