Tailoring the interaction between matter and polarized light with

Invited Paper

Tailoring the interaction between matter and polarized light with plasmonic optical antennas P. Biagioni*a, X. Wub, M. Savoinia, J. Zieglerb, J.-S. Huangc, L. Duòa, M. Finazzia, B. Hechtb CNISM – Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, I-20133 Milano, Italy b Nano-Optics & Biophotonics Group, Department of Experimental Physics 5, Röntgen Research Center for Complex Material Systems (RCCM), Physics Institute, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany c Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan a

ABSTRACT We explore the possibility to control the polarization state of light confined into sub-diffraction volumes by means of plasmonic optical antennas. To this aim, we describe a resonant cross antenna, constituted of two perpendicular two-wire antennas sharing the same gap, which is able to maintain the polarization state in the plane of the antenna. We also discuss how, by proper tuning of the arm length in a slightly off-resonance cross antenna, it is possible to effectively realize a nanoscale quarter-waveplate antenna. We present experimental results for the preparation of individual cross antennas by means of focused ion beam milling starting from single-crystalline Au microflakes, and finally show preliminary characterization results based on two-photon photoluminescence confocal imaging with linearly-polarized light. Keywords: Plasmonics, nanoantennas, polarimetry.

1. INTRODUCTION The analysis and control of polarized fields on the nanometer scale is a crucial issue for many developments in nanoplasmonics, since the scaling down of widely used optical techniques relies upon the availability of polarized near fields [1]. The possibility of shaping the polarization properties of local fields, moreover, would open the road towards controlled interaction between polarized light and matter at the nanoscale. Resonant optical antennas, which have been recognized as one of the most promising way to enhance the interaction between light and nano-objects, have been realized mainly as linear antennas so far [2, 3, 4]. Coupling to matter at the nanoscale is therefore restricted to transitions with dipole moment projections oriented along the antenna axis [5]. For this reason, linear antennas cannot be employed when full control of the light polarization state is required, e.g. in optical techniques relying on polarization modulation or in polarization-encoded data storage [6]. Moreover, linear antennas completely rule out applications involving circularly-polarized fields, e.g. nanoscale mapping of molecular chirality [7] or sub-diffraction all-optical magneto-recording [8]. Here we introduce a cross antenna structure, constituted by two perpendicular dipole antennas with a common feed gap, and show that with this novel configuration an arbitrary polarization state can be created in the shared feedgap in the plane of the antenna [9, 10]. We also present a strategy for the fabrication of cross antennas by focused ion-beam (FIB) milling, based on our previous experience with linear antennas, starting from single-crystalline Au microflakes grown by chemical methods [11]. Such prototype cross antenna structures are characterized by two-photon photoluminescence (TPPL) imaging with linearly-polarized light. Finally, we discuss possible strategies to experimentally characterize the degree of circular polarization in the feed-gap of a cross antenna. * [email protected]; phone +39 02 2399 6170; fax +39 02 2399 6126

Synthesis and Photonics of Nanoscale Materials VIII, edited by David B. Geohegan, Jan J. Dubowski, Frank Träger, Proc. of SPIE Vol. 7922, 79220C · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.876571

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2. POLARIZATION MAINTENANCE WITH SYMMETRIC CROSS ANTENNAS A prototype of the proposed structure is shown in Fig. 1(a). It consists of two perpendicular linear antennas sharing the same feedgap volume. The width and thickness of each arm are about 50 and 40 nm, respectively, with a gap size of about 40 nm. Nanostructuring of the antenna is achieved by FIB milling, starting from single-crystalline Au microflakes grown by well-established chemical methods [11]. Typical lateral sizes of flakes range from 10 to 40 μm, with a thickness of 40-80 nm. After preparation in solution, the Au flakes are drop-casted onto a microscope cover glass coated with a thin ITO layer to avoid charging effects during FIB milling and characterization of the resulting nanostructures by scanning electron microscopy (SEM) imaging. In the chamber of a dual-column Helios Nanolab (FEI Company), a suitable flake is chosen by SEM by considering its lateral dimensions, thickness, and adhesion to the ITO substrate, before FIB is applied to obtain the desired nanostructure. The use of single-crystalline flakes as a substrate for subsequent FIB milling is a key factor in obtaining reproducible and well-defined nanoantennas. This is because grain boundaries in standard multi-crystalline materials are avoided which show anisotropic response to the FIB milling and therefore often limit the quality and precision of resulting structures [11]. The polarization maintenance property of the cross antenna structure stems from the fact that every arbitrarily polarized plane-wave field, propagating along the direction perpendicular to the antenna plane, can be decomposed into two perpendicular components, with suitable amplitudes and a well-defined phase relation. Each of the two perpendicular antennas picks up one of such field components and coherently confines it in the gap volume, where the original polarization state is therefore reconstructed [9]. It is important to note that the key to this behavior is the fact that two linear antennas when arranged at an angle of 90o exhibit two linearly independent, orthogonal resonances.

