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Enhanced Photon Extraction from a Nanowire Quantum
Dot Using a Bottom-Up Photonic Shell
Mathieu Jeannin, Thibault Cremel, Teppo Häyrynen, Niels Gregersen, Edith
Bellet-Amalric, Gilles Nogues, Kuntheak Kheng
To cite this version:
Mathieu Jeannin, Thibault Cremel, Teppo Häyrynen, Niels Gregersen, Edith Bellet-Amalric, et al..
Enhanced Photon Extraction from a Nanowire Quantum Dot Using a Bottom-Up Photonic Shell.
Physical Review Applied, American Physical Society, 2017, 8 (5), pp.054022.
�10.1103/PhysRevAp-plied.8.054022�. �hal-01635907�
bottom-up photoni shell 2 Mathieu Jeannin, 1,
∗
Thibault Cremel, 2,∗
Teppo Häyrynen, 3 Niels 3 Gregersen, 3 Edith Bellet-Amalri , 2 Gilles Nogues, 1,†
and Kuntheak Kheng 2 4
1
Univ. Grenoble Alpes, CNRS, Institut Néel, "Nanophysique 5
et semi ondu teurs" group, F-38000 Grenoble, Fran e 6
2
Univ. Grenoble Alpes, CEA, INAC, PHELIQS, "Nanophysique 7
et semi ondu teurs" group, F-38000 Grenoble, Fran e 8
3
DTU Fotonik, Department of Photoni s Engineering, Te hni al University of 9
Denmark, Ørsteds Plads, Building 343, DK-2800 Kongens Lyngby, Denmark 10
Abstra t 11
Semi ondu tornanowiresoerthepossibilitytogrowhighqualityquantumdotheterostru tures, 12
andinparti ular CdSequantumdotsinsertedinZnSe nanowireshave demonstrated theabilityto 13
emitsinglephotonsuptoroomtemperature. Inthisletter, wedemonstrate abottom-upapproa h 14
to fabri ate a photoni ber-like stru ture around su h nanowire quantum dots by depositing an 15
oxide shell using atomi layer deposition. Simulations suggest that the intensity olle ted in our 16
NA=0.6 mi ros ope obje tive an be in reased by a fa tor 7 with respe t to the bare nanowire 17
ase. Combining mi ro-photolumines en e, de ay time measurements and numeri al simulations, 18
we obtain a 4-fold in rease in the olle ted photolumines en e from the quantum dot. We show 19
thatthis improvement isdue to an in reaseof thequantum dot emissionrateand a redire tionof 20
the emitted light. Our ex-situ fabri ation te hnique allows a pre ise and reprodu ible fabri ation 21
onalarges ale. Itsimprovedextra tione ien yis ompared tostateofthearttop-downdevi es. 22
∗
ontributedequallytothiswork
†
Controlling and enhan ing the spontaneous emission of quantum emitters is one of the 24
urrent key issues in the eld of nanophotoni s. Semi ondu tor quantum dots (QDs) are 25
onsideredaspromisingande ientsingle-photonemittersforquantumopti sappli ations. 26
[16℄ Over the past few years, several approa hes have been pursued to ontrol their emis-27
sion properties, from the use of photoni rystals [7, 8℄ to top-down photoni wires [911℄ 28
and trumpets.[12, 13℄ These strategies are based on the early work of Pur ell[14℄ whi h 29
demonstrated that the spontaneous emission of an emitter an be modi ed by engineering 30
itsele tromagneti environment. They relyonawaveguidingapproa h toin rease the ou-31
pling between a well-de ned propagating opti al mode and the QD while simultaneously 32
redu ing the ouplingbetween the QDand ba kground radiationmodes, oering ontrolof 33
both the opti almode pro leand the QD spontaneousemission rate. 34
In this ontext, the interest of the dot-in-a-nanowire on guration fabri ated using 35
bottom-up methodsnaturally arises be ause itprovides a simple way to ensure the enter-36
ingofasinglequantumemitterinthephotoni stru ture.[15 17℄Thebottom-upfabri ation 37
methodalsoavoidsheavy pro essing,likeet hingthe semi ondu tingmaterial,thatisoften 38
detrimentaltotheQDsopti alproperties. However,themainrealizationsuptonow on ern 39
III-V semi ondu tors, [1517℄ limiting the operation range to the ryogeni temperature. 40
Ta kling this issue, the potentialof II-VI materials,inparti ular CdSe QDs inserted inside 41
ZnSe nanowires (NWs) has been demonstrated in previous studies. They allow for robust 42
high temperature single-photon emission using heteroepitaxial [18℄ or homoepitaxial [19℄ 43
