Two-Dimensional Fano Lineshapes in Ultrafast Vibrational

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Two-Dimensional Fano Lineshapes in Ultrafast Vibrational Spectroscopy of Thin Molecular Layers on Plasmonic Arrays Andrey Gandman, Robert T. Mackin, Bar Cohn, Igor V Rubtsov, and Lev Chuntonov J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01490 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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The Journal of Physical Chemistry Letters

Two-Dimensional Fano Lineshapes in Ultrafast Vibrational Spectroscopy of Thin Molecular Layers on Plasmonic Arrays Andrey Gandman,1‡ Robert Mackin,2‡ Bar Cohn,3 Igor V. Rubtsov,2* and Lev Chuntonov1,3,4,* AUTHOR ADDRESS 1

Solid State Institute, Technion – Israel Institute of Technology, Haifa 32000, Israel.

2

Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, USA.

3

Schulich Faculty of Chemistry, Technion – Israel Institute of Technology, Haifa 32000, Israel.

4

Russel Berrie Nanotechnology Institute, Technion – Israel Institute of Technology, Haifa 32000,

Israel. KEYWORDS Surface-enhanced ultrafast vibrational spectroscopy, Fano lineshape, surface-enhanced 2DIR, plasmonic antennas.

ACS Paragon Plus Environment

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TOC graphics

ABSTRACT

Two-dimensional femtosecond infrared (2DIR) spectroscopy routinely provides insights into molecular structure and ultrafast dynamics in 1-100 µm thick bulk samples. Confinement of molecules to surfaces, gaps, crevices, and other topographic features, frequently encountered on the nanometer length scale, significantly alters their structure and dynamics, affecting physical and chemical properties. Amplification of 2DIR signals by the plasmon-enhanced fields around metal nanostructures can permit structural and dynamics measurements of the confined molecules. Fano resonances, induced by the interaction between laser pulses, plasmon, and vibrational modes significantly distort 2D lineshapes. For different detuning from plasmon resonance, the interference between multiple signal components leads to different lineshape asymmetry, which we demonstrate on a set of linear absorption, transient absorption, and 2DIR spectra. An intuitive model used to describe experimental data points onto the interference’s origin. Our results will facilitate the application of surface-enhanced 2DIR spectroscopy for studies of molecular structure and dynamics in nano-confined environment.


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Studies of molecular dynamics and function under confinement conditions at nanometer length scale are of high importance for heterogeneous catalysis,1 interfacial chemistry,2 proteindrug interactions,3 molecular machines,4 and other fields.5 Vibrational spectroscopy is a sensitive method to study molecular structure, however, its application for thin sample layers is limited by relatively weak cross-sections of the vibrational transitions and poor detection sensitivity. Surface-enhanced infrared linear absorption spectroscopy (SEIRA) allows studies of molecular structure in samples as thin as a molecular monolayer.6-10 While first SEIRA experiments were conducted on the noble metal island films,11,12 modern nanofabrication capabilities (e.g., electron beam lithography) combined with the plasmon fields engineering led to development of the superior substrates, optimized to enhance weak signals.13-17 Vibrational lineshapes play a central role in the interpretation of infrared spectra, including the information on homogeneous and inhomogeneous molecular dynamics. However, reliable quantification of their relative contribution with linear spectroscopy is highly challenging. Two-dimensional infrared spectroscopy (2DIR) uses sequences of three femtosecond laser pulses to construct correlation maps between the excited and probed vibrational frequencies.18-20 Spreading the spectroscopic signal in two dimensions is instrumental when structural information has to be extracted from highly congested and inhomogeneouslybroadened spectra. Such conditions are frequently met in studies of disordered molecules on surfaces, that are highly irregular on the nano-scale.21-25 Furthermore, the femtosecond duration of the excitation pulses can open an access to real-time studies of the structural dynamics of molecules on nano-structured surfaces, which differ significantly from those in the bulk.26-28 Ultrafast structural changes are followed via the relaxation dynamics reflected in the time evolution of 2D lineshapes. For molecules with sufficiently large transition dipole moments,


