Electro-optical switching between polariton and cavity lasing in an InGaAs quantum well microcavity

We report on the condensation of microcavity exciton polaritons under optical excitation in a microcavity with four embedded InGaAs quantum wells. The polariton laser is characterized by a distinct nonlinearity in the input-output-characteristics, which is accompanied by a drop of the emission linewidth indicating temporal coherence and a characteristic persisting emission blueshift with increased particle density. The temporal coherence of the device at threshold is underlined by a characteristic drop of the second order coherence function to a value close to 1. Furthermore an external electric field is used to switch between polariton regime, polariton condensate and photon lasing. ©2014 Optical Society of America OCIS codes: (250.5590) Quantum-well, -wire and -dot devices; (190.5890) Scattering, stimulated; (230.5750) Resonators; (140.3948) Microcavity devices. References and links 1. C. Weisbuch, M. Nishioka, A. Ishikawa, and Y. 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Introduction
Microcavity exciton polaritons are quasi-particles which form in the strong coupling regime as a mixture of matter excitation and cavity photons [1].Because of their small effective mass and their bosonic nature they can form a dynamic condensate at elevated temperatures.Such a condensate is characterized both by its spatial as well as temporal coherence of the emitted photons.Hence the light radiated from the condensate shares many similarities with a conventional photon laser.However, in strong contrast to a conventional photon laser, which relies on strong out of equilibrium conditions, a polariton laser can be operated closer to thermal equilibrium.This indicates the possibility to reduce the threshold power consumption [2,3].The effect of polariton lasing has been shown for different materials [4,5], even up to room temperature in wide bandgap GaN [6], and organic [7][8][9] materials under optical pumping.Furthermore, polariton diodes can be excited electrically in GaAs based vertically emitting microcavities [10][11][12].It is worth mentioning, that it turned out to be challenging to observe polariton laser operation or condensation effects in GaAs based microcavities with implemented InGaAs quantum wells (QWs).However, InGaAs/GaAs QW microcavities bear several significant advantages compared to GaAs/Al(Ga)As QW cavities: a) as discussed in [10][11][12], electrical current injection is much easier due to smaller resistivity of both the doped distributed Bragg reflectors (DBR) and the typically intrinsic cavity; b) the DBRs can be composed of the binary compounds GaAs and AlAs which have the largest possible refractive index difference of the commonly used quasi lattice matched materials on GaAs substrate.Such mirrors are inherently easier to grow, are less afflicted to detrimental roughening, hence allowing to obtain large Q-factors much easier than DBRs based on ternary AlGaAs alloys ; c) the substrate is transparent at the emission frequency, which makes transmission studies straight forward, being particularly appealing in resonant studies; d) compared to GaAs QWs, InGaAs QWs allow for a significantly higher flexibility in the design of the QW bandstructure.Consequently, the strain environment (hence the splitting between heavy hole and light holes), the emission wavelength, the oscillator strength and the tuning behavior in electric (via the quantum confined Stark effect) and magnetic fields can be conveniently adjusted via the QW thickness and the composition of the alloy [13][14][15][16].In fact, it is possible to tune the wavelength band between 830 nm and 1200 nm, which is in particular appealing from the application point of view; e) InGaAs QWs have significantly larger exciton g-factors compared to GaAs/AlGaAs QWs [16], making them very interesting candidates to observe pronounced spinor polariton effects in a magnetic field [17].These points make polariton lasers (i.e non-resonantly injected polariton condensates) based on InGaAs QWs, in addition to their resonantly driven counterparts [18,19] highly desirable.In this work we provide evidence for optically injected polariton lasing in a device based on AlAs/GaAs DBRs sandwiching an intrinsic GaAs cavity with four embedded InGaAs QWs.Additionally, we demonstrate that we can switch between the polartion lasing in the strong coupling regime and the cavity mediated lasing in the weak coupling regime by applying a voltage to the microcavity.

