Integrated broadband and high-efficiency difference frequency generation

Domain: physics
Published: 2025-12-10
Editorial outcome: minor_revision

Abstract
High-efficiencyintegrateddifferencefrequencygeneration(DFG)haslong
beenpursuedforopticalcommunicationsandsignalprocessing.Recent
developmentsofthethin-filmlithiumniobateplatformenablestrongoptical
confinementinnanoscalewaveguides,greatlyenhancingthenonlineareffi-
ciencies.However,theabsoluteDFGconversionefficiencies(CEs)inrecent
studiesarelimitedbecauseoffailtosatisfythephase-matchingcondition
strictly.Here,wedemonstrateanintegratedDFGdevicebasedonanadapted
thin-filmperiodicallypoledlithiumniobatewaveguide.Thegeneratedidler
waveachievesamaximumoutputpowerof13.2dBmandaCEof48.6%.Fur-
thermore,ourdeviceexhibitsflatopticalresponsesandhigh-qualityeyedia-
gramswhenconvertingasignalat1638nmtoanidlerat1556nm,enabling
bringingunique-bandlightintotheamplifier’sgainband.Byovercomingthe
efficiencylimitationsofpreviousDFGimplementations,ourworkopenswider
possibilitiesforpracticalapplicationsinopticalcommunications,wavelength
conversion,andsignalamplification.

Introduction
Globaldatatraffichasemergedasacriticalchallengeamidtheexpo- transformingcoherentlightwithinconventionalgainspectraintolight
nential growth of link capacity requirements in the era of artificial atdesiredwavelengths.Crucially,theycanalsoshiftlightfromoutside
intelligence. Accessing new wavelength spectrabeyond theconven- the amplifier’s gain band into the amplifiable range, thereby over-
tionalcommunicationbandinopticalcommunicationsiscrucialfor comingspectrallimitations.Amongthesetechniques,DFGstandsout
increasingtransmissioncapacity,asitenablesmoredatachannelsto duetoitsbroadwavelengthtunability,makingituniquelyvaluablefor
bemultiplexedwithinthesamefiber.However,coherentlightwaves flexibleandwidebandopticalsignalprocessing.
Periodically poled lithium niobate (PPLN) has been widely
are typically generated through stimulated emission in III-V semi- explored for nonlinear wavelength conversion through parametric
conductor materials, such as InP and InGaAs-based laser sources1–5. processesowingtoitsexcellentopticalproperties,suchasabroad-
The achievable wavelengths are fundamentally constrained by the bandtransparencywindowrangingfrom400nmto5μmandlarge
electronic band structure of these materials, making it difficult to second-ordernonlinearcoefficients(27pm/V@d ).Theemergenceof
33
producecoherentlightatcertaindesiredwavelengths.Additionally, thinfilmlithiumniobate(TFLN)platformwithnano-scaleconfinement
conventional optical fiber amplifiers, such as erbium-doped fiber of opticalfieldsenhanceslight-matterinteractionstrengthforseveral
amplifiers(EDFA),exhibitlimitedgainbandwidth,restrictingtheuseof orders,significantlyreducingthedevicelengthandpumppowerwhen
wavelengths outside their amplification spectrum. Nonlinear para- compared with bulky PPLN28–30. Moreover, compact thin-film PPLN
metric processes, including second-harmonic generation6–12, sum- devices make nonlinear wavelength conversion easier be integrated
frequency generation13–16, and difference-frequency generation with other optical components to realize versatile and functional
(DFG)17–27, provide a powerful solution to these challenges by chips31–34. Consequently, DFG devices based on thin-film PPLN have
attractedsignificantinterestinthepastfewyears.In2019,Liuetal.
demonstratedtheDFGprocessinaTFLNmicrodiskwithaconversion
efficiency (CE) of approximately 10-420. In 2019, Luo et al. demon-
strated the DFG processin a TFLN microring resonatorwith a con-
versionrateof−53dB21.In2021,Mishraetal.proposedanon-chipDFG
device based on a thin-film PPLN waveguide, enabling wavelength
conversionfromnear-infraredregiontomid-infraredregionof3.3μm
with a normalized CE of 200% (W−1cm−2)22. In 2024, Koyaz et al.
demonstrated an ultrabroadband tunable DFG device covering
1418–1862nm using a dispersion-engineered thin-film PPLN
waveguide23.However,highabsoluteCEDFGdevicesbasedonthin-
filmPPLNwaveguideshavebarelybeenreportedbecausethethick-
nessinhomogeneityofTFLNcanseverelydisruptthephase-matching
condition.
In this work, we present a high-efficiency and broadband inte-
gratedDFGdevicebasedonathin-filmPPLNwaveguide.Anadapted-
polingmethodwasadoptedtoovercomethethicknessinhomogeneity
issue[11].Thedeviceachievesremarkableperformancebygenerating
anidlerwavewithamaximumoutputpowerof13.2dBmandarecord
CE of 48.6%, while maintaining a normalized CE exceeding 100%
W−1cm−2 across a 90nm bandwidth. Significantly, the demonstrated
DFG device achieves excellent signal fidelity when converting a
modulated1638nmsignal(outsideconventionalamplifierbands)toa
1556nmidler(withintheEDFAgainspectrum),asevidencedbyflat
optic-opticresponseandwell-preservedeyediagramsafterDFGcon-
version. The breakthroughs in efficiency and bandwidth overcome
previousDFGlimitations,pavingthewayforpracticalapplicationsin
optical communications, wavelength conversion, and amplifier
technologies.

Results
Designandfabrication
Figure1ashowsaschematicillustrationoftheDFGprocessinathin-
film PPLN waveguide. A cross-sectional view of the designed wave-
guidewithawidthwof2μm,waveguidethicknesstof600nm,and
etchdepthhof300nmisshowninFig.1b.Thesidewallangleofthe
waveguidewassetas60°duetotheisotropicetchingprocessofthe
TFLN. It should be noted that the exact thickness t is not strictly
600nm across a TFLN wafer because of imperfect smart-cut
technology.Thethicknessvariationsaremorethan5nmforthestate-
of-the-artTFLNwaferscurrently.Therefore,theexactthicknessofthe
TFLN layer was measured using an optical thickness measuring
instrumentbased on reflectancemeasurements,as shown inFig. 1c
first.Atwo-dimensionalmotorizedmovingstagewasplacedunderthe
thin-film PPLN chip to precisely acquire the thickness distribution.
