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We propose a planar long-period grating filter based on coupling between the long-range surface plasmon mode and a cladding mode of a fully buried metal stripe waveguide. Using a 2.5-mm-long corrugation grating produced along the surface of an epoxy-clad aluminum stripe waveguide, we achieve a rejection band with a contrast of ~18 dB at the wavelength ~1500 nm, which can be tuned by ~25 nm with a temperature change of ~30°C. The experimental results agree closely with the simulation results. The filter could find applications in surface-plasmon-based integrated-optic circuits and biosensors.
©2010 Optical Society of America
1. Introduction
Surface plasmons or surface plasmon polaritons, which are light waves propagating along an interface between a metal and a dielectric material [1], have attracted much interest in recent years for their many potential applications in integrated optics and nanophotonics [2]. A thin metal stripe, in particular, supports a long-range surface plasmon (LRSP) mode that can propagate over a distance of several millimeters with a low loss [3]. LRSP modes have been explored for the development of inexpensive and compact integrated optical devices, such as Y-splitters [4,5], multimode interference devices [4,5], couplers [4–7], modulators [8], switches [8], and attenuators [9]. Bragg grating filters based on periodic corrugation along the width [10] or the thickness [11] of a metal stripe have also been demonstrated. A comprehensive review of LRSP-based optical devices can be found in Ref. 12. In this paper, we propose a new LRSP-based grating, which is formed on the surface of the dielectric cladding of a fully buried metal stripe and designed to couple between the LRSP mode of the metal stripe and a co-propagating cladding mode. The grating requires a long period (10 – 100 μm), compared with a Bragg grating, so it is a kind of long-period waveguide grating (LPWG) [13]. The grating functions as a broadband rejection filter and offers a number of advantages, including easy fabrication, low back reflection, and good wavelength tunability. A special feature of the LRSP mode is that its field extends deeply into the cladding, which makes possible strong coupling to a cladding mode with a shallow grating formed on the cladding surface. Because of that, we can form the grating on the cladding at the last step of the fabrication process, which means that the grating parameters and hence its performance can be determined accurately from a fully characterized waveguide. On the other hand, LPWG filters based on all-dielectric waveguides where the grating is formed in the core [13] do not have this advantage and are more difficult to fabricate reliably. Moreover, the large spot size of the LRSP mode facilitates the launching and the detection of the mode with standard single-mode fibers. To demonstrate the operation principle of the device, we fabricated an epoxy-clad aluminum (Al) stripe waveguide that contained a carefully designed 2.5-mm-long grating engraved on the cladding surface. The transmission spectrum of the filter shows a rejection band of ~18 dB at the wavelength ~1500 nm, which can be tuned by ~25 nm with a temperature change of ~30 °C. Our experimental and simulation results agree well. This filter could be used in integrated-optic circuits and biosensors.
2. Filter configuration and operation principle
The proposed grating filter is shown in Fig. 1 , which consists of a thin metal stripe buried completely in a finite dielectric cladding. Light waves are confined in the x-y plane and propagate along the z direction. A grating with corrugation depth Δh and pitch Λ is formed on the cladding surface to induce coupling between the LRSP mode and a selected cladding mode. For the realization of a practical device, low-loss metal (such as gold or silver) and dielectric material should be used [3–12,14]. Limited by our experimental conditions, however, we employed Al and epoxy OPTOCAST 3505 (Electronic Materials Inc) as the metal stripe and cladding materials and a SiO2/Si wafer as the substrate. Although Al and epoxy are not the optimal materials, they are easy to process and good enough to demonstrate the principle of the filter.
Fig. 1 A planar long-period grating with pitch Λ and corrugation depth Δh formed on the surface of a dielectric-clad metal stripe waveguide.
