Ultrafast Transmission Modulation and Recovery via Vibrational

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Ultrafast Transmission Modulation and Recovery via Vibrational Strong Coupling Adam D Dunkelberger, Roderick Davidson, Wonmi Ahn, Blake S. Simpkins, and Jeffrey C. Owrutsky J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10299 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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The Journal of Physical Chemistry

Ultrafast Transmission Modulation and Recovery via Vibrational Strong Coupling Adam D. Dunkelberger, Roderick Davidson,† Wonmi Ahn,† Blake S. Simpkins, and Jeffrey C. Owrutsky* Chemistry Division, U.S. Naval Research Laboratory, 4555 Overlook Ave SW, Washington, DC 20375 †NRC RAP Associate *[email protected] ABSTRACT Strong coupling between vibrational modes and cavity optical modes leads to the formation of vibrationcavity polaritons, separated by the vacuum Rabi splitting. The splitting depends on the square root of the concentration of absorbers confined in the cavity, which has important implications on the response of the coupled system after ultrafast infrared excitation. In this work, we report on solutions of W(CO)6 in hexane with concentration chosen to access a regime that borders on weak coupling. Under these conditions, large fractions of the W(CO)6 oscillators can be excited, and the anharmonicity of the molecules leads to a commensurate reduction in the Rabi splitting. We report excitation fractions >0.4, depending on excitation pulse intensity, and show drastic increases in transmission that can be modulated on the picosecond timescale. In comparison to previous experiments, the transient spectra we observe are much simpler because excited-state transitions lie outside the transmission spectrum of the cavity, thereby contributing only weakly to the spectra. We find that the Rabi splitting recovers with the characteristic vibrational relaxation lifetime and anisotropy decay of uncoupled W(CO)6, implying that polaritons are not directly involved in the relaxation after the first few ps. The results help corroborate the model we proposed to describe the results at higher concentrations and show that the ground-state bleach of cavitycoupled molecules has a broad, multi-signed spectral response.

1. Introduction Optical modes such as Fabry-Perot cavity modes or propagating plasmon polaritons can couple to quantum transitions in a nearby material, provided the modes are close to resonant and have similar decay

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rates. If the splitting induced by such coupling is larger than the linewidth of either mode, the system is in the strong coupling regime, where two new polariton modes are created that are separated by the so-called Rabi splitting and with each possessing partial character of the optical mode and the material excitation.1-3 Strong coupling has been extensively studied for electronic transitions in systems like strongly absorbing dye molecules and quantum dots,4-11 but only recently was vibrational strong coupling demonstrated.12-15 Strong coupling provides an experimental method of systematically modifying the energetics of particular vibrational modes, making it an attractive tool in the pursuit of tailored chemical reactivity. Fabry-Perot cavities with easily achievable, macroscopic cavity lengths (2-100 µm) have optical modes that overlap many strong vibrational absorbers. Since 2015, strong coupling between cavity optical modes and vibrational modes has been shown for polymers, neat liquids, and compounds in solution,12-20 and considerable theoretical work has also been dedicated to vibration-cavity polaritons.21-27 Ebbesen and coworkers have shown that coupling cavity modes to vibrational modes involving reactive bonds can drastically modify the reactivity of a system, but the mechanism of this action remains unclear.28 Ultrafast laser techniques offer an important window into the microscopic details of vibrational polariton dynamics and could shed light on how polaritons modify vibrational relaxation phenomena and chemical reactions. Previously, we examined the relaxation dynamics of the triply degenerate C-O stretching band of W(CO)6 coupled to a cavity mode with infrared pump-probe spectroscopy.29 We found that, in large part, the transient spectra are dominated by the response of a reservoir of uncoupled excited vibrational states but the results also demonstrated systematically modified relaxation from one of the polariton levels. These findings were complicated by the fact that the anharmonic shift of the mode (15 cm-1) nearly matches half the Rabi splitting (10-20 cm-1) for concentrations that place the system firmly in the strong coupling regime (>10 mM). This inadvertent coincidence leads to severe spectral congestion where the hot-band and polariton transitions lie. In this work, we report on the dynamics of W(CO)6 at the edge of the strong-coupling regime by using comparatively lower (10x) concentrations of W(CO)6, reducing the vacuum Rabi splitting to ~8 2 ACS Paragon Plus Environment