Subsequently,thepolingperioddistributionalongthewaveguidewas
calculated according tothemeasured thickness distribution. There-
fore,thephase-matchingconditioncanbesatisfiedevenwithanon-
uniformthicknessdistributionalongthewaveguide;thus,theabsolute
DFG efficiency limitation can be overcome. The fitted curve of the
measuredthicknessdistribution(dashed)withastepof200μmalong
a1-cm-thin-filmPPLNwaveguideisplottedinFig.1d.Asaresult,the
correspondingpolingperioddistributioninvolvingthreewavemixing
wavelengthsof798nm,1550nm,and1645nminthedesignedwave-
guide can be obtained according to the phase-matching condition.
Figure1dplotsthecalculatedpolingperioddistribution(solid)along
the1-cm-thin-filmPPLNwaveguide(seenote2inthesupplementary
information).
Figure2a–cpresentthesimulatedmodalprofilesoftheinteract-
ingwavesinthethin-filmPPLNwaveguide,thepump,signal,andidler
waveswerefixedat798nm,1607.9nm,and1584.3nm,respectively,to
satisfytheenergy conservationcondition (seenote 1in the supple-
mentary information). Figure 2d–f present the simulated DFG con-
versioncharacteristicsinthethin-filmPPLNwaveguide,obtainedby
solvingthecoupled-waveequations(seenote1inthesupplementary
information). Under ideal conditions with perfect thickness uni-
formity, the simulations (Fig. 2d) show stable and efficient energy
transferfrompumptosignalandidlerwaves.Whenaccountingforthe
actual measured thickness variations in the TFLN layer (Fig. 2e),
although initial power transfer occurs, the signal and idler powers
quicklybeginoscillatingaroundlowvalues,resultinginfundamentally
limited conversion efficiency that cannot be improved by simply
increasing the interaction length (see note 3 in the supplementary
information). However, implementing customized compensation
through adaptive poling period adjustments cancaneffectivelycounteract
the thickness inhomogeneity effects as shown in Fig. 2f, leading to
substantialimprovementsinbothsignalandidleroutputpowers.
Figure2gillustratesthesimulatednormalizedCEacrossvarying
signal wavelengths. Both the adapted-poling and ideal scenarios
exhibit robust difference-frequency generation (DFG) operation
spanningthe1420–1820nmwavelengthrange,yieldingatheoretical
bandwidth of approximately 400nm. The close correspondence
betweentheadapted-polingandidealCEprofilesfurthervalidatesthe
efficacy of our adapted-poling technique in enabling high-
performanceDFG.Toassesstheimpactofmeasurementstepsizein
theadapted poling process,weanalyzed the normalized CEdepen-
denceunderdifferentmeasurementstepsizesasshownFig.2h.The
results reveal a clear degradation in CE with increasing step size,
attributed to the loss of detailed thickness distribution information
duringthecharacterizationprocess.Thishighlightsthecriticalneed
forhigh-resolutionthicknessmeasurementstooptimizedeviceper-
formance.Therefore,astepsizeof200μmtowardstheexperimental
setuplimitationwasadopted.
Devicecharacterization
Wefabricatedthedeviceona600-nmx-cut5%MgO-dopedTFLNona
2-μmsilicawitha525-μm-thickSisubstrate.Figure3ashowsanoptical
microscopeimageofthefacetofthefabricatedthin-filmPPLNwave-
guide for fiber-waveguide coupling. The piezoelectric force micro-
scope(PFM)signaloftheperiodicalferroelectricdomaininversionin
thethin-filmPPLNwaveguideisshowninFig.3b,indicatinguniform
periodical poling of the fabricated waveguides. Figure 3c shows a
scanning electron microscope (SEM) image of the chip facet. The
experimental setup for the DFG performance characterization is
showninFig.3d.AnimageofthechipintheDFGexperimentcaptured
by the cameraisshownin Fig. 3e. The brightvisible light radiating
from the chip indicates the high-power second-order nonlinear
phenomena.
Firstly, the coupling losses of the pump and signal waves were
characterizedbysimultaneouslyinjectingthepumpandsignalwaves
intothethin-filmPPLNwaveguideundertest.Thecouplinglossofthe
pumpwavewasestimatedtobe~6.5dB/facetaftercarefullycalibrating
therelativepositionsbetweenthethin-filmPPLNwaveguideandthe
lensfibers.Thecouplinglossforthesignalwavewasestimatedtobe
~6.8dB/facetundertheconditionthatthepumpwavehasaminimum
couplerloss.Thelossesofpolarizationcontrollers(PCs)wereevaluated
as0.4dBand0.5dBforpumpandsignal/idlerwaves,respectively.The
lossesofthewavelengthdivisionmultiplexers(WDMs)weremeasured
as0.5dBforpump,signal,andidlerwaves.Thecouplinglossofthe
signal wave is higher than that of the pump wave because of the
wavelength-dependentmodalprofilesofthefibersandthewaveguide.
Thecouplinglossesforthesignalandidlerwaveswereestimatedtobe
thesamebecausetheirwavelengthsweresimilarwhencomparedwith
thepumpwave.Undertheon-chipinputpowersofpumpandsignalsof
19.3dBm and 21.2dBm, respectively, the best DFG performance was
observed when the wavelength of the signal wave was tuned to
1607.9nmand thetemperatureofthechip was tunedto50°C.The
spectrum ofthe recorded signaland idlerwaves isshown in Fig. 3f,
wherethepeaksontheleftandrightrefertotheidlerandsignalwaves,
respectively.Inaddition,themeasuredpoweroftheidlerwaveonthe
chipcanreachupto13.2dBm,indicatingthattheCEoftheDFGprocess
in this chip can reach up to 48.6%. The CE of the DFG process was
evaluatedbytheratioofthepowerofthepumpwaveconvertedtothe
signalandidlerwaves.Themeasured(dots)andfitted(line)normalized
CEoftheDFGprocessasafunctionoftemperatureareshowninFig.3g,
indicating that the chip temperature had already been optimized to
obtain the highest efficiency of the DFG process. The peak around
1596nminthespectrummainlyoriginatedfromtheseedwaveinside
thepumplasersourceandwasobservedwhenonlyapumpwavewas
injectedintoaTFLNwaveguidewithoutperiodicalpoling.
