Experimental demonstration of low-loss optical waveguiding at deep sub-wavelength scales

Abstract

Emerging communication applications call for a road map towards nanoscale photonic components and systems. Although metal-based nanostructures theoretically offer a solution to enable nanoscale photonics, the key demonstration of optical modes with deep sub-diffraction-limited confinement and significant propagation distances has not been experimentally achieved because of the trade-off between optical confinement and metallic losses. Here we report the first experimental demonstration of truly nanoscale guided waves in a metal- insulator-semiconductor device featuring low-loss and broadband operation. Near-field scanning optical microscopy reveals mode sizes down to 50×60 nm² at visible and near-infrared wavelengths propagating more than 20 times the vacuum wavelength. Interference spectroscopy confirms that the optical mode hybridization between a surface plasmon and a dielectric mode concentrates the hybridized mode inside a nanometre thin gap. This nanoscale waveguide holds promise for next generation on-chip optical communication systems that integrate light sources, modulators or switches, nonlinear and quantum optics.

ARTICLE

Received 4 Feb 2011 | Accepted 13 Apr 2011 | Published 31 May 2011 DOI: 10.1038/ncomms1315

Volker J. Sorger1,*, Ziliang Ye1,*, Rupert F. Oulton1, Yuan Wang1, Guy Bartal1, Xiaobo Yin1 & Xiang Zhang1,2

Emerging communication applications call for a road map towards nanoscale photonic components and systems. Although metal-based nanostructures theoretically offer a solution to enable nanoscale photonics, the key demonstration of optical modes with deep sub-diffraction-limited connement and signicant propagation distances has not been experimentally achieved because of the trade-off between optical connement and metallic losses. Here we report the rst experimental demonstration of truly nanoscale guided waves in a metal insulatorsemiconductor device featuring low-loss and broadband operation. Near-eld scanning optical microscopy reveals mode sizes down to 5060 nm2 at visible and near-infrared wavelengths propagating more than 20 times the vacuum wavelength. Interference spectroscopy conrms that the optical mode hybridization between a surface plasmon and a dielectric mode concentrates the hybridized mode inside a nanometre thin gap. This nanoscale waveguide holds promise for next generation on-chip optical communication systems that integrate light sources, modulators or switches, nonlinear and quantum optics.

Experimental demonstration of low-loss optical waveguiding at deep sub-wavelength scales

1 NSF Nanoscale Science and Engineering Center, 3112 Etcheverry Hall, University of California, Berkeley, California 94720, USA. 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA. *These authors contributed equally to this work. Correspondence and requests for materials should be addressed to X.Z. (email: [email protected]).

NATURE COMMUNICATIONS | 2:331 | DOI: 10.1038/ncomms1315 | www.nature.com/naturecommunications

2011 Macmillan Publishers Limited. All rights reserved.

ARTICLE

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1315

Photonics has become the key driver in global data communications. The ever growing demand for higher data bandwidth and lower power consumption of photonic devices1,2 has set a

roadmap for reducing the physical photonic component size down to the nanoscale beyond the diraction limit of light, with integrated functionality25. Although various compact technologies have been developed for reducing the physical size of devices1,4,5, strong opti

cal mode connement oers enhanced light-matter interactions614

towards low driving power and fast modulation speeds1520. Opti

cal connement in conventional photonic components is restricted by the diraction limit of the light resulting in weak light-matter interaction that oen demands long device sizes to achieve an eect. For example, ring resonators4 and FabryPerots20 oen require large cavity lengths of 101,000 m leading to relatively large footprints, thermal instabilities5 and high bending-induced radiation losses on downscaling ring resonators21. Surface plasmon polaritons6, collective oscillations of electrons at metal-dielectric interfaces, were proposed as a potential solution for nanoscale photonics as their wavelength can be scaled down below diraction limit68,22. How

ever, the direct experimental demonstration of low-loss propagation of deep sub-wavelength optical modes has not been realized because of the rapid increase in the optical modes propagation loss on scaling down the optical mode, which pushes the electromagnetic eld into the metal810,2225. As a result, the use of plasmon

ics for integrated photonics, in particular for optical interconnects, remains uncertain.

