Abstract-Ultrafast photoexcitation dynamics of ZnTe crystals by femtosecond optical pump‐probe and terahertz emission spectroscopy

Jianrui Liu, Xinzhong Chen,   Ziheng Yao, Xincheng Wu,   Mengkun Liu, Alexey V. Balakin,   Alexander P. Shkurinov, Guanjun You, Yiming Zhu

https://onlinelibrary.wiley.com/doi/abs/10.1002/mop.32392

In this work we perform ultrafast optical pump‐optical probe (OPOP) and optical pump terahertz (THz) emission (OPTE) studies on the ultrafast excitation dynamics in <110> ZnTe crystals. Ultrafast two‐photon absorption and coherent phonon are revealed in OPOP measurements. Pump‐power‐ and polarization‐dependent phonon dynamics are characterized in time‐resolved transmission, reflection, and Kerr rotation using OPOP. The phonon polariton‐induced THz emission is directly observed in the time domain of OPTE dynamics. It is clear that the transverse optical phonon at ~3.7 THz and phonon polariton at ~2.6 THz are evident in OPOP measurement while OPTE only reveals part of the polariton dynamics.

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.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz Nano-imaging of graphene

Jiawei Zhang, Xinzhong Chen, Scott Mills, Thomas Ciavatti, Ziheng Yao, Ryan Mescall, Hai Hu, Vyacheslav Semenenko, Zhe Fei, Hua Li, Vasili Perebeinos, Hu Tao, Qing Dai, Xu Du, Mengkun Liu,

https://pubs.acs.org/doi/abs/10.1021/acsphotonics.8b00190?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std

Accessing the non-radiative near-field electromagnetic interactions with high in-plane momentum (q) is the key to achieve super resolution imaging far beyond the diffraction limit. At far infrared and terahertz (THz) wavelengths (e.g. 300 μm = 1 terahertz = 4 meV), the study of high q response and nanoscale near-field imaging is still a nascent research field. In this work, we report on THz nanoimaging of exfoliated single and multi-layer graphene flakes by using the state-of-the-art scattering-type near-field optical microscope (s-SNOM). We experimentally demonstrated that the single layer graphene is close to a perfect near-field reflector at ambient environment, comparable to that of the noble metals at the same frequency range. Further modeling and analysis considering the nonlocal graphene conductivity indicate that the high near-field reflectivity of graphene is a rather universal behavior: graphene operates as a perfect high-q reflector at room temperature. Our work uncovers the unique high-q THz response of graphene, which is essential for future applications of graphene in nano-optics or tip-enhanced technologies.