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cm-1. Exciting the system with infrared pulses strongly modulates the infrared transmission of the system by effectively switching between the strong- and weak-coupling regimes. The polariton position is separated from the reservoir hot band, thus substantially reducing the influence of excited-state transitions from the transient spectra. This greatly simplifies the response and allows us to more clearly observe and characterize the effects of the ground-state bleach. The spectral signature and temporal evolution of this response are in excellent agreement to results from simple analytical calculations and, in large part, corroborates our proposed model.29 2. Experimental and Theoretical Methods 2.1. Cavity Construction We couple the triply degenerate asymmetric C-O stretching band of W(CO)6 in hexane (70 000 M-1 cm-1, ω01 = 1983 cm-1, FWHM = 3 cm-1) to an optical cavity mode by filling a 25 µm-long FabryPerot cavity with a 1 mM solution of W(CO)6. The cavity is constructed from dielectric mirrors separated by a 25 µm PTFE spacer and mounted in a demountable liquid cell (Harrick). Each mirror is a CaF2 window coated with a proprietary dielectric (Universal Thin Film Lab) specified to give 92% reflection from 4.7 to 5.1 µm. The empty cavity has modes separated by ~140 cm-1 with a full width at half maximum (FWHM) of ~6 cm-1. 2.2. Time-Resolved Infrared Spectroscopy The ultrafast laser instrument has been described in greater detail previously.29-30 Briefly, a regenerative amplifier supplies 800 nm pulses at 1 kHz which are converted to the mid-infrared via optical parametric amplification and subsequent difference frequency generation. A CaF2 window reflects a small portion of the infrared pulse, which serves as the infrared probe, to a computer-controlled delay stage and then to the sample. The remainder of the infrared pulse serves to excite the sample. We take special care to ensure the probe and excitation pulses interact with the sample at the same angle relative to the surface normal of the cavity, thus ensuring that the two pulses interact with the same angle-dependent



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polariton spectrum. The ~150 cm-1 bandwidth of the infrared pulses spans the entirety of the polariton spectrum. The Rabi splitting we measure with the infrared probe pulse without excitation matches the splitting we measure with an FTIR instrument. The instrument used in this work differs from that previously reported in its detection system. Rather than single-wavelength detection with a monochromator and single-channel detector, we now use a 128x128 pixel mercury cadmium telluride (MCT) array31 (Phasetech, Inc.) affixed to a spectrograph (Princeton Instruments). The MCT array collects the spectrum of each infrared pulse and is synchronized to an optical chopper in the excitation path that blocks every other pump pulse. LabVIEW software controls the delay stage and MCT array. 2.3. Calculating Transmission through a Fabry-Perot Cavity The following analytical description essentially summarizes that given in Dunkelberger, et al.29 We calculate the transmission through the cavity containing W(CO)6, using the well-established classical expression given by Equation 1:

(̅ ) =

(1)

) (

In this expression, T and R are the transmissivity and reflectivity of the mirrors, taken to be 8% and 92% across the wavelength range of interest, L is the length of the cavity,

accounts for a phase shift as light

reflects from the mirror, and n and α are the frequency-dependent index of refraction and absorption per unit length of the material within the cavity. To generate n and α, we first describe the dielectric function of W(CO)6 in hexane solution in terms of a background real index nbg and sum of two Lorentzian oscillators given by the equations:

! = "#$ + (

&'( ('( )

'(

) ()

'( )

+ (

&( (( )

(

) ()


( )

(S2)

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! = (

&'( ('( )

'(

) ()

+ (

'( )

&( (( )

(

) ()

( )

(S3)

Here, we choose * = 1983 cm-1, = 1968 cm-1, and Γ* = Γ = 6 cm-1 to match values for the C-O stretch in W(CO)6. We vary the amplitude terms A, (A01 = N0 • µ01 ; A12 = N1 • µ12) which depend on the transition intensity and population of the absorber, to account for depopulation of the ground state and population of the first excited state. With the dielectric constants in hand, we use the following expressions to calculate n and α:

"=+

,( -,( ,

(S4)

,( -,( ,

(S5)

. = 401 = 40+

Finally, we calculate transmission spectra for the unpumped (N0 = 1, N1 = 0) and excited systems (i.e., population moved from N0 to N1), subtract the two, and normalize by the ground state transmission to arrive at calculated transient spectra. 3. Results and Discussion The concentration used and absorption strength of the C-O oscillators lead to an 8 cm-1 separation between the upper (UP) and lower (LP) polaritons, schematically depicted in Figure 1a. The two polariton modes appear as transmission features in the infrared spectrum of the cavity (Figure 1c, blue) that bracket the v = 0 to 1 transition of the uncoupled W(CO)6 (Figure 1c, red). In our previous ultrafast study, we found that the transient response of the system after infrared excitation was described, in part, by derivative-like features at the polariton frequencies due to ground-state bleach-induced reduction of the vacuum Rabi splitting. However, these features were convoluted with excited-state absorptions (ESA) from both reservoir v = 1 and polariton modes bracketing the v = 1 level.29 Our previous results did not show evidence of “ladder climbing” in the v = 2 manifold,32 (i.e., transitions to new states corresponding 5 ACS Paragon Plus Environment


to double excitations of the polaritons that have been demonstrated for excitonic systems), which we attribute to our use of only modest excitation intensities. However, the previously observed ESA did shift to higher frequencies when the ground-state polaritons were tuned to lower frequencies, helping to confirm our assignment of polaritonic transitions.

