Circularly Polarized Photoluminescence from Achiral Dye Molecules

Oct 1, 2018 - *E-mail: [email protected]. ... We report strong dissymmetry between left- and right-handed circularly polarized photoluminescence (PL) ...
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Article Cite This: J. Phys. Chem. C 2018, 122, 24924−24932

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Circularly Polarized Photoluminescence from Achiral Dye Molecules Induced by Plasmonic Two-Dimensional Chiral Nanostructures Khai Q. Le,† Shun Hashiyada,† Masaharu Kondo,‡ and Hiromi Okamoto*,†,§ †

Center for Mesoscopic Sciences, Institute for Molecular Science, 38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585, Japan Graduate School of Engineering, Nagoya Institute of Technology, Gakiso, Showa-ku, Nagoya, Aichi 466-8555, Japan § The Graduate University for Advanced Studies (Sokendai), 38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585, Japan ‡

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S Supporting Information *

ABSTRACT: We report strong dissymmetry between left- and right-handed circularly polarized photoluminescence (PL) enhancement induced by 2D chiral gold nanostructures, which can be utilized to provide a circularly polarized luminescence source. Lightning-boltlike (composed of two displaced rectangles) chiral plasmonic gold nanostructures were fabricated on a glass substrate and were adopted as materials to induce dissymmetry in PL enhancement. We employed achiral IR125 dye as an achiral molecular PL emitter with luminescence that was enhanced by near-field interaction between the chiral plasmon and the molecule. PL decay measurements confirmed that the PL enhancement arose from the plasmonic effect. Large PL enhancement dissymmetry factors g > 0.1 were obtained in the wavelength region near 800 nm. The dissymmetry of PL enhancement showed maximum amplitudes at 800−850 nm, which approximately correspond to the wavelength providing maximal extinction dissymmetry (∼800 nm), and is resonant with a chiral multipolar plasmon mode. The dissymmetry was relatively small at the wavelength resonant with a dipolar plasmon mode.

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metamaterials,15,16 chiral nanoantennas,17−20 and colloidal chiral nanoparticles,21 have been explored for their chiroptical effects. The application of chiral plasmonic nanostructures that enhance the chiro-optical effect significantly, such as the detection of chirality in biomolecules,13 chiral-selective trapping,22 chiral photoluminescence (PL),18 and so forth, is also of emerging interest. It has been also proposed that the chirality of luminescence from chiral molecules can be enhanced when the molecules are placed inside optical resonators of photonic structures that cause strong local field enhancement, thanks to the enhanced chiral Purcell effect.23 The utility of chiral nanostructures as circularly polarized luminescence (CPL) sources is yet another aspect of application research. CPL is a luminescence process that emits left- and right-handed CPL with different intensity. It provides information on excited-state properties of chiral molecules, which may provide a wide range of applications such as sensors, optical storage, display devices, and so forth.24 In addition, there is emerging interest in producing CPL for integrated on-chip applications.25,26 The promising way to achieve this goal is to develop chiral luminescent dye molecules that directly emit CPL. However, the synthesis of fluorescence molecules for chiral luminescence is sometimes quite complicated, and the dissymmetry factors of chiral dyes are, in general, low (e.g., dissymmetry factor of the luminescence was found to be ∼0.02 for bichromophoroic

INTRODUCTION The optical responses to circularly polarized light of chiral molecules such as amino acids, sugars, and helical DNA strands, which depend on their handedness, are known as optical activities; these optical responses include optical rotation and circular dichroism (CD). Such chiral molecules exhibiting CD activities absorb different amounts of lefthanded circularly polarized (LCP) and right-handed circularly polarized (RCP) incident light.1,2 The CD measurements play important roles in various fields associated with biological and pharmaceutical sciences1 and electromagnetics.3 Natural chiral molecules usually exhibit weak CD, in the range of only a few tens of millidegrees (in ellipticity) or less in the ultraviolet region. Attempts to expand the operating regime have led to the realization of artificial chiral nanostructures that exhibit strong optical activities over a broad frequency range, from visible to infrared.4−7 The optical near-field effects of these chiral nanostructured materials have recently become of emerging research interest regarding the fundamental characteristics of geometry- and position-dependent optical activity8−11 as well as the use of these effects in chemical applications and devices. In particular, the chiroptical effects of chiral molecules in the vicinity of metallic chiral nanostructures were found to be significantly enhanced by plasmonic near-field effects,12,13 with a proper design of chiral nanostructures. The enhancement of the sensitivity was estimated to be up to 106-fold. The CD activity of a chiral molecule could also be dramatically enhanced in the resonance wavelength region of a metallic nonchiral (achiral) nanocrystal.14 Various engineered chiral nanostructures, including © 2018 American Chemical Society

Received: July 29, 2018 Revised: September 28, 2018 Published: October 1, 2018 24924

DOI: 10.1021/acs.jpcc.8b07297 J. Phys. Chem. C 2018, 122, 24924−24932

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The Journal of Physical Chemistry C

handed (RH) Au nanostructures were fabricated on a glass substrate using a standard nanofabrication technique based on electron beam lithography and lift-off processing. Typical scanning electron micrograph images of the fabricated nanostructures are shown in Figure 1a−d. Each enantiomeric

