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Unformatted text preview: Reproduced with permission. Copyright {5) 2001 by Scientific American. {no AH rights reserved. Scientific American website: httpzilwwwsmamcem Vi” WAS WE SECEBNQ EXésSPERANNG PHBfiE QALL ”€8,537? MB RECEWEQ. Yet amther group aftheoris’cs was sagging that mg discoverg did act mark. That was distressing. I had spent three long years trying and discarding com ntiass desigas to arrive at what I thought was success, but if the theerists were fight, 3 had tan g0 back Is the lab and continue searching. And maybe what i was trying ta create-w an artificial crystal structure that could manipuiate beams of Eight ia 1“th same way that siliccm and ether mmimnduc- tars centmi alectriccurrems-wwas :30? possible at all. Emmi: scnficonciuctors, of conga, an? at the heart of all the computers ané ozhez devicm that Wade the gicbai 2cm— omy. Semiconductors 0f light conici iead the infmmatian and teiecommunications 48 SfiEEfiTiRC AMERICM {avoiation stiii fuxther by enabling fiigher- Capacisy eptical fibers, nancswpic lasers anti phamnic integrated circuits that might we day regime amiayss microcfiips. indeed, despite a tacky start in the Eat: E9895 and much skepticism fram tine photouics {esearch communizy 332:1}? an? the {mid oi: photoniir crystais has thrived. Amund the world many researchers {in- Ciuding me} have foxmded Companies that are developing commercéai products. TEE key was proving flax? skeptics; wreng by shcwing that it was possébie :0 create {2"}: light the same kind if phemamemm gem in eiectmnic semiccarzduc:or3~—namega a SQ—caiied band gap. The éiemrmiz: band gap is; a forbiédm zone, a narmw range 0? energieiz that e16:— tmns saunas: Oecupy. Whéfi the electrons in the semiconfiucmr EH 2111 the wares available to them beiow the band gap? electric current: cannot flaw, becausa each deciima has nowhcre :0 go. Brmgting. 322 electron abevc: thc gap takes :3 in: (if err ergy. I? {has arr: a few exceisa cicctmns, hawweg the}! aummazécaiiy must 5%: above the gap, when: {My (33:: casiiy mam {hmugh the wide (3pm spams 0? empty smms. Simiiafly, a deficit mg cicctrims Opens up some positiveiy chasgfid “bales” beigw the gap? again pmviéing a way ft}? Cflfffifli :0 flow raadiiy. AB 25% magic of semiconductoyswthc switching; 313$ lfigic functifmsmcsmaa abam item cummfling the avaiiabilirjy' <3? eEezstmns and Emir-:5 aimw: and izcimv this Emmi gap. The existenm Emci properties (3? an deitmnk Emmi gag} éegend cmcialiy fiEfififiBEfi 2381 CLSMHESY (1F EL! YASWSQVITCH (pmmmng pagw} a v; a; (I QMCS {(8}! arm rem SPUCAL FRBERS can use the 5320:0113: bandgap grintiplew guide light. The ctadding 0f severai hundmfi sisica capifiafg tubes farms an spficai band—gap matariai that canfines fight in» we camera! haie, which is 3239222 15 micraas "m diameter Heft} in the design at thee right, Sr: whécb the light 35 (m the E3336: (3f moms in the matsriaE and {Mir :rysra§ fitrucmramthe spacing and shape a? flu” ianicc that they form. By 5:: b— srimzmg various' other moms {cailéd depamg) mm the iaztice or its interstices, engineers can dictate the number a? eke» {runs or hoies in the semiconductor and thereby miim its pmpcrzieg. in siiiwn and gather scmicm‘éducwrs, adjacsm grams are separated by about a quarter of a nanometer. Phamnic baniiw gap materiais inwlvc simiiar