1

Laser mode feeding by shaking quantum dots in a planar microcavity

C. Brüggemann1*, A. V. Akimov2,3, A.V. Scherbakov3, M. Bombeck1, C. Schneider4, S. Höfling4,

A. Forchel4, D. R. Yakovlev1,3, and M. Bayer1

1Experimentelle Physik 2, Technische Universität Dortmund, D-44221 Dortmund, Germany

2School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United

Kingdom

3A. F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg,

Russia

4Technische Physik, Universität Würzburg, 97074 Würzburg, Germany

* christian.brueggemann@tu-dortmund

Semiconductor light emission can be changed considerably in an optical resonator1.

Prerequisite is that the electronic transitions involved in light generation are in resonance

with a cavity mode. While resonance can be arranged through dedicated fabrication, there

are cases where this is virtually impossible. As an example we study a planar microcavity

containing an inhomogeneous quantum dot ensemble with a spectral broadening much

larger than the optical mode width, so that resonance is achieved for a tiny dot fraction

only. Still, the laser threshold can be crossed at moderate optical pumping. We demonstrate

that strain pulses generated by ultrafast acoustics techniques can be used to modulate the

transition energies such that resonance with the optical mode is dynamically induced for a

much larger dot fraction. As a result the emission output can be enhanced by more than

two orders of magnitude, potentially useful for modulating light sources.

This work was published by the Nature Publishing Group in Nature Photonics 6, 30 (2012) and the

final and edited version is online available here:

http://www.nature.com/nphoton/journal/v6/n1/full/nphoton.2011.269.html

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During recent years great progress has been made in fabricating monolithic optical resonators

with micrometer dimensions by combining atomically precise epitaxy with high resolution

lithographic patterning1. Placing a semiconductor light emitter into such a cavity has rendered a

number of spectacular demonstrations. Particularly noteworthy are the enhanced spontaneous

emission of quantum dots (QDs) in micropillars2, the strong coupling regime for a quantum well3

and a QD4,5 in a microcavity, the realization of efficient single photon6 and entangled photon7-9

sources based on QD resonators, and the demonstration of condensation-phenomena for

polaritons in quantum well cavities10-13. All these demonstrations require bringing an electronic

transition in a nanostructure that serves as optically active medium into resonance with an optical

cavity mode. While being demanding in general, there are cases where fulfillment of the

resonance condition is particularly challenging, e.g., for a single QD inside a resonator. Here

typically the initial fabrication is not sufficient but additional measures need to be taken.

Tuning of an electronic transition relative to the optical mode could be demonstrated by

applying electric14 or magnetic fields15 and by varying the sample temperature4,5. All these

tuning methods are of limited versatility, since, for example, high electric fields may lead to

carrier tunneling out of the nanostructure, magnetic field induced shifts are typically small and

temperature increase leads to enhanced carrier scattering. Here we present another, non-

detrimental methodology inducing considerable shifts of the electronic transitions so that

dynamically resonance with an optical mode is obtained. This method is based on injecting a

picosecond strain pulse, generated by ultrafast acoustics methods, into the resonator. The strain

pulse propagates through the resonator at longitudinal sound velocity until it hits the light

emitting quantum structure. Having arrived there, it can shift the energy of an electronic

transition by more than 10 meV during picoseconds16,17. Thus an energy detuning relative to the

optical mode of the same order of magnitude as the shift achievable by strain may be

compensated.

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The effect of the strain pulse is equivalent to ultrafast mechanical shaking of the QDs.

While being relevant for many different nanophotonic systems, we apply the technique here to a

self-assembled QD ensemble placed in a planar cavity. These QDs have a remarkably high

optical quality, but the fabrication method inherently leads to a considerable inhomogeneity in

size, shape and composition. The inhomogeneities are translated into the electronic transition

energies leading to a broadening of the photoluminescence (PL) emission in the few 10 meV

range, exceeding by far the spectral width of the resonator modes in the order of a meV or even

better. Despite the recent finding that also QDs with a detuning up to a few meV can feed the

optical mode18-22 the mismatch in linewidth leads to an inefficient coupling of the QD ensemble

as a whole to the cavity light field, limiting the emission efficiency.

