DTIC ADA517105: Simulation and Micro-Fabrication of Optically Switchable Split Ring Resonators

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PhotonicsandNanostructures–FundamentalsandApplications5(2007)106–112
www.elsevier.com/locate/photonics
Simulation and micro-fabrication of optically switchable
split ring resonators
T.F. Gundogdua,*, Mutlu Go¨kkavasb, Kaan Gu¨venb, M. Kafesakia,
C.M. Soukoulisa,c, Ekmel Ozbayb
aInstituteofElectronicStructureandLaser,FoundationforResearchandTechnology,HellasandDepartmentof
MaterialsScienceandTechnology,UniversityofCrete,Greece
bNanotechnologyResearchCenter,DepartmentofPhysics,DepartmentofElectricalandElectronicsEngineering,
BilkentUniversity,Bilkent,Ankara06800,Turkey
cAmesLaboratory-USDOEandDepartmentofPhysicsandAstronomy,IowaStateUniversity,Ames,IA50011,USA
Received15February2007;receivedinrevisedform3July2007;accepted4July2007
Availableonline12July2007
Abstract
Theeffectofconductivityvariationasaproposedmethodfortheinvestigationofphotoconductiveswitchingpropertiesofsplit
ringresonators(SRRs)issimulated.Threedifferentsystemsthatareapplicableundercertainfabricationand/oropticalexcitation
conditionsaredescribed.Thesimulatedtransmissionspectrumindicatesthatforalargerangeofdarkconductivityvalues,complete
switching of the SRR resonance is possible. One of the simulated systems, involving split ring resonators on Si substrate, was
fabricatedandcharacterized.Thetransmissionspectrumofthatsystemwasmeasured,withtheSiinitshigh-resistivitystate,anda
(cid:2)60dBdip between108and 115GHz, dueto SRRsmagnetic resonance, wasobserved.
#2007PublishedbyElsevier B.V.
Keywords: Lefthandedmaterials;Splitringresonators;Photoconductive;Simulation
Veselago developed the idea of left-handed (LH) theinitialstructureconsistingoftheperiodicarrangement
materialsin1968[1].Thesematerialsexhibittheunique of split ring-resonators (SRRs) and metallic wires
physical property of having both negative electrical remains the mostly studied. The wires provide the
permittivity(e)andnegativemagneticpermeability(m) negativeelectricpermittivity,(e),[7]whereastheSRR-
overacommonfrequencyband.Lefthandedmetamater- elements generate the negative magnetic permeability
ialsgeneratedagreatinterest,partlyduetotheproperties (m)response[2].TheSRRconsistsofmetallicringswith
of these materials which are not observed in naturally gapsthatactlikeacapacitor–inductor(LC)circuitwhena
occurring materials. These include negative refractive magneticfieldcomponentisperpendiculartotheirplane.
index,artificialmagnetism,superfocusing,andreduced Due to the resonant circular currents in the rings they
lensaberrations[1–5].SincethefirstdemonstrationofLH exhibit a resonance at v =1/(LC)1/2 associated with
m
metamaterials [4], different structures that exhibit resonantcircularcurrentsintheSRRrings[8–10].Itwas
equivalent properties have been proposed [6]; however alsodemonstratedthatbesidesforamagneticfieldthatis
perpendicular to the SRR plane, the existence of an
electricfieldparalleltothegapsoftheSRRcancoupleto
themagneticresonanceandexciteanoscillatingresonant
* Correspondingauthor.
currentaroundtherings.Thisphenomenonisdescribedas
E-mailaddress:[email protected](T.F.Gundogdu).
1569-4410/$–seefrontmatter#2007PublishedbyElsevierB.V.
doi:10.1016/j.photonics.2007.07.001
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Simulation and micro-fabrication of optically switchable split ring
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T.F.Gundogduetal./PhotonicsandNanostructures–FundamentalsandApplications5(2007)106–112 107
the electric excitation ofthemagneticresonanceofthe Padilla et al. reported the all-optical switching of the
SRR[11–13]. electromagnetic response of SRRs at THz frequencies
A significant challenge in LH materials research is by the photoexcitation of carriers in the sample
the tuning and switching of such materials. Recently, substrate [14]. Previously, we had demonstrated a LH
Fig.1. Schematicillustratingthethreemodelsusedinthesimulations.
