Martín Sabanés Natalia, Krecinic Faruk, Kumagai Takashi, Schulz Fabian, Wolf Martin, Müller Melanie
Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195Berlin, Germany.
IMDEA Nanoscience, Faraday 9, 28049Madrid, Spain.
ACS Nano. 2022 Sep 27;16(9):14479-14489. doi: 10.1021/acsnano.2c04846. Epub 2022 Aug 26.
Efficient operation of electronic nanodevices at ultrafast speeds requires understanding and control of the currents generated by femtosecond bursts of light. Ultrafast laser-induced currents in metallic nanojunctions can originate from photoassisted hot electron tunneling or lightwave-induced tunneling. Both processes can drive localized photocurrents inside a scanning tunneling microscope (STM) on femto- to attosecond time scales, enabling ultrafast STM with atomic spatial resolution. Femtosecond laser excitation of a metallic nanojunction, however, also leads to the formation of a transient thermalized electron distribution, but the tunneling of thermalized hot electrons on time scales faster than electron-lattice equilibration is not well understood. Here, we investigate ultrafast electronic heating and transient thermionic tunneling inside a metallic photoexcited tunnel junction and its role in the generation of ultrafast photocurrents in STM. Phase-resolved sampling of broadband terahertz (THz) pulses via the THz-field-induced modulation of ultrafast photocurrents allows us to probe the electronic temperature evolution inside the STM tip and to observe the competition between instantaneous and delayed tunneling due to nonthermal and thermal hot electron distributions in real time. Our results reveal the pronounced nonthermal character of photoinduced hot electron tunneling and provide a detailed microscopic understanding of hot electron dynamics inside a laser-excited tunnel junction.
电子纳米器件要在超快速度下高效运行，需要理解并控制飞秒光脉冲产生的电流。金属纳米结中超快激光诱导电流可源于光辅助热电子隧穿或光波诱导隧穿。这两个过程都能在飞秒到阿秒时间尺度上驱动扫描隧道显微镜（STM）内的局部光电流，实现具有原子空间分辨率的超快STM。然而，金属纳米结的飞秒激光激发也会导致瞬态热化电子分布的形成，但热化热电子在比电子 - 晶格平衡更快的时间尺度上的隧穿情况尚未得到很好的理解。在此，我们研究金属光激发隧道结内的超快电子加热和瞬态热电子发射隧穿及其在STM中超快光电流产生中的作用。通过太赫兹（THz）场诱导的超快光电流调制对宽带太赫兹脉冲进行相位分辨采样，使我们能够探测STM针尖内的电子温度演化，并实时观察由于非热和热电子分布导致的瞬时隧穿和延迟隧穿之间的竞争。我们的结果揭示了光诱导热电子隧穿明显的非热特性，并提供了对激光激发隧道结内热电子动力学的详细微观理解。
