Atomically thin layers of transition metal dichalcogenides (TMDs) have unique electronic and optical properties, offering the possibility of novel applications in electronics, optoelectronics and valleytronics. These applications require a fundamental understanding of the valley dynamics involving the carrier radiative and non-radiative recombination, valley polarization, and valley coherence. We will present our study of valley dynamics in monolayer TMD by using optical two-dimensional coherent spectroscopy (2DCS). Compared to conventional one-dimensional spectroscopic techniques, optical 2DCS has many advantages such as separating homogeneous and inhomogeneous linewidths, isolating relaxation pathways, detecting valley coherence and measuring coherence dephasing time. In rephasing 2D spectra, homogeneous and inhomogeneous linewidths are associated with line shape in the cross-diagonal and diagonal directions, respectively. The homogeneous linewidth can be extracted from a cross-diagonal fit to give the coherence dephasing time. The measurement is repeated with various excitation intensities and sample temperatures to extract the intrinsic dephasing time at zero power and temperature. By using various combinations of excitation pulse helicities, our experiment can selectively excite and detect a particular valley population and coherence. This allows to isolate and measure valley exchange and coherence between the two valleys.
Our recent work on optical two-dimensional coherent spectroscopy (2DCS) of semiconductor materials is reviewed. We present and compare two approaches that are appropriate for the study of semiconductor nanostructures. The first one is based on a non-collinear geometry, where the Four-Wave-Mixing (FWM) signal is detected in the form of a radiated optical field. This approach works for samples with translational symmetry, such as Quantum Wells (QWs), or large and dense ensembles of Quantum Dots (QDs). The second method is based on a collinear geometry, where the FWM is detected in the form of a photocurrent. This second approach enables 2DCS of samples where translational symmetry is broken, such as single QDs, nanowires, or nanotubes, and small ensembles thereof. For each method, we provide an example of experimental results obtained on semiconductor QWs. In particular, it is shown how 2DCS can reveal coherent excitonic coupling between adjacent QWs.
Optical multidimensional coherent spectroscopy is a powerful tool for studying structure and dynamics in complex
systems, such as semiconductors. In optical two-dimensional coherent spectroscopy (2DCS), where the spectrum
is presented in a two-dimensional (2D) plane with two frequency axes, an important advantage is the ability
to isolate quantum pathways by unfolding a one-dimensional spectrum onto a 2D plane. For many systems,
however, the quantum pathways are only partially separated in a 2D spectrum. In order to completely isolate
the quantum pathways, we extend 2DCS into a third dimension to generate three-dimensional (3D) spectra
in which the spectrum is further unfolded. A 3D spectrum provides complete and well-isolated information of
the third-order optical response of the system. The information can be used to fully characterize the quantum
pathways and to determine the system’s Hamiltonian. Quantitative knowledge of the Hamiltonian enables
prediction and control of quantum processes. For instance, such information is essential for deterministic control
and improved performance of coherent control schemes.
We have theoretically predicted and experimentally demonstrated an ultra-dispersive atomic prism
made of coherently driven Rb atomic vapor. The prism posses spectral angular dispersion that six
orders of magnetude higher than the prism made from optical glass; it is the highest spectral angular
dispersion that has been ever shown (such angular dispersion allows one to resolve spatially light
separated by a few kHz). The prism is working near the resonant frequency of atomic vapor, and its
dispersion is optically controlled by coherent driving field.
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