Abstract-Photo-induced terahertz near-field dynamics of graphene/InAs heterostructures

Ziheng Yao, Vyacheslav Semenenko, Jiawei Zhang, Scott Mills, Xiaoguang Zhao, Xinzhong Chen, Hai Hu, Ryan Mescall, Thomas Ciavatti, Stephen March, Seth R. Bank, Tiger H. Tao, Xin Zhang, Vasili Perebeinos, Qing Dai, Xu Du, and Mengkun Liu

Fig. 1 Schematic of the near field optical-pump THz-probe (n-OPTP) setup, equally capable of performing optical pump near-field THz emission (n-OPTE) experiments. ①: 800 nm, 300 mW pump pulses pass through the ITO then go through the center of, while parallel to, the THz beam ②. Optical pump ① and THz probe ② are focused onto the AFM tip apex using an off-axis parabolic mirror. ③: THz gate (detection) beam. Tip scattered THz signals can be mapped out in the time domain by modifying the arrival time of THz gate beam ③ to the THz photoconductive antenna detector, thus changing the delay between ② and ③ (t1). Changing the delay between ① and ③ (t2) by modifying arrival time of pump pulse ① probes the photo-excited ultrafast dynamics of the sample (see main text), where in this work, t1 is fixed at the peak position of the scattered THz electric field while t2 is varied to yield n-OPTE (with THz probe ② blocked) and n-OPTP (with THz probe ② unblocked) measurements.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-10-13611

In this letter, we report optical pump terahertz (THz) near-field probe (n-OPTP) and optical pump THz near-field emission (n-OPTE) experiments of graphene/InAs heterostructures. Near-field imaging contrasts between graphene and InAs using these newly developed techniques as well as spectrally integrated THz nano-imaging (THz s-SNOM) are systematically studied. We demonstrate that in the near-field regime (𝜆/6000$\lambda /6000$), a single layer of graphene is transparent to near-IR (800 nm) optical excitation and completely “screens” the photo-induced far-infrared (THz) dynamics in its substrate (InAs). Our work reveals unique frequency-selective ultrafast dynamics probed at the near field. It also provides strong evidence that n-OPTE nanoscopy yields contrast that distinguishes single-layer graphene from its substrate.

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