Dual broadband infrared absorptance enhanced by magnetic polaritons using graphene-covered compound metal gratings

Dual b oadband in a ed abso p ance enhanced by magne ic pola i ons using g aphene-co e ed compound me al g a ings NGHIA NGUYEN-HUU,1,2,3,* JAROMIR PISTORA,4 AND MICHAEL CADA2 1Nano echnology Cen e, VSB - Technical Uni e si y o Os a a, Os a a-Po uba 708 33, Czech Republic 2Depa men o Elec ical and Compu e Enginee ing, Dalhousie Uni e si y Hali ax, No a Sco ia B3J 2X4, Canada 3LED Roadway Ligh ing L d., 115 Chain Lake D i e Hali ax, No a Sco ia B3S1B3, Canada 4Regional Ma e ials Science and Technology Cen e, Facul y o Ma e ial Technologies, VSB -Technical Uni e si y o Os a a, Os a a-Po uba 708 33, Czech Republic *[email p o ec ed] Abs ac : A dual b oadband pe ec abso be based on a g aphene-co e ed compound sil e (Ag) g a ing s uc u e wo king in he in a ed (0.8–2.1 µm) egime is p oposed and in es iga ed nume ically. Two dis inc abso p ion peaks app oxima ely 1.0 a e achie ed by he exci a ion o magne ic pola i ons o e a la ge ange o inciden angles om 0 o 70 deg ees. The physics unde lying he s uc u e is also explained by compu ing in e ac ions o elec omagne ic ields wi h he g aphene and he Ag g a ing. In addi ion, i has shown ha he abso p ion peaks can be uned by changing geome ic pa ame e s o he s uc u e; howe e , hei spec al shape and abso p ion emain unchanged. Fu he mo e, he p oposed compound g a ing wi h a g aphene o e lay p o ides po en ial applica ions o in a ed abso bing de ices. © 2019 Op ical Socie y o Ame ica unde he e ms o he OSA Open Access Publishing Ag eemen 1. In oduc ion Ligh enhancemen in plasmonic nanos uc u es has a ac ed a g ea a en ion o m he esea ch communi y since i p o ides emendous applica ions o sola cells, pho ode ec o s, senso s, nanoimaging de ices, he mal emi e s, and me ama e ial abso be s [1–12]. The enhancemen was heo e ically and expe imen ally demons a ed due o such phenomena o su ace plasmon pola i ons, magne ic pola i ons, Fano esonance, o ca i y esonance [13– 23]. Recen ly, a ious single- equency pe ec abso be s wo king in gigahe z, e ahe z, and in a ed (IR) anges ha e been ac i ely in es iga ed due o hei a ious applica ions such as IR de ec o s, bio/chemical sensing, IR imaging de ices, he mo bolome e s, and so o h [24– 32]. To be de ailed, IR pe ec abso be s we e cons uc ed based on one dimensional (1D) o 2D mul iple laye ed s uc u es comp ising a op me ama e ial laye , a middle dielec ic space laye , and a bo om me al e lec o o silicon subs a e [15–17,23,31]. In addi ion o ha , abso be s based on g a ing s uc u es consis ing o a g aphene laye co e ing a me al/dielec ic/me al o me al g a ing ha e also been s udied physically [19–21,23,33]. The enhanced abso p ion was caused by a ious esonances occu ing ei he in he op me al, in he space , o coupling each o he . Al hough many IR abso be s we e p oposed nume ically and expe imen ally, hei na owly spec al bandwid hs limi ed he po en ial applica ions o single-band abso be s. To add ess he na ow bandwid h p oblem, abso be s ea u ing dual and mul iband abso p ion in he IR egime ha e been p oposed wi h di e en geome ic shapes [34–37]. Fo ins ance, hese abso be s we e designed by uning hei geome ic dimensions in a single, mul iple uni cells, o e ically s acked mul iple laye s [36,38]. Dual-band abso be s we e also cons uc ed based on 1D o 2D mul iple hole s uc u es wi h a g aphene o e lay Vol. 27, No. 21 | 14 Oc 2019 | OPTICS EXPRESS 30182 #376454 h ps://doi.o g/10.1364/OE.27.030182 Jou nal © 2019 Recei ed 27 Aug 2019; accep ed 2 Sep 2019; published 3 Oc 2019 [18,34,35]. Simila