` Figure 1. a) Schematic energy level diagram depicting coupling between the infrared active C-O stretch of W(CO)6 and the cavity optical mode to yield the upper polariton (UP) and lower polariton (LP) mode. b) Schematic energy level diagram depicting the pump and probe pulse interactions with the cavitycoupled system, following the model from Dunkelberger, et al. The pump pulse interacts with the polaritons and reservoir v = 1, reducing the Rabi splitting observed by the probe pulse (blue dashed lines) and enabling red-shifted excited-state absorptions (red dashed lines). c) Absorbance spectrum of 1 mM 6 ACS Paragon Plus Environment

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W(CO)6 in hexane in CaF2 liquid cell (red) and transmission spectrum of the same solution in a 25 µm Fabry-Perot cavity (blue). The dashed lines represent the excited-state absorptions we predict for the cavity-coupled system, as depicted in b. d) Transient difference spectrum of W(CO)6 in hexane in a CaF2 liquid cell 10 ps after infrared excitation (red), showing the characteristic ground-state bleach at 1983 cm1 and excited-state absorption at 1968 cm-1. The transmission of the cavity-coupled system is shaded in grey and the predicted excited-state absorption frequencies of the cavity-coupled system are depicted, again, as red dashed lines.

The lower solution concentrations used in the present study result in the ESA transitions lying outside the transmission spectrum of the coupled system, thus protecting us from their contamination of the ground-state bleach induced effects. We show the predicted ESA frequencies as dashed lines in Figures 1c and 1d, with colors corresponding to those same transitions in the schematic, Figure 1b. This concept is further illustrated by the transient response of W(CO)6 in a CaF2 cell, i.e. without coupling (Figure 1d, red trace), which shows a ground-state bleach at 1983 cm-1 and excited-state absorptions at 1968 cm-1 and 1953 cm-1, in excellent agreement with literature reports.33-36 A representative transmission spectrum of a cavity-coupled sample used in this work is shown in grey, showing that the cavity-coupled samples produce little or no transmission at the ESA frequencies. Without the presence of ESA transitions, our model predicts that the transient response should be dominated by a contraction of the Rabi splitting as the ground-state concentration is reduced due to optical excitation. The vacuum Rabi splitting depends on the square root of the concentration of the absorptive material. Promoting some fraction of W(CO)6 molecules to higher-lying vibrational states effectively negates their contribution to the cavity coupling since these excited molecules no longer absorb at the v = 0 to 1 transition frequency to which the cavity is tuned. It is important to note that this effect is made possible by the nuclear anharmonicity of the molecular potential. Excited states of a harmonic system absorb at the same frequency as the ground state which would not result in a Rabi splitting contraction. As the separation between polariton bands contracts, we predict increased transmission between the two bands and reduced transmission on either edge.



Figure 2 shows the transient response of the cavity-coupled system after infrared excitation, with Figure 2a serving as an overview of the response. To obtain these representative results, we significantly attenuated the energy of the excitation pulses (2 µJ) and used the magic angle (54.7 °) polarization configuration to eliminate effects of rotational diffusion and dipolar exchange. Figure 2a shows the evolution of the transmission of the cavity as a function of the time delay between excitation and probe pulses. Before the pump interacts with the sample, the two polariton modes appear at 1979.0 cm-1 and 1987.5 cm-1. After excitation, the splitting between the modes is reduced and then gradually recovers over the course of ~100s of ps.

Figure 2. a) Transmission through the cavity-coupled system as a function of probe wavelength and time delay between the excitation and probe pulses. The color scale represents the intensity of light at the detector. Black circles are the centers of the polariton modes as determined by fits to Lorentzian lineshapes. b) Transmission through the cavity-coupled system measured before excitation (blue points) and 10 ps after excitation (red points). Dashed lines are fits to a pair of Lorentzian peaks. c) Experimental transient difference spectrum measured 10 ps after excitation (red) and calculated difference spectra (blue). One calculation includes only a reduction of ground-state population (solid blue), while the other includes excited-state absorption from v = 1 to 2 (dashed blue).