108 T.F.Gundogduetal./PhotonicsandNanostructures–FundamentalsandApplications5(2007)106–112
Table1
SimulationparametersforthethreemodelsinFig.1
Model1 Model2 Model3
Substratematerial Si(e=11.9) Glass(e=4.82) Glass(e=4.82)
Substratethickness 150mm 150mm 150mm
Photoconductivelayer Substrate 1mm-thickZnO(e=8.5) 1mm-thickZnO(e=8.5)acrossgaps
PeriodicityinSRRplanea =a 164.2mm 262.7mm 262.7mm
k E
Metalthickness 0.5mm 0.5mm 0.5mm
Splitringgap 4.5mm 7.2mm 7.2mm
Innerringinnerradius 26.9mm 43mm 43mm
Innerringouterradius 42mm 67.2mm 67.2mm
Outerringinnerradius 50.4mm 80.7mm 80.7mm
Outerringouterradius 67.2mm 107.5mm 107.5mm
composite metamaterial operating at 100GHz [15]. In situations where the photogeneration of carriers is
thepresentpaper,wereportourworkconductedonthe localized at the gap region by use of a focused laser
design, simulation and micro-fabrication of optically beam,evenifthephotoconductivelayerorsubstrateis
switchableSRRsaround100GHzoperationfrequency. notpatterned. Models 2 and 3are both appropriatefor
For simulations a commercially available software describing front- and substrate-side optical illumina-
(Microwave Studio) employing the finite integration tion, whereas model 1 is suited only for front-side
method to solve Maxwell’s equations was used [16]. illumination because of the thick photoconducting
Simulationswereperformedforthreedifferentphysical substrate.
configurations (herein after referred to as models). The exact dimensions used in the simulations are
ThesethreemodelsareillustratedinFig.1a–c.Theleft listedinTable1.Formodels2and3,thedimensionsare
panelsshowninFig.1illustratethegeneralviewofone identicaltothestructurein[15].Thisstructureisknown
metamaterial unit cell, whereas the right panels show to yield a magnetic resonance dip (m<0) between 95
the gap region in closer detail. In the first model and 108GHz when fabricated on glass substrates
(Fig. 1a), the effect of a photoconductive substrate is (e=4.82). For model 1, the structure is rescaled such
investigatedbywayofincreasingtheconductivity(s)of thattheresonancewilloccuraround90–100GHzwhen
the substrate region to symbolize illumination with fabricatedonSisubstrates(e=11.9).Inmodels2and3,
higheropticalpowers.Thefabricationsequenceforthis the thin photoconductive layer is modeled by a 1mm-
model is identical to the one described in our earlier thick ZnO (e=8.5) film. For all models, the substrate
work[15],usingphotoconductivesubstratesinsteadof thickness is 150mm, and the 0.5mm-thick conductors
glass. In comparison, model 2 (Fig. 1b) employs
insulating substrates. Split ring resonators are placed
atop thin films of deposited or epitaxially grown
photoconductivematerial.Itisnoteworthythatmodel2
is also applicable to photoconductive substrates for
which the optical absorption coefficient (a) is so high
thatthephotogenerationofcarriersisconfinedtoathin
surface layer. On the other hand, model 1 better suits
photoconductive substrates with low a, due to the
possibility of photogeneration throughout the entire
substrate. The last model includes photoconductive
materialinalocalizedareaslightlylargerthanthesplit
ring gap regions beneath the gaps. To realize such a
structure, a two step micro-fabrication sequence must
be employed: first the photoconductive gap regions
mustbedefinedbywayofchemicalorphysicaletching,
followed by the deposition of the resonator pattern.
Fig.2. Calculatedmagneticresonanceforthethreemodelswhenno
Even when the entire substrate is illuminated, photo-
opticalexcitationispresent.Thepronouncedtransmissiondipsinthe
generation for such a structure is limited to the gap
spectra are due to the SRRs magnetic resonance. Inset shows the
regions only. In addition, model 3 also describesthose orientationoftheE-field,H-field,andwavevector.