Shaking of the QDs by picosecond strain pulses is an elegant solution, as more QDs

come dynamically into resonance with the cavity mode relative to the stationary case. Doing so,

we find a drastic enhancement of emission intensity. The increase is similar to the enhanced

throughput through a sieve filled with flour, where the flour runs through the sieve’s pores much

more quickly, if the sieve is shaken.

The resonator under study is a λ-cavity consisting of a GaAs layer sandwiched between

GaAs/AlAs Bragg mirrors (see Methods for details). In the center of this layer the elecric field

has an antinode at which a sheet of In0.3Ga0.7As QDs (density of ~1010cm-2) is placed. PL spectra

of the QD cavity are shown in Fig. 1a, recorded under quasi-stationary excitation (see Methods)

using a peak power density of P~10 MWcm-2, which is well below the lasing threshold. The

black curve in Fig.1 a, shows the PL spectrum recorded from the MC front, perpendicular to the

cavity plane. All emission is concentrated in the optical mode, confined perpendicular to the

cavity plane at an energy of ħωc =1.367 eV, with a full width at half maximum of 1.2 meV. The

red curve shows the side PL detected from the cleaved edge of the sample, parallel to the cavity

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plane. This spectrum consists of a Gaussian emission band with a spectral width of 11 meV

centered at E0=1.351eV, and corresponds to the inhomogeneously broadened luminescence from

the QD ground states. The maximum of the QD emission is clearly off-resonant with the cavity

mode, shifted by 16 meV to lower energy. The cavity mode emission is therefore fed only by the

far out, high energy flank of the QD ground state distribution, so that only a small dot fraction

contributes, even when taking into account that QDs with an energy detuning in the order of a

few meV can couple to the cavity mode.

The input-output curve of the emission intensity from the cavity mode as a function of

quasi-stationary (pulsed) excitation density is shown by the red (black) symbols in Fig. 1b. As

seen from the superlinear increase of the intensity, the cavity can be pushed into lasing by

increasing the excitation density beyond the threshold value (definition in Supplementary 1)

PTH=22 MWcm-2 in the stationary excitation regime (WTH=5.5 µJcm-2 in the pulsed regime). The

emission kinetics recorded with a streak camera above the threshold (W=70 µJcm-2) is shown in

Fig. 1c. When recording the emission from the cavity mode, contributed by quasi-resonant QDs,

after a rapid rise the signal decays with a short time constant of τc=22 ps only (black curve). On

the other hand, when detecting the spontaneous emission by off-resonant QDs from the side

(ħω=1.351 eV), the decay is much slower with a characteristic decay time τ0=1580 ps (red

curve). The emission from these QDs obviously does not contribute to lasing.

The experimental scheme of the QD shaking experiment is given in Fig. 2a. The strain

pulse used for that purpose is generated on the backside of the sample by the established

technique of ultrafast acoustics23. It is injected into the substrate with a shape that is shown in

Fig. 2b. After propagation through the substrate and the lower Bragg mirror the strain pulse hits

the QD layer. For the used excitation power of the metal film transducer, the strain amplitudes

are so high that non-linear effects become important during strain pulse propagation. These non-

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linearities in combination with multiple reflections of phonons in the Bragg mirrors modify the

shape of the strain pulse, as seen in Fig. 2c, calculated for the moment of arrival, tS, at the QD

layer. Details about this calculation can be found elsewhere24,25.

First we describe the effect of the strain pulse on the emission intensity I(t) for optical

excitation under quasi-stationary conditions. Figure 3a shows the temporal evolution I(t)/I0 of the

emission intensity relative to the intensity I0 without strain, monitored for P=0.9PTH over a time

interval of 1.8 ns. Two pulses separated in time by 1.3 ns are observed. This separation is equal

to the time it takes the strain wave packet after passing the QD layer to move through the upper

Bragg mirror, become reflected at the sample surface, pass the top mirror again and hit the QD

layer a second time. When the second hit occurs a further change of the strain pulse profile (see

Fig. 2d) has taken place caused not only by mirror reflections but also by the π-phase shift for

reflection at the MC open surface. The QDs are shaken during the incident and reflected strain

pulses, and in both cases we observe a considerable increase of the output emission intensity, up

to a factor of 50.

