Dielectric-loaded plasmonic waveguide-ring resonators

Using near-field microscopy, the performance of dielectricloaded plasmonic waveguide-ring resonators (WRRs) operating at telecom wavelengths is investigated for various waveguide-ring separations. It is demonstrated that compact (footprint ∼ 150 μm2) and efficient (extinction ratio ∼ 13 dB) WRR-based filters can be realized using UV-lithography. The WRR wavelength responses measured and calculated using the effective-index method are found in good agreement. © 2009 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (250.5300) Photonic integrated circuits; (250.5460) Polymer waveguides References and links 1. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, 1st ed. (Springer-Verlag, Berlin, 1988). 2. P. Berini, “Plasmon-polariton waves guided by thin lossy metal films of finite width: Bound modes of symmetric structures,” Phys. Rev. B 61, 10484 (2000). 3. T. Nikolajsen, K. Leosson, I. Salakhutdinov, and S. I. Bozhevolnyi, “Polymer-based surface-plasmon-polariton stripe waveguides at telecommunication wavelengths,” Appl. Phys. Lett. 82, 668–670 (2003). 4. S. I. Bozhevolnyi, J. Erland, K. Leosson, P. M. W. Skovgaard, and J. M. Hvam, “Waveguiding in Surface Plasmon Polariton Band Gap Structures,” Phys. Rev. Lett. 86, 3008–3011 (2001). 5. S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, “Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides,” Nat. Mat. 2, 229–232 (2003). 6. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, and T. W. Ebbesen, “Channel plasmon-polariton guiding by subwavelength metal grooves,” Phys. Rev. Lett. 95, 046802 (2005). 7. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J.-Y. Laluet, and T. W. Ebbesen, “Channel plasmon subwavelength waveguide components including interferometers and ring resonators,” Nature (London) 440, 508–511 (2006). 8. V. S. Volkov, S. I. Bozhevolnyi, E. Devaux, J.-Y. Laluet, and T. W. Ebbesen, “Wavelength selective nanophotonic components utilizing channel plasmon polaritons,” Nano Lett. 7, 880–884 (2007). 9. C. Reinhardt, S. Passinger, B. N. Chichkov, C. Marquart, I. P. Radko, and S. I. Bozhevolnyi, “Laser-fabricated dielectric optical components for surface plasmon polaritons,” Opt. Lett. 31, 1307–1309 (2006). 10. B. Steinberger, A. Hohenau, H. Ditlbacher, A. L. Stepanov, A. Drezet, F. R. Aussenegg, A. Leitner, and J. R. Krenn, “Dielectric stripes on gold as surface plasmon waveguides,” Appl. Phys. Lett. 88, 094104 (2006). 11. T. Holmgaard and S. I. Bozhevolnyi, “Theoretical analysis of dielectric-loaded surface plasmon-polariton waveguides,” Phys. Rev. B 75, 245405 (2007). #105527 $15.00 USD Received 18 Dec 2008; revised 9 Feb 2009; accepted 10 Feb 2009; published 12 Feb 2009 (C) 2009 OSA 16 February 2009 / Vol. 17, No. 4 / OPTICS EXPRESS 2968 12. A. V. Krasavin and A. V. Zayats, “Passive photonic elements based on dielectric-loaded surface plasmon polariton waveguides,” Appl. Phys. Lett. 90, 211101 (2007). 13. A. V. Krasavin and A. V. Zayats, “Three-dimensional numerical modeling of photonic integration with dielectricloaded SPP waveguides,” Phys. Rev. B 78, 045425 (2008). 14. S. Massenot, J. Grandidier, A. Bouhelier, G. C. des Francs, L. Markey, J.-C. Weeber, A. Dereux, J. Renger, M. U. Gonzàlez, and R. Quidant, “Polymer-metal waveguides characterization by Fourier plane leakage radiation microscopy,” Appl. Phys. Lett. 91, 243102 (2007). 15. T. Holmgaard, S. I. Bozhevolnyi, L. Markey, and A. Dereux, “Dielectric-loaded surface plasmon-polariton waveguides at telecommunication wavelengths: Excitation and characterization,” Appl. Phys. Lett. 92, 011124 (2008). 16. T. Holmgaard, Z. Chen, S. I. Bozhevolnyi, L. Markey, A. Dereux, A. V. Krasavin, and A. V. Zayats, “Bendand splitting loss of dielectric-loaded surface plasmon-polariton waveguides,” Opt. Express 16, 13585–13592 (2008). 17. T. Nikolajsen, K. Leosson, and S. I. Bozhevolnyi, “Surface plasmon polariton based modulators and switches operating at telecom wavelengths,” Appl. Phys. Lett. 85, 5833–5835 (2004). 18. G. Gagnon, N. Lahoud, G. A. Mattiussi, and P. Berini, “Thermally Activated Variable Attenuation of Long-Range Surface Plasmon-Polariton Waves,” J. Lightwave Technol. 24, 4391–4402 (2006). 19. T. Holmgaard, S. I. Bozhevolnyi, L. Markey, A. Dereux, A. V. Krasavin, P. Bolger, and A. V. Zayats, “Efficient excitation of dielectric-loaded surface plasmon-polariton waveguide modes at telecommunication wavelengths,” Phys. Rev. B 78, 165431 (2008). 20. A. Yariv, “Universal relations for coupling of optical power between microresonators and dielectric waveguides,” Electron. Lett. 36, 321–322 (2000).


