Graphene photonics and optoelectronics

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NATURE PHOTONICS | VOL 4 | SEPTEMBER 2010 | www.nature.com/naturephotonics 611

Electrons propagatng through the bdmensonal struc-ture o graphene hae a lnear relaton between energy andmomentum, and thus behae as massless Drac ermons13.

Consequently, graphene exhbts electronc propertes or atwo-dmensonal (2D) gas o charged partcles descrbed bythe relatstc Drac equaton, rather than the non-relatstcSchrdnger equaton wth an ef ecte mass1,2, wth carrers mm-ckng partcles wth zero mass and an ef ecte speed o lght oaround 106 m s1.

Graphene exhbts a arety o transport phenomena that arecharacterstc o 2D Drac ermons, such as spec c nteger andractonal quantum Hall ef ects4,5, a mnmum conductty o~4e2/h een when the carrer concentraton tends to zero1, andShubnkode Haas oscllatons wth a phase sh due to Berrysphase1. Mobltes () o up to 106 cm2 V1 s1 are obsered n sus-pended samples. T s, combned wth near-ballstc transport atroom temperature, makes graphene a potental materal or nano-

electroncs6,7

, partcularly or hgh-requency applcatons8

.Graphene also shows remarkable optcal propertes. For exam-ple, t can be optcally sualzed, despte beng only a sngle atomthck9,10. Its transmttance (T) can be expressed n terms o the ne-structure constant11. T e lnear dsperson o the Drac elec-trons makes broadband applcatons possble. Saturable absorptons obsered as a consequence o Paul blockng12,13, and non-equlbrum carrers result n hot lumnescence1417. Chemcal andphyscal treatments can also lead to lumnescence1821. T ese prop-ertes make t an deal photonc and optoelectronc materal.

Electronic and optical propertiesElectronic properties. T e electronc structure o sngle-layer graphene (SLG) can be descrbed usng a tght-bndng

Hamltonan2,3

. Because the bondng and ant-bondng -bandsare well separated n energy (>10 eV at the Brlloun zone centre), they can be neglected n sem-emprcal calculatons, retanngonly the two remanng -bands3. T e electronc waeunctonsrom df erent atoms on the hexagonal lattce oerlap. Howeer,any such oerlap between the pz() and the s or pxand pyorbt-als s strctly zero by symmetry. Consequently, the pz electrons,whch orm the -bonds, can be treated ndependently rom theother alence electrons. Wthn ths -band approxmaton t seasy to descrbe the electronc spectrum o the total Hamltonanand to obtan the dsperson relatons E (kx ,ky) restrcted to rst-nearest-neghbour nteractons only:

Graphene photonics and optoelectronicsF. Bonaccorso, Z. Sun, T. Hasan and A. C. Ferrari*

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility andoptical transparency, in addition to fl exibility, robustness and environmental stability. So far, the main focus has been onfundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, wherethe combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and thelinear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronicsis shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors andultrafast lasers. Here we review the state-of-the-art in this emerging fi eld.

E+(kx,k

y)=+0 1+4cos cos +4cos

23k

xa

2 kya

2k

ya

2(1)

where a = 3 acc (wth acc = 1.42 beng the carboncarbondstance) and 0 s the transer ntegral between rst-neghbour-orbtals (typcal alues or 0 are 2.93.1 eV). T e k= (kx,ky) ec-tors n the rst Brlloun zone consttute the ensemble o aalableelectronc momenta.

Wth one pz electron per atom n the *model (the threeother s, px, py electrons ll the low-lyng -band), the () band(negate energy branch) n equaton (1) s ully occuped,whereas the (+) branch s totally empty. T ese occuped and unoc-cuped bands touch at the Kponts. T e Ferm leel EF s the zero-energy reerence, and the Ferm surace s de ned by Kand K.Expandng equaton (1) at K(K) yelds the lnear -and *-bandsor Drac ermons:

E() = F|| (2)

where = k K and Fs the electronc group elocty, whch sgen byF = 3 0a/(2) 106 m s1.

T e lnear dsperson gen by equaton (2) s the soluton to theollowng ef ecte Hamltonan at the K(K) pont H = F ( ),where = i and are the pseudo-spn Paul matrces operatngn the space o the electron ampltude on the AB sublattces ographene2,3.

Linear optical absorption. T e optcal mage contrast can beused to denty graphene on top o a S/SO2 substrate (Fg. 1a)9.T s scales wth the number o layers and s the result o nterer-

ence, wth SO2 actng as a spacer. T e contrast can be maxmzedby adjustng the spacer thckness or the lght waelength9,10. T etransmttance o a reestandng SLG can be dered by applyng theFresnel equatons n the thn- lm lmt or a materal wth a xedunersal optcal conductance22 G0 = e2/(4) 6.08 105 1,to ge:

T= (1 + 0.5)2 1 97.7% (3)

where = e2/(40c) = G0/(0c) 1/137s the ne-structure con-stant11. Graphene only re ects


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612 NATURE PHOTONICS | VOL 4 | SEPTEMBER 2010 | www.nature.com/naturephotonics

the optcal absorpton o graphene layers to be proportonal to thenumber o layers, each absorbng A 1 T 2.3% oer the

sble spectrum (Fg. 1b). In a ew-layer graphene (FLG) sample,each sheet can be seen as a 2D electron gas, wth lttle perturba-ton rom the adjacent layers, makng t optcally equalent toa superposton o almost non-nteractng SLG9. T e absorptonspectrum o SLG s qute at rom 300 to 2,500 nm wth a peak nthe ultraolet regon (~270 nm), due to the excton-sh ed anHoe sngularty n the graphene densty o states. In FLG, otherabsorpton eatures can be seen at lower energes, assocated wthnterband transtons23,24.

Saturable absorption. Interband exctaton by ultraast opt-cal pulses produces a non-equlbrum carrer populaton n thealence and conducton bands (Fg. 1c). In tme-resoled exper-ments25, two relaxaton tmescales are typcally seen: a aster one

o ~100 s that s usually assocated wth carrercarrer ntrabandcollsons and phonon emsson, and a slower one, on a pcosec-ond tmescale, whch corresponds to electron nterband relaxatonand coolng o hot phonons26,27.

T e lnear dsperson o the Drac electrons mples that or anyexctaton there wll always be an electronhole par n resonance.A quanttate treatment o the electronhole dynamcs requresthe soluton o the knetc equaton or the electron and hole dstr-buton unctons, fe(p) and fh(p), p beng the momentum countedrom the Drac pont13. I the relaxaton tmes are shorter than thepulse duraton, then durng the pulse the electrons reach a staton-ary state, and collsons put electrons and holes nto thermal equ-lbrum at an ef ecte temperature13. T e populatons determneelectron and hole denstes, total energy densty and a reducton ophoton absorpton per layer, due to Paul blockng, by a actor o

A/A = [1 fe(p)][1 fh(p)] 1. Assumng e cent carrercarrerrelaxaton (both ntraband and nterband) and e cent coolngo the graphene phonons, the man bottleneck s energy transerrom electrons to phonons13.