structures; im: a: Earger scaies. A typical exampis \muid be a Muck of 5peciai giass driiied thmugiz with a clams: spaced array of cyiindrica} E30163, mah with a dimmtez‘ of 400 nanometers. These Opmings are swab ogous m the atoms in a semiconducton k1 general, but no; aiwa ys, the spacing (3f the array must: he rcascsmifily aims m {he wmeécngrh of the fight (12' the cicammag neric wave% to be CGfitj‘Gfifid. Visibie iigh: has waveémgrhs ranging from about 400 to 70$ nanometers; many 02% phones use waves around 35 cemimcrers Kong. [3ng emering the hoicy materizfi will mfract through 3m} psrriaiiy rrsfiecr off the myriad interim} intedaccs between air and giaxs. The complcx pattern of Over— flapping beams wilE rxziaforcc or cancel {me annshér 0m awarding m the lighfs wavdeugth. its directim 01:1?an through the crystai, the refractive index of the $3553, and the size and an‘angemszm 0f 23H the inks. {3erfeci cancdladcm in 21%? (ii~ rccmms £0: :2 narrow hand a)? wave— ma Amman imgths is iikc: the band gap 50? eiacmma in semimnduczms: that band of Eight mam): pmpagate through the Crystai‘ Moéifymg the band gag structurew—Rfi insta me, by fiiiing 502m hmesmpméucss other effecra simiiar so what can 33»: done by deping {siécrmnic semiconductors. 0?? mm 3 phottmic crysmi i5 madr: 0f an em:- ironic semicondacmr mazcriai, and am the crysra? has bath 2m eiectmnic bane? gap anci 21 phamnic Emmi gap‘ 508,889 Hoies 3341:: QBEST far a phomai‘c band gap miginamd quietly enough in 1987 with two imiependem pmpmsals submitted for publécmiizm inst two monthg apart. one by me am} the mixer by Saieev john, {hen at Princmcm Univemity. W72 had {was very different goais in mind. E was at 1363! Cam» munications Research, {he {ciephxme re- search consortium in New Jamey, and, I was making m makiz miecammunicationx iasers more afflcient. Most of tbs ciccrric current cuxzsumeé m madam: iasing war; wasted as spt’amaneous light amisséim, and the photonic band gag amid sup, press {hat Wk‘ififfi: amms cmmm sp<3nm~ aeoxsiy emit Eight what may are gum Of confined ta the twemicrsn salid care, the fiber is highig noniirzear‘ which can be useful far switcéfing and shaping Sign: guises. an {ha 52mm, 3 pattern of coiers iiiustrates how {he canfinemem pmgeng m‘ a bandwgag: fiber varies fer d¥fferem wavsiengths of iighz‘ a mazerial that forhiés fight pramgmizm. john in contrast, was pursuing; a pure research gcai‘ He proposefi rim phw tonic band gap :0 cream what is knew: 35 fight imaiizaticm. Tbs Electmnm am- k)gt3€ 0? this phmmmcmm, a quanmm e€~ fem caiiefl alertrma iocafimzium (mum; in disordered maieriais such as amorphmux semfiizcmducmrs‘ The éisordcr maps; 0? imalizess, eEethns in fixcé imazicms, oi» stmczing carrem 80w. 30%;} and E had mare; mm. but when we learned of each other’s propusai, we were curious enough to atrange 2: gm» acquaimeé lunch We thanght we were 0mm samething, and we agmeé m use the same termmoksgy: “phomaic band gap" and “phmonic crystai.” I {calmed to my is?) maker {)verconfidem. I rmmght iimt E m§ght create the firs: workmg mode? Wfihifi (mi); :1 153w momhss. Alrh('3ugh “phoumic” refers m Eight‘ tbs priacipie of the baxxd gap apgflies equaily wail m eiectmmagnetic waves; of 3% waveisagths. szscquentiy, i siméd maké trig} crysra} struccures ’5th 3:1}; mm vcniem mw spacing 33153 size 2131;! {has wast than} with eifictmmagnezic waves 0f the appropriate wavelmgth. Indeed, i began EL! YAELQHQWTEH was an invenmr cf the phsmfiic band—gap cancepz and made the first phatanic band-g2? crgsta! whiie at Befi Communicaticns Research in flew 39:389. 