ly, an abso be was buil based on wo di e en dielec ic space ma e ials o by changing di e en hicknesses o he dielec ic ma e ial in a single uni cell [34,37]. Al hough he p e ious designed IR abso be s ha e p o ided e y high abso p ion e iciency, hey owned complex geome ic s uc u es which we e no easible o be manu ac u ed and wo ked in a na ow ange o wa eleng hs as well. The e o e, i is necessa y o design mul iple band pe ec abso be s ea u ing an easily- ab ica ed geome y and nea uni y abso p ion. Subwa eleng h g a ing s uc u es whose dimensions a e smalle compa able wi h inciden ligh a e playing a i al key in ul illing he equi emen o high e icien de ices, which could be used o build mul iple band pe ec abso be s. Mos ecen ly, he subwa eleng h s uc u es combined wi h a g aphene o e lay ha e shown ha he abso p ion enhancemen is g ea ly enhanced, and hey could wo k as single- equency abso be s in he IR egion [22,39]. Howe e , compound g a ings (CG) co e ed by a g aphene shee used o enhance dual band IR abso p ion has no been add essed ye . Acco dingly, in his pape we design and in es iga e a dual band pe ec abso be in he nea IR ange based on g aphene-co e ed CGs. A CG has a pe iod including mul iple noniden ical g a ing pe iods [40], and i was also known as a double-pe iod g a ing, a dual-pi ch g a ing, a dual-pe iod g a ing, and a complex g a ing [41–43]. On he con a y, a simple g a ing (SG) ea u es a single pe iod [3]. In he p esen s udy he abso be exhibi s wo dis inc abso p ion peaks wi h app oxima ely uni y abso p ance and is insensi i e o a la ge ange o inciden angles. The physical phenomenon o enhance he abso p ance is also s udied. Fu he mo e, he esonan wa eleng h o he wo bands can be ailo ed by a ying he geome y o he g aphene-co e ed CG s uc u e. Gene ally, he dual band pe ec abso be is p oposed in he p esen s udy using a compound me allic g a ing wi h a g aphene o e lay ha p o ides p ac ical uses o enhancing abso p ion o be nea ly pe ec . 2. Theo e ical desc ip ion Figu e 1 shows he schema ic illus a ion o a double laye ed compound g a ing (DLCG) s uc u e made o sil e (Ag). The geome y o he DLCG s uc u e is depic ed by he pe iod (Ʌ), he g a ing hickness d1 and he lamella wid h 1Ʌ ( 1 is he illing a io, 0 < 1 <1) o he SG laye , and he g a ing hickness d2, he lamella wid h 2 Ʌ ( 2 is he illing a io, 0 < 2 <1) and he lamella wid h Ʌ ( is he illing a io, 0 < <1) o he CG laye . The inciden ligh including ans e se magne ic (TM) H o ans e se elec ic wa es (TE) E a els h ough a ee space wi h an o ien a ion de ined by he pola angle θ be ween he wa e ec o k and he su ace no mal z. No e ha H and E deno e oscilla ion di ec ions o magne ic and elec ic ields, espec i ely. Fo he one-dimensional g a ing shown in Fig. 1, he elec omagne ic ield is independen o y-axis because he wa e ec o s o all di ac ed wa es lie in he x-z plane, and hus, he e a e no exci a ions in he y di ec ion. In his s udy, he TM wa e is used o analyses because as demons a ed su ace plasmon pola i ons and magne ic pola i ons could only be exci ed when he magne ic ield is in he y di ec ion. These esonances a e solely o al oge he coupled esul ing in enhancing he abso p ion o s uc u es [22–24,28,29]. The Ag base below he g a ing is assumed o be hick enough as an opaque, and acco dingly, he ansmi ance is equal o 0, and he abso p ance (α) can be compu ed om one minus he e lec ance (R), α = 1 – R, whe e he e lec ance was calcula ed by he igo ous coupled-wa e analysis (RCWA) based on Ma Lab p og amming [30]. The esul s ob ained om he RCWA and Comsol Mul iphysics we e alida ed wi h hose in he p e ious pape s [23,28]. I