nanowire growth. Contrary to all the aforementioned systems where the photoni wire 44
stru ture hasadiameter omparabletothewavelength
λ/n
ofthe guidedlightwhi hallows 45forhighlye ient ouplingtothe HE
11
mode[20℄,the diameterof theII-VINW embedding 46the QD (
∼
20 nm
) is mu h smaller than the wavelength of the emitted light (530 nm
). It 47leads tolightemission predominantly intonon-guidedradiationmodes and a low olle tion 48
e ien y. Anadditionalfabri ationeorthasthustobemadetoensureane ient oupling 49
tothe olle tionopti s. 50
In apreviousreport[21℄wehavetheoreti ally investigatedthe potentialofusing anoxide 51
shell deposition onabareZnSe NWtoform athi kphotoni wire stru ture. In this arti le, 52
we experimentally demonstrate the use of atomi layer deposition (ALD) to fabri ate a 53
onformal aluminum oxide (Al
2
O3
) shell around ZnSe NWs ontaining a single CdSe QD. 54Weshowthattheoxideshelldrasti allyenhan esthelightintensityemittedbytheQD,and 55
we use time-resolved mi rophotolumines en eto systemati allystudy the ee t of the shell 56
thi kness onthe nanowire quantum dot (NWQD) emission rate. Our results are ompared 57
to numeri al simulations a ounting for the real NW geometry, eviden ing the dierent 58
physi alme hanismsleadingto the enhan ementof the spontaneous emissionfromthe QD 59
and to the improved light olle tionfromthe emittingstru ture. 60
II. PRINCIPLES OF OPERATION
61
Toillustratethe ee t of the NW and itssurroundingmediumonthe QD emissionrate, 62
let us onsider a QD pla ed inside an in nitely long ylinder as illustrated in Fig. 1(a) 63
radiatinga eld ata wavelength
λ
. The ylinder ismade ofa diele tri material(refra tive 64index
n
) and has a diameterd
. We rst onsider a dipoleorientation perpendi ular to the 65NWaxisinordertousetheNWasapropagationmediumforthe emittedlight. Inthelimit 66
where
d ≪ λ/n
, the diele tri s reening ee t[11 ℄ redu es the spontaneous emission rateγ
67 by a fa tor: 68γ
γ
0
=
4
n(n
2
+ 1)
2
,
(1)where
γ
0
istheradiativeemissionrateinthebulkmaterialofindexn
.[22℄ForaZnSe ylinder 69(
n
ZnSe
= 2.68 atλ
=530 nm
), the s reening fa tor is∼
1/45. If the NW is surrounded by a 70shell of refra tiveindex
n
s
insteadof va uum, equation1 remainsvalidby repla ingn
with 71theindex ontrast
n/n
s
. ForanAl 2O 3
surroundingmedium(
n
s
=
1.77),the s reeningfa tor 72be omes
∼
1/4.1,resulting inan order of magnitudelarger radiativerate. 73Inadditionto hangingthediele tri s reening,theAl 2
O 3
shellalsoin uen estheguiding 74
of light along the NW. We have omputed the total emission rate
γ
and the emission rate 75γ
HE11
intothefundamentalHE11
waveguidemodefromaradialdipoleasfun tionoftheshell 76thi kness
t
s
[see Fig. 1(a)℄ using a semi-analyti al approa h[23℄ ombined with an e ient 77non-uniformdis retizations hemeink spa e.[24 ℄Theresultsare plottedinFigure1(b). We 78
observe that the shell thi kness of
∼
120 nm
not only leads to an in reased total emission 79rate, it also allows for on nement of the fundamental HE
11
mode to the ore-shell NW 80leadingtoapreferential ouplingof theemittedlighttothis mode. Figure1( ) presentsthe 81
spontaneousemission
β
fa torrepresentingthefra tionβ = γ
HE11
/γ
ofemittedlight oupled 82(a)
(c)
Air
Al
2
O
3
ZnSe
d
(b)
HE11
β
Figure 1. (a) Geometry of the in nite NW. (b) Total spontaneous emission rate (bla k
+
) andspontaneousemission rateinto the rst guidedmode HE
11
(red) asa fun tionof shellradius fora radialdipole. ( ) Fra tion
β
ofpower radiated into theHE11
mode.tothe HE
11
mode. Weobserveindeedthat up to71%of theemittedlightis oupledtothis 83mode for
t
s
=120 nm
. The dipole thus be omes oupled to the equivalent of a monomode 84photoni wire[911, 1517℄ pavingthe way to the ontrolof itsfar- eld radiationpattern. 85
III. SAMPLE FABRICATION 86
OuremittersareCdSeQDsembeddedinsideaZnSeNWwithathin,epitaxialpassivation 87
Zn
0.83
Mg0.17