3a 3.5392 he maxed w the eiectrical enginesying degartment at the Baiveysitg 3f Caiifaméa. Les Ar;- gates: where he iaaés the amoeiemroaics gmup, He is a {gander sf (was Compafiies in the burgeaning field sf phstcnic crgstais: Ethertronics and {.uxteraé Befnre he became a few? tg member, Yamaha‘s/itch had emugh time tn saii racing swaps, SE§£NI3F1{ Amgastm 4% my quest for a phmxmic band—gap mate— ria} in a machine shop, carving Stu-names out: of éieEectric plates with a chill. Only human imagination iimited the crystai d§~ Sign anti structure. Therein lay 3 probicm, however, On: '0? thzz inflammable choices avaiiabie, which design W0u§d pmdace 27: photonic band gap? in {$130301} 3:: semiconducxm crystals, the band gap arises because electrons bw haw part3}; Iike a wavg and the waves scattér off the iayers or rows of mm. Rm: cf the waw scattttrs back the way if same? anti if the wavelength is about the same as the Spacing of successive iayme, 33% the backscattercd waves mid 21;} ex» fiereniiy. Consequemiy, {he {:Eecxmn’s waw is refieamd back caingletciy, iiki‘: iigh: himsgg 1x mima For 2. M1 haw gag, ziais perfccz‘ refiectim: mast (mm: (we? 2: range a? waveimgrhs and fm‘ waves heat?» 33;: in an}; éirecém: :hmugh {he tz'ysmi. physicai intuition as micuiativns, my cow workens and i buiit structure after strucw cure, searching for the rigixt mm. In rim course offimr years, my ioyai machinist, john Gum}, driiied more than 500,088 holes ia diéiecrréc {insuiat’mg} pines, ad~ mkmdiy assisted by a numericafiy can» {relied machine. it became Hamming as we produced faiéure afmr faiime. Eye Surprise of Biamand ma Ex vmi‘rm the face~center€d cubic {fee} gtmcmre :0 be particuiarly favor» abie for making alectmmagneiic ham? gaps. Yam can buiié this armature by tak» mg; a checicerboaré and placing a black cube can ask white square. and a white (me an each £31216}; square. {)1} {he seamd iayev, {:{mfinae pfiacixg black cuhas {m white 3113 vice versa, am? 30 rm up. The Mack cums Liam? 3693:3163}? aka tin: whim {mm} farm an fee iattécc. that {)f any Exisiing {rangparem material. \Vithin weeks, ium'evar, {th iowa State group found that; the ciiamond structure, {he tetrahedmi cryszai grams:- tz'y aswciamd with the prficimzs imwi, woaid pmduca :1 33am? gag Tim? farm {hat giws the whites: hind gap Ci'maisfs <3? dielzsctric rods in the pcsitians 0? fix: chemica} bonds between carban amnm with the moms sizrunk m gmmct‘ric paints. ‘Diammnd itseif is :20: a phomnéc band~gap margarita; as fa: as we kaow. Eariier in this pieate i said rim: when me began {32.x research, we knew me maid fiat simpiy cmuiate this SmCiHE crystai structure it: grodnce a pimmuig Emmi gag. How wrung, we were: ssiiéccm’s mys- mi structure ig precigeiy that a? ééas’mazéd. "53%: {ha {arahedrafi sanctum is size best far making a phimmic ipzmd gap £3 staniiug and gpmfcmraé. 86%;”{2 {Eras aaivcm of pianwmic m'ysmisx the dimmmé can in; Unlike tattices efatams, ghatm‘g éfi grggm ES have structura? passéb co- ilataes iimiied 0mg by the human imaginatioa. @539 S $33 $58 can be sculpted ax the lattice sites. 