Dynamic control of the optical emission from GaN/InGaN nanowire quantum dots by surface acoustic waves

The optical emission of InGaN quantum dots embedded in GaN nanowires is dynamically controlled by a surface acoustic wave (SAW). The emission energy of both the exciton and biexciton lines is modulated over a 1.5 meV range at ~330 MHz. A small but systematic difference in the exciton and biexciton spectral modulation reveals a linear change of the biexciton binding energy with the SAW amplitude. The present results are relevant for the dynamic control of individual single photon emitters based on nitride semiconductors.

high frequency applications. While experiments on modulation of the electronic properties of GaN films 17 as well as transport of charge carriers in GaN nanowires 5 by SAWs have been reported, studies on acoustically driven modulation of the optical emission of nitride-based QDs have not yet been demonstrated.
In this letter we use a SAW to periodically modulate the emission wavelength of individual InGaN QDs immersed in GaN nanowires. The dynamic strain field of the SAW transferred to the QDs results in an alternating shift of the QD transition energy at the acoustic frequency of ~330 MHz within a bandwidth up to ~1.5 meV. The small difference in the modulation amplitudes of the exciton (X) and biexciton (XX) QD lines indicate the influences of the SAW fields on the biexciton binding energy. The energy splitting of the X and XX emission scales linearly with the acoustic amplitude, showing that the main effect on the QD electronic structure is due to the strain field of the SAW, with a possible contributions of the SAW piezoelectric field along the nanowire c-axis. The nitride-based dot-in-a-nanowire heterostructures presented here are efficient SPS 18 and can be precisely arranged in periodic two-dimensional arrays 19 . Thus, the present results are an important step towards the development of single photon sources working at high temperature 20 , high repetition rates and broad spectral range 18 , with simultaneous spatial and time control.
The InGaN/GaN nanowire heterostructures were fabricated by plasma-assisted molecular beam epitaxy (PA-MBE) on (0001) GaN-on-sapphire templates 21 . The nanowires have a typical height of ~500 nm and a diameter of ~200 nm. They exhibit hexagonal cross section with lateral facets defined by non-polar m-planes and a pyramidal top profile formed by six semi-polar r-facets. This profile determines the shape of the InGaN nano-disks embedded inside the GaN nanowire tips 18,21 . The transmission electron microscope (TEM) micrograph of an individual nanowire in Fig. 1(a) reveals the presence of two InGaN sections: a thicker one (~30 nm) formed on the polar facet close to the nanowire top and a narrower (~20 nm thick) one on semi-polar side facets. As detailed in our previous work 18 , the QDs under study are formed by fluctuations of the indium content in the topmost InGaN region.
For SAW experiments, the nanowire heterostructures were mechanically transferred onto a SAW delay line consisting of two interdigitated transducers (IDTs) lithographically defined on the surface of a 128° Y-cut lithium niobate (LiNbO 3 ) substrate. A schematic of the device is shown in Fig. 1 (b). We employed floating electrode unidirectional IDTs 22 with a length of ~700 µm and an aperture of ~400 µm (approximately equal to the IDT finger length) designed to generate SAWs with an acoustic wavelength of λ "#$ = 11.67 µm, corresponding to a SAW frequency and period of f "#$ = 338 MHz and T "#$ = 2.96 ns, respectively, at the measurement temperature = 10 . Note that the exciton decay time in these nanowire-QDs is ~1.3 ns 18 , i.e. comparable to T "#$ /2. The amplitude of the SAW will be specified in terms of the nominal radio-frequency (rf) power (P => ) applied to the IDT, without correction for the rf coupling losses. By measuring the rf reflection and transmission spectra using a network analyzer, we determine that only about 30% of the input electrical power applied to the IDT turns into acoustic power. The use of a highly piezo-electric LiNbO 3 crystal provides strong strain and electric fields of the propagating SAW, which extend to the optically active nanowire heterostructures deposited on the surface 5,6,23,24 . The micro-photoluminescence (µ-PL) experiments were performed at 10 K on a sample mounted in a cold-finger liquid helium flow cryostat equipped with rf connections for the excitation of the IDTs. A continuous wave helium-cadmium laser operating at λ ?@A = 442 nm was used for PL excitation. The laser spot was focused by a 100× microscope objective (NA = 0.73) to a < 1.5 µm diameter spot. The emitted PL was collected by the same objective and dispersed by a single grating monochromator (with an overall spectral resolution of ~350 µeV) equipped with a liquid N 2 cooled Si-CCD camera for timeintegrated signal detection. For polarization analysis a rotatable half-wave retardation plate and a fixed linear polarizer were placed in front of the monochromator entrance slit, with 0⁰ denoting the SAW propagation direction ( -axis in Fig. 1(b)).
