Room-temperature valley-selective emission in Si-MoSe_2 heterostructures enabled by high-quality-factor chiroptical cavities

Domain: physics
Published: 2025-11-29
Editorial outcome: minor_revision

Abstract
Transitionmetaldichalcogenidespossessvalleypseudospin,enablingcou-plingbetweenphotonspinandelectronspinforclassicalandquantuminformationprocessing.However,rapidvalley-dephasingprocesseshaveimpededthedevelopmentofscalable,high-performancevalleytronicdevicesoperatingatroomtemperature.Herewedemonstratethatachiralresonantmetasurfacecanenableroom-temperaturevalley-selectiveemissioninMoSe2monolayersindependentofexcitationpolarization.Thisplatformprovidescirculareigen-polarizationstateswithahighqualityfactor(Q-factor)andstrongchiralnear-fieldenhancement.ThefabricatedSichiralmetasurfacesexhibitchiropticalresonanceswithQ-factorsupto450atvisiblewavelengths.Werevealdegreesofcircularpolarization(DOP)reachingarecordhighof0.5atroomtemperature.OurmeasurementsshowthatthehighDOPcanbeattributedtothesignificantlyincreasedchiropticallocaldensityofstates,whichenhancesvalley-specificradiativetransitionratesbyafactorof~13.Ourworkcouldfacilitatethedevelopmentofultracompactchiralclassicalandquantumlightsources.

Introduction
Employingquantummechanicaltwo-levelsystems(qubits)forcomputationnecessitatesresolvingthetensionbetweentwofundamental
yetconflictingcorerequirements:longcoherencetimes,andstrongcouplingtotheirenvironmentforinitializationandreadout.Electronicdegreesoffreedom,suchasangularmomentum(spinororbital),havestrongandtunablecoupling,butcoherencetendstobealimitation.Inparallel,photonspinangularmomentum(circularpolarization)typicallyhasalongcoherencetimebutintrinsicallyweakcoupling.If
electronsandphotonscouldbecoupledtogetherinmaterials,thenit
might be possible to achieve simultaneous strong coupling and
coherenceintwo-levelsystems.
Monolayersoftransitionmetaldichalcogenides(TMDCs)provide
apromisingplatformtoachievesuchelectron-photoncoupling.The
electronsinthevalleysoftheirspin-orbitsplitbandscanbeselectively
excited with circularly polarized light, resulting in valley polarized
excitonswithcorrespondingspinandvalleyindexes1–3. This new
“valleytronic” route of encoding information offers an exciting
approachforclassicalandquantuminformationstorage,processing,
andcomputation4,5.Thespin-valleypolarizationofTMDCmonolayers
can be probed and evaluated using the degree of optical circular
polarization(DOP),DOP=ðIðσ(cid:2)Þ(cid:2)Iðσ+ÞÞ=ðIðσ(cid:2)Þ+Iðσ+ÞÞ,whereIðσ±Þis
the photoluminescence intensity recorded for left-handed or right-handedcircularlypolarizedemission(σ±).
InTMDCs,thepreparedspin-valleypolarizationoftenundergoes
rapid dephasing processes, including intervalley exchange coupling
andphonon-assistedscattering,resultinginfastspindecoherenceat
room temperature6. Such dephasing prevents TMDCs from being
employed as room-temperature on-chip valley-selective devices for
applications in valleytronics7 and quantum information systems8.
Recently, it has been demonstrated that chiral light-matter interac-tionsofTMDCmonolayerscanbeenhancedbyemployingthenear-fieldsofplasmonicanddielectricnanostructures.Insomestudies,the
nanostructuresareachiral,withtheenhancednearfieldcreatedwith
circular polarized excitation9–11. In other studies, the nanostructures
are chiroptical, with broken in-plane or out-of-plane mirror
symmetries12–17. Such chiral nanostructures enable initialization and
readoutoftheTMDCvalleypolarizationwithoutrelyingontheoptical
excitation’s helicity, and henceare especiallypromising for on-chip
electro-optic transduction and quantum integrated photonics. The
superchiral near-fields around nanostructures compete with inter-valleyscatteringattheKorK′valleysintheTMDC,andthusenhance
valley-selective emission12,13,15–18. However, in each of these chiral
metasurface-aided studies,the intrinsicabsorptionloss and/or scat-teringlossoftheplasmonicorMie-resonantnanostructureslimitsthe
quality factor (Q-factor) of the resonance and the achievable chiral
near-fields.Consequently,theseTMDC-interfacedmetasurfaceseither
have to operate at cryogenic temperatures to exhibit a meaningful
DOP(e.g.,DOP=0.43atT=87K)13orexhibitlowDOPatroomtem-perature(e.g.,DOP<0.10)18.
