Emission stimulation in a directional band gap of a CdTe-loaded opal photonic crystal

An anisotropic photonic crystal light source has been realized by impregnation of a thin film latex opal with brightly luminescent CdTe nanocrystals. Its photoluminescence along a given direction has been studied as a function of excitation power. An increase of the photoluminescence saturation threshold in the directional photonic band gap has been observed. This result is interpreted as a stimulation of emission coupled to specific eigenmodes of a directional band gap. A theoretical model exploiting the low group velocity eigenmodes has been elaborated to explain the resonance character of the emission with band gap frequencies.

Figure 1

(a) Schematics of CdTe NCs positioning in octahedral voids of the opal. (b) Experiment layout: $d$, film thickness; $l^{*}$, MFP; $L_B,$ the Bragg attenuation length; $\theta$, angle of detection; $\Omega$, solid angle of emission collection. (c) Normalized angular-resolved PL spectra of a CdTe-loaded opal at different angles of the light collection.

Figure 2

Transmission spectra of bare (1) and CdTe opal (2) for $\theta =0°$. PL spectrum of CdTe opal at the same angle (3).

Figure 3

PL intensity against excitation power recorded at $\theta =30°$ for $\hslash \omega =2.5,2.3,\text{and}2\text{eV}$ represented by squares, circles, and triangles, respectively. Lines show two-parameter fits of these input-output characteristics. Fit parameters are shown at curves.

Figure 4

Parameter spectra at $\theta =70°$, $30°$, and $0°$. $I_0$ and $P_0$ denoted by open and closed circles, respectively. PL spectra (lines) are shown for comparison.

Figure 5

(a) Isoenergy and group velocity contours of opal for a frequency within the Bragg PBG. ${\mathbf{k}}^{\prime }\left(\mathbf{k}\right)$ and ${\mathbf{V}}^{\prime }\left(\mathbf{V}\right)$ are the wave vector and the group velocity of type 1 (2) mode. (b) Group velocities of type 1 (solid curves) and type 2 (dash curves) Bloch modes along the [111] direction. The group velocity is given in units of the speed of light in vacuum.