Spectral and mode properties of surface plasmon polariton waveguides studied by near-field excitation and leakage-mode radiation measurement
© Pan et al.; licensee Springer. 2014
Received: 30 May 2014
Accepted: 15 August 2014
Published: 25 August 2014
We present a method to couple surface plasmon polariton (SPP) guiding mode into dielectric-loaded SPP waveguide (DLSPPW) devices with spectral and mode selectivity. The method combined a transmission-mode near-field spectroscopy to excite the SPP mode and a leakage radiation optical microscope for direct visualization. By using a near-field fiber tip, incident photons with different wavelengths were converted into SPPs at the metal/dielectric interface. Real-time SPP radiation images were taken through leakage radiation images. The wavelength-dependent propagation lengths for silver- and gold-based DLSPPWs were measured and compared. It confirms that silver-based SPP has a propagation length longer than a gold-based one by 1.25, 1.38, and 1.52 times for red, green, and blue photons. The resonant coupling as a function of wavelength in dual DLSPPWs was measured. The coupling lengths measured from leakage radiation images were in good agreement with finite-difference time domain simulations. In addition, the propagation profile due to multi-SPP modes interference was studied by changing position of the fiber tip. In a multimode DLSPPW, SPP was split into two branches with a gap of 2.237 μm when the tip was at the center of the waveguide. It became a zigzag profile when the SPP was excited at the corner of the waveguide.
Surface plasmon polariton (SPP) waveguides allow electromagnetic wave propagating along metal-dielectric interface with a feature size smaller than optical wavelength. Due to the Ohmic loss of the metal, the propagation length of conventional SPP mode is limited to few microns. There are increasing interests in designing SPP waveguides with a longer propagation length [1–3]. A simple way to increase the SPP length and confine light in subwavelength region is to coat a submicron dielectric strip onto the silver or gold thin film; such dielectric-loaded SPP waveguide (DLSPPW)  can increase the length up to tens of microns. Several waveguide devices based on DLSPPWs have been demonstrated, such as waveguide ring resonators , interferometers , and splitters . The excitation of SPP waveguide modes can be done by both electronic and photonic ways. For example, an electron tunneling current can launch free electrons into SPP mode . By controlling the momentum of free electrons, SPP emission with a spectrum from 650 to 800 nm was demonstrated. For the photonic excitation method, the momentum matching with SPP's propagation constant can be achieved by using attenuated total reflection in an optical prism  or grating-coupling effect . A simple way by focusing a laser beam onto the edge of the waveguide can also couple SPPs into waveguides due to the light-scattering effect . The propagation images of SPP modes are often measured by using near-field scanning microscopy . For the above methods, the excitation of SPP modes needs an optical prism and a waveguide coupler to match the SPP momentum. The waveguide device is complicated. The launching position of SPPs is fixed at the end of waveguide, and the focused spot is limited to the diffraction. The launch condition of the SPP mode is hard to be controlled. Besides, the scanning near-field optical measurement is a time-consuming process.
In this paper, we present a near-field excitation system (NFES) to excite the SPP modes. This system provides efficient SPP coupling at any location of the waveguide with various excitation wavelength. The NFES is combined with a leakage radiation microscopy  (LRM). It provides direct visualization of the SPP mode in real time. To demonstrate the functions of the proposed setup, we measured different DLSPPW devices. The DLSPPW fabrication is simple. The dielectric stripe can be easily functionalized to provide thermo-optical, electro-optical, or all-optical functionalities for the development of active plasmonic components.
The system was used to study the propagation properties of the DLSPPW. The SPP mode in the DLSPPW has a propagation constant β = β′ + iβ″ with an effective index (nspp), where nspp = β/k0. The effective index is the equivalent refractive index of the surface plasmon waveguide. It depends on the wavelength, modes, dielectric constants of materials, and geometry of the waveguide. That can be calculated by numerical method  or determined by Fourier plane analysis . For a dielectric stripe with a refractive index similar to the glass substrate, the nspp will be smaller than the index of glass (ng = 1.48). The metallic film thickness is smaller than 100 nm; therefore, the SPP mode will have an evanescent tail in the glass substrate. It results in a small leakage of light, radiating at an angle (θ) of sin- 1(nspp/ng). The angular wave vector of the leakage radiation is the same as nspp and larger than air. Conventional optical microscope with an air lens cannot image the SPP mode. In the system, we applied a high numerical aperture (NA = 1.45) oil objective. The 1.45 NA is larger than the nspp which can collect the leakage radiation from the SPP mode. The intensity distribution of the leakage light is proportional to the SPP mode profile. Therefore, the propagation properties of SPP mode in the DLSPPW can be directly observed by recoding the leakage radiation images from a CCD camera.
