We report the design, fabrication and characterization of compact gradual bends for channel plasmon polaritons (CPPs) being excited at telecom wavelengths. We obtain high-quality near-field optical images of CPP modes propagating along a bent V-groove in gold, which indicate good CPP mode confinement in the groove and efficient guiding around the compact S-bend connecting two 5-µm-offset grooves over a distance of 5 µm. Using averaged cross sections of the CPP intensity distributions before and after the S-bend, the total bend loss is evaluated and found to be close to 2.3 dB for the wavelengths in the range of 1430-1640 nm.
©2006 Optical Society of America
Ever increasing pace of miniaturisation and integration in optics requires further progress in fabrication and integration of several photonic components on a common planar substrate with the purpose of realizing various functionalities such as guiding, bending, splitting, filtering, multiplexing and demultiplexing of optical signals. To support these functions, integrated optics involves a number of technologies with different technological platforms and material systems including semiconductors, silica, silica-on-silicon materials , polymers  and organo-mineral materials  among others. The size and density of optical devices employing these technologies is nonetheless limited by the diffraction limit of light that does not allow localization of electromagnetic waves in regions noticeably smaller than half the wavelength in the structure . Another limitation is the typical guiding geometry (e.g., planar waveguides are limited in their geometry because of radiation leakage at sharp bends). These circumstances result in a much lower level of integration and miniaturization of optical devises, compared to that achieved in modern microelectronics. A new approach to circumvent this problem, which suggests employing surface plasmon polaritons (SPPs) that are light waves coupled to oscillations of free electrons in a metal, meets the requirements of low-cost, simplicity of planar fabrication, combined with good performance. The SPP fields decay exponentially into both media and reach maximum at the interface, a circumstance that makes SPPs extremely sensitive to interface properties . The possibility of using SPPs for miniature photonic circuits has attracted a great deal of attention to the field of SPPs during the last years . So far, many different concepts have been suggested, including photonic bandgap structures , metal stripes in a dielectric environment , nanorods , metal particle waveguides , and the use of dielectric thin-film structures on top of a metal surface . Quite recently, SPP gap waveguides based on the SPP propagation between profiled metal surfaces have been suggested  and various nano-waveguide configurations have been considered [12–14]. The possibility of guiding of electromagnetic waves by a channel cut into an otherwise planar surface of a solid (metal or polar dielectric) characterized by a negative dielectric function was first considered by Maradudin and co-workers (more than 15 years ago) in the electrostatic limit . After this initial paper, a number of publications discussing the channel guided waves propagation in triangular metallic grooves appeared [16–19]. Channel SPP modes, or channel plasmon polaritons (CPPs) , where the electromagnetic radiation is concentrated at the bottom of V-shaped grooves milled in a metal film, have been predicted to have promising subwavelength confinement and relatively low propagation loss , single-mode operation  and efficient transmission around sharp bends . Quite recently, we have employed a collection scanning near-field optical microscope (SNOM) for imaging of the CPP propagation at telecom wavelengths along straight subwavelength grooves milled in gold . High-quality SNOM images were obtained allowing us to directly demonstrate the CPP existence, evaluate the CPP propagation loss and characterize the CPP mode confinement.
In this work, we report the design, fabrication and characterization of compact gradual bends featuring the low-loss CPP guiding at telecom wavelengths. We present near-field optical images of CPP modes propagating along a bent V-groove in gold, which indicate good CPP mode confinement in the groove and efficient guiding around the compact S-bend connecting two 5-µm-offset grooves over a distance of 5 µm.
2. Experimental arrangement
The experimental setup employed for the CPP observation and characterization is essentially the same as that used in our previous experiments with photonic crystal waveguide structures . It consists of a collection SNOM with an uncoated sharp fiber tip used as a probe and an arrangement for launching tunable (1430-1640 nm) TE/TM-polarized (the electric field is parallel/perpendicular to the sample surface plane) radiation into a metal groove by positioning a tapered-lensed polarization-maintaining single-mode fiber (Fig.1).