Sample design and experimental setup
The layer structure of the microresonator consists of 23 (27) doped AlAs/GaAs mirrorpairs which is schematically shown in Fig. 1(a) and it has been described in detail by Schneider et al in [11].The typical normal mode splitting which is a signature of the strong coupling regime was mapped out in the photoreflection geometry, yielding a Rabi splitting of 5.5meV.Despite the high Indium content of 15% in the QWs, we can measure an inhomogeneous broadening of the four integrated QWs, which is as low as 2.7 meV.For these standard macro photoluminescence measurements, we removed the top DBR segment via reactive ion etching.Micropillar cavities with diameters of 20µm were etched into the layer structure, and the sample was planarized by benzocyclobuthene.Afterwards, electrical contacts were evaporated on the backside of the Si-doped GaAs wafer and the top of the micropillar to enable one to apply a voltage to the device.In our experiment, we chose a detuning between the exciton and photon mode of −3.7meV.For optical excitation a pulsed laser with a pulse length of 50ps and repetition rate of 82MHz is used.It is set to the energy of the first Bragg minimum, about 110meV above the polaritonic resonances.Hot excitons, which can relax via phonon interactions and form polaritons, are created inside the microcavity.The relaxation process continues till the energy difference between exciton and photon is in the order of the Rabi-splitting.This makes the photonic component notable and shortens the polaritonic lifetime.At high densities, polariton-polariton interaction gives rise to a new relaxation channel, which facilitates the formation of a macroscopically occupied ground state.Further general information on the formation of polariton condensates can for example be found in [20].Details on the relaxation pathway in this structure will be published elsewhere [21].All experiments were performed at a temperature of 6K.An external voltage of 1. with m e as the electron mass.In agreement with other reports on high-Q microcavities in the strong coupling regime, the upper polariton branch as well as higher excited states of the lower branch are depleted as a result of this process [22,23].The in-plane polariton dispersion is collected with a far field spectroscopy setup [24].The recorded contour plots are presented in Fig. 1 for different excitation powers.The lower polariton mode at 0.5*P th in Fig. 1(b) shows a mode pattern along the dispersion because of the lateral confinement of 20µm originating from the micropillar geometry.The uncoupled exciton (red broken line) and photon (green broken line) are plotted with the resulting lower polariton fit (white broken line).With increasing excitation power the emission is shifted to higher energy because of exciton-exciton-interactions [25] with its polaritonic dispersion being still visible in the background in Fig. 1(c).Almost three times above threshold (which corresponds to a power density of 620 W/cm 2 ) in Fig. 1(d) the dispersion is not observable anymore and the emission is centered around k = 0 approximately 0.4meV below the bare cavity mode.In order to characterize the behavior in more detail the emission of k = 0 ± 0.1 µm −1 was integrated and fitted by a Lorentzian lineshape.The evolution of characteristic parameters with increasing excitation power is shown in Fig. 2. The input-outputcharacteristic in Fig. 2(a) shows a quadratic behavior for small excitation [26][27][28] and a strong non-linearity at the threshold, which coincides with a change in the dispersion.Above 1.5*P th the slope changes again.We note that a second non-linearity, which commonly is attributed to the onset of conventional lasing [29,30], cannot be reached since heating effects set in before this transition can be crossed.As we will show later in this manuscript, the transition to the weak coupling, however, gets accessible via manipulation of the intrinsic electric field.The energy at k = 0 is shown in Fig. 2(b): It continuously shifts towards higher energy because of particle interactions.Around threshold the slope changes from linear to logarithmic in good agreement with previous reports on GaAs based polariton lasers [31, The change of emission energy above the threshold is a hint for the preservation of strong coupling.The laser emission from the cavity mode is typically pinned for all excitation powers, except for minor deviations caused by a carrier dependent refractive index [29,33] or heating [34].The linewidth in Fig. 2(c) shows an increase from 0.31meV in the linear regime to 0.65meV just below the threshold caused by polariton-polariton-interactions, which has been observed in most GaAs based polariton lasers [29,35].At threshold, the linewidth is subject to a sudden drop as a result of temporal coherence in the condensate to a minimum value of 0.18meV around 1.5*P th .Number fluctuations and inter-particle interactions inside the condensate subsequently lead to decoherence and therefore a rise of the linewidth above 1.5*P th [36].   ((τ) measurement slightly above threshold.(b) Excitation power dependence of the g (2) (τ = 0) value with an increase around the threshold.