A hybrid plasmon polariton (HPP) concept has been proposed to overcome this challenge11,26,27. This approach uses a high dielec

tric constant semiconductor strip separated from a metal surface by a nanoscale low dielectric constant gap. As the hybridized plasmon energy is concentrated in the low-loss gap, this novel method oers ultra-small mode connement (2/400) over a broad range of frequencies and simultaneously allows for reduced optical loss compared with a metalsemiconductor interface design11,12. The

physical origin of this quasi-TM highly conned mode stems from the continuity requirement of the vertical component of electric displacement (Dy) at the high-index contrast interfaces between the high-index material (for example, semiconductor), the low-index gap region and plasmonic metal.

ResultsExperimental design to probe sub-wavelength mode. By utilizing semiconductor and metallic structures in an integrated design, in this paper, we report the rst observation of long-propagating optical waveguiding revealing mode sizes signicantly below the diraction limit of light visualized by apertureless near-eld scanning optical microscopy (NSOM; Fig. 1a; Methods and Supplementary Fig. S1a). Moreover, unlike other non-resonant plasmonic designs9,10,2225,

this HPP concept features both ultra strong optical connement over a broad wavelength range ( = 633 nm1.43 m) and relatively low metal losses with propagation distances exceeding 20 times its free-space wavelength at near infrared, making it a promising candidate for nanophotonics2. The measured optical mode sizes down to 5363 nm2 are in excellent agreement with theoretical simulations. To visualize the extremely small scales of the HPP mode distribution, we fabricated HPP-based strips with lateral dimensions of about 200 nm in height, H, with varying width, W (150800 nm). Figure 1b shows a topological atomic force microscopy scan with the shown region being close to the diraction limit of light around the output of such a strip superimposed with the NSOM image acquired simultaneously at an illumination wavelength of 633 nm. It can be seen that the optical connement is indeed as small as about 50 nm (full-width at half-maximum, FWHM) and is situated at the dielectric constant gap region directly conrming the HPP mode.

The HPP strips are excited by illuminating a metal slit at the input (IN) end of the strip (inset of Fig. 1b) from the substrate

side with polarization perpendicular to the slit. As our near-eld imaging technique is most sensitive to electric elds along the axis of the NSOM detection tip, the z-component of the HPP mode is imaged, which is expected to be strongest in the gap region. To gain access to the internal elds, we sliced the strip open (Fig. 1b) allowing the NSOM tip to probe the HPP modes cross-section directly (Methods). When the polarization of illumination is parallel to the metal slit, HPP modes are not excited in the strip and only a background signal was measured by the NSOM (see Supplementary Material).

a

Si tip

Illumination

z

x

y

=45o

w

H

ZnS MgF2

h

Ag

b

MgF2

ZnS

Ag

IN

x

y

OUT

Figure 1 | HPP mode mapping using near-eld scanning optical microscopy (NSOM). (a) Schematic of HPP mode size mapping via apertureless NSOM. The HPP strips consist of a semiconductor strip separated from a metallic surface by a nanometre-scale low dielectric constant gap and are excited by illuminating a metal slit at the input end. (b) Three-dimentional image overlap of the deep sub-wavelength HPP mode signal (red spot) offering optical connement signicantly below the diffraction limit of light. This degree of optical connement indicates the devices potential to create strong light-matter interaction for compact and highly functional photonic components. Scale bar, 125 nm. MgF2 gaph = 10 nm, illumination wavelength = 633 nm. Inset: height prole of tapered strip for free space to HPP strip coupling, scale bar, 1 m. Focused ion beam (FIB) etching was used to dene the illumination port (IN) and the access point for the NSOM probe of the conned optical mode (OUT) (see Methods for fabrication details).

NATURE COMMUNICATIONS | 2:331 | DOI: 10.1038/ncomms1315 | www.nature.com/naturecommunications

2011 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1315

ARTICLE

a

b

1.0

1,000

Mode size (nm)

Signal, |E|2

Signal, |E|2 Signal, |E |2

0.5

100

0.0 200 100 0 100 200

10 100 1,000

z-Direction (nm)

Strip width (nm)

c

VIS NIR IR

1.0

0.0

0.5

0.0

0 100 200 300

0 50 100 200

150 250 0 100 200 300 400

Mode height (nm)

