Document [original]

Appl. Phys. Lett. 106, 193101 (2015); https://doi.org/10.1063/1.4921000 106, 193101

All-optical depletion of dark excitons from a

semiconductor quantum dot

Cite as: Appl. Phys. Lett. 106, 193101 (2015); https://doi.org/10.1063/1.4921000

Submitted: 25 March 2015 . Accepted: 27 April 2015 . Published Online: 11 May 2015

E. R. Schmidgall , I. Schwartz, D. Cogan, L. Gantz, T. Heindel , S. Reitzenstein , and D. Gershoni

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All-optical depletion of dark excitons from a semiconductor quantum dot

E. R. Schmidgall,

I. Schwartz,

D. Cogan,

L. Gantz,

1,2

T. Heindel,

S. Reitzenstein,

and D. Gershoni

The Physics Department and the Solid State Institute, Technion-Israel Institute of Technology, Haifa 32000,

Israel

Department of Electrical Engineering, Technion-Israel Institute of Technology, Hafia 32000, Israel

Institute of Solid State Physics, Technische Universit€

at Berlin, 10623 Berlin, Germany

(Received 25 March 2015; accepted 27 April 2015; published online 11 May 2015)

Semiconductor quantum dots are considered to be the leading venue for fabricating on-demand

sources of single photons. However, the generation of long-lived dark excitons imposes significant

limits on the efficiency of these sources. We demonstrate a technique that optically pumps the dark

exciton population and converts it to a bright exciton population, using intermediate excited biexci-

ton states. We show experimentally that our method considerably reduces the dark exciton popula-

tion while doubling the triggered bright exciton emission, approaching thereby near-unit fidelity of

quantum dot depletion. V

C2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4921000]

On-demand sources of single photons are an essential

component of emerging quantum information technologies.

1–6

Semiconductor quantum dots (QDs) are a leading technology

for achieving these single photon emitters.

7–13

In QDs, the

emission of single photons results from recombinations of

excited QD confined electron-hole pairs (excitons).

7–10

The

QDs are excited non-resonantly either optically

9,10

or electri-

cally

11,14

by a short pulse which leaves, after relaxation, a

confined exciton in the QD. In general, during the relaxation,

the spins of the exciton pair are randomized such that they are

either anti-parallel or parallel. An anti-parallel spin pair forms

a bright exciton (BE), which can efficiently recombine opti-

cally and emit a single photon. In contrast, a parallel spin pair

forms a long lived (1ls) dark exciton (DE),

15–17

which can-

not efficiently decay radiatively, preventing the device from

being an on-demand emitter.

In the case of resonant excitation of the QD, resonantly

or even quasi-resonantly tuned optical pulses can determinis-

tically generate a bright exciton in the QD, providing an on-

demand triggered source of single photons

18,19

and even

entangled photon pairs.

20,21

Moreover, resonant optical exci-

tations can be used to deterministically convert the light

polarization into exciton spin polarization.

However, in

practice, these resonant excitations require that the QD be

totally depleted from charges and excitons prior to the excit-

ing pulse. On-demand single photon QD-based emitters

require a method to deterministically deplete the QD of dark

excitons.

Here, we present a method to deplete a QD of long-lived

dark excitons. We optically pump the dark exciton popula-

tion to the bright exciton population using intermediate

excited biexciton states, and we demonstrate experimentally

both a substantial reduction in the DE population and a dou-

bling of the triggered BE emission, thus proving depletion

with close to unit fidelity.

Excited biexciton levels consist of two electron-heavy

hole pairs, where at least one charge carrier is in an excited

energy level. These levels have been the subject of several

previous studies which provide a comprehensive understand-

ing of their spectrum, spin wavefunctions of carriers in these

levels, and their dynamics.

23–27

Particularly relevant to this

study are the nine states in which both the electrons and the

heavy holes form spin-triplet configurations.

25–27

Such states

will be indicated in this paper by the notation Te

m(Th

m) repre-

senting an electron spin triplet (hole spin triplet) of projec-

tion mon the QD growth direction. We previously used two

of these levels for demonstrating full coherent control of the

bright exciton spin state.

