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. 2024 Mar 4;128(10):1817–1824. doi: 10.1021/acs.jpca.3c07847

Ultrafast Spectroscopy under Vibrational Strong Coupling in Diphenylphosphoryl Azide

Garrek Stemo †, Joel Nishiuchi †, Harsh Bhakta ‡, Haochuan Mao ‡, Garret Wiesehan ‡, Wei Xiong ‡, Hiroyuki Katsuki †,*
PMCID: PMC10945483  PMID: 38437187

Abstract

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Strong coupling of cavity photons and molecular vibrations creates vibrational polaritons that have been shown to modify chemical reactivity and alter material properties. While ultrafast spectroscopy of vibrational polaritons has been performed intensively in metal complexes, ultrafast dynamics in vibrationally strongly coupled organic molecules remain unexplored. Here, we report ultrafast pump–probe measurement and two-dimensional infrared spectroscopy in diphenylphosphoryl azide under vibrational strong coupling. Early time oscillatory structures indicate coherent energy exchange between the two polariton modes, which decays in ∼2 ps. We observe a large transient absorptive feature around the lower polariton, which can be explained by the overlapped excited-state absorption and derivative-shaped structures around the lower and upper polaritons. The latter feature is explained by the Rabi splitting contraction, which is ascribed to a reduced population in the ground state. These results reassure the previously reported spectroscopic theory to describe nonlinear spectroscopy of vibrational polaritons. We have also noticed the influence of the complicated layer structure of the cavity mirrors. The penetration of the electric field distribution into the layered structure of the dielectric-mirror cavities can significantly affect the Rabi splitting and the decay time constant of polaritonic systems.

Introduction

It has long been established that a material transition can be coupled to a resonant confined optical mode, forming new hybrid light-matter states called cavity polaritons.1−3 Strong coupling between a material transition and cavity mode results in the formation of two polariton modes, lower and upper polaritons (LP and UP). These polariton modes are energetically separated by Rabi splitting, ℏΩR. Vibrational polaritons are a kind of polariton composed of infrared-active vibrational transitions and cavity photons.4−9 Following the pioneering works by Ebbesen et al.,10,11 vibrational strong coupling (VSC) is now recognized as a new tool to modulate the vibrational potential energy surface in the electronic ground state, allowing for systematic modification of chemical reactivity and manipulation of bulk material properties by the formation of delocalized collective states.12−15 A number of groups have reported exciting results, including reaction selectivity,10,16,17 increased ionic conductivity,18 enhanced driving of DNA origami coassembly,19 and acceleration and suppression of ground-state reactivity.20,21 Despite continued reports of modified molecular properties and chemical reactivity, the mechanisms behind them have still not been clearly elucidated. One major unsolved problem is, for example, the relation between the coherent and delocalized nature of the vibrational polaritons and the modified properties of VSC systems.

A possible approach to shed light on this problem is the application of ultrafast spectroscopy. Ultrafast transient spectroscopy and two-dimensional infrared (2D IR) spectroscopy have yielded fruitful insights into the dynamics and structure of vibrational polaritons22−30 and have even served as platforms for demonstrating energy transfer between molecular isotopomers and neighboring cavities in a checkerboard pattern.31,32 Most previous works, however, use metal complexes such as tungsten hexacarbonyl (W(CO)6) as the molecular component of the polariton, which features a strong asymmetric stretching (C=O) transition. Additional studies of completely dissimilar molecules are needed to develop a general framework of polariton dynamics and to identify the features that may be instrumental for modifying chemical reactivity and quantum device design.

In this work, we strongly couple the N3 asymmetric stretching mode of diphenylphosphoryl azide (DPPA) (one of the molecules reported in the earliest stage of VSC studies33) to an optical mode of a microcavity. Figure 1a shows an energy schematic illustrating VSC. Azides are functional groups in an uncluttered region of the infrared spectrum around 2000 cm–1 and are commonly used as vibrational reporters (VRs) to probe local environments and study dynamic processes, in proteins and reverse micelles for example.34−37 Studying these useful molecules in the context of vibrational strong coupling may lead to future applications of VRs in-cavity coupling. The transient spectra of the VSC states are investigated by infrared pump–probe spectroscopy at NAIST and 2D IR spectroscopy techniques at UCSD. As is well-known, 2D IR spectroscopy can retrieve all the information obtainable by pump–probe experiments. Our motivation for studying the same chemical species with two independent setups is to confirm the reproducibility of the data, which has been a point at issue several times in previous VSC experiments.38−40 Time-resolved measurements of the coupled system reveal coherent Rabi oscillations at early times at both the LP and UP transmission frequencies. The slow relaxation rates after the early time oscillation are compared to independent noncavity experiments with DPPA. The results are discussed based on polariton-specific phenomena combined with the contribution of uncoupled molecules and dark state absorption.

Figure 1.

Figure 1

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Energy levels and static spectra of DPPA. (a) Vibrational energy levels and cavity photon energy. When the vibrational transition is resonant with the cavity photon energy, the two modes are coupled to form two new modes (UP and LP) separated by the Rabi splitting energy (ℏΩR). Inset is a schematic of a DPPA molecule. (b) The red trace (left axis) is an absorption spectrum of DPPA in toluene (∼0.3 M) in a CaF2 cell with 12 μm spacer (taken by FTIR). The purple trace (right axis) is the transmission spectrum of the same sample in a dielectric-mirror cavity with a 12 μm spacer (taken by MIR laser). The peak at the blue side of the UP is due to the neighboring cavity mode. Cavity mode spectrum (without DPPA) is shown in Figure S2 in the Supporting Information.