Mode width (nm)

d

1.0

0.5

0 100 200 300 0 200 400 600 800 0 200 400 600

Figure 2 | HPP mode connement. (a) Experimental eld intensity (black dots) of the HPP mode compared with different tip-sample separations, t, from its exiting point. The optical mode height of FWHMz = 53 nm is deep sub-wavelength for = 633 nm, HPP strip dimensions H = 230 nm, W = 174 nm. Note, the NSOM operates in tapping-mode having an average separation of t = 10 nm (black line), and the simulated mode prole (t = 2 and 5 nm, red dotted and black dashed line, respectively) at this distance agrees well with the NSOM result. (b) Summary of experimental HPP mode dimensions yielding the smallest measured mode area of 5363 nm2, at = 633 nm. The good agreement between experimental data (mode width and height, black squares and red triangles, respectively) and theoretical simulations (dashed lines, colours, respectively) conrms that the optical HPP mode is indeed squeezed into the low dielectric constant gap: the mode height is independent on the strip width (triangles), whereas the mode width is scaling with the strip width (squares). (c, d) Line scans of the mode height and width for wavelengths of the illumination beam of 633, 808 and 1,427 nm featuring broadband, deep sub-wavelength operation of HPP-based devices. The FWHM (solid line) are Gaussian ts to binned data, yielding measured mode areas of 2/120, 2/59 and 2/157, which are deep sub-wavelength modes for all three wavelengths. As the strip height, H, is optimized for 633 nm, a slightly larger mode height for longer excitation wavelengths is expected.

Broadband HPP mode mapping for height and width. We have compared line-scans of the NSOM eld image (broken white lines in Fig. 1b) with numerical simulations. Taking into account the HPP modes eld diraction along the z-direction, we were able to correlate experiment and theory well at tip-sample separations of about 10 nm (Fig. 2a). A similar correlation is found for the mode width data along the y-direction. The optical mode connement in the y-direction is controllable by the lateral HPP strip width, W, with the smallest measured mode area, AHPP = 5363 nm2, proving

a non-resonant, deep sub-wavelength ( < /10, = 633 nm) nanoscale mode. Figure 2b summarizes this property by comparing the FWHM of the HPP mode for varying HPP strip widths. Whereas the mode height in the z-direction remains essentially constant, the mode width follows the strip width. Whereas deep sub-wavelength mode sizes are expected in the z-direction because of the involvement of surface plasmons, lateral connement can also be strongly sub-wavelength. This arises from the accumulation of polarization

surface charges concentrated around the centre (y-direction) of the HPP strip11 (Supplementary Fig. S2). We have chosen to use FWHM as a measure of the mode size, as it is representative of where the majority of a modes power resides. This is particularly relevant to active applications of plasmonics, where interest resolves around achieving high peak eld intensities within waveguides. We note that this measure can underestimate the mode size in some plasmonic waveguide systems that involve sharp metal corners or large changes in permittivity12. However, in our experiments, diraction of the conned mode into free space and the point spread function of the NSOM tip smooth out the modes sharp features making the FWHM a fair representation of where the majority of the modes power resides. Furthermore, we nd the hybrid plasmon modes connement to be relatively insensitive to the illumination wavelength11. We demonstrate this broadband feature by illuminating the HPP strips with visible and near-infrared light ( = 633, 808 and 1,427 nm) and nd that the vertical mode connement remains

NATURE COMMUNICATIONS | 2:331 | DOI: 10.1038/ncomms1315 | www.nature.com/naturecommunications

2011 Macmillan Publishers Limited. All rights reserved.

ARTICLE

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1315

essentially constant at about 5060 nm (Fig. 2c). Since the chosen HPP strip height, H, is purposely kept constant through all experiments, a subtle mode height increase is expected for changing the illumination wavelength from visible to near infrared. The connement of the mode width is also deep sub-wavelength with /12 and /7 for visible and near-infrared wavelengths, respectively (Fig. 2d). These remarkably small mode sizes are among the smallest for propagating electromagnetic waves demonstrated to date and consequently facilitate reduced waveguide cross-talk leading towards high data bandwidth densities (Supplementary Fig. S3).

Propagation length. In addition to providing strong optical connement, another advantage of the HPP mode, over other non-resonant plasmonic schemes9,10,2225, is reduced optical loss by allo

cating most of the optical energy in the dielectric gap instead of inside the metal. We have measured the transmission through HPP strips of varying length under white light illumination (Fig. 3a). The sub-wavelength-conned HPP mode propagates more than ten times its vacuum wavelength at visible wavelengths near 633 nm, which is more than six times further than plasmonic control strips consisting of a metal-semiconductor interface without the low-index gap region (Fig. 3b). The propagation length increases with the wavelength, and exceeds 20 times its vacuum wavelength at near infrared ( 0.3 dB m 1; inset Fig. 3), thus allowing for sufficiently long interaction lengths, which, combined with strong optical connement, can create strong light-matter interaction eects for active photonic components1517.

