Thermally Induced Nonlinear Optical Absorption in Metamaterial Perfect Absorbers

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by  Sriram Guddala

Thermally Induced Nonlinear Optical Absorption in Metamaterial Perfect
Absorbers
Sriram Guddala,1 Raghwendra Kumar,1 and S. Anantha Ramakrishna1
Department of Physics, Indian Institute of Technology Kanpur, 208016, India.
(Dated: 5 January 2015)
Ametamaterialperfectabsorberconsistingofatri-layer(Al/ZnS/Al)metal-dielectric-metalsystemwithtop
aluminiumnano-disksisfabricatedbylaser-interferencelithographyandlift-offprocessing. Themetamaterial
absorber had peak resonant absorbance at 1090 nm and showed nonlinear absorption for 600ps laser pulses
at 1064 nm wavelength. A nonlinear saturation of reflectance was measured to be dependent on the average
5 laser power incident and not the peak laser intensity. The nonlinear behaviour is shown to arise from the
1 heating due to the absorbed radiation and photo-thermal changes in the dielectric properties of aluminium.
0 The metamaterial absorber is seen to be damage resistant at large laser intensities of 25 MW/cm2.
2

n PACS numbers: Valid PACS appear here
a
J

2 Metamaterials (MTMs), with sub-wavelength unit
cells of metal-dielectric composite structures called

] “meta-atoms”, show unique electro-magnetic resonances
s
c excited by the incident radiation. The resonant interac-
i tioncausestheMTMtohavehighlydispersive“effective”
t
p medium properties. The dispersive material parameters
o and perfect impedance (Z=pµ(ω)/ε(ω)) matching can
.
s give rise to unusual optical phenomena such as negative
c
refractiveindex,sub-wavelengthimagingandperfectab-
i
s sorption, etc., which are not found in the natural ma-
y terials.1 Metamaterial perfect optical absorbers (MPA)2
h
with narrow and broad band resonances over various
p
bandsinthe electromagneticspectrumhaveshowngreat
[
potential for a wide range of applications such as sen-

1 sors,3 imaging devices,4 and solar cells,5 etc.
v
Though the linear optical properties of MTMs have
1
5 been widely studied for applications, the nonlinear op-
3 tical properties have attracted attention only more re- FIG. 1. (a) Schematic unit cell of the tri-layer (Al/ZnS/Al)
0 cently.6 The large local field enhancements within the MPA with structural parameters. (b) SEM top view of hole
0 MTM structures due to resonant interactions can give pattern photoresist on ZnS/Al films with period p= 880±5
. nm and hole diameter of 560±5 nm. (c) AFM image of the
1 rise to enhanced nonlinear response. Especially, there
top pattern Aldisksafter lift-off; (d)AFM image line profile
0 hasbeengreatinterestinparametricamplification,7 har- showstheAldiskdiameter560±5nmandthickness30±3nm.
5 monic generation,8,9 andopticalswitching10 in splitring
1
resonators, fishnet structures and more complex chiral
:
v MTMs.6 Incorporating nonlinear Kerr dielectric media sorbancewith increasingaveragelaserpower incidenton
Xi for switchable MTMs have also been investigated.11,12 the structure, indicating a purely thermal origin for the
The intense absorption of radiation in MPAs mediated nonlinearities. These studies can have significant impor-
r
a by electromagneticorplasmonicresonancescangiverise tance in the application of highly absorbing sensitizing
to strong photo-thermal effects. So, these effects need layers for integrated photonic devices, thermal sensing,
to be well understood to optimize the devices based on photo-detecting and optical imaging. We further show
MTM or plasmonic structures. thatMPAscanbebuiltrobustlytowithstandlargelaser
In this letter, we report on thermally driven nonlin- pulse interactions of several tens of MW/cm2 without
ear absorption of MPAs, where the resistive heating in any sign of damage.
the structure modifies the spectralresponseofthe MPA. The design of the MPAs is based on resonant struc-
Wepresentresultsonnonlinearreflectance/absorbanceof turesthatcanbesimultaneouslydrivenbyboththeelec-
600pspulses at 1064nmfroma resonantMPAconsisting tric and magnetic fields of radiation. This can result in
ofatri-layerstructureofaluminium(Al)disksonZnS/Al resonant absorption of radiation at perfect or optimized
thin films. The constituent materials(Al, in our case)of impedance matching of the effective medium with free
the composite MTM change their material parameters space. One simple design of a MPA13,14(Fig. 1(a)) con-
with increasein temperature, whichleads to a change in sists ofa conducting groundplane separatedby a dielec-
the MTM properties. We find evidence of increasing ab- tric spacer layer from the top structured metallic motif.
2