Methods

Pump–Probe and 2D IR Measurements

Transient absorption and time-resolved spectroscopy were performed at NAIST with a Ti/sapphire regenerative amplifier (Legend Elite, Coherent) operated at 1 kHz repetition rate with a fwhm of ∼35 fs. The output of the amplifier pumped an optical parametric amplifier (OPERA, Coherent) that generated the signal and idler pulses, which were converted to mid-infrared (MIR) pulses centered at 4.6 μm through a difference frequency generation process with a type II AgGaS2 nonlinear crystal. The MIR pulse was split into pump (75%) and probe (25%) pulses by a beam splitter. The pump pulse was mechanically chopped at 500 Hz (model 3501, New Focus) and sent to a delay stage (M-VP-25XA, Newport). Pump and probe pulses were focused by CaF2 lenses (f = 80 and 50 mm, respectively) and overlapped spatially at the sample. The pump power just in front of the sample was typically ∼1.8 μJ per pulse. The sample was mounted on a stage and rotated so that the pump and probe pulses entered the cavity with the same incident angle (∼6.5°). The transmitted probe pulse was collimated by an off-axis parabolic mirror (f = 100 mm) and then focused onto the entrance slit of a spectrometer (MS-3501i, Solar TII) equipped with a gold-coated grating (100 L/mm, blaze = 4.16 μm). The dispersed light was detected with a single-pixel HgCdTe (MCT) detector (FTIR-13-1.0, Infrared Associates) cooled to liquid nitrogen temperature.

Two-dimensional infrared (2D IR) spectra were taken at UCSD with a Ti/sapphire system (Astrella, Coherent) that pumped an OPA (TOPAS, LightConversion) to generate MIR pulses. The details of the 2D-IR experimental setup are explained elsewhere.25 Briefly, after splitting the MIR pulse into pump and probe pulses, the pump pulse was sent through a Germanium acousto-optic modulator (AOM)-based pulse shaper (QuickShape, PhaseTech) and shaped into pulse pairs. The pump and probe pulses were focused by CaF2 lenses (f = 80 and 50 mm, respectively) and overlap spatially at the sample. The same focusing optics were used at NAIST and UCSD, with a similar optical layout around the sample area. The transmitted probe output was collimated with a CaF2 lens and focused with a ZnSe lens onto a spectrometer equipped with a 2D-MCT (PhaseTech) detector that collected an entire spectrum for each laser shot.

Sample Preparation

Liquid DPPA was purchased from Wako Chemical [96+% (NMR grade), NAIST] and Sigma-Aldrich (97%, UCSD) and used without further purification. A fresh solution of DPPA in toluene was prepared for each experiment. The glass bottle containing DPPA was removed from the storage refrigerator and allowed to warm to room temperature. Then it was mixed with toluene at the desired concentration and stirred well to ensure homogeneity and minimize the formation of possible clusters that could complicate the spectra. A very small drop of solution was then pipetted onto one of the mirrors (with a spacer set on top), and then the second mirror is placed on top starting at an angle to allow air to escape as it is laid down. Cavities were constructed from a pair of dielectric mirrors on CaF2 substrates (13 mm diameter at NAIST and 25 mm diameter at UCSD) and separated by 12 μm-thick polytetrafluoroethylene (PTFE) spacers. When filled with only toluene, the cavity mode nearest the DPPA azide stretching (2170 cm–1) has a Q-factor of Q ∼ 180. (Details are given in Supporting Information.) The DPPA in toluene solution-filled cavities were carefully placed in demountable flow cells, either 13 mm or 25 mm in diameter (Harrick Scientific).

Cavity mirrors used at NAIST were purchased from Optical Coatings Japan, designed with an average reflectivity of 92 ± 5% in the region of interest (1900–2380 cm–1). The mirrors used at UCSD were purchased from Universal Thin Film Lab Corp. with a reflectivity of 96% near 2000 cm–1. The cavity length can be tuned by adjusting the screws or panning the cell laterally, relying on inhomogeneity in the cavity length throughout the cell to provide different tunings. We measured the absorption spectra of DPPA in toluene under noncavity condition using a Fourier transform infrared spectrometer (FT/IR-4600, JASCO) with 2 cm–1 resolution.

It should be emphasized here that the pump–probe experiment at NAIST and the 2D-IR experiment at UCSD were conducted with cavity mirrors of different layer designs. The different cavity characteristics can affect the electric field distribution inside the cavity, Rabi splitting parameters, and temporal dynamics. We will discuss these issues in the following section.