has demons a ed ha simple deep g a ings signi ican ly enhance he abso p ance o g aphene [22,23], and hus, his manusc ip i s ly ep esen s an op imiza ion o he high abso p ance o SG and CG s uc u es, and based on ha he cons uc ed g aphene-co e ed CG s uc u es a e hen p oposed wi h dual band abso p ance nea ly uni y in a wa eleng h ange be ween 0.8 and 2.1 µm. Vol. 27, No. 21 | 14 Oc 2019 | OPTICS EXPRESS 30183 In ou calcula ion, he op ical p ope y o Ag is caompu ed using a Lo enz -D ude model [44,45] while he dielec ic unc ion o g aphene is desc ibed as 0 () 1 /( ) s i εω σ εωδ =+ [46] whe e σs, ε0, and ω a e he shee conduc ance, he acuum pe mi i i y, and he angula equency, espec i ely. The shee conduc ance σs (σs = σD + σI) including he con ibu ion o a D ude (in aband) e m σD and an in e band e m σI is desc ibed heo e ically and expe imen ally in [47]. Based on he in aband and in e band exp essions [47], he pa ame e s a e used o he calcula ion such as he chemical po en ial (µ = 0.3 eV), elaxa ion ime (τ = 10−13 s), he empe a u e T = 300K, and he g aphene hickness σ = 0.3 nm. I is no ed ha wi h he condi ion o kBT  μ, he g aphene conduc ance is cons an (σs = σI = e2/4) in he isible and IR egion [47]. No e ha he p ope ies o Ag a e calcula ed based on he Lo enz D ude model ha was cons uc ed by Rakic e al. [45]. The e a e some easons using his model. Fi s ly, he pa ame e s o he Lo en z-D ude unc ion we e i ed in a la ge ange o ene gy om 0.125 eV (o wa eleng h o 12.4 μm) o 6 eV (o wa eleng h o 0.2 μm), and he da a was abula ed based on expe imen al wo k o ou esea ch g oups [48] and was also published in [49]. Secondly, he unc ion used o modeling he op ical p ope ies o 11 me als was a lexible and con enien selec ion o simula ions and op imiza ions. Las ly, he da a was alida ed o i s consis ency o he op ical cons an s based on calcula ions o a elaxa ion ime [50]. The esul has shown ha i s elaxa ion ime was cons an in he ee elec on egion while o he da a displayed an uns able end [50-52]. Fig. 1. Schema ic illus a ion o he double laye ed compound Ag g a ing (DLCG). Thei geome ies a e de ined by he g a ing pe iod Λ, he g a ing hicknesses d1 and d2, and he lamella wid hs 1Λ, 2Λ, and Λ ( 1 is a ied illing a io o SG, 2 and a e a ied and ixed illing a ios o CG, espec i ely). The ans e se magne ic wa e (H) (pa allel o he g a ing g oo es o y-axis) is inciden on he g a ing wi h a wa e ec o k and an angle θ 3. Resul s and discussion To op imize a CG abso be , we i s simula ed a SG s uc u e comp ising an Ag g a ing on op o an Ag subs a e wi h a ixed g a ing pe iod (Ʌ = 400 nm) and a ixed hickness (d1 = 200 nm) as a unc ion o wa eleng h λ and he illing a io 1. Then, we calcula ed he SG s uc u e wi h he same d1 o 200 nm and he illing a io o be ound wi h a high abso p ance ( 1 = 0.95) as a unc ion o wa eleng h λ and g a ing pe iod Ʌ. Finally, we simula ed he SG wi h he ixed pa ame e s including Ʌ = 400 nm and 1 = 0.95 as a unc ion o wa eleng h λ and g a ing hickness d1. Resul s ha e shown ha he SG ea u ing he p ope y o a deep g a ing wi h he illing a io o 0.95 exhibi s high abso p ance in a wa eleng h ange om 1.6 Vol. 27, No. 21 | 14 Oc 2019 | OPTICS EXPRESS 30184 o 1.8 µm when he pa ame e o Ʌ and d1 a e ixed. The esul was also ag eed wi h p e ious s udy on deep me al SG s uc u es [22,23]. When he pe iod o he deep g a ing was a ied om 200 nm o 800 nm, he abso p ance spec um shi ed om he wa eleng h o 2.1 µm o 1.5 µm. Simila ly, when he g a ing hickness was changed om 50 nm o 800 nm, he high abso p ion was ob ained wi h di e en hickness d1. Fo example, a single- equency abso be wi h a high