Chiroptical signal enhancement in quasi-null-polarization-detection geometry: Intrinsic limitations

Despite its unique capability of distinguishing molecular handedness, chiroptical spectroscopy suffers from the weak-signal problem, which has restricted more extensive applications. The quasi-null-polarization-detection (QNPD) method has been shown to be useful for enhancing the chiroptical signal. Here, the underlying enhancement mechanism in the QNPD method combined with a heterodyne detection scheme is elucidated. It is experimentally demonstrated that the optical rotatory dispersion signal can be amplified by a factor of similar to 400, which is the maximum enhancement effect achievable with our femtosecond laser setup. The upper limit of the QNPD enhancement effect of chiroptical measurements could, in practice, be limited by imperfection of the polarizer and finite detection sensitivity. However, we show that there exists an intrinsic limit in the enhancement with the QNPD method due to the weak but finite contribution from the homodyne chiroptical signal. This is experimentally verified by measuring the optical rotation of linearly polarized light with the QNPD scheme. We further provide discussions on the connection between this intrinsic limitation in the QNPD scheme for enhanced detection of weak chiroptical signals and those in optical enantioselectivity and Raman optical activity with a structured chiral field. We anticipate that the present work could be useful in further developing time-resolved nonlinear chiroptical spectroscopy.111Nsciescopu

Melting Domain Size and Recrystallization Dynamics of Ice Revealed by Time-Resolved X-ray Scattering

The phase transition between water and ice is ubiquitous and one of the most important phenomena in nature. Here, we performed time-resolved x-ray scattering experiments capturing the melting and recrystallization dynamics of ice. The ultrafast heating of ice I is induced by an IR laser pulse and probed with an intense x-ray pulse, which provided us with direct structural information on different length scales. From the wide-angle x-ray scattering (WAXS) patterns, the molten fraction, as well as the corresponding temperature at each delay, were determined. The small-angle x-ray scattering (SAXS) patterns, together with the information extracted from the WAXS analysis, provided the time-dependent change of the size and the number of the liquid domains. The results show partial melting (~13 %) and superheating of ice occurring at around 20 ns. After 100 ns, the average size of the liquid domains grows from about 2.5 nm to 4.5 nm by the coalescence of approximately six adjacent domains. Subsequently, we capture the recrystallization of the liquid domains, which occurs on microsecond timescales due to the cooling by heat dissipation and results to a decrease of the average liquid domain size

Photoluminescence Characteristics of p-Phenylene Vinylene and Its Derivatives in Solution and in Nanoaggregates

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Retrieval of frequency spectrum from time-resolved spectroscopic data: comparison of Fourier transform and linear prediction methods

Femtosecond time-resolved signals often display oscillations arising from the nuclear and electronic wave packet motions. Fourier power spectrum is generally used to retrieve the frequency spectrum. We have shown by numerical simulations and coherent phonon spectrum of single walled carbon nanotubes (SWCNT) that the Fourier power spectrum may not be appropriate to obtain the spectrum, when the peaks overlap with varying phases. Linear prediction singular value decomposition (LPSVD) can be a good alternative for this case. We present a robust way to perform LPSVD analysis and demonstrate the method for the chirality assignment of SWCNT through the time-domain coherent phonon spectroscopy.X1133sciescopu

Ultrafast Energy Transfer Process in Confined Gold Nanospheres Revealed by Femtosecond X‑ray Imaging and Diffraction

Femtosecond laser pulses drive nonequilibrium phase transitions via reaction paths hidden in thermal equilibrium. This stimulates interest to understand photoinduced ultrafast melting processes, which remains incomplete due to challenges in resolving accompanied kinetics at the relevant space–time resolution. Here, by newly establishing a multiplexing femtosecond X-ray probe, we have successfully revealed ultrafast energy transfer processes in confined Au nanospheres. Real-time images of electron density distributions with the corresponding lattice structures elucidate that the energy transfer begins with subpicosecond melting at the specimen boundary earlier than the lattice thermalization, and proceeds by forming voids. Two temperature molecular dynamics simulations uncovered the presence of both heterogeneous melting with the melting front propagation from surface and grain boundaries and homogeneous melting with random melting seeds and nanoscale voids. Supported by experimental and theoretical results, we provide a comprehensive atomic-scale picture that accounts for the ultrafast laser-induced melting and evaporation kinetics