perylene bisimide with cyclohexane moiety as the chiral central core24). An alternative approach is the combination of achiral dye molecules with chiral nanostructures. In fact, PL chirality (circular polarization) for fluorescent molecules was found to be induced when the molecules were properly placed near the chiral plasmonic nanostructures.18 A correlation was found between the PL enhancement dissymmetry and the numerically calculated near-field optical chirality with electromagnetic modeling. (Here PL enhancement is defined as IPL(λ)/IPL0(λ) where IPL(λ) and IPL0(λ) represent PL intensities from dyes at a wavelength, λ, with and without nanostructures, respectively.) The feature of such an emitted CPL was attributed to the chirality-selective enhancement of the host chiral nanostructures. The optical field enhancement is dependent on the handedness of the chiral nanostructure. It was also demonstrated that metamaterials enable chirality-selective enhancement of two-photon luminescence from a quantum emitter.27 The handedness of two-photon luminescence from achiral plasmonic nanoparticles embedded in a chiral dielectric host was also selective to the handedness of the host material.28 However, the physics behind the PL enhancement dissymmetry is still unexplored, which calls for further experimental investigation. In the present study, we investigate PL enhancement dissymmetry for achiral dye molecules placed on 2D chiral metal nanostructures. We experimentally demonstrate that the PL enhancement dissymmetry induced by the 2D chiral plasmonic metal nanostructures is correlated with the extinction dissymmetry of the metal nanostructures between left- and right-handed CPL. The dissymmetry factor of the circularly polarized PL enhancement obtained was even higher than that of previous related work.18 We also conducted time-resolved PL decay measurements to confirm that the PL enhancement was arising from the plasmonic effect. Furthermore, we found large dissymmetry in the wavelength region in resonance with the multipolar 2D chiral plasmon mode, whereas it showed only small dissymmetry at the wavelength of the dipolar mode. This is in good agreement with the previous work on plasmon polarimetry, where chiral field structure with steep gradients was considered to enhance the optical activity.13 The finding provides evidence of the strong PL enhancement dissymmetry being induced by the chiral plasmon resonance of the nanostructures in the wavelength region of the luminescence and provides a guideline for the efficient production of CPL.

Figure 1. 2D chiral plasmonic nanostructures (metasurfaces). (a−d) Scanning electron micrograph images of chiral metasurfaces consisting of left-handed (LH) (a,c) and right-handed (RH) (b,d) enantiomers. (e) Extinction spectra of the LH (solid curves) and RH (dotted curves) chiral nanostructures polarized along the x axis (black, “x-pol”) and the y axis (red, “y-pol”). (f) Boundary element method (BEM) simulated extinction cross sections with the boundary element method for the LH enantiomer under x-polarized and ypolarized conditions. A unit cell of the simulation model is given in Figure 8a. (g) Experimental setup for the measurement of extinction spectra shown in panel e.

nanostructure forms a periodic array with a total area of 200 × 200 μm2 and a pitch of 800 nm. The two enantiomer arrays were fabricated on the same sample substrate and separated by a distance of 2 mm to avoid their mutual interaction, which might cause diffraction and scattering by these neighboring nanostructures. The 50 nm thick gold nanostructures were formed by vacuum vapor deposition onto the substrate on a 2 nm chromium adhesion layer. The measured dimensions of the chiral nanostructures are given in Figure 1c,d. Optical Extinction and Extinction Dissymmetry Measurements. Far-field extinction spectra, which include contributions from absorption and scattering, were measured using a standard microscopic absorption measurement setup, as shown in Figure 1g. The extinction was defined as −log(I/ I0) where I and I0 denote light intensities through the nanostructure and in free space, respectively. To measure the extinction spectrum, white light from a stabilized halogen lamp was focused onto the arrayed nanostructure sample via an objective lens (20×, numerical aperture NA = 0.45). The positions of the nanostructures and the focused light spot were monitored with a charge-coupled device (CCD) camera. The transmitted light was collected using another objective lens (20×, NA = 0.40). The collected light was then coupled into a fiber-optic spectrometer through a 50 mm focal length lens to acquire the extinction spectrum.

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METHODS We employed a fluorescent dye with an achiral molecular structure as an achiral emitter placed in the vicinity of the 2D chiral plasmonic nanostructures. 2D chiral nanostructures consisting of two overlapping Au rectangles in a symmetrybroken arrangement (lightning-bolt structure) were adopted as the material to convert the achiral luminescence into a chiral (circularly polarized) light. We observed the PL from the dye molecules enhanced through near-field interaction between the molecules and the chiral plasmons, and dissymmetry between left- and right-handed circularly polarized PL enhancement induced by the plasmonic enhancement effect was analyzed. Sample Design and Fabrication. 2D chiral metamaterials, such as chiral metasurfaces, have been found to induce strong CD.3,6,16,17 In this study, we designed a planar 2D chiral nanostructure with a lightning-bolt-like shape as a structural unit of metasurfaces for circularly polarized PL enhancement measurements. The enantiomeric left-handed (LH) and right24925

DOI: 10.1021/acs.jpcc.8b07297 J. Phys. Chem. C 2018, 122, 24924−24932

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Figure 2. Extinction and extinction dissymmetry spectra of the chiral nanostructures. (a) Optical extinction dissymmetry spectral measurement setup. LP: linear polarizer, OL: objective lens, QWP: quarter-wave plate, 50 mm-L: 50 mm focal length lens, CCD: charge-coupled device camera, FM: folded mirror, NA: numerical aperture. (b,c) Left-handed (solid curves) and right-handed (dashed curves) circularly polarized extinction spectra of the LH (b) and RH (c) enantiomers. (d) Dissymmetry factor (gex) spectra for LH (black curve) and RH (red curve) enantiomers.