T.F.Gundogduetal./PhotonicsandNanostructures–FundamentalsandApplications5(2007)106–112 109
of the resonators are modeled as perfectly conducting.
TheschematicSRRunitcellandtherelativeorientation
oftheelectromagneticexcitationareshownintheinset
ofFig.2.ThemagneticfieldisperpendiculartotheSRR
planeandthedirectionsofelectricfieldandpropagation
areasshown.Forallofthesimulations;thenumberof
unitcellsemployedwaschosenas3inthepropagation
direction and 1 in the other two directions, in order to
limit the computational cost of simulations. Boundary
conditions were defined by setting tangential electric/
magnetic fields equal to zero in the boundaries
perpendicular to the direction of the electric/magnetic
fields.Thecalculatedtransmissionspectraforthethree
SRRmodelswhenthereisnoilluminationareshownin
Fig.2.Amagneticresonancedipisobservedat88GHz
formodel2,at90GHzformodel3,andat95GHzfor
Fig.3. Calculatedtransmissionspectraforsimulationmodel1asa
model 1. Although the in-plane geometry of models 2 functionoftheconductivityofthephotoconductingsubstrate.
and 3 are identical, the 2GHz shift in the resonance
frequency is expected due to the presence/absence of
the thin photoconductive layer which contributes by a Analogous calculations to the one described above
differentdielectricconstant.Themagneticnatureofthe were performed also for models 2 and 3, where the
resonance dip is verified by closing the SRR gaps by effect of a conductivity increase of the thin photo-
perfectelectricalconductor(PEC)insimulationsandin conducting layer was investigated. Fig. 4 shows the
observationofthedisappearanceofthedip.Forallthree transmission spectra that were calculated for model 2.
models the minimum of the transmission dip is The two lower s values (0.3S/m and 5S/m) represent
observed around (cid:2)60dB. the range of conductivity values when the structure is
Next, the effect of photoexcitation was investigated not illuminated with sufficient optical power. It is
forthethreemodelsbyvaryingtheconductivity(s)of observedthatthetransmissiondipoccursat(cid:2)60dBand
the respective photoconductive layers. Because of the (cid:2)36dBfors=0.3S/mands=5S/m,respectively.As
limitations imposed by the simulation toolbox, the theconductivityofthethinfilmphotoconductivelayer
conductivity was assumed to be a constant throughout is increased the resonance is observed to disappear
the photoconductive layer. Although this assumption gradually.Ifthephotoconductivityisincreasedasmuch
maynotmodeltheopticalabsorptionprocessaccurately as 300S/m (dotted purple line), the resonance dip
for some cases, it is a valid approximation for low disappears. Similar to the model 1, the transmission
absorption coefficients and/or thin absorbing films.
Fig. 3 shows the transmission spectra calculated for
model1asafunctionofthesubstrateconductivity.The
twolowersvalues(0.1and1S/m)representthetypical
conductivity values for commercial Si substrates. It is
observedthatthetransmissiondipoccursat(cid:2)53dBand
(cid:2)36dB for s=0.1S/m and s=1S/m, respectively.
When the conductivity of the substrate is increased
gradually (equivalent to a gradual increase of photo-
generation throughout the entire substrate), the reso-
nance is observed to disappear gradually. For a
sufficientlylargeincreaseinconductivity,theresonance
can be completely switched off. For example, an
increase from 1S/m to 30S/m in s results in the total
disappearance (dotted purple curve) of the (cid:2)36dB
resonance (solid red curve). For s=30S/m, the
transmission level drops to (cid:2)15dB as a result of the
Fig.4. Calculatedtransmissionspectraforsimulationmodel2asa
increased total loss in the structure. functionoftheconductivityofthethinphotoconductingfilm.