Seshell grown aroundtheNW.They aregrownby mole ularbeamepitaxyona 88GaAs(111)Bsubstrate. AZnSebuerlayeris rstgrownontheGaAssubstrateafterwhi h 89
a thin layerof Au (less than one monolayerthi k) is evaporatedon the sample surfa e and 90
dewetted at
510
◦
C
to form small (
∼
10 nm
diameter) Au droplets that serve as a atalyst 91fortheNWgrowth. Thesubstratetemperatureisthenset at
400
◦
C
anda uxofZnandSe 92
atomswith anex ess of Se is used,indu ingpreferential growthof verti alZnSe NWs. The 93
NWsareinwurtzitephaseandtheirdiameteristhesameasthedroplet(
∼
10 nm
diameter). 94The thi kness of the initialAu layer is hosen to ensure a low NW density (
≤
1 NW per 95µm
2
). After the growth of a
400 nm
high NW, the atom uxes are stopped to allow the 96eva uationofresidualSeatomsinsidethedroplet. Then,theQDisgrownundera uxofCd 97
and Se atomsfor
20 s
. The uxes are interrupted againbefore theZnSe growth isresumed, 98resulting inan expe ted QD height of 2-
3 nm
inserted in a∼
700 nm
high NW. Finally, an 99epitaxial Zn
0.83
Mg0.17
Se shell (5 nm
thi k) is grown around the NW at220
◦
C
. A s anning 100
ele tron mi ros ope (SEM) image of su ha CdSe/ZnSe/ZnMgSe ore/shell NWQD system 101
is presented in Figure 2(a). The ag-shape termination of the NW is formed during the 102
100nm
(a)
100nm
(d)
20 nm
10 nm
2-3 nm
t
s
(c)
(b)
100 nm
ZnSe
ZnMgSe
CdSe
Al
2
O
3
Figure 2. (a) SEM image of a standing ZnSe/ZnMgSe NW embedding a CdSe QD. The QD
position is marked by the red square. (b, ) Tilted SEM image of a ZnSe NW after a
20 nm
and110 nm
thi k Al2
O3
shell deposition respe tively. The NW is sket hed on the SEM image. Notethe ir ular shape of the shell as well as its hemispheri al termination above the NW apex. (d)
Sket h of theNWQD geometry, indi ating theQD height (2-
3 nm
), theNW diameter (≃10 nm
), theepitaxial shellthi kness(≃5 nm
) andtheALD shellthi knesst
s
.growth of the ZnMgSe shell. It is present insome NWs. 103
The higher bandgap of Zn
0.83
Mg0.17
Se shell prevents the harge arriers to re ombine 104non-radiatively on the ZnSe NW sidewall and hen e improves the quantum yield of the 105
CdSe emitter. In prin iple, it ould dire tly be used to grow a photoni wire of diameter 106
∼
λ/n
ZnSe
around the NWQD. However, during the epitaxial shell growth two phenomena 107are ompeting: the radial growth of the shell around the wurtzite NWs, and the verti al 108
growth of a 2D Zn
0.83
Mg0.17
Se layer on the sample surfa e. The radial shell growth rate is 109verylowbe ause the growth of ZnSeon WZsurfa es is not favourable. Be ause ofthis low 110
shell growth rate, atrade-o has tobefound to avoid buryingthe NWs ina Zn
0.83
Mg0.17
Se 111matrix. As aresult, only thin epitaxialshells an be fabri ated. 112
The omplexity of reating a thi k epitaxialshell is one of the reasons why we fabri ate 113
reason isthat, sin e this pro ess step an bedone separately from the NW growth pro ess, 115
it allows to tune ex situ the shell parameters after a rst opti al hara terization of the 116
QD.Indeed, due toitsslowdeposition rate,the ALD pro essallowstopre isely ontrolthe 117
deposited thi kness, whi h an also be nally veri ed using s anning ele tron mi ros opy. 118
Wehavetestedseveraloxidematerials,andsele ted Al
2
O3
be auseitprodu edverysmooth 119and onformal, amorphous shells. Figure 2(b) and ( ) show two SEM images of the result-120
ing oxide shell deposition (
20 nm
and110 nm
), and the omplete stru ture is sket hed in 121Fig. 2(d). We note that the onformaldeposition allows to end the NW+shell stru ture by 122
analmost perfe t half-sphereas an beseen inFig. 2(b, ). ALD alsoburiesthe Au droplet 123
under the shell. The latter might intera t with the eld emitted by the QD through its 124
lo alized plasmon resonan e. Considering its small diameter it will essentially absorb the 125
in oming eld. Moreover the guided HE11 mode pro le presents a minimum on the NW 126
axis. This iswhy we negle t the dropletin uen e in the following. 127
IV. EXPERIMENTAL RESULTS
128