28,29

In general, the nine Te

mTh

nm¼1;0;1;n¼3;0;3

excited biexciton states are spin blockaded from relaxation

to the ground e-singlet/h-singlet (Se

0Sh

0) biexciton state.

There are, however, efficient spin flip and spin flip-flop

mechanisms that permit this relaxation.

24,27,30

In these proc-

esses, an electron or an electron and a hole flip their spins

due to the enhanced effect of the electron-hole exchange

interaction in the presence of a near resonant electron-

longitudinal optic (LO) phonon Fr€

ohlich interaction.

The process of optical depletion is schematically

described in Figure 1, where Figure 1(a) describes a single

DE in its ground state in the QD. The DEs are generated

by non-resonant optical or electrical excitation, where

electron-hole pairs with high excess energy are generated

in the vicinity of the QD. During their relaxation, the car-

rier spins are randomized, resulting in a stochastic QD

excitonic population that is composed on average of

approximately 50% BEs and 50% DEs. Here, we used a

weak continuous wave (cw) 445 nm pulse of a few nano-

seconds duration to excite the QD and generate a mixed

excitonic population in the QD. Within 2–3 ns of the end of

the non-resonant pulse, the BE population has recombined

radiatively, resulting in an empty QD. On average, this

occurs in about 50% of the cases, corresponding to the

cases in which the QD was populated with a BE. In the

remaining cases, a long-lived DE remains in the QD for

long after the end of the non-resonant excitation. In order

to deplete the DE, an optical resonant excitation

15,17

into

an excited biexcitonic level is used as described in Figure

1(b). These excited biexciton states then quickly relax non-

radiatively to one of the lower energy levels of the biexci-

ton as described in Figure 1(c).

0003-6951/2015/106(19)/193101/4/$30.00 V

C2015 AIP Publishing LLC106, 193101-1

APPLIED PHYSICS LETTERS 106, 193101 (2015)

In roughly half of the cases (as shown in Figure 1and

experimentally demonstrated in Figure 2(b), below), the

relaxation is into the spin blockaded biexciton XX0

T63states,

where further relaxation is prohibited by the spin-parallel

configuration of the heavy holes (Th

63—Figure 1(f)). This

relaxation proceeds by emission of another photon and a

return to a QD containing a DE.

15,17

In the other cases, the

relaxation is to the ground biexciton level from which the

biexciton decays by the well studied two photon radiative

cascade,

20,21,31

leaving the QD eventually empty of charges.

The relatively large branching ratio (approximately 0.5)

between the two processes provides an extremely efficient

way of depleting DE populations from the QD. It can be eas-

ily estimated that, for a measured exciton (biexciton) radia-

tive lifetime of 470 (270) ps,

the QD can be fully emptied

using an excitation pulse of several nanoseconds.

The sample that we study was grown by molecular-

beam epitaxy on a (001)-oriented GaAs substrate. One layer

of strain-induced InGaAs QDs

was grown in a planar

microcavity.

The measurements were carried out in a l-PL

setup at 4.2 K. More details on the sample and the experi-

mental setup can be found in previous publications.

28,34

Few-picosecond pulsed excitation, such as the pulses used to

probe QD population, was performed using a frequency-

tunable, cavity-dumped dye laser pumped by a frequency-

doubled Nd:YVO

(Spectra Physics Vanguard

) laser. For

these experiments, the cavity-dumped dye laser pulse rate

was 9.5 MHz. The longer depletion pulse used grating-

stabilized tunable diode laser emission modulated by an

electro-optic modulator (EOM). This EOM was synchron-

ized to the pulsed dye lasers, and it permitted variable pulse

duration with rise and fall times of less than half a nanosec-

ond. For the non-resonant excitation pulse, a 445 nm diode

laser modulated by a fast accousto-optic modulator was used

to provide a weak excitation pulse with a stochastic popula-

tion of BEs and DEs in the QD.

Figure 2presents the (a) PL and (b) two laser PL excita-

tion (PLE) spectra of the QD.