12 2) 1.2
10 (a) J/cm20 (b) 180KK nce (c) 180KK
% Reflection 468 1e2x cd=e1g064 nm (ed Fluence 1105 53KK orm. Reflecta 01..80 53KK
ct N
e 5
2 efl 0.6
R
0 0
500 1000 1500 2000 2500 3000 0 2 4 6 8 10 12 14 2 3 4 5 6 7 8
Wavelength (nm) Input Fluence (mJ/cm2) Avg. power (mW)
FIG. 2. (a) White light reflection spectrum of the MPA measured at 12o angle of incidence. (b) Reflected fluence verses the
incident pulse fluence for each PRR (600ps, 1064 pulses). (c) Normalized reflectance plotted with respect to incident average
laser power.
Proper choice of the motifs and spacer layer thickness broadening of the resonances. The measured reflection
canresultinsimultaneousresonancesfortheelectricand spectrum[A(ω)=1−R(ω)−T(ω)]oftheMPAisshown
the magnetic excitations at a common frequency. Vari- in Fig. 2(a).
ousfabricationtechniques,especially,expensiveandtime Thermal effects in MPA structure are studied at a
consuminge-beamlithography15,16andfocusedionbeam small angle of incidence (200) with laser pulses of 600ps
millingtechniques17,18 havebeeninwideusetofabricate pulsewidth and1064nmfroma Q-switchedpulsedlaser
motifs of MPAs with sub-micron structural features op- (WedgeHF,BrightSolutions)withpulserepetitionrates
erate at NIR and visible frequencies. We have used laser (PRR) between 3 to 10 kHz. Due to literally no trans-
interference lithographyfollowed by lift-off processing to mission of the structure for 1064 nm, the experimental
fabricatemetallicstructureswithsub-microndimensions measurements are performed in the reflection configu-
(<300nm) over few mm2 areas. ration by focusing the incident laser beam with 10 cm
The fabrication process of MPA involves three princi- focal length plano-convex lens and the reflected beam is
pal steps: first, an Al film (100 nm) and a thin ZnS film collectedonto a photo-diode. The experimentalobserva-
(65nm)aresequentiallydepositedbythermalvapourde- tions of the reflected pulse fluence as a function of the
position on a clean glass substrate. Second, a thin posi- incident pulse fluence (monitored by a reference photo-
tive photoresist (PR) (ma-P 1205, Micro-resist Technol- diode) is plotted in Fig. 2(b). A nonlinear saturation in
ogy) film is spin coated at 3000 rpm. The PR layer was the reflected fluence is observed for various PRRs (3kHz
subsequently exposed to a two-beam laser interference to 10kHz). The nonlinear saturation of the reflected flu-
pattern19 followed by 90o rotation of the sample about ence depends on the PRR, with lesser saturation noted
its surface normal to obtain a 2D square lattice of holes forsmallerPRRatthesamepulseenergy. Thenonlinear
after exposure and development as shown in Fig.1(b). reflectance, normalized to the linear reflectance at small
Finally, a negative replica of the PR pattern is obtained pulseenergies,isalsoplottedwithrespecttotheincident
by physical vapour deposition of thin Al film followed averagepowerforeachPRRinFig. 2(c). Thisshowsthat
by PR lift-off by immersing in acetone for few minutes. the reflectance begins reducing with increase in average
An atomic force microscope (AFM, Park XE 70) image incident power for all the PRR. The rate of reduction is
of the MTM in Fig. 1(c) shows the top Al disks (30 nm larger for large PRR. Thus, a nonlinear increase in the
thickness)withaperiodof880±5nm. Uniformityofthe absorbance of the MPA that appears to be principally
diskdiameters(560±5nm)andheights(30±3nm)canbe determined by the averagelaser power incident is noted.
notedfromtheAFMimagelineprofileinFig. 1(d). Itin- Tounderstandtheabovenonlinearabsorbance,wefirst
dicatesthatourfabricationtechniquehasgoodpotential needto analysethe linear absorbancemechanismsinthe
for fabricating MPAs over large areas with uniformity. system. Numerical simulations of the MPA structure
The reflection spectrum of the MPA, normalized to were performed for the fabricated structural parameters
the reflection (≈ 100%) from a thick and smooth Al usingthecommercialfiniteelementpackage,COMSOLr
film, was measured with collimated white light beam Multiphysics20. In the simulations, the temperature de-
(1mm×3mm) at an angle of incidence of 12o with re- pendent dielectric permittivity values for Al films were
spect to the surface normal. The bottom 100 nm thick considered from Hutnner et.al.21 The refractive index
Alfilm,whichis muchmorethantheskindepth(∼5nm of ZnS22 is considered as n = 2.75 and non-dispersive
at 1090 nm), literally makes the transmission zero. The within the bandwidth considered. A normally incident
optimized impedance matching of the tri-layer system plane wave (θ = 00, Fig. 1(a)) on the MPA was consid-
shows a large drop in the reflected intensity at 1090 nm ered. Perfectelectric and magnetic conductor (PEC and
within a broadband presumably due to inhomogeneous PMC) boundary conditions are applied along the X and
3