For lnear dspersons near the Drac pont, parcarrer coll-sons cannot lead to nterband relaxaton, thereby conserng thetotal number o electrons and holes separately13,28. Interband relax-aton by phonon emsson can occur only the electron and holeenerges are close to the Drac pont (wthn the phonon energy).

Radate recombnaton o the hot electronhole populatonhas also been suggested1417. For graphte akes, the dsperson squadratc and parcarrer collsons can lead to nterband relaxa-ton. T us, n prncple, decoupled SLG can prode the hghestsaturable absorpton or a gen amount o materal 13.

Luminescence. Graphene could be made lumnescent by nducng abandgap, ollowng two man routes. One s by cuttng t nto rbbonsand quantum dots, the other s by chemcal or physcal treatments,to reduce the connectty o the -electron network. Een thoughgraphene nanorbbons hae been produced wth aryng bandgaps7,as yet no photolumnescence has been reported rom them. Howeer,bulk graphene oxde dspersons and solds do show a broad photo-lumnescence1921,29. Inddual graphene akes can be made brghtly

lumnescent by mld oxygen plasma treatment18. T e resultng pho-tolumnescence s unorm across large areas, as shown n Fg. 1d, nwhch a photolumnescence map and the correspondng elastc scat-terng mage are compared. It s possble to make hybrd structuresby etchng just the top layer, whle leang underlyng layers ntact18.T s combnaton o photolumnescent and conducte layers couldbe used n sandwch lght-emttng dodes. Lumnescent graphene-based materals can now be routnely produced that coer the nra-red, sble and blue spectral ranges1821,29.

Een though some groups hae ascrbed photolumnescence ngraphene oxde to bandgap emsson rom electron-con ned sp2slands1921, ths s more lkely to arse rom oxygen-related deectstates18. Whateer the orgn, uorescent organc compounds are omportance to the deelopment o low-cost optoelectronc deces30.

Blue photolumnescence rom aromatc or ole nc moleculess partcularly mportant or dsplay and lghtng applcatons31.Lumnescent quantum dots are wdely used or bo-labellng andbo-magng. Howeer, ther toxcty and potental enronmentalhazard lmt wdespread use and in vivo applcatons. Fluorescentbo-compatble carbon-based nanomaterals may be a more sut-able alternate. Fluorescent speces n the nrared and near-nrared are useul or bologcal applcatons, because cells andtssues show lttle auto- uorescence n ths regon32. Sun et al.exploted photolumnescent graphene oxde or le cell magng nthe near-nrared wth lttle background20.

Wang et al. hae reported a gate-controlled, tunable gap upto 250 meV n blayer graphene23. T s may make new photoncdeces possble or ar-nrared lght generaton, ampl caton

and detecton.Broadband nonlnear photolumnescence s also possble ollow-ng non-equlbrum exctaton o untreated graphene layers (Fg. 1c),as recently reported by seeral groups1417. Emsson occurs through-out the sble spectrum, or energes both hgher and lower than theexctng one, n contrast wth conentonal photolumnescence proc-esses1417. T s broadband nonlnear photolumnescence s thought toresult rom radate recombnaton o a dstrbuton o hot electronsand holes, generated by rapd scatterng between photoexcted carr-ers a er the optcal exctaton1417, ther temperature beng determnedby nteractons wth strongly coupled optcal phonons15. It scales wththe number o layers and can be used as a quanttate magng tool,as well as to reeal the dynamcs o the hot electronhole plasma1417(Fg. 1c). As or oxygen-nduced lumnescence, urther work s neces-sary to ully explan ths hot lumnescence.

OpticalpumpingR

ecombination

Coolingby

phononemission

ee relaxation

ee relaxation

hot ehplasma

Air

Bilayer

Graphene

2.3%

Transmittance(%)

96

98

100

0 25 50Distance (m)

Photoluminescence

Elastic scattering

1L

1L 1

L

1L

1L

a b

c d

Figure 1 | The optical properties of graphene. a, Elastic light scattering

(Rayleigh) image o a graphite fl ake with varying number o graphene

layers9. b, Transmittance or an increasing number o layers. Inset, sample

design or the experiment o re. 11, showing a thick metal support structure

with several apertures, on top o which grahene fl akes are placed.

c, Schematic o photoexcited electron kinetics in graphene, with possible

relaxation mechanisms or the non-equilibrium electron population.

d, Photoluminescence (top) and elastic scattering (bottom) images

o an oxygen-treated fl ake18. 1L indicates single-layer graphene.Figure

reproduced with permission rom: a, re. 9, 2007 ACS; b, re. 11, 2008

AAAS; c, re. 13, 2010 ACS; d, re. 18, 2009 ACS.

REVIEW ARTICLE NATURE PHOTONICSDOI: 10.1038/NPHOTON.2010.186

20 Macmillan Publishers Limited. All rights reserved10


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Electrolumnescence was also recently reported n prstnegraphene33. Although the power conerson e cency s lower thant s or carbon nanotubes (CNs), ths could lead to new lght-emttng deces based entrely on graphene.

Production

Graphene was rst produced by mcromechancal exola-ton o graphte34. T s approach stl l ges the best samples nterms o purty, deects, moblty and optoelectronc proper-tes. Howeer, t s clear that large-scale assembly s needed orthe wdespread applcaton o ths materal. Seeral approacheshae been deeloped to prode a steady supply o graphene nlarge areas and quanttes, amenable or mass applcatons. T esenclude growth by chemcal apour deposton (CVD)3539, seg-regaton by heat treatment o carbon-contanng substrates4042,and lqud phase exolaton4347. In act, most o these methodsdate back seeral decades. T e current nterest n graphene haspushed these early approaches to large yelds, controlled growthand large areas, and made t possble n just sx years to go rommcrometre-szed akes to near-mass-producton o layer-controlled samples.

Micromechanical cleavage. T s method noles peelng of apece o graphte by means o adhese tape34. It has been optmzedto produce SLG o up to mllmetres n sze, and wth hgh struc-tural and electronc qualty. Although ths s the method o choceor undamental research, wth most key results on nddual SLGbeng obtaned on such akes, t has dsadantages n terms o yeld

and throughput, and s mpractcal or large-scale applcatons.Liquid-phase exfoliation. Lqud-phase exolaton (LPE) conssts ochemcal wet dsperson ollowed by ultrasoncaton, n both aque-ous45 and non-aqueous solents44. Up to ~70% SLG can be acheedby mld soncaton n water wth sodum deoxycholate ollowed bysedmentaton-based ultracentrugaton. Ble salt suractants alsoallow the solaton o akes wth controlled thckness, when com-bned wth densty gradent ultracentrugaton48. Exolaton ographte-ntercalated compounds46 and expandable graphte49 hasalso been reported.

