We design, fabricate and investigate compact Z-add-drop (ZAD) filters for long-range surface plasmon polaritons (LR-SPPs) at telecom wavelengths. The ZAD filter for LR-SPPs consists of two ridge gratings formed by periodic gold thickness modulation at the intersections of three zigzag-crossed gold stripes embedded in polymer. We investigate influence of the grating length and crossing angle on the filter characteristics and demonstrate a 10°-ZAD filter based on 80-µm-long gratings that exhibit a 15-dB dip (centered at ~1.55 µm) in transmission of the direct arm along with the corresponding ~13-nm-wide transmission peak in the drop arm.
©2005 Optical Society of America
Surface plasmon polaritons (SPPs) represent surface electromagnetic waves that can propagate along an interface between two media with real parts of permittivity of opposite signs, e.g. a dielectric and a metal . The SPP field components have their maxima at the interfaces and decay exponentially into both media . Small penetration depth (of the order of 100 nm in dielectric and 10 nm in metal) makes SPPs greatly suitable for surface sensing and gives possibilities for controlling the SPP propagation by using surface nanostructures [2–5]. Despite the fact that SPP-based components can transmit only one polarization (the electric field is perpendicular to the stripe surface) , there is growing interest in exploration of SPPs for miniaturized photonic circuits. Over the last years, SPPs have been extensive studied for radiation guiding along straight and sharply bent channels in (periodically) corrugated regions and along narrow metal stripes [4–7]. However, the small propagation length of SPPs (~30 µm in visible and ~300 µm in the near-infrared wavelength range for a silver-air interface ) implies that the realization of photonic components based on SPPs is feasible only for ultra-compact devices (few hundred micrometers). Furthermore, the conventional end-fire coupling of radiation with single-mode fibers cannot be directly applied for SPP excitation and therefore, special techniques, such as prism or grating coupling, have to be used .
However, it was found that the SPP mode supported by a symmetrical structure of a thin metal film embedded in dielectric (e.g., polymer) exhibits a significant increase in propagation length as the thickness of the metal film decreases with the mode approaching the plane wave supported by the dielectric background [8,9]. This mode, called a long-range SPP (LR-SPP), is formed by two coupled SPPs associated with the upper and lower metal-dielectric interfaces. When the metal film thickness decreases, the LR-SPP mode expands farther into dielectric not only decreasing its damping by metal but also facilitating the end-fire excitation . Recently, LR-SPP propagation along thin metal stripes of finite widths embedded in a dielectric was studied both theoretically  and experimentally [11,12] showing low propagation loss and efficient coupling with single-mode fibers (Fig. 1).
LR-SPP stripe waveguides constitute thereby a new alternative for integrated optical (IO) circuits, which is very interesting and promising in a longer perspective due to the possibility of using the same metal-stripe circuitry for both guiding optical radiation and transmitting electrical signals that control its guidance. Indeed, different LR-SPP-based IO components including straight and bent waveguides, Y-splitters and couplers, have been experimentally demonstrated [13,14]. Moreover, efficient LR-SPP-based dynamic devices with low power consumption, including various modulators and switches, have been very recently realized utilizing the thermo-optic effect in the polymer cladding [15,16].
In this paper, we present the design, fabrication and investigation of compact Z-add-drop (ZAD) filters for LR-SPPs at telecom wavelengths and demonstrate a 10°-ZAD filter based on 80-µm-long gratings that exhibit a 15-dB dip (centered at ~1550 nm) in transmission of the direct arm along with the corresponding ~13-nm-wide transmission peak in the drop arm.
2. Design and fabrication
The main components of ZAD filters are diffraction gratings for LR-SPPs that are formed by periodically modulating the thickness of a metal film embedded in dielectric, so that the grating configuration represents periodic metal ridges protruding symmetrically above and below the metal film (Fig. 2). The idea is that the symmetric ridge configuration should minimize the LR-SPP scattering into asymmetric and high-loss (short-range) SPP modes.
The ZAD filter configuration consists of two ridge gratings formed at the intersections of straight stripe LR-SPP waveguides arranged in the zigzag fashion with different angles θ (10°, 20° and 30°) between the stripes (separation D between parallel stripes is 200 µm) (Fig. 3).
