Article pubs.acs.org/JPCC
Antenna-Enhanced Nonlinear Infrared Spectroscopy in Reflection Geometry Ikki Morichika,† Fumiya Kusa,†,‡ Akinobu Takegami,†,‡ Atsunori Sakurai,† and Satoshi Ashihara*,† †
Institute of Industrial Science, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo, 153-8505, Japan Department of Applied Physics, Tokyo University of Agriculture and Technology, 2-24-16, Nakacho, Koganei, Tokyo, 184-8588, Japan
‡
S Supporting Information *
ABSTRACT: We propose and demonstrate antenna-enhanced nonlinear infrared (IR) spectroscopy in reflection geometry. Our approach uses resonant metal nanoantennas to enhance near-fields and to amplify the interaction between molecular vibrations and IR light. We successfully obtain amplified nonlinear vibrational signals in backscattering light of nanoantennas using IR pump energy of 10 nJ, with local signal enhancement of more than 7 orders of magnitude. This ultrasensitive and reflection-type method is useful for characterizing the structure and dynamics of minute volumes of molecules, monolayer materials, and biomolecules in aqueous environments, with additional benefit of surface sensitivity.
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INTRODUCTION Infrared (IR) spectroscopy, enabling direct access to vibrational fingerprints, is a powerful tool that provides detailed information about molecular structures. In the last two decades, the high-power, high-stability femtosecond IR pulsed laser has enabled nonlinear spectroscopy, in particular, IR pump−probe spectroscopy and two-dimensional infrared (2DIR) spectroscopy.1 These techniques use ultrashort pulse sequences to excite and detect molecular vibrations, providing abundant information on molecular structures and dynamics, such as threee-dimensional structure of peptides and proteins, vibrational relaxation, and energy transfer.2−5 Despite their fascinating potential, however, their applications are somewhat limited because of the relatively weak absorption cross sections of molecular vibrations. The experiments require IR pump pulses of typically microjoule level, generated by the complex frequency conversion system based on the regenerative amplifier. A molecular vibration should have relatively large absorption cross section in order that its nonlinear vibrational signal is observed. Furthermore, observation of nonlinear vibrational signals from a small number of molecules is a current challenge. One promising approach for increasing sensitivity is to amplify the interaction of molecular vibrations with IR light by using plasmonic near-field enhancements. In the past, it has been demonstrated that linear vibrational signals of molecules adsorbed on metallic nanostructures were amplified in IR absorption spectroscopy (surface-enhanced infrared absorption spectroscopy: SEIRAS).6−11 In particular, by using rod-shaped nanoantennas resonant with a molecular vibration, signal enhancement of 5 orders of magnitude was achieved.8 The © XXXX American Chemical Society
plasmonic near-field enhancements should be useful for increasing nonlinear vibrational signals as well. Rather, the benefits of the near-field enhancements would be larger for the nonlinear spectroscopy because nonlinear signals increase superlinearly with the field strengths. Recently, it has been demonstrated that nonlinear vibrational signals are amplified using colloidal gold nanoparticles12 and randomly arranged resonant nanoantennas.13 However, further study is necessary to increase the sensitivity and to add new functionalities for wide use in material science and biology. In this paper, we propose a reflection-type of the antennaenhanced nonlinear vibrational spectroscopy, and conduct its proof-of-principle experiments. We demonstrate that nonlinear vibrational signals of molecules are obtained in backscattering light, that is to say, reflected light of nanoantenna arrays, with local signal enhancement of more than 7 orders of magnitude. The demonstrated reflection-type method not only has benefits of substantial signal enhancement and surface sensitivity, but also overcomes the issues of strong background absorption, which tends to restrict the measurements in transmission geometry (e.g., biomolecules under aqueous environments). The reflection-type is also advantageous in that interpretation of the amplified vibrational spectrum of the coupled antenna− molecule system is straightforward. Received: February 23, 2017 Revised: April 26, 2017
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DOI: 10.1021/acs.jpcc.7b01798 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
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complex-valued factor (α̃ AG)2, which we call a linear signal enhancement factor and corresponds to the square of the field enhancement factor. Because α̃ A,α̃ V and G are complex-valued and frequency-dependent, the interference between the antenna’s dipole excited directly by incident light and that excited indirectly via antenna−molecule coupling can be either constructive or destructive, depending on the driving frequency ω. Similarly, eq 2 indicates that the nanoantenna amplifies the nonlinear vibrational polarization by the complex-valued factor (α̃ AG)4, which we call a nonlinear signal enhancement factor and corresponds to the fourth-power of the field enhancement. Reflection from Periodic Nanoantenna Arrays. In our method, resonant nanoantennas are arranged periodically, and the light is incident from the substrate side. The scattering from nanoantennas, containing linear or nonlinear vibrational signals of molecules, is detected as the zeroth-order diffraction or the reflection.10,19 It is known that the elements of periodical nanoantenna arrays interact with each other through scattering light.20,21 When the array pitch in the direction of the radiative dipolar coupling matches with multiples of the effective wavelength (or the wavelength in the substrate), constructive interference happens between incident field and the scattered field at each nanoantenna. Under this diffractive coupling condition, the nanoantenna is driven more strongly, bringing about even larger field enhancement. In transmission geometry, the enhanced signal originates from the extinction of the antenna, which is “a mixture” of absorption and scattering. Here the spectral modulation due to the antenna−molecule coupling is different between absorption and scattering. Furthermore, the spectral modulation changes depending on the scattering−absorption ratio of the antenna and on the antenna−molecule coupling strength.22 In contrast, the enhanced signal in reflection geometry originates from “pure” scattering of the antenna. Therefore, the interpretation of the amplified vibrational spectrum is more straightforward for the reflection geometry.