Results

Linear Spectroscopy of DPPA

The linear spectra of ∼0.3 M DPPA in toluene solutions outside and inside of the cavity are shown in Figure 1b. The uncoupled solvent-subtracted absorption spectrum of DPPA shows a peak centered at ωv = 2170 cm–1 with a full width at half-maximum (fwhm) of 16 cm–1. This concentration gives an absorbance of 1 OD with a 12 μm spacer between two CaF2 plates and is low enough so that not all of the MIR beam component at ωv is absorbed by the sample, and a reasonable nonlinear signal can be measured in ultrafast measurements. When one of the Fabry-Pérot cavity modes with a frequency of ωc is tuned to be resonant with ωv, we observe a double-peaked transmittance spectrum corresponding to the LP and UP branches of the vibrational polariton as shown in Figure 1b (purple curve). This spectrum is measured with cavity mirrors prepared at NAIST. The Rabi splitting in this coupled spectrum is ∼33 cm–1. The strong coupling criteria are satisfied for all cavity experiments [i.e., the Rabi splitting is greater than the fwhm of the cavity (∼12 cm–1) and DPPA absorption (∼16 cm–1)].

Ultrafast Spectroscopy of DPPA under Vibrational Strong Coupling

Pump–Probe Spectroscopy of Uncoupled DPPA

Infrared transient absorption spectra of uncoupled DPPA (∼0.3 M) at three different delay timings are shown in Figure 2a. The pump and probe pulses are linearly polarized at the magic angle with respect to one another. This result provides a basis of comparison to the in-cavity cases. We point out some salient features and compare our measurements to those of other azide species. After excitation with the pump pulse, a negative transient absorption feature appears around 2170 cm–1, which is ascribed to decreased population of the vibrational ground state (ground-state bleach) and stimulated emission from the v = 1 excited state. A positive transient absorption feature near 2142 cm–1 is assigned to the anharmonically shifted v = 2 ← 1 excited-state absorption.

Figure 2.

Figure 2

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Transient absorption and kinetics of uncoupled DPPA. (a) Transient absorption spectra of DPPA in toluene measured at 0.2, 2.0, and 7.0 ps after infrared excitation. (b) Kinetic traces measured at the peak of the ground-state bleach (2170 cm–1) and first hot-band transition (2142 cm–1). Solid black lines are results of biexponential fits to the relaxation dynamics.

Time-resolved traces, plotted in Figure 2b, show the excited-state relaxation measured at 2142 cm–1 and recovery of the ground-state bleach at 2170 cm–1. Both curves are well-reproduced with a double exponential function given by

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The decay of v = 1 state population has a fast component τ1 = 0.17 ± 0.02 ps and a slow component τ2 = 4.55 ± 0.04 ps. The ground-state bleach recovers with a fast time constant τ1 = 0.25 ± 0.02 ps and a slow time constant τ2 = 4.36 ± 0.04 ps. The fast time components are on the order of the temporal width of the MIR pulse, and represents the instantaneous response of the system. Our results are reasonable within the context of previous studies of organic azides in nonaqueous solvents, which are known to have decay times on the order of a few picoseconds.37

Pump–Probe Spectroscopy of Cavity-Coupled DPPA

The nonlinear spectra of DPPA in a cavity are shown in Figure 3a for three different pump–probe delay timings. The linear transmission spectra for the same cavity are shown in Figure S4, with an estimated Rabi splitting of 28 cm–1. The difference in the Rabi splitting from the value of 33 cm–1, shown in Figure 1b, is due to the different cavity assemblies and manual tuning of the cavity length. We observe a positive–negative derivative-like feature with a strong positive peak around 2153 cm–1 together with a slight negative feature on the higher energy side near 2160 cm–1. We also observe a negative–positive derivative-like feature with a strong negative peak at 2180 cm–1 accompanied by a weak positive peak at 2190 cm–1. As shown in Figure 3a, no spectral diffusion nor spectral shift has been observed for DPPA within the measured time scale. Thus, we assume that the measurement at a single probe frequency component is sufficient for temporal decay analysis.

Figure 3.

Figure 3

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Transient spectroscopy and kinetics of cavity-coupled DPPA. (a) Transient absorption spectra of DPPA in a cavity measured 50 fs, 1 ps, and 2 ps after infrared excitation. (b) Kinetics of the decay of the transient response of cavity-coupled DPPA. The red trace is measurement of the transient response near the lower polariton at 2153 cm–1. The blue trace is measurement of the transient response near the upper polariton at 2180 cm–1. Solid black lines represent fits to single exponential decays. (c) Transfer matrix model (black) of the cavity-coupled system (detailed in the text) overlaying experimental data at 50 fs (blue).

Figure 3b shows the kinetic traces of the two main peaks at 2153 and 2180 cm–1 in Figure 3a. The absorptive feature at 2153 cm–1 exhibits oscillations at both negative and positive times around delay zero between the pump and probe pulses. Due to the small oscillatory amplitude at the short time delay, decomposing the oscillatory portion and monotonically decaying portion in the positive delay region gives a large error for the slow decay constant. Instead, we extract decay information after these early time oscillations have vanished: about 2 ps after excitation. The decay at 2153 cm–1 fit to a single exponential yields a time constant of τLP = 3.58 ± 0.07 ps. For the decay of the peak around UP, we fit the negative portion of the derivative feature at 2180 cm–1 with a single exponential curve. We find a time constant of τUP = 3.48 ± 0.16 ps, as shown in Figure 3b.

