扫描隧道显微镜结内超宽带THz脉冲的相位分辨探测
Phase-resolved Detection of Ultrabroadband THz Pulses inside a Scanning Tunneling Microscope Junction
摘要：
将相位稳定的单周期太赫兹（THz）脉冲耦合到扫描隧道显微镜（STM）结，可以实现具有飞秒时间分辨率和飞秒空间分辨率的时空成像。在这种THz门控STM中实现的时间分辨率最终受限于作为超快电压脉冲的尖端增强的THz场的子周期时间变化，并且因此受限于将高频宽带THz脉冲馈送到结中的能力。在这里，我们报告的超宽带（1-30太赫兹）单周期太赫兹脉冲从自旋太赫兹发射器（STE）到金属STM结的耦合。我们演示了通过太赫兹场诱导的超快光电流调制直接在STM结中的太赫兹电压瞬态的宽带相位分辨检测。与未扰动的远场太赫兹波形的比较揭示了STM尖端的天线响应。尽管尖端引起的低通滤波，高达15太赫兹的频率可以检测到在尖端增强近场，导致太赫兹瞬态与115 fs的半周期。我们进一步证明了简单的极性控制的太赫兹偏置通过STE磁化，并表明，高达2 V的太赫兹偏置在1 MHz的重复率，可以实现在当前的设置。最后，我们发现一个几乎恒定的太赫兹电压和波形在很宽的范围内的尖端样本的距离，通过比较数值模拟证实了太赫兹脉冲的准静态性质。我们的结果证明了自旋电子太赫兹发射器对于具有前所未有的太赫兹偏置带宽的超快太赫兹-STM的适用性，并提供了对定义的纳米级结的飞秒响应的深入了解。
英文摘要：
Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatio-temporal imaging with femtosecond temporal and Ångstrom spatial resolution. The time resolution achieved in such THz-gated STM is ultimately limited by the sub-cycle temporal variation of the tip-enhanced THz field acting as an ultrafast voltage pulse, and hence by the ability to feed high-frequency, broadband THz pulses into the junction. Here, we report on the coupling of ultrabroadband (1-30 THz) single-cycle THz pulses from a spintronic THz emitter(STE) into a metallic STM junction. We demonstrate broadband phase-resolved detection of the THz voltage transient directly in the STM junction via THz-field-induced modulation of ultrafast photocurrents. Comparison to the unperturbed far-field THz waveform reveals the antenna response of the STM tip. Despite tip-induced low-pass filtering, frequencies up to 15 THz can be detected in the tip-enhanced near-field, resulting in THz transients with a half-cycle period of 115 fs. We further demonstrate simple polarity control of the THz bias via the STE magnetization, and show that up to 2 V THz bias at 1 MHz repetition rate can be achieved in the current setup. Finally, we find a nearly constant THz voltage and waveform over a wide range of tip-sample distances, which by comparison to numerical simulations confirms the quasi-static nature of the THz pulses. Our results demonstrate the suitability of spintronic THz emitters for ultrafast THz-STM with unprecedented bandwidth of the THz bias, and provide insight into the femtosecond response of defined nanoscale junctions.
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