Selective Suppression of Stimulated Raman Scattering with Another Competing Stimulated Raman Scattering

A three-beam femtosecond stimulated Raman scattering (SRS) scheme is formulated and demonstrated to simultaneously induce two different SRS processes associated with Raman-active modes in the same molecule. Two SR gains involving a common pump pulse are coupled and compete: As one of the Stokes beam intensities increases, the other SRS is selectively suppressed. We provide theoretical description and experimental evidence that the selective suppression behavior is due to the limited number of pump photons used for both of the two SRS processes when an intense depletion beam induces one SRS process. The maximum suppression efficiency was ∼60% with our experimental setup, where the SR gain of the ring breathing mode of benzene is the target SRS signal, which is allowed to compete with another SRS process, induced by an intense depletion beam, of the CH stretching mode. We anticipate a potential of this new switching-off concept in super-resolution label-free microscopy

Origin of the Reversible Thermochromic Properties of Polydiacetylenes Revealed by Ultrafast Spectroscopy

Polydiacetylenes (PDAs) with thermochromic properties undergo colorimetric transitions when the external temperature is varied. This capability has the potential to enable these materials to be used as temperature sensors. These thermochromic properties of PDAs stem from their temperature-dependent optical properties. In this work, we studied the temperature-dependent optical properties of Bis-PDA-Ph, which exhibits reversible thermochromic properties, and PCDA–PDA, which exhibits irreversible thermochromic properties, by UV–visible absorption and femtosecond transient absorption spectroscopy. Our results indicate that the electronic relaxation of PDAs occurs via an intermediate state in cases where the material exhibits reversible thermochromic properties, whereas the excited PDAs relax directly back to the ground state when irreversible thermochromic properties are observed. The existence of this intermediate state in the electronic relaxation of PDAs thus plays an important role in determining their thermochromic properties. These results are very important for both understanding and strategically modulating the thermochromic properties of PDAs

Quantum Beats and Phase Shifts in Two-Dimensional Electronic Spectra of Zinc Naphthalocyanine Monomer and Aggregate

The origin of quantum coherence in two-dimensional (2D) electronic spectra of molecular aggregates and light-harvesting complexes still remains an open question. In particular, it could be challenging to distinguish between electronic and vibrational coherences for a coupled system, where both degrees of freedom can be simultaneously excited. In this Letter, we examine quantum beats in the 2D spectra of zinc naphthalocyanine (ZnNc) aggregate and monomer, and compare their characteristic features in terms of the frequency and relative phase of diagonal and off-diagonal amplitude oscillations. The long-lasting oscillating components (>1 ps) at 600–700 cm^–1 observed in both the aggregate and monomer are found to be attributed to the vibrational coherence. The wide phase variations of the 2D spectral amplitude oscillations are observed not just in the aggregate but also in the monomer state. This suggests that the unusual 90° phase shift may be attributed to neither quantum population-to-coherence transfer nor vibronic exciton coupling