LPE can also produce graphene nanorbbons wth wdths less than10 nm (re. 43) and of ers adantages o scalablty and no requre-ment or expense growth substrates. Furthermore, t s an dealmeans to produce lms and compostes.

a

Graphene

ITO

Arc discharge SWNTs

TiO2/Ag/TiO

2

ZnO/Ag/ZnO

200 400 600 800

20

40

60

80

100

Transmittance(%)

Wavelength (nm)

b

c d

Transmittance(%)

40

60

80

100

Transmittance(%)

40

60

80

100

1

10

100

1,000

Graphene calculated

Graphene CVD

SWNTs

Thickness (nm)

Sheetresistance(/)

0.1 1 10 100 1,000

Ag nanowire mesh

ITO

Graphene calculated

Graphene CVD

SWNTs

Ag nanowire mesh

ITO

n = 1013

cm2

= 2 103 cm2 V1 s1

n = 1013 cm2

= 2 103 cm2 V1 s1

n = 3.4 1012 cm2

= 2 104 cm2 V1 s1

n = 3.4 1012 cm2

= 2 104 cm2 V1 s1

Sheet resistance (/ ) Sheet resistance (/ )1 10 100

n = 3.4 1012 cm2

= 2 104 cm2 V1 s1

101

103

105

107

109

1011

Graphene calculated

MC

PAHs

CVD

RGO

LPE

Figure 2 | Graphene as transparent conductor. a, Transmittance or dif erent transparent conductors: GTCFs39, single-walled carbon nanotubes (SWNTs)77,

ITO75, ZnO/Ag/ZnO (re. 81) and TiO2/Ag/TiO2 (re. 70). b, Thickness dependence o the sheet resistance. The blue rhombuses show roll-to-roll GTCFs

based on CVD-grown graphene39; red squares, ITO75; grey dots, metal nanowires75; green rhombuses, SWNTs77. Two limiting lines or GTCFs are also

plotted (enclosing the shaded area), calculated rom equation (6) using typical values or n and . c, Transmittance versus sheet resistance or dif erent

transparent conductors: blue rhombuses, roll-to-roll GTCFs based on CVD-grown graphene39; red line, ITO75; grey dots, metal nanowires75; green triangles,

SWNTs77. Shaded area enclosed by limiting lines or GTCFs calculated using n and as in b. d, Transmittance versus sheet resistance or GTCFs grouped

according to production strategies: triangles, CVD3739,97; blue rhombuses, micromechanical cleavage (MC)88; red rhombuses, organic synthesis rom

polyaromatic hydrocarbons (PAHs)65; dots, liquid-phase exoliation (LPE) o pristine graphene44,45,48,88; and stars, reduced graphene oxide (RGO)52,54,83,84,96.

A theoretical line as or equation (6) is also plotted or comparison.

REVIEW ARTICLENATURE PHOTONICSDOI: 10.1038/NPHOTON.2010.186



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Graphene oxide. Soncaton o graphte oxde can be used to producegraphene oxde47, ollowng the 50-year-old Hummers method50.T e oxdaton o graphte n the presence o acds and oxdants,proposed n the nneteenth century51, dsrupts the sp2 network andntroduces hydroxyl or epoxde groups52,53 wth carboxylc or carbo-nyl groups attached to the edges. T ese make graphene oxde sheetsreadly dspersble n water and seeral other solents. Although large akes can be obtaned, these are ntrnscally deecte and electr-cally nsulatng. Despte seeral attempts47,52, reduced graphene oxde

does not ully regan the prstne graphene electrcal conductty52,54.It s thereore mportant to dstngush between dsperson-processedgraphene akes, whch retan the electronc propertes o graphene,and nsulatng graphene oxde layers.

Chemical vapour deposition. T e CVD o FLGs was reported morethan 40 years ago35. SLG and FLG can now be grown on arous sub-strates by eedng hydrocarbons at a sutable temperature3539,55,56.T e scale o progress n CVD growth s gen by re. 39, n whchsamples o oer 60 cm were reported. Plasma-enhanced CVD can beappled on substrates wthout a catalyst56. Note that most as-grownCVD samples are multlayered. Een ther Raman spectrum seemssmlar37,38 to that o deal SLG57, ths s just an ndcaton o electroncdecouplng o the layers, not de nte proo o SLG growth.

Carbon segregation. Graphene can also be produced through carbonsegregaton rom slcon carbde40,58,59 (SC) or metal substrates41,55,6062ollowng hgh-temperature annealng. Acheson reported58 a methodo producng graphte rom SC n as early as 1896, and the segrega-ton o graphene rom N(111) was nestgated oer 30 years ago60.Hgh-qualty layers can now be produced on SC n an argon atmos-phere42, and electronc decouplng rom the underlyng SC substratecan be acheed by hydrogen treatment63.

Chemical synthesis. Graphene or carbon nanosheets can be producedthrough chemcal synthess64. otal organc synthess yelds graphene-lke polyaromatc hydrocarbons65. T ese synthetc nanographenescan then be assembled to orm larger layers, or to achee atomcally

precse bottom-up abrcaton o nanorbbons. Supramolecular nter-actons can be used to coer SLG wth polyaromatc hydrocarbons,keepng the sp2 network ntact wthout compromsng the transportpropertes. Nanographenes orm ordered layers, wth precse controlo orentaton and spacng66. T ese nteract wth the graphene back-bone, makng t possble, n prncple, to control and tune ts optoelec-tronc propertes66.

Deterministic placement. A undamental step n the productono useul deces s the determnstc placement o graphene onpre-de ned postons on a substrate o choce. ranser processesare common n the semconductor ndustry, and extense exper-ence o transer has been deeloped or CNs. Rena et al. reportedtranser o SLG and FLG rom SO2/S to other substrates67. A layer o

poly(methyl methacrylate) (PMMA) was coated on graphene depos-ted on SO2, then subsequently detached by partal SO2 etchng67.T e PMMA/graphene membrane was then placed oer the targetsubstrate and PMMA dssoled wth acetone67. Km et al. used a dry-method based on a polydmethylsloxane stamp to transer patterned lms37. Bae et al. scaled the process to a roll-based layer-by-layertranser onto plastc substrates39.

We hae deeloped a procedure or determnstc placement, ol-lowng transer. T s explots a water layer between the PMMA/graph-ene ol and the substrate, whch enables the PMMA to moe. T sallows us to place graphene layers on any substrate n any pre-de nedlocaton, prepare art cal multlayers and create sandwch structureswth other materals (such as BN or MoS2). We wll show an exampleo ths technque n the secton enttled Saturable absorbers and ultra-ast lasers, by placng graphene on the core o an optcal bre.

Large-scale placement o LPE samples can be acheed by spn-coatng and LangmurBlodgett technques49. Surace mod catonby sel-assembled monolayers can enable targeted deposton ographene akes on the large scale. D-electrophoress allows control-led placement o nddual graphene akes between pre-patternedelectrodes68. Inkjet prntng s another attracte technque69, andcould drectly wrte optoelectronc deces or thn- lm transstors.