Mode group index interference measurements. To gain full insight into the HPP mode physics, we also investigated the mode speed, namely the group index, ng. The abrupt change in the eective refractive index at the end of the HPP strip acts as a partial reector for the HPP modes. The resulting optical cavity displays FabryPerot interference fringes, corresponding to longitudinal cavity modes. The group index can be determined from the spectral mode spacing, = (2/2ngL), where L is the strip length, the wavelength, ng=n dn/d the group index and n is the eective mode index. The increasing group index with photon energy conrms the dispersive, plasmonic mode character of the HPP mode, which manifests itself in the deep sub-wavelength connement (Fig. 3c). In contrast, an eective mode group index close to one of the other surface plasmon polariton-based designs indicates that most of the eld sits outside the metal, thus showing only weak optical connement23. The dispersion of the HPP relation lies between the two extreme cases of gap width, h: namely, a semiconductor strip (h) and a semiconductor strip in direct contact with the silver lm (h0). This demonstrates controlled hybridization of the modes of a semiconductor strip and a metaldielectric interface.

Discussion

Plasmonic waveguides can be deployed in two distinct application areas: routing information passively811,2225 and actively alter

ing optical signals1517. For the latter, the optical connement can

strengthen light-matter interactions, thereby reducing the required devices size to utilize a certain eect (for example, a 2nd, 3rd order nonlinearity). The trade-o lies in enhancing such eect, that is, mode connement, and incurring loss over the designed device length providing the eect. With this in mind, deep sub-wavelength metal optics hold promise to produce a signicant nonlinear enhancement, while maintaining sufficient propagation distances leading to unprecedented footprint-performance functions.

In conclusion, we directly demonstrated waveguiding of ultra-small propagating waves11 at visible and near-infrared frequencies using NSOM imaging. We conrm the appropriate optical eld hybridization between a surface plasmon and a dielectric mode by FabryPerot interference spectroscopy and nd a dispersive group

index as a result. This HPP concept has reduced ohmic losses oering reasonably long light-matter interaction lengths, potentially enabling compact and efficient nanoscale photonic components, as it elegantly interfaces plasmonics with semiconductors. This novel

a

IN white Light

OUT

3 m 4 m 6 m 10 m

Strip

Propagation length (m)

b

20

15

10

5

Intensity (a.u.)

1

0

400 600 800

Operating wavelength (nm)

0.1 3 6 9 12

Strip length (m)

c

5

4

3

Group index

2

1

0 1.8 2 2.2 2.4 2.6

Energy eV

Figure 3 | HPP mode propagation length and mode speed. (a) Far eld images of illuminated HPP strips showing incident white light reected from the strip input and the corresponding emission from the distal strip output facet. The strip lengths, L, are indicated on top of each image and dashed lines guide the eye to the output signal. (b) The HPP mode offers propagation lengths of more than ten times its free-space wavelengthat = 633 nm (blue squares). Without the dielectric constant gap-layer (plasmonic control) the propagation length is only about 1 m (black dashed line). Inset: the HPP modes propagation length increases with its operating wavelength and exceeds 20 times its vacuum wavelength (0.3 dB m 1) at near-infrared wavelength ( = 808 nm). The data (blue squares) are in good agreement with the expected trend from numerical calculations (blue dashed line; see Methods). (c) HPP mode group index versus photon energy. Analysis of the HPP strips transmission spectra, that is, FabryPerot interference for a range of strip lengths yields a dispersive group index of the HPP mode falling between the two extreme cases, that is, photonic (without metal, red dotted line) and plasmonic control (black dashed line) conrming the hybrid nature of the HPP mode.

NATURE COMMUNICATIONS | 2:331 | DOI: 10.1038/ncomms1315 | www.nature.com/naturecommunications

2011 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1315

ARTICLE

mode design holds a great potential for truly nanoscale photonic applications, such as intra-chip optical communication19, signal modulation1517, nanoscale lasers13 and biosensing28.