In the measurement of extinction dissymmetry, linearly polarized light, where LCP and RCP components contribute equally, was incident on the sample, and the transmitted LCP and RCP components were detected separately (Figure 2a). The arrayed chiral nanostructures were illuminated with a linearly polarized white light. The transmitted light was collected in the forward direction, and its ellipticity was analyzed using an achromatic quarter-wave plate and a linear polarizer. With the quarter-wave plate set at +45 and −45°, we measured the corresponding left- and right-handed circularly polarized extinction spectra (EXTLCP and EXTRCP, respectively). Next, we calculated the dissymmetry factor (gex) as follows gex = 2(EXTLCP − EXTRCP)/(EXTLCP + EXTRCP)

(1)

The ellipticity (a ratio of minor and major elliptical radius) of the circularly polarized light produced by the linear polarizer and the quarter-wave plate used for the detection system was found to be ∼0.99 on the bare substrate without the nanostructure. The polarization dependence of the fiber-optic spectrometer used was found to be negligible. PL Enhancement Dissymmetry Measurements. The poly(vinyl alcohol) (PVA) polymer film doped with IR125 dye molecules was deposited on the nanostructures by spin coating and baking processes. The IR125 fluorescent dye was purchased from Sigma-Aldrich. We mixed a 2.5 × 10−3 mol dm−3 solution of IR125 dye in methanol with the water solution of PVA (0.25 wt %) at a 1:1 volume ratio. The solution was spin-coated on the nanostructured sample substrate at speeds of 500 rpm for 10 s and then 3000 rpm for 70 s. The sample was then baked for 2 min at 95 °C to evaporate the solvents. The dye-doped PVA solution had an absorption maximum at 785 nm, and the thin film made from the solution showed a fluorescence maximum at 815 nm, as shown in Figure 3a. The optical setup for the PL enhancement dissymmetry measurements is illustrated in Figure 3b. The setup was basically the same as that for the extinction dissymmetry (Figure 2a), except for some filters that were inserted into the beam path to avoid the interference from the excitation light. We define PL enhancement here as the ratio of

Figure 3. Photoluminescence measurement setup. (a) Photoluminescence (in poly(vinyl alcohol) film) and absorption (in water/methanol solution) spectra of dye IR125. (b) Optical measurement setup for measuring photoluminescence from dye molecules. SPF: short-pass filter, LPF: long-pass filter. (See the caption to Figure 2 for other abbreviations.)

the PL intensity from the dye-doped PVA film on the nanostructures to the corresponding reference PL intensity measured at an area without nanostructures. The dissymmetry between left- and right-handed PL enhancement factors was measured and computed in a manner similar to that for the extinction dissymmetry. We pumped the dye molecules with a 670 nm continuous-wave diode laser (power 3 mW) impinging on the sample from the polymer-film side for PL measurements. The pump light was filtered out from the luminescence using a long-pass filter (cutoff wavelength 700 nm). A shortpass filter (cutoff wavelength 675 nm) was inserted to the pump beam path to avoid any influence on the pump light from the background luminescence. Near-Field Extinction Imaging. To assign the spectral bands to plasmon modes, we conducted near-field imaging at the wavelengths of the extinction peaks, which display the spatial structures of the modes, using a home-built 24926

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measured extinction spectra of the two enantiomers, particularly in the wavelength region longer than 700 nm, might be caused by the imperfection of the fabricated nanostructures. When the light is linearly polarized parallel to the x axis, both enantiomers exhibited a strong broad extinction band due to plasmon resonance with a peak at 683 nm. In contrast, when the light was linearly polarized along the y axis, the corresponding extinction intensities were much lower than those of x-polarized extinction and exhibited two separate extinction maxima at 620 and 780 nm. These extinction maxima are associated with the multipolar modes of the localized surface plasmon resonances of the metallic nanostructures, as revealed by near-field extinction images described below. The results of 3D BEM simulations to compute the extinction spectra of the nanostructures are shown in Figure 1f. Because the two enantiomeric nanostructures were perfectly symmetric to the linearly polarized light in the simulation, the simulated extinction spectra were identical. The simulated extinction at ∼620 nm was weaker than that at 780 nm. This result is in excellent agreement with the observed extinction spectra. Figure 5a,b shows the near-field extinction

illumination-mode aperture-type scanning near-field optical microscope (SNOM)29 (Figure 4). Linearly polarized light was

Figure 4. Schematic view of scanning near-field optical microscopy system. BPF: band-pass filter, LP: linear polarizer, HWP: half-wave plate, QWP: quarter-wave plate, NA: numerical aperture.

incident on the sample through the near-field probe aperture to excite the plasmon mode. The transmitted light (into the far-field) was collected with a microscope objective (20×, NA = 0.45), and its intensity was detected with a photodetector. To obtain linearly polarized light at the probe tip, quarter- and a half-wave plates were inserted into the beam path to compensate for the phase retardation arising from the optical fiber. The aperture diameter of the near-field fiber probe was typically 50−100 nm. Time-Resolved Fluorescence Decay Measurements. Details of the experimental setup for the fluorescence decay measurements are described in the previous report.30 In brief, the sample was excited by a femtosecond Ti:sapphire laser at 750 nm, and the emission in the wavelength region between 885 and 825 nm was analyzed with a 30 cm monochromator and a streak camera (Hamamatsu C10627-03). Simulation. The extinction of the chiral nanostructure and the spatial features of the plasmon mode were analyzed with the aid of numerical simulation. To compute the extinction cross-section of the nanostructure, we numerically solved the light−matter interaction problem via the full-wave Maxwell’s equations solver based on the 3D boundary element method (BEM) that is implemented in the MATLAB-based toolbox MNPBEM.31 A unit cell consisting of the nanostructure on a glass substrate (the dimensions taken from the measured ones in Figure 1c) was modeled with a periodic boundary condition to mimic the entire periodic array of the nanostructure. To achieve a fine resolution, the nanostructure was discretized using the function round, which is a MATLAB function compatible with the MNPBEM toolbox. The light impinging on the nanostructure was assumed to be a plane wave polarized in the direction along the x or y axis defined in Figure 1c. The complex refractive index of Au with realistic material losses provided in the refractive index library of MNPBEM was used for the calculation.

