1. Introduction

Several studies have focused on the use of thin metal stripes embedded in a dielectric material to guide light in optical waveguide devices, due to their end-fire excitation and easy fabrication[1–4]. For sufficiently thin metal stripes, surface plasmon polaritons (SPPs) associated with the upper and lower interfaces couple and form a symmetric mode, a long-range SPP (LRSPP), whose propagation loss decreases with a decrease in stripe thickness. Owing to the low-cost fabrication process and their simple structure, LRSPP-based metal stripe waveguides and their application in integrated optical devices such as optical attenuators[5, 6], couplers[7–9], filters [10], modulators[11, 12], and sensors[13] have been widely studied. Recently, high-bitrate optical signal transmissions have been demonstrated using a thin gold stripe waveguide based on the LRSPP[14–15]. To take advantage of these properties of LSRPPs, it is critical to reduce the comparatively high loss in LRSPP waveguides. The propagation loss can theoretically be reduced to less than 1 dB/cm at telecommunication wavelengths[16] and its field distribution can be adjusted to close to that of a single-mode fiber by varying the stripe thickness and width.

LRSPP waveguides consisting of 10-nm-thick gold stripes of 6-10 μm were embedded in a dielectric; they exhibited a coupling loss of ~0.5 dB and a propagation loss of ~6-8 dB/cm at the wavelength range of 1.51-1.62 μm[17]. To reduce the propagation loss of the LRSPP mode, multi-stripe configurations were proposed by Berini et al.[18]. Gold and silver stripe waveguides with multi-stripe configurations were investigated at a wavelength of 1.55 μm. The lowest propagation losses among the silver and gold stripes were 4.2 and 3.2 dB/cm, respectively, both obtained using 20-nm-thick stripes. Recently, propagation losses of ~2 dB/cm and ~ 1.4 dB/cm were realized for 14.5 nm-thick gold stripes at wavelengths of 1.31 and 1.55 μm, respectively[19]. Furthermore, a propagation loss of 1.6 dB/cm at a wavelength of 1.55 μm was achieved for the 4-μm-wide gold stripe with a single layer cladding[20]. The propagation loss increases as the wavelength shortens, due to the higher ohmic loss and the stronger bound of the metal stripe[16, 19].

2. Experiment and results

Theoretical results predict that the propagation loss of the polymer-based LRSPP waveguide can be decreased to less than 1 dB/cm, not only by optimizing the metal stripe structure, but also by lowering the refractive index of the clad material[16]. However, there are some obstacles preventing the achievement of this ideal optical property. The propagation loss of the polymer-based LRSPP waveguide could be attributed to intrinsic loss induced by ohmic loss of the metal and absorption loss of the clad material, and to the extra loss generated by the inhomogeneous metal structure and nonuniformity of the refractive index of the clad[3]. The extra loss results mainly from unstable conditions in the fabrication procedure. To obtain a low-loss LRSPP waveguide, those loss-generating factors should be eliminated or minimized.

To minimize intrinsic loss, we used silver thin films and an ultraviolet (UV) curable fluorinated polymer with a low refractive index and absorption loss. To obtain a uniform refractive index distribution of the polymer used as a clad layer, curing process conditions such as UV light dose, UV irradiation time, and the sequences of the thermal curing condition, were kept constant. The thickness of the clad layers is sufficient to accommodate the LRSPP field.

To form uniform metal stripe patterns, we developed a lift-off process using double layers of photoresist and SiNx. Figure 1 schematically illustrates the fabrication process of the metal stripes using double layer lift-off lithography. The cladding polymer material is a UV-curable fluorinated resin based on acrylate supplied by ChemOptics Inc., trademarked ZPU450. We measured the cladding material loss of 0.1 dB/cm by the liquid immersion method and the reflective index of ~1.451 at 1.31 μm. The lower cladding was spin-cast on a silicon wafer, and then UV curing was performed in a UV light (365 nm) irradiation chamber with an optical power density of 20 mW/cm² (on the wafer) for 10 min in the absence of oxygen (less than 100 ppm) by flowing nitrogen through the chamber (20 L/min). The film thickness was measured (using a surface profiler) to be 30 μm. Because the uniformity of the refractive index in the thick film affects the waveguide loss significantly, we optimized the curing processes to control the index fluctuation within 1x10^-4. In the double layer lift-off process, SiNx and AZ 5206E photoresist (Clariant Inc.) layers were coated on the cladding layer as shown in Fig. 1(a). The SiNx layer was formed by chemical vapor deposition and its thickness was 40 nm. The photoresist was patterned with a UV expose and then developed. The SiN_x was then etched in a buffered oxide etchant (BOE) to form an over-hanging structure (Fig. 1(c)), and a silver film (11 nm thick) was deposited by thermal evaporation. We carried out the silver evaporation process with a relatively low evaporation rate of 0.02 nm/sec to reduce the metal thickness uncertainty and the waveguide loss due to the larger grain size at elevated rates. Then, the photoresist and SiN_x layers were removed by an AZ 340 developer and BOE, respectively. After metal stripe fabrication, a 30-μm-thick upper cladding was formed by spin-coating with ZPU 450 and UV curing under the same conditions as above. For improved stability and perfect curing of the polymer film, the whole waveguides underwent an additional thermal curing process in a nitrogen oven (flow rate 50 L/min) at 160°C for 60 min.