TocharacterizethebroadbandresponseoftheDFGprocess,we
performedawavelengthsweepofthesignalwave.Theon-chipsignal
wave power was maintained at approximately 21dBm (with EDFA
amplification) and 7dBm (without EDFA amplification) across the
entiresweeprange.Theon-chippowersofthesignalandidlerwaves
wereestimatedfromtherecordedspectraldataaftercalibratingthe
exact power of the optical spectrum analyzer (OSA) with a photo-
detector and are presented in Fig. 4a, b. In the absence of EDFA
amplification, the signal wavelength was swept from 1500nm to
1640nm,coveringthefulltuningrangeofthelasersource.Withinthis
range, an idler wave was observed for signal wavelengths between
1505nmand1640nm,confirmingthebroadbandDFGresponseofthe
fabricated thin-film PPLN waveguide. Notably, no idler signal was
detected for signal wavelengths below 1505nm because the corre-
spondingidlerwavelengthsfelloutsidetheoperationalrangeofthe
optical spectrum analyzer (OSA). Future experiments using
instrumentswithabroaderspectralresponseareexpectedtorevealan
evenwiderDFGbandwidth.WhenEDFAamplificationwasapplied,the
sweeprangewasconstrainedto1530–1610nmduetothelimitedgain
bandwidthoftheamplifier.TheabsoluteCEderivedfromtheampli-
fiedon-chippowerisplottedinFig.4c,showingCEaroundorabove
20%between1554nmand1610nm.Additionally,thenormalizedCE
wasmeasuredwithoutEDFAamplificationtomaximizethebroadband
responseasshowninFig.4d.Apeakvalueof>710%(W−1cm−2)andthe
normalized CE remained above 100% (W−1cm−2) over 90nm are
observed,furtherhighlightingthehighperformanceofthethin-film
PPLNwaveguides.
TovalidatethepracticalapplicationofthefabricatedDFGdevice
inopticalcommunicationsystems,weconductedawavelengthcon-
version experiment. The primary objective was to demonstrate the
conversionofsignalwavesoutsidetheEDFAgainspectrumtoidler
waveswithintheamplificationband.Thiscapabilityeffectivelyextends
theusablewavelengthrangebeyondconventionalEDFAlimitations,
enablingamplificationofsignalsthatwouldotherwisefalloutsidethe
amplifier’soperationalbandwidth.Theexperimentalconfigurationfor
evaluating the optical communication performance is illustrated in
Fig.5a.Theexperimentalsetupwasdesignedtocharacterizetheability
of the DFG device to maintain signal integrity while performing
wavelengthconversionacrossdifferentwavelengthspectralregions.
Theelectro-optic(EO)responseoftheidlerwavecanbeobtainedby
modulating the signalwave, but measuring the idler wave after the
DFGprocess.Theoptic-optic(OO)responseoftheDFGprocessS
21,DFG
canbecalculatedbycomparingEOresponsesofboththeconverted
idlerwaveS andtheoriginalsignalwaveS ,andwrittenas
21,idler 21,signal
S =S (cid:1)S (1)
21,DFG 21,signal 21,idler
ThenormalizedOOresponseofDFGS canbeevaluatedas
21,NORM
S =S (cid:1)S (1GHz) (2)
21,NORM 21,DFG 21,DFG
Figure5bpresentsthemeasuredopticalspectrumobtainedfrom
the OSA, demonstrating complete suppression of the signal wave
throughband-passfiltering.ThisconfirmsthattheEOresponserecor-
ded by the vector network analyzer (VNA) originates from the idler
wave.Theinsetprovidesadetailedviewoftheidlerwavespectrum,
observedsidebandsverifysuccessfultransferofthemodulationfrom
thesignaltotheidlerwavesthroughtheDFGprocess.TheEOresponses
ofboththeconvertedidlerwave(redtrace)andtheoriginalsignalwave
(blue trace) are plotted in Fig. 5c. The signal wave response was
acquiredbydirectlycouplingtheelectro-opticmodulator(EOM)out-
puttotheVNA’shigh-speedphotodetector module.The remarkable
similarityinresponsecharacteristicsbetweenthesignalandidlerwaves
demonstratesfaithfulmodulationtransferduringwavelengthconver-
sion.ThenormalizedOOresponseoftheDFGprocess,derivedfromthe
measured S parameters of both waves, is shown in Fig. 5d. The
response exhibits excellent flatness across a broad microwave spec-
trum,indicatingconsistentconversionefficiencyoverawidefrequency
range.Finally,Fig.5edisplayswell-openeyediagramsgeneratedfrom
time-domainanalysisofbothsignalandidlerwaveresponses,withall
eyesremainingclearlyopenaftertheDFGconversionprocess.These
results conclusively demonstrate that our on-chip DFG process pre-
serves modulation integrity while enabling wavelength conversion,
thereby extending the operationalwavelength range ofopticalcom-
municationsystemsbeyondconventionalEDFAamplificationbands.
WefurthercompareourDFGperformancewithlithiumniobate-
basedDFGdevicesasshowninTable1.TheoverallCEandNCEofDFG
exceedthestate-of-the-artofintegratedlithiumniobatedevices.Fur-
ther,theCEandoutputidlerpowercanbeimprovedbyimprovingthe
pumppower.Ultra-broadbandandlownoiseintegratedopticalpara-
metricamplificationwithagainmorethan10dBcanbeexpected.

Methods
Fabrication
Thepolingelectrodeswithcustomizedperiodswerefirstpatterned
byastepofelectron-beamlithography(EBL),andalayerof100/10/
150-nm Al O /Ti/Au electrodes was deposited by steps of electron-
2 3
beam evaporation and lift off. Periodical poling was achieved by
applyingasequenceofshorthigh-voltagepulsesthroughthepoling
electrodes.Next,anotherstepofEBLwasusedtoformthewaveguide
pattern,andthepatternwastransferredintotheTFLNlayerbyAr+
plasmadryetching,inwhichwaythewaveguidehadasidewallangleθ
of ~60°. Finally, the facets of the chip were diced and polished to
couplethelightfromthelensfiberstothewaveguides.ThePFMwas
adoptedtocharacterizeperiodicalferroelectricdomaininversionin
thepoledwaveguide.
