In article number 1800171, Christopher L. Smallwood and Steven T. Cundiff introduce a multidimensional coherent spectroscopy (MDCS) probe of the dynamics of excitons in an asymmetric In0.05Ga0.95As/GaAs double quantum well. Spectral information here is plotted as a correlation between the frequency of a first-order optical excitation interaction (ωτ) and a third-order emission interaction (ωt) at various intermediate-state mixing times (T), providing a window into a semiconductor's inhomogeneity, many-body interactions, and coherent coupling dynamics at femtosecond timescales. Image credit: Brad Baxley, Part to Whole.
In article number 1800171, Christopher L. Smallwood and Steven T. Cundiff introduce a multidimensional coherent spectroscopy (MDCS) probe of the dynamics of excitons in an asymmetric In0.05Ga0.95As/GaAs double quantum well. Spectral information here is plotted as a correlation between the frequency of a first-order optical excitation interaction (ωτ) and a third-order emission interaction (ωt) at various intermediate-state mixing times (T), providing a window into a semiconductor's inhomogeneity, many-body interactions, and coherent coupling dynamics at femtosecond timescales. Image credit: Brad Baxley, Part to Whole.
Abstract Multidimensional coherent optical spectroscopy (MDCS) is an elegant and versatile tool to measure the ultrafast nonlinear optical response of materials. Of particular interest for semiconductor nanostructures, MDCS enables the separation of homogeneous and inhomogeneous linewidths, reveals the nature of coupling between resonances, and is able to identify the signatures of many-body interactions. As an extension of transient four-wave mixing (FWM) experiments, MDCS can be implemented in various geometries, in which different strategies can be used to isolate the FWM signal and measure its phase. I review and compare different practical implementations of MDCS experiments adapted to the study of semiconductor materials. The power of MDCS is illustrated by discussing experimental results obtained on semiconductor nanostructures such as quantum dots, quantum wells, microcavities, and layered semiconductors.
Multidimensional coherent spectroscopy (MDCS) is used to separate coherent and incoherent nonlinear contributions to the population-time dynamics in a GaAs-based semiconductor microcavity encapsulating a single InGaAs quantum well. In a three-pulse four-wave-mixing scheme, the second delay time is the population time that in MDCS probes excited-state coherences and population dynamics. Nonlinear optical interactions can mix these contributions, which are isolated here for the lower- and upper-exciton polariton through the self- and mutual-interaction features. Results show fast decays and oscillations arising from the coherent response, including a broad stripe along the absorption energy axis, and longer time mutual-interaction features that do not obey a simple population decay model. These results are qualitatively replicated by Bloch equation simulations for the 1s exciton strongly coupled to the intracavity field. The simulations allow for separation of coherent and incoherent Pauli-blocking and Coulomb interaction terms within the ${\ensuremath{\chi}}^{(3)}$-limit, and a direct comparison of each feature in one-quantum rephasing and zero-quantum spectra.
Multi-dimensional coherent spectroscopy (MDCS) has become an extremely versatile and sensitive technique for elucidating the structure, composition, and dynamics of condensed matter, atomic, and molecular systems. The appeal of MDCS lies in its ability to resolve both individual-emitter and ensemble-averaged dynamics of optically created excitations in disordered systems. When applied to semiconductors, MDCS enables unambiguous separation of homogeneous and inhomogeneous contributions to the optical linewidth, pinpoints the nature of coupling between resonances, and reveals signatures of many-body interactions. In this review, we discuss the implementation of MDCS to measure the nonlinear optical response of excitonic transitions in semiconductor nanostructures. Capabilities of the technique are illustrated with recent experimental studies that advance our understanding of optical decoherence and dissipation, energy transfer, and many-body phenomena in quantum dots and quantum wells, semiconductor microcavities, layered semiconductors, and photovoltaic materials.
Double-quantum two-dimensional (2D) coherent spectroscopy (MDCS) is a powerful optical method that is used to study the optical properties of atomic and complex molecular systems and semiconductor materials. Double-quantum 2D spectra and particularly the peak line shapes on the spectra can also provide information about many-body interactions. We model 2D spectra by solving the optical Bloch equations and show the effects of correlation between coupled resonances, which also explains the discrepancies between the experimental results reported by multiple groups.