The investigated 160-µm-long groove has been fabricated (using a focused ion-beam milling technique) in a 1.9-µm-thick gold layer deposited on a substrate of fused silica covered with an 80-nm-thick indium-tin-oxide layer. The groove profile turned out being similar to a triangular profile [Fig. 2(a)]. It is also seen that the groove walls are somewhat rough, a feature that can influence the CPP propagation by causing the CPP scattering out of the groove. The width and depth was measured as w~0.6 µm and d~1.1 µm, respectively. For the nearly triangular groove, we have also evaluated the groove angle as θ~300. Considering the dimensions of the groove and taking into account the results of our previous experiments , it is reasonable to suggest that (only) the fundamental CPP mode can be supported by this groove being close to cut-off. The latter would facilitate its observation both with a far-field microscope (weakly confined mode is easier to scatter by surface features) and the SNOM (the detection efficiency of a fiber probe increases for lower spatial frequencies). The set of structures to be investigated included straight reference guides and S-bends, i.e., smoothly curved double bends connecting two parallel waveguides offset with respect to each other. The design of S-bends was based on sine curves allowing for continuous curvature throughout the bend . Anticipating (relatively) low loss for S-bends with small curvature radii because of the subwavelength CPP confinement , we have chosen the S-bend design that connects two 5-µm-offset waveguides over the distance of 5 µm with the smallest curvature radius being in this case ≅2.25 µm [Fig. 2(b)]. In general, such a choice is a subject of trade-off between the propagation and bend losses, since the former decreases and the latter increases for shorter bends. The adjustment of the in-coupling fiber [Figs. 1 and 2(c)] with respect to the sample facet was accomplished when monitoring the light propagation along the sample surface with help of a far-field microscopic arrangement [Figs. 1 and 2(d)]. The idea was to judge upon the CPP excitation and propagation by observing its scattering out of the surface plane by groove imperfections.
We have found that the track of radiation propagating along the groove was visible only for TE polarization of incident light [Fig. 2(d)]. This track was clearly distinguishable for distances of up to~120 µm from the in-coupling groove edge and in the whole range of laser tunability. The far-field observations have confirmed the expected polarization properties of the guided radiation and demonstrated its (relatively) low dissipation. Following these experiments (that include also adjusting the in-coupling fiber position to maximize the coupling efficiency) we moved the whole fiber-sample arrangement under the SNOM head and mapped the intensity distribution near the surface of the groove with an uncoated sharp fiber tip of the SNOM. The near-field optical probe used in the experiment has been produced from a single-mode silica fiber by~120 min etching of a cleaved fiber in 40% hydrofluoric acid with a protective layer of olive oil. The resulting fiber tip has a cone-angle of~40° and curvature radius of less than 80 nm. The tip was scanned along the sample surface at a constant distance of a few nanometers maintained by shear force feedback. It should be borne in mind that this distance could not be maintained in the middle of the groove (given the groove dimensions and the tip size), a circumstance that might influence the characterization of CPP mode cross section. Near-field radiation scattered by the tip was partially collected by the fiber itself and propagated in the form of the fiber modes towards the other end of the fiber, where it was detected by a femtowatt InGaAs photo receiver.
3. Experimental results
Topographical and near-field optical images of CPP guiding by the groove containing the Sbend were recorded at the distance of ~120 µm from the in-coupling groove edge (to decrease the influence of stray light, i.e., the light that was not coupled into the CPP mode) and in the whole range of laser tunability (Fig. 3).
Appearance of the optical images is similar to those reported previously for straight grooves  featuring efficient mode confinement (in the cross section) at the groove and intensity variations along the propagation direction. The latter can be most probably accounted for by the interference between the CPP mode and scattered (including stray light) field components . It should be noted that we have also recorded optical images in constant plane mode at the height of 200 nm above the sample surface. These images exhibited only low-contrast noise-like signal variations with the average level of ~20% from the maximum signal associated with the CPP mode, confirming that the main contribution to the signals detected at the groove (Fig. 3) originates indeed from the CPP (evanescent) fields . The optical signal related to the CPP propagation along the groove decreases noticeably after the S-bend. We can further identify the following channels for the associated bend loss: (i) the modal reflection that gives rise to the interference fringes in the intensity distribution before the bend, (ii) the light scattering out of the bend in the forward direction that is clearly seen for all wavelengths, and (iii) corresponding propagation loss owing to CPP guiding along the bent groove. Note that the light scattering in the bend region results in scattered field components propagating (in air) away from the sample surface. These propagating components are detected with the SNOM fiber tip much more efficiently than the evanescent field components, e.g., associated with the CPP groove mode . This circumstance should be borne in mind in the course of interpretation of SNOM images. Despite the aforementioned signal variations, we could characterize the average full-width-at-half-maximum (FWHM) of the CPP mode (Fig. 4).
