Metamaterials for Multi-spectral Infrared Absorbers

182 Pages·2015·32.62 MB·English

Metamaterials for Multi-spectral Infrared Absorbers
AThesisSubmitted
inPartialFulfilmentoftheRequirements
fortheDegreeof
DOCTOR OF PHILOSOPHY
by
GOVIND DAYAL SINGH
tothe
DEPARTMENT OF PHYSICS
INDIAN INSTITUTE OF TECHNOLOGY KANPUR, INDIA
JUNE, 2014
(cid:5) (cid:5)
Dedication
Thisthesisisdedicatedto
MYFAMILY
[MyBelovedParents]
(cid:5)
INDIAN INSTITUTE OF TECHNOLOGY KANPUR
DEPARTMENT OF PHYSICS, INDIA - 208016
STATEMENT
I hereby declare that the matter manifested in this thesis entitled: "Metamaterials
for Multi-spectral Infrared Absorbers", is the result of research carried out by me in
the Department of Physics, Indian Institute of Technology Kanpur, India under the
supervisionofProf. S.AnanthaRamakrishna.
In keeping with the general practice of reporting scientific observations, due ac-
knowledgement has been made wherever the work described is based on the findings
ofotherinvestigators.
IITKanpur GovindDayal
June,2014 Ph.D.Student
RollNo: Y9109067
INDIAN INSTITUTE OF TECHNOLOGY KANPUR
DEPARTMENT OF PHYSICS, INDIA - 208016
ffi +
ProfessorS.AnanthaRamakrishna O ce: 915122597449
+
Lab: 915122596601
Email: [email protected]
CERTIFICATEI
It is certified that the work contained in this thesis entitled: "Metamaterials for
Multi-spectralInfraredAbsorbers",hasbeencarriedoutbyGovindDayalSinghunder
mysupervisionandthesamehasnotbeensubmittedelsewhereforadegree.
IITKanpur Prof. S.AnanthaRamakrishna
June,2014 (ThesisSupervisor)
Acknowledgement
"It is good to have an end to journey toward; but it is the journey that matters, in the end." - Ernest
Hemingway
This dissertation took almost five years from conception to completion and it’s just like climbing a
highpeak,stepbystep,accompaniedwithbitterness,hardships,countlesscyclesofinquiry,exploration,
confusion and encouragement. Though, it will not be enough to express my gratitude in words to all
thosepeoplewhohelpedmeinthisjourney,Iwouldstillliketogivemymany,manythankstoallthese
people.
First of all, I offer my sincerest gratitude to my supervisor, Prof. S. Anantha Ramakrishna for
exposing me a very exciting and interesting field of research which instilled confidence in me. I owe
a great deal of devotion to him, not only introducing me from the infancy of Metamaterials, but also
for his moral support, ’optics feelings’, optimism and constant encouragement and being the source
of inspiration for me. He has oriented and supported me with promptness and care, and has always
been patient and encouraging in times of new ideas and difficulties; he has listened to my ideas and
discussions which led to key insights. His fresh view on science and positive attitude have always been
verystimulating.
Iamextremelyindebtedto"peers"Prof. H.WanreandProf. R.Vijayawhohavehelpedwiththeir
suggestionsandcriticalcommentswhichenabledmetoshapemythesis. IowespecialthankstoProf. J
Ramkumar,forhisinstruction,insightsandsupervisionforthelasermicromachiningexperiments.
I must acknowledge Prof. V. Subrahmanyam, Prof. Sutapa Mukherji, Prof. Avinash Singh and
Dr. S.Damodaranforteachingmeinterestingandinformativecoursesduringtheperiodofcoursework
here. Ialsowanttoconveymysincerethankstoallmyteachersduringmyschoolandcollegelifeprior
to IIT-K. Through classroom teaching they shaped my mind, nurtured my thoughts and nourished my
researchpotential.
A token of thanks to my Lab Post Docs Dr. Jeyadheepan, Dr. P. Mandal and Dr. Sriram for their
friendlyattitudeandacademicsupport. IoffermyspecialthankstomydearlabmatesDheeraj,Ganga,
Jhuma, Prince, Rameshwari, Raghwendra and Rajesh for their help and nice efforts to make the lab
enthusiastic place for research. They shared with me their intellect and the basic tools of the trade. My
special thanks goes to Jhuma for helping me in many depositions, in particular, with glancing angle
deposition, Dheeraj for AFM measurements and also Nadeem for providing me many initial mask to
startwiththelasermicromachiningexperiments.
I am very thankful to IIT Kanpur for the great research infrastructure and CSIR for providing me
theresearchfellowshipandofferingmefinancialsupport.
