Identifying incoherent mixing effects in the coherent two-dimensional photocurrent excitation spectra of semiconductors

Abstract

A versatile technique to measure two-dimensional (2D) coherent electronic spectra involves coherent excitation of a nonlinear optical response and time- and phase-resolved measurement of a resulting population observable, such as photoluminescence intensity, photocurrent, or photoinduced absorption. This coherent spectroscopy is a unique probe in materials research in that it resolves energy pathways of ultrafast processes by addressing the coupling between electronic states in semiconductors in the complex dielectric environment of operating devices. However, we have previously demonstrated that an \emph{incoherent} nonlinear response due to pairs of photoexcitations produced via linear excitation pathways contribute to the measured signal as unexpected background [Gr\'egoire et al., J.\ Chem.\ Phys. \textbf{147}, 114201 (2017)]. Here, we simulate the effect of such incoherent population mixing in the photocurrent signal collected from a GaAs solar cell by acting externally on the transimpedance amplifier circuit used for phase-sensitive detection. We identify an effective strategy to recognize the presence of incoherent population mixing in 2D data, and we discuss a possible way of isolating the coherent response.

Resolving Nonlinear Recombination Dynamics in Semiconductors via Ultrafast Excitation Correlation Spectroscopy: Photoluminescence versus Photocurrent Detection

“…The measured spectra are interpreted as 2D excitation spectra, but we highlight that there is no well-defined phase resolution in the excitation-pulse wave packets, and the measurements are thus purely incoherent, as in the work presented here. This incoherent response arises from the dependence of the physical observable on the intensity of the excitation due to the population evolution (e.g., trap recombination and exciton–exciton recombination), rather than a coherent nonlinear response as in coherent multidimensional spectroscopies. − We also note that these 2D measurements that implement phase modulation may also contain incoherent contributions due to nonlinear population dynamics picked up by the phase demodulation detection scheme. − We thus underline the difference between the technique presented in this article and 2D coherent excitation. Earlier, ECS-like experiments have been interpreted using Feynman diagrams. , We emphasize that this is not precise since Feynman diagrams indicate optical transitions among states and their coherent correlation but do not include the interactions among their populations.…”

Unifying Nonlinear Response and Incoherent Mixing in Action-2D Electronic Spectroscopy

“…A further point deserving attention is the generality of the incoherent mixing mechanism. Indeed, just as the mixing of linear signals can enter in the fourth-order response, higher-order contributions may appear in the linear signal, e.g., scriptS ( Ω R ) × scriptS * ( Ω 43 ) is modulated at Ω R +Ω 43 = Ω 21 , as recently observed in ref , or the mixing between fourth-order and linear signals may contribute to sixth-order response . Hence, the incoherent mixing of contributions from different orders is intrinsic to A-2DES and should always be considered in spectral assignments and simulations.…”

“…On the other hand, Grégoire et al brought to the attention the phenomenon of incoherent mixing as an unwanted contribution in A-2DES spectra . Incoherent mixing occurs from the combination of linear signals due to nonlinear population dynamics, e.g., exciton–exciton annihilation, bimolecular recombination, and Auger recombination, or due to nonlinearities in the detection process . Since incoherent mixing can hide spectral features of the coherent nonlinear response, efforts have been devoted to distinguishing these two contributions.…”

“…As a result, the product of two linear signals can also be modulated at the same frequency as the rephasing, i.e., scriptS * ( Ω 21 ) × scriptS ( Ω 43 ) is modulated at Ω 21 – Ω 43 = Ω R . Therefore, in this picture, the emergence of cross-peaks is due to the incoherent mixing of linear signals during the detection time. , In this respect, the term “incoherent mixing” may be deceptive. Indeed, the mixing signal inherits and preserves the phase combination of the fourth-order interaction sequence, and for this reason, it is extracted together with the nonlinear response.…”

Uncovering Transport Mechanisms in Perovskite Materials and Devices with Recombination-Induced Action Spectroscopies

“…Recombination-induced nonlinearities are seen as a nuisance in studies of molecular aggregates because they are incapable of resolving femtosecond energy transfer dynamics and do not provide insights into biexciton electronic structure. The type of process shown in Figure b, which has been referred to as incoherent and/or a mixture of linear responses, ,,,,, is not regarded as a “true nonlinear signal” in studies of femtosecond dynamics; , however, this signal generation mechanism can be leveraged in studies of slower, long-range processes in materials and devices. Notably, the nonlinearity represented in Figure b is fundamentally different from a cascade of lower-order responses, such as those observed in off-resonant 2D Raman spectroscopies , because it provides dynamical information that is not available when a material absorbs a single laser pulse.…”