Figure 1. Simulation results for a cross antenna prototype. Panel (a): SEM image of the simulated nanoantenna; panel (b): near-field intensity enhancement spectrum; panels (c) and (d): maps of the local field intensity enhancement on resonance, after circularly-polarized illumination, in a plane cutting mid-height through the antenna structure [panel (c)] or located 10 nm above the upper antenna surface [panel (d)]; panels (e) and (f): maps of the degree of circular polarization C in the same planes as for panels (c) and (d), respectively; panels (g) and (h): maps for the figure of merit f in the same planes as for panels (c) and (d), respectively. Small field asymmetries are due to non perfectly symmetric meshing of the simulated structure.

In order to analyze the expected polarization response of the experimentally realized prototype shown in Fig. 1(a), we perform finite-difference time-domain simulations (FDTD Solutions, Lumerical Solutions, Inc., Canada). To obtain realistic results, we perform simulations for an arrangement as close as possible to the experimentally realized structures.

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Both a 100-nm thick ITO layer and the underlying glass substrate are included in the simulation. As a relevant application case, and without loss of generality, we illuminate the antenna with a circularly-polarized focused Gaussian beam (NA=0.6) and calculate the spatially-resolved, steady-state near-field response of the antenna as well as the spectrum of the near-field intensity enhancement in the feedgap [Fig. 1(b)].The spectrum clearly reveals a single fundamental antenna resonance as expected for two orthogonal and degenerate antenna resonances which therefore do not hybridize [12]. In Fig. 1(c) a field intensity enhancement map is plotted on resonance in a plane parallel to the substrate cutting mid-height through the antenna structure. Intensity localization in proximity of the gap region is clearly observed, with an enhancement factor up to about 140 in the middle of the gap and higher than 300 in the four lateral hot spots that are created between adjacent wires. It is also significant from an experimental point of view to consider the field distribution in a plane located 10 nm above the upper antenna surface [Fig. 1(d)], where a more uniform intensity hot spot with about 60-fold enhancement is obtained. In this plane, the relative weight of the lateral hot spots decreases because such fields possess stronger localization and therefore do not extend significantly above the antenna. In order to evaluate the quality of the polarization state in the gap, we calculate a modified degree of circular polarization C, which also takes depolarization due to longitudinal fields into account, as [9, 10]

C=2 where

E x (t ) E y (t ) sin(δ x − δ y ) E x2 (t ) + E y2 (t ) + E z2 (t )

• denotes time average, the xy plane is the antenna plane, E(t) is the electric field amplitude, and δ denotes the

respective absolute phases. For perfect circular polarization in the xy plane, one therefore expects C = ±1 . Maps of C are shown in Figs. 1(e) and 1(f), calculated in the two relevant planes of Figs. 1(c) and 1(d), respectively. An almost unitary degree of circular polarization is apparent in the central part of the gap region. Also, we consider the figure of merit f = I × C2 [9] in order to evaluate the effective circularly-polarized hot spot that is available for practical applications. Figures 1(g) and 1(h) clearly show that such an antenna can be used to efficiently concentrate (localize and enhance) propagating fields in the subwavelength gap without perturbing their polarization state.

3. POLARIZATION CONTROL WITH ASYMMETRIC CROSS ANTENNAS Plasmonic antennas show strong resonances in the amplitude of the charge oscillation which is accompanied by the typical harmonic-oscillator-like behavior of the phase shift between the driving field and the local charge oscillations close to a resonance. By proper tuning of the length of an antenna for a fixed illumination wavelength, one can therefore tailor its phase response. In this context, cross nanoantennas offer the unique opportunity of independently controlling the phase of two orthogonal resonances, thus deterministic polarization shaping can be achieved [10]. For example, one can design an asymmetric cross antenna to generate circularly-polarized near fields starting from a linearly-polarized far-field illumination. In order to achieve this, we make one of the two linear antennas longer and the other one shorter than the resonance length, with the aim of maintaining the same local field enhancement for the two field components but add a 90° phase shift between them [10]. Fig. 2 shows the simulated degree of circular polarization C for an asymmetric nanoantenna, designed with different optimized arm lengths in order to achieve quarter-wave plate behavior at 830 nm wavelength, but otherwise with the same parameters as in Fig. 1. The antenna is illuminated with linearly-polarized light oriented at 45° with respect to the antenna axes. It is clearly demonstrated that a spot of circularly polarized light is generated in the middle of the gap thanks to the accumulated phase shift between the two components.

Figure 2. Simulated map of the degree of circular polarization for an asymmetric cross antenna after linearly-polarized far-field illumination. The arm lengths are optimized in order to achieve a quarter-wave plate behavior at 830 nm.

