{{Use dmy dates|date=April 2020}} A '''metamaterial absorber'''<ref name=landy-n/> is a type of [[metamaterial]] intended to efficiently absorb [[electromagnetic radiation]] such as [[light]]. Furthermore, metamaterials are an advance in [[materials science]]. Hence, those metamaterials that are designed to be absorbers offer benefits over conventional absorbers such as further miniaturization, wider adaptability, and increased effectiveness. Intended applications for the metamaterial absorber include emitters, [[photodetectors]], [[sensors]], [[spatial light modulators]], infrared camouflage, [[wireless communication]], and use in [[solar photovoltaics]] and [[thermophotovoltaics]].
For practical applications, the metamaterial absorbers can be divided into two types: narrow band and broadband.<ref name=":0">{{Cite journal|last1=Yu|first1=Peng|last2=Besteiro|first2=Lucas V.|last3=Huang|first3=Yongjun|last4=Wu|first4=Jiang|last5=Fu|first5=Lan|author5-link=Lan Fu (engineer)|last6=Tan|first6=Hark H.|last7=Jagadish|first7=Chennupati|last8=Wiederrecht|first8=Gary P.|last9=Govorov|first9=Alexander O.|title=Broadband Metamaterial Absorbers|journal=Advanced Optical Materials|volume=7|issue=3|article-number=1800995|language=en|doi=10.1002/adom.201800995|issn=2195-1071|year=2018|doi-access=free|hdl=1885/213159|hdl-access=free}}</ref> For example, metamaterial absorbers can be used to improve the performance of [[photodetectors]].<ref name=":0" /><ref>{{cite journal | last1 = Li | first1 = W. | last2 = Valentine | first2 = J. | year = 2014 | title = Metamaterial Perfect Absorber Based Hot Electron Photodetection | journal = Nano Letters | volume = 14 | issue = 6| pages = 3510–3514 | doi=10.1021/nl501090w| pmid = 24837991 | bibcode = 2014NanoL..14.3510L }}</ref><ref>{{Cite journal|last1=Yu|first1=Peng|last2=Wu|first2=Jiang|last3=Ashalley|first3=Eric|last4=Govorov|first4=Alexander|last5=Wang|first5=Zhiming|date=2016|title=Dual-band absorber for multispectral plasmon-enhanced infrared photodetection|journal=Journal of Physics D: Applied Physics|language=en|volume=49|issue=36|article-number=365101|doi=10.1088/0022-3727/49/36/365101|issn=0022-3727|bibcode=2016JPhD...49J5101Y|s2cid=123927835 |url=https://discovery.ucl.ac.uk/id/eprint/1522579/1/JPD%20final%20version.pdf}}</ref><ref>{{cite journal |last1=Awad |first1=Ehab |title=Graphene Metamaterial Embedded within Bundt Optenna for Ultra-Broadband Infrared Enhanced Absorption |journal=Nanomaterials |date=21 June 2022 |volume=12 |issue=13 |page=2131 |doi=10.3390/nano12132131 |pmid=35807966 |pmc=9268047 |ref=5|doi-access=free }}</ref> Metamaterial absorbers can also be used for enhancing [[absorption (electromagnetic radiation)|absorption]] in both [[solar photovoltaic]]<ref>{{cite journal | last1 = Vora | first1 = A. | last2 = Gwamuri | first2 = J. | last3 = Pala | first3 = N. | last4 = Kulkarni | first4 = A. | last5 = Pearce | first5 = J.M. | last6 = Güney | first6 = D. Ö. | year = 2014 | title = Exchanging ohmic losses in metamaterial absorbers with useful optical absorption for photovoltaics | journal = Sci. Rep. | volume = 4 | article-number = 4901 | doi = 10.1038/srep04901 | pmid = 24811322 | pmc = 4014987 | bibcode = 2014NatSR...4.4901V | arxiv = 1404.7069 }}</ref><ref>{{cite journal | last1 = Wang | first1 = Y. | last2 = Sun | first2 = T. | last3 = Paudel | first3 = T. | last4 = Zhang | first4 = Y. | last5 = Ren | first5 = Z. | last6 = Kempa | first6 = K. | year = 2011 | title = Metamaterial-plasmonic absorber structure for high efficiency amorphous silicon solar cells | journal = Nano Letters | volume = 12 | issue = 1| pages = 440–445 | doi=10.1021/nl203763k| pmid = 22185407 | bibcode = 2012NanoL..12..440W }}</ref> and [[thermo-photovoltaic]]<ref>{{cite journal | last1 = Wu | first1 = C. | last2 = Neuner III | first2 = B. | last3 = John | first3 = J. | last4 = Milder | first4 = A. | last5 = Zollars | first5 = B. | last6 = Savoy | first6 = S. | last7 = Shvets | first7 = G. | year = 2012 | title = Metamaterial-based integrated plasmonic absorber/emitter for solar thermo-photovoltaic systems | journal = Journal of Optics | volume = 14 | issue = 2| article-number = 024005 | doi=10.1088/2040-8978/14/2/024005| bibcode = 2012JOpt...14b4005W | s2cid = 120371536 }}</ref><ref>{{cite journal | last1 = Simovski | first1 = Constantin | last2 = Maslovski | first2 = Stanislav | last3 = Nefedov | first3 = Igor | last4 = Tretyakov | first4 = Sergei | year = 2013 | title = Optimization of radiative heat transfer in hyperbolic metamaterials for thermophotovoltaic applications | journal = Optics Express | volume = 21 | issue = 12| pages = 14988–15013 | doi=10.1364/oe.21.014988| pmid = 23787687 | bibcode = 2013OExpr..2114988S | doi-access = free }}</ref> applications. Skin depth engineering can be used in metamaterial absorbers in [[photovoltaic]] applications as well as other optoelectronic devices, where optimizing the device performance demands minimizing resistive losses and power consumption, such as [[photodetectors]], [[laser diodes]], and [[light emitting diodes]].<ref>{{cite journal | last1 = Adams | first1 = Wyatt | last2 = Vora | first2 = Ankit | last3 = Gwamuri | first3 = Jephias | last4 = Pearce | first4 = Joshua M. | last5 = Guney | first5 = Durdu Ö. | editor1-first = Ganapathi S | editor1-last = Subramania | editor2-first = Stavroula | editor2-last = Foteinopoulou|editor2-link=Stavroula Foteinopoulou | year = 2015| title = Controlling optical absorption in metamaterial absorbers for plasmonic solar cells | url = https://www.academia.edu/15416546 | journal = Proc. SPIE 9546, Active Photonic Materials VII | series = Active Photonic Materials VII | volume = 9546 | pages = 95461M | doi = 10.1117/12.2190396 | bibcode = 2015SPIE.9546E..1MA | s2cid = 8271761 }}</ref>
In addition, the advent of metamaterial absorbers enable researchers to further understand the [[metamaterial|theory of metamaterials]] which is derived from [[electrodynamics|classical electromagnetic wave theory]]. This leads to understanding the material's capabilities and reasons for current limitations.<ref name=landy-n/>
Unfortunately, achieving broadband absorption, especially in the THz region (and higher frequencies), still remains a challenging task because of the intrinsically narrow bandwidth of surface plasmon polaritons (SPPs) or localized surface plasmon resonances (LSPRs) generated on metallic surfaces at the nanoscale, which are exploited as a mechanism to obtain perfect absorption.<ref name=":0" />
==Metamaterials== [[Metamaterial|'''Metamaterials''']] are artificial materials which exhibit unique properties which do not occur in nature. These are usually arrays of structures which are smaller than the wavelength they interact with. These structures have the capability to control [[electromagnetic radiation]] in unique ways that are not exhibited by conventional materials. It is the spacing and shape of a given metamaterial's components that define its use and the way it controls electromagnetic radiation. Unlike most conventional materials, researchers in this field can physically control electromagnetic radiation by altering the geometry of the material's components. Metamaterial structures are used in a wide range of applications and across a broad frequency range from [[radio frequencies]], to [[microwave]], [[Terahertz radiation|terahertz]], across the [[infrared]] spectrum and almost to [[visible spectrum|visible wavelengths]].<ref name=landy-n/>
==Absorbers== "An electromagnetic absorber neither reflects nor transmits the incident radiation. Therefore, the power of the impinging wave is mostly absorbed in the absorber materials. The performance of an absorber depends on its thickness and morphology, and also the materials used to fabricate it."<ref name=Alici>{{cite journal |doi=10.1063/1.3493736 |url=http://www.fen.bilkent.edu.tr/~bora/articles/article21_alici_JAP_2010.pdf |format=Free PDF download |title=Experimental verification of metamaterial based subwavelength microwave absorbers |year=2010 |last1=Alici |first1=Kamil Boratay |last2=Bilotti |first2=Filiberto |last3=Vegni |first3=Lucio |last4=Ozbay |first4=Ekmel |journal=Journal of Applied Physics |volume=108 |issue=8 |pages=083113–083113–6 |bibcode = 2010JAP...108h3113A |hdl=11693/11975 |s2cid=51963014 |hdl-access=free }}</ref>
