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Near-Field Nonlinear CD Imaging of Single Gold Nanostructures Yoshio Nishiyama, and Hiromi Okamoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07315 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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The Journal of Physical Chemistry

Near-field Nonlinear CD Imaging of Single Gold Nanostructures Yoshio Nishiyama,†§* Hiromi Okamoto†‡* † Institute for Molecular Science, Myodaiji, 38 Nishigonaka, Okazaki, Aichi 444-8585, Japan ‡ The Graduate University for Advanced Studies (Sokendai), Myodaiji, 38 Nishigonaka, Okazaki, Aichi 444-8585, Japan § Present address: Faculty of Chemistry, Institute of Science and Engineering, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan * Email: [email protected] (YN); [email protected] (HO).

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ABSTRACT

We demonstrate near-field nonlinear circular dichroism (CD) imaging of single rectangular (achiral) gold nanostructures using a two-photon excitation method. The gold rectangles were illuminated by pulses of circularly polarized light (CPL) to generate two-photon excitation images of the longitudinal plasmon modes, and the images consisted of oval-shaped spatial features (corresponding to the antinodes of the plasmon modes) tilted from the long axis of the rectangles. The tilting direction depended on the handedness (left or right) of the CPL used for illumination, which resulted in the observation of a strong local dissymmetry of the two-photon excitation signals. The tilts of the oval features were not observed under linearly polarized pulse illumination with any polarization direction. The nonlinear CD images constructed from the differential two-photon excitation probability for left- and right-CPL pulses exhibited spatial features that were reasonably explained by the multipolar characters of the excited plasmon modes.


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I. Introduction Surface plasmon resonances in noble metal nanostructures allow us to confine optical fields on the nanoscale, resulting in the local enhancement of the fields.1 Confined optical fields have not only been utilized for surface-enhanced spectroscopies2 but have also provided various new applications in the fields of photonics and nanotechnologies.3 In recent years, it has been theoretically demonstrated that plasmons generate highly twisted optical fields,4,5 attracting much attention to the chiroptical properties of plasmonic materials, such as circular dichroism (CD). Most chiral molecules exhibit only weak optical activity such that their CD signals, defined as differential absorbance for left- and right-circularly polarized light (CPL), are ~1/1000 times the intensity of their absorbance, or even less.6 In contrast, the CD signals of chiral metal nanostructures are sometimes orders of magnitude greater than the typical CD signals of molecules.7,8 It has also been demonstrated that the detection sensitivity of chiral molecules is drastically improved by a polarimetry technique based on plasmonic excitation.9,10 Highly twisted optical fields in the vicinity of the metal nanostructures are a promising basis for experimental techniques capable of ultrasensitive chiroptical detection.11 To understand and exploit the characteristics of plasmon-induced chiral optical fields, spatially resolved observation of local optical activity on the metal nanostructures is informative. Recently, our group developed a nanoscale CD imaging system based on scanning near-field optical microscopy (SNOM) and investigated the local optical activity of metal nanostructures.12 The local optical activity of the nanostructures observed in the near-field CD images was found to depend strongly on position on the nanostructures and CD signals with both directions of handedness coexisting in single nanostructures.12 The CD signals were locally very large (as high as ~1/10 times the extinction strength at the same wavelength) even in chiral metal