The net increase of the PL intensity during QD strain shaking is the central result reported

here. Figure 3b shows the temporal evolution of I(t)/I0 due to the incident and reflected strain

pulses for different excitation densities P. The largest relative increase is observed when the

optical excitation density P has a value close to the lasing threshold PTH=22 MWcm-2. The

amplitude of I(t)/I0 is smaller when the device is operating above the lasing threshold. However,

even at P=3PTH the maximum intensity is still increased several times. At P>PTH there is a time

interval where a decrease of emission intensity up to complete quenching is observed, see purple

and black curves in the time interval from 50 to100 ps after strain pulse arrival at time tS in

Fig.3b. This decrease, which is only a minor correction to the net increase of output intensity,

will be explained below.

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The intensity enhancement is even stronger for femtosecond laser pulse excitation of the

QDs. The black curves in Fig. 4 (main panel and inset) show the time-dependent emission

intensity I(t) without shaking. The output intensity increases at early times and reaches the

maximum I0 about 100 ps after the femtosecond optical excitation pulse at t0. Thereafter the

intensity decreases with the decay time τc governed by the lasing kinetics of the cavity mode.

When the strain pulse arrives at the QD layer (t=tS), the intensity I(t) shows a considerable

enhancement. The relative increase is the highest for strain pulse arrival at the QD layer when

the emission intensity has its maximum I0 (tS-t0≈100 ps) and for an optical excitation density

W=WTH. The signal under these conditions is shown by the red curve in Fig. 4, where the shaking

occurs from the incident strain pulse. Here we observe a 200-fold increase of the emission

intensity. The relative increase of I(t) is still observed when W exceeds WTH (see red curve in the

inset of Fig. 4), but the enhancement is significantly less than around threshold. The increase of

I(t) is observed also when the shaking occurs at later times, after the emission maximum, as

shown by the purple curve in the inset of Fig. 4.

The physics underlying these observations is similar to modulation of the Q-factor in

lasers, but here, in contrast to Q-switching and cavity dumping, the cavity parameters are kept

constant and the modulation is applied to the optical transition energy of the lasing medium. The

shaking, which is maintained over times in the order of 100 ps (see Fig. 2c and 2d), has different

consequences for the spontaneous and stimulated emission regimes below and above the

threshold, respectively. For a given excitation power, the optical excitation creates a certain

population of the QDs by carriers. This population can decay for all QDs into guided optical

modes along the resonator plane, while only the QDs with transition energies in a narrow

window of a few meV around the optical mode can emit into the cavity mode. The strain pulses

consist of compression and tension parts (see Figs. 2c and 2d), both resulting in modulation of

the QD spectrum by a value of ~ 10 meV16,17. The amount of modulation can be tailored further

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by the strain pulse amplitude and shape (Supplementary Figure S1). Thus, when the cavity mode

is located in the spectral wing of the QD distribution (see Fig. 1a), the compressive part of the

strain pulse causes a larger QD density at the cavity spectral mode during its duration. In other

words shaking brings nominally off-resonant QDs into the energy window relevant for coupling

to the cavity mode, so that photons from exciton decay in these QDs are not only emitted into

guided modes but are funneled also into the cavity mode. We need to discuss three regimes of

shaking which are classified by various QD excitation densities:

(i) P<<PTH . In this regime the intensity is weak and for stationary excitation it increases

only by a few times during shaking, resulting from the shift of the maximum of the QD spectral

distribution towards the cavity mode.

(ii) P≈PTH . The increase of QD density at the cavity mode during shaking corresponds to

an increase of the excited QD population that is equivalent to a shift of the gain spectrum, which

may be sufficient to exceed the threshold for lasing. Due to the dynamical crossing of this

threshold the emission intensity during the strain pulse is increased drastically as observed

experimentally for both the stationary and pulsed excitation regimes (Figs. 3 and 4).

