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Enhancement of the evanescent wave coupling effect in a sub-wavelength-sized GaAs/AlGaAs ridge structure by low-refractive-index surface layers

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Abstract

We have investigated the three-dimensional emission patterns of GaAs/AlGaAs ridge structures with a sub-wavelength-sized top-flat facet by angle-resolved photoluminescence (PL). We found that the integrated PL intensity, and hence the light-extraction efficiency, can be enhanced by about 34% just by covering the ridge surface with a thin SiO2 layer. A double-coupling effect of evanescent waves that occurs at both the semiconductor–SiO2 and SiO2–air interfaces is suggested to be responsible for the improvement, based on a finite-difference time-domain simulation of the electromagnetic field around the ridge top.

© 2014 Optical Society of America

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Figures (6)

Fig. 1
Fig. 1 (a) Schematic drawing of the experimental setup for the measurement of 3D emission patterns. (b) Schematic illustration of the rotation angles θ and φ with respect to the ridge axis.
Fig. 2
Fig. 2 Comparison of the 15-K PL spectra of samples with and without the thin SiO2 layer.
Fig. 3
Fig. 3 (a) 3D plot of the measured emission pattern of the sample with the SiO2 layer. (b) 3D plot of the measured emission pattern of the sample without the SiO2 layer. The Cartesian coordinate (X, Y, Z) for the 3D plot was converted from the measured PL intensity I and the rotation angles θ and φ by using the following equations: X = I sin θ cos φ , Y = I sin θ sin φ , Z = I cos θ . (c) Polar plot of the 2D emission patterns of the two samples measured in two planes parallel with (φ = 0°, indicated by // in the figure) or perpendicular to (φ = 90°, indicated by ⊥ in the figure) the ridge axis direction. The theoretical Lambertian pattern for a flat-surface sample was also shown in the figure as a reference.
Fig. 4
Fig. 4 FDTD simulation image showing the distribution of the magnitude of the Poynting vector temporally integrated over one cycle inside (a) the ridge sample with the SiO2 layer and (b) the ridge sample without the SiO2 layer.
Fig. 5
Fig. 5 (a) Simulated image of the electric field intensity in the as-grown ridge sample. (b) Simulated image of the magnitude of Poynting vector temporally integrated over one cycle in the as-grown ridge sample. (c) Simulated image of the electric field intensity in the ridge sample covered with the thin SiO2 layer. (d) Simulated image of the magnitude of Poynting vector temporally integrated over one cycle in the ridge sample covered with the thin SiO2 layer.
Fig. 6
Fig. 6 Theoretically calculated light-extraction efficiency of the ridge and the flat-surface sample covered with a SiO2 layer as a function of the thickness of the SiO2 layer. The “×” symbols indicate the experimentally measured light-extraction efficiencies.

Tables (1)

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Table 1 Summary of total PL intensity, excitation laser intensity, and light-extraction efficiency of the 3 samples

Equations (4)

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I total = 0° 90° I ave ( θ )sinθdθ.
I ave ( θ )=( φ=0° φ=90° I( θ,φ ))/m,
η ext ridge ( d ) = ( I ( d ) / I ( 0 ) ) × η ext ridge ( 0 ) ,
η ext flat ( d ) = ( 1 R ) × 2 . 24 % ,
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