Methods

Sample fabrication and experimental details. Lithographically dened hybrid plasmon mode strips are prepared by evaporating a high-index semiconductor, ZnS, n = 2.2 onto an Ag lm separated by a thin, h = 10 nm, MgF2 lm, both on a quartz substrate followed by li o in acetone. The root-mean-square of the Ag lm roughness is about 1.01.5 nm, measured using atomic force microscopy.

The strip input was chosen to be tapered to act as an optical funnel to increase the signal strength and to reduce the impedance mismatch between free space and the HPP mode (inset Fig. 1b). Focused ion beam milling creates the input and the 45 angled output port to enable the scanning NSOM tip to access the HPP modes eld prole. HPP strip illumination for the NSOM experiments is achieved by focusing a laser beam from the sample backside (quartz) with the following wavelengths: a HeNe laser at = 633 nm and two solid-state laser diodes at 808 and 1,427 nm wavelength, respectively (Fig. 1a). For the mode decay length and speed measurements a white light (Xe lamp, 150 W) illuminates the HPP strip in a reection mode setup (Supplementary Fig. S1b). The signal to noise ratio was increased by placing an aperture in the secondary plane of the image, that is, over the endof the strip. The transmitted signal is then collected by an objective lens (100, numerical aperture = 0.9), sent to a spectrometer and recorded by an N2-cooled

CCD camera. All measurements were carried out at room temperature. The modes decay length is measured by the change in the absolute intensity with the variation of the strip length. The experimental results from Figure 3b are in good agreement with that obtained by simulation. Data from ref. 29 were used for the wavelength-dependent propagation length calculation showing good agreement with the experimental results (Fig. 3b). In an earlier work, we found that the permittivity values from Johnson and Christy29 were overestimations by about 40% (ref. 30), which, together with additional scattering losses, could explain the small dierence between experiment and simulation results. For the group index extraction from the measurement data, the Drude-model was used with the following parameters; the refractive indexes for air, zinc sulphide and magnesium oride were 1, 2.2 and 1.4, respectively; for silver we used Ag ( = 600 nm) = 16.100.44i (ref. 29).

References

1. Welch, D. F. etal. The realization of large-scale photonic integrated circuits and the associated impact on ber-optic communication systems. J.LightwaveTech. 24, 46744683 (2006).

2. Kirchain, R. & Kimerling, L. A roadmap for nanophotonics. Nat.Phot. 1,

303305 (2007).

3. Brongersma, M. L. & Shalaev, V. M. The case for plasmonics. Science 328,

440441 (2010).

4. Liao, L. etal. High speed silicon Mach-Zehnder modulator. Opt.Express 13, 31293135 (2005).

5. Guha, B., Kyotoku, B. B. C. & Lipson, M. Minimizing temperature sensitivity of silicon Mach-Zehnder interferometer. Opt.Express 18, 18791887 (2010).

6. Maier, S. A. Plasmonics,FundamentalsandApplications (Springer, 2007).7. Pyayt, A. L., Wiley, B., Xia, Y., Chen, A. & Dalton, L. Integration of photonic and silver nanowire plasmonic waveguides. Nat.Nanotech. 3, 660665 (2008).

8. Gramotnev, D. K. & Bozhevolnyi, S. I. Plasmonics beyond the diraction limit. Nat.Phot. 4, 8391 (2010).

9. Maier, S. A., Kik, P. G. & Atwater, H. A. Observation of coupled plasmonpolariton modes in Au nanoparticle chain waveguides of dierent lengths: estimation of waveguide loss. Appl.Phys.Lett. 81, 17141716 (2002).

10. Krasavin, A. V. & Zayats, A. V. Silicon-based plasmonic waveguides. Opt. Express 18, 1179111799 (2010).

11. Oulton, R. F., Sorger, V. J., Genov, D. A., Pile, D. F. P. & Zhang, X. A hybrid plasmonic waveguide for subwavelength connement and long-range propagation. Nat.Phot. 2, 495500 (2008).

12. Oulton, R. F., Bartal, G., Pile, D. F. P. & Zhang, X. Connement and propagation characteristics of subwavelength plasmonic modes. J.NewPhys. 10, 105018 (2008).