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nanostructures with considerably weak macroscopic optical activity13 or even in achiral metal nanostructures with no macroscopic optical activity.14 As revealed in these works, the local CD of nanostructures are governed by local geometrical symmetry of the surrounding area of the measured position, rather than the chirality of the whole nanostructure. The local site of the material is in general optically active if the mirror image of the site is not superimposable with the original one. To further improve the sensitivity and reliability of the near-field CD spectroscopy and imaging, the introduction of a nonlinear optical method, specifically a two-photon excitation method, is promising because of the background-free character in signal detection and the improved spatial resolution due to the nonlinearity. Recently, Valev et al. reported far-field second-harmonic (SH) imaging of metal nanostructures under left- and right-CPL illumination to characterize the chiral nature of the nanostructures, in which they found spatial features in SH images that were dependent on the handedness of the incident CPL.15 In the present study, we report the near-field nonlinear CD imaging of single rectangular gold nanostructures. Using this imaging, we detected two-photon-induced photoluminescence (TPI-PL) from gold in the visible region after excitation by a near-infrared pulse. The two-photon excitation process of the photoluminescence is drastically accelerated by the plasmon resonance.16 This effect of resonance on TPI-PL can be applied to plasmon mode imaging, and our group has previously demonstrated high-contrast imaging of the spatial structure of plasmon modes in various gold nanostructures by near-field two-photon excitation imaging based on TPI-PL detection.17,18,19 As a representative case, a gold nanorod (or elongated rectangle) has longitudinal plasmon modes that yield standing waves of the collective oscillation of conduction electrons along the long axis of the rod (or the rectangle). The spatial structures of the multipolar longitudinal plasmon modes


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were visualized using near-field two-photon excitation imaging.16,17 Regarding the optical activity of the rod (or rectangle), these plasmon mode structures are geometrically achiral and have no net macroscopic optical activity because of the geometrical symmetry. However, even achiral nanostructures generate chiral optical fields locally through plasmonic excitation20 and in fact show strong local optical activity as described above.14 It is highly probable that combination of near-field two-photon excitation imaging method and CD measurement provides unique technique for analysis of characteristics of chiral plasmons. In this work, we developed a new method for near-field CD imaging based on a nonlinear optical process (two-photon excitation) and investigated local optical activity on elongated, rectangular gold nanostructures at the resonant plasmon wavelengths. The CD imaging measurements revealed the strong optical activity of the nanorectangles and spatial features that reflect the symmetry of the rectangles and the excited plasmon-mode structures. We have found that the spatial features obtained has a close relevance to optical chirality associated with an oscillating dipole. II. Experimental Section The near-field nonlinear CD imaging measurement system used in the present study was based on an aperture-type SNOM that was essentially the same as that reported previously.16,17,18,19,21 The SNOM was equipped with an optical fiber probe with an aperture diameter of 100 nm for near-field illumination. To obtain near-field images, the distance between the tip apex and the sample surface was maintained at ~10 nm using a shear-force feedback mechanism. For twophoton excitation of the gold nanostructures, we used narrow spectral-band femtosecond pulses from a wavelength-tunable Ti:Sapphire laser (wavelength range 680‒1080 nm, spectral full


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width at half maximum < 5 nm) as a light source. The TPI-PL (in the wavelength range of 500‒ 650 nm) from the rectangular gold nanostructures was detected as the optical signal. The polarization of the incident pulse was controlled by half- and quarter-waveplates before the fiber probe. To analyze the circular polarization state of the incident pulse at the probe aperture, the circularly polarized pulse transmitted through the sample substrate was converted into linearly polarized light by a quarter-waveplate and the polarization degree was measured by a linear polarizer and a photodetector. The purities of incident circular polarization were found to be over 95% for both left- and right-CPL pulses. The remaining linearly polarized and/or unpolarized components do not affect seriously the CD signal because the signals obtained in the gold nanostructures are sufficiently large (several tens of percent in g value in eq. (1)), as described below. To obtain information on the resonant wavelength of the plasmon modes, we performed static near-field transmission measurements. We used a Xe discharge lamp as a light source and analyzed the light transmitted through the probe and the sample using a spectrometer. We prepared elongated, rectangular gold nanostructures with two different dimensions (A and B) on a glass substrate. These nanostructures were fabricated by an electron beam lithography and lift-off technique. The 20 nm thick gold films that formed the nanorectangles were vapor deposited on an underlying 2 nm thick Cr adhesion layer. The lengths and widths of the rectangles were evaluated using scanning electron micrographs (Fig. 1) and listed in Table 1. III. Result and Discussion Figure 2(a) shows the near-field extinction spectrum of the rectangular nanostructure A (120 nm in length). An extinction peak was observed at 830 nm with the linear polarization parallel to the long axis of the rectangle. A near-field transmission image at the peak wavelength (Fig. 2(b))