Figure 5. Near-field extinction images of the LH nanostructure and theoretically simulated images. (a,b) Extinction images measured by an aperture-type scanning near-field optical microscope at 600 and 780 nm, respectively. (c,d) Simulated x-component of the magnetic field amplitudes induced by the incident light at 600 and 780 nm, respectively. (e,f) Schematic electric fields associated with the plasmon modes resonant with 600 and 780 nm light, respectively.

images of the nanostructures at 600 and 780 nm, respectively, under the y-polarized excitation measured by a SNOM system sketched in Figure 4. We used a bandpass filter at 780 and 600 nm (transmission full width at half-maximum of 10 nm) to obtain the near-field image arising from the y-polarized plasmon resonances of the nanostructure at longer (780 nm) and shorter (620 nm) wavelengths, respectively. The simulated x components of the magnetic local field distributions at 20 nm above the nanostructure at these wavelengths (Figure 5c,d) closely resemble the observed near-field extinction images. This is consistent with the previously reported results on the close resemblance between the magnetic field distribution and the near-field extinction image of plasmonic nanoantennas.32 The dark regions of the near-field extinction images correspond to antinodes of the resonant plasmon modes. From the simulation, we can identify the phase relations among the antinodes of the modes. We found that all of the

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RESULTS AND DISCUSSION Extinction and Extinction Dissymmetry Spectra of the Metal Nanostructures. Under linearly polarized excitation conditions, the two fabricated enantiomeric nanostructures (Figure 1a−d) showed the same extinction spectra because they behaved symmetrically to the incident light, as shown in Figure 1e. The small discrepancy in the 24927

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The Journal of Physical Chemistry C antinodes observed in the near-field images (two antinodes for Figure 5b and three for Figure 5a) are in-phase (i.e., the polarizations are in the same directions). The y-polarized extinction peaks at 780 and 620 nm are thus attributable to the multipolar plasmon modes, as depicted in Figure 5f,e (we call them modes A and B), respectively. The x-polarized extinction peak at ∼680 nm is attributed to a transverse dipolar plasmon mode (we call this mode C). In the extinction dissymmetry measurements, we illuminated the arrayed chiral nanostructures with the linearly polarized white light and measured intensities of transmitted LCP and RCP components to obtain extinctions of the respective components. The resulting LCP and RCP extinction spectra of the LH enantiomer are shown in Figure 2b. The RCP extinction peak was shifted to blue relative to the LCP one. The situation is reversed for the RH counterpart, as shown in Figure 2c. The corresponding extinction dissymmetry factor spectra for the nanostructures are shown in Figure 2d, and their essential features are in common with the results obtained for the CD measurements (Supporting Information). Comparing the dissymmetry spectra with the extinction spectra and the mode assignments, the dissymmetry showed large amplitudes at the wavelength region of mode A (∼780 nm). In the wavelength region of mode C (∼680 nm), the dissymmetry amplitudes were small relative to their large extinction peak intensities, and in the wavelength region of mode B (∼620 nm), the dissymmetry signals were inverted. It has been discussed theoretically that even if the optical excitation of a material resonantly induces a single oscillating point dipole, local chiral optical fields are generated in the vicinity of the dipole.33 The local chiral fields are accompanied by local CD; in fact, the local CD induced by the oscillating dipole has been experimentally demonstrated.34,35 However, for a single oscillating dipole, the space-averaged total optical chirality is null, and it does not generate macroscopic extinction dissymmetry. This is supposed to be the reason why the extinction dissymmetry amplitudes were small in the wavelength region resonant with mode C of dipole character. When multiple oscillating dipoles are in two-dimensionally chiral spatial arrangement (as in Figure 5e,f) and the interaction among them causes imbalance between left- and right-handed circularly polarized fields, the space-averaged optical chirality remains at a nonzero value and yields macroscopic extinction dissymmetry. The large amplitudes of dissymmetry in the wavelength region resonant with mode A probably originated from this effect. It was pointed out for gammadion structures with a four-fold symmetry that 3D structural chirality arising from asymmetry in the z direction (substrate and/or adhesion layer, round surface structure, etc.) is necessary to produce the CD activity.36 In the present case, the z-direction asymmetry, in addition to the 2D chiral arrangement of the dipoles, might be also essential to explain the absorption dissymmetry behavior, although further detailed discussion is necessary. Plasmon-Induced PL Enhancement. Figure 6a shows the PL enhancement spectrum for the fluorescent dye molecules (IR125) placed in the vicinity of the fabricated LH nanostructures. The sample was prepared as described in the Methods section. The sample was irradiated with a laser output at 670 nm linearly polarized along the longitudinal (y) direction of the nanostructures. The PL enhancement spectrum shows the maximum amplitude at ∼850 nm, which is approximately resonant with the extinction peak (mode A)

Figure 6. PL enhancement spectrum and time-resolved PL decay curves. (a) Photoluminescence enhancement spectra under linearly polarized light excitation on the LH nanostructures (the direction of the linearly polarized excitation light is indicated by the arrow). (b) Time-resolved fluorescence decay measurements conducted on IR125 dye in the region with (circles) and without (triangles) the LH Au nanostructure. Solid curves are the results of fitting to double exponential functions.