13. Oulton, R. F. etal. Plasmon lasers at deep subwavelength scale. Nature 461, 629632 (2009).

14. Hill, M. T. etal. Lasing in metallic-coated nanocavities. Nat.Phot. 1, 589594 (2007).

15. Cai, W., White, J. S. & Brongersma, M. L. Compact, high-speed and power-efficient electrooptic plasmonic modulators. NanoLett. 9, 44034411 (2009).

16. Dionne, J. A., Diest, K., Sweatlock, L. A. & Atwater, H. A. PlasMOStor: a metal oxideSi eld eect plasmonic modulator. NanoLett. 9, 897902 (2009).

17. Pacici, D., Lezec, H. J. & Atwater, H. A. All-optical modulation by plasmonic excitation of CdSe quantum dots. Nat.Phot. 1, 402406 (2007).

18. Togan, E. etal. Quantum entanglement between an optical photon and a solid-state spin qubit. Nature 466, 730735 (2010).

19. Miller, D. A. B. Optics for low-energy communication inside digital processors: quantum detectors, sources, and modulators as efficient impedance converters. Opt.Lett. 14, 146148 (1989).

20. Green, W. M. J., Rooks, M. J., Sekaricand, L. & Vlasov, Y. A. Ultra-compact, low RF power, 10 Gb/s silicon Mach-Zehnder modulator. Opt.Express 15, 1710617113 (2007).

21. Shainline, J., Fernandes, G., Liu, Z. & Xu, J. Subwavelength silicon microdisks with high quality factor. FWZ4,FrontiersinOptics (San Jose, California, USA, 2009).

22. Verhagen, E., Spasenovic, M., Polman, A. & Kuipers, L. Nanowire plasmon excitation by adiabatic mode transformation. Phys.Rev.Lett. 102, 203904 (2009).

23. Bozhevolnyi, S. I., Volkov, V. S., Devaux, E. & Ebbesen, T. W. Channel plasmonpolariton guiding by subwavelength metal grooves. Phys.Rev.Lett. 95, 046802 (2005).

24. Dionne, J. A., Lezec, H. J. & Atwater, H. A. Highly conned photon transport in subwavelength metallic slot waveguides. NanoLett. 6, 19281932 (2006).

25. Pile, D. F. P. etal. Theoretical and experimental investigation of strongly localized plasmons on triangular metal wedges for subwavelength waveguiding. App.Phys.Lett. 87, 061106 (2005).

26. Zhang, X.- Y. etal. Numerical analysis of deep sub-wavelength integrated plasmonic devices based on Semiconductor-Insulator-Metal strip waveguides. Opt.Express 18, 1894518959 (2010).

27. Chu, H.- S., Li, E.- P. & Hedge, R. Optical performance of single-mode hybrid dielectric-loaded plasmonic waveguide-based components. Appl.Phys.Lett. 96, 221103 (2010).

28. Anker, J. N., Hall, W. P., Lyandres, O., Shah, N. C., Zhao, J. & Van Duyne, R. P. Biosensing with plasmonic nanosensors. Nat.Mater. 7, 442453 (2008).

29. Johnson, P. B. & Christy, R. W. Optical constants of the noble metals. Phys.Rev. B 6, 43704379 (1972).

30. Sorger, V. J., Oulton, R. F., Yao, J., Bartal, G. & Xiang, X. Plasmonic fabry-perot nanocavity. NanoLett. 9, 34893493 (2009).

Acknowledgments

We acknowledge nancial support from the National Science Foundation Nano-Scale Science and Engineering Center (NSF-NSEC) under the award CMMI-0751621.

Author contributions

V.J.S. and Z.Y. developed the experimental approach. V.J.S. fabricated the devices, performed optical far eld measurements, device metrology and data analysis. Z.Y. and Y.W. conducted NSOM experiments and data analysis. R.F.O. developed theoretical modal and conducted simulations. X.Z., G.B. and X.Y. provided guidance on theory and measurements. All authors discussed the results and contributed to the manuscript.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/ naturecommunications

Competing nancial interests: The authors declare no competing nancial interests.

Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/

How to cite this article: Sorger, V. J. etal. Experimental demonstration of low-loss optical waveguiding at deep sub-wavelength scales. Nat.Commun. 2:331 doi: 10.1038/ncomms1315 (2011).

NATURE COMMUNICATIONS | 2:331 | DOI: 10.1038/ncomms1315 | www.nature.com/naturecommunications

2011 Macmillan Publishers Limited. All rights reserved.