Here,wedesignandfabricateSinanostructuresthatbreakbothin-planemirrorandinversionsymmetries,resultinginhigh-Qchiroptical
resonancesatvisiblewavelengths(Fig.1a).Wheninterfacedwithlarge-areaMoSe monolayers,wedemonstratestrongvalley-selectiveemis-2
sion(i.e.,amaximumDOPof0.5atroomtemperatureindependentof
other excitationpolarization)throughsteady-statephotoluminescence(sPL)
measurements at elevated temperatures spanning 100K up to room
temperature.Ournanostructuresarebasedonchiralquasi-boundstates
in the continuum (q-BICs), which localize light in the radiation con-tinuum, resulting in both high Q-factor resonances19 and maximum
chirality20,21. Recent demonstrations of high-Q planar and three-dimensional chiral metasurfaces show that chiral q-BIC modes can
drivemaximumcirculardichroism(CD)22–26,high-efficiencychirallight
sources22,opticalmodulators27,andnonlinearoptics24.Inourstudy,the
chiral q-BIC localizes valley-selective emission in the vicinity of the
photonicГ-point,asdemonstratedthroughreciprocalspacemappingof
theTMDCPLemission.Time-resolvedPL(TRPL)measurementsfurther
revealthatthestrongchiralnear-fieldenhancestheradiativerecombi-nation of excitons/trions at the K’ valley over the K valley through a
valley-selectivePurcelleffect.Comparedwithpreviousworksbasedon
either achiral individual10 and extended cavities9,17,28 or conventional
chiral metasurface cavities13,16,18,29, our heterostructured platform
enabledbychiralq-BICsexhibitsbothhighQ-factorsandrecord-high
DOPsatroomtemperature(Fig.1b).Takentogether,thehigh-Qchir-opticalresonancesandstrongnear-fieldvalley-selectiveenhancement
showninourworkarethekeytoachievingsuchsuperiorperformance,
enroutetoscalablehigh-performancevalleytronicdevices.

Results
Chiralq-BICmetasurfacesforvalley-selectiveemission
To achieve photon-spin-valley locking in MoSe monolayers, we
2
designedacrystallineSi(c-Si)metasurfaceonaglasssubstrate.The
metasurfaceiscomposedofperiodicallyarrangednano-squareswith
etched corners across the anti-diagonal of the square (Fig. 1a). In
contrast with conventional plasmonic and dielectric chiral
metasurfaces12,13,16,18,30–35, q-BIC-driven chiral metasurfaces leverage
symmetry-protected photonic states that exhibit, in principle, infi-nitelyhighQ-factorsandstronghelicity-selectivenear-fieldenhance-ment.TheBICstateinourdesignisavortexpolarizationsingularity
enclosed by elliptical eigen-polarizations24 and consequently is
uncoupledtofar-fieldCPL.Tuningtheasymmetryparametersbreaks
thein-planeinversionandmirrorsymmetriesandenablesthecoupling
ofchiralBICmodestofree-spaceradiation,thusformingachiralq-BIC
resonance.Allout-of-planesymmetriesarepreservedwithanindex-matchinglayer(i.e.,polymethylmethacrylate(PMMA))ontopofthe
metasurface(Fig.1a)toachievethemaximalplanarchiropticaleffects.
Doingsocreatesacirculareigen-polarizationstateattheГ-pointsur-roundedbyellipticaleigen-polarizations(SupplementaryFig.1).Fig-ure1cshowsthesimulatedtransmissionspectraoftheJonesmatrix
elementsonthecircularpolarizationbasis(T ,wherei=r,l;j=r,l;r
ij
representsright-handedcircularpolarizationorRCP,lrepresentsleft-handedcircularpolarizationorLCP;iandjrepresentcircularpolar-izationstatesofoutgoingandincominglight,respectively)foroneof
our designed c-Si metasurfaces. A high-Q chiroptical resonance at
772nmisshownintheT -componentspectrumwhichisnotseenin
rl
therestofcomponentspectra(T ,T ,andT ).FittingT totheFano
lr ll rr rl
expression(SupplementaryFig.2a)yieldsaQ-factorof677.Notethat
the transmission CD does not reach unity, mainly due to the inter-ference between the chiral q-BIC mode and background scattering
channels(e.g.,higher-orderMiemodes).Multipoleexpansionofthe
chiral q-BIC suggests that the mode is mainly characterized by an
electricquadrupole(EQ)(SupplementaryFig.3).
Inourdesign,theetchedlengthandwidthdifferencesinthex
andydirections(ΔL=L1−L2,ΔW=W1–W2)areengineeredtoadjust
Q-factorsandchiropticaleffectsofthechiralq-BIC.Theoveralledge
length (L) and height (H) of the nano-square determine the reso-nancewavelengthsandultimatelythedetuningbetweenchiralq-BIC
andvalleyexcitonresonanceinmonolayerMoSe .Figure1dreveals
2
that smaller ΔW, ΔL yield Q-factors exceeding 1000, which would
otherwise scaleuprapidlyinaninverselyquadraticrelationshipwith
the asymmetry parameter (e.g., α=ΔW/W1) assuming zero non-radiativelossatvisiblewavelengths(SupplementaryFig.4).Toverify
theroleofchiralq-BICmodeinenablingvalley-selectiveemission,an
ensemble of in-plane emitting dipoles are employed to excite the
chiralq-BICmode(SupplementaryNote1).Thesedipolesareachiral
andthereforepopulateKandK′valleysequally,butvalleydepolar-ization is ignored in these simulations. Figure 1e shows how our
structureexhibitsanearlyunitaryemissionCD,suggestingthatthe
chiral q-BIC mode favors σ(cid:2) emission over σ+ emission in the
metasurface’supperspace,andjusttheoppositeinthelowerspace
(SupplementaryFig.5).ThisbehaviorpersistsaswealterΔW(Sup-plementaryFig.6).Figure1fshowsthestronghelicity-selectivenear-field enhancement (also see Supplementary Fig. 7, shown for a
ΔW=22nm,ΔL=2nm,L=297nm,H=220nm).Astrikingcontrastin
thecross-sectionalelectricfieldenhancementisseenwhenthechiral
q-BICmodeisexcited(e.g.,at772nm)byLCPversusRCP,confirming
the role of the chiral q-BIC in differentiating the states of circular
polarization.Inordertoevaluatethenear-fieldchiralitythroughthe
chiralmetasurfacestructure,wecomputedopticalchiralitydensity
(OCD)usingOCD= (cid:2) ω ImðE*(cid:3)HÞ36,whereEandHrepresentcom-2c2
plexelectricandmagneticfields,andωistheangularfrequencyof
light. Complementary to both unity emission CD and near-field
chiropticalenhancementisthenetnear-fieldchiralityidentifiedon
the plane justatopthemetasurface structurewhereMoSe mono-2
layersreside(seeFig.1g,h).Takentogether,thechiralq-BICmodein
our design readily enables valley-selective emission in MoSe
2
monolayers.