DFGperformancecharacterization
Thepumpwavewasgeneratedbyahigh-powercontinuous-wave(CW)
fiberlasersourcewithafixedwavelengthof798nm.Thesignalwave
wasprovidedbyatunableCWlasersourceattelecomwavelengths
ranging from 1500nm to 1640nm. An EDFA is used to amplify the
signal wave when its wavelength sweeps from the C-band to the
L-band.Apairoflensfiberswasusedtocoupleallinteractingwavesof
theDFGprocessintoandoutofthewaveguidesonthechip.Forthe
inputpart,twoPCswereusedtoensurethatthepumpwaveandsignal
wave propagating in the waveguides were fundamental transverse-
electric(TE)modes,ensuringthatthelargestsecond-ordernonlinear
coefficientd ofTFLNwasusedto obtainthe highestefficiencyof
33
theDFGprocess.Thepumpandsignalwaveswerecombinedusing
a fiber-based WDM. For the output part, the pump wave and the
signal/idlerwaveweresplitbytheotherfiber-basedWDM.Theoutput
pumpwavewascoupledintoaphotodetector(HP81530A)tomonitor
thecouplingconditionsbetweenthefibersandthechip.Theoutput
signalandidlerwavesweresenttoanOSA(YOKOGAWAAQ6370C)to
observetheDFGperformance.Moreover,thetemperatureoftheDFG
chip was tuned by a thermoelectric cooler to change the effective
refractive indices of all interacting waves in the DFG process, thus
the slight phase-mismatching caused by fabrication errors can be
compensated.
OOresponsecharacterization
Thesignallightfromatunablelaserwithawavelengthof1638nm,
adjustedbyapolarizationcontroller,wassentintoanEOM,andthe
EOMimposedhigh-speeddatafromaVNAonthesignalwave.Sub-
sequently,themodulatedsignalandpumpwaves,adjustedbytwoPCs
independently,werecombinedbyaWDMandcoupledinandoutof
thethin-filmPPLNwaveguidethroughapairoflensfibers.Another
WDMseparatedthepump,signal,andidlerwaves,andtheleftpump
wavewassentintoaphotodetectortoassistfiber-waveguidecoupling.
On the 1550nm port, only the idler wave is left after transmitting
through a band-pass optical filter. An EDFA and a variable optical
attenuatorwereusedtoamplifytheidlerwavetoasuitablepower.A
50:50fiber-basedsplittersplitstheidlerwaveintotheOSAandthe
high-speedphotodetectoroftheVNAopticalmodule.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper describes an integrated device for on-chip generation of difference frequency mixing based on the use of thin film lithium niobate. The use of lithium niobate waveguides modified by poling for wavemixing processes is fairly widely used, including in commercially available devices. This approach, however, has modest efficiency and requires large pump power, resulting in [practical limitations. The work described in the manuscript overcomes this limitation and achieves unprecedented conversion efficiency of 31.7%. The approach is based on overcoming the major limitation of poling the waveguide with phase matching approaching the theoretical values. This is achieved by matching the poling to the unavoidable inhomogeneity of the waveguide thickness. The authors devised a metrology apparatus to measure the nm- scale thickness inhomogeneity and implement the poling period for optimized phase matching. As a result, their device can generate a maximum power output of12.26 dBm with normalized conversion efficiency exceeding 100% /(W cm^2).\nThis is a well-written paper that provides comprehensive technical details for the experimental measurements. It also demonstrates the ability to convert information outside the bandwidth of an EDFA (at1638 nm) to idler at 1556 nm, which falls within the EDFA bandwidth. This capability is useful for applications in PIC-based optical communication.\nI recommend the paper for publication and urge the authors to clarify what are the sources of loss in their experimental setup and if they are accounted for in reporting their efficiency numbers.", "reviewer_2": "The authors employ a recently invented technique, adapted poling, to maintain phase matching along the length of a PPLN waveguide for difference frequency generation (DFG). They demonstrate a real conversion efficiency of ~30 % and on-chip idler power nearly 20 mW, in addition to wide tunability.\nThe adapted poling method is new and powerful, only utilized in, to my knowledge, a few works to date. This, along with the significantly enhanced power/efficiency compared to other works, make the paper nearly worthy of publication in Nature Communications. Below, I raise some issues that I feel should be addressed before reconsidering the paper for publication.\nMy main concern is how the power and efficiency numbers are reported. Results which stress \"record high\" powers and/or efficiencies should be careful with the significant digits carried in various quantities and report errors/uncertainties. For instance, the authors claim a conversion efficiency of 31.75 %, but I am sure the uncertainty on this figure exceeds several percent, as there are numerous assumptions and sources of error. For instance, the chip coupling losses are assumed to be symmetric (in-coupling equals out-coupling), and the signal/idler losses are assumed equal.\nHow are the various optical powers measured? It seems implied that they are extracted from an optical spectrum analyzer (OSA). Is the setup calibrated for other losses between the chip and OSA? Is the OSA itself calibrated (they can be inaccurate)?\nGiven that the adapted poling method is central to the work, I feel the authors should provide more detail around its implementation. How does the poling period profile shown in Fig. 1(d) connect to the thickness various? There is a clear correlation, but no analysis is given. Were any different poling period profiles tried?\nWhy is Fig. 2(f) not closer to Fig. 2(d)? i.e., what are the limits of the adapted poling method? On a related note, I think Fig. 2(h) could be explained better. Will the normalized CE continue increasing with smaller step sizes? Is 200 um the smallest achievable step size?"}, "author_response": "General comment: This paper describes an integrated device for on-chip generation of difference frequency mixing based on the use of thin film lithium niobate. The use of lithium niobate waveguides modified by poling for wave mixing processes is fairly widely used, including in commercially available devices. This approach, however, has modest efficiency and requires large pump power, resulting in practical limitations. The work described in the manuscript overcomes this limitation and achieves unprecedented conversion efficiency of 31.7%. The approach is based on overcoming the major limitation of poling the waveguide with phase matching approaching the theoretical values. This is achieved by matching the poling to the unavoidable inhomogeneity of the waveguide thickness. The authors devised a metrology apparatus to measure the nm-scale thickness inhomogeneity and implement the poling period for optimized phase matching. As a result, their device can generate a maximum power output of 12.26 dBm with normalized conversion efficiency exceeding 100% /(W cm^2).