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exhibited a strong extinction at the center of the rectangle, and no nodes were observed. This maximum extinction at the center corresponds to the antinode of the wavefunction of the excited plasmon mode. From this feature, the extinction band was identified as a dipolar longitudinal plasmon mode. We obtained near-field two-photon excitation images of the rectangle at this wavelength. The two-photon excitation image with incident linear polarization parallel to the long axis (Fig. 2(c)) exhibited a spatial feature attributable to the dipolar mode in the same manner as the transmission image. Similar features were also observed when the incident polarizations were rotated by ±45° (Figs. 2(d) and (e)). At ±45° polarizations, two-photon excitation process in TPI-PL can be mediated by either the longitudinal or the transverse resonant plasmon modes of the rectangle. However, the transverse resonance appears at the shorter wavelength (~520 nm),21 which is sufficiently distant from the incident wavelength. Therefore, the longitudinal plasmon resonance makes a dominant contribution to the TPI-PL under irradiation by light with a polarization rotated from the long axis of the rectangle, which rationalizes the observation of essentially the same spatial features in the three images described above (Figs. 2(c), (d), and (e)). In contrast, when left- and right-CPL pulses irradiated the sample, the spatial features of the two-photon excitation images were clearly tilted (Figs. 2(f) and (g)). Given that the spatial features are independent of the incident linear polarization direction, as mentioned above, these spatial features can never be explained as a result of linear dichroism, and their origin must be a result of the chiral interaction between CPL and the rectangles. The same trend was observed for another elongated rectangle B (240 nm in length). As shown in Fig. 3(a), the rectangle has a plasmon resonance at 840 nm. The two-photon excitation image at the resonant wavelength shows the spatial structure of the 2nd plasmon mode with a node at the center (Fig. 3(b)). The spatial features were independent of the direction of the incident


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polarizations (Figs. 3(c) and (d)) as long as the irradiation was linearly polarized. In contrast, two-photon excitation images under left- and right-CPL illumination showed tilted spatial features that were not observed under linearly polarized illumination, in a manner similar to rectangle A (Figs. 3(e) and (f)). The difference between the TPI-PL intensity via left-CPL and that via right-CPL at a given point on the sample represents the magnitude of local optical activity. We constructed near-field nonlinear (two-photon excitation) CD images with the dissymmetry factor gobs defined as follows.6 g obs = 2

obs obs I LCP − I RCP obs obs I LCP + I RCP

,

(1)

obs obs where I LCP and I RCP are the TPI-PL intensities excited by left- and right-CPL pulses,

respectively. In reality, the intensity of radiation emitted from the near-field probe tip depends on the polarization condition: the left-CPL intensity at the tip is sometimes different from the rightCPL intensity. Thus, the observed TPI-PL intensity was corrected for the incident CPL intensity. Considering that the TPI-PL intensity is proportional to the squared incident light intensity, the obs obs TPI-PL intensity at each polarization ( I LCP and I RCP ) was normalized by the square of the

measured intensity of the transmitted light from the probe through the substrate. We obtained a left-CPL-excited TPI-PL image and a right-CPL-excited image separately. There was a slight positional deviation between the two images. We compensated for the positional deviation by comparing the central positions of the rectangle observed in the topographic images that were obtained simultaneously with the two-photon excitation images.