We calculate the modes of the waveguide with a finite difference method [15] using the following experimental parameters: Al thickness = 15 nm, Al width = 2 μm, Al permittivity = − 253.93 − 46.08j [16], epoxy thickness = 11.5 μm, epoxy refractive index = 1.51161 (measured at 1536 nm), and SiO2 substrate index = 1.444. We assume an infinitely wide cladding and an Al stripe placed at the center of the cladding in the y direction. The LRSP mode is a transverse-magnetic (TM) mode, which has a magnetic-field component in the x direction and a major electric-field component in the y direction. Our calculation shows that the waveguide supports six TM cladding modes confined in the y direction. In principle, the grating can be designed to couple light from the LRSP mode to any of the cladding modes [17]. Here we choose the fourth TM cladding mode. The electric-field distributions of the LRSP mode and the chosen cladding mode at the wavelength 1536 nm are shown in Fig. 2(a) and (b) , respectively. As shown in Fig. 2(a), the LRSP mode has a large and simple field pattern and, therefore, can be excited efficiently with a single-mode fiber. For the present waveguide design, an excitation efficiency of ~75% can be achieved with a standard single-mode fiber SMF-28, which has a core radius of 4.1 μm and a numerical aperture of 0.12. The excitation efficiency can be further improved by optimizing the dimensions of the metal stripe and the cladding [14]. On the other hand, as shown in Fig. 2(b), the cladding mode has a complicated pattern that cannot be excited efficiently with a single-mode fiber. The wavelength at which the coupling between the two modes is strongest (the resonance wavelength) is determined by the phase-matching condition [13,17], λ 0 = (N LRSP − N cl)Λ, where N LRSP and N cl are the real parts of the mode indices of the LRSP mode and the cladding mode, respectively. The calculated values of N LRSP and N cl are 1.5105 and 1.4784, respectively, which gives a grating pitch of 47.85 μm at the resonance wavelength 1536 nm. The calculated propagation loss for the LRSP mode (which only accounts for the ohmic loss of the Al stripe) is ~2.5 dB/cm. We choose a grating length of 2.5 mm, which is long enough to contain a large number of grating periods and short enough to limit the loss. Given the grating length, we can determine the corrugation depth Δh needed to produce a strong rejection band [17]. According to our calculation with the coupled-mode theory [17], to achieve zero transmission at the resonance wavelength 1536 nm, the corrugation depth required is 406 nm with a 2.5-mm-long grating, which is only 3.5% of the cladding thickness and should not introduce significant losses to the modes.
Fig. 2 Electric-field distributions of (a) the LRSP mode and (b) the fourth vertically confined cladding mode of a 15-nm thick and 2-μm wide Al stripe waveguide buried in an 11.5-μm thick epoxy cladding.
3. Device fabrication
To fabricate the device, a layer of epoxy was first spin-coated on a SiO2/Si wafer, where the SiO2 layer was 3 μm thick. The epoxy was exposed to ultra-violet (UV) light at a power density of 350 mW/cm2 for 5 minutes and post-cured at 130 °C for 30 minutes. The thickness and the refractive index of the epoxy layer, measured with a step profiler (Ambios Technology, Model XP-2) and a prism-coupler system (Metricon, Model 2010), respectively, were 5.75 μm and 1.51161 at 1536 nm for the TM polarization. By means of a negative mask and standard photolithographic processes, a set of 2-μm-wide openings separated by 60 μm was patterned on the epoxy layer with photoresist AZ 5206E (Clariant Inc). A 15-nm-thick aluminum film was then deposited on the surface of the patterned sample with a thermal evaporation system (Edwards, Model Auto 306) and the excess metal was removed with a lift-off process. A second layer of epoxy was spin-coated on the sample to the same thickness as the first layer and cured. To ensure full curing of the epoxy and thus improve its stability, the sample went through additional thermal curing at 130°C for 60 minutes.
The real parts of the mode indices of the waveguide were measured directly with the prism-coupler system. The mode spectrum at 1536 nm from the output of the prism-coupler system together with the measured values are shown in Fig. 3 , which indicates that the waveguide supports an LRSP mode and six cladding modes. The grating pitch for coupling to the fourth cladding mode is Λ = 1.536/(1.51029 – 1.47819) = 47.9 μm, which agrees closely with our simulation result. The actual pitch used was 48.0 μm. We next spin-coated a photoresist film on the sample to a thickness of ~1 μm and formed a grating mask by photolithography. We then etched the masked sample with a plasma reactive ion etching (RIE) machine (Plasma-Therm, Model 790 Series), where the gas used was O2 and SF6 and the discharge pressure and power were set at 20 mTorr and 75 W, respectively. The sample was etched for 150 seconds. The resultant grating had a length of 2.5 mm and a width of 1 mm, which covered a number of waveguides. Figure 4(a) is an optical image of the sample, where the locations of the Al stripes are highlighted. The corrugation depth of the grating, measured with the step profiler, was 366 nm, as shown in Fig. 4(b). The uniformity of the grating was good. Finally, the two ends of the sample were cleaved. The total length of the waveguide was 6 mm.