of the LH nanostructure in the PVA environment, although the peak is shifted toward a longer wavelength side. On the basis of the previous studies on plasmon-enhanced fluorescence,37,38 the PL enhancement observed here is attributed to the nearfield interaction between the plasmon and the molecule. To confirm that the PL enhancement observed here was arising from the plasmonic origin, we conducted time-resolved PL decay measurements. The fluorescence was excited at 750 nm, and the emission in the wavelength region between 885 and 825 nm was analyzed. The observed fluorescence decay curves are shown in Figure 6b. The circles and triangles are measured PL decay curves for the IR125 dyes with and without Au nanostructures, respectively. The curves represent the results of fitting to double-exponential functions. The fitting parameters are summarized in Table 1. For the dye fluorescence on the substrate with Au nanostructure, the contribution of the faster (sub-100 ps) decay component was enhanced, and overall fluorescence decay rate became much higher than that without the nanostructure. This result indicates that the plasmonic structure promoted the radiative decay of the fluorescent molecules IR125. The PL enhancement for the dye molecules is thus considered to be of plasmonic (near-field interaction) origin. Circularly Polarized Luminescence Induced by Chiral Plasmons. We investigate the circular polarization of the enhanced PL from the achiral fluorescent dye molecules placed in the vicinity of the fabricated enantiomeric LH and RH nanostructures. The nanostructured sample was irradiated with a laser output at 670 nm linearly polarized along the longitudinal (y) direction of the nanostructures. Transversely (x-) polarized irradiation of light produced only weak PL enhancement, and its dissymmetry could not be determined. The PL intensity at the nanostructured area of the sample was divided by that at the nearby area without the nanostructure to obtain the PL enhancement spectrum. The PL enhancement spectra for left- and right-handed circularly polarized components of emitted light are shown in Figure 7a for the LH nanostructure array, and those for the RH are shown in Figure 7b. For both LH and RH enantiomers, clear differences between the LCP and RCP PL enhancement are found. For the LH nanostructure, the magnitude of the LCP PL 24928

DOI: 10.1021/acs.jpcc.8b07297 J. Phys. Chem. C 2018, 122, 24924−24932

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The Journal of Physical Chemistry C Table 1. Summary of Double-Exponential Fitting Parameters for the Fluorescence Decay Curvesa without nanostructure with nanostructure

A1

τ1/ns

A2

τ2/ns

0.344 ± 0.018 0.513 ± 0.011

0.081 ± 0.011 0.036 ± 0.003

0.656 ± 0.018 0.487 ± 0.011

0.695 ± 0.063 0.529 ± 0.029

Fitting function: I = A1 exp(−(t − t0)/τ1) + A2 exp(−(t − t0)/τ2) + I0, where t0 and I0 denote offset time and intensity, respectively.

a

Figure 7. Photoluminescence enhancement and its dissymmetry. Photoluminescence enhancement spectra of LCP (solid curves) and RCP (dashed curves) components for (a) the LH and (b) the RH enantiomers under linearly polarized light illuminations (the polarization of the excitation light is indicated by the arrows). (c) Photoluminescence enhancement dissymmetry factor (gpl) spectra for the LH (black curve) and RH (red curve) enantiomers. (d) Extinction dissymmetry factor (gex) spectra for the LH (black curve) and RH (red curve) enantiomers (the same spectra as those in Figure 2d).

denser distribution of the unit nanostructure. (The unit cell areas are (500 nm)2 and (800 nm)2 for the preceding and the present studies, respectively.) The dissymmetry amplitudes of PL enhancement showed maxima at 800−850 nm, which approximately correspond to the wavelength of maximum extinction dissymmetry (∼800 nm), as seen in Figure 7d. The dissymmetry appears to be small at ∼700 nm. Because of the long-pass filter to eliminate the excitation light, PL enhancement could not be observed in the wavelength range shorter than 700 nm, and thus we cannot investigate the behavior of PL enhancement dissymmetry in this wavelength region. In the wavelength region longer than 700 nm, the amplitudes of PL enhancement dissymmetry are large at the wavelength resonant with mode A, whereas they are small at the wavelength resonant with mode C. As described in the discussion in the previous subsection on extinction dissymmetry, mode C is attributed to the transverse plasmon mode of dipolar character; as a result, the total optical chirality averaged over the whole structure may be small. This is probably the reason why the PL enhancement dissymmetry is small in this wavelength region. In contrast, for mode A, two of the in-phase dipoles are arranged in a 2D chiral geometry, and thus the mode may interact strongly with chiral electromagnetic fields. This characteristic of the mode is supposed to be the origin of the strong PL enhancement dissymmetry in the wavelength region resonant with mode A. Suggestions from the Electromagnetic Field Simulation on the Mechanism of Circularly Polarized Luminescence. To gain insight into the mechanism of the

enhancement is higher than that of the RCP PL enhancement. In addition, a small PL enhancement peak shift due to the different plasmonic field enhancements to LCP and RCP is observed. The PL enhancement peak roughly corresponds to the longer wavelength peak of the y-polarized extinction in Figure 1e and is attributable to the plasmon mode A depicted in Figure 5f. The resulting dissymmetry factor between the LCP and RCP PL enhancements for the LH nanostructure, gpl, defined as g pl = 2(IPL,LCP − IPL,RCP)/(IPL,LCP + IPL,RCP)

(2)

where IPL,LCP and IPL,RCP represent LCP and RCP luminescence intensities, respectively, is shown in Figure 7c. The LCP PL enhancement induced by the RH enantiomer (Figure 7b) is almost equivalent to the RCP PL enhancement induced by the LH enantiomer. The handedness dependence of the circularly polarized PL enhancement is inverted when the handedness of the enantiomer is reversed. As a consequence, the PL enhancement dissymmetry is mirrored when the handedness of the enantiomer is inverted, as shown in Figure 7c. Compared with the related preceding work,18 the dissymmetry factor gpl at the peak wavelength of PL enhancement induced by our chiral nanostructure is higher than that of the preceding work, although the difference between LCP and RCP PL enhancements with our chiral nanostructure is smaller: The maximum gpl factor in the present work is ∼0.14 at ∼830 nm, whereas that in ref 18 was ∼0.11. The PL enhancement in the preceding work18 was larger than that of the present study, partly because of the 24929