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2DIR spectroscopy can be extended from bulk to the molecules adsorbed on the surface.26-32 However, in order to expand 2DIR to studies of a wide range of surface molecules and elucidate their structure and dynamics in nano-confined samples, it is desirable to conduct the experiments on the plasmonic substrates tailored to maximize the corresponding signal enhancement.33 Recently, Rezus and co-workers demonstrated 2DIR spectroscopy of thin polymer layers on random plasmonic substrate.32 Note, that unlike the surface-specific methods,31,34,35 where the signal originates from the discontinuity at the interface, plasmonic fields can be designed to target specified regions on the subwavelength scale. Asymmetric (Fano) lineshapes36-38 are typical for SEIRA, arising from the interference between the bright continuum band of the plasmon and dark narrow molecular transitions.39 The lineshape asymmetry parameter q can be translated into the phase acquired at the dark mode frequency.40 In SEIRA, molecular excitation occurs indirectly, via the plasmon-enhanced nearfield, while the detected far-field signal originates from the interference between the excitation field, plasmon scattering, and a phase-shifted molecular signal scattered off the plasmon.41 In 2DIR the signal detected by spectral interferometry involves, in principle, both absorptive and dispersive parts.18 The highest spectral resolution is achieved when the dispersive part contributions are eliminated in the absorptive spectrum, obtained as the real part of the sum of the rephasing and non-rephasing 2DIR spectra.42 This ansatz has proven to significantly facilitate retrieval of structural and dynamical information.18 For example, the elongation along the diagonal in 2D spectrum represents the inhomogeneous broadening of the spectral line, while the anti-diagonal linewidth - homogeneous broadening. When surface-enhanced 2DIR spectroscopy of molecules adsorbed on plasmonic substrate is considered, two-dimensional Fano lineshapes arise.32 Detailed understanding of these


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lineshapes is required in order to benefit from plasmon field enhancements in 2DIR studies. In this work we analyze spectra of few-nanometer-thin poly-methyl methacrylate (PMMA) samples on periodic two-dimensional arrays of gold bars, designed for optimal signal enhancement.13,14 Different detuning between the plasmon and molecular resonances gives rise to different q values, reflected in the corresponding lineshapes. In 2DIR, the effect of all four electric fields (three excitation pulses and the non-linear signal) on the molecule-plasmon interaction should be considered. We rationalize experimental results by theoretical modelling of 2DIR signals with third-order perturbative response functions describing the non-linear interaction,43 which were modified ad-hoc to account for the near-field interaction between the molecule and the plasmon. The additional phase acquired by the molecule is estimated based on models known to describe well Fano lineshapes in linear absorption and, recently, in stimulated Raman experiments.38,40,4447

Example of the gold bars array spin-coated by 20 nm thick PMMA layer used in our experiments is visualized in Figure 1 by electron microscopy and electromagnetic calculations (see more details in SI). SEIRA spectroscopy of the carbonyl stretching vibrational mode of PMMA ( ωmol =1735 cm-1) was studied in great detail.13,14 The near-field coupling between the vibrational mode and the plasmon resonance results in the frequency-dependent phase shift of the signal around the vibrational transition. The phase shift depends on the detuning between the molecular transition ωmol and plasmon resonance ω pl as seen in the first row of Figure 2a-d, where the linear absorption spectra are shown for four arrays of different bar length, featuring different detuning. The associated q -values, which can be obtained by either fitting the spectra to the Fano profile40,45 or direct calculation,40,44 are indicated in the figure caption. For