Photonics and optoelectronics applications

Transparent conductors. Optoelectronc deces such as dsplays,touch screens, lght-emttng dodes and solar cells requre mater-als wth low sheet resstance Rs and hgh transparency. In a thn lm,Rs = /t, where t s the lm thckness and = 1/ s the resstty,beng the d.c. conductty. For a rectangle o length L and wdth W,the resstance R s:

R= = Rs

tL

WL

W

T e term L/Wcan be seen as the number o squares o sde Wthatcan be supermposed on the resstor wthout oerlappng. T us, eenRs has unts o ohms (as R does), t s hstorcally quoted n ohmsper square (/ ).

Current transparent conductors are semconductor-based70: dopedndum oxde (In2O3)71, znc oxde (ZnO)72 or tn oxde (SnO2)70, aswell as ternary compounds based on ther combnatons70,72,73. T edomnant materal s ndum tn oxde (IO), a doped n-type sem-conductor composed o ~90% In2O3 and ~10% SnO2 (re. 70). T eelectrcal and optcal propertes o IO are strongly af ected by mpu-rtes70. n atoms uncton as n-type donors70. IO has strong absorp-ton aboe 4 eV due to nterband transtons 70, wth other eatures atlower energy related to scatterng o ree electrons by tn atoms orgran boundares70. IO s commercally aalable wth T 80% andRs as low as 10 / on glass72, and ~60300 / on polyethyleneterephthalate73. Note that Ts typcally quoted at 550 nm, as ths swhere the spectral response o the human eye s hghest 70.

IO suf ers seere lmtatons: an eer-ncreasng cost due to ndum

scarcty70

, processng requrements, d cultes n patternng70,73

and asenstty to both acdc and basc enronments. Moreoer, t s brt-tle and can easly wear out or crack when used n applcatons nol-ng bendng, such as touch screens and exble dsplays74. T s meansthat new transparent conductor materals are needed wth mproedperormance. Metal grds75, metallc nanowres76 or other metaloxdes73 hae been explored as alternates. Nanotubes and graphenealso show great promse. In partcular, graphene lms hae a hgherToer a wder waelength range than sngle-walled carbon nanotube(SWN) lms7779, thn metallc lms75,76 and IO70,72 (Fg. 2a).

We now present a relaton between Tand Rs or FLG lms o ary-ng dopng leels. From equaton (3), Tdepends on the optcal con-ducttyG0:

T= 1+ NG020c

2

(4)

where Ns the number o layers. T e sheet resstance Rs s lnked tothe bdmensonal d.c. conductty2D by:

Rs = (2DN)1 (5)

Combnng equatons (4) and (5) and elmnatng Nges:

T= 1+

Z0

2Rs

G0

2D

2

(6)

where Z0

= 1/0

c = 377 s the ree-space mpedance, 0

s the




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ree-space electrc constant and c s the speed o lght. In graphene1we can take 2D = ne, where n s the number o charge carrers.Note that or n 0, 2D does not go to zero, but assumes a constantalue1 o approxmately 4e2/h, gng Rs 6 k/ or an deal ntrn-sc SLG wth T 97.7%. T us, an deal ntrnsc SLG would beat thebest IO only n terms oT, not Rs. Howeer, real samples depos-ted on substrates, n thn lms or embedded n polymers are neerntrnsc. Exolated SLG typcally has n 1012 cm2 (see re. 80, orexample), and much smaller Rs. T e range o Tand Rs that can be

realstcally acheed or graphene layers o aryng thckness canbe estmated by takng n = 10121013 cm2 and = 1,00020,000 cm2 V1 s1, whch s typcal or lms grown by CVD.Fgure 2b,c shows that graphene can achee the same Rs as IO,ZnO/Ag/ZnO (re. 81), O2/Ag/O2 and SWNs wth a smlaror een hgher T. Fgure 2c plots Tersus Rs or IO (re. 75), Agnanowres75, SWNs77 and the best graphene-based transparentconducte lms (CFs) reported so ar 39, agan showng that thelatter s superor. For nstance, takng n = 3.4 1012 cm2 and = 2 104 cm2 V1 s1, we get T= 90% and Rs = 20 / .

Note that equaton (6) s ntended as a gudelne or CF desgnand optmzaton not as a statement on the transport physcs ographene. For CF desgn, emprcal expressons o 2D as a unc-ton o carrer concentraton and dopng are enough, whateer the

orgn and precse quant caton o the mnmal conductty, ando the dependence oRs on dopng, deects, electronhole puddlesand so on.

Df erent strateges were explored to prepare graphene-basedCFs (GCFs): sprayng82, dp83 and spn coatng84, acuum ltra-ton54 and roll-to-roll processng39. Consderable progress has beenmade snce the rst attempts to produce graphene-oxde-basedCFs (GOCFs). Because graphene oxde s nsulatng, t must bereduced to mproe Rs (re. 47). Glje et al.82 decreased Rs rom40 G/ to 4 M/ ollowng reducton wth dmethylhydrazne.Graphtzaton84, hydrazne exposure and low-temperature anneal-ng54, or hgh-temperature acuum annealng85 urther decreased Rsdown to 800 / or T= 82% (re. 85).

Dspersons o graphte-ntercalated compounds86 and hybrd

nanocompostes (graphene oxde sheets mxed wth slca sols orCNs87) were also attempted, wth a mnmum Rs = 240 / orT= 86% (re. 87). Graphene lms produced by chemcal synthesscurrently show Rs = 1.6 k/ or T= 55% (re. 65).

Blake et al.88 hae reported the best GCF so ar, rom LPE ographte. T s was abrcated by acuum ltraton ollowed byannealng, acheng Rs = 5 k/ and T 90%. T e hgh Rs s mostlkely due to the small ake sze and lack o percolaton48,88. T e roleo percolaton can be seen n re. 48, where Rs and T went rom6 k/ and ~75%, to 2 k/ and ~77% wth ncreasng ake sze.

A key strategy to mprong perormance s stable chemcal dop-ng. Blake et al.88 prepared GCFs, produced by mcromechancalcleaage, wth T 98% and Rs = 400 / , usng a layer o polynylalcohol to nduce n-type dopng. Bae et al.39 acheed Rs 30 /

and T 90% by ntrc acd treatment o GCFs dered rom CVD-grown akes one order o magntude lower n terms oRs thanpreous GCFs rom the wet transer o CVD lms37.

Fgure 2d s an oerew o current GCFs and GOCFs. It showsthat GCFs dered rom CVD akes, combned wth dopng, couldoutperorm IO, metal wres and SWNs. Note that GCFs andGOCFs produced by other methods, such as LPE, although currentlyhang hgher Rs at T= 90%, hae already been tested n organc lghtemtters85,89 and solar cells83,90. T ese are cheaper and easer to scalethan mcromechancal cleaage or CVD lms, and must be consd-ered or applcatons n whch cost reducton s crucal.

Photovoltaic devices. A photooltac cell conerts lght to elec-trcty91. T e energy conerson e cency s = Pmax/Pnc, whereP

max

= VOC

ISC

FF and Pnc

s the ncdent power. Here, ISC

s the

maxmum short-crcut current, VOC s the maxmum open-cr-cut oltage and FF s the ll actor, de ned as FF = (Vmax Imax)/(VOC ISC), where Vmax and Imax are the maxmum oltage and cur-rent, respectely. T e racton o absorbed photons conerted tocurrent de nes the nternal photocurrent e cency.

