An experimental investigation of long-ranging surface plasmon-polariton waves guided along thin finite width Au structures embedded in a homogeneous background dielectric is reported. The operation of key passive integrated optics elements such as straight waveguides, s-bends, y-junctions and couplers is demonstrated at a free space optical wavelength of 1550 nm. The influence of some important design parameters on the performance of these elements is presented and discussed.
©2005 Optical Society of America
The optical modes supported by thin metal films of finite width, of generic design sketched in Fig. 1, have recently been computed and discussed for metal films in homogeneous  and inhomogeneous (asymmetric)  background dielectrics. The transverse electric field component of the modes that is normal to the width of the metal film dominates for all modes when the aspect ratio w/t of the film is greater than 1, so they are mainly, though not purely, TM in nature. The evolution of the modes with metal thickness often bears resemblance to the evolution of the modes supported by infinitely wide films [3, 4]. Four fundamental modes were identified for the finite width structure (along with numerous higher order modes) and labeled , , and .
The mode supported by a metal film of finite width in an optically infinite homogeneous background dielectric (ε1=ε2) is of interest here. In this situation, the mode exhibits a significant decrease in attenuation as the metal film vanishes (thickness and/or width go to zero) with the mode evolving into the plane wave supported by the background. Following convention, this mode is also termed a long-ranging surface plasmon-polariton (LRSPP). As the metal film vanishes, the mode also expands, becoming less tightly bound to the metal film . Expanded modes are often well-matched with modes of dielectric waveguides (like single mode fibre) leading to efficient end-fire coupling , but the reduced confinement leads to larger bending radii in order to keep radiation losses low. Modal analysis has also highlighted the importance of matching the index of refraction above and below the metal film  in order for the mode to propagate properly.
The existence of the mode, and thus the ability of thin metal films of finite width to propagate SPP’s over appreciable lengths, was first demonstrated experimentally in  using an Au film embedded in SiO2. The ability of the mode to round bends was first demonstrated experimentally in  also using Au in SiO2. Y-junctions, couplers and Mach-Zehnder interferometers based on the propagation of the mode were first demonstrated experimentally in , again using Au in SiO2. This thesis also describes the application of the cut-back technique to measure the attenuation of the mode and its coupling loss to single mode fibre, and presents the first attenuation measurements for this mode.
More recently, experimental results for the attenuation of the mode and its coupling loss to single mode fibre have been reported for polymer cladded straight Au films . A demonstration of thermo-optic modulation and switching of the mode using a Mach-Zehnder interferometer and a coupler, respectively, is reported in .
Near-field techniques have also been employed to experiment with SPP’s on thin metal stripes and nanowires; see  for a review and references therein. It is worth noting here that the SPP modes typically excited using near field techniques are not long-ranging, since the structures usually investigated have a large dielectric asymmetry, and though long-ranging modes can exist in such structures, they are located near a cut-off point, rendering their existence very sensitive to geometrical and material variances .
In this paper, we demonstrate the operation of important integrated optics elements based on the long-ranging mode and show experimentally how the performance of the elements varies as a function of a design parameter. We present results for straight waveguides, s-bends, y-junctions and couplers. The devices are fabricated using a simple approach in which Au films are deposited on SiO2 and covered with an index-matched polymer.
2. Structures investigated and Fabrication Approach
We designed a series of experiments in which an important design parameter for a particular element was varied and the resulting effects investigated. Specifically, the experiments included straight waveguides of various widths, s-bends of various radii of curvature, y-junctions using s-bends of various radii of curvature and four-port couplers having different separations. The elements are sketched in Fig. 2.
Straight metal films of different widths (2, 4, 6 and 8 µm), 25 nm thick and 7 mm long were spaced 120 µm apart in order to investigate the confinement offered by the different structures.
All other structures investigated, as described below, were designed such that the mode supported by the waveguides resembles that of the input PM fibre leading to efficient butt coupling at the input and output ports. For excitation at the free-space wavelength of 1550 nm, and for a background index near 1.444 (SiO2), an Au film that is 8 µm wide and 25 nm thick provides efficient coupling to fibre, thus justifying the selection of these dimensions.