\nThis is a well-written paper that provides comprehensive technical details for the experimental measurements. It also demonstrates the ability to convert information outside the bandwidth of an EDFA (at1638 nm) to idler at 1556 nm, which falls within the EDFA bandwidth. This capability is useful for applications in PIC-based optical communication.\nI recommend the paper for publication and urge the authors to clarify what are the sources of loss in their experimental setup and if they are accounted for in reporting their efficiency numbers.\nResponse: We sincerely thank Reviewer #1 for constructive suggestions and the positive assessment of our work. We appreciate the opportunity to provide a detailed response and revisions addressing the raised questions.\nFor a comprehensive evaluation of interacting wave power values, we conducted systematic experiments to characterize the insertion loss (IL) of components in the experimental setup and the coupling losses for pump, signal, and idler waves.\nAt the input side, we first measured the pump power output from the laser using a visible to near-infrared light photodetector based on silicon, obtaining a value of 26.7 dBm. Subsequent measurements revealed the following power reductions: 26.3 dBm after passing through a polarization controller (PC), 25.8 dBm after a wavelength division multiplexer (WDM), and 25.3 dBm after an additional WDM component, as illustrated in Fig. R1(a). Similarly, for the signal wave from the erbium-doped fiber amplifier (EDFA), we measured 29.0 dBm using a communication-band photodetector, which decreased to 28.5 dBm after a PC and further reduced to 27.5 dBm after passing through two WDMs. These measurements indicated that the IL of a PC and a WDM for the pump (signal) wave were 0.4 (0.5) dB and 0.5 (0.5) dB, respectively.\nFor waveguide-fiber coupling, we employed two lens fibers as shown in Fig. R1(b), achieving a maximum power of 12.3 dBm for the pump wave after careful calibration between the lens fibers and waveguides under the condition of turning on the pump light and turning off the signal light. The coupling loss for the signal wave was estimated under the condition that the pump wave has a minimum coupler loss, so we achieved a power of 13.8 dBm for the signal wave between the lens fibers and waveguides under the condition of turning on the signal light and turning off the pump light. Assuming symmetric coupling losses, we calculated the coupling loss as 6.5 dB/facet for the pump and 6.8 dB/facet for the signal. Consequently, the input pump and signal powers were determined to be 26.7 dBm – 0.4 dB – 0.5 dB – 6.5 dB= 19.3 dBm and 29.0 dBm – 0.5 dB – 0.5 dB – 6.8 dB = 21.2 dBm, respectively.\nThe revised on-chip pump power showed a slight decrease compared to previous results, primarily due to the inclusion of PC and WDM ILs, which accounted for an approximately 1 dB deviation in pump power evaluation. Additionally, the updated fiber-waveguide coupling loss was slightly lower than previously reported, as we implemented power calibration for the optical spectrum analyzer (OSA) to ensure consistency with photodetector measurements.\nThrough these refined measurements, we obtained updated on-chip power values of 13.2 dBm for the idler wave using the calibrated OSA. Based on these measurements, we calculated the difference frequency generation (DFG) conversion efficiency (CE) to be 48.6%. After addressing all potential error sources and accounting for component losses in the experimental setup, we conducted further experiments and updated the power and efficiency values in the manuscript accordingly.\nFigure R1. (a) Experimental setup for measuring ILs of PCs and WDMs. (b) Experimental setup for measuring coupling loss between a waveguide and fiber.\n◼ Revisions:\n(In Abstract, Page 1, Line 21-22): The generated idler wave achieves a maximum output power of 13.2 dBm and a record conversion efficiency (CE) of 48.6%.\n(In Introduction, Page 1, Line 67-69): The device achieves remarkable performance by generating an idler wave with a maximum output power of 13.2 dBm and a record CE of 48.6%, while maintaining a normalized CE exceeding 100% W⁻¹cm⁻² across a 90 nm bandwidth.\n(In DEVICE DESIGN AND FABRICATION, Page 5, Line 136-139): The coupling loss for the signal wave was estimated to be ~6.8 dB/facet under the condition that the pump wave has a minimum coupler loss. The loss of PC was evaluated as 0.4 dB and 0.5 dB for pump and signal/idler wave, respectively. The loss of the WDM was measured as 0.5 dB for pump, signal and idler wave.\n(In DEVICE DESIGN AND FABRICATION, Page 5, Line 142): Under the on-chip input powers of pump and signals of 19.3 dBm and 21.2 dBm, respectively, the best DFG performance was observed when the wavelength of the signal wave was tuned to 1607.9 nm and the temperature of the chip was tuned to 50 ℃.\n(In DEVICE DESIGN AND FABRICATION, Page 5, Line 146-147): In addition, the measured power of the idler wave on the chip can reach up to 13.2 dBm, indicating that the CE of the DFG process in this chip can reach up to 48.6%.\n(In DEVICE DESIGN AND FABRICATION, Page 6, Figure 3 (f)-(g)):\nFig. 3. (a) Optical microscope image of waveguide facets. (b) The measured PFM signal of the periodical ferroelectric domain inversion in the thin-film PPLN waveguide. (c) SEM image of a waveguide facet. (d) Experimental setup for DFG characterization. (e) Captured image of the chip in the DFG experiment. (f) Measured spectrum of signal and idler waves. (g) Simulated (line) and measured (dots) normalized CE of DFG at different temperatures. TEC, thermoelectric cooler. PD, photodetector. PC, polarization controller. WDM, wavelength division multiplexer.\n(In DEVICE DESIGN AND FABRICATION, Page 6, Line 162-163): The on-chip powers of the signal and idler waves were estimated from the recorded spectral data after calibrating the exact power of optical spectrum analyzer (OSA) with a PD and are presented in Figs. 4a and 4b.\n(In DEVICE DESIGN AND FABRICATION, Page 7, Line 173): A peak value of >710 %(W⁻¹cm⁻²) and the normalized CE remained above 100 %(W⁻¹cm⁻²) over 90 nm are observed, further highlighting the high performance of the thin-film PPLN waveguides.\n(In DEVICE DESIGN AND FABRICATION, Page 7, Figure 4 (a)-(d)):\nFig. 4. (a)–(b) Measured on-chip power of signal and idler waves with (a) and without (b) EDFA at different wavelengths in the DFG process. (c) Measured absolute CE at different signal wavelengths in the DFG process. (d) Measured normalized CE at different signal wavelengths in the DFG process.\n(In DEVICE CHARACTERIZATION, Page 9, Table 1):\nTable 1. Performance comparison of DFG in lithium niobate\nPlatform CE (%) NCE (%W-1cm-2) Idler power (mW)\nBulk 17 0.0038 ~ 2\nBulk 18 0.016a 0.042b 0.012\nBulk 19 ~13.4 ~ >1000\nMicro resonator 20 0.01c ~ 0.0008c\nMicro resonator 21 0.014a ~ 0.00000048\nTFLN 22 ~2.6a 200 ~0.42a\nTFLN 23 0.0031 4-7a 0.000013\nTFLN 24 ~0.15a 100 0.0015\nThis work (TFLN) 48.6 710 20.94\naCalculated from relevant data demonstrated in paper. bNCE in unit of %W-1. cCE of cascaded SHG/DFG process.