  We create exciton-polaritons in a zero-dimensional (0D) microcavity filled
with organic ladder-type conjugated polymer in the strong light-matter
interaction regime. Photonic confinement at wavelength scale is realized in the
longitudinal direction by two dielectric Bragg mirrors and laterally by a
sub-micron Gaussian-shaped defect. The cavity is separated into two parts
allowing nanometer position control and enabling tuning of exciton and photon
fractions of the polariton wavefunction. Polariton condensation is achieved
with non-resonant, picosecond optical excitation at ambient conditions and
evidenced by a threshold behavior with non-linear increase of the emission
intensity, line narrowing and blue-shift of the emission peak. Furthermore,
angular emission spectra show that condensation occurs in the ground state of
the 0D-cavity, and first order coherence measurements reveal the coherent
nature. These experiments open the door for polariton quantum fluids in complex
external potentials at room temperature.
