Beating signals in CdSe quantum dots measured by low-temperature 2D spectroscopy

Advances in ultrafast spectroscopy can provide access to dynamics involving nontrivial quantum correlations and their evolutions. In coherent 2D spectroscopy, the oscillatory time dependence of a signal is a signature of such quantum dynamics. Here we study such beating signals in electronic coherent 2D spectroscopy of CdSe quantum dots (CdSe QDs) at 77 K. The beating signals are analyzed in terms of their positive and negative Fourier components. We conclude that the beatings originate from coherent LO-phonons of CdSe QDs. No evidence for the quantum dot size dependence of the LO-phonon frequency was identified.Comment: 18 page

Direct time-frequency response of electronic coherences in assemblies of colloidal CdSe quantum dot dimers probed at room temperature by 2-dimensional electronic spectroscopy

The advantages of the directly measured time-frequency maps are discussed as a useful representation of the coherent output in a 2 dimensional electronic spectroscopy (2DES). We demonstrate the theory by a detailed application to the fast femtosecond beatings of a wide variety of electronic coherences in dimers of size-dispersed (9%) 3nm quantum dots (QDs). The observed and computed results can be consistently characterized directly in the time-frequency domain by probing the polarization in a 2DES set-up. Experimental and computed time-frequency maps are found in very good agreement and several electronic coherences are characterized at room temperature in solution before extensive dephasing due to the size-dispersion kicks in. As compared to the frequency-frequency maps that are commonly used in 2DES, the time-frequency maps allow for exploiting electronic coherences without additional post processing and with fewer 2DES measurements of polarization. Towards quantum technology applications, we also report on the modeling of the time-frequency photocurrent response of these electronic coherences, which opens the way to integrating QD devices with classical architectures thereby enhancing the quantum advantage of such technologies for parallel information processing at room temperature

Time-frequency signatures of electronic coherence of colloidal CdSe quantum dot dimer assemblies probed at room temperature by 2-dimensional electronic spectroscopy

Electronic coherence signatures can be directly identified in the time-frequency maps measured in 2 dimensional spectroscopy (2DES). We demonstrate the theory and discuss the advantages of this approach by a detailed application to the fast femtosecond beatings of a wide variety of electronic coherences in dimers of size-dispersed (8%) 3nm quantum dots (QDs). The observed and computed results can be consistently characterized directly in the time-frequency domain by probing the polarization in a 2DES set-up. Experimental and computed time-frequency maps are found in very good agreement and several electronic coherences are characterized at room temperature in solution before extensive dephasing due to the size-dispersion kicks in. As compared to the frequency-frequency maps that are commonly used in 2DES, the time-frequency maps allow for exploiting electronic coherences without additional post processing and with fewer 2DES measurements. Towards quantum technology applications, we also report on the modeling of the time-frequency photocurrent response of these electronic coherences, which opens the way to integrating QD devices with classical architectures thereby enhancing the quantum advantage of such technologies for parallel information processing at room temperature

Harvesting a wide spectral range of electronic coherences with disordered quasi-homo dimeric assemblies at room temperature

A wide variety of photoinduced electronic coherences are shown to be robust with respect to dephasing in ensembles of quasi-homodimers assembled with sub-nm ligands from colloidal 3 nm CdSe quantum dots (QDs) with controlled 9% size dispersion, both in solution and in solid-state. Coherence periods ranging from 40 to 300 fs are consistently characterized by multi-dimensional electronic spectroscopy in the Vis range in solution and solid-state samples. A theoretical model that includes size dispersion, spin orbit coupling, and crystal field splitting supports the assignment of electronic coherences. Further, this model provides a guide for optimizing the coherences by tuning the interplay between dimer electronic delocalization, optical activity and size dispersion. The experimental persistence of many QD electronic coherences at the level of the size dispersed ensemble in the solid-state and in solution opens the way for building versatile bottom-up materials well suited to quantum technology applications

Excited States and Their Dynamics in CdSe Quantum Dots Studied by Two-Color 2D Spectroscopy

Quantum dots (QDs) form a promising family of nanomaterials for various applications in optoelectronics. Understanding the details of the excited-state dynamics in QDs is vital for optimizing their function. We apply two-color 2D electronic spectroscopy to investigate CdSe QDs at 77 K within a broad spectral range. Analysis of the electronic dynamics during the population time allows us to identify the details of the excitation pathways. The initially excited high-energy electrons relax with the time constant of 100 fs. Simultaneously, the states at the band edge rise within 700 fs. Remarkably, the excited-state absorption is rising with a very similar time constant of 700 fs. This makes us reconsider the earlier interpretation of the excited-state absorption as the signature of a long-lived trap state. Instead, we propose that this signal originates from the excitation of the electrons that have arrived in the conduction-band edge