resonance (1 . 484 eV ). Similar results were obtained on the confined UP branch. Imaging wavefunctions of confined polaritons is possible by means of optical microscopy. Indeed, due to their very small effective mass (four orders of magnitude smaller than the free electron mass), polaritons can be confined in traps of sizes in the micrometer range, above the optical resolution limit. Images of the two-dimensional distribution of the confined state probability densities can be realized either using a tomography technique $^{15}$, or by directly imaging the coherent emission of a given state when it is resonantly excited with a continuous wave laser $^{16}$. The emission intensity gives direct information on the probability density of the polariton states. To image the wavefunction rather than the probability density only, one needs a phase- resolved detection scheme. In this perspective, we used a homodyne detection setup, where we split the cw pump laser into two. One part was focalized on the back side of the sample using a camera objective to resonantly excite the polariton states. The camera objective provided us a diffraction limited gaussian spot of ∼ 15 µm diameter (of the same order of magnitude than the size of the mesa) and allowed for a good control of the excitation angle. Both these features are important for the selective excitation of polariton states, as it will be discussed further. The other part of the laser served as a phase reference. The sample was held in a cold-finger cryostat at a temperature of ∼ 4 K . The coherent emission of the polariton state was collected from the front side using a 0 . 5 N.A. microscope objective. The image of the emission was then interferred with the reference beam on a CCD. Figure 1 (b) shows the emission of the ground state ( n = 1 , m = 0) of the lower confined polariton [indicated with a plain arrow in Fig. 1 (a)] interfering with the reference laser. A slightly different incidence angle is used for the signal and the reference to provide straight interference fringes. As the ground state is expected to have a constant phase, the phase gradient obtained with this state is used as a phase reference for the other interferograms. Using numerical Fourier transform and digital off-axis filtering $^{17}$, one can then extract the amplitude and phase of the polariton states. In circular coordinates, confined states can be described by two numbers ( n, m ), the well known radial and orbital quantum numbers. In cylindrically symmetric systems, + m and − m states are degenerate. In most of the mesas, due to a small ellipticity of the trap, there is a small degeneracy lift of the ± m doublet into two new eigenstates 1 √ 2 [ ψ$_{+}$$_{m}$ ± ψ$_{−}$$_{m}$ ] $^{12,15,16}$. For example, the first excited state characterized by the quantum numbers ( n = 1 , m = ± 1) 3