Wefabricatedthec-Sichiralmetasurfacesonaglasssubstrate.
Thesec-Sithinfilmshavemuchlowerlossinvisibleandnear-infrared
frequencies than amorphous and polycrystalline Si37–39. To fabricate
c-Si chiral metasurfaces on an isotropic substrate and gain optical
accesstotheMoSe -integratedstructuresonbothsides,wedeveloped
2
awaferdirect-bondingprocesstotransferc-Sithinfilms(thickness:
220nm)fromasilicon-on-insulator(SOI)waferontoadouble-sided
polished borofloat glass wafer (see “Methods” and Supplementary
Fig.8).Theresultingc-Sifilmsshowmuchlowerlossinthespectral
region of interest (Supplementary Fig. 9), which is consistent with
literaturereports37,38.Wethenfabricatedc-Simetasurfacesontheglass
substrateusingelectron-beamlithography(see“Methods”).Allofthe
fabricatedmetasurfaceshaveaheightof220nmdefinedbythebon-dedc-Sithinfilm.Figure1ishowsoneofthefabricatedmetasurfacesin
top-viewandtilted-viewscanningelectronmicroscope(SEM)images,
inwhichP=430nm,L=297nm,ΔL=2nm,ΔW=22nm.Nonoticeable
differencebetweensharp-cornernanostructures(Fig.1a,f)andround-cornernanostructuresduetouncertaintyinpatterningsharpedges
(Fig. 1i) is observed in simulated transmittance and electric field
enhancement distribution (Supplementary Fig. 10). We utilized a
home-builtopticalsetup(SupplementaryFig.11andSupplementary
Note 2) to measure the transmission spectra of the metasurfaces
coatedwithPMMA.Ahigh-Qchiropticalresonanceisseenat772nm
withaFanolineshape(greencurve)asshowninFig.2a,matchingthe
T -componentspectrumshowninFig.1c.Fittingthespectrumtothe
rl
Fano expression (Supplementary Fig. 2b) yields a Q-factor of 304.
Overall,thefabricatedmetasurfacesexhibitexperimentallyfittedQ-factors ranging from 200 to 450, and the resonances span the
A-excitonemissionwavelengthrangeofMoSe monolayers(Supple-2
mentary Fig. 12). These sets of high-Q metasurfaces allow us to
examine the influence of resonance detuning between chiral q-BIC
modeandA-excitonuponvalley-selectiveemission.

Valley-selectiveemissionatelevatedtemperatures
Weexfoliatedlarge-area MoSe single-crystalmonolayers frombulk
2
crystalsusingthegoldtapeexfoliationmethod(see“Methods”)40and
subsequentlytransferredthemonolayersontoourchiralmetasurfaces
andcoatedthemwithPMMA.AnimageforoneoftheMoSe -meta-2
surfaceintegrateddevicesisshowninFig.2b,withmonolayers(pink
andgreen)spanningboththemetasurfacearray(yellowsquare)and
glasssubstrate.Theincompletemonolayercoverageonthemetasur-faceisduetorelativelyweakadhesionbetweenthetransferredMoSe
2
monolayerandc-Simeta-atomscomparedtothatbetweenthetrans-ferredmonolayerandtherelativelyflatglasssubstrate.Theintegrated
devices were characterized at elevated temperatures (100–294K)
usingahome-builtspin-resolvedcryogenicPLsetup(Supplementary
Fig. 13 and Supplementary Note 2). Central to assessing the valley-selective Purcell effect imposed solely by the metasurfaces is initi-alizing exciton population equally at K and K′ valleys. In our mea-surements,weexcitedtheintegrateddeviceswithlinearlypolarized
lightat633nm(or640nm)andmeasuredPLemissionofσ+ andσ(cid:2)
photons(seeFigs.1aand2c)throughacombinationofaquarter-wave
plateandaWollastonprismoralinearpolarizer(see“Methods”).We
conductedhyperspectralPLimagingoftheintegrateddevices,which
allowsustoassesstheheterogeneityofthePLintensity,peakwave-length, and linewidth. These properties are associated with both
monolayers’ quality and the interplay between monolayers and
meta-atoms.
By enforcing the chiral q-BIC mode resonant with valley-dependentexcitontransitionsinmonolayerTMDCs,ourchiralmeta-surface can enhance the σ(cid:2) radiative transition rate from K′ valley
(Fig. 2c), regardless of the state of photoexcitation polarization.
