Multicolor Quantum Control for Suppressing Ground State Coherences in Two-Dimensional Electronic Spectroscopy

The measured multidimensional spectral response of different light harvesting complexes exhibits oscillatory features which suggest an underlying coherent energy transfer. However, making this inference rigorous is challenging due to the difficulty of isolating excited state coherences in highly congested spectra. In this work, we provide a coherent control scheme that suppresses ground state coherences, thus making rephasing spectra dominated by excited state coherences. We provide a benchmark for the scheme using a model dimeric system and numerically exact methods to analyze the spectral response. We argue that combining temporal and spectral control methods can facilitate a second generation of experiments that are tailored to extract desired information and thus significantly advance our understanding of complex open many-body structure and dynamics.

Figure 1

(a) In a single-color scheme, a sample is excited by three pulses and then generates signal, with time intervals denoted by coherence $\tau$, waiting $T$ and rephasing $\tau$ times. (b) A multicolor scheme for suppressing ground state coherences where the second pulse is blueshifted by $\mathrm{\Delta }\mathrm{\Omega }>0$ from the first pulse, while the third pulse is redshifted by $\mathrm{\Delta }\mathrm{\Omega }$. (c) Electronic eigenstates of a dimer. (d) Energy-level structure of a vibronic model including ground $|g,n⟩$ and excited $|{\psi }_k⟩$ states. The generation of ground state coherences from (e) global ground state $|g,0⟩$ and (f) vibrationally excited ground state $|g,1⟩$, and (g) excited state coherences $|{\psi }_2⟩⟨{\psi }_1|$ and $|{\psi }_1⟩⟨{\psi }_2|$ between vibronic eigenstates, induced by the first two pulses in 2D measurements.

Figure 2

2D spectral response of a model vibronic dimer. (a),(b) Rephasing spectra obtained by single-color scheme: (a) 2D line shape at $T\approx 100\mathrm{fs}$ and (b) the transients of peaks $R11$, $R12$, $R21$, $R22$ marked in (a) and (c). Excited and ground state signals are shown in blue solid and red dashed lines, respectively. (c),(d) Rephasing spectra obtained by the multicolor scheme. Compared to the single-color scheme, the amplitude of the ground state oscillations is significantly reduced compared to that of the excited state and the nonoscillatory component of the total signal is strongly suppressed. Hence, while there is a 50-fold reduction in overall signal intensity, the ratio of signal to noise for the oscillatory excited state component is improved compared to the single-color scheme and not contaminated by ground state oscillations.