(iii) P>PTH . In this regime the exciton decay for QDs, which can contribute to lasing, is

strongly shortened, as shown in Fig. 1c. As a result, the emission of these quasi-resonant QDs

occurs dominantly into the cavity, while the guided mode emission becomes negligible relatively

to the laser output. In particular the emission dynamics becomes much faster than the time

during which strain modulation is present. The emission into the cavity mode burns a spectral

hole at the cavity mode into the electron-hole pair population of the QDs. Due to shaking this

spectral hole is broadened over the modulation range of ~10 meV. Far above the threshold the

increase of emission intensity is determined only by the number of excited QDs in the modulated

energy window relative to those in the static window coupling to the cavity mode.

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So far we have focused on the impact of the compressive part of the strain pulse, now we

consider the tensile part, by which the QD distribution is shifted to lower energies, so that its

separation from the optical resonator mode increases. Consequently the emission intensity drops

and can even be completely quenched, as observed in Fig. 3 (purple and black curves). However,

the enhancement of intensity during compression is much larger than this drop, so that integrated

over the whole strain pulse a net increase of the emitted intensity is obtained.

The modulation of the QD optical transition energy by shaking is the main reason for the

enormous intensity increase at excitation densities near the lasing threshold. However, taking a

closer look at Fig. 3 and 4, one notes that the increase of emission intensity begins already before

the strain pulse hits the QD layer (t<tS), more than 50 ps earlier. Most likely the related increase

arises from the propagation of the strain pulse through the cavity and, in particular, the Bragg

mirror below the QD sheet26. The strain perturbation of the cavity structure leads to a modulation

of the optical mode energy relative to the QD distribution which is still stationary at these times.

The magnitude of modulation by < 0.5 meV is much less than that for the QD resonance,

resulting still in an notable increase of the intensity. This effect is predominant around the lasing

threshold. However, in this regime even small modifications of the quasi-resonant QD density

can make the difference between spontaneous and stimulated emission,

In conclusion, we have demonstrated that the emission output of a quantum dot cavity,

where a significant fraction of dot structures is per se out of resonance can be enhanced

drastically using ultrafast acoustics techniques. Independent of the specific example, this

technique can be used for bringing electronic transitions in resonance with an optical cavity

mode. Examples for devices where such a resonance is hard to achieve are QD cavities for single

or entangled photon emission or high efficiency lasers where population inversion can be

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achieved for off-resonance, e.g. by electrical pumping. Upon shaking the device a large QD

fraction is pushed into cavity resonance so that stimulated emission channel sets in.

It can also be applied more traditionally for modulation of light sources at frequencies in

the GHz-range, comparable with the current limit for such modulation. Up to now, this

modulation of coherent light emission from lasers is obtained by varying resonance frequency or

quality factor of the optical cavity. In lasers with external cavities this is realized by inserting

devices like acousto-optical modulators etc into the cavity. In microlasers with dimensions in the

order of the emission wavelength, usage of such tools is challenging27,28, so that external

modulators need to be used. Even higher modulation frequencies might be obtained by designing

the energy detuning between the optically active medium and the cavity mode such that only the

high amplitude parts of the strain pulse affect their coupling.

Modern methods of ultrafast acoustics are approaching the stage where shaking may be

obtained without using external femtosecond laser systems. Recent work on the development of

THz sasers (the acoustic analogue of lasers)29 on the basis of GaAs/AlAs superlattices controlled

by an external electrical bias will allow one to fabricate a single chip where the “shaker” is

integrated into a microlaser and the device is controlled purely electrically. Such devices might

also be much more energy efficient than the strain generation applied here.

Methods

Sample. The microcavity was grown by molecular-beam epitaxy on a (001)-oriented GaAs

substrate. The distributed Bragg-reflectors of the cavity are composed of 23 (27) pairs of

alternating layers of AlAs and GaAs, each with λ/4-thickness, in the upper (lower) mirror.

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Sandwiched between the mirrors of the λ-cavity is a GaAs layer with a sheet of In0.3Ga0.7As QDs

(sheet density of ~1010cm-2) in its center. For the sample piece studied here, the optical resonance

of the cavity is located at a photon energy ħωc =1.367 eV (see emission spectrum in Fig. 1a).