8):" m3 eleammagnetic {mad gap‘ i knew one timid not simpiy émuiate a sii- icon crystal. $01“ fight, this scattering; is caused by changes of refractiw: index {fur instance, bezwceza air am? giass}, and an mmraction éirecfiy aaalogeus m {3160 trons and silimn storm wouid [squire a mareria§ wifil 2m extraorciinary refractive, index. Nor comic? one simpi’y deduce: a smac— mre {mm theory: the band gap depends on how rht‘: waves interact wizh many hundreds of huies, a very complicated process. Theorists haé devebpcd com~ purer models for (icing the caEcuIatixms Em semicandncmrs, bait these gregmmg maid was he mad for pixmms. first, the (:4; madam of motion 21:6 fifferemmSchrié» ding€r’s equation governs electmna but Maxwefi’s‘: flqzzatims ckssfibe the beham iar'mf Eight, Second, with phomns one cannot, safeiy :zegiect mimimfimz {he way am: can with ciectrtms, (kartsequemiy‘ i haé :20 way to {iemrmifie whether a pm» posed structure would have a phmanic band gap. And 50, guided as much Say 50 summit: mgmgm That sanctum stiii haves 2m iafinim variety of chaims becauss yet: car} with 313th any oihm‘ geometric 53322198 for rim biack cui‘res, which 3km how {fie light waves wii‘i bi: refracted and refiecteé. A19 {er {we yfiars? WE arrix’ec‘i at Sinmazhing that SECH‘IC‘d to work: an fcc structure Ea which each biack tube was replaced by a spherical mid in the material. I pubiished this result but 3' was mistaken 33’ now the theorists had started it) camh upQ and a few of them had rcmoEed {Emir band—«strucmrc computer programs {Q werk with fight. Stvcmi {Mary groupg, fixiuding {11052 32215 in; K. Mingfwung of Poiymchnic {Enivcrsity and Kai Ming H0 {31‘ Iowa State Uglivcrsi‘fg, began making {E1056 dreaded phcme c3315. My 2011;;— sought fcc sanctum had 923135 a pscucia- gap: 21 {orifiddcn “hand” having mm width” meaning that inst one {mu wave— iengfl: 9f Eight wag fcrbiéden. After our 3:33:33 0:? effort, it appeared {haz nature migifl not parmif a phzmmic band gap :0 axis: at aii. i’crfiaps it rcqaimd a snip 3m :30: with a rcfmczivc index, F3: keyimfl uratimz was mat-2%.}; az‘xzashcr minm’aé simcmm, arising (mi (.3? 21 cumpkzx $3,:ch play an" amms, chemicai honcés ami may" g}? minimization under suitabie mndim {inns uf tezzmeratme and measure. in; mi? izy for §orn1§ng :1 phoumic band gzm‘ which emerges entirfiiy and mid}; §mm .Pv’iachii’s equations {the iawsa of elec— tricky, magnezism and light}, shmwg that {he {1533103113 crmfiguratims aiscz has fun» damenmi s§gnificancc in {simian m sim— tmmagnetism and the gcmnczry (,2? three— dimsmicmai space. fiianwnd’s terrahedmi szraicmm takgs (3:: many éificrent appearzmccs accnrdiag m whaf sham: is pissed £21 each Earthy: size and fimm which angk the crysmi 89 viewed. The bin: an the apposim page in- ciuéss two very dissimiiar phatoniz crys- tzfis {he}: are based {3:} rim (figment? SUfiC- tm‘e, My grim}; made {31:3 fifsi succcssfm thouic bzami«gap crystai kins; mm: for rmi} in I99] using; a variant :3? {he dizz~ mam? structure flow tafizsé yaifionavim. Nature is kind firm 213%: :1 bané gap mmg‘é; in the cziiammfid strucmm fur a fairways BEHMBER 2081 ELEM 5711‘ MS MAKWG BAND GAPS 1N ALLBtMENSIONS {IRE mMENSiBR { ~ 3 ma wavamsm m BAND BA? r , A wave incident an a bandwgag materiai {13 yaniaéiy refieas eff eaizh iager a? the 51111::er {21m reflected waves are in amass: 33d reinforce {me anather. They sunshine with the incidem wave to produce a standing wave {31mm dues mt nave: through the maxeriai. 