Ramakrishnan Gopakumar, Ramanandan Gopika K P, Adam Aurèle J L, Xu Man, Kumar Nishant, Hendrikx Ruud W A, Planken Paul C M
Optics Research Group, Department of Imaging Science and Technology, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands.
Opt Express. 2013 Jul 15;21(14):16784-98. doi: 10.1364/OE.21.016784.
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.
我们报道了在飞秒激光脉冲照射下，沉积在金上的非晶硅、锗和多晶氧化亚铜等超薄半导体层发出的相干太赫兹脉冲具有令人惊讶的强宽带发射。发射强度令人惊讶，因为这些材料充其量被认为是较差的（非晶硅和多晶氧化亚铜）或中等的（非晶锗）太赫兹发射体。我们表明，发射强度部分可由腔增强光吸收来解释。这使得大部分光在半导体/金属界面的耗尽区被吸收，而太赫兹波正是在该区域产生的。对于800nm的激发波长，在金上，发现25nm厚的非晶锗层、40nm厚的非晶硅层和420nm厚的氧化亚铜层发出的太赫兹辐射最强。氧化亚铜的发射强度与从300μm厚的磷化镓晶体通过光整流获得的发射强度相似。作为我们研究结果的一个应用，我们展示了如何利用这种薄膜将标准光学元件，如抛物面镜，转变为自聚焦太赫兹发射体。