Electro-optical switching
We will now extend our study and investigate the electro-optical properties of our polariton condensate.First, we drive the polariton condensate slightly above the threshold for polariton lasing, as shown in Fig. 3(a) (1.3*P th , corresponding to 2.2 mW).Under these conditions, already minor deviations from the flatband conditions result in increased losses via carrier tunneling out of the QWs.This is causing a decrease of the carrier capturing rate in the QWs due to the built-in electric field which drags the injected carriers to the DBR segments.Consequently, the effective injection rate decreases with an applied voltage lower than flatband condition.Therefore the condensate threshold density can no longer be reached, and the emission is expected to follow the lower polariton branch.This scenario is directly reflected in Figs.3(b) and 3(c).For even stronger tilted bands, one would expect a transition to the weak coupling regime resulting from electrostatic quenching of the exciton oscillator strength and fast tunneling losses exceeding the Rabi frequency.This scenario is shown in Figs.3(d)-3(f).Here, in order to ensure a sufficiently strong photoluminescence signal even in the tunnel current regime, we drive the condensate 7 times above flatband threshold.
Once an internal voltage of −2.9 V is reached, along with a redshift of the condensate as a result of the quantum confined stark effect, we observe the appearance of a second mode in the emission spectrum.This feature is blueshifted with respect to the condensate and located on the emission energy of the uncoupled cavity resonance.Further increasing the reverse bias to −3.1 V, the condensate related emission is vanished, and only the emission from the bare cavity mode persists.This leads us to the interpretation that the strong coupling is lost under these conditions, and the system enters the weak coupling regime.This is visualized in more detail in Fig. 3(g), which shows the emission energy dependence on the applied voltage.One notes that far above threshold both, the condensate and the photon mode are recorded, which we attribute to the time integration of different regimes caused by the pulsed excitation.The intensity of the two modes show oppositional behavior as the condensate diminishes and the photon mode brightens while applying a stronger reverse bias.This is visualized with the visibility of the markers fillings.

Photon statistics
To investigate the temporal coherence of the emission from the condensate in greater detail, we characterize our system via photon statistic measurements.The second order correlation function is defined as (2) (τ) I(t τ)I(t) / ( I(t) I(t τ) = + + g with I(t) as the emission intensity at time t and .. as time average.We record g (2) (τ = 0) values with a Hanbury Brown and Twiss setup using avalanche photo diodes with a time resolution of 400ps [4,37,38].Therefore the dispersion ground state was filtered by a monochromator with an energy

Fig. 1 .
Fig. 1.(a) Scheme of used structure.False colour energy dispersions for (b) lower polariton with mode patern caused by 20µm confinement (c) polariton condensate at the threshold with dispersion in the background and (d) condensate three times above the threshold with emission from k = 0.
3 V was applied to the sample to reach flat band conditions, except for the electro-optical studies presented in this manuscript.Throughout the paper all threshold values refer to flatband condition.Because of the high Q-factor of around 6300, polaritons can cool into the bottom of the lower polariton branch, which features a parabolic dispersion with an effective mass of 4

Fig. 2 .
Fig. 2. Excitation power dependence of (a) the integrated ground state intensity of k = 0 ± 0.1 µm −1 (b) the ground state emission energy and (c) the linewidth.The threshold is characterized by a non-linearity, stronger energy shift and drop of linewidth.

Fig. 3 .
Fig. 3. Energy dispersions for an excitation of 1.3*P th with an applied Voltage of (a)1.3V(equals flatband condition) (b) 0.9V and (c) 0.6V.As the bandstructure is tilted the condensate is destroyed and normal polariton emission sets in.Energy dispersions for an excitation of 7*P th with an applied Voltage of (d)1.3V(equals flatband condition) (e) −2.9V and (f) −3.1V.The electric field breaks the strong coupling regime and emission jumps to the photon mode.(g) Energy vs. voltage to further demonstrate the loss of the condensate regime.Markers filling visibility for 7*P th visualizes the emission intensity.