Asample fromasingle epitaxialgrowth pro ess is ut inpie es,and photoni stru tures 129
with dierentoxide shell thi knesses are fabri ated. Taking advantage of the lowNW den-130
sity, individualstru tures are opti ally hara terized dire tly onthe growth substrate. The 131
samplesare mountedon the old nger ofa He- ux ryostat and ooled down to
4 K
. Indi-132vidualphotoni stru tures are probed using onfo almi rophotolumines en e (µPL). They 133
are ex itated by a super ontinuum pulsed laser (Fianium WhiteLase,
10 ps
pulse duration, 134repetiton rate
76 MHz
) and a spe trometer sele ting a10 nm
bandwidth entered around 135485 nm
. This ex itationenergy, belowthe ZnSegap, allows ustoindu e rossedtransitions 136between delo alizedstatesinthe NW1D ontinuumanda dis rete on ned 0D stateinthe 137
NWQD band stru ture[25 ℄. In this on guration, the NW axis is aligned with the opti al 138
axis and emission from the QD is olle ted by a
NA = 0.6
obje tive. A typi al NWQD 139spe trum is presented in Figure 3(a) as a fun tion of the pump laser power. Three lines 140
an be identi ed and are attributed to the ex iton (X), the harged ex iton (CX) and the 141
bi-ex iton(XX) respe tively. The total emissionintensity of the X lineas afun tion of the 142
pumppowerisreportedinFigure3(b). Itshowsalinearin reaseatlowpumpingpower,and 143
525 530 535 540
Wavelength
(nm)
0.0
0.5
1.0
1.5
2.0
2.5
PL
In
ten
sit
y (
arb
. u
nit
s)
X CX XX
(a)
10
-2
10
-1
10
0
Pump Power (mW)
10
0
10
1
Ex
cit
on
in
ten
sity
(x
10
3
s
−
1
)
(b)
0
20
40
60
80 100 120
Oxide shell thickness (nm)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
No
rm
ali
ze
d In
ten
sit
y
(c)
axial
radial
Figure 3. (a)µPL spe trum ofa NWQD with
120 nm
thi k photoni shellfor dierent pumpingpowers. Itshowsaex iton(X), hargedex iton(CX)andbiex iton (XX)lines. The orresponding
pumpingpowersarereportedinpanel(b). Thebla kre tangleindi atestheintegrationbandwidth
usedtoextra t thetotal ex itonemissionintensity(Xline). (b)Integrated ex itonemission
inten-sity as a fun tion of pumping power, in a log-log s ale. ( ) Blue rosses: Total ex iton emission
intensity for dierent NWQDs as a fun tion of the oxide shell radius. Red diamonds: average of
the experimental datapoints. Data are normalized to the average intensity at
t
s
=110 nm
Bla k lines: results ofthenumeri alsimulationsfor aradial (solidline)andanaxial(dashedline)dipole.Theyarenormalized to the axial intensityat
t
s
=110 nm
a onstant plateau at high pumping powers orresponding to the saturation of the ex iton 144
level.[26℄ Under pulsed ex itation, we note that hanging the shell thi kness might modify 145
the laser power in the NW and the ex itation probability of the QD. Hen e if ae ts the 146
slopeatlowpowerinFig.3(b). Ithashowevernoee tonthesaturationplateauwhi honly 147
depends on the QD emission rate and light olle tione ien y. This allows usto ompare 148
statisti alsets of nanostru tures with dierent oxide shellthi knesses. The total integrated 149
emissionat saturation asa fun tion of the oxide shell thi kness is reported in blue markers 150
for ea h NWQD in Figure 3( ). The values have been normalized to the average intensity 151
0 2 4 6 8
t (ns)
10
-2
10
-1
10
0
No
rm
ali
ze
d PL
In
ten
sity
(a)
(b)
0 20 40 60 80 100 120
Shell thickness (nm)
0
2
4
6
8
10
De
ca
y t
im
e (
ns)
axial
radial
semi-analytical
Figure4. (a)Example ofTRPL signalversus timefor 2 NWswith a
t
s
=20 nm
shell. Ba kground ountsaremeasuredfort < 0
andsubstra ted. Amplitudeof ountsarenormalizedto1to ompare the2 datasets. Red lines aremono-exponential ts, whose orresponding pointsin(b) areshowmbyarrows. (b)Blue rosses: experimentalex iton de aytimesfor several QDsasa fun tionof the
oxideshellradius. Theverti al errorbarsrepresent the terror. Bla klines: numeri al simulation
results for a radial dipole (solid line), or an axial dipole (dashed line). Red dashed-dotted line:
Semi-analyti al al ulations for the in niteNW.