Current photooltac technology s domnated by slcon cells91,wth up to ~25% (re. 92). Organc photooltac cells rely on poly-mers or lght absorpton and charge transport93. T ey can be manu-actured economcally compared wth slcon cells, or example by

a roll-to-roll process94, een though they hae lower . An organcphotooltac cell conssts o a transparent conductor, a photoac-te layer and the electrode93. Dye-senstzed solar cells use a lqudelectrolyte as a charge-transport medum95. T s type o solar cellconssts o a hgh-porosty nanocrystallne photoanode, comprsngO2 and dye molecules, both deposted on a transparent conduc-tor95. When llumnated, the dye molecules capture the ncdentphoton, generatng electronhole pars. T e electrons are njectednto the conducton band o the O2 and are then transported tothe counter-electrode95. Dye molecules are regenerated by capturngelectrons rom a lqud electrolyte. At present, IO s the most com-mon materal or use both as a photoanode and cathode, the latterwth a platnum coatng.

Graphene can ul l multple unctons n photooltac deces: as

the transparent conductor wndow, photoacte materal, channel orcharge transport, and catalyst. GCFs can be used as wndow elec-trodes n norganc (Fg. 3a), organc (Fg. 3b) and dye-senstzedsolar cell deces (Fg. 3c). Wang et al. used GCFs produced bychemcal synthess, reportng 0.3% (re. 65). A hgher o ~0.4%was acheed usng reduced graphene oxde96, wth Rs = 1.6 k/nstead o 5 k/ (re. 65), despte a lower T(55% nstead o 80%).De Arco et al. acheed better perormance ( 1.2%) usng CVDgraphene as the transparent conductor, wth Rs = 230 / andT= 72%(re. 97). Further optmzaton s certanly possble, consderng theperormance o the best GCF so ar39.

Graphene oxde dspersons were also used n bulk-heterojuncton photooltac deces, as electron-acceptors wthpoly(3-hexylthophene) and poly(3-octylthophene) as donors,

acheng 1.4% (re. 90). Yong et al. clam that > 12% should bepossble wth graphene as photoacte materal98.Graphene can coer an een larger number o unctons n dye-

senstzed solar cells. Wang et al. reported a sold-state solar cell basedon the organc compound spro-OMeAD1 (as the hole transportmateral) and porous O2 (or electron transport) usng a GCFanode, wth 0.26% (re. 83). Graphene can be ncorporated ntothe nanostructured O2 photoanode to enhance the charge transportrate, preentng recombnaton, thus mprong the nternal photo-current e cency99. Yang et al. used graphene as a O2brdge, ache-ng aster electron transport and lower recombnaton, and leadng to 7%, whch s hgher than they acheed wth conentonal nanoc-rystallne O2 photoanodes n the same expermental condtons99.Another opton s to use graphene, wth ts hgh spec c surace area,

to substtute or the platnum counter-electrode. A hybrd poly(3,4-ethylenedoxythophene):poly(styrenesulphonate) (PEDO:PSS)/graphene oxde composte was used as counter-electrode, to obtan = 4.5%, comparable to the 6.3% or a platnum counter-electrodetested under the same condtons100 but now wth a cheaper materal.

Light-emitting devices. Organc lght-emttng dodes (OLEDs)hae an electrolumnescent layer between two charge-njectng elec-trodes, at least one o whch s transparent101. In these dodes, holesare njected nto the hghest occuped molecular orbtal (HOMO) othe polymer rom the anode, and electrons are njected nto the low-est unoccuped molecular orbtal (LUMO) rom the cathode. Fore cent njecton, the anode and cathode work unctons shouldmatch the HOMO and LUMO o the lght-emttng polymer101.Because o ther hgh mage qualty, low power consumpton and




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ultrathn dece structure, OLEDs nd applcatons n ultrathn tel-esons and other dsplay screens such as computer montors, dg-tal cameras and moble phones. radtonally, IO, wth ts workuncton o 4.44.5 eV, s used as the transparent conducte lm.Howeer, besdes cost ssues, IO s brttle and lmted as a exblesubstrate73. In addton, ndum tends to df use nto the acte OLEDlayers, whch reduces dece perormance oer tme70. T us, there sa need or alternate CFs wth optcal and electrcal perormancesmlar to IO, but wthout ts drawbacks. Graphene has a work

uncton o 4.5 eV, smlar to IO. T s, combned wth ts promseas a exble and cheap CF, makes t an deal canddate or an OLEDanode (Fg. 3d), whle also elmnatng the ssues related to ndumdf uson. GCFs anodes enable an out-couplng e cency compa-rable to IO85. Consderng that the Rs and To re. 85 were 800 /and 82% at 550 nm, t s reasonable to expect that urther optmza-ton wll mproe perormance.

Matyba et al.89 used a GOCF n a lght-emttng electrochemcalcell. Smlar to an OLED, ths s a dece n whch the lght-emttngpolymer s blended wth an electrolyte102. T e moble ons n theelectrolyte rearrange when a potental s appled between the elec-trodes, ormng layers wth hgh charge densty at each electrodenterace, whch allows e cent and balanced njecton o electronsand holes, regardless o the work uncton o the electrodes 102.

Usually, the cells hae at least one metal electrode. Electrochemcalsde-reactons, nolng the electrode materals, can cause prob-lems n terms o operatonal letme and e cency89. T s alsohnders the deelopment o exble deces. Graphene s the dealmateral to oercome these problems. Matyba et al.89 demonstrateda lght-emttng electrochemcal cell based solely on dsperson-processable carbon-based materals, pang the way towards totallyorganc low-oltage, nexpense and e cent LEDs.

Photodetectors. Photodetectors measure photon ux or optcalpower by conertng the absorbed photon energy nto electrcal cur-rent. T ey are wdely used n a range o common deces103, suchas remote controls, telesons and DVD players. Most explot thenternal photoef ect, n whch the absorpton o photons results ncarrers excted rom the alence to the conducton band, outputtng

an electrc current. T e spectral bandwdth s typcally lmted bythe materals absorpton103. For example, photodetectors based on ivand iiiv semconductors suf er rom the long-waelength lmt, asthese become transparent when the ncdent energy s smaller thanthe bandgap103. Graphene absorbs rom the ultraolet to terahertzrange11,13,104,105. As a result, graphene-based photodetectors (GPDs;see Fg. 3e) could work oer a much broader waelength range. T eresponse tme s ruled by the carrer moblty103. Graphene has hugemobltes, so GPDs can be ultraast.