Additional file 1: Leakage radiation images of SPP waves. Leakage radiation images of SPP waves when the fiber tip was moved along the width direction of the waveguide. (MOV 2 MB)
Additional file 2: Leakage radiation images of SPP waves. Leakage radiation images of SPP waves when the incident wavelength was scanned from red to blue wavelength. (MOV 4 MB)
Results and discussion
Propagation length of DLSPPW
Dual DLSPPW coupler
When two waveguides are very close to each other, their mode fields overlap and optical energy is transferred from one waveguide to the other. This dual waveguide coupler has been applied for many kinds of devices, such as power splitter, wavelength filter, and optical modulator. Understanding the coupling property is an important issue in the applications. The proposed setup can be well applied to the measurement of the plasmonic coupling between dual DLSPPWs. Figure 4a shows a scanning electron microscopy (SEM) image of a dual DLSPPW coupler. The coupler was consisted of two 90-nm wide and 300-nm high DLSPPW, which supported only fundamental TM00 mode at wavelengths from λ = 480 to 800 nm. The gap of both waveguides was 420 nm. Figure 4b shows the leakage radiation images of SPP mode from λ = 700 to 800 nm wavelengths. Due to the directional coupling effect, period oscillation of the SPP mode was observed. The coupling length (Lc) was defined by the length needed for optical power transferred from one waveguide to the other. As indicated in the blue dash line in Figure 4b, the coupling length decreased with the increase of excited wavelength. The coupling length in a dual DLSPPW coupler can be considered as a symmetric and an anti-symmetric modes propagating in the coupler with different propagation constants β+ and β-. The phase shift φ± is β±L, where L is the propagation distance. Mode power in one of waveguide will transfer to the other waveguide when Δφ = φ+ - φ- = π. The coupling length is defined as the distance for the π phase difference, where Δβ = β+ - β-, Δnspp = nspp+ - nspp-. Since the Lc is related to nspp. It will depend on the wavelength, modes, dielectric constants of materials, and geometry of the waveguide. The reason is that increase of the wavelength will increase the SPP mode size. It has a longer evanescent tail overlapping between neighboring waveguides. The coupling becomes stronger; thus, the coupling length is shorter. To verify the measurement of propagation properties in the directional coupler, both symmetric and asymmetric modes, the mode solver through vector finite-difference method was used. We found the coupling length, Lc = 5.37 μm at wavelength λ = 700 nm. The length was decreased to Lc = 3.99 μm at wavelength λ = 800 nm. Figure 4c shows the comparison between the measured and calculated results. The results are in good agreement between calculated lengths and the measured leakage radiation images.
We proposed a new optical setup that provides tunable spectral and modal excitation for surface plasmon polariton waveguide. The SPP images with broadband and single wavelength excitation at different excitation positions were demonstrated. The waveguides with different layouts and materials can be quickly compared by this setup. We confirmed the better SPP mode for longer wavelength excitation on silver film-based waveguides. The coupling length of dual plasmonic coupler was studied by using tunable wavelength mode. An increase of SPP coupling with the increase of wavelength was observed and identified with the calculation results. This setup takes advantages of nanoscale excitation, lower background, wavelength selectivity, and controllable excitation positions for direct visualization. In addition to the proposed DLSPPW devices, this technique can be applied to study other types of plasmonic waveguides and devices, such as ring oscillators , interferometers , plasmonic logic gates , etc.
This work was supported by National Science Council, Taipei, Taiwan, under Contract No. NSC-100-2120-M-007-006, NSC-100-2221-E-001-010-MY3 and NSC-101-2218-E-001-001. Technical support from NanoCore, the core facilities for nanoscience and nanotechnology at Academia Sinica in Taiwan, is acknowledged.
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