at
t
s
=110 nm
. ForNWswithout anoxide shell, the lumines en e intensity is very lowand 152we were never able to rea h the saturation regime, this is why we donot report the orre-153
sponding points in Fig. 3( ). Forea h shell thi kness, we observe a large spread inex iton 154
saturation intensity. However, we note a general trend of in reasing saturation intensity 155
within reasingshellthi kness, asdemonstratedby theredmarkerswhi hshowtheposition 156
of the average intensity of our measurements for ea h shell thi kness. On average, the de-157
position ofa
110 nm
thi k shell resultsin the experimentsin analmost4-foldenhan ement 158of the olle ted intensity with respe t to the
20 nm
thi k shell ase. The semi-analyti al 159al ulationsshowthatthis enhan ementis10-foldwhenwe omparetoaNWwithoutoxide 160
shell. 161
The observed in rease in intensity at saturation orresponds to the ombination of im-162
proved olle tion e ien y through light redire tion from the stru ture and enhan ement 163
of the spontaneous emission rate. In the latter ase, a modi ation of the QD dynami s is 164
expe ted tobedete ted by measuring the ex iton de ay rate. Time-resolved measurements 165
were arried out using a low pump power as ompared to the ex iton saturation power to 166
avoid any repopulation of the X level. The measured de ay transients are thus monoex-167
ponential. The tted de ay onstant is the total ex iton de ay time
τ
. The experiment 168one of gure 3( ). The same ex itation laser wasused, the QD uores en e was spe trally 170
ltered in a spe trometer (500gr/mm grating) and integrated on an avalan he photodiode 171
ina photon orrelation setup,using the exit slitof the spe trometeras aspe tral bandpass 172
lter. Theresultsofthese measurements,presented inFigure4showalsoa greatdispersion 173
in de ay time. One observes however that longer lifetimes are observed for smaller shell 174
thi kness (up to
5.9 ns
). In reasing the shell thi kness leads to an overall de rease in the 175measured ex iton lifetime, hen e an enhan ement of the ex iton de ay rate in agreement 176
with the results of the numeri al simulations. For systems without an oxide shell, only a 177
fewNWQDsgivealargeenoughsignaltobeproperlymeasured. Theyyieldamu hsmaller 178
dispersion of short de ay times. 179
V. DISCUSSION AND COMPARISON TO NUMERICAL SIMULATIONS
180
A. Dispersion of the results 181
For ea h oxide shell thi kness, the large variations of the experimental results in both 182
Figs. 3( ) and 4 have several possible origins. First, the presen e of non-radiative re om-183
bination hannels an redu e the intensity at saturation and hange the de ay time. The 184
non-radiative re ombination rate an vary fromQD to QD be ause of fabri ation inhomo-185
geneities,leadingtoaspreadinthemeasuredvalues.[27℄Se ond,variationsintheQDaspe t 186
ratio and piezoele tri elds indu ed internal strain appliedby both the ZnSe ore and the 187
Zn
0.83
Mg0.17
Se shell lead to dierent overlap of ele tron and hole wavefun tions and hen e 188dierentex iton os illatorstrengths. Finally, onsidering the QD aspe t ratio and internal 189
strain, we expe t a heavy-holeex iton type for our QDs.[2831℄ Heavy-hole ex iton re om-190
bination results in a mixture of ir ularly polarized emission, omposed of two degenerate 191
out-of-phase radial dipoles. However, strain and on nement ee ts might lead to valen e 192
band mixingbetween lighthole and heavy hole levels,[3234℄ resulting inan emission om-193
posed of a mixture between axialand radial dipoles and hen eto a spreadin total emitted 194
intensity, aswe dis uss later. Additionalmeasurementson NWQDs grown insimilar ondi-195
tions and me hani allydispersed ona substrate (i.e. lying horizontallyon it)revealed that 196
oneNWQDoutof6emitlightpolarizedalongtheNWaxis,whileothersemitlightpolarized 197
the Zn
0.83
Mg0.17
Se shell and low temperature of observation, we expe t that non-radiative 199ee ts play a minor role. The epitaxial shell prevents non-radiative de ay hannels owing 200