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completeness, the linear spectrum of a thick PMMA layer without plasmon array is also shown in Figure 2e. The second row in Figure 2 shows transient infrared absorption spectra of the PMMA samples. Here, in addition to the transition to the first vibrational excited state (negative peak for the bulk samples representing the ground state bleach (GSB) and stimulated emission (SE)), excited state absorption (ESA) to the second vibrational excited state is observed (positive peak). The anharmonicity constant ∆ 20 cm-1, reflected in the ESA transition frequency, was found by fitting of the transient absorption spectrum of the thick polymer layer shown in Figure 2e. The intrinsic electronic transient response of noble metal nanostructures is known to be significantly shorter than the duration of 80 fs laser pulses used in our experiments. Because the experimental data were all collected at the waiting time of 200 fs, the broadband plasmon peak is not observed in our surface-enhanced transient absorption measurements, as evident from the corresponding spectra (Figure 2a-d, second row). However, the effect of the near-field coupling between the plasmon and the molecule is clearly seen in the phase-shifted lineshapes. The positive and negative peaks in the transient absorption spectra in columns (a-d) change their signs and relative amplitudes depending on the detuning from the plasmon resonance, as can be seen from the comparison to the spectrum collected from thick-layer PMMA spectrum without plasmonic enhancement in column (e). When ω pl is blue-shifted from ωmol , the negative peak appears at the lower detection frequency, and its magnitude is higher than that of the positive peak. For the case where ω pl

ωmol , the negative and positive peaks change their signs, while

their relative magnitudes are retained, while for the red-shifted ω pl the positive peak appears at the lower detection frequency and its magnitude increases. Importantly, these peaks do not


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represent pure GSB/SE and ESA lineshapes with inverted signs, but originate from the interference of their mixed absorptive-dispersive lineshape components. Spreading the spectral data in two dimensions in 2DIR experiments reveals the details of the vibrational dynamics not accessible by linear spectroscopy. The lineshape of the linear absorption peak in Figure 2e is nearly Lorentzian with the width of 20 cm-1, whereas the information on the polymer microscopic inhomogeneity is hidden. The corresponding absorptive 2DIR spectrum, measured by a three-beam 2DIR spectrometer with heterodyned detection (see SI for details),48 reveals the homogeneous component of ca. 10 cm-1, as measured from the antidiagonal width of the peak, while the inhomogeneous component is ca. 20 cm-1, as measured from the diagonal width. Consistently with the linear and the transient absorption data, the 2DIR spectra collected from the polymer deposited on the plasmonic arrays shown in the third row of Figure 2 have asymmetric lineshapes. As expected, the trends in the 2DIR spectra are similar to those of the transient absorption, because the corresponding signals are related by the Fourier transform projection slice theorem.18 Because interpretation of the 2DIR spectra is heavily based on lineshape details, it is necessary to understand the origin of the corresponding phase-shifts. Theoretical description of the surface-enhanced vibrational spectroscopy requires proper treatment of the weak interaction between the plasmon and the molecule using either the configurational interaction formulation of Fano36 or the Green function method of Jortner and Rice.49 Alternatively, in order to readily provide qualitative interpretation of the experimental data in SEIRA, its mechanical analogue is commonly used.44,45,50 Typically, vibrational transition is assumed to be optically dark, because the corresponding signal is much weaker than that observed with plasmon enhancement. Thus, it is assumed that the molecular state is excited only as a consequence of its weak coupling to the bright plasmon mode via the near-field. Despite the


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quantum nature of the system, the asymmetry of the SEIRA lineshapes can be described based on equations of motion of two coupled mechanical oscillators 51 && xb + γ b x&b + ωb2 xb + g 2 xd = E ( t ) && xd + γ d x&d + ωd2 xd + g 2 xb = 0

(1)

where xb and xd are the bright and dark oscillators’ amplitudes, γ b and γ d are the associated linewidths, ωb and ωd are natural frequencies, E ( t ) = aeiωt is the periodic force, g = µd ξ is the coupling strength, µd is the transition dipole moment, and ξ is the plasmon local field enhancement, assumed to be constant around ωd . The power absorption spectrum is calculated as P = − Im {cb (ω )} , where xb = cb (ω ) eiϕb (ω )+iωt and xd = cd (ω ) eiϕd (ω )+ iωt are solutions to equations (1). In typical plasmonic systems,44 the finite bandwidth of the bright mode γ b and its

ωd2 − ωb2 detuning from the dark mode determine the value of q = , and at the resonance γ bωd frequency ϕd (ωd ) = tan −1 ( q ) . However, generally, accounting for the frequency dependence of