1 iNCIQEN? WAVfi “ELECIRSC SLAB /"'""‘”\ [fix / R / mW € 2 REFLECKB WAVES iN 9HA$£ AWN ”a (1/, ’ ( \y’ 3 PG? WAVELENGTR N8? 3‘3 BAND 8A? At a wavelength outside the band gay {13, the reflected waves are cut af phase and cancei nu: ane anemia: {2}. The tight prepagazes through the materiai onig siiglmg attenuazed {3}. 1 §NCIB£I~ET WAVE v” 3W8 EMENSIQRS for a {we-dimensional band gap, each unit cefi of the structure {1} prsduces refiected waves inn: Shawn} aad refratted waves that must cembéae $0 cancel out the incoming wave {2} no matter what diremian it is traveling [33. A full three~dimensigna§ band-gap \: ‘ 3 wrksthe samewa butin afiithree ctimensians. “‘3 ‘ g » THREE ammsxuas 1 i g Biamand's xetrahedrai casfigurafloa {13 is ‘g i me mas: effective geamexrg far amazing three» j. dimensimfl hams-gag: mazarfia’fis, This gecmwg i scams én éEsgaised form in gabionsvite {see pages 45 and 4?}, the “staxzk nf iogs" £2}, anti zhia design {33‘ wrfich uses sfiima Giaxide chanmfis {light} 4 in sister; {dark}. The scaffoid simmers i4} i3 1 a rare examgle that has a different andefiging 1 sgmmatm, but it has enig a smaii hané gap, ixzciex as smaii as 1.8?) and many optics} materiais are avaiiabic with refractive it» dices as high as 3.6. "i‘hs diamcmd structure isn’t: tbs uni}: stmcums having a photonic band gap, in 5992 {heatissgiaseph W. Hausa then a: Rmsseiaer Peiytcch nit: Insrimtc, showsci rim: Wfl had discarded the fcc strucmrcs too quickiy. Scientists had searched the fix simctsres for band gaps only at wave— imgths for which aisom half a wave firs srfsi wings and in, the hairs of a worm» iiice Creature caiieci the sea mouse. Each of tiaese has a phnmnic band stmcmre, though no: a fuli bans gap, in that fight can sail pmpagate in some éirectitms. A compiete band gap has causes Cami-cg psrhags because it requires mo much rc~ fractive—index centrast. {\irzwrii‘teisss3 an incompiete sand gap cm} b6 wry usefui. For example} ii~ {311mm dioxide psfiiciss smaller than a Emsgrstsd Ci i‘fiiii tsthammbine Canventionsi ieatmnics and photonic crgsiai s wouid represent the sit tima‘ie iimii of @giggififiii‘ififiifi in (mi: Ceii of the {3:13:13 {somewhat iiiu“: {hrs fundsmemai vibrsmm {)f 3 guitar striizgi. As we saw»; oniy 21 swuflogsp oc» was at that frequmcy. Haas hmwveg aiso ccmsidered a higher frsqnency, for which a fui! wavelength fizs is a ceii (803245th Bios the first hafnium: sf the guitar string}, and prams shat an fa: band gap would indeed emerge them In adéitian, he ciismveméxhat war: the Sitar pie cubic configuration imam"; as the scaffoid structure: {it}? izs simiisrity m scaffoiciing) maid have a band gap, aibsit a smaii mas. Butterflies and Microchips Wi? HAVE“: NCHX’ LEA RN31) Char 11312392 already makes piunonic crystais in rim sparkling gsm opai, in a fimcrfifs c010 swims; muss: , mamas sssiasss, S4 SCiENTSFiC AMERIU‘N micmfi can be maés t0 ssifwsssemiaie in {he {)in structure. Titanium dioxide: is $18 éntenssiy whim pigment usmi in min: smi m msks papa white. The coi’mrest scattering of Eight“ that scams from Em 13d— gapstmamed titaaésm dioxide can im— parz mare whitesess far isss mass si ti— mnium diaxidsx One day phsmmc crys- {ais may be 31% around us in {he painmi wsiis and in the stack of paper clutter- ing, our desks. Anothey vary ascfui