S-bends of various radii of curvature R were designed and distributed in a row. We reduced the radius of curvature of the different s-bends by increasing the lateral offset between the input and output of the structure while maintaining the length of the curved sections to 4 mm. Straight segments of waveguides 1.5 mm long were added to the input and output of the s-bends to facilitate fibre to waveguide coupling yielding an overall device length of 7 mm (the propagation path length is slightly longer). A straight waveguide was placed next to these s-bends and served as a reference.
Y-junctions were designed by simply joining two mirror-imaged s-bends. The separation between the output branches is 250 µm for all y-junctions tested, such that two fibres could be butt-coupled side-by-side to capture the optical output power simultaneously if desired. Compressing the length of the y-junction reduces the radius of curvature R of the bends comprising the junction. Straight waveguide sections were added at the input and output of the structure such that the overall length of the devices remained at 7 mm.
Finally, the coupler experiment was devised such that the separation S between two parallel waveguides was varied from 2 µm to 8 µm in increments of 1 µm while the coupling length CL remained at 1.5 mm. The branches of the input and output ports consist of s-bends with a radius of curvature of 16.95 mm, which is sufficiently large that radiation losses are negligible for the selected waveguide dimensions. The separation between the input and isolated ports, and between the through and coupled ports is greater than 60 µm for all structures. The couplers were designed to be 4 mm in length, with 1.5 mm long straight waveguides at all ports of the device for an overall length of 7 mm.
The devices were fabricated on native 15 µm thick thermal oxide (SiO2) on a Si wafer. The wafer was spin-coated with a bi-layer of resist, exposed in a contact aligner and then developed in a chemical bath using standard microfabrication techniques. A 25 nm thick film of Au was e-beam evaporated under vacuum (<10-6T) directly onto the cleaned SiO2 surface and subsequently lifted off using a combination of still and ultrasonic baths to reveal the metal structures. Samples were cleaved to desired lengths and an index matching curing gel (Nye Lubricants Inc., OCK-433) was deposited above the waveguide to form the upper clad. Using gel instead of SiO2 as the upper cladding reduces the fabrication effort (cost and time) and is suitable for demonstration prototypes.
3. Experimental results
The specimens were placed on a thermo-electric cooler and the temperature adjusted such that the index of refraction of the gel upper clad matched the index of refraction of the SiO2 substrate. The match point was determined by observing the output of a low confinement straight waveguide (2 µm wide, 25 nm thick Au film) and adjusting the temperature to achieve a nicely symmetrical output mode. A polarization maintaining (PM) fibre with TM incident polarization was used to excite the waveguides in an end-fire approach. The input light source consisted of a highly polarized semiconductor laser operating at a free space wavelength of 1.55 µm. The output light was observed using a two-lens setup consisting of a microscope objective located immediately following the sample and a long focal-length plano-convex lens to focus the image onto an infrared (IR) sensitive camera. The alignment of the input fibre was adjusted to maximize the intensity of the recorded output mode profile. A neutral density filter was placed in front of the camera to obtain the images for the y-junctions and couplers in order to prevent the camera from saturating; for the straight waveguides and s-bends, the laser current was reduced instead. In all figures presenting mode outputs, with the exception of the y-junctions (Fig. 5), the experimental settings were unaltered such that the outputs in a given sequence of images may be compared directly. In the case of the y-junctions, the absorption provided by the neutral density filter was reduced in only the bottom-most image (most aggressive radius of curvature) in order to better reveal the optical radiation described in the text.
3.1 Straight waveguides
Straight waveguides are a fundamental building block for any passive element and as such it is important to understand their behavior. Modal analysis suggests that attenuation and confinement decrease with decreasing metal width and thickness. In this investigation, the width of the metal waveguide was varied and the mode output was recorded and is presented in Fig. 3. The waveguides were on the same die and placed next to each other. The optical magnification provided by the setup remained constant during this experiment, thus facilitating comparisons.
It is clear from Fig. 3 that weak confinement is observed in the case of the 2 µm wide waveguide as the mode appears large and diffuse in comparison to the modes of the wider structures. The mode becomes more tightly bound to the waveguide as the width of the metal increases. The mode becomes flatter and wider to accommodate the wider physical structure, in agreement with results of modal simulations . The mode is more circular in profile in the 4 µm wide film, while it is more elliptical in the 8 µm wide case. In addition, the mode output intensity diminishes for increasing metal width due the higher propagation loss of wider waveguides, in agreement with theory. Furthermore, single long-ranging mode operation has been verified for all metal widths investigated.
