Controlling Quantum Pathways in Molecular Vibrational Polaritons

Abstract

Molecular vibrational polaritons, hybrid quasi-particles formed by molecular vibrations and cavity photons through strong light−matter coupling, have attracted much attention due to their potential to alter photonic properties and to change the course of chemical reactions. The ultrafast dynamics of molecular vibrational polaritons have been studied to advance our understanding of these interesting hybrid quasiparticles. For example, the roles of dark states, hot vibrational mode relaxation in cavities, and manipulation of nonlinear polariton-polariton interactions have been investigated by ultrafast spectroscopy. However, the coherent nature of polaritons makes many viable quantum pathways able to contribute to ultrafast spectroscopic signals, such as two-dimensional infrared spectroscopy (2D IR), complicating its interpretation. Here, using a mid-IR pulse shaper, we report an experimental way to directly measure polariton nonlinear responses involving specific quantum pathways by eliminating others. For example, either coherence or population states may be exclusively probed. This work is useful for the development of polariton-based quantum technologies, and research into the role of coherence in energy transportation in artificial and natural energy materials, and cavity chemistry.

Excited-State Vibration-Polariton Transitions and Dynamics in Nitroprusside

“…4). Previous reports have shown early time oscillations 19 , and very recently, Yang, et al confirmed that the oscillations arise from coherence exchange between modes 24 . By using half-pump pulses, as described above, we were able to suppress the early time oscillations of cavity-coupled SNP and recover the oscillations when exciting both polaritons with a broadband pulse.…”

“…Selective pumping is an established technique 24,49,50 which we implement in the 2D IR apparatus via a pulse shaper. The pulse shaper disperses the pump beam in frequency across the AOM.…”

“…For the following results, we chose to obtain pump-probe data at a series of waiting times rather than collecting full 2D IR spectra, which reduced data collection time by two orders of magnitude while still preserving comparable signal-to-noise ratios. We maintain state-selective excitation by using spectrally filtered pump pulses, which has been used before to suppress oscillations in 2D IR measurments 24,49,50 . The spectrally filtered pulses are created with the mid-IR pulse shaper, which allows us to impose an arbitrary frequency mask on the pump pulse.…”

“…In our initial reports, we found that the transient spectra were dominated by the response of a reservoir of uncoupled excited vibrational states near the LP, which leads to a large absorptive feature there and a characteristic shift of the UP. There was also evidence that the LP spectral region includes polariton-related transitions 17,[19][20][21][22][23][24] , but it was difficult to distinguish reservoir and polariton excitations. Furthermore, the extremely strong and narrow vibrational bands of W(CO) 6 can lead to efficient population of both the first and second vibrational excited states 25 further congesting the excited-state spectra.…”

“…Additionally, the degeneracy of this CO mode leads to rapid reorientation that may compete with short-lived polariton processes. Despite these challenges, nonlinear infrared spectroscopy of W(CO) 6 has continued to yield fascinating results 22,24 , including demonstrations of remote communication between distinct optical cavities and strong optical nonlinearities that arise from coherence exchange 21,23 and very recently, intermolecular vibration energy exchange 26 .…”