\n(In DISCUSSION AND CONCLUSION, Page 9, Line 226): The experimental results demonstrate the remarkable performance of the DFG process, with an absolute CE of up to 48.6%.\nGeneral comment: The authors employ a recently invented technique, adapted poling, to maintain phase matching along the length of a PPLN waveguide for difference frequency generation (DFG). They demonstrate a real conversion efficiency of ~30 % and on-chip idler power nearly 20 mW, in addition to wide tunability.\nThe adapted poling method is new and powerful, only utilized in, to my knowledge, a few works to date. This, along with the significantly enhanced power/efficiency compared to other works, make the paper nearly worthy of publication in Nature Communications. Below, I raise some issues that I feel should be addressed before reconsidering the paper for publication.\nResponse: We sincerely thank Reviewer #2 for constructive suggestions and the positive assessment of our work. We appreciate the opportunity to provide a detailed response and revisions addressing the raised questions. First, we implemented a comprehensive recalibration of the optical spectrum analyzer (OSA) to ensure precise alignment between its power measurements and those obtained from the photodetector. This recalibration process led us to re-evaluate the insertion losses (IL) associated with the polarization controller (PC), wavelength division multiplexer (WDM), and coupling losses for the interacting waves as well as accurately recharacterize the on-chip powers, absolute efficiencies, and normalized efficiencies of wavelength conversion. Second, we have expanded the discussion of our poling implementation in the supplementary materials, including a detailed mathematical expression for poling period calculation. Furthermore, we have provided a thorough explanation of the technical constraints that establish 200 μm as the minimum achievable step size in our experimental setup. In the following sections, we address each of the reviewer’s comments individually.\nComment 1: My main concern is how the power and efficiency numbers are reported. Results which stress \"record high\" powers and/or efficiencies should be careful with the significant digits carried in various quantities and report errors/uncertainties. For instance, the authors claim a conversion efficiency of 31.75 %, but I am sure the uncertainty on this figure exceeds several percent, as there are numerous assumptions and sources of error. For instance, the chip coupling losses are assumed to be symmetric (in-coupling equals out-coupling), and the signal/idler losses are assumed equal. How are the various optical powers measured? It seems implied that they are extracted from an optical spectrum analyzer (OSA). Is the setup calibrated for other losses between the chip and OSA? Is the OSA itself calibrated (they can be inaccurate)?\nResponse: Thank you for the comment. For coupling loss symmetry, the chip facets used edge couplers that had identical designing and processing procedures. Besides, lens fibers of same type were used to couple light between fibers and waveguides on-chip. So, it is reasonable to assume that the chip coupling losses are symmetric. Unlike couplers based on gratings or inverse taper structures, the edge couplers used here were based on broadened waveguides and had advantages in broadband coupling performances. The coupling efficiency mainly dominated by the modal overlap between the output light spot from lens fiber and the fundamental TE mode in waveguides. And the modal profiles between signal and idler wave are quite similar because their wavelengths are so close, as shown in Fig. R2. At the edge coupler, the effective modal field areas for signal (1608 nm) and idler (1584 nm) wave were calculated as 2.082 μm2 and 2.063 μm2, respectively. The slight difference (<1%) in modal field area ensures the similar coupling losses for signal and idler wave.\nFigure R2. Simulated modal profiles of signal (left) and idler (right) at edge couplers.\nFor a comprehensive evaluation of interacting wave power values, we conducted systematic experiments to characterize the insertion loss (IL) of components in the experimental setup and the coupling losses for pump, signal, and idler waves.\nAt the input side, we first measured the pump power output from the laser using a visible to near-infrared light photodetector based on silicon, obtaining a value of 26.7 dBm. Subsequent measurements revealed the following power reductions: 26.3 dBm after passing through a polarization controller (PC), 25.8 dBm after a wavelength division multiplexer (WDM), and 25.3 dBm after an additional WDM component, as illustrated in Fig. R3(a). Similarly, for the signal wave from the erbium-doped fiber amplifier (EDFA), we measured 29.0 dBm using a communication-band photodetector, which decreased to 28.5 dBm after a PC and further reduced to 27.5 dBm after passing through two WDMs. These measurements indicated that the IL of a PC and a WDM for the pump (signal) wave were 0.4 (0.5) dB and 0.5 (0.5) dB, respectively.\nFor waveguide-fiber coupling, we employed two lens fibers as shown in Fig. R3(b), achieving a maximum power of 12.3 dBm for the pump wave after careful calibration between the lens fibers and waveguides under the condition of turning on the pump light and turning off the signal light. The coupling loss for the signal wave was estimated under the condition that the pump wave has a minimum coupler loss, so we achieved a power of 13.8 dBm for the signal wave between the lens fibers and waveguides under the condition of turning on the signal light and turning off the pump light. Assuming symmetric coupling losses, we calculated the coupling loss as 6.5 dB/facet for the pump and 6.8 dB/facet for the signal. Consequently, the input pump and signal powers were determined to be 26.7 dBm – 0.4 dB – 0.5 dB – 6.5 dB= 19.3 dBm and 29.0 dBm – 0.5 dB – 0.5 dB – 6.8 dB = 21.2 dBm, respectively.\nThe revised on-chip pump power showed a slight decrease compared to previous results, primarily due to the inclusion of PC and WDM ILs, which accounted for an approximately 1 dB deviation in pump power evaluation. Additionally, the updated fiber-waveguide coupling loss was slightly lower than previously reported, as we implemented power calibration for the optical spectrum analyzer (OSA) to ensure consistency with photodetector measurements.\nThrough these refined measurements, we obtained updated on-chip power values of 13.2 dBm for the idler wave using the calibrated OSA. Based on these measurements, we calculated the difference frequency generation (DFG) conversion efficiency (CE) to be 48.6%. After addressing all potential error sources and accounting for component losses in the experimental setup, we conducted further experiments and updated the power and efficiency values in the manuscript accordingly.