"A near unity absorber is a device in which all incident radiation is absorbed at the operating frequency–transmissivity, reflectivity, scattering and all other light propagation channels are disabled. Electromagnetic (EM) wave absorbers can be categorized into two types: resonant absorbers and broadband absorbers.<ref name=":0" /><ref name=Watts>{{cite journal |doi =10.1002/adma.201200674 |pmid =22627995 |title =Metamaterial Electromagnetic Wave Absorbers |year =2012 |last1 =Watts |first1 =Claire M. |last2 =Liu |first2 =Xianliang |last3 =Padilla |first3 =Willie J. |journal =Advanced Materials |pages =OP98–OP120 |volume=24|issue =23 |bibcode =2012AdM....24P..98W |doi-access =free }}</ref>
==Principal conceptions== A metamaterial absorber utilizes the effective medium design of metamaterials and the loss components of [[permittivity]] and [[magnetic permeability]] to create a material that has a high ratio of electromagnetic radiation absorption. Loss is noted in applications of negative refractive index ([[photonic metamaterials]], [[Metamaterial antennas|antenna systems metamaterials]]) or transformation optics ([[metamaterial cloaking]], celestial mechanics), but is typically undesired in these applications.<ref name=landy-n/><ref name=optics-express-16/>
[[Complex number|Complex permittivity and permeability]] are derived from metamaterials using the [[Photonic metamaterials|effective medium]] approach. As effective media, metamaterials can be characterized with complex ε(w) = ε<sub>1</sub> + iε<sub>2</sub> for effective permittivity and μ(w) = μ<sub>1</sub> + i μ<sub>2</sub> for effective permeability. Complex values of permittivity and permeability typically correspond to attenuation in a medium. Most of the work in metamaterials is focused on the real parts of these parameters, which relate to wave propagation rather than attenuation. The loss (imaginary) components are small in comparison to the real parts and are often neglected in such cases.
However, the loss terms (ε<sub>2</sub> and μ<sub>2</sub>) can also be engineered to create high attenuation and correspondingly large absorption. By independently manipulating resonances in ε and μ it is possible to absorb both the incident electric and magnetic field. Additionally, a metamaterial can be impedance-matched to free space by engineering its permittivity and permeability, minimizing reflectivity. Thus, it becomes a highly capable absorber.<ref name=landy-n>{{Cite journal |last1=Landy |first1=N. I. |last2=Sajuyigbe |first2=S. |last3=Mock |first3=J. |last4=Smith |first4=D. |last5=Padilla |first5=W. |title=Perfect Metamaterial Absorber |journal=Phys. Rev. Lett. |volume=100 |page=207402 (2008) [4 pages] |date=21 May 2008 |url=http://www2.bc.edu/~padillaw/PDF/PRL_100_207402_2008.pdf |doi=10.1103/PhysRevLett.100.207402 |access-date=22 January 2010 |pmid=18518577 |bibcode=2008PhRvL.100t7402L |name-list-style=vanc |display-authors=1 |issue=20 |arxiv=0803.1670 |s2cid=13319253 |archive-url=https://web.archive.org/web/20110604233731/https://www2.bc.edu/~padillaw/PDF/PRL_100_207402_2008.pdf |archive-date=4 June 2011 }}</ref><ref name=optics-express-16>{{Cite journal|last1=Tao|first1=Hu|last2=Landy|first2=Nathan I.|last3=Bingham|first3=Christopher M.|last4=Zhang|first4=Xin|last5=Averitt|first5=Richard D.|last6=Padilla|first6=Willie J.|title=A metamaterial absorber for the terahertz regime: Design, fabrication and characterization|journal=Optics Express|volume=16|pages=7181–7188|date=12 May 2008|url=http://www2.bc.edu/~padillaw/PDF/Opt_Exp_16_7181_2008.pdf|format=Free PDF download|doi=10.1364/OE.16.007181|access-date=22 January 2010|pmid=18545422|bibcode=2008OExpr..16.7181T|display-authors=1|issue=10|arxiv=0803.1646|s2cid=15714828 |archive-url=https://web.archive.org/web/20110604222138/https://www2.bc.edu/~padillaw/PDF/Opt_Exp_16_7181_2008.pdf|archive-date=4 June 2011}}</ref><ref>{{Cite journal|last1=Yu|first1=Peng|last2=Besteiro|first2=Lucas V.|last3=Wu|first3=Jiang|last4=Huang|first4=Yongjun|last5=Wang|first5=Yueqi|last6=Govorov|first6=Alexander O.|last7=Wang|first7=Zhiming|date=6 August 2018|title=Metamaterial perfect absorber with unabated size-independent absorption|journal=Optics Express|language=EN|volume=26|issue=16|pages=20471–20480|doi=10.1364/OE.26.020471|pmid=30119357|bibcode=2018OExpr..2620471Y |issn=1094-4087|doi-access=free}}</ref>