Introduction
In order to develop compact nanophotonic integrated circuits, it is essential to realize various individual components with specific functionalities to offer and requirements to meet.A possible solution for realizing compact integrated nanophotonic circuits is by utilizing surface plasmon-polaritons (SPPs), which are surface waves in the plasma of a metal coupled to light waves.SPPs are bound to, and propagate along a metal-dielectric interface, with the fields decaying exponentially away from its maximum at the interface [1].This implies a strong inherent confinement in the direction perpendicular to the interface, and thus the design task consists of achieving single-mode waveguides with strong lateral confinement, and propagation loss as low as possible.Several proposals for plasmonic waveguides and integrated components have been made, e.g. by using metal stripes embedded in a dielectric [2,3], SPP band gap structures [4], chains of metal particles [5], and V-groves cut into an otherwise planar metal surface [6,7,8].
An alternative, and technologically simple, approach to achieve ultra-compact plasmonic integrated components is to utilize dielectric stripes deposited on a smooth metal film as waveguides [9,10].This type of waveguides has been thoroughly investigated by utilizing the effective index method (EIM) and the finite element method (FEM) [11,12,13].Near-field characterization of fabricated dielectric-loaded SPP waveguides (DLSPPWs) has revealed the realization of single-mode waveguides with sub-wavelength confinement and relatively low propagation loss [14,15].In addition compact basic plasmonic components such as bends, splitters and Mach-Zehnder interferometers have been fabricated and characterized, demonstrating efficient bending, splitting and recombination of DLSPPW modes [16].Although the metal film (or stripes) supporting the dielectric ridges greatly enhances the propagation loss (compared to conventional dielectric waveguides), the presence of metal adjacent to the waveguides is also one of the greatest virtues of the DLSPPW technology.The main apparent advantage is in having simultaneously at hand a plasmonic and electrical circuit, whose metal stripes can be employed, e.g., for heating the dielectric ridges, thereby inducing a thermo-optical effect in the plasmonic waveguides by changing the refractive index of the ridge.This process is very efficient since, similarly to the case of thermo-optical components utilizing long-range SPPs [17,18], the DLSPPW mode field is maximal at the metal-dielectric interface and thereby at the heating electrode, unlike in most conventional dielectric waveguides, where the mode power extends far out in the surrounding cladding and the proximity of electrodes should be avoided to minimize the absorption loss.In this manner one can envisage the realization of wavelength tunable filters, controlled by applying a voltage over the underlying electrodes.As the DL-SPPW technology is naturally compatible with different dielectrics, the realization of active plasmonic components utilizing electro-optical, magneto-optical, acousto-optical, or nonlinear optical effects is furthermore possible.In the present work we report on the design, fabrication and characterization of passive wavelength-band selective filters based on waveguide-ring resonators (WRRs), operating in the telecommunication range by utilizing the DLSPPW technology.
Samples are fabricated by deep UV lithography using a vacuum contact mask aligner with either a home-made mask or a manufactured commercial mask.The DLSPPW-ring resonators consist of ∼ 550-nm-high and ∼ 500-nm-wide poly-methyl-methacrylate(PMMA) ridges deposited on a 60-nm-thick gold film, which is supported by a thin glass substrate.These waveguide dimensions ensure single-mode propagation and close to optimum lateral mode confinement [11], at telecommunication wavelengths, which imply low bend losses imperative in the realization of compact WRRs.When preparing the samples for near-field characterization, they are placed on an equilateral prism using index matching immersion oil.DLSPPW modes are excited at telecommunication wavelengths by utilizing the Kretschmann-Raether configuration [1].By focusing a Gaussian beam on the metal-prism interface opposite the waveguide structures, at an angle above that of total internal reflection, the lateral component of the incident wave-vector is matched to the effective mode index of the SPP in the dielectric ridge.In this particular case, a high-index prism is applied as funnel in-coupling structures (with high mode effective index), connected to the ridge waveguides, are utilized for more efficient DLSPPW excitation [Fig.1(a)].The near-field characterization is performed using a scanning near-field optical microscope (SNOM) operating in collection mode.In this setup an etched uncoated fiber tip is raster scanned across the sample, thus recording the topography and near-field optical signal as described in detail elsewhere [16,19].