In Figure 2(a), relevant

FIG. 1. Schematic description of the optical depletion process. "(#)[*(+)]

represents a spin-up (spin-down) electron [hole]. Blue (red) color is used for

describing a ground (excited) state carrier. Upward (downward) arrows

describe optical excitation (emission). (a) Following a non-resonant excita-

tion pulse, the QD is populated with a long lived DE (only one spin projec-

tion is shown for clarity). (b) Optical excitation generates an excited

biexciton state. The biexciton then relaxes nonradiatively with almost equal

probabilities either to its ground state XX

ble state XX0

T63(f). In the first case, the biexciton gives rise to a radiative cas-

cade (c) and (d) which leaves the QD empty (e). In the second case, one

photon is emitted (f) and after hole relaxation (g) the QD remains with a

dark exciton (h). The process then repeats from (a), as long as the optical ex-

citation lasts. Black arrows represent nonradiative processes. Radiative

recombination is indicated by oval-matching the recombining electron-hole

pair, and oval colors are matched to the arrows indicating corresponding

emission lines in Figure 2(a).

FIG. 2. (a) PL spectra showing horizontally (H, blue) and vertically (V, red) linearly polarized emission from the QD. Relevant emission lines are marked

above the spectral line by the initial state of the optical transition. Arrow colors match the PLE spectra in (b), which were obtained while monitoring the emis-

sion from the indicated spectral lines. (b) PLE spectra of the indicated lines in (a) showing the e-triplet h-triplet biexciton resonances relevant for the optical

depletion. The PLE spectra were obtained using two pulsed lasers. Spectra indicated by solid (dashed) lines were obtained with one laser tuned to a dark

(bright) exciton resonance while the energy of the second laser was scanned and the emission from the color matched PL line was monitored. (c) Schematic

description of the optical transitions observed in (b). Optical transitions from the DE X0

D(BE, X0

B) are indicated by solid (dashed) arrows and the added carriers

are indicated in the spin state diagram by an oval-matched pair. The spin configuration of the state is provided on the left, where Te

m(Th

m) represents an electron

spin triplet (hole spin triplet) with projection mon the QD ^zaxis. Roman numerals match the observed resonances in (b). Resonances labeled (ii) and (iii),

which initiate from the DE and equally contribute to the DE and BE populations, can be used for the optical depletion process outlined in Figure 1.

193101-2 Schmidgall et al. Appl. Phys. Lett. 106, 193101 (2015)

emission lines corresponding to optical recombination from

the ground state exciton (X

), ground state biexciton (XX0Þ,

and the relevant spin-blockaded biexciton (XX0

T63) are indi-

cated by colored arrows. The energy is measured from the

BE (X

) spectral line at 1.283 eV. The spin wavefunctions of

the initial biexciton states, and the final exciton states that

give rise to these transitions, were described in the previous

publications.

15,17,26

We note here that optical transitions

from the ground state biexciton XX

and the m¼0 spin-

blockaded biexciton XX0

T0result predominantly in BEs and

lead to sequential emission of a photon due to BE recombi-

nation, while the XX0

T63biexciton line heralds the presence of

a DE in the QD. The arrow color corresponds to the moni-

tored PL spectral line used to obtain the PLE spectra in

Figure 2(b).

Figure 2(b) presents cw excited PLE measurements.

One cw laser is used to excite the BE (dashed lines) or DE

(solid lines).

The energy of the second laser is varied while

the PL emission from the indicated emission line is moni-

tored. Four exciton-biexciton resonances marked by Roman

numerals are observed. Resonances indicated (i) and (iv) ini-

tiate from the BE, while resonances indicated (ii) and (iii)

initiate from the DE. These optical transitions, which were

previously discussed,

25,27

are schematically presented in

Figure 2(c).

Figure 2clearly demonstrates that while the resonances

(i) and (iv), which initiate from the BE, contribute mainly to

BE emission, resonances (ii) and (iii), which initiate from

DE (solid lines), contribute almost equally to the XX0

T63

emission and to the XX

emission. This means that in about

50% of these excitations the DE population is transformed

into a BE population and depleted, by subsequent optical

recombination, from the QD. We chose resonance (iii) for

the optical depletion outlined in Figure 1above, though reso-

nance (ii) performs similarly.