The full width at half maximum of this resonance due to emission perpendicular to the MC plane

is 1.2 meV, corresponding to a Q-factor of slightly more than 1000. We attribute this moderate

Q-factor to the cavity mode being positioned in the quantum dot absorption band due to excited

state transitions, leading to considerable damping of the resonance. Due to the wedge shape of

the cavity the optical mode energy varies considerably across the sample wafer. Therefore also

situations can be obtained where the optical mode energy is located well below the QD ground

state transition. Then we find a mode linewidth of less than 0.3 meV, corresponding to a Q-factor

of 5000.

We note that for such planar cavities no enhancement of the spontaneous emission rate through

the Purcell effect is observed, so that in the spontaneous emission regime the QD decay time is

much longer than the strain pulse duration, both on- and off-resonant with the cavity mode. This

also limits the enhancement of emission intensity by strain pulse application for the regime

below the laser threshold.

We also note that we do not observe emission from higher lying quantum dot states over the

whole excitation power range. This is because the laser threshold is passed before complete

ground state filling is reached and occupation of excited states due to the Pauli principle would

take place. Above the threshold electron-hole pairs decay so fast, that carriers can relax into the

ground states, from where they decay, independent of the excitation power level.

Experiment. The experiments are performed at cryogenic temperatures by inserting the sample

into helium gas at T= 5 K. When recording the quasi-stationary PL spectra in Fig. 1, a pulsed

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diode laser (photon energy E=2.33 eV, pulse duration 24 ns, repetition rate 100 kHz), was used

for excitation. The laser was focused onto the front MC surface (spot size 35 µm). The peak

excitation density P~10 MWcm-2 is below the lasing threshold. For recording the emission

kinetics, we used 150 fs pulses, taken from a Ti:Sapphire laser pumped regenerative amplifier

(E=1.55 eV, repetition rate 100 kHz) for excitation. The emission was dispersed by a 0.5 m

spectrometer and detected by a streak camera with a time resolution of 25 ps.

For the strain pulse experiments no spectrometer was used to increase the sensivity. To select

only the cavity emission, a lowpass filter transmitting light with energy <1.46 eV was positioned

in front of the streak camera.

For the strain pulse excitation a 100 nm thick Al film, deposited on the GaAs substrate opposite

to the MC, is illuminated by 100-femtosecond laser pulses, centered at 800 nm wavelength with

an energy density per pulse of ~10 mJcm-2. Photoexcitation of the metal film results in its rapid

thermal expansion. As a consequence a strain pulse is injected into the substrate with a shape that

is shown in Fig. 2b.

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14. Kistner, C. et al. Demonstration of strong coupling via electro-optical tuning in high-

quality QD-micropillar systems. Opt. Express 16, 15006-15012 (2008) 15. Reitzenstein, S. et al. Control of the strong light-matter interaction between an elongated

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16. Scherbakov, A.V. et al. Chirping of an optical transition by an ultrafast acoustic soliton train in a semiconductor quantum well. Phys. Rev. Lett. 99, 057402 (2007)

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22. Winger, M. et al. Explanation of photon correlation in the far-off-resonance optical emission from a quantum-dot-cavity system. Phys. Rev. Lett. 103, 207403 (2009)

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Aknowledgements

We acknowledge financial support by the Deutsche Forschungsgemeinschaft through the project

BA 1549/14-1, the Russian Academy of Science and the State of Bavaria.

Author contributions

A.V.A., M.Ba., and D.R.Y. developed the idea of the experiment. C.B., M.Bo., and A.V.S.

performed the experiment. C.B. processed the data. C.B., A.V.A., and M.Ba. analysed and

interpreted the results and wrote the manuscript. C.S., S.H., and A.F. fabricated the microcavity

sample. All authors discussed the results and the manuscript.

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Figure captions.

Figure 1 | Characteristics of the quantum dot microcavity. a, Photoluminescence spectra

recorded at T=5 K under quasi-stationary excitation conditions. The emission was collected

either normal to the cavity from its front (black trace) or parallel to the cavity from the side (red

trace). b, Output intensity versus excitation energy density W (bottom) and power density P

(top) for pulsed and quasi-stationary excitation, respectively. The arrows indicate the laser

threshold densities. c, Photoluminescence transients after pulsed excitation with W=70 µJcm-2

corresponding to emission from the front (black) and from the side (red).