abso p ance a a peak wa eleng h o 1.1 µm o 1.6 µm could be c ea ed wi h a hickness o 400 nm o 600 nm, espec i ely. Finally, om he ob ained esul s a single SG could no o e a dual bandwid h and maximum abso p ion al hough i was able o o e a high abso p ion based on he cha ac e is ics o a deep g a ing. Acco dingly, a single CG on op o an Ag subs a e consis ing o wo lamellae wi h di e en illing a ios ( 2 and ), he g a ing pe iod o 400 nm, and he g a ing hickness d2 o 200 nm was in es iga ed. In e es ingly, he maximum abso p ance o he CG was achie ed in a wide ange o 2 om 0.05 up o 0.50. No e ha he single CG g a ing o be simula ed in he p esen s udy includes wo g a ing pe iods wi h a ixed lamella o 180 nm ( = 0.45) and ano he is a ied ( a ied 2) wi h a condi ion sa is ied o be a deep g a ing. F om he op imized esul s o he SG and CG s uc u es, i can be concluded ha he double laye ed compound g a ing made o a single SG and a single CG on op o an Ag subs a e was sui ably selec ed o he design o dual bandwid h pe ec abso be s. Fig. 2. Abso p ance (α) con ou s o TM wa es a no mal incidence o he double laye ed compound Ag g a ing (a) wi h he ixed pa ame e s including Λ = 400 nm, d1 = 200 nm, and 1 = 0.95 as a unc ion o wa eleng h λ and 2, (b) wi h he ixed pa ame e s includes d1 = 200 nm, d2 = 200 nm, 1 = 0.95, and 2 = 0.45 as a unc ion o wa eleng h λ and g a ing pe iod Λ, (c) wi h he ixed pa ame e s includes Λ = 400 nm, d1 = 200 nm, 1 = 0.95, and 2 = 0.45 as a unc ion o wa eleng h λ and g a ing hickness d2 Figu e 2(a) exhibi s he abso p ance o TM wa es a θ = 0° o he DLCG wi h he ixed pa ame e s including Ʌ = 400 nm, d1 = d2 = 200 nm, and 1 = 0.95 as a unc ion o he wa eleng h λ and he illing a io 2. I is no ed ha he pa ame e s such as Ʌ = 400 nm, d1 = 200 nm, and 1 = 0.95 o he SG a e ixed since he op imal abso p ance is ound o be maximum as analyzed abo e. F om Fig. 2(a), i can be seen ha he no mal abso p ance o he DLCG is ob ained wi h a maximum alue in a wide ange o he illing a io om 0.1 o 0.45; howe e , a 2 = 0.45 hey a e wo dis inc peaks appea ing a wa eleng hs o a ound 1.17 µm and 1.7 µm. The illing a io, 2 = 0.45, is he maximum alue in a ange o 0 and 0.45 o be op imized o he DLCG, and he DLCG becomes a double laye ed simple g a ing (DLSG) s uc u e when 2 is g ea e han 0.45 ( he DLSG is no objec i e o his s udy). Fo he pu pose o designing dual band pe ec abso be s, om he esul compu ed in Fig. 2(a) he DLCG wi h Λ = 400 nm, d1 = 200 nm, d2 = 200 nm, 1 = 0.95, and op imal 2 = 0.45 was selec ed o a de ailed in es iga ion since i exhibi ed dual band abso p ance. Figu e 2(b) illus a es he abso p ance (α) con ou s o he DLCG o TM wa es a no mal incidence wi h he ixed pa ame e s includes d1 = 200 nm, d2 = 200 nm, 1 = 0.95, and 2 = 0.45 as a unc ion o wa eleng h λ and g a ing pe iod Λ. I can be seen ha when he g a ing pe iod Vol. 27, No. 21 | 14 Oc 2019 | OPTICS EXPRESS 30185 inc eases om 200 nm o 800 nm, he dual abso p ance peak o he DLCG shi s o sho wa eleng hs. Meanwhile, Fig. 2(c) shows he no mal abso p ance o he DLCG wi h he ixed pa ame e s includes Λ = 400 nm, d1 = 200 nm, 1 = 0.95, and 2 = 0.45 as a unc ion o wa eleng h λ and g a ing hickness d2. The ea u e o he abso p ance spec um shown in Fig. 2(c) has wo bands while he e is only one band o a SG s uc u e (no shown he e); acco dingly, wi h di e en g a ing hickness he abso be -based he CG could display a dual band abso p ance spec um in a sho and long wa eleng h ange. The e o e, he op imal pa ame e s o he DLCG including Λ = 400 nm, d1 = d2 = 200 nm, 1 = 0.95, and 2 = 0.45 we e selec ed since he DLCG p o ided a dual band pe ec