Tuesday, September 26, 2017

Abstract-On-grating graphene surface plasmons enabling spatial differentiation in the terahertz region




Yisheng Fang, Yijie Lou, and Zhichao Ruan

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-42-19-3840

We propose a graphene-on-grating nanostructure to enable second-order spatial differentiation computation in the terahertz (THz) region. The differentiation operation is based on the interference between the direct reflected field and the leakage of two excited surface plasmon polaritons counter-propagating along the graphene sheet. With the spatial coupled-mode theory, we derive that the requirement for the second-order spatial differentiation is the critical coupling condition. We numerically demonstrate such an analog computation with Gaussian beams. It shows that the spatial bandwidth of the proposed differentiator is large enough such that even when the waist radius of the Gaussian beam is as narrow as w0=0.68λ(λ is the free-space wavelength), the accuracy of the differentiator is higher than 95%. The proposed differentiator is ultra-compact, with a thickness less than 0.1λ, and useful for real-time imaging applications in THz security detections.
© 2017 Optical Society of America

Abstract-Transient terahertz photoconductivity of insulating cuprates


J. C. Petersen, A. Farahani, D. G. Sahota, Ruixing Liang, and J. S. Dodge


We establish a detailed phenomenology of photocarrier transport in the copper oxide plane by studying the transient terahertz photoconductivity of Sr2CuO2Cl2 and YBa2Cu3O6. The peak photoconductivity saturates with fluence, decays on multiple picosecond timescales, and evolves into a state characterized by activated transport. The time dependence shows little change with fluence, indicating that the decay is governed by first-order recombination kinetics. We find that most photocarriers make a negligible contribution to the dc photoconductivity, and we estimate the intrinsic photocarrier mobility to be 0.6–0.7 cm2/Vs at early times, comparable to the mobility in chemically doped materials.
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Monday, September 25, 2017

Abstract-Terahertz Light–Matter Interaction beyond Unity Coupling Strength




Andreas BayerMarcel PozimskiSimon SchambeckDieter SchuhRupert HuberDominique Bougeard, and Christoph Lange

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b03103

Achieving control over light–matter interaction in custom-tailored nanostructures is at the core of modern quantum electrodynamics. In strongly and ultrastrongly coupled systems, the excitation is repeatedly exchanged between a resonator and an electronic transition at a rate known as the vacuum Rabi frequency ΩR. For ΩR approaching the resonance frequency ωc, novel quantum phenomena including squeezed states, Dicke superradiant phase transitions, the collapse of the Purcell effect, and a population of the ground state with virtual photon pairs are predicted. Yet, the experimental realization of optical systems with ΩRc ≥ 1 has remained elusive. Here, we introduce a paradigm change in the design of light–matter coupling by treating the electronic and the photonic components of the system as an entity instead of optimizing them separately. Using the electronic excitation to not only boost the electronic polarization but furthermore tailor the shape of the vacuum mode, we push ΩRc of cyclotron resonances ultrastrongly coupled to metamaterials far beyond unity. As one prominent illustration of the unfolding possibilities, we calculate a ground state population of 0.37 virtual photons for our best structure with ΩRc = 1.43 and suggest a realistic experimental scenario for measuring vacuum radiation by cutting-edge terahertz quantum detection.

Abstract-The new concept of nano-device spectroscopy based on Rabi-Bloch oscillations for THz-frequency range


Ilay Levie, and Gregory Slepyan

https://arxiv.org/ftp/arxiv/papers/1706/1706.10014.pdf


We considered one-dimensional quantum chains of two-level Fermi particles coupled via the tunneling driven both by ac and dc fields in the regimes of strong and ultrastrong coupling. The frequency of ac field is matched with the frequency of the quantum transition. Based on the fundamental principles of electrodynamics and quantum theory, we developed a general model of quantum dynamics for such interactions. We showed that the joint action of ac and dc fields leads to the strong mutual influence of Rabi- and Bloch oscillations one to another. We focused on the regime of ultrastrong coupling, for which Bloch- and Rabi-frequencies are a significant values of the frequency of interband transition. The Hamiltonian was solved numerically with account of anti-resonant terms. It manifests by the appearance of great number of narrow high-amplitude resonant lines in the spectra of tunneling current and dipole moment. We proposed the new concept of THz spectroscopy promising for different applications in future nanoelectronics and nano-photonics.

Abstract-Stochastic geometry analysis of interference and coverage in Terahertz networks


Xin-Wei Yao, Chao-Chao Wang, Wan-Liang Wang,  Chong Han,

http://www.sciencedirect.com/science/article/pii/S1878778917300261


Terahertz (THz) band communication has been envisioned as a key wireless technology for providing unprecedented high data rates. However, due to the severe path loss, the transmission distance in THz band is very limited. Therefore, the beamforming techniques are explored to enlarge the communication range in THz networks, particularly employed at Access Points (APs). Interference is a critical factor affecting the performance of THz networks. In this paper, the interference and the coverage probability are investigated for the THz band communications with beamforming APs. First, based on the Line-of-Sight (LoS) and Non-Line-of-Sight (NLoS) ray propagation models of THz signals, the interferences from neighboring users and beamforming APs are modeled in closed forms based on stochastic geometry methods. Then, the Signal-to-Interference-plus-Noise-Ratio (SINR) and the coverage probabilities are further derived based on the THz channel and interference model. Extensive simulations are carried out to evaluate the interference and coverage for the THz band communications with beamforming APs, for the different system parameters, which include SINR threshold, density of APs, transmission frequency, gain of beamforming AP, density of users and radius of interference area. Learning from the simulation results, high density of APs with a small beam-width and the transmission at THz frequencies with low absorption efficient, such as 0.67 THz, are recommended to mitigate the interference and achieve a better coverage performance.