Because the excitation energy (E ?@A = 2.805 eV) is lower than the radiative transitions related to the GaN nanowire 25 and the InGaN region formed on semi-polar side facets 18 , carriers are only generated in the apex of the InGaN disk. A typical lowtemperature µ-PL spectrum recorded at laser excitation power P ?@A = 6 µW from dispersed nanowires consists of a series of sharp QD lines on top of a broader background emission. The sharp PL features are attributed to the recombination of photogenerated carriers in QD-like localizing potentials in the topmost InGaN regions 18 . The background emission is due to regions in the InGaN apex without QD-like confinement, as well as different nanowires simultaneously probed by the laser spot. For the experiments discussed in this paper, we focus on a group of QD lines depicted in Fig. 2(a). In the absence of a SAW (upper trace), we observe two narrow (FWHM < 500 µeV) PL peaks appearing at ~2.564 and ~2.587 eV corresponding to the recombination of the exciton (X) and the biexciton (XX) of the same nanowire-QD. This is corroborated by polarizationresolved PL measurements ( Fig. 2(b)), which show collinear polarization of the two emission lines with a similar polarization degree (~64%). The linearly polarized emission is due to the valence band mixing induced by the in-plane anisotropy of the QD confinement potential 26 . The peak at 2.585 eV (marked with asterisk in Fig. 2(b)) originates from a different QD, as: (i) it does not couple to the SAW field (see below) and (ii) it exhibits different polarization direction and ratio (~80%) (cf. Fig. 2(b)).
The latter is consistent with the random asymmetry of our nanowires QDs formed by InGaN alloy fluctuations 27 .
The X and XX assignment is attested by measuring their integrated PL intensities as a function of the optical pump power (cf. Fig. 2(c)). Below the saturation limit of ~6 µW, the 2.564-eV (2.587-eV) emission line develops linear (quadratic) power dependence, expected for the ground-state neutral exciton (biexciton). The energy difference between the X and XX peaks When the SAW is applied (lower trace in Fig. 2(a)), the emission spectrum undergoes an apparent splitting of the X and XX emission lines into two components. Instead, the PL peak at 2.585 eV remains practically unchanged under the SAW, indicating that it originates from a different nanowire heterostructure within the laser spot, which is weakly affected by the SAW. Actually, the fraction of dispersed nanowires which couple efficiently to the SAW is low (a few percent), probably due to their random orientation resulting in random mechanical contact between the nanowire top (containing the QD) and the underlying substrate. The evolution of the X and XX transition energies with increasing SAW amplitude (which is directly proportional to the square root of the rf power, P => ) is summarized in Figs. 3(a) and 3(b), respectively. In order to discriminate SAW effects from those induced by heating of the sample at high P RF , the laser excitation and the rf signal were chopped at the same frequency of 250 Hz 17 . By recording, for each acoustic power, the PL spectrum with the optical and rf excitations inphase (SAW plus thermal effects) and out-of-phase (only thermal effects), we detected no significant rf-induced thermal contribution on the PL peak position and intensity. The observed changes of the X and XX emission energies can, therefore, be solely attributed to the dynamic modulation of the QD optical transitions induced by the strain and piezoelectric fields accompanying the SAW in LiNbO 3 substrate, as reported for (In,Ga)As QDs in Refs. 11,24 . The strain field and its corresponding hydrostatic pressure create alternating regions of maximum compression and tension separated by λ "#$ /2. This, in turn, induces a deformation potential modulation of the QD energy levels, causing a periodic shift of the transition energies with respect to their equilibrium energy in the absence of a SAW. The energy shifts induced by the SAW strain should increase linearly with the acoustic amplitude 11,24 and are expected to be a dominant tuning mechanism at low SAW intensities.