e
c 480
an1.0 (a) (b) (c)
ctance, Absorb000...468 RAT Transmittance Temp (K) 334469250000 180KK Reflectance00..24 TTT334050008
Refle0.2 330 53KK TTe44x67c07
0.0 300 0.0
540 720 900 1080 1260 1440 1620 1800 2 3 4 5 6 7 8 1000 1040 1080 1120 1160 1200
Wavelength (nm) Avg. Power (mW) Wavelength (nm)
FIG.3. (a)SimulatedspectralresponseoftheMPA.(b)CalculatedtemperaturesfromEq.1forthesameincidentaveragepowers
at different PRRs. c) Simulated spectral response of MPA at theelevated temperatures (Tlow).
YdirectionsrespectivelyfortheincidentTEandTMpo-
larizations. Thesimulatedreflectancespectrumobtained
from the S-parameters of the finite element simulations,
shows an absorbance band at 1090 nm along with an-
other resonance band centred at 960 nm. This explains
the absorption maximum (∼ 98%) noted in the samples
at 1090 nm. The two theoretical separate resonances
are not visible in the measurements, while an asymme-
try canstillbe discernedindicating the presenceofmore
than one resonance within the broad absorption band.
The experimental observations indicate a large level of
inhomogeneous broadening, presumably due to the fab-
ricational inaccuracies and limitations.
To understand the nature of the resonances,we calcu-
late the electromagnetic fields excited in the MPA. The
normalized electric field excited at the top Al metallic
disk(Fig. 4(a))indicatestheexcitationofthethirdorder
cavity-likemodesupportedbyanantennawithanoptical
length of mλ/2 = nAl.(2r) for (m = 3).14 The magnetic
FIG. 4. (a)Electric field distribution in the top Al disk and
resonance at 1090 nm and its normalized magnetic field
(b)magnetic field distribution in thespacer ZnS layer for the
confinement(Fig. 4(b))inthemiddledielectricZnSlayer 3rd order magnetic resonance at 1090 nm. The displacement
results from the anti-parallel currents excited in the top
current density in the top and bottom Al layers are shown
and bottom metallic layers (white arrows in Fig. 4(b)). by the white arrows. (c) Electric field distribution at the
The simultaneous excitationofthe electric andthe mag- top MPA surface for propagating plasmon mode at 960 nm.
netic resonances at 1090 nm results in the absorbance (d)Simulated heat dissipation in the top Al disk and ground
band shown in Fig.2(a). Moreover, a propagating sur- Alfilms for the3rd order resonance at 1090 nm.
face plasmon polariton mode supported by the periodic
disk array and the ground plane at 960 nm is suggested
by the field distribution simulations shown in Fig.3(a). ground plane corresponding to the 3rd order mode. The
The broad band in the experimental reflection spectrum elevated temperatures can modify the optical constants
(Fig.2(a)) arises from the overlap of these resonances at of the heated metallic components, thereby change the
1090nm and 960 nm due to inhomogeneous broadening. spectralresponseoftheMPA.Arigorousestimateofthe
The first order dipole resonance of the MPA would be transientelevatedtemperaturesforgivenlaserintensities
