Observation of Enhanced Beaming from Photonic Crystal Waveguides Steven K. Morrison and Yuri S. Kivshar Nonlinear Physics Centre and Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), Australian National University, Canberra ACT 0200, Australia We report on the experimental observation of the beaming effect in photonic crystals enhanced via surface modes. We experimentally map the spatial field distribution of energy emitted from a subwavelength photonic crystal waveguide into free-space, rendering with crisp clarity the diffrac- tionless beaming of energy. Our experimental data agree well with our numerical studies of the beamingenhancementinphotoniccrystalswith modulated surfaces. Withoutloss of generality,we studythebeamingeffectinaphotoniccrystalscaledtomicrowavefrequenciesanddemonstratethe 8 technological capacity to deliver long-range, wavelength-scaled beaming of energy. 0 0 PACSnumbers: 42.70.Qs,42.60.Jf,42.25.Fx,78.68.tm 2 n a Free-space propagation of light is inherently linked to tant factors required for beaming can be obtained [4, 6]. J diffraction; especially so when the beam emerges from a In this Letter we verify experimentally earlier theo- 1 subwavelength aperture [1]. A consequence of tight con- retically predictions and demonstrate a substantially en- 1 finement of electromagnetic waves is the large angular hanced beaming of light from a photonic crystal waveg- diffraction experienced by the waves abruptly released uide of subwavelength width. We highlight the meth- ] s from the strong confinement. This substantial diffrac- odsusedtoimprovecouplingbetweenthephotoniccrys- c tion produced by subwavelength apertures is contradic- tal waveguide mode and surface modes, and illustrate i t tory to the formation of a highly collimated beam. This how this coupling is compatible with setting the correct p o contradiction can, however, be brought to accord and a phase delay and emission direction of the radiative sur- . narrowbeamfashionedthroughthe coherentcouplingof face modes to achieve highly collimated beaming. Fur- s c additional slowly radiating surface waves on the portal thermore, we demonstrate the substantial distance over i surface surrounding the diminutive aperture. The ra- which the beam remains diffractionless and reveal the s y diating surface waves behave as Huygens-styled sources Bessel-like beam profile supporting this extensive beam- h that combine and interfere constructively and destruc- ing range. p tively with all the emissions from the aperture to form Wecommenceourinvestigationintothebeamingeffect [ a narrowly confined beam that exhibits diffraction free propagationover substantial distances [2]. 1 v Beaming of light from a subwavelength aperture was 2 first experimentally demonstrated in metallic thin films 1 flanked by periodic corrugations, with surface plasmons 7 employed to transform the emissions into a confined 1 beamoflight[3]. Throughinsightfulcomparisons,beam- . 1 ing from subwavelength waveguides in photonic crystals 0 was theoretically predicted [4] and independently ob- 8 servedinexperiment[5]. Furtherresearchwasconducted 0 into the enhancement of the beaming effect from pho- : v tonic crystal waveguides through the engineering of the Xi modulated surfaces of photonic crystals [6, 7]. Important factors in the attainment of free-space r FIG. 1: (a) Square lattice, two-dimensional photonic crystal a beaming from photonic crystal waveguides are the ef- beaming model, illustrating the crystal’s geometry, includ- ficient coupling from waveguide mode to the surface ingasingle-rowdefectwaveguideandcorrugatedmono-defect modes and the slow coupling and steering of radiated surface layer in which surface cylinders have a radius of half surface modes, such that their collective spatial phase the bulk photonic crystal’s cylinder radius. (b) Dispersion and amplitudes superimpose and interfere destructively relationship of the photonic crystal depicted in (a) along the everywhere but directly in front of the waveguide aper- direction of the surface where even number cylinders, num- bered symmetrically and sequentially away from the waveg- ture [4]. Monatomic photonic crystals support surface uide are moved: (dashed line) towards the photonic crystal; modes through an introduction of a defect structure to (dotted line) positioned in line with odd number cylinders, the surface layer. These surface modes occur below the removingthesurfacecorrugation; and,(solid line)awayfrom light line (the free-space continuum of states) and decay the photonic crystal. The dark upper triangle denotes the evanescently away from the surface, and like the waveg- free-space continuum of states above the light line, whereas uide mode, occur within the photonic crystal’s bandgap. theothershadedregionsdepicttheprojectedBlochmodesof If the surface defect structure of a photonic crystal is thebulk photonic crystal. also periodically modulated or corrugated, these impor- 2 by modeling an archetype photonic crystal using rigor- ous numerical methods. The model we employ is a pho- tonic crystal with a square lattice populated with high dielectriccylinders,withdielectricconstantofǫr =11.54 (equaltothedielectricconstantofaluminaat∼10GHz, as used in the experiment confirmation) and of nor- malized radii r = 0.18a, where a is the lattice period. Photonic crystals of this configuration produce in-plane