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In the near-field two-photon CD image of rectangle A, the positive and negative extrema of the local CD signals were observed at the four corners of the rectangle (Fig. 4(a)). The local CD signal distribution showed a point-symmetric feature about the center of the rectangle, which correctly reflects the geometrical symmetry of the nanostructure. The extremal CD signals had the same sign at the diagonal corners and opposite signs at adjacent ones. The spatial feature of the CD images can be interpreted in terms of local geometrical symmetry of the rectangle. Local environment around the upper-left corner of the rectangle is the same as a mirror image of that around the upper-right corner. These two corners are geometrically not superimposable, which means that the corners are locally chiral. Therefore, both corners of the rectangle show strong local CD signals with opposite signs. The gobs values at the extremal positions were as high as several tens of percent. The dissymmetry factor increases in terms of two-photon excitation. (When the dissymmetry is much smaller than unity, the dissymmetry factor for two-photon process will be two times that for one-photon process.) Strong local CD signals were reported previously for metal nanostructures,12,13,14 and they also yielded large dissymmetry factors for the nonlinear CD. In the nonlinear CD image of rectangle B, positive and negative extrema were observed not only at the corners but also near the center of the rectangle (Fig. 4(b)). Notably, the spatial structure of the signal distribution observed in the CD images of both rectangles A and B tends to be nearly point-symmetric with respect to the antinode of the excited plasmon modes (i.e., the center of rectangle A and the positions located slightly on the inner sides from both ends in rectangle B). This finding strongly suggests that the observed spatial features of CD images are correlated with the chiral nature of the longitudinal plasmon modes of the rectangular gold nanostructures.


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To examine the effect of plasmon resonance on the observed CD images, we conducted electromagnetic simulations using the finite-difference time-domain (FDTD) method. In the simulation, we adopted an optical configuration that was quasi-reciprocal to the experimental one to facilitate the calculation (the reciprocal arrangement is presumed to yield the same optical response to the original arrangement under an ideal case). Thus, a longitudinal plasmon resonance of a rectangular nanostructure was excited by propagating light with a linear polarization parallel to the long axis of the rectangle and left- and right-CPL components of the local optical field in the vicinity of the nanostructure were evaluated. In the experiment, left- and right-CPL pulses were locally irradiated on samples through the aperture probe and the plasmonmediated TPI-PL was detected. This arrangement is not exactly equivalent to the reciprocal optical configuration adopted in the simulation, in which the sample is locally irradiated with CPL and the propagating light with the linear polarization parallel to the long axis of the rectangle is detected. However, the plasmonic excitation process for the experimental optical configuration is equivalent to that for the reciprocal configuration of the simulated one. The simulated result might be therefore useful to qualitatively understand the experimental result. A rectangular gold nanostructure with the same dimension as rectangle A (Table 1) was located on the glass substrate. A radiation field linearly polarized along the long axis of the rectangle, at a wavelength of 830 nm, was incident on the system, and the local electric fields on the plane 20 nm below the rectangle were evaluated (Fig. 5(a)). Figures 5(b) and (c) show intensity cal cal distributions of left- and right-CPL components ( I LCP (x, y), I RCP (x, y)) of the calculated optical

fields on the evaluation plane, and Fig. 5(d) shows the spatial map of the dissymmetry factor for

(

cal cal − I RCP the calculated left- and right-CPL intensities, g cal ( x , y) = 2 I LCP

) (I

cal LCP

)

cal + I RCP . The

cal cal intensities I LCP and I RCP at each position were averaged over a circular area with a diameter of


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100 nm corresponding to the probe aperture diameter in the experiment. The spatial features of the calculated intensity and CD maps qualitatively reproduced the experimental two-photon excitation images (Figs. 2(f) and (g)) and the near-field nonlinear CD image (Fig. 4(a)). We also calculated intensity maps of linear polarization components (shown in Fig. S3). The maps do not show tilted spatial feature, unlike those of circular polarization components, which is also consistent with the experimental observation. Furthermore, the present calculation showed that the observed local CD can be interpreted qualitatively in terms of mutual configuration of locally generated electric and magnetic dipole moments (shown in Fig. S2). The inner product of electric and magnetic transition dipoles, that may describe the CD intensity, as analogous to molecules, is null at the center of the rectangle, while it is non-zero at the corners. When the wavelength of the incident light was shifted to a much longer wavelength (1000 nm) that was off-resonant with the plasmon mode, the spatial distributions of the left- and right-CPL components were nearly cal identical to each other and the dissymmetry g was vanishingly small (Figs. 5(e), (f) and (g)).