This work covers two main results united by multidimensional coherent spectroscopy (MDCS): one experimental and one theoretical. MDCS is a nonlinear optical technique in which a sample is probed using a series of ultrashort pulses. Measurements are made while varying the relative time delays between the pulses, and the resulting spectra yield a much richer set of information compared to traditional linear spectroscopies. The experimental work concerns the negatively charged silicon-vacancy (SiV) center in diamond. We use two-pulse correlation (TPC) spectroscopy and rephasing MDCS to probe a diamond sample with a high density of implanted SiV centers. These spectra reveal a large number of spectral peaks, which can be grouped into two families of SiV centers using the MDCS spectra. By comparing spectra from two polarizations of the incident light, we associate the two families with two orientation groups within the diamond. We link the differences in the frequencies of the spectral peaks to strain intrinsic to our sample, and use the peak locations from both families to solve for the full strain tensor local to the laser spot. By measuring TPC spectra at multiple points on the sample, we track changes in the measured strain. We observe non-zero strain on the order or 1e-5 at every measured location, and observe variation in both the normal and shear strain in the sample. We interpret the strain as likely to be due to the high implantation density of silicon in the diamond. These results could be useful when using SiV centers as a strain gauge. The theoretical work uses simulations to calculate spectra. The simulations developed here begin by recursively generating a list of Feynman diagrams for a given signal pathway and system. The contributions due to each Feynman diagram are calculated and combined to find the complete spectrum. The code is designed to be very flexible, and can be used to simulate arbitrary types of MDCS spectra and energy level diagrams. We then apply the simulations to investigate the effects of correlated dephasing due to scattering events in the Markovian limit on MDCS of interacting systems. We derive a mathematical expression to represent the dephasing rate of a coherence in terms of the dephasing rates of the coherences from the constituent energy transitions and the correlation matrix of these transitions. This expression is applied to simulate double-quantum and higher-order n-quantum spectra of multiple interacting systems. For double-quantum spectra, correlated dephasing results in a higher double-quantum dephasing rate, and anticorrelated dephasing results in a lower double-quantum dephasing rate. Similar results are found for the n-quantum dephasing rate. Generally, for certain configurations, the many-body dephasing rate can be arbitrarily low, and the n-quantum linewidth can be arbitrarily narrow, although this requires some form of anticorrelation. These results could be useful in creating quantum sensors with higher sensitivities, since one limit of the sensitivity is the decoherence time of the system.
Light-matter interactions associated with excitons and exciton related complexes are explored in bulk GaAs and semiconductor microcavities using multidimensional coherent spectroscopy (MDCS). This approach provides rich spectra determining quantum excitation pathways, structural influences on the excitons, and coherence times.;Polarization, excitation density, and temperature-dependent MDCS is performed on excitons in strained bulk GaAs layers, probing the coherent response for differing amounts of strain. Biaxial tensile strain lifts the degeneracy of heavy-hole and light-hole valence states, leading to an observed splitting of the associated excitons at low temperature. Increasing the strain increases the magnitude of the heavy-/light- hole exciton peak splitting, induces an asymmetry in the off-diagonal interaction coherences, increases the difference in the heavy- and light- hole exciton homogenous linewidths, and increases the inhomogeneous broadening of both exciton species. All results arise from strain-induced variations in the local electronic environment, which is not uniform along the growth direction of the thin layers. For cross-linear polarized excitation, wherein excitonic signals give way to biexcitonic signals, the high-strain sample shows evidence of bound light-, heavy- and mixed- hole biexcitons.;2DCS maps the anticrossing associated with normal mode splitting in a semiconductor microcavity. For a detuning range near zero, it is observed that there are two diagonal features related to the intra-action of exciton-polariton branches and two off-diagonal features related to coherent interaction between the polaritons. At negative detuning, the line shape properties of the diagonal intra-action features are distinguishable and can be associated with cavity-like and exciton-like modes. A biexcitonic companion feature is observed, shifted from the exciton feature by the biexciton binding energy. Closer to zero detuning, all features are enhanced and the diagonal intra-action features become nearly equal in amplitude and linewidth. At positive detuning the exciton-like and cavity-like characteristics return to the diagonal intra-action features. Off-diagonal interaction features exhibit asymmetry in their amplitudes throughout the detuning range. The amplitudes are strongly modulated as the lower polariton branch crosses the bound biexciton energy determined from negatively detuned spectra.