present at 4.5 µm (not shown here). The higher order and PRRs is required to obtain the material parameters
th
resonance corresponding to 5 order can be noticed at at the incident laser powers.
675 nm in the simulated spectrum. We note that the peak temperature (T ) in the
high
The large absorption of radiation results in resis- structure will depend on the pulse energy focused into
tive/ohmic heating and elevated temperatures of the the MPA while the lowest temperature (T ), that the
low
MPA. The intense localized resistive heating in Al com- structure will relax to, will depend on the PRR. Each
ponents ofthe structure is showninFig.4(d)by simulat- pulse will encounter the material parameters at T as
low
ing the total heat dissipation in the PA. Fig. 4(d) shows the laser pulse width is much smaller than the thermal
thattherearelocalizedhotregionssymmetricallylocated relaxation rates and the inter-pulse period. For an in-
on both the disk and regions close to the disk on the cident pulse energy Q, we have Q = C(T −T ),
high low
4
therreversesaturableabsorptionorsaturableabsorption
TABLEI.nandkofAlatλ=1064nmanddifferenttemper-
dependingonwhetherthelaserisblueorred-shiftedwith
atures.21
respect to the MTM resonance.
Temp(K) n(±0.005) k(±0.005) Insummary,we havedemonstratedthermally induced
300 1.589 10.334 resonance mediated nonlinear optical absorbance in a
360 1.726 10.533
MPA.Theintenseandlocalizedabsorptionoflaserradia-
420 1.837 10.807
tionresultinresistiveheatingoftheMPA.Thethermally
450 1.893 10.923
modified dielectric permittivity of Al is responsible for a
480 1.960 11.055
dynamical tuning of the MPA resonance. Moreover, our
fabricationtechniqueusinglaser-interferencelithography
shows great potential for the manufacture of sub-micron
where C is the heat capacity of the MTM in the focused sized MTM unit cells for operation at visible and NIR
area given by C = mAlCAl +mZnSCZnS, where m and frequencies. Our pulsed laser measurementsof nonlinear
C stand for the mass and specific heat of the material absorption in MPA also demonstrates the ruggedness of
components. T , is given by T =T exp(−τ /τ ), the structures, which are able to withstand peak pulse
low low high p r
where τ =(ρ C )R2/k is the relaxation time24 and intensities exceeding 25MW/cm2 without any damage.
r Al Al Al
τ istheinter-pulseperiod. Hereρisthedensity,Cisthe ThisworkwassupportedbyDRDO,Indiaundergrant
p
specific heat, k is the thermal conductivity and R is the no. DECS/15/15124/D(R&D)/CARS−1. SG thanks
radius of the beam (40µm) at the MTM. We take ρ = the IIT Kanpur for a fellowship. RK thanks the Council
2.50g/cm3,3.70g/cm3,C = 0.9J/gm−K,0.47J/gm−K for Scientific and Industrial Research, India, for a fel-
andk =2.05W/cm−K,0.27W/cm−Kforaluminium21 lowship. Authors acknowledge Prof. Goutam Deo, IIT
andZnS22 respectively. ThecontributionofZnSlayerto Kanpur for the NIR spectrometric measurements.
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