3.2 S-bend waveguides
S-bends are important elements since they are often used as building blocks for more complex components such as y-junctions and couplers. Their characterization provides insight on the strength of confinement provided by the waveguide. We investigated the effect on the output power resulting from a reduction in the bend radius. We present, in Fig. 4, a sequence of images comparing the IR output of s-bends of various radii of curvature with the IR output of a straight waveguide.
The relative intensity of the various IR images in this figure remains constant for radii of curvature greater than 12.5 mm. A reduction in power is first noted for the s-bend designed with a 10.0 mm radius of curvature, and significant radiation losses are observed for the s-bends with smaller radii of curvature. It is clear that the confinement offered by thin metal waveguides propagating the long-ranging mode can be sufficiently strong to allow the design of s-bends. For the metal film dimensions used here, and the operating wavelength and background material selected, the radius of curvature should be kept at about 12 mm or greater to maintain low radiation losses. It is stressed that the radius corresponding to the onset of significant radiation is dependant on all of the waveguide variables, including the operating wavelength.
Y-junctions are of interest since they provide means for dividing optical power into two branches. Additionally, y-junctions are broadly utilized in integrated optics as splitter and combiner elements in the implementation of other devices such as, for example, Mach-Zehnder interferometers.
We investigated the effect on the output power of decreasing the radii of curvature of the S-bends used in the y-junctions. In Fig. 5, a sequence of images consisting of the IR outputs of various y-junctions is presented.
We observe two circularly symmetric spots representing the outputs of the two branches of the y-junctions. We note, by comparing the relative intensity between the y-junctions, that the outputs are similar for bend radii of 12.5 mm or greater, and that significant dimming of the spots occurs for smaller bend radii. This is consistent with the results reported in Fig. 3. Slight power imbalances have been noted for some of the structures. Significant background radiation is observed in the case of the most aggressive radius (4.53 mm).
Another passive optical element of interest is the coupler, which divides the power from the input port between the through and the coupled output ports. By adjusting various parameters in the design of a coupler, it is possible to vary the power ratio between the output ports. In our experiment, we have chosen to vary only the separation S between the coupled waveguide sections while the coupling length CL and all other waveguide dimensions remained constant. In Fig. 6, we present a sequence of images in which we compare the IR outputs of couplers designed with various separations, and for which excitation is always provided into the left input waveguide.
From Fig. 6, we note that only a fraction of the input power is transferred to the coupled port for the coupler designed with a waveguide separation of 8 µm. A larger fraction of the incident power is transferred from the input to the coupled port as the separation S decreases. It is noted that approximate equal power division into the through and coupled ports is observed for a waveguide separation of 5 µm, approximate total coupling to the coupled port is observed for a separation of 3 µm, and approximate equal power division is again observed for a separation of 2 µm indicating passage through the first null of the input - through transfer characteristic. This experiment verifies the synchronous operation of these couplers.
We have demonstrated the operation of key passive optical components designed using thin Au films of finite width in an SiO2/polymer background and supporting a single long-ranging ss0b mode. A simple fabrication approach was used to construct the devices and end-fire excitation was achieved using a polarization maintaining fibre butt-coupled to the input of the samples.
We have verified that the mode confinement offered by the waveguides is sufficient to enable the design of s-bends and have determined experimentally the radius of curvature where the onset of significant radiation occurs for our selected waveguide dimensions. Furthermore, we have demonstrated the operation of y-junctions designed using mirrored s-bends of different radii. Finally, we have presented results confirming optical power coupling between two adjacent parallel waveguides, and have verified the synchronous operation of these couplers.
Detailed comparisons with quantitative theoretical expectations are currently being made and will be reported in a subsequent paper.
The authors would like to acknowledge the fabrication work performed by A. Burns and J. Hempinstall, previously with Spectalis Corp, and would like to thank L. Berndt and Professor G. Tarr of Carleton University for their support.
References and links
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