Similarly, σ+ emission can be enhanced by redesigning the meta-surfaces with the opposite chirality. Figure 2d shows the intensity
mapsofσ+ andσ(cid:2) emissionobtainedfromsPLmeasurements,sug-gestingstrongvalley-selectiveemission.However,thisbehaviorisnot
seenforbaremonolayersontheglassorc-Sigivenverycomparable
intensity maps between σ+ and σ(cid:2) emission (see Supplementary
Fig.14a,b).PleasenotethatthePLintensityvariationacrossthemaps
isprimarilyattributedtotheincompletemonolayercoverageonthe
metasurfaceandthespectralvariationacrossthesurfacemetasurfacearrayis
negligible given a narrow spatial distribution of q-BIC resonance
wavelengths(seeSupplementaryFig.15).Toruleoutanychiroptical
effectsneartheexcitationwavelengthsthatmaycontributetochirally
selective absorption, we simulated the transmission spectra of the
Jones matrix elements and near-field electric field profiles. No chir-optical resonances are observed, and comparable electric field
enhancements are obtained for both LCP and RCP excitation (Sup-plementaryFigs.16and17).Takentogether,itisthechiralq-BICmode
at the excitonic emission wavelength that drives valley-selective
emission. High-intensity pixels are correlated to the monolayer-covered areasshown in Fig. 2b. Further characterization using SEM
revealsthatmonolayerflakesaremoreinclinedtobeintactandcon-sequentlyforminterfacialdomains(SupplementaryFig.18).

Figure3adisplayssPLemissionspectraforasinglepixel(5×5μm)
obtained from photon-spin-resolved hyperspectral imaging of the
MoSe -metasurface integrated device shown in Fig. 2b at different
2
temperatures(100,140,200,and294K).Weobserveseveralchanges
inthePLwithtemperature,includingred-shiftedemissionlineshapes,
decreased intensity, and increased linewidth with increasing tem-perature, consistent with the literature reports on TMDC
monolayers41,42. However, we also observe valley-selective emission
behavior across all temperatures including notably, room tempera-ture. We also compare PL emission of MoSe monolayers on the
2
metasurfacestothatofbaremonolayersontheglasssubstrate.The
exciton/trionemissionintensityisenhancedbyafactorofcloseto10
attemperaturesrangingfrom100Ktoroomtemperature(seeSup-plementary Fig. 19), suggesting c-Si metasurfaces’ Purcell enhance-ment.EachPLpeakoriginatesfromanintrinsicLorentzianlineshape
broadened by a Gaussian distribution due to inhomogeneous
broadening43.WefitthemainexcitonpeaktoaVoigtfunction,i.e.,a
convolutionofaLorentzianandaGaussian44,andcapturetheshoulder
feature (which is assigned to trion recombination) via a secondary
Gaussian (see Supplementary Fig. 20). In order to quantify valley-selectiveemission,weusethesingleemissionintensityvaluesatpeak
wavelengthsfromthefitstocomputemaximumDOPsforindividual
pixels in the hyperspectral images. This analysis yields the spatial
distribution of DOPs for the integrated devices at various tempera-tures(seeFig.3b).UnlikenearlyzeroDOPsseenforbaremonolayers
ontheglasssubstrate(seeSupplementaryFigs.14cand21),highDOPs
areobtainedformonolayersonthemetasurfaces.Asthetemperature
increases,theoverallDOPincreasesandreachesarecordhighof0.5at
294K.Thisbehaviorisattributedtotheresonancedetuningbetween
excitonsandchiralq-BICmode,whichiscontrolledbytemperature-dependentexcitonenergy.
WehavedeterminedspatiallyaveragedDOPsforallcharacterized
integrateddevices.Figure3cshowsspatiallyaveragedDOPversusthe
resonancedetuning(δλ)atelevatedtemperatures,whereδλ=λ (cid:2)λ
0 1
is the difference between the chiral q-BIC mode resonance wave-lengthsthatweareabletomeasureonlyat294K(λ )usingouroptical
0
setup (see “Methods” and Supplementary Fig. 11) and the exciton
resonance wavelengths at elevated temperatures ðλÞ. For the inte-1
grateddevicepresentedinFig.3b,whenthetemperatureincreases,δλ
approacheszeroandconsequentlythespatiallyaveragedDOPreaches
itsmaximum(seegreenstarsinFig.3c).ThespatiallyaveragedDOPs
peakatacertaindetuningwhentheintegrateddevicesaremeasuredat
various temperatures. Only when measured at 294K are the DOPs
peakednearδλ=0,whichmaybefurtherboostedifthechiralq-BIC
resonanceisslightlytunedtotheexcitonresonancewavelength.At
lowertemperatures,thedetuningatwhichmaximumDOPisachieved
shiftstowardamorepositivedetuning.Thisdetuningisattributedto
theshiftedchiralq-BICmoderesonanceatdifferenttemperatures.The
refractive index of Si decreases asthe temperaturedecreases45 and
chiralq-BICmoderesonancewavelengthisultimatelyblue-shifted(see
Supplementary Fig. 22). That being said, at a lower temperature, a
maximumspatiallyaveragedDOPisseenatamorepositivedetuning.
Theextenttowhichthechiralq-BICmoderesonanceisblue-shifted
observedinoursimulationsislesspronouncedthanmaximumDOP’s
detuningshiftdeterminedinourexperiments.Thestraininducedby
PMMA’sthermalresponsemaycontributetothisdisparity,whichis
difficultto incorporate into simulations.These results highlight the
importanceofaligningthechiralq-BICmodetoexcitonresonancefor
maximizing valley-selective emission. Additionally,mitigation ofthe
unfavorablestraineffectinducedbyPMMA’sthermalresponsemay
explaintheobservedincreaseinspatiallyaveragedDOPswithrising
temperature. As a further test, we altered the state of excitation
polarization (i.e., applying linear and circular polarizations) and
examined the resulting valley-selective emission properties. We
observealmostthesameDOPspatialdistributionandspatiallyaver-agedDOPsirrespectiveoftheexcitationpolarization(Supplementary
Fig. 23). In other words, the photon-spin-valley locking is robust
againstthewaythatvalleyexcitonpopulationisinitialized.