This approach can be used to create thin absorbers. Typical conventional absorbers are thick compared to wavelengths of interest,<ref name=APL>{{Cite journal| last1 =Yang| first1 =Z.| last2 =Dai| first2 =H. M.| last3 =Chan| first3 =N. H.| last4 =Ma| first4 =G. C.| last5 =Sheng| first5 =Ping| title=Acoustic metamaterial panels for sound attenuation in the 50–1000 Hz regime| journal=Appl. Phys. Lett.| volume=96|page=041906 [3 pages] | year =2010| doi=10.1063/1.3299007|bibcode = 2010ApPhL..96d1906Y| display-authors =1| issue =4 | s2cid =123233731}}</ref> which is a problem in many applications. Since [[metamaterials]] are characterized based on their subwavelength nature, they can be used to create effective yet thin absorbers. This is not limited to electromagnetic absorption either.<ref name=APL/>
==See also== *[[Negative index metamaterials]] *[[History of metamaterials]] *[[Metamaterial cloaking]] *[[Nonlinear metamaterials]] *[[Photonic crystal]] *[[Seismic metamaterials]] *[[Split-ring resonator]] *[[Acoustic metamaterials]] *[[Plasmonic metamaterials]] *[[Superlens]] *[[Terahertz metamaterials]] *[[Transformation optics]] *[[Theories of cloaking]]
==References== {{Reflist|2}}
==Further reading== *{{cite journal |doi=10.1364/OE.19.014260 |title=Optically thin composite resonant absorber at the near-infrared band: A polarization independent and spectrally broadband configuration |year=2011 |last1=Alici |first1=Kamil Boratay |last2=Turhan |first2=Adil Burak |last3=Soukoulis |first3=Costas M. |last4=Ozbay |first4=Ekmel |journal=Optics Express |volume=19 |issue=15 |pages=14260–7 |pmid=21934790|bibcode = 2011OExpr..1914260B |hdl=11693/12111 |url=http://repository.bilkent.edu.tr/bitstream/11693/21853/1/Optically%20thin%20composite%20resonant%20absorber%20at%20the%20near-infrared%20band%20A%20polarization%20independent%20and%20spectrally%20broadband%20configuration.pdf |doi-access=free }} *{{cite journal |doi=10.6028/jres.117.001|pmid=26900513 |pmc=4553869 |title=The Interaction of Radio-Frequency Fields with Dielectric Materials at Macroscopic to Mesoscopic Scales |year=2012 |last1=Baker-Jarvis |first1=James |last2=Kim |first2=Sung |journal=Journal of Research of the National Institute of Standards and Technology |volume=117 |pages=1–60 }} *{{cite journal |doi=10.1109/TAP.2010.2044329 |title=Analysis and Design of Ultra Thin Electromagnetic Absorbers Comprising Resistively Loaded High Impedance Surfaces |year=2010 |last1=Costa |first1=Filippo |last2=Monorchio |first2=Agostino |last3=Manara |first3=Giuliano |journal=IEEE Transactions on Antennas and Propagation |volume=58 |issue=5 |pages=1551–1558|bibcode = 2010ITAP...58.1551C |arxiv = 1005.1553 |s2cid=26617084 }} **The above PDF download is a self-published version of this paper. *{{cite book | last = Munk | first = Benedikt A. | title = Frequency Selective Surfaces: Theory and Design | publisher = John Wiley & Sons | year = 2000 | location = New York | pages = 315–317 | url = https://books.google.com/books?id=9pNMhRQrpScC&pg=PA315 | isbn = 978-0-471-37047-5}} The '''Salisbury screen''', invented by American engineer [[Winfield Salisbury]] in 1952. *Salisbury W. W. "Absorbent body for electromagnetic waves", United States patent number [https://patents.google.com/patent/US2599944 2599944] 10 June 1952. Also cited in [https://books.google.com/books?id=9pNMhRQrpScC&pg=PA315 Munk]
==External links== *Images - A simple schematic of [http://www.fen.bilkent.edu.tr/~bora/w2_research.html Miniaturized Microwave Absorbers] from [http://www.fen.bilkent.edu.tr/~bora/index.html Kamil Boratay Alıcı] (Ph. D., Physics) of the Nanotechnology Research Center, [[Bilkent University]].
[[Category:Metamaterials]]