to surfa e traps. Additional measurements as a fun tion of temperature show that both 201
the emission intensity and the de ay time donot hange signi antly up to150-
200 K
(not 202presented here). This indi ates that the non-radiative ee ts are not dominating at low 203
temperature, as in the present experiment. While we annot yet ompletely rule out the 204
ontributionofnon-radiativeee ts,wethinkthatthe majoree ttoexplainthe dispersion 205
of the results omes from variationsin valen e band mixingand os illator strength due to 206
the lo alenvironmentof the QD.Finally letusstress that the shortest de ay times (1-
2 ns
) 207we measure remain longer than the de ay time of CdSe self-assembled QD embedded in 208
bulk ZnSe (<
1 ns
)[35℄. The redu tion of the diele tri s reening ee t is a main ee t we 209eviden e. 210
B. Colle ted intensity and radiative lifetime 211
Tobetterunderstandtheee toftheshelldepositionontheNWQDemission,weperform 212
numeri alsimulationsof the photoni stru ture formedby the fullNW +oxide shell geom-213
etry[see Fig.2(d)℄. It takesintoa ount thepresen e of the ZnSesubstrate, and the Al 2
O 3 214
shell and layer deposited on the NWs and substrate. The QD is modeled as an os illating 215
ele tri dipole, either in the axial dire tion (along the NW axis) or in the radial dire tion 216
(orthogonaltothe NW axis). Weperform nite-elementmethodsimulations(Comsolv4.1) 217
to ompute the total eld radiated by the dipole.[34℄ For ea h shell thi kness and dipole 218
orientation,we evaluate the power radiatedtowards the obje tiveby omputingthe uxof 219
the Poynting ve tor over a surfa e limited by its numeri al aperture (NA=0.6) in a region 220
far from the NW where near eld an be negle ted. The results of these simulations are 221
reported inFigure 3( ) inbla k lines for an axial(dashed line) orradial (solid line) dipole. 222
The results are normalized tothe axial intensity at
t
s
=110 nm
. Comparing the simulated 223integrated intensity in the ase of a
20 nm
and110 nm
reveals an enhan ement fa tor less 224than2-foldforanaxialdipoleandalmost4-foldforaradialdipole. The4-foldenhan ement 225
observed in our measurements suggests that on average, the dominant emitting dipole in 226
our stru ture is radial, ingoodagreementwith the re ombinationof a heavy hole ex iton. 227
ulations by integrating the total power radiated over every dire tion for the two dipole 229
orientations(radial and axial)
P
. Wenormalize this value by the same quantity omputed 230foradipoleinbulkZnSe
P
0
. ForapurelyradiativesystemwehaveP/P
0
= γ/γ
0
= τ
0
/τ
[36℄, 231where
τ
andτ
0
aretheradiativelifetimeforthenanostru tureandforbulkZnSerespe tively. 232Radiativetimes are presented in bla k lines in Figure4, where we have hosen
τ
0
= 300 ps
233ingoodagreementwithpreviously reported radiativelifetimeofCdSeQDinbulkZnSe[37℄. 234
Theaxialdipoleradiateswithanalmost onstantde aytimeasafun tionoftheoxideshell 235
thi kness, while the radial dipole de ay time strongly de reases with in reasing oxide shell 236
thi kness
t
s
. Additionally,we ompare thede ay timefortheradialdipole omputedforthe 237full geometry to the semi-analyti al al ulations for the in nite NW presented in g. 1(b) 238
with the same
τ
0
value. The agreement is ex ellent indi ating that interferen e ee ts due 239tore e tions fromthe substrate and from the top hemispheri alterminationare negligible. 240
Comparingthetrends ofthe simulations,we an on rmthat ouremittersbearastrong 241
radialdipole hara ter. Themeasurementsdispersion anbewellunderstoodby onsidering 242
thattherealemittersareamixtureofradialandaxialdipolesradiatingwitha hara teristi 243
de ay time omprised between the simulated lifetimes of the pure radial and axial dipole. 244
Wedo not observe long de ay time for NWQDs withoutan oxide shell in Fig. 4. Forthese 245
systems,itisverydi ultto ndemitterswhi harebrightenoughtobedete tedisbe ause 246