PRINCIPLES Linear and Nonlinear Responses of the Antenna− Molecule System. Gold rod-shaped nanoantennas of a few microns in length exhibit their half-wave dipole antenna resonances in the IR spectral range.14 Molecular vibrational signals, coupled with such resonant antennas, are significantly amplified with Fano-like spectral features.8,9 These features are explained as a result of interference between the antenna’s dipole excited directly by an incident light and that excited indirectly via antenna−molecule coupling (Figure 1). By using
Figure 1. Schematic of the coupled point-dipole model. Eincident denotes the incident field and Esignal denotes the scattered field.
the coupled point−dipole model,15−18 the effective linear polarizability of the nanoantenna, α̃ LA,eff(ω), and the effective nonlinear polarizability, in particular, for the pump−probe spectroscopy, α̃ NL A,eff(ω), can be written as αA,eff ̃ L (ω) = αà + (αà G)2 αṼ
(1)
* αA,eff ̃ NL (ω) = (αà G)4 αṼ (3)EpuEpu
(2)
(see Supporting Information “linear response of antenna− molecule system” and “nonlinear response of antenna− molecule system”). Here, ω is optical frequency, α̃ A and α̃ V are the linear polarizabilities of the nanoantenna and the molecule, respectively, α̃ V(3) is the third-order nonlinear polarizability of the molecule, Epu is an incident electric field of the pump, and G is the Green function describing an electric field at the position of the molecule (the antenna) produced by a radiating unit dipole located at the position of the antenna (the molecule). Note that we describe Equations 1 and 2 for the case that a single nanoantenna is coupled with a single molecule, for simplicity. When a nanoantenna is coupled with multiple molecules, we should simply sum up contributions from these molecules. Equation 1 indicates that the nanoantenna amplifies the linear vibrational polarization by the
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EXPERIMENTAL METHODS Fabrication of Gold Nanoantenna Arrays. Gold nanorod 2D arrays of rectangular lattice are fabricated on a CaF2 substrate (20 mm in diameter, 1 mm in thickness). Electronbeam resist (FEP-171) is spin-coated with a thickness of 300 nm, exposed by electron beam, and developed. Then, a 5-nmthick Cr-adhesion layer followed by a 30-nm-thick Au layer are thermally evaporated, and lifted off by acetone. The nanorods are 100 nm wide, 30 nm high, and 1100 nm long. The period of
Figure 2. Schematic diagram of the pump−probe reflection spectroscopy. OPA and DFG denote an optical parametric amplifier and difference frequency generator, respectively. Both the pump and probe pulses are linearly polarized within the plane of the paper. B
DOI: 10.1021/acs.jpcc.7b01798 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C arrangement along x-axis (parallel to the longer axes of the nanorods) and y-axis (parallel to the shorter-axes of the nanorods) is set to be dx = 1.6 μm and dy = 2.7 μm, respectively. The value of dx is set so that the near-field coupling among the nanorods is negligible, and the value of dy is set to be slightly smaller than, but close to, the diffractive coupling condition for the resonance frequency of the target molecular vibrational mode. FT-IR and IR Pump−Probe Spectroscopy. We choose triply degenerated T1u CO stretching mode of W(CO)6 as our target mode. A toluene solution of W(CO)6 and poly(methyl methacrylate) (PMMA) is spin-coated on the nanoantenna arrays (6000 rpm, 60 s), and a 200-nm-thick PMMA film dispersed with W(CO)6 is formed. Linear reflection spectra are measured for unpolarized light with 4 cm−1 resolution by FT-IR spectrometer (VERTEX 70v, Bruker) coupled to IR microscope (HYPERION 3000, Bruker). As a reference, a transmission spectrum of the sample film without nanoantennas is measured for linearly polarized light with 4 cm−1-resolution by FT-IR spectrometer (FT/IR4000, JASCO). Note that the transmission spectrum of the sample film without nanoantennas is the same for polarized and unpolarized light. These reflection/transmission FT-IR measurements are performed under the normal incidence. Single-color pump−probe reflection spectroscopy is performed by using 4 μJ, 5). Chem. Phys. Lett. 2004, 392, 156−161.
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DOI: 10.1021/acs.jpcc.7b01798 J. Phys. Chem. C XXXX, XXX, XXX−XXX