type {)f ismzw piers Cami gap materisi is {hat of two—(ii msnsiomi 'phsmnic crystais which cam kiosk Eight fram Cravsiing mm s pizzas. Such a structure can be stretched aiong the shim dimension, forming a new kind of optics? fiber. Csnvsmisnai Optical fibets have a- high refractive isdex as their neamyteie 3 3} banégag materia? usuailg patterned as dpai stmtiure ‘ , sandengas «minute cat: attract light verg efficisnflg {berm than 323%} (30%., which cxmfincs light by rats? Emmi” n31 reflccfim. Phiiis Si“. j. Russcii of the University of 23th in Engismi damn" strsssci in 1999 haw :0 main: phewsic band—gap fibers. is one versinn, iigiir Crawls slang a centmi iwic in {ha fibers confined there by dis mm-ciimensicmsi band gap 01‘ ‘ths ssrrmmding maserigi. Mme aptimi sows"? can be ssm Ihmugh such 21 centres} voiti than through glass, Cnsbiing greater informstiomcariying iiiiaiiii” Zfi‘iigii. capacity, psri’xsps 180 firms {has mi ms- vessiosai miscsmmsnicszimzs Shem. Spscisizg fibers have“ saivssccd size mas? as wmmsrcisi pimwnic is: névgsp was" acts. Cempazzies is {)cnmsrk am? the BK. have sircad}? dissributsé ssmsis quamitics and wiii 5mm begirz 31:33:1an productiun‘ insisad {3f stretching (ms :1 rwsueii mmsionai {mini-gag) strucsurs {0 make 22 fiber, am: Can go m the mixes sxrmme and make a mwdmmnsiansi rhiwifim ping,» minis crystal? as was flat csiwisrcd is $997 izy Simnhui Fan and join: it}. jazzmmposissa Chen bark at {he Masss chusetts Instimts 0f TCChnCiogy. Thin- fiim phoronic Crystsis can ix: easily psi; temsd in; standard mezhsds used to pmw Cities imegmmci Circuits huraducisg mms ’ Earigvgrsims tress mmerciaiizéfi'r 5 ,flemonstrateé; icmcost mamfiamuring ‘ mathmfs an der deveispmem r flemonstréted, bu! mast seminars with V timer-methods of achieving the sams gm! BEEEMBER 23$: RAWRAL mamas: BmflfiAf’Snccm in same bmterflg wings {fair} and én apals {right}. in Seth cases, We bané gap iséncompietew-it is net effectiva in every digectéanubut 5t pmduces iridescent comm Aminmgragah a? a defficis :0 a bandvgap strucmm is com- parabie m (ioping in 2213 fiectmnic Semi~ wnciucmr and ()penfi up 2: ms: range of magmas. One cxampie {3? a doipzmt ig the germ-3i huie in phmcnic crystai opt? ca} fibers. Simiiariy, plugging one of the bakes $12, :1 thimfilm crystai pmducss a criticai fiiemem of iasag nameiy a smaii “cavity“ that can hoié a local eieczrw magnetic nlodGHimaginc a iitfie stamiing wave {bf iigh: trapped between mirrors, ‘Rccentiy Axe! Sammy’s group at the Cal~ Emma Institute of "i”echnuiagy aseci thsse $313635: c'zf opiicai cavixics to make 333m; inst {3.33 cubic micmn if} minim, the smaficfit ever, 1’21 naming phmmsic crystai thin fiims mm (,mrical circuits weuki represent the ukimate iémit of (aptaeéectronic minim mrizaritm. Many researchers believe the? integrated circuits that camin’ne conven- rionai cicczmnics ami photonics stané ready to extend the integrated~circuir rev» niuficm into the domaira of highbandw width opticai sigmis. This 58k} (‘1fbrmd~ gap device deveiopmens wilf probably draw the must attentian in {he ncxt few years, but mmmercia} pmduczs an: stiii mo to {firms years away Yen might not expect electmmagner‘ 3c bum—gap crysmha £0 be an? much use f0? mafia waves, because excessively farge crystaEs thid seem to he reqaircd. C£§~ iniar ...
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