Fig. 3 Mode spectrum of the fabricated Al stripe waveguide obtained from the output of the prism-coupler system.
Fig. 4 (a) Microscopic image of the grating sample and (b) corrugation profile of a typical grating.
4. Measurement results and discussion
The LRSP mode was launched into and extracted from the waveguide with standard single-mode fibers placed at the two ends of the waveguide. The propagation losses for the LRSP mode, measured with and without the grating, respectively, at 1536 nm (off resonance) with a transmission method [18], were both ~4.6 dB/cm, which confirms that the grating caused negligible loss to the LRSP mode. The loss was larger than the calculated value that ignores the material loss of the epoxy (~2 dB/cm). The transmission spectrum for the TM polarization was measured with a broadband light source (KOHERAS, Model SuperK Compact) and an optical spectrum analyzer. A heat pump was placed under the waveguide to control its operating temperature. The normalized transmission spectra at three temperatures are shown in Fig. 5 . We can see a clear rejection band at 1500 nm at 24.2 °C that has a contrast of ~18 dB and a 3-dB bandwidth of ~19 nm. By taking into account the effect of reducing the average cladding thickness by corrugation, the calculated resonance wavelength and contrast (with Λ = 48 μm and Δh = 366 nm) are 1493 nm and 18.8 dB, respectively, which compare well with the experimental results. To provide a more direct demonstration of the operation of the grating, we took the output near-field images of the filter (at 24.2°C) with an infrared vidicon camera (Hamamatsu, Model C2741-03) at the resonance wavelength 1500 nm and an off-resonance wavelength 1536 nm, respectively, and display the results as insets in Fig. 5. As shown by the images, light is coupled from the LRSP mode into the cladding mode at the resonance wavelength, while stays in the LRSP mode at the off-resonance wavelength.
Fig. 5 Normalized transmission spectra of the filter measured at 24.2, 34.5, and 46.7°C with the insets showing the output near-field images (taken at 24.2°C) at the resonance wavelength 1500 nm and an off-resonance wavelength 1536 nm.
Figure 6 shows the change of the resonance wavelength with the temperature from 24.2 to 55.3°C. The resonance wavelength shifts towards a shorter wavelength by ~25 nm as the temperature changes by ~30°C. The variation of the rejection contrast with the temperature is due to the change of the refractive index of the epoxy, which affects the mode-field distributions (and hence the overlapping of the coupled modes in the grating area) and thus leads to a change in the grating contrast [17]. The temperature sensitivity of the filter is comparable to that of some particularly sensitive all-dielectric LPWG filters [13].
5. Conclusion
In summary, we propose a planar long-period grating filter based on coupling between the LRSP mode and a cladding mode of a fully buried metal stripe waveguide. Our experimental filter using a 2.5-mm-long corrugation grating on an epoxy-clad Al stripe waveguide shows a rejection of 18 dB at 1500 nm (at ~24°C), which can be tuned by ~25 nm with a temperature control of ~30°C. It should be possible to improve the performance of the filter by using low-loss metal (gold or silver) and dielectric as the waveguide materials. Compared with conventional all-dielectric LPWG filters [13], this filter allows the grating to be formed on the cladding surface at the last step of the fabrication process, which offers significant advantages in terms of the convenience in the fabrication process and the accuracy in the control of the grating parameters. It is possible to fabricate the device using an even simpler technique, such as a laser-writing technique (for photosensitive cladding) or an imprinting technique (for polymer cladding). The filter could find applications in surface-plasmon-based integrated-optic circuits. Its large sensitivity to environmental parameters could be further explored for the realization of optical biosensors.
Acknowledgments
This research was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China [Project CityU 111907].
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