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The reduced splitting induced by the excitation pulse allows us to easily determine the fraction of molecules excited. Figure 2b shows transmission spectra measured before excitation (blue) and 10 ps after excitation (red). We fit the spectra to two Lorentzian features to extract peak positions for the upper and lower polaritons and find, in this case, a 15% reduction in the Rabi splitting (Ω). Since Ω ~ N1/2, we calculate the square of the reduction of the splitting, (1-0.852) ~ 28%, to estimate the fraction of molecules excited to v = 1. Stimulated emission and the ground-state bleach act in concert to reduce the 0 to 1 absorption intensity,37 resulting in zero absorption at 50% excitation, and so we surmise that the actual excitation fraction is 14%. This degree of excitation yields the transient transmission spectrum shown in Figure 2c (red) corresponding to a 150% increase in transmission at 1983 cm-1 and concurrent decreases at either edge of the polariton spectrum. The proportion of molecules excited in this case far exceeds the one or two percent excitation we previously reported for higher concentrations of W(CO)6. We surmise that these lower concentrations give Rabi splittings close enough to the weak coupling regime to allow multipass absorption at 1983 cm-1. Based on a mirror reflectivity of 92%, we estimate that the effective cavity length is increased over an order of magnitude (2 33 ≈

√

≈ 12).38 As we have

shown previously, path length does not affect the Rabi splitting.13 We consider the implications of this efficient excitation below, but first turn to a more detailed analysis of the transient response. As described in our previous work, we use the analytical expression for transmission through a filled Fabry-Perot cavity to calculate transient spectra for comparison to the experimental results. In this case, we take advantage of the measured degree of excitation to guide the calculation. The blue traces in Figure 2c are spectra calculated for a 28% decrease in ground state population with either no v = 1 population (solid) or commensurate filling of v = 1 (dashed). Both calculated spectra qualitatively match the experimental spectrum, but the intensity of the calculated response is about one third the measured values. The calculation is highly sensitive to the width of the cavity mode, dictated by the reflectivity and transmissivity of the mirrors, and we suspect that the discrepancy in signal magnitude arises from imperfect modeling of this width. The calculated spectrum that includes v = 1 population more accurately 9 ACS Paragon Plus Environment


captures the asymmetry in the measured response, i.e. the larger negative response on the lower polariton branch, albeit while overestimating the contribution from the excited-state absorption. We now turn to the time evolution of the transient response of the cavity-coupled system. Kinetic cuts at specific probe frequencies, two of which are shown in Figure 3a, reveal considerably simpler dynamics than observed in the previously reported higher-concentration case.29 The results are presented as the quantity -log(T/T0), which depends linearly on absorber concentration in traditional transient absorption experiments. When probed at 1982 cm-1 (Fig 3a, blue), the excitation-induced transmission decays exponentially with a lifetime of 140 ps, in excellent agreement with literature reports for the W(CO)6 in hexane v = 1 lifetime.34, 39 However, the kinetics can appear markedly different at other probe frequencies. For instance, the decay of the response at 1977 cm-1 (Fig 3a, red circles) fits well to a 180 ps decay. Further evaluation of this variation is seen in Figure 3b which shows the exponential decay lifetime of the response plotted against the probe frequency at which the decay was fit. The extracted lifetime can vary by as much as 30% from the 140 ps value one would expect if reservoir v = 1 dynamics truly dominate the response of the system. This large variation most likely arises because the UP and LP features shift in frequency as population decays from v = 1 to v = 0.


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Figure 3. a) Evolution of the transient response of cavity-coupled W(CO)6 probed at 1982 cm-1 (blue circles) and 1977 cm-1 (red circles), after excitation with 2 µJ. The probe is polarized at the magic angle relative to the pump. Transients are reported in terms of –log(T/T0), where T is the transmission measured with the pump pulse present and T0 is the transmission without the pump pulse. Solid lines are fits to the data comprising single exponential decays. b) Exponential decay lifetimes, τ, as a function of probe wavelength. The error bars represent the 10% uncertainty we estimate from averaging multiple scans across several samples. The transmission spectrum of the sample is superimposed on the data in grey. c) Temporal evolution of Ω2, the square of the separation between the UP and LP modes as determined by fitting the pump-on transmission spectrum to two Lorentzian peaks (red) and temporal evolution of A, the amplitude of the ground-state absorption Lorentzian in the calculated transmission (blue). The inset shows the anisotropy decay of Ω2, obtained as described in the main text.

As demonstrated above, simple exponential fits of transient recoveries measured at a single frequency can give misleading results due to the spectral evolution of cavity-coupled spectra. Here, we 11 ACS Paragon Plus Environment