Ourmetasurfacedesignexhibitsacirculareigen-polarizationat
the Г-point surrounded by elliptical eigen-polarizations in the
momentum space (Supplementary Fig. 1). To interrogate how the
chiralq-BICmodeshapesradiationpatternsfromMoSe monolayers,
2
weconductedsPLmeasurementsinmomentumspaceandobtained
angle-resolved emissionofσ+ and σ(cid:2) photons from the integrated
devices.PLemissionfromtheMoSe monolayerintegratedonmeta-2
surfacesislocalizedinthevicinityoftheГ-pointinadditiontostronger
σ− emission, as shown in Fig. 4a. In contrast, the angular emission
intensities for σ+and σ− photons are almost the same across the
momentum space for MoSe monolayers on the glass substrate
2
(Fig.4b).Whenthetemperatureincreasesfrom140to294K,thePLis
morelocalizedinthemomentumspacedespiteitsbroadeningalong
thewavelengthaxis(SupplementaryFig.24).Theseobservationsare
consistentwiththeaboveanalysisandconfirmthecapabilityofthe
chiral q-BIC mode for the simultaneous control of valley-selective
emissionandradiationpattern.

Valley-selectiveemissiondecaydynamics
Weevaluatedtheradiativerecombinationratesfromvalleypolarized
excitons/trionsatKandK′valleysinourintegrateddevices,inorderto
determine the valley-selective Purcell enhancement enabled by the
chiral q-BIC mode. The experiments were carried out with photon-spin-resolvedTRPL,whichmeasuresthedecayprofilesofσ+ andσ(cid:2)
emissions.Thetotalpopulationrelaxationdynamicsisasumof
radiative and nonradiative recombination, respectively. Figure 5a
shows PL decay profiles at elevated temperatures for a single pixel
obtained from the spin-resolved hyperspectral imaging of the inte-grateddevice.Asthetemperatureincreases,PLemissionofbothσ+
and σ(cid:2) photons decay more rapidly. We fitted the TRPL data to a
biexponential decay model, A e(cid:2)t=τ 1+A e(cid:2)t=τ 2, convoluted with the
1 2
instrumentresponsefunction(IRF),whereτ andτ arethefastand
1 2
slowdecaytimeconstants,respectively.τ capturesPLdecaysforboth
1
excitonsandtrions,whereasτ isattributedtotherecombinationof
2
defect-trappedexcitons9.OursPLmeasurementsabovedemonstrate
thatPLemissionofvalley-polarizedexcitonsandtrionsarestrongly
modifiedbythechiralq-BICmode.Thus,wefocusonhowthechiral
q-BICmodealtersτ (hereafterasτ).Althoughtheintrinsicradiative
1
recombinationrateforabrightexcitoninTMDCmonolayersliesinthe
rangeofaps−1,forourtemperaturerange(≥100K),onlyasmallfrac-tion of the excitons in a thermal distribution lie in the light cone,
leadingtoamuchreducedeffectiveradiativedecayrateontheorder
ofns-146,47.Thislongerradiativedecaytimeleadstogreatersensitivity
tononradiativedecaychannels,suchastrappingofexcitonsatdefects
and,athigherdensities,exciton-excitonannihilationprocesses46.The
latter,however,isunlikelytobesignificantforourconditionsofweak
continuous-wavelaserexcitation.
Weobservethatσ(cid:2)emissiondecaysfasterthanσ+ emission,with
morepronouncedcontrastatelevatedtemperaturessuchat294K,as
showninFig.5a.Overall,theemissionlifetimeτ formonolayerson
metasurfacesisfoundtobesignificantlyshorterthanthatformono-layersontheglasssubstrate(0.189nsversus0.398nsat294K)(see
Supplementary Fig. 25). Note that although the dominant decay
channels are nonradiative, particularly at 294K, the reduction of
emissionlifetimesiscausedbyenhancedradiativetransitionrates.The
nonradiativedecayprocessesareindependentofchiropticaleffects.
