Sub-100 nm Focusing of Short Wavelength Plasmons in Homogeneous 2D Space

Bergin Gjonaj Electrical Eng, Technion-Israel Institute of Technology, Haifa, Israel Asaf David Electrical Eng, Technion-Israel Institute of Technology, Haifa, Israel Yochai Blau Electrical Eng, Technion-Israel Institute of Technology, Haifa, Israel Grisha Spektor Electrical Eng, Technion-Israel Institute of Technology, Haifa, Israel Shimon Dolev Electrical Eng, Technion-Israel Institute of Technology, Haifa, Israel Meir Orenstein Electrical Eng, Technion-Israel Institute of Technology, Haifa, Israel Guy Bartal Electrical Eng, Technion-Israel Institute of Technology, Haifa, Israel

We present a direct high resolution measurement of subwavelength focusing with a focal spot of 60 nm, using short wavelength SPPs from a 532 nm laser and propagating in a 50nm Si3N4 layer between silver and air [1]. The supported SPP mode contains strong enough electric field in the dielectric-air interface to sufficiently interact with a metallic tip to allow direct sub-wavelength measurement, i.e. using a scattering near-field scanning optical microscope (s-SNOM).

Fig 1. a) Silver-nitrade-air platform for short wavelength plasmons and the near field detection. b) Measured near-field maplitude of focusing SPPs at sub 100 nm.

Fig. 1 a) Short wavelength plasmons are excited and focused using curved slits. The mode profile (in yellow) can be detected with a near-field tip. b) measured near-field map showing sub 100 nm focusing.

Our sample consists of a silver layer on a 50 nm thick Si3N4 membrane supporting a propagating TM SPP mode (Fig. 1a). We use curved subwavelength slits to couple the laser to the mode and to focus the SPPs. For a laser wavelength of λ0=532 nm, a SPP wavelength of λsp=241 nm and propagation length Lsp=888 nm are expected. The calculated field distribution of this mode (depicted in yellow in the Fig.1a), shows a strong evanescent tail in the air above the membrane layer that can be used to interact with a scattering tip. To map the near-field of the plasmonic mode we use a transmission mode s-SNOM (apertureless). A metallic tip scatters the electric field into a pseudo heterodyne detection unit to provide amplitude and phase information with a resolution of 10-20 nm [2].

Near-field phase measurements determine the the plasmon wavelength at λsp = 240±20 nm, in excellent agreement with our calculations. To improve the focal spot, we use two concentric arcs of circles with a radius mismatch of λsp /2 [3]. The measured near-field amplitude is shown in Fig. 1(b). Line cuts of the plasmonic focus and their relative Gaussian fits determin the focus size to be 104±20 nm and 66±20 nm FWHM. This constitutes the first direct measurement of such sub-wavelength focusing of polaritons, i.e., 2D surface waves propagating in homogeneous plane and not assisted by structural elements, e.g., nanofocusing or nanoantennas.

[1] B. Gjonaj, et al., Nano Lett., 14, 10, 5598, (2014)

[2] N. Ocelic, et al., Appl. Phys. Lett., 89, 10, 101124, (2006).

[3] Z. Fang, et al., Nano Lett., 11, 2, 893, (2011).

bgjonaj@ee.technion.ac.il









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