employ two approaches that provide a more accurate analysis of the time evolution. The first relates the time-dependent polariton separation to the ground state population and the second fits each transient spectrum to a population-dependent function describing the transmission through a Fabry-Perot cavity. If we neglect the contribution of excited-state absorption and assume zero detuning between the optical and vibrational modes, the difference between the center frequencies of the polariton modes, extracted from a two-Lorentzian fit, is the vacuum Rabi splitting, Ω. Again, since Ω ~ N1/2, Ω2 should be directly proportional to the concentration of v = 0 molecules. Each spectrum of Figure 2a were fit to a twoLorentzian function, yielding Ω. This value is squared and plotted against the delay between excitation and probe (red points) in Figure 3c. We find clear evidence for a biexponential decay, with a fast decay lifetime of about 2 ps, discussed in more detail below, and a slow decay lifetime of 140 ps. This type of analysis is dramatically more difficult for the higher concentration experiments reported previously, due to the influence of ESA, but reinforces that ground state bleach and recovery should be present, and depending on system details, may even dominate the transient response. Although this treatment reveals a ground-state recovery time, 140 ps, in excellent agreement with that known for the free molecule, the general efficacy of this approach is suspect since the assumption that Ω ~ N1/2 is strictly only applicable for the strong coupling regime. In the weak coupling regime, (Ω ≤ Γ, where Γ is the oscillator or cavity linewidth), this scaling may not hold. Instead, we now fit each transient spectrum to that predicted from Equation 1and include the ground state population by way of a single Lorentzian oscillator in the dielectric. The amplitude, A, is proportional to ground state population and acts as a fitting parameter. We plot the value of A extracted at each time delay (Fig. 3c, blue) and again find clearly biexponential kinetics with characteristic times of 3 and 145 ps, in excellent agreement with the v = 1 lifetime of the bare molecule and the above analysis based purely upon Rabi splitting recovery. That the slow decay lifetime is ~140 ps reinforces our interpretation that the contraction and subsequent recovery of the Rabi splitting depend, primarily, on the excitation and relaxation of reservoir v 12 ACS Paragon Plus Environment

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= 1 molecules. As opposed to previous work with larger Rabi splittings, a considerable amount of light is transmitted at ω01 = 1983 cm-1, allowing strong absorption at that frequency. We speculate that this direct excitation of reservoir molecules, in addition to the more standard excitation pathways through which light excites the UP and LP and population quickly transfers to the reservoir,23 explains why the uncoupled kinetics appear in the cavity-coupled experiment. Multidimensional spectroscopic techniques may be well suited to further address this issue. In our initial work, we found no evidence for rotational anisotropy in the response of the cavitycoupled system. At lower concentrations, however, we see a strong polarization dependence in the response. To quantify the rotational reorientation time, we measure Ω2 as a function of time with both parallel and perpendicular relative polarization between the excitation and probe pulses. To measure the time it takes for the initial distribution of excited molecules to randomize its orientation, we use the standard form for rotational anisotropy, given by (Ω2∥ -Ω2⊥ )/(Ω2∥ +2Ω2⊥ ),30 and find an exponential decay that fits well to a 4 ps lifetime, again in good agreement with literature reports on the uncoupled mode in W(CO)6.40 Again, we speculate that direct excitation of the reservoir v = 0 to 1 transition leads to the recovery of the uncoupled anisotropy decay. At higher concentrations where such excitation is less favorable, the dynamics of the polaritons could become more important. In the Ω2 decay and some of the –log(T/T0) decays, we observe the 2 ps decay component mentioned above. The origin of this decay is not completely clear, but the fact that it can be measured in the Ω2 recovery as well as the ground state recovery suggests it is a process that involves population relaxation rather than spectral changes. We speculate that this fast decay could be from optical loss of population that is enabled by the mixed optical-vibrational character of the polariton modes, i.e. some of the molecules are excited to the LP and UP states and can very quickly relax as photons leaking from the cavity. This is a consequence of the mixed vibration-photon nature of the polariton. Del Pino, et al., have calculated that much of the polariton population decays through dephasing, with only a small portion reaching the reservoir modes.23 The 2 ps lifetime is consistent with the presumably homogenous width of 13 ACS Paragon Plus Environment


the polariton modes, from which we anticipate a dephasing lifetime of 2-3 ps. This case may be different from that studied by del Pino, et al., in that the population lifetime of the reservoir (140 ps) is much longer than the dephasing of the polaritons.23 In summary, the kinetics of the recovery of the polariton splitting indicate that the relaxation of this system is dominated by relaxation of reservoir v = 1-excited molecules. The strong transmission modulation of the system after excitation could potentially be useful for photonic switching applications in the infrared, in analogy to proof-of-concept results shown for hybrid exciton systems.5,41 To show the limits of the modulation, we examine transient spectra measured with the excitation and probe pulses parallel to one another and coincident in time. Under these conditions, shown in Figure 4a, it is possible to induce a nearly total coalescence of the two polariton modes with easily accessible pulse intensities. The coalescence induces a six-fold increase in the transmission of the cavity at the uncoupled oscillator frequency as the Rabi splitting is drastically reduced by the depopulation of the ground state. The large excitation fractions we observe are surprising in light of the very small fractions observed when working with higher molecular concentrations, and subsequently, larger Rabi splittings. We suspect enhanced excitation is made possible because the small Rabi splitting allows light to enter the cavity at the reservoir v = 0 to 1 transition frequency (see the non-zero transmission at ω01 in Figure 4a) and the cavity-enhanced effective pathlength.