ϕd (ω ) is important. Because the detected signal originates from the bright state, ϕb (ω ) should be considered, and we assume the phase of the signal electric field ϕsig (ω ) = ϕb (ω ) . Intuitively, ϕ sig (ω ) can be understood via decomposition of the SEIRA signal generation process into two steps: excitation of the dark molecular mode via its coupling to the plasmon mode and a consequent secondary excitation of the plasmon by the molecule from the near-field.52-54 Therefore,

ϕb (ω ) = ϕd (ω ) + ϕexc (ω ) ,

(2)


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where the first term is associated with the near-field excitation of the molecule, and the second term

–

with

the

off-resonant

driving

the

 γ bω  . Practically, because γ d 2 2   ω − ωb 

ϕ exc (ω ) = tan −1 

plasmon

by

the

excited

molecule,

γ b , ϕexc (ω ) ϕexc (ωd ) holds.

Next, we describe the linear Fano-lineshape with two interfering first-order response ∞

functions R ( ) ( ω ) = ∫ e − iωt dtR ( ) ( t ) of the uncoupled plasmon and molecular states, where 1

1

0

R( ) (ω ) = µab2 Iba (ω ) , I ba (ω ) = (ω − ωba + iγ 1

)

−1

is the averaged homogeneous Liouville space

Green function43 associated with the transition between states a and b , given by the Fourier transform of I ba ( t ) = θ ( t ) e− iωbat −γ t , and θ ( t ) is the Heaviside step function. The effect of the configuration interaction is introduced via the relative phase ϕ sig (ω ) between the two response () b functions Rb = µb2,01 I 01 (ω ) and Rd(1) (ω ) = µd2,01 ξ 2 I 01d (ω ) eiϕsig (ω ) describing the bright and dark 1

modes respectively. The detected signal is given by

{

}

S ( ) (ω ) ∝ − Im Rb( ) (ω ) + Rd( ) (ω ) . 1

1

1

(3)

Indeed, linear absorption spectra obtained with equation (3), shown in the first row of Figure 3, are in a good agreement with the experimental data and are fully consistent with the exact solution of (1) in the weak coupling limit and for γ d

γb .

Equation (2) is useful also for understanding the lineshape asymmetry in the surfaceenhanced non-linear spectroscopy. Because the transient absorption spectrum can be directly obtained by integration of the 2DIR data along the excitation frequency axis, we focus on the


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( 2DIR spectrum S

3)

(ω1 , t2 , ω3 )

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in detail. In the impulsive excitation regime and for a short

waiting time, the 2DIR spectrum is given by

S(

3)

{

r reph r ⋅r S

(ω1 , ω3 ) = Re eik

r non-reph r ⋅r

( ) Rreph (ω1 , ω3 ) + eikS 3

}

( ) Rnon-reph (ω1 , ω3 ) , 3

(4)

( 3) ( 3) where Rreph (ω1 , ω3 ) = ∑ Rn(3) (ω1 , ω3 ) and Rnon-reph (ω1 , ω3 ) = ∑ Rn(3) (ω1 , ω3 ) . Here, Rn( ) (ω1 , ω3 ) 3

n

n

are the third-order response functions corresponding to the rephasing and non-rephasing signals.43 In the homogeneous limit and in the absence of the coupling to the plasmon mode,

Rn( ) (ω1 , ω3 ) are conventionally derived in the time domain using, for example, products of the 3

( ) 2 µcd2 I cd ( t3 ) I ab ( t1 ) , which are Fourieraveraged Liouville space Green functions Rn ( t1 , t3 ) = µab 3

∞

∞

0

0

transformed in two dimensions as Rn( ) (ω1 , ω3 ) = ∫ e− iω1t1 dt1 ∫ e− iω3t3 dt3 Rn( ) ( t1 , t3 ) .43 Because the 3