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Figure 8. Electromagnetic model analysis for the local optical chirality of the LH nanostructure. (a) BEM simulation setup in which the electromagnetic fields were evaluated at the detection plane 5 nm above the metal nanostructure top surface. (b,c) Steady-state spatial distributions of the electric field enhancement and the normalized optical chirality, respectively, under excitation with x-linearly polarized light (LPL) and (d,e) those under excitation with y-LPL. The incident wavelength is 800 nm.

in ref 23, the similar dissymmetry effect is anticipated. On the basis of the theoretical framework,23 the dissymmetry of spontaneous emission probability for molecules in an optical cavity is predicted to be proportional to local optical chirality, C, at the position of the molecule. The luminescence from the achiral molecule at the site with large C is thus expected to be enhanced for either LCP or RCP component. Strong field enhancement is found under the y-polarized excitation at the upper right and lower left corners. At these corners, strong negative optical chirality is found. On average, over the whole calculation area, the optical chirality is biased toward a negative value (−0.090), as we found by integrating C for the ypolarized excitation. In the x-polarized excitation case, the absolute value of averaged C was much smaller (−0.044), which may indicate that the dissymmetry of PL enhancement is arising mainly from mode A, which is polarized along the y axis. The imbalance between the LCP and RCP luminescence from the achiral dye molecules can be interpreted as arising from the averaged C over the whole system being biased to negative values. On the upper-left and lower-right arms of the LH nanostructure, optical chirality forms four lobes, two with positive values of C and two with negative C. This feature of optical chirality with four lobes corresponds to that expected in the circumference of an optically induced oscillating dipole. The wavelength of excitation for the calculation is nearly resonant with mode A, with an extinction peak at ∼780 nm. As demonstrated in Figure 5, this mode possesses two antinodes of oscillating polarization in the upper-left and lower-right arms. The four-lobe features of optical chirality are understood as arising from the oscillating polarizations in both arms. If the two oscillating polarizations are arranged axial-symmetrically, then the positive and negative values of optical chirality balance to zero on average over the whole structure. We may consider that the chiral nanostructure broke this balance, thereby yielding negative optical chirality on average. In contrast, the field intensity and optical chirality are weak in the

CPL from the dye molecules in the vicinities of the chiral plasmonic nanostructures, electromagnetic field simulations of local field enhancement and optical chirality may be informative. We performed an electromagnetic modeling of a linearly polarized optical field interacting with the chiral nanostructure using the 3D BEM31 and evaluated the optical chirality33,39 distribution in the vicinity of the nanostructure. In this simulation, the effect of the substrate on the local fields was taken into account. We present here only the results for the LH nanostructure. The results for the RH enantiomer are equivalent to those for the LH, but the sense is reversed. The structure model for the simulation was the same as that for the simulation of the extinction spectra. We assumed linearly polarized light impinging on the nanostructure from the substrate side as a plane wave at 800 nm, which is close to the mode A peak wavelength. We evaluated the electromagnetic field and the optical chirality at a detection plane 5 nm above the top surface of the gold nanostructure. We should note that the dye molecules were dispersed only in the one side of the metal nanostructure. The electromagnetic field at the other side of the nanostructure (i.e., in the substrate) has no direct effects on PL enhancement. The geometrical arrangement for the simulation is illustrated in Figure 8a. Figure 8b,d depicts the steady-state distributions of electromagnetic field enhancement (|E|/|Efree|, where E is the electric field evaluated and Efree is that at the free space) under x- and y-polarized illuminations. The optical chirality (C) was computed as C = −(1/ 2)ε0ωIm(E*·B).30,38 Figure 8c,e shows that the optical chirality at each position is normalized with that for left circularly polarized light in free space, CLCP = ε0ω|E|2/(2c), where c denotes the light velocity. It was theoretically proposed that CD of chiral dye molecules in an achiral nanostructured cavity is enhanced due to the Purcell effect.23 Although what we deal with in the present study is fluorescence dissymmetry for achiral dye molecules under interaction with chiral nanostructures, which is a different system from that discussed 24930

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The Journal of Physical Chemistry C entire area over the nanostructures under the x-polarization excitation. From these results, it may be suggested that the dye molecules with the transition moment directions oriented parallel to the y axis (in particular, those in the vicinities of upper-right and lower-left corners) interact with mode A and emit CPL.