Next, we analyze the early time oscillatory structures in Figure 3b to confirm their origin. From the oscillations at ω = 2153 cm–1, we find a period of ∼1.4 ps which corresponds to a Rabi splitting of ∼23 cm–1 (Figure S4c). This value is comparable to the Rabi splitting of 28 cm–1 measured in the linear transmittance spectrum. The difference of ∼17% may be ascribed to the weak oscillatory structure in DPPA. In addition, we performed another set of measurements with different DPPA concentrations. We prepared samples with Rabi splitting values of 34 cm–1 (Figure S4d) by increasing the concentration of DPPA since the Rabi splitting is proportional to the square root of the sample density. In this case, analysis of the early time oscillation gives a beat frequency of 33 cm–1 (Figure S4f). The different periods observed for these two Rabi splittings confirm that these early time oscillatory structures correspond to coherent oscillation between the LP and UP states.

2D IR Spectroscopy of Cavity-Coupled DPPA

2D IR spectroscopy of DPPA in toluene (∼0.3 M) under VSC is performed using the ultrafast experimental setup at UCSD. The advantage of 2D IR is the ability to visualize the correlation between the pump frequency and the response at a specific probe frequency. 2D IR can also provide detailed information about interstate interactions and coherences unavailable with pump–probe spectroscopy. In this report, we focus on the system’s response for different pump frequencies corresponding to the LP and UP transitions. Figure 4a–c shows contour plots of the 2D IR spectra for cavity-coupled DPPA with Rabi splitting ΩR = 47 cm–1 taken at three different pump–probe delay times: 0.9, 3.9, and 6.9 ps, respectively. The different Rabi splitting from the pump–probe measurements at NAIST is ascribed to the different layer structures of the cavity mirror coatings. As has been discussed in exciton polaritons fabricated with metallic and DBR (distributed Bragg reflector) cavities, the penetration depth of the electric field for each cavity mode affects the Rabi splitting.1 It is reasonable to consider that the dielectric-mirror cavity with a different layer structure also results in different Rabi splitting values.

Figure 4.

Figure 4

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2D IR spectroscopy of cavity-coupled DPPA. Two-dimensional infrared spectra of the N3 stretching mode strongly coupled to a Fabry–Pérot mode of a cavity measured at (a) 0.9, (b) 3.9, and (c) 6.9 ps. Corresponding probe-only transmittance spectra are shown directly below the 2D IR spectra for each slice. (d–f) Correspond to spectral cuts in panels (a–c). (Blue) Cuts along the UP pump frequency. (Red) Cuts along the LP pump frequency.

Spectral cuts at pump frequencies resonant with the LP and UP transitions are shown in Figure 4d–f for each delay. The cut at the UP pump frequency of 0.9 ps (Figure 4d) shows the negative–positive derivative feature near the UP (∼2200 cm–1). These structures are more clearly seen in 2D IR measurements than in the pump–probe results shown in Figure 3a. Also, we were able to observe a weak negative feature (∼2155 cm–1) at the foot of the large absorptive feature at the LP pump frequency. These two features decay until the LP and UP spectral cuts resemble one another, as seen in the 6.9 ps time slice. The appearance of these features in the transient spectra and their time-dependent behavior is consistent with other 2D IR studies on metal complexes.24,30 It should be noted that the smaller structures (negative peak around the LP and positive peak around the UP) are more prominent when the corresponding LP or UP transitions are excited.

We note that off-diagonal features indicate interactions between distinct modes in traditional 2D IR spectroscopy of uncoupled molecules. However, this is not necessarily the case for cavity-coupled systems because the response spans a large portion of the probe frequency range. For example, in the context of this work, the off-diagonal features at ωpump = ωUP and ωprobe = ωLP can be ascribed to the excited population of the UP transferred to the first excited state of the dark modes, whose spectral peak is close to or overlapped with the LP transition. Thus, this cross peak cannot be directly interpreted as a nonlinear interaction between the LP and UP states.30

Time-dependent measurements of the features near the LP and UP after the early time oscillation decay are given in the Supporting Information (Figure S5). The results show that the large positive feature near the LP decays in ∼4.66 ps and the negative feature near the UP decays in ∼4.91 ps. Compared to the pump–probe results shown in Figure 3b, the lifetime is longer. This is related to the different coating characteristics of the cavity mirrors. Qualitatively, as described in the sample preparation, the cavity mirrors used at UCSD have a higher reflectivity than those used at NAIST, meaning a longer photon lifetime. This difference is reflected in the different decay lifetimes of the polaritonic states.

Discussion and Conclusions

Ultrafast spectroscopy of systems under VSC has been intensively studied in W(CO)6 and sodium nitroprusside (SNP).22−27,30 In these papers it has been argued that in addition to normal transient phenomena, such as excited-state absorption and ground-state bleach as shown in Figure 2a, polariton-specific phenomena (e.g., Rabi splitting contraction and polariton bleach) are important components of the transient spectra of polaritonic systems. These features have been discussed at length in previous works.22,32,41 Here we analyze our results by following similar concepts.