The developed double layer lift-off process has some advantages for the formation of metal stripe patterns on polymer layers. The thickness near the edge of the polymer layer is thicker than that at the center when spin-coating the polymers on a substrate with a rotational speed of less than 1000 rpm; this effect is called an edge bead. This edge bead induces unstable contact conditions between the substrate and photomask, and results in degradation of the metal stripe patterns, because an over-hanging structure disappears in photoresist patterns. In the double layer lift-off process, however, the over-hanging structure could be formed without dependence on the photoresist patterns, as depicted in Fig. 1. Furthermore, the reactive ion etching (RIE) process for removing residual photoresist could be performed after developing the photoresist. The SiN_x layer acts as a stop layer, protecting the polymer surface from roughening by the RIE process. After the formation of the metal stripe patterns, it is unnecessary to have an additional process that eliminates the residual photoresist on the polymer layer. We obtained the lowest propagation loss by reducing the scattering loss, originated from the soft photoresist and by using the clad polymer which is the lower refractive index and loss than that used in previous works[3, 19].

Metal stripe widths in the range of 1.5 ~ 10 μm (with a 0.5 μm step) were fabricated. The thickness was measured with a carefully calibrated thickness monitor using a quartz-crystal microbalance. The thickness was calibrated with an atomic force microscope (AFM) and an Alpha step instrument. Figure 2 shows an AFM scan of the silver stripe with a width of 4 μm and a thickness of 11 nm. The fabricated stripe exhibits a smooth sidewall profile due to the well-established double layer lift-off process. A surface roughness of less than 1 nm was obtained, and the thickness was measured as 11 nm.

 

Fig. 1. Schematic showing the fabrication processes of the metal stripe waveguides.

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To measure the mode field diameters (MFDs) of the silver stripe waveguides, the waveguide input was excited via end-fire coupled light (1.31 μm) with a polarization maintaining fiber (PMF). Using a polarization controller, the input light was polarized to be perpendicular to the waveguide surface (TM polarization). The output face of the waveguide was imaged with a 50-times magnification lens on a CCD camera. Using a beam view analyzer, the MDF was evaluated by fitting the captured mode profile to a Gaussian distribution. Figure 3 shows the dependence of the horizontal and vertical MFDs on the silver stripe width (with a fixed thickness of 11 nm); the insets are typical images of captured modes for silver stripe widths of 1, 5, and 9 μm. The captured modes were well-fitted to Gaussian distributions without intensity spikes of side modes, which can originate from side-wall roughness of the metal stripe and nonuniformity of the refractive index of the cladding. As the metal width narrows, the optical fields shrink and then spread out along the horizontal direction, and gradually become larger along the vertical one. In particular, the MFD for the stripes of width less than 2 μm expands abruptly both directions. This means that the modes are very weakly confined.

 

Fig. 2. Atomic force microscope (AFM) scan of a 4-μm-wide silver stripe on a Si wafer. (a) Topographic images of a 10*20 μm2 area plotted on a 16-nm vertical scale (b) Average thickness profile of the silver stripe.

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Fig. 3. Horizontal and vertical mode field diameters of the silver stripe waveguides as a function of the width. The thickness is the fixed value of 11 nm. The insets are the captured mode images for the 1-, 5- and 9-μm-wide silver stripes, respectively.

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To investigate the propagation losses of the silver stripe waveguides and their coupling characteristics with the PMF, the waveguide output power was coupled into the PMF, and the transmitted power was then measured with an optical power meter. The propagation loss measurements were carried out with a cut-back method. The inset in Fig. 4 gives the cut-back measurement for the 4-μm-wide metal stripe. Figure 4 shows the dependence of the waveguide loss on width of the silver stripe with a fixed thickness of 11 nm. The propagation and coupling losses for all stripes were obtained by linear fitting of the cut-back measurement. The propagation loss decreases as the stripe width narrows. We obtained a propagation loss of 0.8 dB/cm for silver stripes of width less than 4.5 μm and 0.5 dB/cm for those less than 3 μm. Under a weakly guided condition at the metal stripe width of 2.0 μm, we measured a propagation loss of 0.4 dB/cm. These results could be further improved if we use a more transparent cladding material with a lower refractive index. This propagation loss is much lower than the previous record of 3.2 dB/cm demonstrated by Berini in silver waveguides[18], even though we measured it at a shorter wavelength (1.31 μm instead of 1.55 μm).

The coupling loss of the PMF increases as the metal width narrows (Fig. 4), due to the mode field mismatch between the silver stripe waveguide and the fiber. Thus, in the case of in- and out-coupling applications of the silver stripe waveguide with PMFs, there is a trade-off between the propagation and coupling loss. A coupling loss of ~ 1.0 dB was obtained for the silver stripes with width greater than 4.0 μm.

 

Fig. 4. Propagation losses and coupling losses of the silver stripe waveguides as a function of width. The thickness is the fixed value of 11 nm. The inset gives the cut-back measurement for a 4 μm width.

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3. Conclusion

We realized a sub-dB/cm propagation loss for 11-nm-thick silver stripe waveguides by embedding them in a low-loss optical polymer at the wavelength of 1.31 μm. Our success results from the well-established double layer lift-off process for the uniform stripe patterns and from the optimization of the curing condition under which the uniform polymer claddings minimize the surface roughness of the metal films. The lowest propagation loss of ~0.4 dB/cm was obtained for the 2-μm-wide stripe waveguide. These results could be further improved by using a more transparent cladding material with a lower refractive index.