Ideas taking shape – worldwide. Plasmon Focusing on Single Crystalline Gold Platelets ULTRAFAST SOLID STATE SPECTROSCOPY, MAX PLANCK INSTITUTE FOR SOLID STATE RESEARCH, STUTTGART, GERMANY • 4TH PHYSICS INSTITUTE, UNIVERSITY OF STUTTGART, GERMANY • STUTTGART RESEARCH CENTER OF PHOTONIC ENGINEERING (SCOPE)

The manipulation of highly localized fi elds of surface plasmon polaritons (SPPs) forms the backbone for a vast fi eld of applications. We investigate the SPPs excit- ed on a single crystalline gold nanoplatelet milled with a plasmonic structure using a scattering type scanning near fi eld optical microscope. SPPs are excited at different in the visible regime employing a new tunable continu- ous wave source. Wave vector selection of the SPPs by the gold structures cor- responded well with the numerically calculated dispersion relationship.

Background simple approach to achieve SPP focusing dispersion relationship as can be seen Surface plasmon polaritons (SPPs) are is the use of periodic circular slits (plas- in Fig. 2. The dispersion relationship has electromagnetic excitations propagating monic lens) etched on metal fi lms to guide been obtained by simulations for a 20 nm along a metal-dielectric interface with an the SPP waves to the geometrical centers Au platelet on a SiO2 substrate. The SPP evanescent electric fi eld perpendicular of the structure [4,5]. A cw laser source is fi elds were mapped by an apertureless to the surface. Due to their local fi eld usually employed for exciting SPPs in such scattering type scanning near fi eld optical enhancements, high sensitivity to sur- experiments. microscope (s-SNOM) [6]. The experi- face inhomogeneities and the ease with mental setup can be seen in Fig. 3. A which they can be manipulated, SPPs are Experiment source with appropriate polarization is excellent candidate’s in many fi elds of In our experiment we investigate the ex- focused onto the tip of the s-SNOM which application such as near-fi eld imaging [1], citation and focusing of SPPs on a single raster scans the sample. The scattered surface enhanced Raman scattering [2], crystalline gold (Au) nanoplatelet with a light from the sample is directed back sensing, nanolithography, plasmonic plasmonic lens structure milled on it. Fig- through the same pathway and is detected based integrated circuits, memory devi- ure 1 shows a scanning electron micros- in the far fi eld. Topographical and optical ces [3] etc. One of the major requirements copy (SEM) and an atomic force measure- images are generated simultaneously. for such integrated optical devices and ment (AFM) image of a typical platelet. In order to map the SPP fi elds at sever- sensors is the possibility to focus the SPP The excitation energies were chosen close al excitation energies close to the SPP fi elds to sub diffraction limit sized spots. A to the SPP resonance according to the resonance, a continuous wave source with

Figure 1: SEM and AFM images of a typical platelet which are triangular Figure 2: Plasmon dispersion relationship for a 20 nm Au platelet on a Si

and hexagonal in shape. substrate coated with a 2.5 nm SiO2 layer.

␭ [nm] 0 314 157 104.7 78.5

2.5

y: 5.1 µm 68 nm

2.0 Output C-Wave 0 nm SR-SPP Energy (eV) LR-SPP x: 5.2 µm 1.5 EXP

0 20 40 60 80 Re (k) [µm-1] Figure 3: Experimental setup (s-SNOM): The incoming beam is split into Figure 4: The fourth harmonic near fi eld signal at 630 nm excitation and the two beams by the beam splitter (BS) partly sent to the reference mirror corresponding line profi le of the SPP waves near the edges of the platelet. (M1) and partly focused to the tip via the parabolic mirror (M2). The tapping frequency of the tip modulates the nearfi eld signal. The backscattered fi eld 4.0 µV is collected and detected using a pseudo heterodyne detection scheme.

3.0 µV

2.0 µV

1.0 µV

0.0 µV

1.6 Profile 1

1.4 y [µV] 1.2

0 0.1 0.2 0.3 0.4 0.5 x [µm] a tuning range between 450-650 nm is in Fig. 4. As the tip is operated in tapping and geometry of the excitation medium, necessary. The C-WAVE by Hübner offers mode the near fi eld in the vicinity of the sources at different wavelengths are re- tunability in this range making tip is modulated according to the tapping quired in such experiments. it possible to conduct spectrally resolved frequency. This enables to reach a higher The C-WAVE offers a tuning in the visible measurements with a single light source. signal to noise ratio in higher harmonics by (450-650 nm) and in IR (900-1300 nm) Wavelength switching is computer con- fi ltering the unmodulated signal following a range, which are the most important trolled: The wavelength is set in the GUI pseudo-heterodyne detection scheme [7]. regimes in most plasmonic experiments. and C-WAVE tunes to the set wavelength SPPs excited at the edges of the platelet The compact design, high spectral quality automatically. The high output power, as well as the effect of the plasmonic lens of the beam, wide tuning range and the good beam quality and the narrow line- structure on the direction of SPP propaga- ease and speed with which wavelength widths allows capturing of sharp images. tion can be seen clearly in the fi gure. The switching is achieved makes the C-WAVE No changes to the optical beam path were wavelength of the SPP waves obtained a very convenient and easy to integrate necessary after switching wavelengths from the experimental data matched well tool for such experiments. owing to its high pointing stability. with the numerical predictions. For additional information, Results Outlook please contact: With the help of C-WAVE we were able to Probing and manipulating SPP fi elds Divya Virmani, Max Planck Institute for excite and probe the SPP fi elds at several at resonance is an important aspect in Solid State Research, Stuttgart, Germany, excitation energies ranging from 530 to many experiments designated to the e-mail: [email protected], or 650 nm. The fourth harmonic optical signal improvement and development of many Dr. Niklas Waasem, Hübner GmbH & Co. of the SPP fi eld at 630 nm as well as the nanophotonic devices. Due to the strong KG, Kassel, Germany, niklas.waasem@ line profi le of the SPP waves can be seen dependency on the nature, dimensionality hubner-germany.com

References:

1 W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Sur- 3 E. Ozbay, „Plasmonics: merging photonics and 6 S. Patane, P. G. Gucciardi, M. Labardi, and M. Al- face plasmon subwavelength optics,” Nature 424, electronics at nanoscale dimensions,“ Science 311, legrini, “Apertureless near-fi eld optical ,” 824-830 (2003). 189-193 (2006). Riv. Nuovo Cimento 27, 1-46 (2004).

2 K. Kneipp, M. Moskovits, and H. Kneipp, Surface- 4 Y. Fu and X. Zhou, „Plasmonic : a review,” 7 7. N. Ocelic, A. Huber, and R. Hillenbrand, “Pseu- enhanced Raman scattering: physics and applica- Plasmonics 5, 287-310 (2010. doheterodyne detection for background-free near- tions (Vol. 103). (Springer Science and Business fi eld spectroscopy,” App. Phys. Lett. 89 (101124). 5 Media, 2006). Z. Liu, J. M. Steele, W. Srituravanich, Y. Pikus, C. Sun, and X. Zhang, “Focusing surface plasmons with a plasmonic lens,” Nano Lett. 5, 1726-1729 (2005).

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