3

weak coupling between the molecular and plasmon states results in the frequency-dependent phase ϕd (ω ) , which shapes the amplitude of the time-dependent signal, we extended the frequency-domain notation of equation (3) for description of 2DIR spectrum. ( Consider a third-order rephasing response function R1

3)

(ω1 , ω3 ) .18 The phase matching

r r between the wave vectors of the signal k S and the excitation pulses k1− 3 , enumerated in the order r r r r of their arrival at the sample, and the energy conservation require kSreph = −k1 + k2 + k3 and r r r r kSnon-reph = k1 − k2 + k3 , as well as −ωSreph = ω1 − ω2 − ω3 and −ωSnon-reph = −ω1 + ω2 − ω3 . Therefore, molecular interaction with the first two pulses involves different components of the complex r r

r r

r r

electric field e−ik1 ⋅r +iω1t and eik2 ⋅r −iω2t in the case of the rephasing signal, while eik1 ⋅r −iω1t and


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r r

e−ik2 ⋅r +iω2t in the case of the non-rephasing signal. The interaction with the third pulse and the r r

consequent signal emission in both cases correspond to the electric field components eik3 ⋅r −iω3t and r r

eikS ⋅r −iωS t . When the molecular vibrational transition is excited via the plasmon-induced near field, each excitation and signal emission process contributes to the response function additional frequency-dependent phases, which leads to a total factor of e

iϕ1 (ω ) + iϕ2 (ω ) + iϕ3 (ω ) + iϕexc (ω )

. Using

mechanical model (1) we assume that ϕ1 (ω ) = −ϕ2 (ω ) = ϕ3 (ω ) = ϕd (ω ) and express the phasemodified third-order response functions as ( 3) Rreph (ω1 , ω3 ) = 2µ014  I 01 (ω01 ) I10 (ω10 ) eiϕsig (ω10 ) − I 01 (ω01 ) I 21 (ω21 ) eiϕsig (ω21 )  , iϕ sig (ω10 ) iϕ sig (ω21 ) ( 3) 4   Rnon-reph , = 2 I I e − I I e ω ω µ ω ω ω ω ( 1 3) 01  10 ( 10 ) 10 ( 10 ) 10 ( 10 ) 21 ( 21 ) 

(5)

where we used harmonic scaling for the transition dipole moments’ magnitudes, µ12 = 2 µ 01 .18 To account for the inhomogeneous broadening of the signals in Figure 2, we averaged the ( homogenous spectrum S

3)

(ω1 , ω3 )

over the 20 cm-1 bandwidth (fwhm) of Gaussian distribution

of the molecular transitions around ω01 =1735 cm-1 and ω21 =1715 cm-1. The corresponding results for the cases of different detuning from the plasmon resonance are shown in Figure 3 (third row), while the associated transient absorption spectra are shown in the second row of the figure. Both results are in a good qualitative agreement with the experimental data, indicating that our model captures the main mechanism behind the interference between the absorptive and dispersive components of the 2DIR lineshape.


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Careful inspection of the experimental 2DIR spectra in Figure 2 reveals that maximal magnitude of the peak at the higher detection frequency for both blue and red plasmon detuning (panels (a) and (d), respectively) corresponds to excitation frequency that is ca. 10 cm-1 higher than that of the peak at the lower detection frequency, indicating that the cancelation of the ( phases ϕ1 (ω1 ) and ϕ2 (ω1 ) is not complete. Clearly, our model for S

3)

(ω1 , ω3 )

is

oversimplified, however, we anticipate that this cancelation would still be expected, if the configuration interaction was properly accounted for using appropriate theory, because of the symmetry in the quantum-mechanical description of the interaction between the system’s density matrix and the first and second excitation pulses. The reason for the apparent phase difference may be in plasmon excitation relaxation process. Slow (10 – 100 ps) thermal cooling of metal nanostructures,55 can lead to a difference in the plasmon response to the first and the second excitation pulses. Further studies will help to better understand this effect. In conclusion, we have conducted a systematic study of Fano lineshapes in surfaceenhanced 2DIR spectroscopy. In order to obtain best spectral resolution and allow for simple interpretation of two-dimensional spectra, purely absorptive spectra, where dispersive parts of the lineshape are eliminated are commonly used in 2DIR. In surface-enhanced 2DIR, the phase associated with the near-field interaction prevents such elimination in all cases, except when the vibrational and plasmon transition frequencies coincide. Because typical 2DIR spectrum involves multiple vibrational transitions leading to different lineshape asymmetry, the physical understanding of its origin facilitated by this work will be particularly helpful in studies of molecules confined to metal nanostructures as well as in designing plasmonic substrates for surface-enhanced 2DIR spectroscopy.