band-gapsinthenormalizedfrequencybandofω =0.3to 0.42×2πc/awhentheincidentfieldispolarizedwiththe electricfieldparalleltotheaxisofthecylinders. Through the removal of a single row of cylinders perpendicular to the surface, a single mode waveguideis created; whereas thedefectsurfacestructureusedtoinduceasurfacemode FIG.2: Finite-differencetime-domainsimulationsofthefield is composed of cylinders with a radius reduced by half. intensityfromthebeamingmodelingwith(a)illustratingthe fullanglediffusedoutputfromthesub-wavelengthwaveguide To co-join the surface modes and the directly transmit- exitwhenthesurfacestructureissimplyterminatedinarow tedfield,asurfacecorrugationisformedbydisplacingal- ofcylindersequalinradiustothoseofthebulkphotoniccrys- ternative surface defect cylinders away from their lattice tal, and (b) illustrating the highly collimated beam emitted sites onthe portaldefect surface ofthe photonic crystal. from thewaveguidehavingbeen strongly fashioned byradia- Theimportantgeometricfeaturesofthephotoniccrystal tivesurface states. model are illustrated in Fig. 1(a). To reveal the critical properties of the photonic crys- tal model, we calculate the dispersion relationships of thequasi-periodic,corrugatedsurfacestructuresusingan sion analysis, we control the spatial phase positioning ab initio finite-difference time-domain (FDTD) method. of the radiative surface modes by altering the corruga- The method represents the fields in complex notation tion displacement in accordance with the results given and varies the phase relationship exp(ikr) at the peri- by Fig. 1(b). This corrugationdisplacement is, however, odicboundariesandacrosstheone-dimensionalBrillouin tempered and fine-tuned by the radius of the surface de- zone. The width of the super-cell used is 2a, such that fect cylinders that, in concert with the corrugation dis- it includes the double-periodicity of the surface corruga- placement, also contribute to the scattering direction of tion. Toremedybandfoldingthatoccursduetothedou- the leaky surface mode, and ipso facto, the operational bled super-cell width, sources are introduced into every frequencyandgroupvelocitiesofthesurfacemodes. Op- unit cell of the super-cell with phase values determined timal displacement of the surface corrugation cylinders byBlochtheorem. Usingthismethod,wedeterminethat for this photonic crystal is 0.4a away from their lattice a red shift occurs in the surface mode frequency for a sites. Figure2demonstratesthestrongcontrastbetween positive displacement(awayfromthe crystal)ofthe cor- (a) the simple terminated photonic crystal with full-size rugation forming cylinders, and conversely,a minor blue surfacecylindersand(b)theoptimalbeaming,occurring frequency shift occurs for negatively displaced corruga- at ω = 0.378×2πc/a, with half-diameter surface cylin- tion forming cylinders, as shown in Fig. 1(b). ders, alternatively displaced positively by 0.4a, starting Next, we study the important issue of coupling to fromthewaveguideapertureandwithaperiodicityof2a. the surface modes. Again, using the FDTD method we From these results, we determine that 82% of the trans- measure the power coupled into the surface mode, un- mitted field is conveyed into the narrow directed emis- der steady state and numerically converged conditions, sion that in comparison to the un-altered surface yields as we alter the position of the surface cylinders closer a258%increaseinpowerwithinthe areaofthecentrally to the waveguide. Movement of these surface cylinders directed beam. produces maximum coupling for a forward displacement Experimental fabrication of this photonic crystal for (away from the crystal). This forward displacement cre- operation at optical wavelengths is very challenging, re- ates both a cavity-like mode that captures a large quan- quiringsignificantfabricationresources. Inorderto con- tityofthefield,aswellasdeliveringastrongre-scattering firmenhancedbeaming, yetavoidthese fabricationcom- ofthesurfacemodefromthesubsequentsurfacecylinders plexities, we scale the crystal to microwave wavelengths in a direction close to the waveguide aperture, enabling centeredaround10GHz,wherethelatticeperiodis11mm strong initial shaping of the transmitted beam. and use alumina cylinders with a refractive index of To achieve optimal beaming we note the need to ∼ 3.4. Figure 3 shows a schematic of the experimental achieve both strong coupling to the surface modes along setupwithbroadbandsourceanddetectionfacilitatedby with directional control and phase alignment of the re- a 20GHz vector network analyzer (Rohde and Schwartz scattered surface modes. From the numerical analysis ZVB20) interfaced to half-wavelength dipole antennas. of the serial coupling to the surface modes, we imple- To achieve a low impedance transition from the unbal- ment a positive surface corrugation. From the disper- anced coaxial cable connected from the vector network 3 the results at 10.4GHz with a corrugation displacement of 4.44mm. Owing to impedance-mismatch return-losses between the 50Ω vector network analyzer, antennas and Blochimpedanceofthephotoniccrystal’swaveguide,the fieldisplottedinlinear,normalizedarbitraryunitsasin- dicated by the color scale. Clearly shown in Fig. 5 is the crisp clarity of the mapped intensity field, revealing the long distance over which anomalously low divergence of the transmitted beam occurs, as well as demonstrating a strong corre- lationtothe numericalresult. AlsoclearlyvisibleinFig. 