IexpressmyprofoundthankstoMrs. Kanchan(Ma’am)andJyotitoprovidemeahomelyatmosphere
andlivelycompanyonvariousoccasions.
Completing this work would have been all the more difficult were it not the invaluable network of
supportive, forgiving, generous and loving friends Sunil, Nikhil, Dushyant, Gyanendra, Prabhakar,
Subhankar, Gopal, Upkar, Bahadur, Pranati, Vandana, Seema, Nimisha, Reeta, Shraddhaetc. atIITK.
I have always been fortunate enough to have very good friend like Arun, Ratnesh, Gopi, Subhash with
whom I have shared many a tense moment and bouts of joy and they always wished well for me. I am
eternallygratefultoSushmaforherconstantencouragement,affection,careandsupport.
Nowordsofappreciationcouldexpressmygratitudeformyfamily,whohavealwaysbeenwithme
although they were miles away from me. Whatever I am today is because of their unconditional love,
care, guidance, encouragement andmoralsupport. Toputinone line, thescriptof mysuccessremains
incompletewithouttheirhugesacrifices.
Lastly but definitely not the least, I owe a debt of gratitude to the Almighty to sail the journey and
madethispossiblebybeingwithmeallthetime.
I seek pardon for all whose names are missed out unintentionally in spite of their immense and
persistent support. While grateful to the people who have bring this thesis to fruition, all the mistakes
andflawsthatremainaremyown.
ThankYou!
i
Synopsis
Electromagnetic metamaterials are composite arrays of resonant structures of sub-
wavelength size designed to have specific optical properties. The electromagnetic
properties of a metamaterial depend mainly on the geometry of individual unit cells
ff
throughtheresonancesofthestructure. Manyuniquee ectssuchasnegativerefractive
index,sub-wavelengthimaging,cloaking,perfectabsorptionetc,thatarenotpossible
to achieve with natural materials, have been obtained by carefully designing the unit
cellforthespecifiedfrequencies. Thesub-wavelengthsizeoftheunitcellsallowsmeta-
ff
materials to be characterized by a complex valued e ective medium parameters such
as the electric permittivity and the magnetic permeability. Due to resonant nature of
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themetamaterials,thee ectivemediumparametersareusuallyfrequencydispersive.
ff
Much of the work in metamaterials has focused on the e ects that arise due to the
real part of electric permittivity and magnetic permeability, for example, which can
be manipulated to be negative form a material with a negative index of refraction.
However, the imaginary part of the electric permittivity and magnetic permeability
canalsobemanipulatedtocreateunusualproperties. Inparticular,theelectricpermit-
tivity and magnetic permeability of metamaterials can be manipulated to create very
strong absorbers. By adjusting the electric permittivity and magnetic permeability, a
metamaterialcanbeimpedance-matchedtofreespace,minimizingreflectivity. Byma-
ff
nipulating electric and magnetic resonances independently, it is possible to e ectively
absorbradiationthroughtheelectricandmagneticfieldcomponents. Theuseofperfect
absorbers may provide a method of exceeding the blackbody radiation limit imposed
by most current uncooled thermal detectors. Metamaterials with near unity absorp-
ii
tion are highly desirable for many applications including micro-bolometers, sensors,
thermalimagers,andabsorbersusedinthermalphoto-voltaicsolarenergyconversion.
Landy et. al. (Phys. Rev. Lett 100 (20): 207402 (2008)) proposed and demonstrated the
firstmicrowavemetamaterialabsorber.
ffi
Metamaterials based on metallic elements are particularly e cient as absorbing
media, because both the electrical and the magnetic properties of a metamaterial can
be tuned by structural design. One example of highly absorbing metamaterial design,
extensively investigated in this thesis, is that of a tri-layer structure, which consists
of a conducting metallic patch separated from a conducting (ground) plane by a di-
electric spacer layer. The tri-layer structure supports a series of cavity-like resonances
analogoustoagroundedpatchantenna,inwhichtheelectromagneticfieldislocalized
within the gap between the ground plane and the metallic patch element. The fun-
damental mode of sub-wavelength size metallic resonator is a dipole mode, however,
withamoderatorsizeresonatorfundamentalaswellashigherordermodescanalsobe
excited. Thestructuredunitsonthetopactaselectricresonatorsdrivenbytheelectric
field of the incident radiation. Assuming the metallic patch as a polarizable dipole,
an image dipole is induced in the bottom metallic film in response to the presence of
a metallic patch. The in-plane (parallel to the film) dipole moments of the image are
oppositetothoseofthemetallicpatch. Thesetwoanti-parallelcurrentsalongwiththe
displacement field in the intervening dielectric act to form circulating current loops
with a confined magnetic field in between to give rise magnetic resonance. When the
electricandmagneticdipoleresonancesoccuratsamefrequency,thenastronglocaliza-
tion of electromagnetic energy results in the metamaterial structure and a consequent
strongabsorptionoccursinthepresenceofmetallicregions.