T e photoelectrcal response o graphene has been wdely nest-gated both expermentally and theoretcally106110. Responses at wae-lengths o 0.514, 0.633, 1.5 and 2.4 m hae been reported110. Muchbroader spectral detecton s expected because o the graphene ultraw-deband absorpton. Xa et al. demonstrated a GPD wth a photore-sponse o up to 40 GHz (re. 109). T e operatng bandwdth o GPDs smanly lmted by ther tme constant resultng rom the dece resst-ance,R, and capactance,C. Xaet al. reported anRC-lmted bandwdtho about 640 GHz (re. 109), whch s comparable to tradtonal pho-todetectors111. Howeer, the maxmum possble operatng bandwdtho photodetectors s typcally restrcted by ther transt tme, the nteduraton o the photogenerated current103. T e transt-tme-lmtedbandwdth o a GPD could be well oer 1,500 GHz (re. 109), surpassngstate-o-the-art photodetectors.

Although an external electrc eld can produce e cent photocur-rent generaton wth an electronhole separaton e cency o oer30% (re. 107), zero sourcedran bas and dark current operatonscould be acheed by usng the nternal electrc eld ormed near themetal electrodegraphene nteraces109,110. Howeer, the small ef ec-te area o the nternal electrc eld could decrease the detectone cency109,110, as most o the generated electronhole pars wouldbe out o the electrc eld, thus recombnng, rather than beng sepa-rated. T e nternal photocurrent e cences (1530%; res 107,108)and external responsty (generated electrc current or a gennput optcal power) o ~6.1 mA per watt so ar reported110 or GPDsare relately low compared wth current photodetectors103. T s smanly due to lmted optcal absorpton when only one SLG s used,short photocarrer letmes and small ef ecte photodetecton areas(~200 nm n re. 109).

Light

Transparentgraphene electrode

Back reflectorelectrode

p layerIntrinsic layer

n layer

Transparentgraphene electrode

Electrode

Electronblockinglayer Polymer/

grapheneactive layer

SiOutput

Substrate

Graphene

Organic light-emitting layers

Cathode

a

d

b c

e

Graphene

SiO2

Light

Metal contact

LightLight

Transparentgrapheneelectrode

Graphene counter-electrode

I

I3

Graphenebridgestructure

Figure 3 | Graphene-based optoelectronics. ac, Schematics o inorganic (a), organic (b) and dye-sensitized (c) solar cells. I and I3 are iodide and tri-

iodide, respectively. The I and I3 ions transer electrons to the oxidized dye molecules, thus completing the internal electrochemical circuit between the

photoanode and the counter-electrode. d,e, Schematics o an organic LED (d) and a photodetector (e). The cylinder in d represents an applied voltage.




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T e photothermoelectrc ef ect, whch explots the conersono photon energy nto heat and then electrc sgnal103, may play anmportant part n photocurrent generaton n graphene deces107,112.T us photothermoelectrc GPDs may be possble.

Touch screens. ouch screens are sual outputs that can detect thepresence and locaton o a touch wthn the dsplay area, permttngphyscal nteracton wth what s shown on the dsplay tsel113. ouchpanels are currently used n a wde range o applcatons such as cel-lular phones and dgtal cameras because they allow quck, ntuteand accurate nteracton by the user wth the dsplay content.

Resste and capacte touch panels are the most common(Fg. 4a). A resste touch panel comprses a conducte substrate, alqud-crystal dece ront panel and a CF113. When pressed, the

ront-panel lm comes nto contact wth the bottom CF, and thecoordnates o the contact pont are calculated on the bass o therresstance alues. T ere are two categores o resste touch screens:matrx and analogue113. T e matrx has strped electrodes, whereasthe analogue has a non-patterned transparent conducte electrodewth lower producton costs. T e CF requrements or resstescreens are Rs 5002,000 / and T> 90% at 550 nm (re. 113).Faourable mechancal propertes, ncludng brttleness and wearresstance, hgh chemcal durablty, no toxcty and low productoncosts, are also mportant. Cost, brttleness, wear resstance and chem-cal durablty are the man lmtatons o IO70,73, whch cannot wth-stand the repeated exng and pokng noled wth ths type oapplcaton. T us, or resste touch screens there s an ef ort to ndan alternate transparent conductor.

GCFs can satsy the requrements or resste touch screensn terms o Tand Rs, as well as exhbtng large-area unormty.Bae et al.39 recently produced a graphene-based touch panel dsplayby screen-prntng a CVD-grown sample (Fg. 4b). Consderngthe Rs and T requred by analogue resste screens, GCFs orGOCFs produced by LPE are also able alternates, urtherreducng costs.

Capacte touch screens are emergng as the hgh-end technol-ogy, especally snce the launch o Apples Phone. T ese consst o annsulator such as glass, coated wth IO113. As the human body s alsoa conductor, touchng the surace o the screen results n an electro-statc eld dstorton, whch s measurable as a change n capactance.Although capacte touch screens do not work by pokng wth a pen(makng mechancal stresses lower than or resste screens), the useo GCFs can stll mproe perormance and reduce costs.

Flexible smart windows and bistable displays. Polymer-dspersedlqud-crystal (PDLC) deces were ntroduced n the early 1980s114.T ese consst o thn lms o optcally transparent polymers wthmcrometre-szed lqud-crystal droplets contaned wthn pores othe polymer. Lght passng through the lqud-crystal/polymer sstrongly scattered, producng a mlky lm. I the lqud crystalsordnary reracte ndex s close to that o the host polymer, apply-ng an electrc eld results n a transparent state74. In prncple, anytype o thermotropc lqud crystal may be used n PDLC decesor applcatons not requrng hgh swtchng speeds. In partcular,the ablty to swtch rom translucent to opaque makes them attrac-te or electrcally swtchable smart wndows that can be actatedwhen pracy s requred. Conentonally, IO on glass s used asthe conducte layer to apply the electrc eld across the PDLC.

Howeer, one o the reasons behnd the lmted market penetratono smart wndows s the hgh cost o IO. Furthermore, exblty shndered when usng IO, reducng potental applcatons such asPDLC exble dsplays74. ransparent or coloured/tnted smart wn-dows generally requre T to be 6090% or hgher and Rs to be1001,000 / , dependng on producton cost, applcaton andmanuacturer. In addton to exblty, the electrodes need to be aslarge as the wndow tsel and must hae long-term physcal andchemcal stablty, as well as beng compatble wth the roll-to-rollPDLC producton process. Lqud crystals could also be used ornext-generaton zero-power monochromatc and coloured exblebstable dsplays, whch can retan an mage wth no power con-sumpton. T ese are attracte or sgns and adertsements or ore-readers, and requre a transparent exble conductor or swtch-

ng the mage. T e present IO deces are not deal or ths appl-caton, owng to the lmtatons dscussed aboe.All these de cences o IO electrodes can be oercome by

GCFs. Fgure 4c,d shows ther workng prncple, and Fg. 4e showsa prototype o a exble smart wndow wth polyethylene terephtha-late used as a substrate.