\nFigure R3. (a) Experimental setup for measuring ILs of PCs and WDMs. (b) Experimental setup for measuring coupling loss between a waveguide and fiber.\n◼ Revisions:\n(In Abstract, Page 1, Line 21-22): The generated idler wave achieves a maximum output power of 13.2 dBm and a record conversion efficiency (CE) of 48.6%.\n(In Introduction, Page 1, Line 67-69): The device achieves remarkable performance by generating an idler wave with a maximum output power of 13.2 dBm and a record CE of 48.6%, while maintaining a normalized CE exceeding 100% W⁻¹cm⁻² across a 90 nm bandwidth.\n(In DEVICE DESIGN AND FABRICATION, Page 5, Line 136-139): The coupling loss for the signal wave was estimated to be ~6.8 dB/facet under the condition that the pump wave has a minimum coupler loss. The loss of PC was evaluated as 0.4 dB and 0.5 dB for pump and signal/idler wave, respectively. The loss of the WDM was measured as 0.5 dB for pump, signal and idler wave.\n(In DEVICE DESIGN AND FABRICATION, Page 5, Line 142): Under the on-chip input powers of pump and signals of 19.3 dBm and 21.2 dBm, respectively, the best DFG performance was observed when the wavelength of the signal wave was tuned to 1607.9 nm and the temperature of the chip was tuned to 50 ℃.\n(In DEVICE DESIGN AND FABRICATION, Page 5, Line 146-147): In addition, the measured power of the idler wave on the chip can reach up to 13.2 dBm, indicating that the CE of the DFG process in this chip can reach up to 48.6%.\n(In DEVICE DESIGN AND FABRICATION, Page 6, Figure 3 (f)-(g)):\nFig. 3. (a) Optical microscope image of waveguide facets. (b) The measured PFM signal of the periodical ferroelectric domain inversion in the thin-film PPLN waveguide. (c) SEM image of a waveguide facet. (d) Experimental setup for DFG characterization. (e) Captured image of the chip in the DFG experiment. (f) Measured spectrum of signal and idler waves. (g) Simulated (line) and measured (dots) normalized CE of DFG at different temperatures. TEC, thermoelectric cooler. PD, photodetector. PC, polarization controller. WDM, wavelength division multiplexer.\n(In DEVICE DESIGN AND FABRICATION, Page 6, Line 162-163): The on-chip powers of the signal and idler waves were estimated from the recorded spectral data after calibrating the exact power of optical spectrum analyzer (OSA) with a PD and are presented in Figs. 4a and 4b.\n(In DEVICE DESIGN AND FABRICATION, Page 7, Line 173): A peak value of >710 %(W⁻¹cm⁻²) and the normalized CE remained above 100 %(W⁻¹cm⁻²) over 90 nm are observed, further highlighting the high performance of the thin-film PPLN waveguides.\n(In DEVICE DESIGN AND FABRICATION, Page 7, Figure 4 (a)-(d)):\nFig. 4. (a)–(b) Measured on-chip power of signal and idler waves with (a) and without (b) EDFA at different wavelengths in the DFG process. (c) Measured absolute CE at different signal wavelengths in the DFG process. (d) Measured normalized CE at different signal wavelengths in the DFG process.\n(In DEVICE CHARACTERIZATION, Page 9, Table 1):\nTable 1. Performance comparison of DFG in lithium niobate\nPlatform CE (%) NCE (%W-1cm-2) Idler power (mW)\nBulk 17 0.0038 ~ 2\nBulk 18 0.016a 0.042b 0.012\nBulk 19 ~13.4 ~ >1000\nMicro resonator 20 0.01c ~ 0.0008c\nMicro resonator 21 0.014a ~ 0.00000048\nTFLN 22 ~2.6a 200 ~0.42a\nTFLN 23 0.0031 4-7a 0.000013\nTFLN 24 ~0.15a 100 0.0015\nThis work (TFLN) 48.6 710 20.94\naCalculated from relevant data demonstrated in paper. bNCE in unit of %W-1. cCE of cascaded SHG/DFG process.\n(In DISCUSSION AND CONCLUSION, Page 9, Line 226): The experimental results demonstrate the remarkable performance of the DFG process, with an absolute CE of up to 48.6%.\nComment 2: Given that the adapted poling method is central to the work, I feel the authors should provide more detail around its implementation. How does the poling period profile shown in Fig. 1(d) connect to the thickness various? There is a clear correlation, but no analysis is given. Were any different poling period profiles tried?\nResponse: Thank you for your comment. We have added some details around adapted poling implementation in the supplementary material. At first, the effective refractive indices of the pump (n ), signal (n ), and idler (n ) waves as a function of film thickness t are plotted in Figs. R4(a)-(c), which can be expressed as n = 1.987+0.0001795t, n = 1.671+0.0004048t, and n = 1.680+0.0003996t when the film thickness variation is less than 20 nm.\nFig. R4. (a) Calculated effective refractive index of pump (a), signal (b), and idler (c) waves under different TFLN layer thickness.\nThe quasi-phase matching requirement along the waveguide can be expressed as\nn_eff,p(lambda_p, t) - n_eff,s(lambda_s, t) - n_eff,i(lambda_i, t) = m * lambda_p / Lambda(t)\nwhere m = ±1, ±2, ±3…is the index of phase-matching order. Considering the first order of the phase-matching condition, the poling period distributions along the periodically-poled TFLN waveguide can be expressed as:\nLambda(t) = lambda_p / (n_eff,p(lambda_p, t) - n_eff,s(lambda_s, t) - n_eff,i(lambda_i, t))\nTherefore, the relationship between poling period and film thickness can be expressed as shown in Fig. R5(a).\nFig. R5. (a) Calculated poling period of DFG with different TFLN layer thickness. (b) Simulated effective propagation constant mismatch with different TFLN layer thickness.\nAs for different poling period profiles, we have so far only tried a measurement step size of 200 μm. In adapted-poling implementations, the conversion efficiency limit mainly originates from the thickness measuring resolution along the waveguide, due to the uncertainty in the exact waveguide thickness between the adjacent measured points. This can result in phase mismatches in practical adapted-poling devices, ultimately disrupting the DFG process. Therefore, smaller measurement step sizes will have higher conversion efficiency. In our experimental setup, the smallest achievable step size is limited to 200 μm since the light spot from the measuring instrument exposed on the chip surface has a diameter of around 100~200 μm. As a result, the measured thickness represents thickness information over a 200×200 μm2 area. Nevertheless, further reduction of the step size cannot significantly improve conversion efficiency because the already achieved efficiency remains relatively high.\n◼ Revisions:\n(In Supplemental material, Page 3, Line 53-56): The effective refractive indices of the pump (n ), signal (n ), and idler (n ) waves as a function of film thickness t are plotted in Figs. S1a–S1c, which can be expressed as n = 1.987+0.0001795t, n = 1.671+0.0004048t, and n = 1.680+0.0003996t when the film thickness variation is less than 20 nm.\n(In Supplemental material, Page 3, Figure S1(a)-(c)):\nFig. S1. Calculated effective refractive index of pump (a), signal (b), and idler (c) waves under different TFLN layer thickness.