The dynamics of spatial coherence and momentum distribution of polaritons in the regime of Bose-Einstein condensation are investigated in a GaAs microcavity with embedded quantum wells under nonresonant excitation with picosecond laser pulses. It is shown that the onset of the condensate first order sparial coherence is accompanied by narrowing of the polariton momentum distribution. At the same time, at sufficiently high excitation densities, there is significant qualitative discrepancy between the dynamic behavior of the width of the polariton momentum distribution determined from direct measurements and that calculated from the coherence spatial distribution. This discrepancy is observed at the fast initial stage of the polariton system kinetics and, apparently, results from the strong spatial nonuniformity of the phase of the condensate wave function, which equilibrates on a much longer time scale.Polaritons in semiconductor microcavities with embedded quantum wells (QWs), are quasiparticles corresponding to mixed exciton-photon states in these heterostructures, represent a unique system for studying Bose-Einstein condensation (BEC). The effective mass of microcavity polaritons near the bottom of the lower dispersion branch is ∼ 10 −4 of the free-electron mass, which makes possible polariton BEC at high temperatures, even up to the room temperature [1]. After the demonstration of BEC in CdTe microcavities in 2006 [2], a number of interesting effects related to this phenomenon were investigated, including quantum vortices [3], the superfluidity of the polariton condensate [4], the Josephson effect [5], the spin-Meissner effect [6], polariton solitons [7], and the optical spin Hall effect [8]. Very recently, an electrically pumped polariton laser working at room temperature was demonstrated [9].One of the main characteristics of a Bose condensate is spatial coherence, i.e., the constancy of the phase of the condensate wavefunction over distances exceeding the thermal de Broglie wavelength. In this context, the dynamics of the onset of coherence in the process of the condensate formation is an interesting issue, which was investigated in [10][11][12]. In particular, in Ref. [12] it was found that, in the process of formation of the polariton condensate, coherence expands with a constant velocity of ∼ 10 8 cm/s.In the present work, for the first time, the dynamics of the spatial distribution of coherence and the corresponding dynamics of the polariton momentum distribution are investigated simultaneously. The relationship * belykh@lebedev.ru between these distributions is discussed. The structure under study is a GaAs microcavity with the top and bottom Bragg mirrors consisting of 32 and 36 pairs of AlAs/Al 0.13 Ga 0.87 As layers, respectively. 12 GaAs/AlAs 7-nm-wide QWs are embedded in the microcavity. The Rabi splitting and the Q factor of the structure are 4.5 meV and about 7000, respectively. The detuning between the photon and exciton modes is about -6 meV. In all experiments, the sample temperature is 10 K....
