Highly directed emission from self-assembled quantum dots into guided modes in disordered photonic crystal waveguides

TL;DR

This study demonstrates highly directed emission from self-assembled quantum dots into guided modes in disordered photonic crystal waveguides, showing control over emission rate and directionality despite disorder effects.

Contribution

It provides new insights into emission dynamics and directionality control of quantum dots in disordered photonic crystal waveguides, including measurements of the beta-factor and Purcell enhancement.

Findings

Emission rate varies from 0.25 to 1.55 ns^{-1} near slow-light band edge.

Maximum beta-factor of approximately 90% achieved near slow-light edge.

Disorder broadens slow-light spectral width but emission remains directed into waveguide modes.

Abstract

We explore the dynamics and directionality of spontaneous emission from self-assembled In(Ga)As quantum dots into TE-polarised guided modes in GaAs two-dimensional photonic crystal waveguides. The local group velocity of the guided waveguide mode is probed, with values as low as $\sim 1.5\%\times c$ measured close to the slow-light band edge. By performing complementary continuous wave and time-resolved measurements with detection along, and perpendicular to the waveguide axis we probe the fraction of emission into the waveguide mode ($\beta$-factor). For dots randomly positioned within the unit cell of the photonic crystal waveguide our results show that the emission rate varies from $\geq 1.55$ $ns^{-1}$ close to the slow-light band edge to $\leq 0.25$ $ns^{-1}$ within the two-dimensional photonic bandgap. We measure an average Purcell-factor of $\sim 2\times$ for dots randomly distributed within the waveguide and maximum values of $\beta\sim 90 \%$ close to the slow light band edge. Spatially resolved measurements performed by exciting dots at a well controlled distance $0-45$ {\mu}m from the waveguide facet highlight the impact of disorder on the slow-light dispersion. Although disorder broadens the spectral width of the slow light region of the waveguide dispersion from $\delta E_{d}\leq 0.5$ meV to $>6$ meV, we find that emission is nevertheless primarily directed into propagating waveguide modes. The ability to control the rate and directionality of emission from isolated quantum emitters by placing them in a tailored photonic environment provides much promise for the use of slow-light phenomena to realise efficient single photon sources for quantum optics in a highly integrated setting.