abso p ance wo king in he nea IR egime. Fig. 3. (a) Schema ic illus a ion o he designed DLCG s uc u e ea u ing geome ic pa ame e s as same as hose in Fig. 1 wi h a g aphene o e lay; and (b) No mal abso p ance spec um o he DLCG wi h and wi hou a g aphene o e lay Figu e 3 displays he p oposed DLCG wi h he g a ing pe iod (Ʌ = 400 nm), he g a ing hicknesses o he SG and CG laye s (d1 = d2 = 200 nm), and he illing a ios 1 = 0.95, and 2 = 0.45 wi h a g aphene o e lay ha ing a hickness o 0.3 nm. Figu e 3(b) illus a es he abso p ance spec um o TM wa es a θ = 0° wi h wo esonances, MP1 a λMP1 = 1.73 μm and MP2 a λMP2 = 1.09 μm co esponding he undamen al mode and he second ha monic o magne ic pola i ons, espec i ely. The highe o de o MP is, he peak abso p ion inc eases [19]. I is e ealed ha he abso p ance peaks a λMP2 = 1.09 µm and λMP1 = 1.73 µm in Fig. 3(b) a e enhanced up o a maximum alue o 1.0 when he DLCG is co e ed by a g aphene shee . In e es ingly, abso p ance spec a o he DLCG o be simula ed wi h di e en g a ing pe iods (e.g., Λ = 600 nm and 800 nm) and ixed o he g a ing pa ame e s showed ha hey simila ly ea u e wo modes o MPs and hei spec al bandwid hs emain unchanged al hough he peaks shi o sho e wa eleng hs ( he esul s we e no shown he e). Figu e 4 shows he abso p ance con ou s o he DLCG and g aphene-co e ed DLCG s uc u es as a unc ion o wa eleng h λ and angle o incidence θ. I can be seen om Figs. 4(a) and 4(b) ha he abso p ance a he sho -wa eleng h peaks (λMP2 = 1.09 μm) o he wo g a ing s uc u es is ound o be app oxima ely maximum in a wide ange o inciden angles om 0 o 60°. On he con a y, he abso p ance o he CG a he long-wa eleng h peak (λMP1 = 1.73 μm) shown in Fig. 4(a) is no high in a na ow ange o inciden angles while ha o he g aphene-co e ed DLCG shown in Fig. 4(b) is ob ained o be nea uni y o 50° and dec eases o 0.7 o 70°. Gene ally, wi h a g aphene o e lay, he abso p ance a dual peaks o he DLCG inc eases o he maximum alue o 1.0 and is insensi i e o a wide ange o angles o incidence. Vol. 27, No. 21 | 14 Oc 2019 | OPTICS EXPRESS 30186 Fig. 4. Abso p ance con ou s o (a) he DLCG and (b) g aphene-co e ed DLCG as a unc ion o he wa eleng h and angles o incidence To manu ac u e he p oposed DLCG s uc u e wi h such geome ies, one could i s ly ab ica e g aphene using chemical apo deposi ion on a coppe oil and secondly e ch he deep ench (g oo e) g a ing s uc u es based on a c yogenic e ching me hod [53–55]. To be de ailed, he ab ica ion p ocess s a s by pa e ing g a ing lines on a polyme esis laye on an Ag subs a e using nanoimp in and hen using ch omium shadowing and eac i e ion e ching (RIE) o e ch he esis laye o o m an e ch mask. Con inually, deep enches o he g a ings a e c ea ed by he c yogenic RIE, and he mask ma e ials a e ianally emo ed by a plasma cleaning p ocess. P e ious s udies ha e demons a ed ha his cos -e icien manu ac u ing me hod was success ully implemen ed o c ea e deep enches wi h a deep wid h o wall hickness a io (aspec a io) up o 40 [53] and a deep ench wid h o 20 nm wi h an aspec a io o 8 [54]. Figu es 5(a)-( ) desc ibe he magne ic and elec ic ield dis ibu ions a he esonance wa eleng hs o magne ic pola i ons (λMP2 = 1.09 μm, λMP1 = 1.73 μm a θ = 0°) o he DLCG wi h and wi hou a g aphene o e lay. In Figs. 5(a), 5(b), 5(d), and 5(e), he colo con ou ep esen s he magne ic ield and he a ows indica e no malized elec ic ields. Figu es 5(c) and 5( ) show he no malized magni ude o elec ic ield o he DLCG a λMP1 = 1.73 μm and θ = 0° while he inse igu es display he magni ied elec ic ield a he opening o he g a ing enches. I is seen om Figs. 5(a), 5(b), 5(d), and 