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Figure 4. a) Transmission spectra of 1 mM W(CO)6 coupled to a Fabry Perot cavity without infrared excitation (black) and with concurrent excitation by a 1.5 µJ (red) and 3 µJ (blue) infrared pulse polarized parallel to the probe pulse. b) Fractional reduction of the square of the Rabi splitting (1 – (Ωpumped2 / Ωunpumped2) as a function of pump pulse energy (open circles). Solid line is a linear fit intended to demonstrate linearity. c) Maximum (T-T0)/T response recorded as a function of pump pulse energy. Solid line is a linear fit intended to demonstrate linearity.

By fitting the polariton modes to Lorentzian peaks, just as described above, we can measure the reduction in Ω2 and directly correlate this reduction to the fraction of oscillators excited. At the highest pulse energy we used, 3 µJ (approximately 0.25 µJ cm-2), we measure an 85% reduction in Ω2, i.e. an 85% reduction of the total intensity of the 0 to 1 transition that contributes to the Rabi splitting. This reduction corresponds to a 42.5% excitation fraction in the simple case of a two-level system, but since the value is extracted from early-time spectra measured before the polaritons decay, this may be an overly simplistic estimate. A full quantum-mechanical treatment of the early-time excitation and relaxation dynamics is required to quantitatively determine the role of stimulated emission from the polariton modes.



A large portion of the Rabi-splitting reduction rapidly decays with the 2 ps component described above and the 4 ps anisotropy decay of the W(CO)6, while the rest decays with the characteristic 140 ps lifetime of v = 1 excited W(CO)6, indicating that the transmission can be rapidly modulated (> GHz). Both the fraction excited and maximum transmission modulation vary linearly with the energy of the excitation pulse over the range investigated, and the lifetimes do not appear to change with increasing pump power. A representative plot of Ω2 obtained with 3 uJ excitation is presented in the Supporting Information. This energy is the highest obtainable with our experimental apparatus, but we note that the polariton features are barely resolvable when pumping with 3 µJ (Fig 4a, blue). We add here that optical contrast could be increased with larger Rabi splitting accompanying higher molecular concentrations, however, the efficiency of excitation in such a system will likely be reduced since transmission at ω01 will be weaker. Finally, we note that the results discussed above were obtained in transmission. Schwartz, et al., demonstrated that, for polaritons comprising electronic modes and optical modes, transient transmission and reflection measurements are required to unravel the dynamics of the polariton system.7 For the higher concentration W(CO)6-cavity system in our previous work, the reflection and transmission data showed no significant differences.29 We speculate that the similarity between transmission and reflection is a consequence of how narrow of the vibrational transitions are compared to electronic transitions, leading to less congested spectra, but it is not clear that this result should be general to all vibration-cavity polariton systems. Supplementary Note 2 and Supplementary Figure 2 show results from this lowconcentration W(CO)6 system measured in reflection, which show the same Rabi splitting contraction and indistinguishable kinetics compared to those from the transmission measurements. 4. Conclusion In this work, we have shown that carefully choosing the concentration of an absorber in a cavity can allow access to a regime where the polariton modes do not overlap higher-lying excited-states of the


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system and greatly simplifies the transient spectra and dynamics analysis. We report strong evidence that reservoir v = 1 population dominates the transient response of cavity-coupled W(CO)6 in this regime, with little direct evidence of polariton population. Even with this simplified system, fitting the kinetics of the dynamic response is complicated by peak shifting and must be treated with a proper model. Finally, we have shown that vibrational polariton systems can support large modulations in transmission for potential photonic applications. 5. Acknowledgements This work was supported by the Office of Naval Research through internal funding at the U. S. Naval Research Laboratory. R.D. and W. A. acknowledge fellowships administered by the National Academy of Sciences. 6. References

