Noise- and dynamics-sustained patterns in a nonlinear photorefractive system

We analyze experimentally the dynamics of optical patterns emerging from a photorefractive two-wave mixing geometry where a tilted single-feedback mirror generates an advectionlike effect. Depending on the nonlocal coupling (introduced by the tilting angle) between the two counterpropagating beams, the strength of the nonlocal response of the nonlinear photorefractive bulk medium, and the distance between the mirror and the crystal, we observe the initiation of different pattern geometries, the inversion of pattern transverse phase velocity, and the bifurcation from convective to absolute instabilities.

Figure 1

Schematic of the setup composed of a photorefractive crystal (PRC) in a single-feedback configuration. M, mirror; VM, virtual mirror; L, distance between the virtual mirror and the crystal; $\beta$, mirror tilt angle; $H=L\sin \beta$, transverse shift (in $\mu$m); F and B correspond respectively to the forward and backward beams responsible for the pattern formation.

Figure 2

Different patterns obtained for different transverse shifts $H$ (in $\mu$m) and for a virtual mirror located outside the crystal ($L\approx 1$ mm). The pump beam intensity is fixed to the value $I_p\simeq 45$ mW. Top, near field; bottom, far field.

Figure 3

Different patterns obtained for different transverse shifts $H$ (in $\mu$m) and for a virtual mirror located inside the crystal ($L\approx -2$ mm). The pump beam intensity is fixed to the value $I_p\simeq 45$ mW. Top, near-field; bottom, far-field.

Figure 4

Time evolution of the transverse profile of the squeezed hexagon from Fig. 2(C). From left to right, drifting pattern passing from a positive transverse phase velocity $v\simeq +\pi /3$ rad ${\mathrm{s}}^{-1}$ (A) to null (B) then to a negative value $v\simeq -\pi /3$ rad ${\mathrm{s}}^{-1}$ (C) for different $H$.

Figure 5

Spatiotemporal evolution of the transverse profile of the stripe pattern obtained in Fig. 2(B)($H=10$ $\mu$m) for different powers of the pump beam: (A) 15 mW, (B) 30 mW, (C) 55 mW. The dotted lines depict the available space for the growing process of the pattern. From blue to red, the colors indicate respectively an evolution from low to high intensities in the near-field pattern. (Inset) Corresponding far field.

Figure 6

Evolution of near-field pattern size versus the pump power. NP, noisy precursor regime; CI, convective instability regime; AI, absolute instability regime.