Figure a,b displays the PL images excited by a tightly focused laser spot close to the end of the nanowire facing the acoustic transducer (cf. diagrams on the left of Figure ) in the absence (Figure a) and presence (Figure b) of a SAW. In the upper and lower part of the images, the spectrally (horizontal scale) and spatially (vertical scale) resolved nanowire emission polarized parallel and perpendicular to the nanowire axis are represented respectively. The corresponding spatial PL profiles integrated from 796.6 to 823.3 nm are depicted in Figure c,d. Without acoustic power (Figure a), the emission is restricted to the region close to the excitation spot (position G in the diagrams on the left of Figure ) and is enlarged due to diffusion processes along the nanowire. When a SAW is applied, the PL intensity at G reduces and a second emission at a remote point (position R) appears a few micrometers away from G along the SAW propagation direction. The reduction at G is due to the induced spatial separation of electrons and holes by the SAW piezoelectric field, which decreases the radiative recombination probability at G. The emission at R is attributed to the transport of the photoexcited electrons and holes along the nanowire axis and their recombination at the remote position, as we have recently demonstrated [1
]. The slight PL emission appearing between G and R are associated to the recombination of some carriers at traps as stacking faults, impurities, or defects.
Figure 2 Linear polarized PL emission with SAW off/SAW on. Polarized PL with spatial resolution excited by a tightly focused laser beam close to the nanowire edge facing the acoustic transducer (position G, cf. diagrams on the left side). (a) In the absence of (more ...)
Without acoustic power, the broad emission band at G is highly polarized perpendicular to the nanowire axis (see Figure a,c). When a SAW is applied, the emission at G maintains its polarization behavior while the recombination at R is polarized parallel to the nanowire axis (see Figure b,d). The emission at G is consistent with the optical selection rules expected for the wurtzite GaAs nanowires [3
] where the band edge emission is dipole allowed only if the electric dipole moment is perpendicular to the wurtzite c
]. The emission at R, however, does not coincide with these selection rules. Since the recombination of the transported charge carriers take place at a remote position the crystal structure of that region may be different. Indeed, the emission energy of approximately 818 nm - which is exactly the band gap energy of zinc blende bulk GaAs [5
] - suggests that recombination takes place in a zinc blende region at the top of the nanowire, opposite to the acoustic transducer. This is supported by transmission electron microscopy measurements, which reveal a zinc blende section in the top region of the nanowires probably created during the cooling down after growth, as previously observed in CBE- and MBE-grown GaAs nanowires [6
]. The emission polarized parallel to the nanowire axis at R is consistent with the emission characteristics observed in zinc blende GaAs nanowires [8
]. Here, the emission is preferentially polarized along the nanowire axis due to the dielectric mismatch between the nanowire and its surroundings. The degree of linear polarization, P
) - where I
are the PL polarization intensities detected perpendicular and parallel to the nanowire axis, respectively - at the excitation spot G, is approximately + 88%
(perpendicular) in the absence of a SAW, whereas at the remote position R the polarization becomes equal to approximately −70%
(parallel) with acoustic power.
Similar polarization results have been obtained for all probed nanowires, independent of the polarization of the exciting laser. This also implies that the electron spins are not conserved during transport. To further support the last conclusion, we have carried out transport experiments where spin-up and spin-down electrons were generated on one extreme of the nanowire using right-handed (σ+) and left-handed (σ−) circularly polarized light, respectively. The circular polarization of the PL was detected with spatial resolution using λ/4 plate and a birefringence prism (see Figure ). For simplicity, only the right-handed, circularly polarized emission (I+) is considered in the following discussion. For incident right-handed, circularly polarized light (σ+), the spatially resolved I+profiles in the absence and presence of a SAW are shown in Figure a,b, respectively. Figure c,d presents the I+ profiles along the nanowire axis integrated around the spectral positions 811 nm (blue) and 818 nm (red), in the absence and presence of a SAW for σ+ (lines) and σ− (circles). Without acoustic power, the PL is emitted around 811 nm at the excitation spot G (Figure a). When a SAW is applied, a remote spot appears emitting at 818 nm (Figure b), as observed in the linear polarization measurements. From Figure c,d one sees that I+ is the same for σ+and σ−excitations (cf. solid lines with open circles) at G. This implies that the initially generated spin-up or spin-down electrons have lost their spin polarization in the wurtzite phase at G leading to evenly distributed spin states. The same result is observed for the recombination along the wire and at R where no difference of I+ of the transported electrons between σ+and σ− can be observed.
Figure 3 Spin transport along nanowires. Spatially resolved right-handed circularly polarized PL emission (I+) excited by a tightly focused, right-handed circularly polarized laser beam (σ+) in the absence (a) and presence (b) of a SAW. The PL intensity (more ...)