1. Khitrova, G.; Gibbs, H. M.; Jahnke, F.; Kira, M.; Koch, S. W., Nonlinear Optics of NormalMode-Coupling Semiconductor Microcavities. Rev. Mod. Phys. 1999, 71 (5), 1591-1639. 2. Ebbesen, T. W., Hybrid Light–Matter States in a Molecular and Material Science Perspective. Acc. Chem. Res. 2016, 49 (11), 2403-2412. 3. Flick, J.; Ruggenthaler, M.; Appel, H.; Rubio, A., Atoms and Molecules in Cavities, from Weak to Strong Coupling in Quantum-Electrodynamics (Qed) Chemistry. P. Natl. Acad. Sci. USA 2017, 114 (12), 3026-3034. 4. Khitrova, G.; Gibbs, H. M.; Kira, M.; Koch, S. W.; Scherer, A., Vacuum Rabi Splitting in Semiconductors. Nature Physics 2006, 2 (2), 81-90. 5. Vasa, P.; Pomraenke, R.; Cirmi, G.; De Re, E.; Wang, W.; Schwieger, S.; Leipold, D.; Runge, E.; Cerullo, G.; Lienau, C., Ultrafast Manipulation of Strong Coupling in Metal−Molecular Aggregate Hybrid Nanostructures. ACS Nano 2010, 4 (12), 7559-7565. 6. Vasa, P.; Wang, W.; Pomraenke, R.; Lammers, M.; Maiuri, M.; Manzoni, C.; Cerullo, G.; Lienau, C., Real-Time Observation of Ultrafast Rabi Oscillations between Excitons and Plasmons in Metal Nanostructures with J-Aggregates. Nat. Photonics 2013, 7 (2), 128-132. 7. Schwartz, T.; Hutchison, J. A.; Léonard, J.; Genet, C.; Haacke, S.; Ebbesen, T. W., Polariton Dynamics under Strong Light–Molecule Coupling. ChemPhysChem 2013, 14 (1), 125-131. 8. Hutchison, J. A.; Schwartz, T.; Genet, C.; Devaux, E.; Ebbesen, T. W., Modifying Chemical Landscapes by Coupling to Vacuum Fields. Angew. Chem. Int. Ed. 2012, 51 (7), 1592-1596. 9. Coles, D. M.; Somaschi, N.; Michetti, P.; Clark, C.; Lagoudakis, P. G.; Savvidis, P. G.; Lidzey, D. G., Polariton-Mediated Energy Transfer between Organic Dyes in a Strongly Coupled Optical Microcavity. Nat. Mater. 2014, 13 (7), 712-719. 10. Tischler, J. R.; Scott Bradley, M.; Zhang, Q.; Atay, T.; Nurmikko, A.; Bulović, V., Solid State Cavity QED: Strong Coupling in Organic Thin Films. Org. Electron. 2007, 8 (2–3), 94-113. 17 ACS Paragon Plus Environment


11. Törmä, P.; Barnes, W. L., Strong Coupling between Surface Plasmon Polaritons and Emitters: A Review. Rep. Prog. Phys. 2015, 78 (1), 013901. 12. 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 (1), 130136. 13. Simpkins, B. S.; Fears, K. P.; Dressick, W. J.; Spann, B. T.; Dunkelberger, A. D.; Owrutsky, J. C., Spanning Strong to Weak Normal Mode Coupling between Vibrational and Fabry–Pérot Cavity Modes through Tuning of Vibrational Absorption Strength. ACS Photonics 2015, 2 (10), 1460-1467. 14. George, J.; Shalabney, A.; Hutchison, J. A.; Genet, C.; Ebbesen, T. W., Liquid-Phase Vibrational Strong Coupling. J. Phys. Chem. Lett. 2015, 6 (6), 1027-1031. 15. 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. 16. Ahn, W.; Vurgaftman, I.; Dunkelberger, A. D.; Owrutsky, J. C.; Simpkins, B. S., Vibrational Strong Coupling Controlled by Spatial Distribution of Molecules within the Optical Cavity. ACS Photonics 2017, ASAP. 17. Shalabney, A.; George, J.; Hiura, H.; Hutchison, J. A.; Genet, C.; Hellwig, P.; Ebbesen, T. W., Enhanced Raman Scattering from Vibro‐Polariton Hybrid States. Angew. Chem. Int. Ed. 2015, 54 (27), 7971-7975. 18. Vergauwe, R. M.; George, J.; Chervy, T.; Hutchison, J. A.; Shalabney, A.; Torbeev, V. Y.; Ebbesen, T. W., Quantum Strong Coupling with Protein Vibrational Modes. J. Phys. Chem. Lett 2016, 7 (20), 4159-4164. 19. Casey, S. R.; Sparks, J. R., Vibrational Strong Coupling of Organometallic Complexes. J. Phys. Chem. C 2016, 120 (49), 28138-28143. 20. Muallem, M.; Palatnik, A.; Nessim, G. D.; Tischler, Y. R., Strong Light-Matter Coupling and Hybridization of Molecular Vibrations in a Low-Loss Infrared Microcavity. J. Phys. Chem. Lett. 2016, 7 (11), 2002-2008. 21. Kowalewski, M.; Mukamel, S., Manipulating Molecules with Quantum Light. P. Natl. Acad. Sci. USA 2017, 114 (13), 3278-3280. 22. Saurabh, P.; Mukamel, S., Two-Dimensional Infrared Spectroscopy of Vibrational Polaritons of Molecules in an Optical Cavity. J. Chem. Phys. 2016, 144 (12), 124115. 23. del Pino, J.; Feist, J.; Garcia-Vidal, F. J., Quantum Theory of Collective Strong Coupling of Molecular Vibrations with a Microcavity Mode. New Journal of Physics 2015, 17, 081001. 24. Wu, N.; Feist, J.; Garcia-Vidal, F. J., When Polarons Meet Polaritons: Exciton-Vibration Interactions in Organic Molecules Strongly Coupled to Confined Light Fields. Phys. Rev. B 2016, 94 (19), 195409. 25. Herrera, F.; Spano, F. C., Dark Vibronic Polaritons and the Spectroscopy of Organic Microcavities. Phys. Rev. Lett. 2017, 118 (22), 223601 26. Herrera, F.; Spano, F. C., Absorption and Photoluminescence in Organic Cavity QED. Phys. Rev. A 2017, 95 (5), 053867. 27. Herrera, F.; Spano, F. C., Cavity-Controlled Chemistry in Molecular Ensembles. Phys. Rev. Lett. 2016, 116 (23), 238301. 28. 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 (38), 11462-11466. 29. 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. 30. Zhong, Q.; Baronavski, A. P.; Owrutsky, J. C., Reorientation and Vibrational Energy Relaxation of Pseudohalide Ions Confined in Reverse Micelle Water Pools. J. Chem. Phys. 2003, 119 (17), 91719177.


