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The electronic structure of multiexcitons significantly impacts the performance of nanostructures in lasing and light-emitting applications. However, these multiexcitons remain poorly understood due to their complexity arising from many-body physics. Standard transient-absorption and photoluminescence spectroscopies are unable to unambiguously distinguish effects of sample inhomogeneity from exciton-biexciton interactions. Here, we exploit the energy and time resolution of two-dimensional electronic spectroscopy to access the electronic structure of the band-edge biexciton in colloidal CdSe quantum dots. By removing effects of inhomogeneities, we show that the band-edge biexciton structure must consist of a discrete manifold of electronic states. Furthermore, the biexciton states within the manifold feature distinctive binding energies. Our findings have direct implications for optical gain thresholds and efficiency droop in light-emitting devices and provide experimental measures of many-body physics in nanostructures.

Semiconductor nanocrystals emit light from excitons confined to their core, as well as from their surfaces. Time-resolving the emission from the core yields information on the band edge exciton, which is now well understood. In contrast, the emission from the surface is ill-characterized and remains poorly understood, especially on long time scales. In order to understand the kinetics of charge trapping to the surface and electronic relaxation within the surface, we perform time-resolved emission spectroscopy on CdSe nanocrystals with strong surface emission. The time-resolved spectra reveal a time scale of electron transfer from core to surface much slower than previously thought. These spectra also unveil electron dynamics in the surface band, which gives rise to an average lifetime spectrum. These dynamics are explained by invoking two surface states. This simple model further rationalizes the role of ligands in tuning the surface emission of nanocrystals. These experimental results provide a critical ...

We report on the performance of Ar-filled hollow-core fibers (HCFs) capillary driven by long pulses ( > 100 fs) of moderate energy ( < 600 μ J) for coherent multidimensional spectroscopy in the visible range. The source is characterized by high overall transmission and excellent spatial mode. Broadening is achieved by self-phase modulation. Notably, the uncompressed visible output is shorter than the input pulse. This peculiar observation is explained by the structure of self-phase modulated pulses, coupled with spectral filtering. By virtue of its simplicity, low requirements, spectral stability, and the excellent properties of its spatial output, HCFs can provide an interesting alternative to achieve broadband visible pulses for broadband optical excitation and multidimensional spectroscopy applications.

Ultrafast coherent multi-dimensional spectroscopies form a powerful set of techniques to unravel complex processes, ranging from light-harvesting, chemical exchange in biological systems to many-body interactions in quantum-confined materials. Yet these spectroscopies remain complex to implement at the high frequencies of vibrational and electronic transitions, thereby limiting their widespread use. Here we demonstrate the feasibility of two-dimensional spectroscopy at optical frequencies in a single beam. Femtosecond optical pulses are spectrally broadened to a relevant bandwidth and subsequently shaped into phase coherent pulse trains. By suitably modulating the phases of the pulses within the beam, we show that it is possible to directly read out the relevant optical signals. This work shows that one needs neither complex beam geometries nor complex detection schemes in order to measure two-dimensional spectra at optical frequencies. Our setup provides not only a simplified experimental design over standard two-dimensional spectrometers but its optical readout also enables novel applications in microscopy.

We report on a setup for coherent multidimensional spectroscopy based on visible continuum generation obtained by propagating 130 fs