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AUTHOR INFORMATION

Corresponding Author *corresponding authors: [email protected] (I.R.) and [email protected] (L.C.)

Author Contributions ‡These authors contributed equally.

ACKNOWLEDGMENT This work is supported by the Russell Berrie Nanotechnology Institute, Technion, Israel Science Foundation (grant 1118/15 to L.C.) and by the United States National Science Foundation (CHE-1462075 to I.R.). Plasmonic arrays were prepared at the Micro- and NanoFabrication Unit, Technion.

ASSOCIATED CONTENT

Supporting Information Experimental methods. This material is available free of charge via the Internet at http://pubs.acs.org.


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(17) Huck, C.; Toma, A.; Neubrech, F.; Chirumamilla, M.; Vogt, J.; De Angelis, F.; Pucci, A. Gold Nanoantennas on a Pedestal for Plasmonic Enhancement in the Infrared ACS Photon. 2015, 2, 497. (18) Hamm, P.; Zanni, M. T. Concepts and Methods of 2D Infrared Spectroscopy; Cambridge University Press: Cambridge, 2011. (19) Ge, N.-H.; Zanni, M. T.; Hochstrasser, R. M. Effects of Vibrational Frequency Correlations on Two-Dimensional Infrared Spectra J. Phys. Chem. A 2001, 106, 962. (20) Khalil, M.; Demirdöven, N.; Tokmakoff, A. Coherent 2D IR spectroscopy: molecular structure and dynamics in solution J. Phys. Chem. A 2003, 107, 5258. (21) Li, Z.; Wang, J.; Li, Y.; Xiong, W. Solving the “Magic Angle” Challenge in Determining Molecular Orientation Heterogeneity at Interfaces J. Phys. Chem. C 2016, 120, 20239. (22) Muller, E. A.; Pollard, B.; Raschke, M. B. Infrared chemical nano-imaging: Accessing structure, coupling, and dynamics on molecular length scales J. Phys. Chem. Lett. 2015, 6, 1275. (23) Bian, H.; Li, J.; Chen, H.; Yuan, K.; Wen, X.; Li, Y.; Sun, Z.; Zheng, J. Molecular conformations and dynamics on surfaces of gold nanoparticles probed with multiple-mode multiple-dimensional infrared spectroscopy J. Phys. Chem. C 2012, 116, 7913. (24) Yan, C.; Yuan, R.; Pfalzgraff, W. C.; Nishida, J.; Wang, L.; Markland, T. E.; Fayer, M. D. Unraveling the dynamics and structure of functionalized self-assembled monolayers on gold using 2D IR spectroscopy and MD simulations Proc. Nat. Acad. Sci. 2016, 201603080. (25) Donaldson, P. M.; Hamm, P. Gold Nanoparticle Capping Layers: Structure, Dynamics, and Surface Enhancement Measured Using 2D‐IR Spectroscopy Angew. Chem. 2013, 125, 662. (26) Kraack, J. P.; Kaech, A.; Hamm, P. Molecule-specific interactions of diatomic adsorbates at metal-liquid interfaces Struct. Dyn. 2017, 4, 044009. (27) Rosenfeld, D. E.; Nishida, J.; Yan, C.; Kumar, S. K.; Tamimi, A.; Fayer, M. D. Structural Dynamics at Monolayer–Liquid Interfaces Probed by 2D IR Spectroscopy J. Phys. Chem. C 2013, 117, 1409. (28) Li, J.; Qian, H.; Chen, H.; Zhao, Z.; Yuan, K.; Chen, G.; Miranda, A.; Guo, X.; Chen, Y.; Zheng, N.; Wong, M. S.; Zheng, J. Two distinctive energy migration pathways of monolayer molecules on metal nanoparticle surfaces Nat. Comm. 2016, 7, 10749. (29) Kraack, J. P.; Lotti, D.; Hamm, P. Ultrafast, Multidimensional Attenuated Total Reflectance Spectroscopy of Adsorbates at Metal Surfaces J. Phys. Chem. Lett. 2014, 5, 2325. (30) Yan, C.; Yuan, R.; Pfalzgraff, W. C.; Nishida, J.; Wang, L.; Markland, T. E.; Fayer, M. D. Unraveling the dynamics and structure of functionalized self-assembled monolayers on gold using 2D IR spectroscopy and MD simulations Proc. Nat. Acad. Sci. 2016, 113, 4929. (31) Kraack, J. P.; Hamm, P. Surface-Sensitive and Surface-Specific Ultrafast TwoDimensional Vibrational Spectroscopy Chem. Rev. 2016. (32) Selig, O.; Siffels, R.; Rezus, Y. L. A. Ultrasensitive Ultrafast Vibrational Spectroscopy Employing the Near Field of Gold Nanoantennas Phys. Rev. Lett. 2015, 114, 233004.