This result strongly supports the idea that the observed CD images originate in the longitudinal plasmon resonances of the rectangles. In general, highly twisted chiral optical fields are generated when the dissymmetry factor at the position is prominent. Schäferling et al. characterized the spatial structure of a chiral optical field generated by an oscillating dipole with optical chirality defined via an inner product of electric and magnetic fields.20 The spatial features of our near-field CD image show a close similarity to the calculated spatial distribution of optical chirality. These results indicate that near-field nonlinear CD imaging is a valuable method for the characterization of the spatial feature of chiral optical fields. In summary, we conducted near-field nonlinear CD imaging measurements on single rectangular gold nanostructures and analyzed the local optical activity under resonance with the


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longitudinal plasmon modes. When left- and right-CPL pulses were used to irradiate and excite the rectangles, the obtained two-photon excitation images consisted of oval spatial features tilted from the long axis of the rectangles that were not observed under linearly polarized pulse irradiation. The nonlinear CD images exhibited spatial features that reflected spatial characters of the excited plasmon modes and indicated that local CD signal was induced by the local chirality of the rectangular nanostructure. The spatial features of the CD signals were qualitatively well explained by those of optical chirality in the periphery of an oscillating point dipole (for the dipolar mode) or an assembly of that (for the multipolar mode). These results indicate that the present method provides a useful tool for the sensitive detection and imaging of locally twisted optical fields, primarily because of the background-free character of detection and the improved spatial resolution. The sensitive imaging of local chirality will be helpful to application of chiral plasmon resonances to, for example, ultrasensitive chiroptical detection systems. In the future, this method will allow the characterization of chiral natures of not only plasmonic nanostructures but also various other materials with smaller extinction coefficients, such as nanocarbons. Acknowledgement The authors thank Dr. T. Narushima for his assistance in developing the present SNOM system and Ms. A. Ishikawa (IMS) for the nanostructured sample fabrication. This work was supported in part by Grants-in-Aid for Scientific Research (JSPS KAKENHI) (Grant Nos. JP22225002, JP25810013, JP15H02161, JP16H06505) from the Japan Society for the Promotion of Science. Corresponding Author *E-mail: [email protected]. Fax: +81-564-54-2254. Tel.: +81-564-55-7320.


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Supporting Information Available: Near-field nonlinear CD images of gold rectangular nanostructures with the same designed dimensions as rectangle A, and simulated CD distribution and intensity maps of linear polarization components for rectangle A based on FDTD calculation. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES: (1) Schuller, J. A.; Barnard, E. S.; Cai, W.; Jun, Y. C.; White, J. S.; Brongersma, M. L. Plasmonics for Extreme Light Concentration and Manipulation. Nat. Mater. 2010, 9, 193−204.

(2) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection Using Surface-enhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667−1670.

(3) Maier, S. A. Plasmonics: Fundamentals and Applications: Fundamentals and Applications; Springer: Berlin, 2007. (4) Hendry, E.; Mikhaylovskiy, R. V.; Barron, L. D.; Kadodwala, M.; Davis, T. J. Chiral Electromagnetic Fields Generated by Arrays of Nanoslits. Nano Lett. 2012, 12, 3640−3644.

(5) Davis, T. J.; Hendry, E. Superchiral Electromagnetic Fields Created by Surface Plasmons in Nonchiral Metallic Nanostructures. Phys. Rev. B 2013, 87, 085405. (6) Barron, L. D. Molecular light scattering and optical activity; Cambridge University Press, Cambridge, England, 2004. (7) Vallius, T.; Jefimovs, K.; Turunen, J.; Vahimaa, P.; Svirko, Y. Optical Activity in Subwavelength-period Arrays of Chiral Metallic Particles. Appl. Phys. Lett. 2003, 83, 234−236.