Saturable absorbers and ultrafast lasers. Materals wth nonln-ear optcal and electro-optcal propertes are needed n most phot-onc applcatons. Laser sources producng nano- to subpcosecondpulses are a key component n the portolo o leadng laser manu-acturers. Sold-state lasers hae so ar been the short-pulse source ochoce, beng deployed n applcatons rangng rom basc researchto materals processng, rom eye surgery and prnted crcut-board manuacturng to metrology and the trmmng o electronc

VV

Graphene-basedtransparent electrodes

Polymer-dispersedliquid crystals

Flexible,transparentpolymer support

a b c

d e

Capacitive sensing circuit

Glass/polymersubstrate

Liquid-crystal display

Of On

Polymer-dispersedliquid crystal

Graphene-basedtransparentelectrode

Flexible, transparentpolymer support

Antireflective coating

Figure 4 | Graphene touch screen and smart window. a, Schematic o a capacitive touch screen. b, Resistive graphene-based touch screen. c, Schematic o

a PDLC smart window using a GTCF. d, With no voltage, the liquid-crystal molecules are not aligned, making the window opaque. e, Graphene/nanotube-

based smart window in either an of (let) or on (right) state. Image in b reproduced with permission rom re. 39, 2010 NPG.




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components such as resstors and capactors. Regardless o wae-length, the majorty o ultraast laser systems use a mode-lockngtechnque, whereby a nonlnear optcal element, called a saturable

absorber, turns the contnuous-wae output nto a tran o ultraastoptcal pulses115. T e key requrements or nonlnear materals areast response tme, strong nonlnearty, broad waelength range,low optcal loss, hgh power handlng, low power consumpton, lowcost and ease o ntegraton nto an optcal system. Currently, thedomnant technology s based on semconductor saturable absorbermrrors115. Howeer, these hae a narrow tunng range, and requrecomplex abrcaton and packagng12,115. A smple, cost-ef ectealternate s to use SWNs12,116, n whch the dameter controls thegap and thus the operatng waelength. Broadband tunablty s pos-sble usng SWNs wth a wde dameter dstrbuton12,116. Howeer,when operatng at a partcular waelength, SWNs not n resonanceare not used and contrbute unwanted losses.

As dscussed aboe, the lnear dsperson o the Drac electrons ngraphene of ers an deal soluton: or any exctaton there s always an

electronhole par n resonance. T e ultraast carrer dynamcs25,117,combned wth large absorpton and Paul blockng, make graphenean deal ultrabroadband, ast saturable absorber. Unlke semconduc-tor saturable absorber mrrors and SWNs, there s no need or band-gap engneerng or chralty/dameter control.

So ar, graphenepolymer compostes12,13,118120, CVD-grown lms121,122, unctonalzed graphene (or example, graphene oxdebonded wth poly(m-phenylenenylene-co-2,5-doctoxy-p-phenylenenylene)119 and reduced graphene oxde akes123,124 hae

been used or ultraast lasers. Graphenepolymer compostes arescalable and, more mportantly, easly ntegrated nto a range ophotonc systems12,13,118. Another route or graphene ntegraton sby determnstc placement n a pre-de ned poston on a substrateo choce, or example a bre core or caty mrrors. Fgure 5a showsthe transer o such a ake onto an optcal bre core. T s s acheedby usng a water layer between the PMMA/graphene ol and theoptcal bre, whch enables the PMMA to moe. Graphene decentegraton s nally acheed a er precse algnment to the optcal bre core by a mcromanpulator (Fg. 5b) and dssoluton o thePMMA layer (Fg. 5c).

A typcal absorpton spectrum s shown n Fg. 6a12,13,118. T s seatureless apart rom the characterstc ultraolet peak, and the hostpolymer only contrbutes a small background or longer waelengths.

Fgure 6b plots Tas a uncton o aerage pump power or sx wae-lengths. Saturable absorpton s edent rom the T ncrease wthpower at all waelengths.

Varous strateges hae been proposed to ntegrate graphene satu-rable absorbers (GSAs) n laser cates or ultraast pulse generaton.T e most common s to sandwch a GSA between two bre con-nectors wth a bre adaptor, as shown schematcally n Fg. 5d12,13,118.Graphene on a sde-polshed bre has also been reported, amed athgh power generaton by eanescent eld nteracton123. A quartzsubstrate coated wth graphene has been used or ree-space sold-state lasers124.

T e most common waelength o generated ultraast pulses so ars ~1.5 m, not because GSAs hae any preerence or a partcularwaelength, but because ths s the standard waelength o optcal

telecommuncatons. A sold-state laser mode-locked by graphenehas been reported at ~1 m (re. 124). Fgure 6c shows a GSA mode-locked laser, made rom erbum-doped bre and tunable rom 1,526to 1,559 nm, wth the tunng range manly lmted by the tunable lter,not the GSA118. Fgure 6d,e shows the pulse rom a graphene-oxde-based saturable absorber. T e possblty o tunng the GSA propertesby unctonalzaton or by usng df erent layers or composte concen-tratons of ers consderable desgn reedom. able 1 ges a perorm-ance comparson o graphene-based ultraast lasers and the mancarbon-nanotube-based deces125,126.

Optical limiters. Optcal lmters are deces that hae hgh trans-mttance or low ncdent lght ntensty, and low transmttance orhgh ntensty127. T ere s a great nterest n these or optcal sensors

and human eye protecton, as retnal damage can occur when ntens-tes exceed a certan threshold127. Passe optcal lmters, whch use anonlnear optcal materal, hae the potental to be smple, compactand cheap127. Howeer, so ar no passe optcal lmters hae beenable to protect eyes and other common sensors oer the entre sbleand near-nrared range127. ypcal materals nclude semconductors(or example ZnSe, InSb), organc molecules (or example phthalo-cyannes), lqud crystals and carbon-based materals (or example,carbon-black dspersons, CNs and ullerenes)127,128. Fullerenes andther derates129,130 and CN dspersons130 hae good optcal lm-tng perormance, n partcular or nanosecond pulses at 532 and1,064 nm (re. 130).

In graphene-based optcal lmters the absorbed lght energy con-erts nto heat, creatng bubbles and mcroplasmas128, whch resultsn reduced transmsson. Graphene dspersons can be used as

PMMA oil

Graphene

Fibre core

a

Fibre

Connector

EDFWDM

LD

PC

ISO

b

d

c

Graphene SA

Fibre connectors

Outputcoupler

Figure 5 | Graphene integration in fi bre lasers. a, An optical fi bre is

mounted onto a holder. Once detached rom the original substrate, a

polymer/graphene membrane is slid and aligned with the fi bre core.

b, Flake originally deposited on SiO2/Si. c, The same fl ake ater

deterministic placement and dissolution o the polymer layer. d, Graphene

mode-locked ultraast laser: a graphene saturable absorber (SA) is

inserted between two fi bre connectors. An erbium-doped fi bre (EDF) is

the gain medium, pumped by a laser diode (LD) with a wavelength-division

multiplexer (WDM). An isolator (ISO) maintains unidirectional operation,

and a polarization controller (PC) optimizes mode-locking.




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wdeband optcal lmters coerng sble and near-nrared. Broadoptcal lmtng (at 532 and 1,064 nm) by LPE graphene was reportedor nanosecond pulses128. It has also been shown131 that unctonalzedgraphene dspersons could outperorm C60 as an optcal lmter.

