Ultrafast Hyperspectral Transient Absorption Spectroscopy: Application to Single Layer Graphene

Abstract

We describe the basic principles and the experimental implementation of the hyperspectral transient absorption technique, based on femtosecond laser sources. In this technique the samples were optically "pumped" using the femtosecond tunable pulse delivered by an Optical Parametric Amplifier, and "probed" for changes in transmission in a broad spectral range with a "white light" laser-generated supercontinuum. The spectra were collected by a pair of multichannel detectors which allowed retrieval of the absorbance change in a wide spectral range in one time. The use of the supercontinuum probe introduced artifacts in the measured 2D data set which could be corrected with a proper calibration of the chirp. The configuration with crossed polarization for pump and probe pulse extended the spectral measured range above and below the pump energy within the same experiment. We showed the versatility of the technique by applying it to the investigation of the charge carrier dynamics in two-dimensional single layer graphene.

Topological resonance in graphene-like materials

“…Since the total phase of the effective interband coupling includes, in addition to the dynamic phase, the topological phase, the resonance determined by equation ( 12) is called the topological resonance [10,17]. To analyze the conditions under which the topological resonance can be observed, we rewrite equation (12) in the following form:…”

“…The effective interband coupling determines the mixing of the bands. The strength of the mixing is roughly described by the integral ´t D αα1 , which is exactly the condition of topological resonance (12). If the magnitude of D (eff ) αα1 depends on time through the time dependent wave vector, k(t), then the condition of the strongest mixing should be modified.…”

“…If the magnitude of D (eff ) αα1 depends on time through the time dependent wave vector, k(t), then the condition of the strongest mixing should be modified. Namely, the right hand side in the topological resonance equation (12) will get some extra frequency term of the order of d ln |D…”

“…Such strong pulses can close the bandgap in an insulator and strongly enhance its conductivity, which results in finite transfer of electric charge through the system [3]. The optical properties of solids, such as optical absorption, light transmission, and high harmonic generation, can be also controlled by ultrastrong optical pulses [4][5][6][7][8][9][10][11][12][13]. The response of the system to such strong pulses is highly nonlinear and depends both on the amplitude of the pulse and its polarization.…”

High harmonic generation in graphene quantum dots

“…In the context of solid state physics, a strong short optical pulse modifies both the transport and optical properties of solids [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. In terms of electron dynamics, an ultrashort optical pulse induces two distinct dynamics known as interband and intraband electron dynamics which unfolds the process of HHG through the interactive interactions of these two distinct dynamics [12-14, 18, 19, 34].…”