110 T.F.Gundogduetal./PhotonicsandNanostructures–FundamentalsandApplications5(2007)106–112
Fig.5. Calculatedtransmissionspectraforsimulationmodel3asa
Fig. 6. Calculated resonance depth (dB) for the three models as a
functionoftheconductivityofthephotoconductingmaterialcovering
functionofconductivity.
theSRRgaps.
level of the structure drops to (cid:2)10dB for this high efficient when required photoconductivity change
conductivityvalueasaresultofthelossassociatedwith multiplied by the photo-absorbing volume is consid-
the thin conducting layer. Fig. 5 shows the calculated ered.Thisistheproperwayofcomparison,becausethe
transmissionspectraformodel3.Itisobservedthatthe product of the required photoconductivity change and
non-illuminatedtransmissiondipoccursat(cid:2)54dBand optically absorbing volume is directly proportional to
(cid:2)35dB for s=1S/m and s=10S/m, respectively. thenumberofabsorbedphotons,hencetotherequired
Whentheconductivityofthephotoconductivematerial incident power. Fig. 7 compares calculated resonance
across thegaps isincreased, the resonance isobserved depthsforthethreemodelsasafunctionoftheproduct
todisappeargraduallyinsimilarfashiontotheothertwo ofphotoconductivityandopticallyabsorbingvolume.It
models. Finally, a 100-fold increase of conductivity, is observed from Fig. 7 that model 3 is approximately
from the non-illuminated value of 10S/m (solid red four and two orders of magnitude more efficient than
curve) to 1000S/m (solid purple curve), causes the models 1 and 2, respectively.
completeswitchoffoftheresonance.Incontrasttothe Having defined the required designs and conditions
first two models, where the transmission levels away to achieve satisfactory switching, we proceeded to the
from the magnetic resonance showed a considerable
drop by increasing conductivity, here the transmission
level remains at around (cid:2)5dB even for 1000S/m
conductivity.
Comparing the results of Figs. 3–5, several
observations are made: First, the conductivity value
to achieve complete switching is highest for model 3
and lowest for model 1. Similarly, the proportional
increase in conductivity required to achieve any fixed
amount(indB)ofswitchingisalsohighestformodel3
and lowest for model 1.These observations are better
displayedinFig.6,whereinthedepthoftherespective
resonance dips are plotted for the three models as a
function of conductivity. It is seen that the resonance
depth is highest for model 3 (least switching), and
lowestformodel1(largestswitching).Thisisexpected
since the volume where the material conductivity is
Fig. 7. Calculated resonance depth (dB) for the three models as a
modulatedishighestinmodel1andlowestinmodel3.
functionoftheproductofopticallyabsorbingvolumeandphotoinduced
However,ithastobenotedthatmodel3isstillthemost conductivity(thisproductisproportionaltotheabsorbedopticalpower).
T.F.Gundogduetal./PhotonicsandNanostructures–FundamentalsandApplications5(2007)106–112 111
Fig.8. Fabricatedsamples:(a)larger,lowerfrequencysamples,andthestackofplanarsamplesfor100GHzoperation.(b)Photomicrographofsample.
fabrication of those systems and the experimental observe the SRR resonance when the stack was
validationoftheresultsdescribedabove.Here,wewill disassembledandasinglelayerofsubstratewastested.
report on the fabrication and the non-illumination This is because a single layer is too thin to cover the
characterizationofthemodel1describedabove.Split- antenna aperture. We are currently investigating
ring resonator arrays were fabricated on 450mm-thick different stack geometries where we will be able to
Sisubstrateswith2S/mconductivity.First,themodel1 effectively cover the antenna aperture as well as have
SRR pattern described in Table 1 was defined by enough optical clearance to test switching character-
photolithography. Then, 100A˚ Ti/5000A˚ Au metal istics.
patterns were deposited by thermal evaporation and Inconclusion,wesimulatedtheeffectofconductiv-
subsequentlift-off.Fig.8(a)showsseveralexamplesof ityvariationasaproposedmethodfortheinvestigation
fabricated samples. The larger samples to the left and of the photoconductive switching properties of SRRs.