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(8) 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. (9) 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.

(10) Abdulrahman, N. A.; Fan, Z.; Tonooka, T.; Kelly, S. M.; Gadegaard, N.; Hendry, E.; Govorov, A. O.; Kadodwala, M. Induced Chirality though Electromagnetic Coupling between Chiral Molecular Layers and Plasmonic Nanostructures. Nano Lett. 2012, 12, 977−983.

(11) Tang, Y.; Cohen, A. E. Optical Chirality and Its Interaction with Matter. Phys. Rev. Lett. 2010, 104, 163901. (12) Narushima, T.; Okamoto, H. Circular Dichroism Nano-imaging of two-dimensional Chiral Metal Nanostructures. Phys. Chem. Chem. Phys. 2013, 15, 13805−13809.

(13) Narushima, T.; Okamoto, H. Strong Nanoscale Optical Activity Localized in Twodimensional Chiral Metal Nanostructures, J. Phys. Chem. C, 2013, 117, 23964-23969. (14) Hashiyada, S; Narushima, T.; Okamoto, H. Local Optical Activity in Achiral Twodimensional Gold Nanostructures. J. Phys. Chem. C, 2014, 118, 22229–22233 (15) Valev, V. K.; Baumberg, J. J.; De Clercq, B.; Braz, N.; Zheng, X.; Osley, E. J.; Vandendriessche, S.; Hojeij, M.; Blejean, C.; Mertens, J. Nonlinear Superchiral Meta-surfaces:


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Tuning Chirality and Disentangling Non-reciprocity at the Nanoscale. Adv. Mater. 2014, 26, 4074– 4081 (16) Imura, K.; Nagahara, T.; Okamoto, H. Near-field Two-photon-induced Photoluminescence from Single Gold Nanorods and Imaging of Plasmon Modes. J. Phys. Chem. B 2005, 109, 13214 −13220.

(17) Imura, K.; Nagahara, T.; Okamoto, H. Plasmon Mode Imaging of Single Gold Nanorods. J. Am. Chem. Soc. 2004, 126, 12730−12731.

(18) Imura, K.; Nagahara, T.; Okamoto, H. Photoluminescence from Gold Nanoplates Induced by Near-field Two-photon Absorption. Appl. Phys. Lett., 2006, 88, 023104 (19) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Visualization of Localized Intense Optical Fields in Single Gold-nanoparticle Assemblies and Ultrasensitive Raman Active Sites. Nano Lett., 2006, 6, 2173–2176 (20) Schäferling, M.; Dregely, D.; Hentschel, M.; Giessen, H. Tailoring Enhanced Optical Chirality: Design Principles for Chiral Plasmonic Nanostructures. Phys. Rev. X 2012, 2, 031010. (21) Imura, K.; Nagahara, T.; Okamoto, H. Imaging of Surface Plasmon and Ultrafast Dynamics in Gold Nanorods by Near-field Microscopy. J. Phys. Chem. B 2004, 108, 16344−16347.


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Figure Captions Figure 1. (a) and (b) Scanning electron micrographs of rectangular gold nanostructures A and B, respectively. Scale bars: 100 nm. Figure 2. (a) Near-field extinction spectrum of gold nanorectangle A. (b) Near-field transmission image of rectangle A observed at a wavelength of 830 nm. Incident polarization is parallel to the long axis of the rectangle in both (a) and (b). (c)‒(g) Near-field two-photon excitation images of rectangle A at the excitation wavelength of 830 nm. Incident pulses are (c) linearly polarized, parallel to the long axis of the rectangle, (d) linearly polarized, rotated by 45° from the long axis, (e) linearly polarized, rotated by −45° from the long axis, (f) left-circularly polarized, and (g) right-circularly polarized. The dotted lines represent the approximate shape of the rectangle. Scale bars: 100 nm. Figure 3. (a) Near-field extinction spectrum of gold nanorectangle B. (b)‒(f) Near-field twophoton excitation images of rectangle B at the excitation wavelength of 840 nm. Incident pulses are (b) linearly polarized, parallel to the long axis of the rectangle, (c) linearly polarized, rotated by 45° from the long axis, (d) linearly polarized, rotated by −45° from the long axis, (e) leftcircularly polarized, and (f) right-circularly polarized. The dotted lines represent the approximate shape of the rectangle. Scale bars: 100 nm. Figure 4. (a) and (b) Near-field two-photon CD images of rectangles A and B, respectively. The CD signals (gobs) were not evaluated for the black areas outside the rectangular nanostructures because of low TPI-PL intensity. The dotted lines represent the approximate positions of the rectangles. Scale bars: 100 nm.


