Ultra-resolution coherent Raman spectroscopy with quantum light

(a) Schematic diagram of the entangled double photons as an ultrafast particle probe, where down-shifting is presented by a barium beta-borate (BBO) crystal and multi-photon detection. (B) Microscopic model level diagram in quantum fast cars. (c) The quantum fast motor signal, taking into account the four Raman-active modes A1, E and T2 in methane (CH4). (d) Microscopic model level diagram in QFRS for electronically excited states. (e) Comparison of the intensity-related QFRS and the case of the classical probe pulse of the time-evolving electronic coherence as a function of the delay T between the probe photons and the resonant pump pulse. Credit: Zhedong Zhang et al

In recent years, entangled photons – a common quantum light source – have been widely used in quantum imaging, optical interferometry, quantum computing, quantum communications, and other fields. The spontaneous down-conversion generates entangled photon pairs while conserving energy and momentum, such that the quantum correlation is encoded in space and time. This property enables a quantitative advantage that overcomes the diffraction limit of classical pulses in the field of imaging and detection.

One of the long-standing bottlenecks in molecular spectroscopy is the discovery of ultra-fast electronic processes on the femtosecond scale. The dynamics of electronic coherence is of particular interest. However, the current Raman technique cannot be used to this end, due to the limitation of the temporal frequency resolution and the incoherent channels of the excited states.

In a paper recently published in Light: science and applicationsProfessor Zhedong Zhang of the Department of Physics at City University of Hong Kong and colleagues have developed a time-resolved coherent Raman spectroscopy with entanglement. Photons It leads to QFRS (femtosecond quantum Raman spectroscopy).

Specifically, the ultra-resolution nature of the Raman signal resulting from the manipulation of photon entanglement appears in their work—both temporal and spectral resolutions can be achieved simultaneously. QFRS is sensitive to electronic coherence only.

This makes it uniquely suited for detecting electronically excited state dynamics within a short ~50 fs time scale. This feature is not achievable in the previously studied Raman techniques, which are throttled by either rapid decay or time-frequency resolution. The work offers a new perspective to investigate ultrafast processes in complex materials such as molecules, 2D materials, excitons and polaritons from where we can extract the required relaxation and radiative processes.

Quantum Raman spectroscopy replaces the classical probe pulse with a photon signal beam from the entangled photon source. The idling photon beam acts as the declared beam to measure the coincidence. So the temporal and spectral resolution can be controlled independently. This results in a superfine nature that goes beyond the time-frequency relationship coupling. Further heterogeneity detection can be done in order to observe the phase of the electrons. The highlights of their work are summarized as follows:

“We design a quantum version of femtosecond Raman spectroscopy for three purposes: (1) for high-resolution Raman anti-stroke performance. Spectroscopy in the real time domain; (2) To be able to depict electron dynamics over a very short period of time; and (3) to be sensitive to the molecular excitation phase so as to allow the detection sensitivity to overcome the quantum limit. “

“Our work significantly expands the horizon of entangled light and complements the spectral advances made by entangled light in the context of the two optimal photon absorption processes in complex molecules. This work will help future experimental and theoretical efforts,” the scientists said.

Spatiotemporal manipulation of femtosecond light pulses for wafer devices

more information:
Zhedong Zhang et al, Entangled photons enabled Raman coherent spectroscopy for time-frequency resolution and its applications to femtosecond-scale electronic coherence, Light: science and applications (2022). DOI: 10.1038/s41377-022-00953-y

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