Optical frequency converters. Optcal requency conerters are usedto expand the waelength accessblty o lasers (or example, requencydoublng, parametrc ampl caton and oscllaton, and our-waemxng)127. Calculatons suggest that nonlnear requency genera-

ton n graphene (harmoncs o nput lght, or example) should bepossble or su cently hgh external electrc elds (>100 V cm1)132.Second-harmonc generaton rom a 150 s laser at 800 nm has beenreported or a graphene lm133. In addton, our-wae mxng to gen-erate near-nrared waelength tunable lght has been demonstratedusng SLG and FLG134. Graphenes thrd-order susceptblty |3| wasmeasured to be ~107 e.s.u. (re. 134) up to one order o magntudelarger than that reported so ar or smlar measurements on CNs134.Howeer, photon-countng electroncs s typcally needed to measurethe output133, ndcatng a low conerson e cency. Other eatures ographene, such as the possblty o tunng the nonlnearty by chang-ng the number o layers134, and waelength-ndependent nonlnearsusceptblty134, stll could be potentally used or arous photoncapplcatons (optcal magng134, or example).

Terahertz devices. Radaton n the 0.310 Hz range (30 m to1 mm) s attracte or bomedcal magng, securty, remote sens-ng and spectroscopy135. Much unexplored terrtory stll remansor terahertz technology, manly owng to a lack o af ordableand e cent sources and detectors135. T e requency o grapheneplasma waes136 les n the terahertz range, as well as the gap ographene nanorbbons, and the blayer graphene tunable band-gap, makng graphene appealng or terahertz generaton anddetecton. Varous terahertz sources hae been suggested basedon electrcal136 or optcal136 pumpng o graphene deces. Recentexpermental obseratons o terahertz emsson137 and ampl ca-ton138 n optcally pumped graphene hae shown the easblty ographene-based terahertz generaton. wsted multlayers, retan-

ng the electronc propertes o SLG, could also be ery nterestngor such applcatons.Graphene deces can be used or terahertz detecton and re-

quency conerson. T e possblty o tunng the electronc andoptcal propertes by external means (or example through electrcor magnetc elds, or usng an optcal pump) makes SLG and FLGsutable or nrared and terahertz radaton manpulaton. T epossble deces nclude modulators, lters, swtches, beamsplttersand polarzers.

PerspectiveGraphene lms and compostes hae deal electronc and optcalpropertes or photoncs and optoelectroncs. Graphene s an attrac-te replacement or IO and other transparent conductors. In manycases (touch screens or OLEDs, or example), ths ncreases abrca-ton exblty, n addton to hang economc adantages. Present

PVAGraphene-PVA

0.01 0.1 1

~1.3%

-6 -4 -2 0 2 4 6

1,559 nm

1,553 nm

1,547 nm

1,541 nm

1,534 nm

1,526 nm

20 m

~1.3%

1,558 nm

1,568 nm

1,553 nm

1,563 nm

1,548 nm

1,560 nm

500 1,000 1,500 2,000

Measurement

Sech2

fit

0.0

0.4

0.8

1.2

1.6

64.0

64.5

65.0

65.5

66.0

66.5

0

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Time delay (ps)

1,520 1,540 1,560 1,580

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1,530 1,5 45 1,5 60 1,575

Normalizedspectralintensity

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intensity

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a b

e

c d

Wavelength (nm)

Average pump power (mW)

Figure 6 | Graphene mode-locked laser performance. a, Absorption o

graphenePVA (polyvinyl alcohol) composite and reerence PVA. Inset:

micrograph o the composite. b, Typical transmittance as a unction o

pump power at six dif erent wavelengths. Transmittance increases with

power. c, Tunable (>30 nm) fi bre laser mode-locked by graphene.

d,e, Autocorrelation (d) and spectrum (e) o output pulses o a graphene

oxide mode-locked laser, with a ~743 s pulse duration. Figure a,b

reproduced with permission rom re. 13, 2010 ACS.

Table 1 | Graphene-based mode-locked lasers

Laser type

Coupling

method

Fabrication

type

Laser parameters

ReferenceWavelength () Pulse width () Frequency Power

EDFLSandwiching

LPE1,557 nm 800 s 121,559 nm 464 s 19.9 MHz 13

1,5251,559 nm 1 ps 8 MHz 1 mW 118

CVD

1,565 nm 756 s 1.79 MHz 2 mW 119

1,576 nm 415 s 6.84 MHz 50 mW 121

1,5701,600 nm 40140 ps 1.5 MHz 122

FG1,5701,600 nm 1.08 ps 6.95 MHz 119

1,590 nm 700 s 6.95 MHz 50 mW 120

Evanescent ieldRGO

1,561 nm 1.3 ps 6.99 MHz 51 mW 123

Nd:YAG Free space 1,064 nm 4 ps 88 MHz 0.1 W 124

CNT 12 m 68 s2 ns 177 kHz17 GHz 1.6 W 12,116,125,126

EDFL, erbium-doped ibre laser; FG, unctionalized graphene; Nd:YAG, neodymium-doped yttrium aluminium garnet solid-state laser.




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lqud-crystal-based deces ace hgh abrcaton costs assocatedwth the requrement or large transparent electrodes. T e moe toa graphene-based technology could make them more able. Neworms o graphene-based transparent electrodes on exble sub-strates or solar cells can add alue and a leel o operatonal exbl-ty that s not possble wth current transparent conductors and rgdglass substrates. Recent progress n growth and dsperson process-ng o graphene hae de ntely made ths materal come o age, thusencouragng ndustral applcatons. Determnstc placement o

graphene layers on arbtrary substrates, and the creaton o multlay-ers by the nddual assembly o monolayers at gen angles, are nowpossble. Future ef orts n the eld o nonlnear optcal deces wllocus on demonstrators at df erent waelengths to make ull use ographenes ultrawde broadband capablty. T s could nclude hgh-speed, transparent and exble photosenste systems, whch couldbe urther unctonalzed to enable chemcal sensng. Ultraast andtunable lasers hae become a realty, wth an eer-growng numbero groups enterng ths eld. T e combnaton o graphene photoncswth plasmoncs could lead to a wde range o adanced deces.

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AcknowledgementsWe thank S. A. Awan, D. M. Basko, E. Ldorks, A. Hartschuh, J. Coleman,

A. Dyadyusha, D. P. Chu, . Etchermeyer, . Kulmala, A. Lombardo, D. Popa, G. Prtera,F. orrs, O. rushkeych, F. Wang, . Seyller, B. H. Hong, K. S. Nooselo andA. K. Gem or dscussons. We acknowledge undng rom EPSRC grants EP/G042357/1and EP/G030480/1, ERC grant NANOPOS, a Royal Socety Bran Mercer Award orInnoaton, the Cambrdge Integrated Knowledge Centre n Adanced Manuacturngechnology or Photoncs and Electroncs, and Cambrdge Noka Research Centre. F.B.acknowledges undng rom a Newton Internatonal Fellowshp and .H. rom KngsCollege, Cambrdge. A.C.F. s a Royal Socety Wolson Research Mert Award holder.

Additional informationT e authors declare no competng nancal nterests.


Graphene photonics and optoelectronics

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