Figure 2 | Experimental setup and strain pulse propagation. a, Scheme of the ultrafast

acoustics experiment and in particular of the strain pulse propagation through the sample. Panels

b , c and d give the temporal evolution of the strain pulses: initially injected form the Al film at

t=0 (b); in the QD layer arriving from the substrate at t=tS (c); and in the QD layer after

reflection from the open surface of the MC (d).

Figure 3 | Intensity modulation under stationary optical excitation. a, Change of the

emission intensity from the quantum dot microcavity by the incident and the reflected strain

pulses. - CW optical excitation conditions of the QDs with the excitation power just below the

lasing threshold (P=0.9 PTH=21 MWcm-2). b, Temporal evolution of the emission intensity

caused by the incident strain pulse for various QD optical excitation densities. The inset shows

the same for the reflected strain pulse action. T=5 K.

The larger emission intensity enhancement for the reflected strain pulse compared to the incident

one is due to the stronger weight of the compression parts in this pulse (Supplementary 2)

15

Figure 4 | Intensity modulation under pulsed optical excitation. Time-resolved emission

intensity from the microcavity after pulsed optical excitation of the QDs with an energy density

around the lasing threshold. The black trace gives the transient without strain pulse application;

the red curve is the transient with simultaneous strain pulse application. The times of optical

excitation (t=t0) and the strain pulse arrival (t=tS) are indicated by the vertical arrow and dashed

line, respectively . The inset shows similar data on a linear intensity scale and with optical

excitation slightly above the laser threshold. In addition an emission transient is shown for which

the strain pulse was delayed by another 80 ps (purple).

5 6 7 8 9 10 11

100

101

102

103

104

20 25 30 35 40 45

100

101

102

103

s tationary pulsed

0 1 2 3

10-2

10-1

100

1.33 1.34 1.35 1.36 1.37

0.0

0.5

1.0

Emis

sio

n In

ten

sity

(a.u

.)

Emis

sio

n In

ten

sity

(a.u

.)

Energy (eV) Time (ns)

CW power density P (MW cm-2)

Pulsed energy density W (μJ cm-2)

Emis

sio

n in

ten

sity

pu

lsed

Emis

sio

n in

ten

sity

CW

a c

b

front view

side view

�0 = 1580 ps

�C = 22 ps

PTH

WTH

QD Layer

Microcavity

AlTransducer

StrainExcitation

PL Excitation

Cavity Emission

Substrate

Strain

a

b c d

Time t (ps) Time t-tS (ps) Time t-tS (ps)0 50 100 150 200 1300 1350 1400 1450 15000 50 100 150 200

injected incident reflected

Stra

in (1

0-3)

1

-1

0

Emis

sio

n in

ten

sity

I(t)

/I0

Emis

sio

n in

ten

sity

I(t)

/I0

P=21 MW/cm² (0.9 PTH )P=28 MW/cm² (1.2 PTH )P=70 MW/cm² (3.0 PTH )

Emis

sio

n in

ten

sity

I(t)

/I0

incident

reflected

incident

reflected

a

b

Time t-tS (ps)

Time t-tS (ps)

Time t-tS (ps)0 500 1000 1500

0

10

20

30

40

50

-50 0 50 100 150 2000

5

10

15

20

1300 1350 1400 1450 15000

10

20

30

40

50

-100 0 100 200 300 400

0.1

1

10

100

Emis

sio

n in

ten

sity

I(t)

/I0

Emis

sio

n in

ten

sity

I(t)

/I0 1

2

3 W=6.1 μJ cm-2 = 1.1WTH

W = 5.5 μJ/cm² = WTH

�C

= 62 ps

without strainwith strainstrain delayedby 80 ps

Time t-tS (ps)