Theworkpresentedinthisthesisaimsatthedesign,fabricationandcharacterization
ofmetamaterialabsorberstructuresasnovelmaterialsforthemulti-spectralabsorption
at infrared frequencies. Numerical and experimental approaches are used to demon-
iii
stratemulti-spectralabsorptionacrosstheinfra-redspectrumfromshortwaveinfrared
(SWIR) to long wave infrared (LWIR) and the mechanisms that allows such structures
to produce near unity absorption via the excitation of electromagnetic resonances in
structures are elucidated. The salient results of this thesis are the development of
multi-band metamaterial absorbers, ITO based broadband IR absorber and thermally
switchable metamaterials. Some novel micro fabrication techniques based on the ex-
cimer laser micromachining and shadow mask deposition have been utilized for the
fabricationofthemetamaterials.
Chapter 1 presents a theoretical background to metamaterial and the control of
electromagnetic radiation by such artificial structured materials, and is intended to
establish a context for the detailed discussion of metamaterial absorbers that follows.
ff
Section 2 is dedicated to exploring in detail the di erent mechanisms of resonances
which underpin the selective absorption possible with metamaterials. The charac-
teristics required for a material to be absorbing are discussed, and the conventional
approaches to designing such materials are covered. This is followed by a review of
currentresearchintheareaofabsorbingmaterials.
Chapter2presentsthemethodsusedbyusforthedesign,fabricationandcharacter-
izationofmetamaterialabsorbers. Thefirstsectionfocusesontheuseoffiniteelement
modeling as a tool for the design of metamaterial absorbers and specifically on COM-
SOLMultiphysicssoftware. Thebasisofthefiniteelementmethodisdescribedandthe
mannerinwhichCOMSOLMultiphysicssoftwareappliesthefiniteelementapproach
to simulate electromagnetic problems is detailed. The specific modeling tactics em-
ployed to simulate the behavior of each variant of the metamaterial is also covered in
detail. Inthenextsection,wepresentadetaileddescriptionofthefabricationmethods
usedinthisthesis. Themetamaterialswithmicro-sizedunitcellshavebeenfabricated
with shadow mask techniques. The free standing shadow masks have been fabricated
using Excimer laser micro-machining. The laser micro machining of high aspect ratio
iv
featuresonpolymersaswellasonthinplasmonicmetalfilmsisdiscussedindetail.
In Chapter 3, we investigate the optimal designs of highly absorbing metamate-
rial at infra-red frequencies optimized from both the point of view of metamaterial
performance as well as fabrication process. A detailed study of the behavior of these
structuresasafunctionoffrequency,polarizationstateandincidentanglesispresented.
Thefiniteelementmodelisusedtoinvestigatetheformoftheresonantmodesexcited.
ff
In particular, the e ect of ground plane thickness and the physical phenomenon of
electromagnetic resonance in the tri-layer system is exhaustively considered. We also
further extended our design strategy for extending the usable bandwidth of metama-
terial absorber whereby a more detailed understanding of the phenomenon resulted.
ff
Stackedmetal-dielectric-metalmulti-layeredstructureswithdi erentdielectricspacers
can support multiple resonant modes and are presented as building blocks of multi-
bandmetamaterials.
All the design studies in literature have considered only the fundamental electric
and magnetic dipole resonances or LC resonances of the tri-layer structure. In chapter
4, we demonstrate a simple metamaterial absorber design that behaves as a multi-
band perfect absorber at infra-red frequencies due to excitation of the fundamental
as well as higher order electromagnetic resonances in the tri-layer structure. The
metamaterialswerefabricatedusingshadowmaskdepositiontechniques,anattractive
low-costtechnologyformakinglarge-areasamples. Adetailednumericalanalysiswas
carriedoutforfurtherunderstandingthenatureofthesemodesforperfectimpedance
matchingtothatofvacuumforthespectrallyselective“perfect”absorptionofinfrared
light. In the next section, we demonstrated a metamaterial absorber design fabricated
onflexiblepolymersubstrates. Themetamaterialswerefabricatedusingglancingangle
deposition (GLAD) on pre-patterned polymer substrates that were fabricated using
excimer laser micromachining. The principle of the GLAD is based on the shadowing
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e ectsuchthatonlytheexposedareasarecoatedbyadirectionalincomingvaporflux