Lateral cross sections of the SNOM images obtained in the course of this investigation showed the average FWHM of the CPP mode to be between 0.8 and 1 µm, being almost independent on the wavelength for a given groove, i.e. values that are even slightly smaller than those obtained previously . The fact (observed here and in the previous work ) that the fundamental CPP mode width is wavelength independent can be explained by the circumstance that this field is squeezed by and reaches its maximum at the groove walls [16–18] that set the mode lateral extent (apart from those modes that are very close to cut-off extending significantly out of the groove–these modes should also exhibit very large propagation losses). Considering the groove width ~0.6 µm and taking into account the limited resolution of our SNOM with an uncoated fiber tip, we find the obtained FWHM values corresponding to the expected ones. The average cross sections of near-field optical images (obtained at different wavelengths) made before and after the S-bend [as illustrated in Fig. 4(a)] provide a direct way of the total insertion loss evaluation. Fig. 4(b) shows the cross sections obtained with the optical images shown in Fig. 3 by averaging over ~3-µm-along straight groove regions. It is seen that the optical signal outside the grooves amounts to ~13% from the maximum signal associated with the CPP mode at the input regions. This background signal is also present in the optical images recorded on constant plane mode at the height of ~200 nm above the sample surface . However, it is not clear to what extent it influences the signal measured in the middle of a groove with the SNOM fiber tip being actually below the sample surface by ~40 to 100 nm, depending on the tip shape. With this in mind, the bend transmission T S was found slightly increasing (with the wavelength) at short wavelengths, i.e., from T S≅0.45 at the wavelength of 1430 nm to T S≅0.49 at 1640 nm. It is clear that at least some of the power loss, which is estimated being at the level of ~3.2 dB in the wavelength range 1430–1640 nm, should be attributed to the propagation loss due to the internal damping. One should also bear in mind the loss contribution due to scattering out of the groove plane by roughness and imperfections of groove walls. Evaluating the propagation loss  for a 12-µm-long groove region (this region is schematically shown in Fig. 4(a) as a hatched groove area between lines marked “In” and “Out”) allowed us to determine the bend loss that includes also the scattering loss contribution (Fig. 5).
Figure 5 shows that the average bend loss of the S-bend is close to ~2.3 dB per double bend in the wavelength range 1430–1640 nm. However, it can be seen that the losses were increasing slightly at short wavelengths (1430–1460 nm). We would like to point out that the loss level attained already in these experiments indicates that the CPP waveguide circuits having relatively large (tens of degrees) bend angles can be realized with relatively small (<3dB) bend losses, allowing for high integration level. It should be noted that the CPP intensity distribution inside the groove was found to be noticeably varying for different adjustments of the in-coupling fiber with respect to the fiber displacement parallel to the surface plane. The typical SNOM images recorded at the wavelength λ≅1620 nm with the in-coupling fiber being moved along the sample facet (and displaced with respect to the central line of the groove) are shown in Fig. 6.
For the case where the input fiber was correctly aligned and centered to the groove [Fig. 6(b)], the CPP propagation along the groove is clearly seen. The recorded intensity distribution is well confined to the groove area exhibiting efficient single-mode guiding around the S-bend. When the fiber was ~150 nm moved along the sample facet [Fig. 6(c)], the recorded SNOM image became perturbed displaying two distinct maxima (that correspond most probably to the fundamental CPP mode and/or other SPP modes propagating along the groove edge) as well as strong CPP scattering in the bend region. The contribution of the fundamental CPP mode decreased rapidly with further displacement of the in-coupling fiber [Fig. 6(d)]. At the same time, the additional SPP mode(s) was still observed in the groove area before and after the bend. Finally, when the fiber was not aligned to the groove, i.e., moved over ~500 nm along the sample facet [Fig. 6(e)], only some light scattering in the bend region was seen on the SNOM image without any sign of light coupled into the CPP mode. The evolution of the CPP mode intensity distribution across the groove before and after the S-bend (for different adjustments of the in-coupling fiber with respect to the groove center) is shown in Fig. 7.
Complicated intensity patterns observed for the different adjustments of the in-coupling fiber are probably related to the excitation of SPP mode(s) localized at and propagating along the groove edge, which can be considered as a wedge. Recently, it has been suggested based on numerical simulations corroborated with experimental observations that SPP modes can propagate along the top of triangular silver wedges . However, we would rather desist from definite conclusions before conducting further detailed investigations of the possibility of excitation and observation of wedge SPP modes in this configuration.
In this paper, using the collection SNOM we have directly observed the CPP propagation along the (triangular) 0.6-µm-wide and 1.1-µm-deep V-groove in gold containing the rather compact S-bend connecting two 5-µm-offset grooves over a distance of 5 µm (which is equivalent to a double 45° sharp bend). High-quality SNOM images of the groove excited at telecom wavelengths have been obtained and used to characterize the CPP waveguide structure studied. Thus, the bend loss has been directly evaluated using averaged cross sections of the intensity distributions before and after the S-bend and found to be close to 2.3 dB in the wavelength range of 1430–1640 nm. We have also identified the loss channels for the S-bend in question.
The issues discussed in this work have implications for the SNOM characterization of any CPP waveguide structure. Thus, we have demonstrated the importance of careful adjustment of the in-coupling fiber with respect to the central line of the groove. We have observed drastic changes of the CPP mode with only a 150-nm-large displacement of the in-coupling fiber parallel to the surface plane. The field intensity profile recorded in this configuration showed two distinct maxima that can be associated with one or several SPP modes localized near and propagating along the groove edge. These modes might be related to wedge SPP modes considered recently when illuminating (in visible) triangular silver wedges .
Finally, we believe that further investigations and optimization of the structural parameters (in the first place the grooves quality) will allow us to decrease the level of bend loss in the CPP waveguide components and thereby optimize their performance.
This work was supported by the European Network of Excellence, PLASMO-NANODEVICES (FP6-2002-IST-1-507879).
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