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Figure 5. (a) The model for the FDTD analysis of the optical fields. The dotted line represents the position of the plane where the optical fields were evaluated. (b) and (c) Intensity maps of left-CPL and right-CPL components, respectively, and

(d) dissymmetry factor gcal, under

illumination at 830 nm. (e) and (f) Intensity maps of left-CPL and right-CPL components, respectively, and (g) dissymmetry factor gcal, under illumination at 1000 nm.


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Table 1. Dimensions of the gold rectangular nanostructures. The gold nanostructures, with a thickness of 20 nm, were formed on a 2 nm thick Cr adhesion layer.

A

Length / nm 120

Width / nm 60

B

240

70


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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TOC GRAPHICS


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Figure 1. (a) and (b) Scanning electron micrographs of rectangular gold nanostructures A and B, respectively. Scale bars: 100 nm. 66x122mm (300 x 300 DPI)



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Figure 2. (a) Near-field extinction spectrum of gold nanorectangle A. (b) Near-field transmission image of rectangle A observed at a wavelength of 830 nm. Incident polarization is parallel to the long axis of the rectangle in both (a) and (b). (c)‒(g) Near-field two-photon excitation images of rectangle A at the excitation wavelength of 830 nm. Incident pulses are (c) linearly polarized, parallel to the long axis of the rectangle, (d) linearly polarized, rotated by 45° from the long axis, (e) linearly polarized, rotated by −45° from the long axis, (f) left-circularly polarized, and (g) right-circularly polarized. The dotted lines represent the approximate shape of the rectangle. Scale bars: 100 nm. 129x234mm (300 x 300 DPI)


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Figure 3. (a) Near-field extinction spectrum of gold nanorectangle B. (b)‒(f) Near-field two-photon excitation images of rectangle B at the excitation wavelength of 840 nm. Incident pulses are (b) linearly polarized, parallel to the long axis of the rectangle, (c) linearly polarized, rotated by 45° from the long axis, (d) linearly polarized, rotated by −45° from the long axis, (e) left-circularly polarized, and (f) right-circularly polarized. The dotted lines represent the approximate shape of the rectangle. Scale bars: 100 nm. 177x217mm (300 x 300 DPI)



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Figure 4. (a) and (b) Near-field two-photon CD images of rectangles A and B, respectively. The CD signals (gobs) were not evaluated for the black areas outside the rectangular nanostructures because of low TPI-PL intensity. The dotted lines represent the approximate positions of the rectangles. Scale bars: 100 nm. 87x118mm (300 x 300 DPI)


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Figure 5. (a) The model for the FDTD analysis of the optical fields. The dotted line represents the position of the plane where the optical fields were evaluated. (b) and (c) Intensity maps of left-CPL and right-CPL components, respectively, and (d) dissymmetry factor gcal, under illumination at 830 nm. (e) and (f) Intensity maps of left-CPL and right-CPL components, respectively, and (g) dissymmetry factor gcal, under illumination at 1000 nm. 215x182mm (300 x 300 DPI)



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85x46mm (300 x 300 DPI)


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Near-Field Nonlinear CD Imaging of Single Gold ... - ACS Publications

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