Structure and zero-dimensional polariton spectrum of natural defects in GaAs/AlAs microcavities

Abstract

We present a correlative study of structural and optical properties of natural defects in planar semiconductor microcavities grown by molecular beam epitaxy, which are showing a localized polariton spectrum as reported in Zajac et al., Phys. Rev. B 85, 165309 (2012). The three-dimensional spatial structure of the defects was studied using combined focussed ion beam (FIB) and scanning electron microscopy (SEM). We find that the defects originate from a local increase of a GaAs layer thickness. Modulation heights of up to 140 nm for oval defects and 90 nm for round defects are found, while the lateral extension is about 2µm for oval and 4µm for round defects. The GaAs thickness increase is attributed to Ga droplets deposited during growth due to Ga cell spitting. Following the droplet deposition, the thickness modulation expands laterally while reducing its height, yielding oval to round mounds of the interfaces and the surface. With increasing growth temperature, the ellipticity of the mounds is decreasing and their size is increasing. This suggests that the expansion is related to the surface mobility of Ga, which with increasing temperature is increasing and reducing its anisotropy between the [110] and [110] crystallographic directions. Comprehensive data consisting of surface profiles of defects measured using differential interference contrast (DIC) microscopy, volume information obtained using FIB/SEM, and characterization of the resulting confined polariton spectrum are presented.

Linear Wave Dynamics Explains Observations Attributed to Dark Solitons in a Polariton Quantum Fluid

“…The presence of the defect has the effect to modify the effective thickness of the cavity layer, resulting for example in a red-shift of the photonic dispersion inside the defect [24]. As a result, the resonance frequency inside the defect shifts from ω 0 to ω 0 with respect to the bare cavity and accordingly the in-plane wave vector k to k , with k > k (Fig.…”

“…As discussed above, the appearance of the intensity minima and phase dislocations is a result of interference which is sensitive to the intensity and relative phase of contributing waves. The increase of the energy of the excitation beam by 2 meV causes an increase of the in-plane wave vector of the propagating The wave vector dependence of such transitions will depend on the defect structure and the related bound polariton states [24], so that they could also be observed with decreasing wave vector for other defects. Intensity profiles measured at a fixed distance from the defect [Figs.…”

“…In a recent study [23] of the structural and optical properties of GaAs/AlAs microcavities grown by molecular beam epitaxy it was shown that the most common pointlike defects were characterized by a circular or elliptical shape [24], due to Gallium droplets emitted occasionally during the growth [25,26]. The presence of the defect has the effect of modifying the effective thickness of the cavity layer, which typically results in an attractive potential for the cavity mode inside the defect [24]. Consequently, the wave vector of the photonic mode in the region of the defect is higher than in the rest of the cavity.…”

“…To model the defect parameters, which are not experimentally known, we use a disk shape with a radius of 3 µm and a polariton potential of −2.3 meV (consistent with Ref. [24]). Maxwell's equations are then solved using an expansion of the fields into the planar cavity eigenmodes in cylindrical coordinates fulfilling the boundary conditions for tangent components of electric and magnetic fields on the interface between the cavity and the defect (see [20], S5).…”

Multicomponent polariton superfluidity in the optical parametric oscillator regime

“…We describe the defect via a potential V d (r) acting on the photonic component; this can either be a defect in the cavity mirror or a localized laser field [25,29,30]. In the conservative, homogeneous, and linear regime [γ X,C = 0 = V d (r) = g X ], the eigenvalues ofĤ are given by the lower (LP) and upper polariton (UP) energies, 2ω…”