Such a folded double-grating configuration represents a simple wavelength add-drop filter, in which an optical signal at a particular wavelength λ0 can be added in or dropped from an input optical signal propagating in the direct arm. Since LR-SPPs are very close to the light line [8–10], i.e., λLR-SPP≈λ/n (n is the dielectric refractive index), the wavelength λ0 and the grating period Λ can be related via the Bragg condition : λ0≈2Λn/cos(θ/2).
To fabricate ZAD filter structures for LR-SPPs, a silicon substrate was first spin-coated with a 15-µm-thick BCB polymer (Benzocyclobutene) layer. Then, electron-beam lithography was used for the patterning of parallel ridges. After reactive-ion etching (RIE) of the BCB layer with the electron-beam resist as a mask, ridges were formed by gold deposition (thickness of htotal-see Fig. 2) and lift-off. To ensure the vertical symmetry of the structure, which is very important for the LR-SPP propagation , the etch depth was carefully measured and the appropriate gold thickness was chosen in order to obtain 20-nm-high ridges (htotal/2) placed symmetrically above and below the LR-SPP propagation plane. 40-µm-long ridges of different widths w (filling factors defined as a ratio between the ridge width and the grating period, w/Λ, ~0.3 and 0.4) were arranged in 500-nm-period gratings of different length having from 20 to 160 periods. After the grating fabrication, zigzagged stripes were patterned using standard UV lithography. A second metallization and lift-off was used to make 15-nm-thick (t) and 8-µm-wide gold stripes that were then spin-coated with another 15-µm-thick BCB layer. The resulting stripe LR-SPP waveguides were characterized at telecom wavelengths showing low propagation loss (~6 dB/cm) and efficient radiation (in/out) coupling with single-mode polarization-maintaining fibers (loss per coupling ~0.5 dB) .
It should be noted that, in general, the diffraction grating while being efficient should be sufficiently short, i.e., much shorter than the length of intersection of the corresponding LR-SPP modes (incident and reflected one), otherwise the reflected radiation might become too wide to be efficiently coupled into the reflected LR-SPP mode. This circumstance implies that the grating efficiency per period should be maximized. Our preliminary investigations of the individual gratings oriented perpendicular to LR-SPP stripe waveguides showed that the grating efficiency increases with the increase of the ridge height until the height of ~20 nm, after which both transmission and reflection characteristics rapidly deteriorate when further increasing the ridge height.
3. Experimental arrangement and results
Optical characterization of the fabricated ZAD filters has been carried out using standard spectrally resolved transmission measurements. The light from a broadband source based on EE-LED diode (1.55 µm) was sent through a polarization controller (to adjust the electric field polarization perpendicular to the stripe-waveguide plane) and launched into the LR-SPP waveguide via butt-coupling from a polarization-maintaining single-mode fiber. 1 km of coiled standard single-mode fiber was used as out-coupling fiber in order to strip off all light coupled into the fiber cladding. The output signal was detected by an optical spectrum analyzer.
Transmission spectra were recorded in the wavelength range from 1500 to 1600 nm with a resolution of 0.5 nm and sensitivity of -80 dBm, and subsequently normalized to the transmission through a straight stripe waveguide (without gratings). Typical transmission spectra for the ZAD filter direct arm (Fig. 3) are shown in Fig. 5 for different grating lengths. It was found that, in general, the transmission exhibits the pronounced Bragg-grating behavior with the transmission dip increasing with the increase of the grating length . For example, for the considered 20°-configuration, the transmission dip increases from ~0.5 dB for a 20-µm-long grating to ~17 dB for an 80-µm-long grating (Fig. 5). The evolution of the transmission spectra with the grating length when compared to the dielectric (Bragg) grating behavior  showed that the effective-index modulation in the investigated gratings was of the order of 0.01.