The Rabi splitting contraction is a phenomenon related to ground-state bleaching. The pump pulse promotes some fraction of the ground state population to excited states. As a result, the number of molecules occupying the vibrational ground state or, more precisely, the number of molecules that can couple with the cavity photon mode is decreased after the pump pulse irradiation. Since the Rabi splitting is proportional to the square root of the density of the cavity-coupled molecules, the Rabi splitting detected by the probe pulse is slightly smaller than in the static case. The result is a derivative line shape appearing around the LP and UP transition frequencies, as in Figure 3a. One conspicuous feature common to the spectra in Figures 3a and 4d–f is the large positive absorption around the LP transition, which destroys any symmetry that would exist between the structures around both the LP and UP. This absorptive feature is partly ascribed to coincidental overlap of the LP transition and the v = 2 ← 1 transition of DPPA in the dark state reservoir. In many chemical species, the anharmonic red shift of the v = 2 ← 1 transition is comparable to half of the Rabi splitting so that the two contributions overlap in the transient spectrum. Strictly speaking, the contribution from higher energy levels that includes both polaritonic states and dark states and their coupling with cavity photon modes is necessary for a complete assignment of the nonlinear spectrum, as has been discussed recently.29,42,43 Because of the overlap of the LP modes and the excited-state absorption, independent analysis of the decay dynamics of the LP branch is not straightforward. From the coherent oscillatory structure trace taken at 2153 cm–1, shown in Figure 3b, we can estimate that polariton decoherence occurs within ∼2 ps after excitation. In our experiments, the Rabi splitting is smaller than the thermal energy of ambient temperature. Therefore, it is reasonable to consider that the relaxation path from both LP and UP states is mediated by the dark states. If we assume that population in the LP and UP states transfers to the dark states on a time scale similar to the decay of the oscillatory component, the remaining slowly decaying part after 2 ps would represent decay of the dark state population. The time constants τLP ∼ 3.58 and τUP ∼ 3.48 ps are slightly faster than the noncavity slow decay constant of the ground- state bleach, τ2 ∼ 4.36 ps. This difference may be considered a direct consequence of VSC. In the future, we will study LP-specific decay dynamics utilizing a chemical species that shows larger Rabi splitting values >150 cm–1. In such systems, the behavior of the LP modes can be studied independent of v = 2 ← 1 excited-state absorptions. One example is the ionic liquid, EMIMDCA, for which we have reported a Rabi splitting of ∼180 cm–1 for the Inline graphic mode.9

To model the transient spectrum shown in Figure 3c, we estimate the fraction of molecules that are excited by the pump pulse by comparing the positions of the LP and UP peaks in the pump-off versus pump-on spectra. For the ∼0.3 M sample, we found a reduction of ∼1% in the Rabi splitting. Then, since ΩR ∝ N1/2, we estimate that ∼2% of the molecules are excited by the pump pulse. Following the classical model introduced by Dunkelberger et al.,22 we calculate the transient absorption using an analytical expression for a Fabry–Pérot cavity with an absorbing medium (details in the Supporting Information). We first fit the Lorentzian model expression for the dielectric function (eqs S2 and S3) to the transmission spectrum of the ground-state system with a single absorber with a frequency ν01 and an amplitude A01, which are chosen to match the experimentally measured Rabi splitting. Excited-state spectra are then calculated by reducing the ground state population and adding a second absorber with frequency ν12 and amplitude A12. The result of the model calculation is plotted in Figure 3c. As shown in previous research, these two nonlinear features (large positive absorption around LP and derivative-like feature around UP) are only reproduced when both ground state population is depleted and v = 1 population is increased. The model reproduces the experimental data well, including a 1.6% ground-state population reduction, which migrates to the vibrational excited state.

Another important phenomenon discussed in ultrafast spectroscopy of W(CO)6 vibrational polaritonic systems is polariton bleach.26 Polariton bleach is a phenomenon in which the transmission of the probe light at the LP and UP transition frequencies decreases by the occupation of those LP and UP levels because an incoming photon of the same energy cannot couple to the excited polaritonic states. This is reminiscent of the Coulomb blockade in the field of semiconductors.44 In our pump–probe signal (Figure 3a), however, such polariton bleach is not observed (i.e., the absorptive feature around the UP transition near delay zero timing is not apparent). On the other hand, there is a clear positive signal around the UP at 0.9 ps in the spectral cut at the LP frequency in the 2D IR data in Figure 4d (red curve). After a 3.9 ps delay, the structure around the UP has a derivative shape. The early time response measured at pump frequency 2150 cm–1 may be ascribed to the polariton bleach phenomenon. We have performed a transfer-matrix-based phenomenological model simulation to confirm the spectral difference between the pump–probe versus 2D IR signals.26 The details of the simulation are given in Supporting Information. As shown in Figure S6, our simulation suggests that the polariton bleach phenomenon is dependent on the reflectivity of the cavity mirror. The higher reflectivity of the cavity enables more efficient coupling of the vibration and photon mode; thus, the blockade phenomenon, for which higher occupation of the polaritonic state is mandatory, are observed only in the 2D IR data.

In summary, we have observed the ultrafast transient response of cavity-coupled diphenylphosphoryl azide molecule with mid-infrared pump–probe and 2D IR spectroscopies. Although the transient response decays much faster than W(CO)6, which is a standard sample in ultrafast vibrational polariton research, the main features of the vibrational polaritons—the Rabi splitting contraction, transient coherent oscillation of the polaritonic states—are observed. The temporal decay constants of the LP and UP components are observed to be slightly faster than the decay constants of the uncoupled DPPA molecules. The observed characteristics of polaritonic states, such as Rabi splitting and temporal decay constants, are affected by the different cavity mirror dielectric coatings. This result suggests that the formation of the vibrational strong coupling can be easily influenced by the cavity parameters and care must be taken with the design of cavity mirrors. In addition, vibrational polariton dynamics in low temperature environments where thermal activation energy is much less than the Rabi splitting will be of great importance to highlight the role of the coherent nature of the polariton states.