Page 18 of 20

Page 19 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31. Ghosh, A.; Serrano, A. L.; Oudenhoven, T. A.; Ostrander, J. S.; Eklund, E. C.; Blair, A. F.; Zanni, M. T., Experimental Implementations of 2D IR Spectroscopy through a Horizontal Pulse Shaper Design and a Focal Plane Array Detector. Opt. Lett. 2016, 41 (3), 524-527. 32. Fink, J. M.; Goppl, M.; Baur, M.; Bianchetti, R.; Leek, P. J.; Blais, A.; Wallraff, A., Climbing the Jaynes-Cummings Ladder and Observing Its Nonlinearity in a Cavity QED System. Nature 2008, 454 (7202), 315-318. 33. Dougherty, T. P.; Arrivo, S. M.; Heilweil, E. J., Femtosecond Vibrational-Relaxation Dynamics of Group-VI Metal-Carbonyls. Abstr. Pap. Am. Chem. S. 1994, 208, 216-PHYS. 34. Dougherty, T. P.; Heilweil, E. J., Ultrafast Transient Infrared-Absorption Studies of M(CO)6 (M=Cr, Mo or W) Photoproducts in n-Hexane Solution. Chem. Phys. Lett. 1994, 227 (1-2), 19-25. 35. Tokmakoff, A.; Sauter, B.; Fayer, M. D., Temperature-Dependent Vibrational-Relaxation in Polyatomic Liquids - Picosecond Infrared Pump-Probe Experiments. J. Chem. Phys. 1994, 100 (12), 9035-9043. 36. Tokmakoff, A.; Sauter, B.; Kwok, A. S.; Fayer, M. D., Phonon-Induced Scattering between Vibrations and Multiphoton Vibrational Up-Pumping in Liquid Solution. Chem. Phys. Lett. 1994, 221 (56), 412-418. 37. Ribeiro, R. F.; Dunkelberger, A. D.; Xiang, B.; Xiong, W.; Simpkins, B. S.; Owrutsky, J. C.; Yuen-Zhou, J., Theory for Nonlinear Spectroscopy of Vibrational Polaritons. arXiv preprint arXiv:1711.11242 2017. 38. Romanini, D.; Ventrillard, I.; Méjean, G.; Morville, J.; Kerstel, E., Introduction to Cavity Enhanced Absorption Spectroscopy. In Cavity-Enhanced Spectroscopy and Sensing, Springer: 2014; pp 1-60. 39. Arrivo, S. M.; Dougherty, T. P.; Grubbs, W. T.; Heilweil, E. J., Ultrafast Infrared-Spectroscopy of Vibrational Co-Stretch up-Pumping and Relaxation Dynamics of W(CO)(6). Chem. Phys. Lett. 1995, 235 (3-4), 247-254. 40. Banno, M.; Iwata, K.; Hamaguchi, H.-o., Intra- and Intermolecular Vibrational Energy Transfer in Tungsten Carbonyl Complexes W(CO)5(X) (X=CO, CS, CH3CN, and CD3CN). J. Chem. Phys. 2007, 126 (20), 204501. 41. Houdré, R.; Gibernon, J. L.; Pellandini, P.; Stanley, R. P.; Oesterle, U.; Weisbuch, C.; O’Gorman, J.; Roycroft, B.; Ilegems, M., Saturation of the Strong-Coupling Regime in a Semiconductor Microcavity: Free-Carrier Bleaching of Cavity Polaritons. Phys. Rev. B 1995, 52 (11), 7810-7813.



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Ultrafast Transmission Modulation and Recovery via Vibrational

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