Time t-tS (ps)-100 0 100 200

t0

t0

Supplementary Informationto

Laser mode feeding by shaking quantum dots in a planarmicrocavity

C. Bruggemann, A. V. Akimov, A.V. Scherbakov, M. Bombeck,

C. Schneider, S. Hofling, A. Forchel, D. R. Yakovlev, and M. Bayer

Abstract

Semiconductor light emission can be changed considerably by placment inan optical resonator. Prerequisite is that the electronic transitions involvedin light generation are in resonance with a cavity mode. While resonancecan be arranged through dedicated fabrication, there are cases where this isvirtually impossible. As an example we study a planar microcavity containingan inhomogeneous quantum dot ensemble with a spectral broadening muchlarger than the optical mode width, so that resonance is achieved for a tinydot fraction only. Still, the laser threshold can be crossed at moderate opticalpumping. We demonstrate that strain pulses generated by ultrafast acousticscan be used to modulate the transition energies such that resonance with theoptical mode is dynamically induced for a much larger dot fraction. As a resultthe emission output can be enhanced by more than two orders of magnitude,which may be useful for modulating light sources.

1 Definition of the laser threshold

The laser threshold is the excitation power at which the cavity emission changes fromspontaneous to stimulated. A quantitative threshold value is obtained, by

(1) extrapolating the linear dependence of emission output as function of excitationpower density P (energy density W) for stationary (pulsed) excitation.

(2) approximating the regime of the superlinear increase by a linear form, too.

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The laser threshold is then given by the excitation power density (energy density), atwhich the two dependencies cross.

2 Difference between emission intensity modulation

by the incident and reflected strain pulse

The emission intensity modulation for stationary excitation conditions in Fig. 3 of themain text shows a larger enhancement for the reflected strain pulse compared to theincident one. While the origin cannot be exactly assessed at the moment, the mostplausible explanation is the following, for which one has to compare two time scales:

(1) The first one is the time for photon emission through the cavity mode, which isabout 1.5 ns in the spontaneous emission regime and is reduced down to 20 ps inthe lasing regime (main text Fig. 1b).

(2) The second one is the time during which quantum dots loaded with an exciton aremoved across the optical mode. This time can be assessed from the incoming andreflected strain pulses shown in Fig. 2c and Fig. 2d of the main text.

Only the compressive parts of these pulses (strain < 0) shift the quantum dot transitionenergies towards the optical mode, so that the resonator mode can be feeded. For theincoming pulse one sees, that at the leading edge the strain pulse contains a train ofsolitary peaks with duration on the order of a ps each, much shorter than the emissiontime, so that these parts move the quantum dots across the optical mode too fast forefficient cavity mode feeding. Upon reflection these parts become tensile. The compressivepart of the pulse is now given by a large number of peaks whose duration is longer thanthat of the leading edge peaks, so that the quantum dots are moved across the opticalmode not as fast. Therefore the reflected strain pulse is more favourable for high intensitymodulation.

3 Dependence of emission intensity modulation on

strain amplitude

Figure S1 shows the dependence of the modulation of the emission output for three differ-ent excitation powers of the metal film. The different excitation powers result in differentstrain pulse profiles at the quantum dot layer as shown Figure S1a. In particular the strainamplitudes scale roughly with excitation power. Figure S1b shows the emission output

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modulation for the three different pulses, and one sees that the modulation amplitude isabout proportional to the strain amplitude.

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Em

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T ime t-tS (ps )

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P=21 MW/cm2 (0.9 PTH

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13.0 mJ/cm2

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Figure S1: a, Temporal evolution of the incident strain pulse at the quantum dot layer forthree different optical excitation energy densities on the metal transducer film. Note theearlier arrival of the strain pulse at the QD layer for higher strain amplitudes because of theaccelerated propagation of the solitons at the leading edge of the strain pulses comparedto the sound velocity. b, Emission intensity modulation by the incident strain pulse forthe three different strain pulses shown in a. The optical excitation power density of theQDs, P=21 MW cm−2, is just below the laser threshold, where we observe the largestemission enhancement by the strain pulses. The strain amplitude is a measure of theachieved QD energy modulation and thus determines the fraction of QDs that can coupleto the cavity mode.

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This work was published by the Nature Publishing Group in Nature Photonics 6, 30 (2012) and the

final and edited version is online available here:

http://www.nature.com/nphoton/journal/v6/n1/full/nphoton.2011.269.html