PicoQuant MultiHarp 150 - Photon Counting Detectors; Photomultiplier Tube (PMT); Micro Channel Plate PMT (MCP); Single Photon Avalanche Photo Diode (SPAD)

PicoQuantGmbH MultiHarpSoftwareV.3.1.0.0
2.3. Photon Counting Detectors
2.3.1. Photomultiplier Tube (PMT)
APMTconsistsofalight–sensitivephotocathodethatgenerateselectronswhenexposedtolight.Theseelec-
tronsaredirectedontoachargedelectrodecalleddynode.Thecollisionoftheelectronswiththedynodepro-
ducesadditionalelectrons.Sinceeachelectronthatstrikesthedynodecausesseveralelectronstobeemitted,
thereisamultiplicationeffect.Afterfurtheramplificationbymultipledynodes,theelectronsarecollectedatthe
anodeofthePMTandoutputasacurrent.Thecurrentisdirectlyproportionaltothelightintensitystrikingthe
photocathode.Becauseofthemultiplicativeeffectofthedynodechain,thePMTisaphotoelectronamplifier
withhighsensitivityandremarkablylownoise.Thehighvoltagedrivingthetubemaybevariedtochangethe
sensitivityofthePMT.CurrentPMTshaveawidedynamicrange,i.e.theycanalsomeasurerelativelyhighlev-
elsoflight.Furthermore,theyareveryfast,sothatrapidsuccessiveeventscanbereliablymonitored.Onepho-
tononthephoto cathodecanproduceashortoutputpulsecontainingmillionsofphotoelectrons.PMTscan
thereforebeusedassinglephotondetectors.Inphotoncountingmode,individualphotonsthatstrikethephoto
cathodeofthePMTareregistered.Eachphotoneventgivesrisetoanelectricalpulseattheoutput.Thenum-
berofpulses,orcountspersecond,isproportionaltothelightimpinginguponthePMT.Asthenumberofpho-
toneventsincreaseathigherlightlevels,itwillbecomedifficulttodifferentiatebetweenindividualpulsesandthe
photoncountingdetector’sbehaviorwillbecomenon–linear.Thisusuallyoccursbetween1and20Mcps,de-
pendingonthedetectordesign.Similarly,inTCSPCapplications,individualphotonpulsesmaymergeintoone
asthecountrateincreases.Thisleadstopulsepile–upanddistortionsofthecollectedhistograms.
Thetiminguncertaintybetweenphotonarrivalandelectricaloutput(transittimespread)isusuallysmallenough
topermittime–resolvedphotoncountingatasub–nanosecondscale.Insinglephotoncountingmodethetubeis
typicallyoperatedataconstanthighvoltagewherethePMTismostsensitive.
PMTsusuallyoperatewithinthebluetoredregionsofthevisiblespectrum,withgreatestquantumefficiencyin
theblue–greenregion,dependinguponphoto–cathodematerials.Typicalquantumefficienciesareabout25%.
Forspectroscopyexperimentsintheultraviolet/visible/nearinfraredregionofthespectrum,aPMTisvery
wellsuited.
Becauseofnoisefromvarioussourcesinthetube,theoutputofthePMTmaycontainpulsesthatarenotre-
latedtothelightinput.Thesearereferredtoasdarkcounts.Thedetectionsystemcantosomeextentreject
thesespuriouspulsesbymeansofelectronicdiscriminatorcircuitry.Thisdiscriminationisbasedontheproba-
bilitythatsomeofthenoisegeneratedpulses(thosefromthedynodes)exhibitlowersignallevelsthanpulses
fromatruephotonevent.Thermalemissionfromthecathodethatundergoesthefullamplificationprocesscan
usuallynotbesuppressedthisway.Inthiscasecoolingofthedetectorismorehelpful.
2.3.2. Micro Channel Plate PMT (MCP)
AMicroChannelPlatePMTconsistsofanarrayofglasscapillaries(5–25µminnerdiameter)whoseinsidesare
coatedwithanelectron–emissivematerial.Thecapillariesarebiasedatahighvoltage.LikeinaPMT,anelec-
tronthatstrikestheinsidewallofoneofthecapillariescreatesanavalancheofsecondaryelectrons.Thiscas-
cadingeffectcreatesagainof10
3
to10
6
andproducesacurrentpulseattheoutput.Duetothenarrowandwell
definedelectronpathinsidethecapillaries,thetransittimespreadoftheoutputpulsesismuchreducedcom-
paredtoanormalPMT.ThetimingjitterofMCPsisthereforesufficientlysmalltoperformtime–resolvedphoton
countingonapicosecondscale,usuallyoutperformingPMTs.GoodbutalsoexpensiveMCPscanachievetim-
inguncertaintiesaslowas25ps.Inthisrespect,microchannelplatesareagoodmatchfortheMultiHarpbut
theyarequitelimitedinpermittedcountrateandprovidelowersensitivitytowardstheredendofthespectrum
comparedtosuitablyoptimizedSPADs.
2.3.3. Single Photon Avalanche Photo Diode (SPAD)
AvalanchePhotoDiodes(APDs)aresemiconductordevices,usuallyrestrictedtooperationinthevisibletoin-
fraredpartsofthespectrum.Generally,APDsmaybeusedforultra–lowlightdetection(opticalpowers<1pW),
andcanalsobeusedasphoton–countersintheso–called"Geiger"mode(biasedslightlyabovethebreakdown
voltage).Inthelattercase,asinglephotonmaytriggeranavalancheofabout10
8
carriers.Inthismodethede-
vicecanbeusedasadetectorforphotoncountingwithveryaccuratetimingofthephotonarrival.Inthiscontext
theyarealsoreferredtoasSinglePhotonAvalanchePhotoDiodes(SPAD).Selecteddeviceswithsmallactive
surfacesmayachievetimingaccuraciesdownto30ps,butareusuallyhardtoalignanddifficulttofocusinto.
SPADsaresometimesnoisierthanPMTs,butcanhaveagreaterquantumefficiencyespeciallytowardsthered
sideofthespectrum.Maximumquantumefficienciesareabout70%.Suchsensitivedevicesprovideatiming
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