Acknowledgments

Pump–probe experiments were performed at NAIST and 2D IR experiments were performed at UCSD. The work at NAIST was supported by JSPS KAKENHI grant nos JP21H01890 and JP23K17904 and by the NAIST Fellowship for Materials Science. The work at UCSD was supported by the following: H.H.B. by DOE QIS #DE-SC0022134, G.D.W. by NSF CHE 2101988, and H.M. and W.X. by AFOSR MURI #FA9550-22-1-0317.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.3c07847.

  • Molecular and cavity mode line width measurements, transmission model to calculate population changes in-cavity-coupled data, determination of the frequency of early time oscillations in-cavity-coupled data, time-resolved measurements corresponding to 2D IR data, description of the filtering algorithm used to process the 2D IR spectra, and description of the simulation of the reflectivity dependence of the polariton bleach (PDF)

The authors declare no competing financial interest.

Supplementary Material

jp3c07847_si_001.pdf (2.4MB, pdf)

References

  1. Skolnick M. S.; Fisher T. A.; Whittaker D. M. Strong Coupling Phenomena in Quantum Microcavity Structures. Semicond. Sci. Technol. 1998, 13, 645–669. 10.1088/0268-1242/13/7/003. [DOI] [Google Scholar]
  2. Lidzey D. G.; Bradley D. D. C.; Skolnick M. S.; Virgili T.; Walker S.; Whittaker D. M. Strong Exciton–Photon Coupling in an Organic Semiconductor Microcavity. Nature 1998, 395, 53–55. 10.1038/25692. [DOI] [Google Scholar]
  3. Agranovich V.; Benisty H.; Weisbuch C. Organic and Inorganic Quantum Wells in a Microcavity: Frenkel-Wannier-Mott Excitons Hybridization and Energy Transformation. Solid State Commun. 1997, 102, 631–636. 10.1016/S0038-1098(96)00433-4. [DOI] [Google Scholar]
  4. Long J. P.; Simpkins B. S. Coherent Coupling between a Molecular Vibration and Fabry–Perot Optical Cavity to Give Hybridized States in the Strong Coupling Limit. ACS Photonics 2015, 2, 130–136. 10.1021/ph5003347. [DOI] [Google Scholar]
  5. Shalabney A.; George J.; Hutchison J.; Pupillo G.; Genet C.; Ebbesen T. W. Coherent Coupling of Molecular Resonators with a Microcavity Mode. Nat. Commun. 2015, 6, 5981. 10.1038/ncomms6981. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Cohn B.; Das K.; Basu A.; Chuntonov L. Infrared Open Cavities for Strong Vibrational Coupling. J. Phys. Chem. Lett. 2021, 12, 7060–7066. 10.1021/acs.jpclett.1c01438. [DOI] [PubMed] [Google Scholar]
  7. Hertzog M.; Munkhbat B.; Baranov D.; Shegai T.; Börjesson K.; Börjesson K. Enhancing Vibrational Light–Matter Coupling Strength beyond the Molecular Concentration Limit Using Plasmonic Arrays. Nano Lett. 2021, 21, 1320–1326. 10.1021/acs.nanolett.0c04014. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Hertzog M.; Rudquist P.; Hutchison J. A.; George J.; Ebbesen T. W.; Börjesson K. Voltage-Controlled Switching of Strong Light-Matter Interactions Using Liquid Crystals. Chem.—Eur. J. 2017, 23, 18166–18170. 10.1002/chem.201705461. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Yamada H.; Stemo G.; Katsuki H.; Yanagi H. Development of a Spacerless Flow-Cell Cavity for Vibrational Polaritons. J. Phys. Chem. B 2022, 126, 4689–4696. 10.1021/acs.jpcb.2c02752. [DOI] [PubMed] [Google Scholar]
  10. Thomas A.; George J.; Shalabney A.; Dryzhakov M.; Varma S. J.; Moran J.; Chervy T.; Zhong X.; Devaux E.; Genet C.; et al. Ground-State Chemical Reactivity under Vibrational Coupling to the Vacuum Electromagnetic Field. Angew. Chem., Int. Ed. 2016, 55, 11462–11466. 10.1002/anie.201605504. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Lather J.; Bhatt P.; Thomas A.; Ebbesen T. W.; George J. Cavity Catalysis by Cooperative Vibrational Strong Coupling of Reactant and Solvent Molecules. Angew. Chem., Int. Ed. 2019, 58, 10635–10638. 10.1002/anie.201905407. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Kohler B.; Krause J. L.; Raksi F.; Wilson K. R.; Yakovlev V. V.; Whitnell R. M.; Yan Y. Controlling the Future of Matter. Acc. Chem. Res. 1995, 28, 133–140. 10.1021/ar00051a006. [DOI] [Google Scholar]