The LR-SPP grating behavior was found to have some characteristic features that were different from that of one-dimensional dielectric gratings, viz., the asymmetry in the spectra with respect to the Bragg wavelength, the Bragg wavelength shift with the increase in the grating length and different transmission levels outside of the Bragg resonance. These features can be related to the influence of the LR-SPP absorption in the stripe and by the ridges and, especially, to the effect of the out-of-plane scattering. The latter can be decomposed into the directional scattering, i.e., the LR-SPP diffraction on the grating structure, and the diffuse scattering of the incident LR-SPP by ridges. The first process removes the radiation out of the propagation plane only for wavelengths shorter than the Bragg wavelength. The diffuse scattering is also expected to increase (though gradually) towards shorter wavelengths. Notice also from Fig. 5 that the overall transmission outside the transmission dip decreases with grating length. This is related to the accumulating effect of absorption and out-of-plane scattering losses.
The influence of the zigzag angle θ on the transmission through the ZAD filter direct arm is illustrated in Fig. 6, comparing the ZAD filter spectra with the transmission spectrum measured for the grating oriented perpendicular to LR-SPP stripe waveguide (θ=0°). It is seen that the transmission dip decreases with the increase of angle θ. Such a behavior is related to the finite LR-SPP mode width resulting in the decrease of the overlap between the interacting LR-SPPs and the grating (see also the discussion after Fig. 4). It should be noted that the transmission through the ZAD filter drop arm exhibited a peak at the wavelength corresponding the transmission dip for the direct arm. However, it was not possible to obtain a series of drop-arm signals for all angles. Due to the angled double-reflection configuration, the drop signal depends critically on the grating efficiency. More precisely, for angles more than 20 degrees the drop signal was too weak to be measured experimentally (it was not possible to perform the output-fiber alignment).
The overall performance of a 10°-ZAD filter configuration based on 80-µm-long gratings (filling factor ~0.4) is shown in Fig. 7. The transmission spectrum through the direct arm exhibits a 15-dB dip centered at ~1.55 µm, while the drop-arm transmission built up by double reflection from two gratings features a peak at around the same wavelength with a full-width-at-half-maximum (FWHM) of ~13 nm.
For this device, the loss introduced by the two gratings (propagation loss in stripe waveguides was subtracted) was found to be ~8.8 dB at the Bragg wavelength, changing to ~9 and 6 dB in the short-(1.52 µm) and long-wavelength (1.58 µm) regions, respectively. A decrease in the total loss for the Bragg wavelength (1.55 µm) compared to the loss for wavelengths just outside the Bragg condition (1.54 and 1.56 µm) is a characteristic feature for LR-SPP structures that can be related to the fact that at the resonance, the field cannot penetrate the grating, and less overlap between field and ridges gives less absorption. Moreover, the roughness and imperfection of ridges may also contribute similar to an absorption loss, so that the observed decrease in the total loss for the Bragg wavelength is due to both reduced absorption and reduced out-of-plane scattering.
Summarizing, we have designed, fabricated and investigated compact Z-add-drop (ZAD) filters for LR-SPPs at telecom wavelengths based on the folded double-grating configuration. The gratings represented ridge gratings formed by periodic gold thickness modulation at the intersections of three zigzag-crossed gold stripes embedded in polymer. We have investigated influence of the grating length and crossing angle on the transmission spectra and demonstrated the 10°-ZAD filter based on 80-µm-long gratings that exhibit a 15-dB dip (centered at ~1.55 µm) in transmission of the direct arm along with the corresponding peak in the drop arm having the FWHM of ~13 nm.
The effective index contrast in the investigated gratings was found (by comparing with dielectric Bragg gratings ) to be ~0.01, which is large enough for designing efficient and compact Bragg gratings, offering thereby certain advantages over their fiber counterparts. As an alternative to the integrated Bragg gratings, LR-SPP grating structures can provide both efficiency and compactness and can be easily integrated with couplers, splitters, and other LR-SPP-based waveguide components to be used for wavelength selective functions. We foresee therefore a great potential in the devices based on Bragg gratings involving LR-SPPs for variety of applications ranging from lasers with distributed feedback (DFB) and wavelength filters to biosensors. Moreover, for some applications (e.g. sensors based on light interference or DFB lasers), it might be advantageous that the LR-SPP-based components transmit only one polarization.
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