both the laser absorption and the emission rate of a radial dipole are very weak for su h 247
small NW diameters. We think that the emitters whi h have been sele ted orrespond to 248
NWQDshavingalargefra tionofaxialdipole hara ter astheyarethebrightestoneswhen 249
nooxideshell is present. 250
C. Radiation pattern 251
Toanalyze the me hanismsleading tothe in rease in olle ted intensity with in reasing 252
shell thi kness, we present in Figure5 several simulated radiation patterns. They are rep-253
resented as polar plots of the far- eld intensity
I(θ)
in the top(x, z)
plane,θ
is the angle 254between the dire tion of observation and the verti al
z
axis. Simulations are made using 255respe tivelyaradialdipole[along
x
,Figures5(a- )℄oranaxialdipole[alongz
,Figures 5(d-256f)℄. 257
1
1
1
1
1
1
Air
NW
t
s
=110nm
t
s
=70nm
t
s
=30nm
NW
z
z
z
z
z
z
(a)
(b)
(c)
(d)
(e)
(f)
Figure 5. Radiation patterns from numeri al simulations for a radial dipole pla ed at
470 nm
abovethesubstrate. TheexperimentalNAregionisshadedandindi atedinred. (a,d)Comparison
betweenthe aseofafree-standingemitterinair(bla kdashes),andembeddedinsidetheNW(blue
solidline)foraradial(a)oraxial(d)dipole. Theyeviden etheee tofthediele tri s reeningfrom
theNW on the radial dipole and theabsen e of s reening for the axialdipole. (b, e)Comparison
of the total emitted intensity versus
t
s
for a radial (b) or axial (e) dipole. A ombined ee t of redu eddiele tri s reening andlightguiding and redire tiontowardssmall angles isobserved. ( ,f) Ee t oftheshelllayertermination shape fora radial ( )or axial(f)dipole. The hemispheri al
shape in reases the fra tionoflight thatis redire ted towardsthe
z
dire tion.Figures5(a,d) show the ee t of the NW stru ture alone (no oxide shell being present) 258
on su h dipoles by omparing it to the ase of a free standing dipole in va uum above 259
the same ZnSe substrate. One an see that the presen e of the NW does not ae t the 260
shapeof radiationdiagram,whi hisessentiallydeterminedbytheinterferen esbetween the 261
dire tlyradiated eldanditsre e tiononthesubstrate. Mostremarkably,inthe aseofthe 262
radial dipole the presen e of the NW dramati ally redu es the emission intensity through 263
the diele tri s reening ee t dis ussed earlier. Simulationsshowa radiativerate redu tion 264
by afa tor
∼
1/16 ≃ n
ZnSe
/45
inagreementwiththe diele tri s reeningvaluepredi ted by 265Eq. (1). In ontrast, in the ase of the axial dipole it an be seen that the presen e of the 266
NW only slightlyin reases the emittedintensity. 267
shell thi kness
t
s
. In the ase of the radial dipole, the shell rst redu es the index ontrast 269between the NWand thesurroundingmedium( f. Eq. 1),resultinginastrongredu tionof 270
the emitter lifetimeand thus in an in reased total emitted intensity as seen in in Fig. 3( ) 271
andFig.4. Notethatthetheintensitypatternshowninthepolarplotmustbemultipliedby 272
thesolidangle
sin θdθ
ifonewantstoevaluatethepowerradiatedinthenumeri alaperture. 273This is why the intensity for an axial dipole an be larger than for a radial one, as seen in 274
Fig. 3( ). Se ond, as shown in Figure 1( ), the shell presen e ensures preferential emission 275
intotheguidedHE
11
mode forin reasingshellthi kness. As a onsequen e anear-Gaussian 276far- eldemissionpattern orrespondingtothefar- eldemissionpro leoftheHE
11
mode[38℄ 277is observed for
t
s
=110 nm
, ontrary to the stru tures with a smaller oxide shell thi kness 278where one observesthe presen e of two losely-spa ed lobesat small emissionangles (
±
10
◦
279with respe t to the
z
-axis). The resulting emission into the 0.6 NA one is maximum for 280t
s
=110 nm
, where the emission into the HE11
mode is nearly maximum [ f. Fig. 1(b)℄. 281The ee t of the oxideshell thi kness on the axialdipoleis ompletelydierent. Whilethe 282
totalemittedintensitydoesnotvarymu h,andhen etheemitterlifetimestays onstant(as 283
notedinFig.4),the lightemittedby theaxialdipoledoesnot oupletothe HE
11