bottom of the figure are designed for lower frequency Wedescribedthreedifferentsimulationmodelsthatare
experiments. The stack of planar samples sandwiched applicable under certain fabrication and/or optical
betweenglassspacersisthebulk100GHztestsample. excitation conditions. We calculated that for a large
A detail of one of the planar samples can be seen in range of dark conductivity values, it is possible to
Fig. 8(b). The millimeter wave measurements were achieve switching by the careful selection of the
carried out by using a HP 8510C network analyzer. appropriate model and excitation conditions. We
First, the signal from the network analyzer was fabricatedSRRstructuresonSisubstratesandmeasured
converted up in frequency to the 75–115GHz range a (cid:2)60dB resonance dip between 108–115GHz. Next,
by using a multiplier source unit. The signal was these structures will be tested under optical excitation.
transmitted by a horn antenna and the transmission
through a stack of samples was measured by a second
horn antenna and mixing detector combination. Stack-
ingofapproximately20sampleswasnecessaryinorder
to cover the entire antenna aperture. Fig. 9 shows the
measuredtransmissionthrough36consecutiveSRRsin
thepropagationdirection.Themeasurementorientation
was same as that is indicated in Fig. 1. A (cid:2)60dB
resonance dip was observed between 108GHz and
115GHz.Thespectralshiftfromsimulationsisdueto
deviation from the design during fabrication. The
transmission level for non-resonant frequencies was
around(cid:2)20dB,whichisconsistentwiththeincreased
loss associated with the large number of SRRs in the
propagation direction. To test the optical switching, it
was necessary to be able to illuminate the sample.
However, the stack of samples was too tightly packed
Fig. 9. Measured transmission spectra of SRRs fabricated on Si
and had no optical clearance. We were not able to substratesaccordingtomodel1.
112 T.F.Gundogduetal./PhotonicsandNanostructures–FundamentalsandApplications5(2007)106–112
ThisworkissupportedbytheEuropeanUnionunder [5] D.Schurig,D.R.Smith,Phys.Rev.E70(2004)065601(R).
the projects EU-NoE-METAMORPHOSE, EU-NoE- [6] V.M.Shalaev,W.Cai,U.K.Chettiar,H.K.Yuan,A.K.Sarychev,
V.P.Drachev,A.P.Kildishev,Opt.Lett.30(2005)3356.
PHOREMOST, by TUBITAK under Projects Nos.
[7] J.B. Pendry,A.J. Holden,W.J. Stewart,I. Youngs,Phys.Rev.
104E090, 105E066, 105A005, 106A017, by Ames
Lett,.76(1996)4773.
Laboratory (Contract No. W-7405-Eng-82) and [8] N. Katsarakis, T. Koschny, M. Kafesaki, E.N. Economou, E.
DefenceAdvancedResearchProjectsAgency(Contract Ozbay,C.M.Soukoulis,Phys.Rev.B70(2004)201101(R).
No.MDA972-01-2-0016),bytheAFOSRunderMURI [9] K.Aydin,K.Guven,M.LeiZhang,C.M.Kafesaki,E.Soukoulis,
Ozbay,Opt.Lett.29(2004)2623.
GrantFA9550-06-1-0337,andbytheGreekMinistryof
[10] R.A.Shelby,D.R.Smith,S.C.NematNasser,S.Schultz,Appl.
Education Pythagoras Project. The research of C.M.
Phys.Lett.78(2001)489.
Soukoulis is further supported by the Alexander von [11] P.Gay-Balmaz,O.J.F.Martin,J.Appl.Phys.92(2002)2929.
Humboldt Senior Scientist Award 2002. One of the [12] N.Katsarakis,T.Koschny,M.Kafesaki,E.N.Economou,C.M.
authors (E.O.) also acknowledges partial support from Soukoulis,Appl.Phys.Lett.84(2004)2943.
[13] N.Katsarakis,G.Konstantinidis,A.Kostopoulos,R.S.Penciu,
the Turkish Academy of Sciences.
T.F. Gundogdu, M. Kafesaki, E.N. Economou, T.H. Koschny,
C.M.Soukoulis,Opt.Lett.30(2005)1348–1350.
References [14] W.J.Padilla,A.J.Taylor,C.Highstrete,M.Lee,R.D.Averitt,
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