  13. Herrera F.; Owrutsky J. Molecular Polaritons for Controlling Chemistry with Quantum Optics. J. Chem. Phys. 2020, 152, 100902. 10.1063/1.5136320. [DOI] [PubMed] [Google Scholar]
  14. Simpkins B. S.; Dunkelberger A. D.; Owrutsky J. C. Mode-Specific Chemistry through Vibrational Strong Coupling (or A Wish Come True). J. Phys. Chem. C 2021, 125, 19081–19087. 10.1021/acs.jpcc.1c05362. [DOI] [Google Scholar]
  15. Xiong W. Molecular Vibrational Polariton Dynamics: What Can Polaritons Do?. Acc. Chem. Res. 2023, 56, 776–786. 10.1021/acs.accounts.2c00796. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Thomas A.; Lethuillier-Karl L.; Nagarajan K.; Vergauwe R. M. A.; George J.; Chervy T.; Shalabney A.; Devaux E.; Genet C.; Moran J.; et al. Tilting a Ground-State Reactivity Landscape by Vibrational Strong Coupling. Science 2019, 363, 615–619. 10.1126/science.aau7742. [DOI] [PubMed] [Google Scholar]
  17. Sau A.; Nagarajan K.; Patrahau B.; Lethuillier-Karl L.; Vergauwe R. M. A.; Thomas A.; Moran J.; Genet C.; Ebbesen T. W. Modifying Woodward–Hoffmann Stereoselectivity Under Vibrational Strong Coupling. Angew. Chem., Int. Ed. 2021, 60, 5712–5717. 10.1002/anie.202013465. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Fukushima T.; Yoshimitsu S.; Murakoshi K. Inherent Promotion of Ionic Conductivity via Collective Vibrational Strong Coupling of Water with the Vacuum Electromagnetic Field. J. Am. Chem. Soc. 2022, 144, 12177–12183. 10.1021/jacs.2c02991. [DOI] [PubMed] [Google Scholar]
  19. Zhong C.; Hou S.; Zhao X.; Bai J.; Wang Z.; Gao F.; Guo J.; Zhang F. Driving DNA Origami Coassembling by Vibrational Strong Coupling in the Dark. ACS Photonics 2023, 10, 1618–1623. 10.1021/acsphotonics.3c00235. [DOI] [Google Scholar]
  20. Hirai K.; Takeda R.; Hutchison J. A.; Uji-i H. Modulation of Prins Cyclization by Vibrational Strong Coupling. Angew. Chem., Int. Ed. 2020, 59, 5332–5335. 10.1002/anie.201915632. [DOI] [PubMed] [Google Scholar]
  21. Ahn W.; Triana J. F.; Recabal F.; Herrera F.; Simpkins B. S. Modification of Ground-State Chemical Reactivity via Light–Matter Coherence in Infrared Cavities. Science 2023, 380, 1165–1168. 10.1126/science.ade7147. [DOI] [PubMed] [Google Scholar]
  22. Dunkelberger A. D.; Spann B. T.; Fears K. P.; Simpkins B. S.; Owrutsky J. C. Modified Relaxation Dynamics and Coherent Energy Exchange in Coupled Vibration-Cavity Polaritons. Nat. Commun. 2016, 7, 13504. 10.1038/ncomms13504. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Dunkelberger A. D.; Davidson R. B.; Ahn W.; Simpkins B. S.; Owrutsky J. C. Ultrafast Transmission Modulation and Recovery via Vibrational Strong Coupling. J. Phys. Chem. A 2018, 122, 965–971. 10.1021/acs.jpca.7b10299. [DOI] [PubMed] [Google Scholar]
  24. Xiang B.; Ribeiro R. F.; Dunkelberger A. D.; Wang J.; Li Y.; Simpkins B. S.; Owrutsky J. C.; Yuen-Zhou J.; Xiong W. Two-Dimensional Infrared Spectroscopy of Vibrational Polaritons. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 4845–4850. 10.1073/pnas.1722063115. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Xiang B.; Ribeiro R. F.; Chen L.; Wang J.; Du M.; Yuen-Zhou J.; Xiong W. State-Selective Polariton to Dark State Relaxation Dynamics. J. Phys. Chem. A 2019, 123, 5918–5927. 10.1021/acs.jpca.9b04601. [DOI] [PubMed] [Google Scholar]
  26. Xiang B.; Ribeiro R. F.; Li Y.; Dunkelberger A. D.; Simpkins B. B.; Yuen-Zhou J.; Xiong W. Manipulating Optical Nonlinearities of Molecular Polaritons by Delocalization. Sci. Adv. 2019, 5, eaax5196 10.1126/sciadv.aax5196. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Dunkelberger A. D.; Grafton A. B.; Vurgaftman I.; Soykal Ö. O.; Reinecke T. L.; Davidson R. B.; Simpkins B. S.; Owrutsky J. C. Saturable Absorption in Solution-Phase and Cavity-Coupled Tungsten Hexacarbonyl. ACS Photonics 2019, 6, 2719–2725. 10.1021/acsphotonics.9b00703. [DOI] [Google Scholar]