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(33) Vogt, J.; Huck, C.; Neubrech, F.; Pucci, A. Plasmonic Light Scattering and Infrared Vibrational Signal Enhancement. In Frontiers of Plasmon Enhanced Spectroscopy Volume 2; ACS Publications, 2016; pp 1. (34) Bredenbeck, J.; Ghosh, A.; Smits, M.; Bonn, M. Ultrafast Two DimensionalInfrared Spectroscopy of a Molecular Monolayer J. Am. Chem. Soc. 2008, 130, 2152. (35) Xiong, W.; Laaser, J. E.; Mehlenbacher, R. D.; Zanni, M. T. Adding a dimension to the infrared spectra of interfaces using heterodyne detected 2D sum-frequency generation (HD 2D SFG) spectroscopy Proc. Nat. Acad. Sci. 2011, 108, 20902. (36) Fano, U. Effects of Configuration Interaction on Intensities and Phase Shifts Phys. Rev. 1961, 124, 1866. (37) Luk'yanchuk, B.; Zheludev, N. I.; Maier, S. A.; Halas, N. J.; Nordlander, P.; Giessen, H.; Chong, C. T. The Fano resonance in plasmonic nanostructures and metamaterials Nat. Mater 2010, 9, 707. (38) Miroshnichenko, A. E.; Flach, S.; Kivshar, Y. S. Fano resonances in nanoscale structures Rev. Mod. Phys. 2010, 82, 2257. (39) Priebe, A.; Sinther, M.; Fahsold, G.; Pucci, A. The correlation between film thickness and adsorbate line shape in surface enhanced infrared absorption J. Chem. Phys. 2003, 119, 4887. (40) Ott, C.; Kaldun, A.; Raith, P.; Meyer, K.; Laux, M.; Evers, J.; Keitel, C. H.; Greene, C. H.; Pfeifer, T. Lorentz Meets Fano in Spectral Line Shapes: A Universal Phase and Its Laser Control Science 2013, 340, 716. (41) Rezus, Y.; Selig, O. Impact of local-field effects on the plasmonic enhancement of vibrational signals by infrared nanoantennas Opt. Express 2016, 24, 12202. (42) Khalil, M.; Demirdöven, N.; Tokmakoff, A. Obtaining Absorptive Line Shapes in Two-Dimensional Infrared Vibrational Correlation Spectra Phys. Rev. Lett. 2003, 90, 047401. (43) Mukamel, S. Principles of nonlinear optical spectroscopy; Oxford University Press: New York, 1995. (44) Gallinet, B.; Martin, O. J. Ab initio theory of Fano resonances in plasmonic nanostructures and metamaterials Phys. Rev. B 2011, 83, 235427. (45) Joe, Y. S.; Satanin, A. M.; Kim, C. S. Classical analogy of Fano resonances Phys. Scr. 2006, 74, 259. (46) Frontiera, R. R.; Henry, A.-I.; Gruenke, N. L.; Van Duyne, R. P. Surfaceenhanced femtosecond stimulated Raman spectroscopy J. Phys. Chem. Lett. 2011, 2, 1199. (47) Buchanan, L. E.; Gruenke, N. L.; McAnally, M. O.; Negru, B.; Mayhew, H. E.; Apkarian, V. A.; Schatz, G. C.; Van Duyne, R. P. Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy at 1 MHz Repetition Rates J. Phys. Chem. Lett. 2016, 7, 4629. (48) Leger, J. D.; Nyby, C. M.; Varner, C.; Tang, J.; Rubtsova, N. I.; Yue, Y.; Kireev, V. V.; Burtsev, V. D.; Qasim, L. N.; Rubtsov, G. I. Fully automated dual-frequency three-pulseecho 2DIR spectrometer accessing spectral range from 800 to 4000 wavenumbers Rev. Sci. Instr. 2014, 85, 083109. (49) Jortner, J.; Rice, S. A.; Hochstrasser, R. M. Radiationless Transitions in Photochemistry Adv. Photochem. Volume 7, 149. (50) Lovera, A.; Gallinet, B.; Nordlander, P.; Martin, O. J. Mechanisms of Fano resonances in coupled plasmonic systems ACS Nano 2013, 7, 4527. (51) Wu, X.; Gray, S. K.; Pelton, M. Quantum-dot-induced transparency in a nanoscale plasmonic resonator Opt. Express 2010, 18, 23633.


