Search Results
Terahertz radiative coupling and damping in multilayer graphene
The nonlinear interaction between intense terahertz (THz) pulses and epitaxial multilayer graphene is studied by field-resolved THz pump-probe spectroscopy. THz excitation results in a transient induced absorption with decay times of a few picoseconds, much faster than carrier recombination in single graphene layers. The decay times increase with decreasing temperature and increasing amplitude of the excitation. This behaviour originates from the predominant coupling of electrons to the electromagnetic field via the very strong interband dipole moment while scattering processes with phonons and impurities play a minor role. The nonlinear response at field amplitudes above 1 kV cm-1 is in the carrier-wave Rabi flopping regime with a pronounced coupling of the graphene layers via the radiation field. Theoretical calculations account for the experimental results.
Characterizing half-a-degree difference: a review of methods for identifying regional climate responses to global warming targets
The Paris Agreement long-term global temperature goal refers to two global warming levels: well below 2°C and 1.5°C above preindustrial. Regional climate signals at specific global warming levels, and especially the differences between 1.5°C and 2°C, are not well constrained, however. In particular, methodological challenges related to the assessment of such differences have received limited attention. This article reviews alternative approaches for identifying regional climate signals associated with global temperature limits, and evaluates the extent to which they constitute a sound basis for impacts analysis. Four methods are outlined, including comparing data from different greenhouse gas scenarios, sub-selecting climate models based on global temperature response, pattern scaling, and extracting anomalies at the time of each global temperature increment. These methods have rarely been applied to compare 2°C with 1.5°C, but some demonstrate potential avenues for useful research. Nevertheless, there are methodological challenges associated with the use of existing climate model experiments, which are generally designed to model responses to different levels of greenhouse gas forcing, rather than to model climate responses to a specific level of forcing that targets a given level of global temperature change. Novel approaches may be required to address policy questions, in particular: to differentiate between half degree warming increments while accounting for different sources of uncertainty; to examine mechanisms of regional climate change including the potential for nonlinear responses; and to explore the relevance of time-lagged processes in the climate system and declining emissions, and the resulting sensitivity to alternative mitigation pathways. WIREs Clim Change 2017, 8:e457. doi: 10.1002/wcc.457. For further resources related to this article, please visit the WIREs website.
The role of the Kramers-Henneberger atom in the higher-order Kerr effect
We discuss the connection between strong-field ionization, saturation of the Kerr response and the formation of the Kramers-Henneberger (KH) atom and long-living excitations in intense infrared (IR) external fields. We present a generalized model for the intensity-dependent response of atoms in strong IR laser fields, describing deviations in the nonlinear response at the frequency of the driving field from the standard model. We show that shaping the driving laser pulse allows one to reveal signatures of the excited KH states in the Kerr response of an individual atom.
Non-instantaneous third-order optical response of gases in low-frequency fields
It is commonly assumed that for low-intensity short optical pulses far from resonance, the third-order optical nonlinear response is instantaneous. We solve the three-dimensional time-dependent Schrödinger equation for the hydrogen atom and show that this is not the case: the polarization is not simply proportional to the cube of the electric field even at low intensities. We analyze the fundamental-frequency and third-harmonic nonlinear susceptibilities of hydrogen, investigate their dependence on intensity, and find that the delays in the Kerr response rapidly approach the femtosecond time-scale at higher intensities, while the delays in the third harmonic generation remain much lower. We also propose an experimental scheme to detect and characterize the above effects.