Weestimatethatradiativedecayratesformonolayersonmetasurfaces
increasebyafactorof~13comparedtothatformonolayersonthe
glasssubstratebyassumingnonradiativedecayis10timesfasterthan
radiative decay as reported by the literature48. Such strong valley-selectivePurcellenhancementisenabledbyhigh-Qchiropticalreso-nances and strong chiral near-field enhancement. The spatial dis-tributionofτforσ+ andσ(cid:2)emissionatdifferentlocationsisdisplayed
in Fig. 5b. Despite its heterogeneity, the lifetime of σ(cid:2) emission is
shorterthanthatofσ+ emissionbyafactorofuptonearlytwoat
294K, indicating valley-selective decay dynamics at elevated tem-peratures. In contrast, there is no meaningful difference in the life-timesforσ+ andσ(cid:2)emissionfrommonolayersontheglasssubstrate
(seeSupplementaryFig.26).Furthermore,toquantifyvalley-selective
Purcell enhancement and valley-selective emission, we estimate the
time-integrated valley polarization using the following phenomen-ological equation, ρ=ðτðσ+Þ(cid:2)τðσ(cid:2)ÞÞ=ðτ v+τðσ+Þ+τðσ(cid:2)ÞÞ18. In this
equation, τ v is the intervalley scattering time (see Supplementary
Note3foradditionaldetails),whichis<10psat≥100K18,44andthere-fore is minimal compared to τðσ(cid:2)Þ and τðσ+Þ. The expression then
reduces to ρ=ðτðσ+Þ(cid:2)τðσ(cid:2)ÞÞ=ðτðσ+Þ+τðσ(cid:2)ÞÞ. Figure 5c shows the
statisticaldistributionofspatiallyaveragedρversusδλidentifiedinsPL
measurementsforalltheintegrateddevicesmeasuredusingTRPL.ρis
temperature-dependentandshowsaverysimilartrendasDOPversus
δλinFig.3c.ρisalsomaximizedatcertaindetuningwhentheinte-grated devices are measured at individual temperatures, additional
evidence that chiroptical effects enhance valley-selective PL decay
dynamics.

Methods
Samplefabrication
SOI and borofloat glass wafers (4”) were first cleaned in a piranha
solution.Thewafers’surfaceswereactivatedthroughN /O reactive
2 2
ionetchingbeforetheyweredirectlybondedusingawaferbonder
(ElectronicsVision501)at400°Cunderapressureof2000Nanda
highvacuum(10−6torr).Thebondedwaferswereprocessedviaawafer
grinder (Disco Backgrinder DAG810) to remove the majority of Si
handlinglayerandfinishedbyXeF etching(Xactixe-1)toleavethe
2
thermaloxideonthethin-filmSilayer.Thethermaloxidewasremoved
by 6:1 BOE etchant to yield 220-nm Si-on-glass wafers. Diced chips
fromtheSi-on-glasswaferswereexposedusingelectronbeamlitho-graphy (Raith EBPG5200). Specifically, an electron-beam resist,
hydrogen silsesquioxane (HSQ), was spin-coated on 220-nm Si-on-glasschipsandbakedat90°Cfor2min.Wecoatedalayerofelectron
conductivelayer(espacer)forexposurebeforebeingdevelopedina
saltydevelopsolution(1%NaOH:4%NaCl).Thesamplewasthenetched
byaninductivelycoupledplasmaofCl /HBrandthenHBr(OxfordIII-V
2
etcher)andtheresistwasremovedina2%HFsolutionafterwards.
The monolayer MoSe is prepared with a gold tape exfoliation
2
technique40. In brief, a 100nm thick flat and clean gold layer is
depositedonacleanSiwafer,whichislaterusedasthegoldtape.The
gold tape is covered with a layer of polyvinylpyrrolidone (PVP) for
protection against contaminations and a thermal release tape as a
handlinglayer.Aflatandcleangoldsurfaceisobtainedfrompeeling
theTRT/PVP/AulayerofftheSisubstrate.Thefreshlypreparedgold
surface is brought in contact with the MoSe bulk crystal (HQ gra-2
phene)andexfoliatesoffamonolayer.Thestackistransferredontoa
SiO /Sichipfollowedbyheatingtoremovethethermalreleasetape.
2
ThePVPlayerisdissolvedwithwater,andthegoldlayerisdissolved
with KI/I etchant solution. After preparation, the monolayers were
2
thentransferredtometasurfaceswithcelluloseacetatebutyrate(CAB)
usingawedgingtransfertechnique57.PMMAwascoatedbeforeand
aftertransferringMoSe monolayersfortransmissioncharacterization
2
andcryogenicPLmeasurements,respectively.

Numericalsimulations
The transmission spectra of the Jones matrix elements and
momentum-spaceeigen-polarizationsweresimulatedwiththefinite
element method (COMSOL Multiphysics Wave Optics module). c-Si
thinfilmsandborofloatglasssubstratewerecharacterizedusingan
ellipsometer (J. A. Woollam) to obtain optical constants (n and k),
whichwereusedthroughoutthesimulations.ThevalueforPMMAis
setasn=1.485.Multipoleexpansionanalysiswasperformedwiththe
FDTDmethod(Lumerical).

Transmissioncharacterizationoffabricatedmetasurfaces
A supercontinuum laser (NKT Photonics Extreme Supercontinuum)
deliveredaVis/NIRbroadbandlightsource(500–1100nm)throughan
outputofaspectralsplitter(SuperKSplit).Thelightsourcefirstpassed
through a pinhole to achieve a 1-mm beam size and then a set of
a linear polarizer (Union Optic PGT5012) and a quarter-wave plate
(Union OpticWSA4220-380-1100-M25.4)togeneratecircularlypolar-ized light. The beam was weakly focused by a low-NA objective
(Olympus 10x NA 0.25) onto a sample to achieve a beam size of
~80μm. The effective NA is significantlyless than 0.25 to eliminate
spin-orbit coupling and excitation of higher-order modes at high
k-vectors(seeSupplementaryNote2formoredetails).Thesamplewas
mounted on a motorized rotation stage. The transmitted light was
capturedbyasecondlow-NAobjectiveandpassedthroughapolar-izationanalyzercomposedofanothersetofalinearpolarizeranda
quarter-wave plate. The transmitted light was focused onto the
entranceslitofaspectrophotometer(PrincetonInstrumentsIsoPlane
160) and imaged by a Si CCD camera (Princeton Instruments PIXIS
400BRX).