5(e) ha he s ong magne ic ield aps inside he enches (g a ing g oo es) o bo h s uc u es wi h and wi hou g aphene o e lay; howe e , he magne ic ield occupies in he longe ench s onge han ha in he sho ench. In addi ion o ha , he magne ic ields a λMP2 = 1.09 μm apping in he ench much mo e han ha a λMP1 = 1.73 μm; his esul s a highe abso p ance a he highe o de MP as seen in Figs. 3 and 4. As an illus a ion shown in Figs. 5(c) and 5( ), he elec ic ield simila ly aps inside he enches o he bo h g a ing s uc u es. I can be seen ha he elec ic ield shown in Fig. 5(c) aps mo e inside he ench o he DLCG wi hou a g aphene o e lay while ha o he DLCG co e ed by a g aphene shee shown in Fig. 5( ) concen a es less. Howe e , aking a close look a he g aphene o e lay on he g a ing opening as seen in he inse igu e o Fig. 5( ), one can see he elec ic ield ocsila eing s onge a he in e ace. As a esul , his makes g aphene abso bing mo e ene gy ha is also well-ag eed wi h demons a ions in he p e ious s udies [22,39]. Vol. 27, No. 21 | 14 Oc 2019 | OPTICS EXPRESS 30187 Fig. 5. Elec omagne ic ields o DLCG s uc u es o TM wa es a no mal incidence: (a) and (b) he DLCG wi h and w/o g aphene a he peak λMP2 = 1.09; (d) and (e) he DLCG wi h and w/o a g aphene shee a he peak λMP1 = 1.73 μm; (c) and ( ) he colo indica es he no malized magni ude o he elec ic ield o he DLCG wi h and w/o a g aphene shee a he esonance wa eleng h λ1 = 1.73 μm. The colo in Figs. (a), (b), (d), and (e) show no malized magni ude o he magne ic ield, while he a ows indica e he no malized elec ic ield. The inse igu es in Figs. (c) and ( ) show he enhancemen o elec ic ields (blue spo s) a he opening o he g a ing ench. Gene ally, Figs. 5(a)-(e) e eal he magne ic pola i ons (MPs) o be exci ed in he DLCG wi h and wi hou a g aphene laye . The MP is a s ong coupling be ween a magne ic esonance inside g a ing s uc u es and ex e nal elec omagne ic wa es. As shown in hese igu es, he ime- a ying magne ic ield pa allel o he y di ec ion gene a es a closed cu en loop a ound he enches in he s uc u es whe e he s ong localized magne ic ield is c ea ed, and i hen o ms he MP (i is known as Lenz’s law). In addi ion o ha , he e is no occu ence o su ace plasmon pola i ons based on he elec omagne ic calcula ion shown in he abo e igu es. The e o e, he enhancemen o abso p ance in he g aphene-co e ed DLCG s uc u es was caused by he magne ic pola i on exci a ion wi h wo modes o esonances. 4. Conclusion The manusc ip has p esen ed he simple design o he no el plasmonic s uc u e, namely he g aphene-co e ed double laye ed compound Ag g a ing wo king as a dual band pe ec abso be . I s abso p ance was ound o be 100% and i s dual band spec um was insensi i e o a la ge ange o inciden angles om 0 o 70°. The physics behind he DLCG wi h he g aphene o e lay was demons a ed due o he exci a ion o he magne ic pola i ons. Fu he mo e, he p oposed s uc u e easible o be manu ac u ed p o ides emendous applica ions o designing mul iple band pe ec IR abso be s based on g aphene-co e ed compound me allic g a ings. Vol. 27, No. 21 | 14 Oc 2019 | OPTICS EXPRESS 30188 Funding RMSTC (CZ.1.05/2.1.00/19.0387). Acknowledgmen s The au ho s a e much g a e ul o he inancial suppo s unde he p ojec No. CZ.1.05/2.1.00/19.0387 “De elopmen o esea ch and de elopmen basis o RMSTC” wi hin he ame o he ope a ion p og am, Resea ch and De elopmen o Inno a ions, and he S uc u al Funds and he s a e budge o he Czech Republic. Re e ences 1. K. R. Ca chpole and A. Polman, “Plasmonic sola cells,” Op . Exp ess 16(26), 21793–21800 (2008). 2. J. N. Munday and H. A. 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