Frontier Network Program of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

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(1) Berova, N.; Nakanishi, K.; Woody, R. W. Circular Dichroism: Principles and Applications; VCH: New York, 1994. (2) Barron, L. D. Molecular Light Scattering and Optical Activity; Cambridge University Press, Cambridge, England, 2004. (3) Zvezdin, A. K.; Kotov, V. A. Modern Magnetooptics and Magnetooptical Materials; Taylor & Francis, New York, 1997. (4) Valev, V. K.; Baumberg, J. J.; Sibilia, C.; Verbiest, T. Chirality and Chiroptical Effects in Plasmonic Nanostructures: Fundamental, Recent Progress, and Outlook. Adv. Mater. 2013, 25, 2517−2534. (5) Yeom, J.; Yeom, B.; Chan, H.; Smith, K. W.; DominguezMedina, S.; Bahng, J. H.; Zhao, G.; Chang, W.-S.; Chang, S.-J.; Chuvilin, A.; et al. Chiral Templating of Self-Assembling Nanostructures by Circularly Polarized Light. Nat. Mater. 2015, 14, 66−72. (6) Konishi, K.; Nomura, M.; Kumagai, N.; Iwamoto, S.; Arakawa, Y.; Kuwata-Gonokami, M. Circularly Polarized Light Emission from Semiconductor Planar Chiral Nanostructures. Phys. Rev. Lett. 2011, 106, 057402. (7) Kuwata-Gonokami, M.; Saito, N.; Ino, Y.; Kauranen, M.; Jefimovs, K.; Vallius, T.; Turunen, J.; Svirko, Y. Giant Optical Activity in Quasi-Two-Dimensional Planar Nanostructures. Phys. Rev. Lett. 2005, 95, 227401. (8) Narushima, T.; Okamoto, H. Circular Dichroism Nano-Imaging of Two-Dimensional Chiral Metal Nanostructures. Phys. Chem. Chem. Phys. 2013, 15, 13805−13809. (9) Hashiyada, S.; Narushima, T.; Okamoto, H. Local Optical Activity in Achiral Two-Dimensional Gold Nanostructures. J. Phys. Chem. C 2014, 118, 22229−22233. (10) Narushima, T.; Hashiyada, S.; Okamoto, H. Nanoscopic Study on Developing Optical Activity with Increasing Chirality for TwoDimensional Metal Nanostructures. ACS Photonics 2014, 1, 732−738. (11) Narushima, T.; Okamoto, H. Strong Nanoscale Optical Activity Localized in Two-Dimensional Chiral Metal Nanostructures. J. Phys. Chem. C 2013, 117, 23964−23969. (12) Tullius, R.; Karimullah, A. S.; Rodier, M.; Fitzpatrick, B.; Gadegaard, N.; Barron, L. D.; Rotello, V. M.; Cooke, G.; Lapthorn, A.; Kadodwala, M. Superchiral Spectroscopy: Detection of Protein Higher Order Hierarchical Structure with Chiral Plasmonic Nanostructures. J. Am. Chem. Soc. 2015, 137, 8380−8383. (13) Hendry, E.; Carpy, T.; Johnston, J.; Popland, M.; Mikhaylovskiy, R. V.; Lapthorn, A. J.; Kelly, S. M.; Barron, L. D.; Gadegaard, N.; Kadodwala, M. Ultrasensitive Detection and Characterization of Biomolecules Using Superchiral Fields. Nat. Nanotechnol. 2010, 5, 783−787. (14) Govorov, A. O. Plasmon-Induced Circular Dichroism of a Chiral Molecule in the Vicinity of Metal Nanocrystals. Application to Various Geometries. J. Phys. Chem. C 2011, 115, 7914−7923. (15) Oh, S. S.; Hess, O. Chiral Metamaterials: Enhancement and Control of Optical Activity and Circular Dichroism. Nano Convergence 2015, 2, 24. (16) Wang, Z.; Cheng, F.; Winsor, T.; Liu, Y. Optical Chiral Metamaterials: A Review of the Fundamentals, Fabrication Methods, and Applications. Nanotechnology 2016, 27, 412001. (17) Valev, V. K.; Smisdom, N.; Silhanek, A. V.; De Clercq, B.; Gillijns, W.; Ameloot, M.; Moshchalkov, V. V.; Verbiest, T. Plasmonic Ratchet Wheels: Switching Circular Dichroism by Arranging Chiral Nanostructures. Nano Lett. 2009, 9, 3945−3948. (18) Meinzer, N.; Hendry, E.; Barnes, W. L. Probing the Chiral Nature of Electromagnetic Fields Surrounding Plasmonic Nanostructures. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 041407. (19) Cotrufo, M.; Osorio, C. I.; Koenderink, A. F. Spin-Dependent Emission from Arrays of Planar Chiral Nanoantennas Due to Lattice and Localized Plasmon Resonances. ACS Nano 2016, 10, 3389−3397. (20) Kruk, S. S.; Decker, M.; Staude, I.; Schlecht, S.; Greppmair, M.; Neshev, D. N.; Kivshar, Y. S. Spin-Polarized Photon Emission by

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CONCLUSIONS We experimentally investigated the circularly polarized PL from achiral dye molecules placed within the near-field regime of plasmonic chiral metasurfaces. The metasurfaces consisted of nanostructures with lightning-bolt-like shapes, where two rectangles are arranged in a 2D chiral geometry. We found strong dissymmetry between the left- and right-handed circularly polarized PL enhancement induced by plasmonic enhancement effects. The extinction spectral bands of the 2D chiral nanostructure were assigned to dipolar and multipolar plasmon modes with the aid of near-field imaging and electromagnetic simulation. The extinction dissymmetry was measured to analyze the correlation between the PL enhancement and extinction dissymmetries. In particular, by comparing the longitudinal multipolar plasmon mode (mode A) and the transverse mode of dipolar character (mode C), the multipolar mode was found to yield stronger dissymmetry of PL enhancement. This observation is presumably attributable to the chiral arrangement of oscillating polarization in the multipolar mode, although further experimental investigation on the generality of the phenomenon and theoretical analyses is necessary. When the general correlation between the plasmon mode characters and the PL enhancement dissymmetry is established, it may provide a valuable guideline and strategy for designing nanoscale circularly polarized light sources, which may be exploited to construct on-chip optical devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07297.