modebut 284is emittedex lusively intoradiation modes. Thus the fra tion of intensity emitted towards 285
the olle tionlens in reases onlyslightly asthe oxideshell thi kness in reases [ f. Fig.5(e)℄. 286
This intensity in rease for the axialdipolealsopresented inFig.3( ) isnot due to a hange 287
in the spontaneous emission rate of the emitter, but rather to a slight redire tion of the 288
emittedlight. 289
Finally, Figures5( ,f ) ompare the a tual hemispheri al geometryof the oxide shell ter-290
mination to the at end of a simple lateral shell. They show that the presen e of the 291
hemisphere is bene ial to the radiation pattern for both kinds of dipole. For the radial 292
dipole, the hemisphere enablesa near-adiabati expansionof the HE
11
mode[38℄ leading to 293a narrowing of the far- eld emission pattern and an in reased olle tion by the numeri al 294
aperture. The axialdipolebene ts less from the hemispheri altermination of the photoni 295
stru turesin enolightfromthisdipoleis oupledtotheHE11mode. Wealsonotethathalf 296
oftheemittedlightpropagatestowards thegrowthsubstrateanddue totheindex-mat hing 297
ondition between the NW and the substrate, this lightis predominantly lost. 298
In order to assess the performan es of our devi e we ompute the ratio
η
between the 299parameter is agood gure of merit for the antenna redire tion ee t althoughit annotbe 301
dire tlyrelatedtotheoverall olle tione ien y be auseofthe powerlostinthesubstrate. 302
Forourfullphotoni stru tureandaradialdipoleonehas
η ≃
80%fort
s
=110 nm
. Thisvalue 303redu es to
≃
66%fora atterminated ore-shellphotoni wireillustratingtheimportan eof 304theadiabati expansionoftheHE
11
guidedmodeattheendofthewire. Forthedipoleinthe 305NW without shell
η ≃
55%. We havealso simulated astru ture inspiredby state-of-the-art 306devi esfabri atedby top-down methodsinRef.[9℄. Inthis asewesimulatea
110 nm
oxide 307shell photoni wire where the hemispheri al termination is repla ed by a oni al tapper of 308
Al 2
O 3
whose radius progressively de reases from 120 to
10 nm
in1.5 µm
. In this ase one 309has
η ≃
94%,showingthatalthoughbene ialourhemispheri alterminationisnotoptimal. 310VI. CONCLUSION
311
In summary, we have presented a bottom-up approa h to fabri ate a diele tri antenna 312
around a QD inserted inside a NW. This method allows for both reprodu ible and very 313
pre isefabri ationofthestru tureonalargeensembleofemittersaton e. Itisbasedonthe 314
depositionofathi k oxideshellaroundtheNWusing atomi layerdeposition. Experiments 315
show a 4-fold enhan ement of the QD photolumines en e shown in Fig. 3( ) between a 316
20 nm
and a110 nm
thi k shell. Semi-analyti al al ulations and numeri alsimulations of 317thestru ture revealthattheoxideshell thi kness stronglya ts onthe radialdipoleemission 318
through twomain phenomena: the redu tionof thediele tri s reening,whi hin reases the 319
spontaneousemission ratefrom theQD, and the redire tionof lightthrough a waveguiding 320
ee t. Simulationssuggestthatthe olle tedintensityismultipliedbyafa tor7withrespe t 321
tothebareNW ase. Thefabri ationpro essofthe photoni shellisverysimpleand anbe 322
appliedtoQDsemittingsinglephotonsup toroomtemperature. Althoughnotoptimal,the 323
resulting stru ture is a step towards the best nanowire single photon sour es operating at 324
lowtemperature[9℄. Diele tri s reening ould be furtherredu ed by growingan oxideshell 325
of higher index mat hing
n
ZnSe
like TiO 2. We note alsothat in our system a large fra tion 326
of the emitted power is radiated in the substrate. This loss hannel ould be redu ed by 327
having a mirror at the bottom of the stru ture.[15 , 39℄ Moreover, to fully bene t from the 328
waveguiding approa h, a better ontrol on the intrinsi QD properties has to be rea hed 329
olle tionaperture. 331
ACKNOWLEDGMENTS 332
This work was supported by the Fren h National Resear h Agen y under the ontra t 333
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