We present a simple yet elegant Mueller matrix approach for controlling the Fano interference effect and engineering the resulting asymmetric spectral line shape in anisotropic optical system. The approach is founded on a generalized model of anisotropic Fano resonance, which relates the spectral asymmetry to two physically meaningful and experimentally accessible parameters of interference, namely, the Fano phase shift and the relative amplitudes of the interfering modes. The differences in these parameters between orthogonal linear polarizations in an anisotropic system are exploited to desirably tune the Fano spectral asymmetry using pre-and post-selection of optimized polarization states. We begin with a phenomenological model where a Fano-type spectral asymmetry in the scattered intensity naturally arises due to the interference of the complex Lorentzian field ( ( )) of a narrow resonance with a broad spectrum of relative field amplitude ( ) assumed to be independent of frequency (ideal continuum):Here, = ( ) = −The resulting expression for the scattered intensity becomes The first term represents the Fano-type asymmetric line shape with an effective asymmetry parameter = ⁄ . The second term corresponds to a Lorentzian background, widely reported in Fano resonance from diverse optical systems [6,25]. It is

We study the exciton gas-liquid transition in GaAs/AlGaAs coupled quantum wells. Dipolar excitons in coupled quantum wells (CQW) offer an interesting test bed for studying collective effects of an interacting quantum degenerate system [1, 2]. Their relatively light mass, which is smaller than that of a free electron, implies that the necessary conditions for achieving quantum degeneracy can occur already at cryogenic temperatures, and their strong dipole-dipole interaction may give rise to formation of ordered phases. Extensive attempts have been made over the past two decades to observe these phases and to determine the phase diagram of this system [3][4][5][6][7][8][9][10][11][12].In recent years there are mounting evidences for a phase transition that occurs at low temperatures in this system, yet its nature and thermodynamics remain open questions.Many of the studies are performed in a trap geometry, which confines the excitons to a narrow region around the illuminated spot [13,14], and evidences for condensation are found through photoluminescence (PL) anomalies: The appearance of spontaneous coherence [7], onset of non-radiative recombination ("PL darkening") [9] and large blueshift of the PL energy [10,11]. An alternative approach to study this phase transition uses an open geometry, where photo-excited carriers are free to move away from the illumination spot, and their diffusion is limited only by the mesa boundary. We have recently studied the behavior of indirect excitons in such an open geometry, and found an abrupt phase transition at a critical temperature and excitation power density [12]. The PL separates into two spatial regions, one which consists of electron-hole plasma and another that has a set of properties of a high density liquid.In this work we investigate this phase transition using spatially resolved PL and resonant Rayleigh scattering (RRS). Measuring the threshold power density as a function of temperature we determine the phase diagram of the system over the temperature range 0.1 -4.8K. Pump probe measurements, in which the liquid is created by a focused pump beam and studied by a much weaker probe, reveal that the liquid is dark and diffuses to large distances away from the illuminated spot, filling the entire area of the mesa at low temperatures. We find that the RRS spectrum narrows significantly and becomes uniform over macroscopic distances at the liquid phase, indicating that the disorder in the sample is effectively screened.The sample structure is identical to that used in [12] and consists of two GaAs quantum wells with well widths of 12 and 18 nm, separated by a 3-nm Al 0.28 Ga 0.72 As barrier. Top and bottom n-doped layers allow the application of voltage that shifts the energy levels of the wide well (WW) relative to the narrow well (NW) [15]. To create the liquid we apply voltages exceeding -2.4V and excite the system with a laser diode at energy of 1.590eV, focused to a Gaussian spot with 22µm half width at half maximum (HWHM) [16]. Figure 1 shows the evolution...

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We show that a Bose–Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin-flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements under an in-plane magnetic field allow us to preferentially modify the bright exciton density and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rate-equations model, which well reproduces the observed temperature, power, and magnetic-field dependence of the exciton density.