Dominant Effect of Polariton-Polariton Interactions on the Coherence of the Microcavity Optical Parametric Oscillator

The importance of interaction effects in determining the temporal coherence of spectrally and spatially isolated single modes of the microcavity optical parametric oscillator (OPO) is demonstrated. As a function of macroscopic occupancy, the coherence time (${\tau }_c$) first increases linearly and then exhibits saturation behavior, reaching maximum values of up to 500 ps. Good agreement is found with a model including fluctuations in polariton number and polariton-polariton interactions between the OPO states. ${\tau }_c$ is a property of the coupled OPO system, a result confirmed by the finding of equal coherence times for signal and idler, even though the idler is subject to strong additional scattering.

Figure 1

(a) Polariton dispersion curve showing scattering into signal and idler states. (b) Schematic diagram of the experimental setup. The positions of mirrors $P$ and $D$ in the Mach-Zehnder interferometer determines the course delay time and the phase shift between beams $A$ and $B$, respectively. (c) Spectra corresponding to stimulated polariton emission from the bottom of the LP branch at excitation power $P=31\mathrm{mW}$, recorded from two spatially separated slices of $5\times 40\mu \mathrm{m}$ dimension across the excitation spot. (d)–(h) Spatial images of the polariton emission for (d) $P=1.5\mathrm{mW}$ and (e)–(h) $P=31\mathrm{mW}$. The detection energies $E_0–E_3$ for images (e)–(h) are given on the images and are marked on the spectra. The center of the pump is indicated by the black solid circle in (e)–(h).

Figure 2

Signal spectra from the spatially filtered area of radius $\sim 5\mu \mathrm{m}$ versus excitation power $P$. (b) Intensity of the signal emission versus pump power $P$.

Figure 3

(a) Interference fringes as a function of phase shift between the beams in the two arms of the Mach-Zehnder interferometer for the pump laser, the signal (open squares) and the idler (solid squares). (b) A typical variation of the $g^1$ function versus delay time for an isolated signal mode above threshold. (c) Coherence times ${\tau }_{\mathrm{coh}}$ of the individual signal (open squares) and corresponding idler (solid squares) modes versus signal intensity $I_s$. (d) Calculation of the signal coherence time versus signal polariton population $N_s$.