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REFERENCES

CD spectroscopic measurements (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-564-55-7320. ORCID

Khai Q. Le: 0000-0002-3272-6477 Shun Hashiyada: 0000-0002-3229-538X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ms. A. Ishikawa (IMS) for the nanostructured sample fabrication. We also thank Dr. T. Narushima (IMS) and Prof. T. Dewa (Nagoya Inst. Tech.) for useful discussion and support. This work was supported by Grants-in-Aid for Scientific Research (KAKENHI) (nos. JP22225002, JP15H02161, JP15K13683, and JP16H06505 to H.O. and nos. JP15J01261 and JP17H07330 to S.H.), by the JSPS Coreto-Core Program (A. Advanced Research Networks) from the Japan Society for the Promotion of Science, and by the Photon 24931

DOI: 10.1021/acs.jpcc.8b07297 J. Phys. Chem. C 2018, 122, 24924−24932

Article

The Journal of Physical Chemistry C Resonant Multipolar Nanoantennas. ACS Photonics 2014, 1, 1218− 1223. (21) Shen, X.; Asenjo-Garcia, A.; Liu, Q.; Jiang, Q.; García de Abajo, F. J.; Liu, N.; Ding, B. Three-Dimensional Plasmonic Chiral Tetramers Assembled by DNA Origami. Nano Lett. 2013, 13, 2128−2133. (22) Zhao, Z.; Saleh, A. E.; van de Haar, M. A.; Baum, B.; Briggs, J. A.; Lay, A.; Reyes-Becerra, O. A.; Dionne, J. A. Nanoscopic Control and Quantification of Entantioselective Optical Forces. Nat. Nanotechnol. 2017, 12, 1055−1059. (23) Yoo, S.; Park, Q.-H. Chiral Light-Matter Interaction in Optical Resonators. Phys. Rev. Lett. 2015, 114, 203003. (24) Kumar, J.; Nakashima, T.; Kawai, T. Circularly Polarized Luminescence in Chiral Molecules and Supramolecular Assemblies. J. Phys. Chem. Lett. 2015, 6, 3445−3452. (25) He, L.; Li, M. On-Chip Synthesis of Circularly Polarized Light Emission of Light with Integrated Photonic Circuits. Opt. Lett. 2014, 39, 2553−2556. (26) Li, W.; Coppens, Z. J.; Besteiro, L. V.; Wang, W.; Govorov, A. O.; Valentine, J. Circularly Polarized Light Detection with Hot Electrons in Chiral Plasmonic Metamaterials. Nat. Commun. 2015, 6, 8379. (27) Rodrigues, S. P.; Cui, Y.; Lan, S.; Kang, L.; Cai, W. Metamaterials Enable Chiral-Selective Enhancement of Two-Photon Luminescence from a Quantum Emitter. Adv. Mater. 2015, 27, 1124− 1130. (28) Singh, H. J.; Saumitra; Singh, V. R.; Sikdar, S. K.; Jayaprakash, B.; Ghosh, A. Circular Differtial Two-Photon Luminescence from Hellical Arranged Plasmonic Nanoparticles. ACS Photonics 2016, 3, 863−868. (29) Imura, K.; Okamoto, H. Reciprocity in Scanning Near-Field Optical Microscopy: Illumination and Collection Modes of Transmission Measurements. Opt. Lett. 2006, 31, 1474−1476. (30) Noji, T.; Suzuki, T.; Kondo, M.; Jin, T.; Kawakami, K.; Mizuno, T.; Oh-oka, H.; Ikeuchi, M.; Nango, M.; Amao, Y.; et al. LightInduced Hydrogen Production by Photosystem I−Pt Nanoparticle Conjugates Immobilized in Porous Glass Plate Nanopores. Res. Chem. Intermed. 2016, 42, 7731−7742. (31) Hohenester, U.; Trugler, A. MNPBEM − A Matlab Toolbox for the Simulation of Plasmonic Nanoparticles. Comput. Phys. Commun. 2012, 183, 370−381. (32) Denkova, D.; Verellen, N.; Silhanek, A. V.; Valev, V. K.; Dorpe, P. V.; Moshchalkov, V. V. Mapping of Magnetic Near-Field Distributions of Plasmonic Nanoantennas. ACS Nano 2013, 7, 3168−3176. (33) Schäferling, M.; Yin, X.; Giessen, H. Formation of Chiral Fields in a Symmetric Environment. Opt. Express 2012, 20, 26326−26336. (34) Nishiyama, Y.; Okamoto, H. Near-Field Nonlinear CD Imaging of Single Gold Nanostructures. J. Phys. Chem. C 2016, 120, 28157− 28162. (35) Hashiyada, S.; Narushima, T.; Okamoto, H. Imaging Chirality of Optical Fields near Achiral Metal Nanostructures Excited with Linearly Polarized Light. ACS Photonics 2018, 5, 1486−1492. (36) Arteaga, O.; Sancho-Parramon, J.; Nichols, S.; Maoz, B. M.; Canillas, A.; Bosch, S.; Markovich, G.; Kahr, B. Relation between 2D/ 3D Chirality and the appearance of Chiroptical Effects in Real Nanostructures. Opt. Express 2016, 24, 2242−2252. (37) Abadeer, N. S.; Brennan, M. R.; Wilson, W. L.; Murphy, C. J. Distance and Plasmon Wavelength Dependent Fluorescence of Molecules Bound to Silica-Coated Gold Nanorods. ACS Nano 2014, 8, 8392−8406. (38) Li, J.; Krasavin, A. V.; Webster, L.; Segovia, P.; Zayats, A. V.; Richards, D. Spectral Variation of Fluorescence Lifetime Near Single Metal Nanoparticles. Sci. Rep. 2016, 6, 21349. (39) Tang, Y.; Cohen, A. E. Optical Chirality and Its Interaction with Matter. Phys. Rev. Lett. 2010, 104, 163901.

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DOI: 10.1021/acs.jpcc.8b07297 J. Phys. Chem. C 2018, 122, 24924−24932