Coupling to ring resonators
The WRRs considered in this work consist of a straight waveguide, extended in one end by a funnel (used for coupling in SPPs), and a circular resonator placed in proximity of the straight waveguide to allow for coupling between the two [Fig. 1 carefully consider several parameters impacting the performance.The bandwidth of the WRR is determined by the radius of the ring resonator along with the wavelength dependent mode effective index of the bent waveguide, whereas the extinction ratio (ratio between minimum and maximum signal output) is determined by the coupling strength to the ring and the attenuation due to SPP propagation and bend loss around the ring.In the design of the fabricated waveguides a fixed ring radius of R = 5 μm has been used, whereas the gap between waveguide and ring is varied in order to investigate the effect on the extinction ratio.A weak coupling is expected for waveguide-ring gaps larger than ∼ 0.5 μm due to the very strong mode confinement [12,15,19] and short interaction region, which, by SNOM characterization of WRRs with gaps of g 2.5 μm, g 1.5 μm, and g 0.5 μm, indeed is found to be the case [  The near-field characterization of the g 0.5 μm WRR reveals an extinction ratio of ∼ 13 dB, giving promise of realization of very effective band selective filters.An SEM image of the g 0.5 μm WRR reveals that the gap is not completely resolved as some residual PMMA resist still exists in the gap [Fig.2(a)], which, by perfecting the fabrication, or by using lithography techniques more advanced than a simple contact mask aligner, is likely to be avoided.The effect of this nonresolved gap could be larger coupling to the ring than expected, and the effect of this is further investigated in the following.

Transmission through WRRs
In order to further investigate the periodic behavior of the WRR transmission, the measured transmission values are fitted to an analytical expression for the power transmission taken from [20] where scattering losses at the interaction region have been disregarded.The first factor reflects the propagation loss from cross section A to B (l = 10 μm).α = exp(−2πRβ )σ is the field attenuation factor per roundtrip around the ring (β being the imaginary part of the propagation constant of the DLSPPW mode and σ being a parameter accounting for the pure bend loss).t is the field transmission through the interaction region in the straight waveguide.θ = (2π/λ )N eff (λ )2πR is the phase change around the ring, where λ is the free space excitation wavelength, and R is the ring radius.In the calculations the wavelength dependent power propagation length L SP (λ ) and mode effective index N eff (λ ) are taken from EIM calculations [11].The fitting parameters are thus the ring radius R, determining the period of oscillations and position of transmission bands, and the ring bend loss factor σ , and transmission t determining the amplitude and offset of the oscillations.
By minimizing the total deviation from the measured transmission values a best fit is obtained for the g 0.5 μm WRR, where R = 5.39 μm, σ = 0.71 (giving α values in the range 0.49 to 0.52 due to the wavelength dependence of L SP ), and t = 0.66 [Fig.3].The realized ring radius is close to the designed, and gives a transmission bandwidth of ∼ 20 nm, with a period of 45 μm.At resonance, i.e., for θ = m2π (m being an integer), Eq. ( 1) simplifies to implying that critical coupling occurs for α = t.For the g 0.5 μm WRR this is quite close, and along with the relatively large α and t values, this gives the large extinction ratio of ∼ 13 dB.In order to achieve even larger extinction ratios a slight increase in α or decrease in t is necessary.By decreasing the gap between the waveguide and the ring a larger coupling ratio is expected, implying a smaller t value, however, the bend loss factor σ is not expected to change significantly due to unchanged ring radius and waveguide widths in the design.
A WRR with a gap of g 0.3 μm has been designed and fabricated using a second mask, and by characterization with the SNOM [Figs.4(a)-(e)] transmission values were obtained [Fig.4(f)].
Due to imperfect fabrication, the near-field optical images show more scattering than for the previous sample, implying that lower transmission values are to be expected as compared to the g 0.5 μm WRR.This trend is further enhanced due to the expectance of a lower t value, and by observing Fig. 4(f) this is indeed found to be the case.The best fit to the measured

