Pseudoheterodyne near-field imaging at kHz repetition rates via quadrature-assisted discrete demodulation

Scattering-type scanning near-field optical microscopy enables the measurement of optical constants of a surface beyond the diffraction limit. Its compatibility with pulsed sources is hampered by the requirement of a high-repetition rate imposed by lock-in detection. We describe a sampling method, called quadrature-assisted discrete (quad) demodulation, which circumvents this constraint. Quad demodulation operates by measuring the optical signal and the modulation phases for each individual light pulse. This method retrieves the near-field signal in the pseudoheterodyne mode, as proven by retraction curves and near-field images. Measurement of the near-field using a pulsed femtosecond amplifier and quad demodulation is in agreement with results obtained using a CW laser and the standard lock-in detection method

Fifth-order two-quantum absorptive two-dimensional electronic spectroscopy of CdSe quantum dots

Two-quantum variants of two-dimensional electronic spectroscopy (2DES) have previously been used to characterize multi-exciton interactions in molecules and semiconductor nanostructures though many implementations are limited by phasing procedures or non-resonant signals. We implement 2DES using phase-cycling to simultaneously measure one-quantum and two-quantum spectra in colloidal CdSe quantum dots. In the pump–probe geometry, fully absorptive spectra are automatically acquired by measuring the sum of the rephasing and nonrephasing signals. Fifth-order two-quantum spectroscopy allows for direct access to multi-exciton states that may be obscured in excited state absorption signals due to population relaxation or third-order two-quantum spectra due to the non-resonant response

Temperature behavior of the Kohlrausch exponent for a series of vinylic polymers modelled by an all-atomistic approach

The Kohlrausch-Williams-Watt (KWW) function, or stretched exponential function, is usually employed to reveal the time dependence of the polymer backbone relaxation process, the so-called α relaxation, at different temperatures. In order to gain insight into polymer dynamics at temperatures higher than the glass transition temperature T g , the behavior of the Kohlrausch exponent, which is a component of the KWW function, is studied for a series of vinylic polymers, using an all-atomistic simulation approach. Our data show very good agreement with published experimental results and can be described by existing phenomenological models. The Kohlrausch exponent exhibits a linear dependence with temperature until it reaches a constant value of 0.44, at 1.26T g , revealing the existence of two regimes. These results suggest that, as the temperature increases, the dynamics progressively change until it reaches a plateau. The non-exponential character then describes subdiffusive motion characteristic of polymer melts

Pseudoheterodyne near-field microscopy at kHz repetition rates

We present quadrature-assisted discrete demodulation, which circumvents constraints imposed on the repetition rate by lock-in detection. The method enables pseudo-heterodyne near-field microscopy with kHz fs laser systems

Investigating the electronic structure of confined multiexcitons with nonlinear spectroscopies

Strong confinement in semiconductor quantum dots enables them to host multiple electron–hole pairs or excitons. The excitons in these materials are forced to interact, resulting in quantum-confined multiexcitons (MXs). The MXs are integral to the physics of the electronic properties of these materials and impact their key properties for applications such as gain and light emission. Despite their importance, the electronic structure of MX has yet to be fully characterized. MXs have a complex electronic structure arising from quantum many-body effects, which is challenging for both experiments and theory. Here, we report on the investigation of the electronic structure of MX in colloidal CdSe QDs using time-resolved photoluminescence, state-resolved pump–probe, and two-dimensional spectroscopies. The use of varying excitation energy and intensities enables the observation of many signals from biexcitons and triexcitons. The experiments enable the study of MX structures and dynamics on time scales spanning 6 orders of magnitude and directly reveal dynamics in the biexciton manifold. These results outline the limits of the simple concept of binding energy. The methods of investigations should be applicable to reveal complex many-body physics in other nanomaterials and low-dimensional materials of interest

Two-dimensional electronic spectroscopy reveals liquid-like lineshape dynamics in CsPbI₃ perovskite nanocrystals

Lead-halide perovskites have attracted tremendous attention, initially for their performance in thin film photovoltaics, and more recently for a variety of remarkable optical properties. Defect tolerance through polaron formation within the ionic lattice is a key aspect of these materials. Polaron formation arises from the dynamical coupling of atomic fluctuations to electronic states. Measuring the properties of these fluctuations is therefore essential in light of potential optoelectronic applications. Here we apply two-dimensional electronic spectroscopy (2DES) to probe the timescale and amplitude of the electronic gap correlations in CsPbI3 perovskite nanocrystals via homogeneous lineshape dynamics. The 2DES data reveal irreversible, diffusive dynamics that are qualitatively inconsistent with the coherent dynamics in covalent solids such as CdSe quantum dots. In contrast, these dynamics are consistent with liquid-like structural dynamics on the 100 femtosecond timescale. These dynamics are assigned to the optical signature of polaron formation, the conceptual solid-state analogue of solvation

Development of a Model for Analyzing the Temperature Dependence of the Viscosity of Ion Conducting Polymers and Ionic Liquids

The bond strength-coordination number fluctuation (BSCNF) model of the viscosity developed by the authors considers that the viscous flow occurs by breaking and twisting the connections between the structural units that form the melt. The analytical expression of the viscosity that results from such processes is written in terms of the average bond strength, the average coordination number, and their fluctuations of the structural units. In the present study, we use the BSCNF model to investigate the temperature dependence of the viscosity of ion conducting polymers LiClO_4-PPG and NaCF_3SO_3-PPG, and ionic liquids[bmim][PF_6], [bpy] [BF_4], [bmpro][(CF_3SO_2)_2N], [bpy] [(CF_3SO_2)_2N] and [bmim][(CF_3SO_2)_2N]. For ion conducting polymers, the analysis of the α-relaxation process is also presented. A case study done for ionic liquids indicates that the cooperativity for molecular motion which is evaluated from the viscosity analysis can be correlated with the diffusion coefficients and the ionic conductivities. The results of this study indicate that the BSCNF model is an effective model that could be used to analyze and interpret the measured temperature dependence of the viscosity