Spectral shifts associated with quantum confined Stark effect (QCSE) governed by the oscillating piezo-electric field of the SAW exhibit in general a quadratic dependence on SAW amplitude 24 , and are likely to contribute mostly at large SAW amplitudes. However, in polar c-plane III-Nitride QDs with large polarization-related fields (typically of several MV/cm), electric fields applied parallel 29,30 to the QD polar axis give rise to "linear" Stark effect. In our experiment, according to calculations based on the method described in Ref. 31 , a SAW excited at Z[ = 25 produces a dominant strain component (0.05%) along the wave direction (x-axis in Fig. 1(b)). At times during the acoustic cycle corresponding to maximum compressive or tensile strain, only the transverse component (of the order of few kV/cm) of the oscillating SAW piezoelectric field (i.e. along z-axis in Fig. 1(b)) is present. To try to distinguish between the effects of strain and piezoelectric field during the SAW half-cycles corresponding to maximum tension and compression, the emission spectrum in Fig. 2(a) was recorded for different laser excitation powers in the absence and presence of a SAW (Fig. 3(c)). When no SAW is applied the X emission energy undergoes an apparent blueshift (up to 300 µeV) with increasing pump power (empty squares), indicating partial screening of the large internal electric field by carriers photo-generated in the InGaN regions. For nanowire-QDs with the caxis parallel to the SAW piezoelectric field, the energy splitting induced by the acousto-electric effect is also expected to change at high optical pumping. No change in splitting is observed in our case even at high excitation intensities (well above the X and XX saturation limit), as shown for the X line in Fig. 3(c) (filled squares). This suggests that the electric field parallel the c-axis has a minor influence on the observed energy splitting of the X and XX peaks. Considering that the excitonic transitions in our nanowire-QDs are polarized in the growth plane (i.e. perpendicular to the nanowire axis) 18 , the polarization-resolved measurements in Fig. 2(b) imply that the nanowire under study is perpendicular to the SAW propagation direction. However, its exact orientation is difficult to determine due to the presence of serval nanowires within the excitation spot. We can exclude, in principle, any effect of the SAW piezoelectric field perpendicular to the QD polar axis, if we assume QD symmetry around the c-axis. In such case, the effect of this field should not depend on its direction (i.e. positive or negative field) 32 , which is contrary to the observed symmetric splitting of the X and XX peaks around their equilibrium energies when no SAW is applied (Fig 3(a) and (b)). can be ascribed to both quantum confinement-induced (also responsible for linearly polarized X and XX emission) and SAW induced valence band mixing. In analogy to GaAs/AlGaAs QWs 2 , for mixed hole states, the modulation of the valence-bandedge deviates from a sinusoidal shape resulting in the observed behavior.
The small difference between ∆E N and ∆E NN in Fig. 3 Consequently, changes in E NN O with increasing SAW intensity (Fig. 3(e)) are consistent with the effect of lateral size variation of the confinement potential on the biexciton binding energy, as reported for III-Nitride QDs 27,34,35 . The observed trend can thus be explained by the effect of the SAW strain field and some possible contribution of the SAW-induced electric field parallel to the c-axis on the QD confining potential 30 , affecting the Coulomb interactions within the four-particle biexciton complex.
In summary, we have demonstrated the SAW-driven modulation of the optical emission of a single GaN/InGaN nanowire-QD. We show that the acousto-mechanical coupling shifts the QD energy levels giving rise to a characteristic splitting (up to 1.5 meV) of the excitonic transition energies. The SAW fields also induce a monotonic change of the biexciton binding energy without any appreciable degradation of the emission, thus providing a reliable tool for in situ control of the exciton-biexciton system. By collecting photons in a narrow energy range, the dynamic spectral tuning reported here can be readily used to control the QD emission times and obtain triggered single photon sources operating at acoustic frequencies without the need for a pulsed laser, as reported for III-As QDs in Ref. 8 .