Multidimensional coherent spectroscopy (MDCS) is a technique that unfolds the optical response of a mixed
substance in two or more spectral dimensions. MDCS reveals the correlations between features to separate the signatures of each individual substance from the mixture. This separation is analogous to analyzing overlapped fingerprints to pick out each unique fingerprint or signature.
In the top figure we illustrate the difference in information provided by MDCS versus typical optical spectroscopy. On the left we plot six absorption dips one might measure with the linear absorption or photoluminescence. Unlike these methods, which measure a mixture of constituents as a forest of overlapping features, MDCS measures the correlations between individual substances leading to unique signature mapping of each component. Using the MDCS methodology immediately determines the signatures of all of individual substances in the sample, which can be compared to a database of substance signatures for easy and rapid identification of the sample components.
This idea of isolating the spectra of substances in a mixture applies to all types of materials. It is well known that optical spectra of nanostructures (e. g. quantum wells in a laser diode and monolayer materials) are dominated by spatial inhomogeneity. MDCS is also useful for separating the homogeneous spectra of an inhomogeneous distribution. In the adjacent MDCS spectrum, the top-left feature is elongated along the diagonal, which results from spatial inhomogeneity. Despite this inhomogeneity, MDCS is able to measure the homogeneous linewidth.
MDCS is a third-order nonlinear spectroscopy, which is the lowest order of nonlinear spectroscopy for centrosymmetric material, i. e. all materials have a third order response. What makes MDCS so much more powerful than linear spectroscopy techniques is that the use of multiple light-matter interactions is particularly well suited for measuring the intrinsic properties of an optical resonance.
One MDCS application, called the photon-echo sequence, is shown in the adjacent cartoon. If a largely inhomogeneous distribution of optical resonances is measured with a linear spectroscopy technique, the inhomogeneous broadening will dominate the spectrum. By using a photon-echo MDCS pulse sequence, it is possible measure the intrinsic properties of each frequency group including homogeneous dephasing and coupling.
There are yet other techniques for measuring other complex material properties, all of which are possible with the same hardware and require only small software/electronics reconfiguration. One of these other useful MDCS pulse sequences is the two-quantum technique, which is a background-free measurement of many-body effects.
Applications and Links
Multidimensional coherent spectroscopy (MDCS), the optical analog of NMR, has found its way into several scientific fields. The research community currently applies MDCS to semiconductors, molecules, drug-protein interactions, living bacteria, atoms, and even oil painting degradation. Here we list many of these applications with some links to the original work. Most of these researchers are physicists and chemists that have invested tremendous time and effort into building the setups they use to achieve their results.
Materials studied with MDCS and other four-wave-mixing-based techniques:
- Transition metal dichalcogenides (MoSe2, WSe2, MoS2, WS2, hBN-encapsulated monolayer, heterostructures)
- Perovskites (lead iodide, lead bromide)
- Quantum wells (GaAs, GaN, CdSe)
- see also: in diode structure
- Self-assembled and interfacial quantum dots
- see also: isolated in cavity
- Colloidal quantum dots
- Confined polariton devices
- Plasmonic antennas
- Oil paints (four-wave-mixing is much more sensitive to degradation than linear spectroscopy)
Phenomena directly measurable with MDCS:
- Many-body effects
- Homogeneous linewidth of inhomogeneously broadened resonances
- Biexcitons, trions, and other high order interactions
- Spectral diffusion
- Rabi flopping of an inhomogeneous ensemble of quantum dots
- Vibrational coupling
The above is just a sampling of the measurements that are possible with MDCS. One rule of thumb to know is, if you can measure pump-probe, you can measure MDCS. The difference is that MDCS is a much more controlled and generalized technique that will provide you with more information about the system. If you have another material science application or interest, let us know.