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-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.

Abstract-A review of non-linear terahertz spectroscopy with ultrashort tabletop-laser pulses

DOI:

10.1080/09500340.2014.918200

Harold Y. Hwang, Sharly Fleischer, Nathaniel C. Brandt, Bradford G. Perkins Jr., Mengkun Liu, Kebin Fan, Aaron Sternbach, Xin Zhang, Richard D. Averitt & Keith A. Nelson,

Publishing models and article dates explained

• Received: 2 Oct 2013
• Accepted: 20 Apr 2014
• Published online: 12 Jun 2014

http://www.tandfonline.com/doi/abs/10.1080/09500340.2014.918200?journalCode=tmop20#.U5nYW5RdV8E

Over the past decade, breakthroughs in the generation and control of ultrafast high-field terahertz (THz) radiation have led to new spectroscopic methodologies for the study of light-matter interactions in the strong-field limit. In this review, we will outline recent experimental demonstrations of non-linear THz material responses in materials ranging from molecular gases, to liquids, to varieties of solids – including semiconductors, nanocarbon, and correlated electron materials. New insights into how strong THz fields interact with matter will be discussed in which a THz field can act as either a non-resonant electric field or a broad bandwidth pulse driving specific resonances within it. As an emerging field, non-linear THz spectroscopy shows promise for elucidating dynamic problems associated with next generation electronics and optoelectronics, as well as for demonstrating control over collective material degrees of freedom.

Abstract & Presentation -University of Minnesota - Exploring the fundamental time and length scales of Strongly Correlated Electron Materials

Speaker: Mengkun Liu, UC-San Diego

Thursday, January 30th 2014 -

1:25 pm:

Condensed Matter Seminar in 210 Physics

http://www.physics.umn.edu/events/calendar/spa.all/thisweek/index/calendar.html?item=26922

ABSTRACT: In strongly correlated electron materials (CEMs), the delicate interplay between spin, charge, and lattice degrees of freedom often leads to extremely rich phase diagrams exhibiting intrinsic phase inhomogeneities. The key to studying and disentangling such complexities usually lies in characterization and control of these materials at their fundamental energy, time and length scales. Using the prototypical correlated insulator vanadium dioxide (VO2) as a case study, I will show in this talk that ultrafast and ultrasmall optical spectroscopy offers unique insights into this electronic/structural interplay with unprecedented spatial and temporal resolutions. Specifically, with scanning near-field infrared microscopy we resolved the long-lasting enigma of electronic anisotropy in VO2 and revealed three distinct stages of the insulator to metal transition (IMT) at nanoscopic length scales. Using ultrafast terahertz pump terahertz probe spectroscopy we have also unambiguously demonstrated that the IMT occurs at picosecond time scales via electric field-induced electron liberation. These results set the stage for future spectroscopic investigations to access the fundamental time and length scales of CEMs.

Abstract-Nonlinear Terahertz Metamaterials via Field-Enhanced Carrier Dynamics in GaAs

Kebin Fan¹, Harold Y. Hwang², Mengkun Liu³, Andrew C. Strikwerda³, Aaron Sternbach³, Jingdi Zhang³, Xiaoguang Zhao¹, Xin Zhang¹, Keith A. Nelson², and Richard D. Averitt^3 
1Department of Mechanical Engineering, Boston University, 110 Cummington Street, Boston, Massachusetts 02215, USA
²Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

We demonstrate nonlinear metamaterial split ring resonators (SRRs) on GaAs at terahertz frequencies. For SRRs on doped GaAs films, incident terahertz radiation with peak fields of ∼20–160  kV/cm drives intervalley scattering. This reduces the carrier mobility and enhances the SRR LC response due to a conductivity decrease in the doped thin film. Above ∼160  kV/cm, electric field enhancement within the SRR gaps leads to efficient impact ionization, increasing the carrier density and the conductivity which, in turn, suppresses the SRR resonance. We demonstrate an increase of up to 10 orders of magnitude in the carrier density in the SRR gaps on semi-insulating GaAs. Furthermore, we show that the effective permittivity can be swept from negative to positive values with an increasing terahertz field strength in the impact ionization regime, enabling new possibilities for nonlinear metamaterials.

© 2013 American Physical Society