Steady-statephotoluminescencecharacterization
In our sPL measurements a continuous-wave (cw) laser beam (CNI
MDL-III-633L) at 633nm first propagated through a half-wave plate
(Newport10RP42-2),alinearpolarizer(UnionOpticPGT5012),anda
quarter-wave plate (Union Optic WSA4220-380-1100-M25.4) to gen-eratelinearlyorcircularlypolarizedlight.Thebeamwasthenfocused
to a diffraction-limited spot (see Supplementary Note 2 for more
details) by overfilling the back aperture of a high-NA objective
(OlympusMPLFLN50xNA0.8)installedinCryoAdvance100(Montana
Instruments).Theexcitationpowerwasmeasuredtobe5μWpriorto
enteringtheobjectiveandkeptconstantthroughoutallofthemea-surements.PLwascollectedthroughthesameobjectiveandpassed
througha50:50beamsplitter,anotchfilter(EdmundOptics632.8nm)
withatransmissionwavelengthrangeof475–845nm,along-passfilter
(EdmundOptics725nm),andthenaquarter-waveplate(UnionOptic
WSA4220-380-1100-M25.4).CircularlypolarizedPLemissions(σ+ and
σ(cid:2))wereconvertedtos-andp-polarizedlight,whichco-propagated
through two different achromatic doublet lenses (f =200mm and
1
f =150mm)beforepassingaWollastonprism(ThorlabsWPQ10).The
2
σ+ andσ(cid:2)emissionswerethenspatiallyseparated.Forreal-spacePL
spectroscopy,spatiallyseparatedemissionswerefocusedthrougha
thirdachromaticdoubletlens(f =100mm)ontotheentranceslitofa
3
spectrophotometer(PrincetonInstrumentsActonSP2500)andthen
detectedbyaliquid-N cooledSiCCDcamera(PrincetonInstruments
2
PyLon400BeXcelon).Thus,σ+ andσ(cid:2)lightemissionsweremeasured
usingasinglecameraforbalanceddetection.Raster-scanningasample
using the Rook nanopositioner stack while acquiring PL emission
enableshyperspectralimagingoftheintegrateddevices.ForFourier-space spectroscopy, spatially separated emissions entered the
entranceslitofthespectrophotometerwhichwaspositionedatthe
backfocalplaneofthesecondlens.

Time-resolvedphotoluminescencecharacterization
Afiber-coupledpulsedlaser(PicoQuantPrima)at640nm(pulsewidth
100ps) was co-aligned with the cw 633-nm laser and propagated
through the same optical components for excitation. The average
excitationpowerwasmaintainedat0.21μW,whichtranslatedtoan
intensityof8.3μJ/cm2onthesample,whichwasemployedthroughout
alloftheTRPLmeasurements.Thepulserepetitionratewas20MHz.
The IRF was obtained by measuring laser light reflected on the
sample. The emissions propagated through the same beam splitter
and quarter-wave plate before entering a second channel for
TRPL measurements. A linear polarizer (Union Optic PGT5012)
mountedonamotorizedrotationstage(ThorlabsPRM1Z8)wascon-trolledforselectings-orp-polarizedlightemissionstoimpingeontoa
fiber launching system and then detected by a fiber-coupled single
photon avalanche photodiode (Excelitas SPCM AQRH14 SPAD). PL
decayprofileswereobtainedbytaggingthearrivaltimeofeachPL
photon to the laser trigger pulse using a time-correlated single
photon counting module (PicoHarp 300) with a time resolution of
4ps.Thedatawerere-binnedbyafactorof4,whichequalsatemporal
resolutionof16ps.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have tried to address some of the questions raised previously. However as noted in the response, the present structure does not have net near field chirality and so it is unclear what advantage the present structure provides. Most of the data is in the SI because a large body of this work has already been reported in other works. This work should be published, but I am not convinced it is suitable for Nature Communications.", "reviewer_3": "The authors have tried to address some of the questions raised previously. However as noted in the response, the present structure does not have net near field chirality and so it is unclear what advantage the present structure provides. Most of the data is in the SI because a large body of this work has already been reported in other works. This work should be published, but I am not convinced it is suitable for Nature Communications."}, "author_response": "We thank the reviewers for re-reading our revised manuscript. We appreciate Reviewer 1 and 2 recommending our paper for publication. We noticed that Reviewer 3 appears to have misunderstood our newly provided evidence. The significant advances presented in our manuscript and the distinctions between our work and prior studies have unfortunately been overlooked. Please see below our point-by-point response to Reviewer 3’s comments. In addition, we have slightly modified the order of Figures 1d–f in the Main text and revised the accompanying text accordingly (see marked changes in the Main text).\n\nReviewer 3\nThe authors have tried to address some of the questions raised previously. However as noted in\nthe response, the present structure does not have net near field chirality and so it is unclear what\nadvantage the present structure provides. Most of the data is in the SI because a large body of this\nwork has already been reported in other works. This work should be published, but I am not\nconvinced it is suitable for Nature Communications.\nAuthor Reply: We appreciate the reviewer’s recognition that the significant findings and\nconclusions presented in our work warrant publication.\nRegarding the net near-field chirality, our new evidence—namely, calculations of near-field\noptical chirality density (OCD)—indeed demonstrates a net near-field chirality at the top surface\nof the structure (see Figure R1a), where the MoSe monolayer is situated. Specifically, we observe\n2\na positive surface-integrated OCD (see Figure R1b). This net near-field chirality is both critical\nand sufficient for the direct observation of room-temperature valley-selective emission, as noted\nin our work, when a MoSe monolayer with a thickness of less than 1 nm is coupled to the\n2\nmetasurface. In other words, achieving such high performance does not require net volumetric\nnear-field chirality throughout the entire structure. It is important to note that the MoSe\n2\nmonolayers are in contact only with the very top surface of the structure, rather than occupying\nthe entire internal volume. Unfortunately, the reviewer appears to have overlooked this critical\npoint. We have strengthened this critical point for better clarity by revising this sentence and\nproviding a modified figure (see Figure R1 below or Figure 1g and 1h in the Main text).