5 are the side lobes resulting from the grating effect of the surface corrugation, thus clearly confirming the the- ory that leaky surface modes shape the outgoing emis- sion and transform the normally diffused output into an anomalously low diverging beam. Through mapping of the field without corrugation, we observer and affirm a strong, rapidly diffracting emission, absence of beaming. It is worth commenting that the long range diminishing oftheexperimentaldirectedemissionistoalargedegree aresultofthediffractionofthebeaminthenon-confined, out-of-planedirectionresultingfromthetwo-dimensional nature of the photonic crystal. Furthermore, the finite height of the experimental crystal ( 5λ) that is bounded FIG. 3: Schematic representation of the pertinent elements bylowdielectricconstantPerspexplatescausesspurious, andexperimentalconfigurationusedtomapthefieldintensity destructiveinterferenceofthe beamatapproximately20 of the beaming effect. wavelengths in front of the crystal’s surface. Cross-sectional slices of the normalized field intensity forboththenumericalandexperimentalresultsaretaken analyzer (the conductors of the coax do not connect to at a distance of 17λ0 in front of the portalsurface of the photoniccrystalsandarepresentedinFig. 6,withlinear the antenna in the same way and are thus unbalanced) normalization taken with reference to the central inten- to the symmetrical, balanced dipole antenna, a minia- sity peak of the numerical results. Also clearly seen in ture sleeve balun with a 1 : 1 impedance transform was Fig6. istheBessel-likebeamprofile,knowntopropagate used. The source dipole antenna inserted into the crys- tal’swaveguideexcitesawaveguidemodewithanelectric field parallel to both the antenna and cylinder axis after approximately four periods along the waveguide,as con- firmed by FDTD simulations. Electromagnetic energy not coupled into the waveguide mode exits the rear of the waveguide. We mount the detector antenna to a tri- pod in front of the photonic crystal. Finally, to map the transmitted field, we mount the photonic crystals to an X-Y translation stage and use a phase maintaining ca- ble between the source dipole antenna and the network analyzer, thus removing any phase noise occurring due to the disturbance ofthe cablingasthe translationstage moves while mapping the transmitted field. Figure 4 shows the surface structure of the crystal formed with 1mm radius cylinders with a corrugation period of 2a. The surface corrugationdisplacement, ∆z, is slide adjustable as illustrated in the inset of Fig. 4. This adjustable corrugation displacement allows adjust- ment and fine-tuning of the beaming effect to overcome FIG.4: Photographoftheexperimentalphotoniccrystaland experimental discrepancies in, for example, the dielec- portal surface. (Inset) Close-up of the photonic crystal’s de- tric constant of the alumina cylinders. We sample the fect portal surface, highlighting the surface corrugation that, field on a 5mm grid, equating to approximately 6 points due to momentum conservation of the surface states, allows perwavelength,andtheninterpolatethemeasuredresult thesurface modes to radiate into free space. using a two-dimensional cubic spline; Fig. 5 illustrates 4 FIG. 5: Experimental map of the intensity field from the photonic crystal waveguide demonstrating the beaming effect at 10.4GHzinaplanecentraltotheheightofthecrystal’scylindersandinfrontofthecrystal’sportalsurface(spatialdimensions are given in free space wavelengths equivalent to 28.82mm). without diffractionoversubstantialdistances,formedby the leaky surface modes. In conclusion, we have studied, theoretically and ex- perimentally,theconditionsfordramaticenhancementof the free-space beaming of electromagnetic energy from a subwavelengthphotoniccrystalwaveguide. These condi- tions, that have a tendency to contend with each other, can be brought to accord such that optimal superposi- tion of all emissions, both direct and re-scattered from the surface of the crystal, produce a highly collimated beamofwavelengthdimensions. Wehaveclearlydemon- strated,withastrongcorrelationbetweenourtheoretical andexperimentalresults,theroleoftheradiativesurface modes in the fashioning of the directed emission with a Bessel-likebeamprofilecapableofpropagationoversub- stantial distance without divergence. Our results hold FIG.6: Experimentallyandnumericallydeterminedcrosssec- tions of the normalized field intensity at a distance of 17 λ0 great technological promise and perspectives for tailor- ing the beam profiles from subwavelength apertures in in front of the portal surface of the photonic crystal, demon- stratingbothvirtuallyflatfielddistributionintheabsenceof microwave,millimeter wave and optical systems. radiating surface modes and highly collimated transmission, This work has been supported by the Australian Re- with Bessel-like profile, resulting from thebeaming effect. searchCouncil through the Discovery and Center of Ex- cellenceprojects. WethankDr. IlyaShadrivovforuseful discussions and help with experiments. [1] H.A. Bethe, Phys.Rev. 66, 163 (1944). Muller, R.B.Wehrspohn,U.Gosele, andV.Sandoghdar, [2] D. Monroe, Phys. Rev.Focus. 13, 11 (2004). Phys. Rev.Lett. 92, 113903 (2004). [3] T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. 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