  28. Yang Z.; Xiang B.; Xiong W. Controlling Quantum Pathways in Molecular Vibrational Polaritons. ACS Photonics 2020, 7, 919–924. 10.1021/acsphotonics.0c00148. [DOI] [Google Scholar]
  29. Duan R.; Mastron J. N.; Song Y.; Kubarych K. J. Isolating Polaritonic 2D-IR Transmission Spectra. J. Phys. Chem. Lett. 2021, 12, 11406–11414. 10.1021/acs.jpclett.1c03198. [DOI] [PubMed] [Google Scholar]
  30. Grafton A. B.; Dunkelberger A. D.; Simpkins B. S.; Triana J. F.; Hernández F. J.; Herrera F.; Owrutsky J. C. Excited-State Vibration-Polariton Transitions and Dynamics in Nitroprusside. Nat. Commun. 2021, 12, 214. 10.1038/s41467-020-20535-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Xiang B.; Ribeiro R. F.; Du M.; Chen L.; Yang Z.; Wang J.; Yuen-Zhou J.; Xiong W. Intermolecular Vibrational Energy Transfer Enabled by Microcavity Strong Light–Matter Coupling. Science 2020, 368, 665–667. 10.1126/science.aba3544. [DOI] [PubMed] [Google Scholar]
  32. Xiang B.; Wang J.; Yang Z.; Xiong W. Nonlinear Infrared Polaritonic Interaction between Cavities Mediated by Molecular Vibrations at Ultrafast Time Scale. Sci. Adv. 2021, 7, eabf6397 10.1126/sciadv.abf6397. [DOI] [PMC free article] [PubMed] [Google Scholar]
  33. George J.; Shalabney A.; Hutchison J. A.; Genet C.; Ebbesen T. W. Liquid-Phase Vibrational Strong Coupling. J. Phys. Chem. Lett. 2015, 6, 1027–1031. 10.1021/acs.jpclett.5b00204. [DOI] [PubMed] [Google Scholar]
  34. Zhong Q.; Baronavski A. P.; Owrutsky J. C. Vibrational Energy Relaxation of Aqueous Azide Ion Confined in Reverse Micelles. J. Chem. Phys. 2003, 118, 7074–7080. 10.1063/1.1562608. [DOI] [Google Scholar]
  35. Sando G. M.; Dahl K.; Owrutsky J. C. Vibrational Relaxation Dynamics of Azide in Ionic and Nonionic Reverse Micelles. J. Phys. Chem. A 2004, 108, 11209–11217. 10.1021/jp0463363. [DOI] [Google Scholar]
  36. Sando G. M.; Dahl K.; Owrutsky J. C. Vibrational Spectroscopy and Dynamics of Azide Ion in Ionic Liquid and Dimethyl Sulfoxide Water Mixtures. J. Phys. Chem. B 2007, 111, 4901–4909. 10.1021/jp067143d. [DOI] [PubMed] [Google Scholar]
  37. Chalyavi F.; Schmitz A. J.; Fetto N. R.; Tucker M. J.; Brewer S. H.; Fenlon E. E. Extending the Vibrational Lifetime of Azides with Heavy Atoms. Phys. Chem. Chem. Phys. 2020, 22, 18007–18013. 10.1039/D0CP02814B. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. Imperatore M. V.; Asbury J. B.; Giebink N. C. Reproducibility of Cavity-Enhanced Chemical Reaction Rates in the Vibrational Strong Coupling Regime. J. Chem. Phys. 2021, 154, 191103. 10.1063/5.0046307. [DOI] [PubMed] [Google Scholar]
  39. Wiesehan G. D.; Xiong W. Negligible Rate Enhancement from Reported Cooperative Vibrational Strong Coupling Catalysis. J. Chem. Phys. 2021, 155, 241103. 10.1063/5.0077549. [DOI] [PubMed] [Google Scholar]
  40. Thomas P. A.; Tan W. J.; Kravets V. G.; Grigorenko A. N.; Barnes W. L. Non-Polaritonic Effects in Cavity-Modified Photochemistry. Adv. Mater. 2023, 36, 2309393. 10.1002/adma.202309393. [DOI] [PubMed] [Google Scholar]
  41. Dunkelberger A. D.; Simpkins B. S.; Vurgaftman I.; Owrutsky J. C. Vibration-Cavity Polariton Chemistry and Dynamics. Annu. Rev. Phys. Chem. 2022, 73, 429–451. 10.1146/annurev-physchem-082620-014627. [DOI] [PubMed] [Google Scholar]
  42. Simpkins B. S.; Yang Z.; Dunkelberger A. D.; Vurgaftman I.; Owrutsky J. C.; Xiong W. Comment on “Isolating Polaritonic 2D-IR Transmission Spectra. J. Phys. Chem. Lett. 2023, 14, 983–988. 10.1021/acs.jpclett.2c01264. [DOI] [PMC free article] [PubMed] [Google Scholar]
  43. Duan R.; Mastron J. N.; Song Y.; Kubarych K. J. Reply to “Comment on: ‘Isolating Vibrational Polariton 2D-IR Transmission Spectra. J. Phys. Chem. Lett. 2023, 14, 1046–1051. 10.1021/acs.jpclett.2c02823. [DOI] [PubMed] [Google Scholar]
  44. Khitrova G.; Gibbs H. M.; Jahnke F.; Kira M.; Koch S. W. Nonlinear Optics of Normal-Mode-Coupling Semiconductor Microcavities. Rev. Mod. Phys. 1999, 71, 1591–1639. 10.1103/RevModPhys.71.1591. [DOI] [Google Scholar]

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