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Figure 1. Plasmonic bar array. (a, b) Scanning electron microscopy of the gold bar array with bar length 2 micron, width 260 nm, and height 80 nm. The image (a) was collected with normal electron beam incidence, while a tilt angle of 50 degrees was used in (b) showing an individual bar. (c) Image of the cross-section obtained by the focused ion beam milling of the bar spincoated with a PMMA layer, after the consequent deposition of additional layers of chromium and platinum. The measured thickness of the polymer is 20 nm. (d) Near-field distribution (absolute value) near the bar edge, as simulated by the FDTD method. Shown is a vertical slice cut through the center of the bar along its long dimension. White line shows the contour of the bar profile.


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Figure 2. Surface-enhanced spectra of PMMA layer on plasmonic arrays. First row – linear extinction spectra. Black line – total extinction; red line – Fano lineshape of the vibrational transition. The total peak extinction magnitude is ca. 2 OD, the peak-to-peak Fano signal is ca. 40 mOD. Second row – infrared transient absorption spectra. Third row – 2DIR spectra at the waiting time of t 2 =200 fs of a 20 nm PMMA layer. The corresponding lengths of the gold bars, plasmon resonance frequencies, and the Fano asymmetry parameters are: (a) L=2050 nm, ω pl =1810 cm-1, q =0.6; (b) L=2100 nm, ω pl =1780 cm-1, q =0.4; (c) L=2150 nm, ω pl =1710 cm-1, q =0.2; (d) L=2300 nm, ω pl =1630 cm-1, q =-0.6; (e) Spectrum of the 150 nm-thick PMMA layer measured without plasmonic array.


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Figure 3. Modeling of surface-enhanced spectra of PMMA layer on plasmonic arrays. First row – linear absorption spectra obtained based on equation (3). Second row – infrared transient absorption spectra obtained by integration of the 2DIR spectra along the excitation frequency axis. Third row – 2DIR spectra calculated based on equation (4). The corresponding plasmon resonance frequencies used as parameters in the model are: (a) ω pl =1810 cm-1; (b) ω pl =1780 cm1

; (c) ω pl =1710 cm-1; (d) ω pl =1630 cm-1; (e) Spectrum of the PMMA layer modeled without

plasmonic array using Lorentzian profile.


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Two-Dimensional Fano Lineshapes in Ultrafast Vibrational

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