For demonstrating optical depletion, a sequence of three

laser pulses was used, schematically illustrated above Figure

3. The first pulse was a weak 20 ns pulse duration of 445 nm

laser light. A few ns after the pulse ended, a second pulse of

20 ns duration tuned to resonance (iii) of Figure 2was

launched. The third pulse was an 8 ps long pulse, and it was

tuned to the XX0

T63absorption line. It was turned on 5 ns after

the depletion pulse ended, its intensity corresponded to a p-

pulse, and the PL emission that resulted from this pulse

monitored the population of the DE in the QD.

Figure 3(a) presents the PL intensity from the XX0

T63line

as given by the color bar, as a function of time (horizontal

axis) and the power of the depleting pulse tuned to the (iii)

resonance of Figure 2(c) (vertical axis). As can be readily

seen in Figure 3(a), as the power of the depleting pulse

increases, the population of the DE in the QD decreases. A

quantitative measure of the efficiency of the depleting pulse

is provided by comparing the PL intensity during the probe

pulse (solid blue line) in Figure 3(b), where the depletion

pulse power was very weak (lower part of Figure 3(a))to

that in (c) where the power of the depletion pulse is maximal

(upper part of Figure 3(a)). The integrated PL intensity dur-

ing the probe pulse is reduced under these conditions to less

than 5%.

A complementary verification of the efficiency of the

depletion is provided by comparing the PL emission from the

BE (dashed green line in Figures 3(b) and 3(c)), at low (b) and

high (c) depletion pulse power. For these measurements, the

probe pulse was tuned to one of the BE resonances.

The inte-

grated BE emission during the probe pulse increases by almost

a factor of 2, indicating that the QD is approximately 50%

occupied by DEs in the absence of the depletion pulse and pre-

dominantly empty when the depletion pulse is present. Under

these conditions, we were able to get more than 8500 counts/s

on the BE detector during the probe pulse. Taking into account

the repetition rate of our experiment (9.5 MHz) and the meas-

ured overall light harvesting efficiency of the sample and the

setup (approximately 1/1000

) one gets depletion fidelity of

close to 90%. We note here that, by increasing the depletion

pulse power, we were able to achieve similar depletion fidel-

ities with pulses as short as 3 ns (not shown).

In conclusion, we have demonstrated an all-optical

depletion method of semiconductor QDs which deterministi-

cally converts dark excitons to bright excitons, thereby

increasing the fidelity of these on-demand triggered light

sources. We do this using optical pumping via intermediate

excited biexciton states which efficiently transfer dark exci-

ton populations to bright excitons and subsequently to an

empty QD, on a nanosecond timescale. This fast depletion

mechanism is essential for on-demand sources of single pho-

tons as well as for deterministic writing of the bright

and

dark exciton

spin states using picosecond optical pulses.

These abilities may be important for further development of

QD-based devices for quantum information and communica-

tion applications.

The support of the Israeli Science Foundation (ISF), the

Israeli Nanotechnology Focal Technology Area on

“Nanophotonics for Detection,” and the German-Israeli

Foundation (GIF) was gratefully acknowledged.

FIG. 3. (a) PL emission intensity from the XX0

T63spectral line (as given by

the color bar) as a function of the power of the depleting pulse to the (iii) reso-

nance (Figure 2) and time. The temporal sequence of the non-resonant excita-

tion pulse, depleting pulse, and the probe pulse tuned to the XX0

T63ðX0Þ

absorption line are shown above the figure. (b) [(c)] The solid blue line

describes the XX0

T63PL emission intensity as a function of time at very low

[maximal] depleting pulse power. More than 95% depletion is clearly

observed at high power. The overlaid dashed green line presents the intensity

of the PL from the X

spectral line while the ps probe pulse is tuned to the BE

resonance. The BE PL increases between (b) and (c) by nearly a factor of 2,

yet another indication for the efficient depletion of the QD from DEs.

193101-3 Schmidgall et al. Appl. Phys. Lett. 106, 193101 (2015)

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