Wavelength (nm) WRR transmission
Measurements Fitted values Fig. 3. Transmission through the WRR with a gap of g 0.5 μm, measured by taking averaged cross sectional profiles of the optical images at the input A and output B (Fig. 2(b)), similar to those shown in Fig. 2(j).A fit to the measured transmission values is obtained by utilizing Eq. ( 1).transmission values is found to be for R = 4.90 μm, σ = 0.66 (giving α values in the range 0.47 to 0.50), and t = 0.23, indicating that the coupling change significantly by decreasing the gap by 200 nm.As expected the bend loss factor is almost unchanged, and the small variation is likely to be caused by scattering, or simply due to the slightly smaller ring radius of the fabricated WRR, originating from the second mask used.
On the same sample a WRR with a designed gap of g = 0.0 μm, i.e., intersection between ring and waveguide, has been realized [Fig.5].In this case the coupling mechanism between the waveguide and the ring differs significantly, as the geometry in the interaction region resembles that of a double-width waveguide instead of two closely spaced waveguides.This double width waveguide is split into two arms, one continuing straight and one bending off as part of the ring resonator, by means of a Y-splitter like separation, which implies that a scattering loss in the intersection region is to be expected [16].Due to the non-symmetric splitting of the double width waveguide a significantly smaller fraction of the mode power will be coupled to the ring resonator as this requires a change in wavevector in the intersection region.This implies that although the gap size is the smallest of the investigated WRRs a smaller coupling (larger transmission) is expected due to the different coupling mechanism.From the obtained nearfield optical images [Fig.5(b)-(e)] it is apparent the the coupling to the ring is much weaker than for the other investigated WRRs, and that the Y-splitting separation of the waveguide in the interaction region gives rise to a scattering loss.As the WRR is fabricated using the same mask as the g 0.3 μm WRR, similar ring characteristics are expected, which is confirmed as the best fit to the measured transmission values is found to be R = 4.89 μm, σ = 0.65 (giving α values in the range 0.47 to 0.49), and t = 0.79.The larger transmission implies a higher throughput, but lower extinction ratio, relative to the g 0.5 μm WRR, as apparent from Eq. (2).

Conclusion
Compact wavelength selective waveguide-ring resonators have been designed and fabricated by using large-scale compatible UV lithography to deposit PMMA ridges on a smooth gold surface.Characterization of the fabricated WRRs with a SNOM imaging system, at telecommunication wavelengths, shows a strong coupling to the ring resonator for waveguide-ring gaps smaller than ∼ 0.5 μm, whereas gap sizes larger than ∼ 1.5 μm does not give rise to any noticeable coupling to the ring resonator.Near-field characterization of a g 0.5 μm WRR with a designed ring radius of R = 5 μm reveals an extinction ratio of ∼ 13 dB and transmission bandwidth of ∼ 20 nm, giving promise of realization of very effective band selective filters based on DLSPPWs.Characterization of a g 0.3 μm WRR reveals a much stronger coupling to the ring resulting in lower transmission values and smaller extinction ratio, whereas a g 0.0 μm WRR, i.e., waveguide-ring intersection, shows weak coupling due to the change in coupling mechanism.A good correspondence is found between measured transmission values and values obtained from an analytical expression for the transmission.This expression explains the features of each WRR well, with regard to coupling to the ring and attenuation in the resonator, and can thus be used in the design of WRRs to obtain the desired bandwidth and extinction ratio.

Fig. 1 .
Fig. 1. (Color online) Waveguide-ring resonators with different gaps between the waveguide and the ring.(a) SEM image showing four WRRs with different gap sizes.[(b) and (c)] Topographical and near-field optical images, respectively of a WRR with a gap of g 2.5 μm, [(d) and (e)] of a WRR with a gap of g 1.5 μm, and [(f) and (g)] of a WRR with a gap of g 0.5 μm.The images (b)-(g) share the scale shown in (b), and all near-field images are recorded for a free-space excitation wavelength of λ = 1550 nm.

Fig. 1 ]
. It is observed that the output signal is clearly damped in the g 0.5 μm WRR [Fig.1(g)], as compared to the two other WRRs, indicating strong coupling and good resonator performance in this case.For the WRR with the largest gap (g 2.5 μm) a strongly confined DLSPPW mode propagates in the straight waveguide without any noticeable change in the interaction region [Fig.1(c)].This is also the case for the g 1.5 μm WRR [Fig.1(e)],whereas the g 0.5 μm WRR exhibits strong coupling to the ring resonator [Fig.1(g)].A wavelength analysis of the g 0.5 μm gap WRR reveals that the output signal is strongly modulated with a period of Λ 45 nm [Fig.2].Averaged cross sectional profiles of the near-

Fig. 2 .
Fig. 2. (Color online) WRR with a gap of g 0.5 μm.(a) SEM image of the whole WRR (b) Topographical image recorded with the SNOM.[(c)-(i)] Near-field optical images with free-space excitation wavelength varying from 1530 nm to 1590 nm in steps of 10 nm, which share the scale shown in (b).(j) Averaged cross sectional profile of the near-field optical images (g) and (i), taken at lines A and B as marked in (b).