\n(In Supplemental material, Page 3-4, Line 60-71): The quasi-phase matching requirement along the waveguide can be expressed as\nn_eff,p(lambda_p, t) - n_eff,s(lambda_s, t) - n_eff,i(lambda_i, t) = m * lambda_p / Lambda(t) (S1)\nwhere m = ±1, ±2, ±3…is the index of phase-matching order. Considering the first order of the phase-matching condition, the poling period distributions along the periodically-poled TFLN waveguide can be expressed as:\nLambda(t) = lambda_p / (n_eff,p(lambda_p, t) - n_eff,s(lambda_s, t) - n_eff,i(lambda_i, t)) (S2)\nTherefore, the relationship between poling period and film thickness can be expressed as shown in Fig. S2a. The simulated effective propagation constant mismatch varies linearly with the thickness of the TFLN layer with a slope of -1754 (m·nm)-1 as shown in Fig. S2b. The extreme sensitivity of the effective propagation constant mismatch to the changes of TFLN layer thickness makes it essential to compensate for the negative effects induced by the inhomogeneity of TFLN layer thickness.\n(In Supplemental material, Page 4, Figure S2(a)):\nFig. S2. (a) Calculated poling period of DFG with different TFLN layer thickness. (b) Simulated effective propagation constant mismatch with different TFLN layer thickness.\n(In Supplemental material, Page 4, Line 75-81): For adapted-poling method, the raw data of thickness of the TFLN layer was acquired with a step of 200 μm. Then the detailed TFLN layer thickness distribution along the waveguide was interpolated using makima interpolation (a modified Akima piecewise cubic Hermite interpolation method) based on the raw data of thickness [S2], as shown in Fig. S3. Once the thickness of the TFLN layer was measured through the instruments, the poling period of can be referred using equation (S8). After then, the poling period distribution along the waveguide was obtained.\n(In Supplemental material, Page 4, Figure S3):\nFig. S3. Fitted TFLN layer thickness profile (line) and measured TFLN layer thickness raw data (dots).\nComment 3: Why is Fig. 2(f) not closer to Fig. 2(d)? i.e., what are the limits of the adapted poling method? On a related note, I think Fig. 2(h) could be explained better. Will the normalized CE continue increasing with smaller step sizes? Is 200 um the smallest achievable step size?\nResponse: Thank you for the comment. We adopted makima interpolation method to fit the thickness distribution (line) from measured raw data (dots) as shown Fig. R6a. However, we used linear interpolation method instead of makima interpolation method to fit the adapted-poling period as shown in Fig. R6(b), which causing phase-mismatching along the waveguide because the poling period does not perfect match the waveguide thickness. Therefore, the simulated DFG conversion efficiency in Fig. 2(f) of the previous manuscript is slightly below than the perfect condition. To be consistent with the actual implementation process, we used the makima interpolation method to fit adapted-poling period from the raw thickness data and obtained new simulation results as shown in 2(f) of the revised manuscript, achieving similar conversion efficiency to ideal conditions in Fig. 2(d). In addition, though we corrected the simulation conditions, the simulated conversion efficiency of DFG under adapted poling method was still a little bit lower to the ideal conditions, as shown in Figs. 2(d) and 2(f). This is due to the thickness inhomogeneity within in one period of poling electrodes (4~5 μm), which was not able to be compensated by changing poling periods. The tiny phase mismatches accumulated by this finally caused tiny difference between the simulation results between ideal conditions and adapted poling conditions.\nFig. R6. (a) Thickness raw data (dots) and fitted thickness distribution (line) using makima interpolation method. (b) Thickness raw data (dots) and fitted thickness distribution (line) using linear interpolation method.\nIn adapted-poling implementations, the conversion efficiency limit mainly originates from the thickness measuring resolution along the waveguide, due to the uncertainty in the exact waveguide thickness between the adjacent measured points. This can result in phase mismatches in practical adapted-poling devices, ultimately disrupting the DFG process. Figure 2(h) in the manuscript clearly explained the influence of thickness measurement resolution. The phase mismatches can be better compensated under smaller step sizes because more detailed thickness distribution information can be acquired under smaller step sizes, hence increasing normalized conversion efficiency to a higher level. Therefore, the simulated results in Fig. 2(h) clearly indicates measurement step size of 200 μm has much higher conversion efficiency than that with measurement step sizes of 300 μm and 400 μm.\nThe smallest achievable step size is limited to 200 μm because the light spot from the measuring instrument exposed on the chip surface has a diameter of around 100~200 μm. So, the measured thickness information is the average thickness of a 200×200 μm2 area. Nevertheless, further reduction of the step size cannot significantly improve conversion efficiency because the already achieved efficiency remains relatively high.\n◼ Revisions:\n(In DEVICE DESIGN AND FABRICATION, Page 4, Figure 2 (f)):\nFig. 2. (a)–(c) Simulated modal profiles of (a) pump, (b) signal, and (c) idler waves in the designed thin-film PPLN waveguide. (d)–(f) Simulated DFG conversion characteristics of pump, signal, and idler waves under conditions of ideal phase matching (d), without adapted-poling method (e), and with adapted-poling method (f) when inhomogeneity of TFLN layer is considered. (g) Simulated normalized CE at different signal wavelengths in conditions of ideal phase matching and with the adapted-poling method. (h) Simulated normalized CE at different signal wavelengths under different thickness measuring step sizes.\n(In DEVICE DESIGN AND FABRICATION, Page 5, Line 118-122): To assess the impact of measurement step size in the adapted poling process, we analyzed the normalized CE dependence under different measurement step sizes as shown Fig. 2h. The results reveal a clear degradation in CE with increasing step size, attributed to the loss of detailed thickness distribution information during the characterization process. This highlights the critical need for high-resolution thickness measurements to optimize device performance. Therefore, a step size of 200 μm towards the experimental setup limitation was adopted."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have done a careful and thorough job of responding to my question. It is nice too see that responding to the question indicated their approach has even a higher efficiency than what was originally reported. I recommend publication of the modified manuscript as is.", "reviewer_2": "The authors have sufficiently addressed my comments and I feel the paper can be accepted for publication."}, "author_response": ""}]

Decision letter