Coherent and thermal light: Tunable hybrid states with second-order coherence without first-order coherence

We demonstrate the realization of a new hybrid state of light that is simultaneously spectrally broadband, i.e., in-coherent in first order, and exhibits a laserlike normalized intensity correlation coefficient of 1.33, reflecting high coherence in second order. This is achieved by temperature-tuned light emission from an optoelectronic quantum dot superluminescent diode where the condensation of injected charge carriers into the globally lowest energy state of the strongly inhomogeneously broadened semiconductor quantum dot ensemble gives rise to a particular balance between spontaneous and stimulated emission.

Figure 1

(a) Single quantum dot energy level scheme. Carriers enter the ground-state (GS), excited-state (ES), and second-excited-state (SES) optical transitions through the wetting layer (WL) and recombine with the emission of photons. For low pump currents, only the GS transition emits. With increasing current, the ES and SES transitions also contribute to the emission. (b) Two-photon absorption setup. The QD SLD is operated inside a liquid-nitrogen-cooled cryostat. The emission enters the Michelson interferometer consisting of a beamsplitter, BS, a fixed and a variable mirror with the displacement, Δ$x$. The combined beams pass a long-pass filter (λ > 850 nm) and are tightly focused onto the photomultiplier tube (PMT) where the TPA signal is generated.

Figure 2

Optical spectra of QD SLD at a pump current of 1 A for various temperatures. With reducing temperature, the optical spectra shift to shorter wavelengths and the relative intensity changes from an ES-dominated emission at 290 K to a GS-dominated emission at 90 K.

Figure 3

SLD coherence properties. (a) TPA interferogram (left scale) and second-order coherence function $g^{\left(2\right)}$(τ) (right scale) at 290 K, with $g^{\left(2\right)}$(0) $=$ 2.04 ± 0.05 indicating the photon bunching. (b) TPA interferogram at 190 K, with $g^{\left(2\right)}$(0) $=$ 1.33 ± 0.05 indicating a more regular photon emission.

Figure 4

Reduced photon bunching. Second-order correlation coefficient $g^{\left(2\right)}$(0) at various temperatures. Error bars are shown exemplarily. The limiting cases of fully coherent laser emission reflected by $g^{\left(2\right)}$(0) $=$ 1 and totally incoherent thermal emission with $g^{\left(2\right)}$(0) $=$ 2 confine the shaded region.

Figure 5

Charge-carrier condensation. (a) Schematic visualization of the thermal coupling of three identical quantum dots embedded in the wetting layer for various temperatures. Top: Strong coupling occurs at 290 K. Middle: Reduced thermal coupling at 190 K and thermally isolated dots at 90 K (bottom). (b) Mean carrier occupation per energy level averaged over the whole QD ensemble. Broadened bands are chosen to illustrate the inhomogeneous broadening of all levels. The number of available carrier sites increases for higher states due to an increased level degeneracy. Gray circles denote states occupied by carriers; white circles indicate empty states. At 190 K, charge-carrier condensation maximizes the excited-state occupation. (c) Excited-state spectral peak power with a local maximum close to 190 K.