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4. EXPERIMENTAL CHARACTERIZATION OF CROSS ANTENNA PROTOTYPES In order to test the behavior of symmetric cross antenna prototypes, we prepared an array of structures with nominal length ranging from 220 to 360 nm in steps of 20 nm. A SEM image of the whole array is shown in Fig. 3(a). We address individual cross antennas by means of TPPL scanning confocal microscopy [13]. The experimental setup exploits ultrashort laser pulses (duration around 90 fs) from a standard Ti:sapphire laser (Tiger-200, Time Bandwidth Products, CH) centered at 830 nm with a repetition rate of 80 MHz and an average power of 20 μW. The pulses are coupled into a short piece (about 75 cm) of optical fiber and then delivered to an inverted confocal microscope equipped with an oilimmersion objective (Zeiss, Plan-Apochromat, NA=1.4). The pulse duration at the exit of the fiber is about 650 fs. The excitation beam is linearly polarized with a cube polarizer and a half-wave plate and then focused onto the sample. TPPL is collected in reflection geometry by the same objective, spectrally filtered by a short-pass filter (cut-off around 785 nm), and eventually collected by a single-photon avalanche photodiode (Perkin-Elmer, APCM-ARQ 13). A representative TPPL map, acquired after linearly-polarized excitation, is shown in Fig. 3(b). In order to preliminarily test the polarization maintenance properties of the cross antennas, we acquire a TPPL map for each of five different orientations of the impinging electric field, namely 0°, 30°, 45°, 60°, and 90° with respect to the horizontal antenna arms in Fig. 3(a). For a polarization maintaining antenna, an isotropic response would be expected for different angles of linear polarization.

Figure 3. Experimental TPPL characterization of cross antenna prototypes. Panel (a): SEM image of the investigated sample; panel (b): TPPL map after linearly-polarized excitation (field polarization is horizontal in this image); panel (c): TPPL intensity as a function of the antenna length (dots) and simulated near-field intensity enhancement (solid line); panel (d): representative polar plot for the square root of the TPPL counts CTPPL from a resonant cross antenna [dashed circle in panels (a) and (b)] as a function of the polarization angle of the excitation beam.

We first integrate the photon counts over each antenna spot in Fig. 3(b) and plot them as a function of the antenna length in Fig. 3(c). Each point corresponds to one antenna, while the solid line shows the simulated near-field intensity enhancement for comparison. The resonant behavior, when varying the antenna length for a fixed illumination wavelength, is fairly reproduced, with a very small shift which can be attributed to differences between nominal and actual geometries and/or small corrections to the dielectric functions used for simulations. In order to evaluate the polarization performances of each antenna, we now consider the TPPL response as a function of the orientation of the linear polarization. Since TPPL is a two-photon process, we take the square root of the TPPL counts CTPPL to directly gain insight into local field intensities. A representative polar plot for TPPL for the 280-nmlong resonant antenna is shown in Fig. 3(d). Since for a perfectly isotropic antenna one would obtain a constant TPPL efficiency for different orientations of the linear polarization of the excitation beam, we consider the ratio R between the largest and lowest value in the polar plot to get a preliminary evaluation of the antenna polarization performances. For an

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ideally symmetric cross antenna, one would expect R=1 (i.e. constant TPPL emission for different polarization angles). By calculating the anisotropy factor R for 25 nominally symmetric cross antennas, we get a mean value of 1.42 with a standard deviation of 0.24, compared e.g. to standard linear antennas which have typical anisotropy factors R > 50 [2].

5. DISCUSSION AND CONCLUSIONS We have discussed the use of cross-shaped nanoantennas for polarization maintenance and control, and presented preliminary characterization results of symmetric Au cross antennas by probing the anisotropy of the TPPL antenna response as a function of the linear polarization angle of the excitation beam. While most of the measured cross antennas already show a fair level of isotropy, it should be stressed that such a response does not yet represent a definite proof of polarization maintenance, since in order to locally build a circularly-polarized near field with high efficiency not only should the response be isotropic in terms of TPPL counts, but also the hot spots generated by the two perpendicular antennas should spatially overlap so that the two field components can be effectively superimposed. In our case, simulation results in Fig. 1(c) seem to indicate that the largest field enhancement does not appear in the middle of the gap, but rather in the four side gaps between adjacent wires, where the field is mostly linearly polarized. Nevertheless, it is also worth noting that a circularly-polarized hot spot should still be present in the middle of the gap [as predicted in Fig. 1(e)], although with lower field enhancement. In applications that are sensitive to circularly-polarized fields, the four linearly-polarized side hot spots would represent an unwanted background but might not hinder the use of the central circular spot. In the near future, we plan to characterize the antenna response to circularly-polarized fields by covering the antenna with a thin layer of chiral molecules [7] or by structuring cross antennas on tips [14] and acquire circulardichroism maps in order to directly probe the degree of circular polarization of the antenna near field.

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