\nOriginal\n“Complementary to this observation is the near-field chirality identified on the plane just atop the\nmetasurface structure where MoSe monolayers reside (see Supplementary Fig. 8).”\n“In order to evaluate the near-field chirality through the chiral metasurface structure, we computed\noptical chirality density (OCD) using 𝑂𝐶𝐷 = − 𝜔 𝐼𝑚(𝐸∗ ∙𝐻),36 where E and H represent\n2𝑐2\ncomplex electric and magnetic fields, and 𝜔 is the angular frequency of light. Complementary to\nboth unity emission CD and near-field chiroptical enhancement is the net near-field chirality\nidentified on the plane just atop the metasurface structure where MoSe monolayers reside (see\nFigure R1. (a) Cross-sectional distribution of normalized OCD atop the chiral metasurface\nstructure at Z = 110 nm. Note that the MoSe2 monolayers are in contact only with the top surface\nof the structure at Z = 110 nm, rather than occupying the entire internal volume. The origin of the\ncoordinate is at the center of the nanostructure. (b) Surface-integrated OCD over the xy-plane along\nthe z-axis. The location where MoSe monolayer resides shows a positive surface-integrated OCD.\ntemperature valley-selective emission.\nWe also respectfully disagree with the subjective assessment that a large portion of this work has\nalready been reported in prior studies. While previous works have indeed demonstrated various\nphotonic platforms, the heterogeneous integration of TMDCs with chiral quasi-bound states in the\ncontinuum (q-BIC) metasurfaces—exhibiting both high Q-factors and maximal chirality—has not\nbeen previously reported. We conducted a meticulous comparison between our work and previous\nstudies (see Figure 1b, Supplementary Fig. 27, and Table S1). To the best of our knowledge,\nour heterostructured platform outperforms all state-of-the-art demonstrations across several\naspects, including the highest degree of polarization (DOP), room-temperature operation, large-\narea scalability, control over the radiation pattern, and the absence of optical spin injection, which\nis desirable for electro-optic transduction.\nRegarding the advantages of our structure, first, it exhibits significantly higher-Q chiroptical\nresonances than any previous designs aimed at enhancing valley-specific light-matter interactions\nin transition metal dichalcogenides (TMDCs). Second, while achieving net near-field chirality\nacross the entire structure is intriguing, previous approaches typically require either two-step\nlithography (Light Sci. Appl. 2023, 12, 250) or stringent slant etching techniques (Science 2022,\n377, 1215; Nature 2023, 613, 474) to break out-of-plane mirror symmetries and attain chirality. In\ncontrast, our patterning procedure is much simpler, utilizing a one-step lithography process and\nstandard dry etching methods. We have also added the following text to emphasize the advantages\nof our structure.\n“Our structural design with maximal planar chirality and significantly high-Q chiroptical\nresonances, in combination with simpler patterning and etching procedures, render our platform\nmore advantageous in enhancing valley-specific light-matter interactions in monolayer TMDCs\nthan other reported metasurfaces with three-dimensional chirality.22,23,25”\nLastly, as demonstrated in our work, we achieve record-high DOP at room temperature (see Figure\n3b), precise control over the radiation pattern to enhance photon collection (see Figure 4), and\nindependence from excitation polarization (see Supplementary Fig. 23), all of which make our\nstructure more advantageous and appealing. Notably, the independence from excitation\npolarization is crucial for the development of large-area spin-polarized electroluminescent devices.\nIn summary, the unprecedented properties demonstrated in our work have not been reported in\nprevious studies. Specifically, our platform achieves:\n1. the highest DOP at room temperature among all reported heterogeneously integrated\nTMDC-metasurface works;\n2. chiroptical resonances with the highest Q-factors for enhancing valley-specific light–\nmatter interactions;\n3. unprecedented scalability enabled by nonlocal chiral q-BIC modes for large-area valley-\nselective emission;\n4. precise control of the radiation pattern for efficient photon extraction; and\n5. strong potential for electro-optical transduction due to spin-free injection.\nThus, we believe our platform represents a significant advancement toward optically addressable\nvalley pseudospins in monolayer TMDCs for both classical and quantum information processing\nusing high-Q nanophotonic architectures. Moreover, it is poised to stimulate broad interest in\nleveraging high-Q chiroptical resonances for enabling chiral electro-optic transduction, single-\nphoton emission encoded with spin angular momentum, and spin-polarized chiral polaritons."}, {"round": 2, "reviewer_comments": {"reviewer_2": "I believe the authors have provided sufficient details and made appropriate revisions in response to the reviewer comments. The manuscript is well written and presents novel results. I recommend it for publication in Nature Communications."}, "author_response": "We thank the reviewer for carefully reading our revised manuscript and response, and we greatly appreciate the reviewer’s recommendation to publish our work in Nature Communications."}]

Decision letter