Self-Turning Self-Confined Light Beams in Guest-Host Media

We demonstrate power-dependent steering of self-confined light beams propagating in the nonparaxial regime in a guest-host nonlinear medium. Exploiting the Janossy enhancement in dye-doped liquid crystals with reorientational response, we observe sub-mW spatial soliton self-steering as large as 39 degrees, accompanied by trapping of the light spontaneously emitted by the excited dye. The self-turning is modeled in terms of an effective transverse force stemming from the longitudinal electric field in the tightly confined beams, with good agreement between data and simulations.

Figure 1

Sketch of the cell and nonlinear beam propagation in NLCs. The green ellipses represent the NLC molecules. (a)–(c) Light-beam evolution based on a plane-wave model. The 2D graphs show walk-off versus propagation in three cases. (a) Beam propagation in the linear (diffractive) regime. (b) Self-confinement: the excitation generates a soliton with unaltered walk-off $\delta =\tilde{\delta }$. (c) Nonlinear change in walk-off. (d) Pictorial rendering of the obtained experimental results at higher excitations: the soliton gets steered towards negative $y$ (4th quadrant in the plane $yz$), with the appearance of a weaker ordinary component propagating along $z$.

Figure 2

(a) Ordinary and (b) extraordinary wave propagation for an excitation $P_0=150\mu \mathrm{W}$. the $e$ wave propagates with $\varphi (z)=\tilde{\delta }(z)$.

Figure 3

Evolution of an $e$-polarized beam in $yz$ for various excitations $P_0$. (a)–(c) Input powers are $80–180–220\mu \mathrm{W}$, respectively, with Poynting vector spanning angles from 9° to 0° at large $z$. (d)–(f) Input powers are 260, 300, $400\mu \mathrm{W}$, respectively, with negative angles and the appearance of an ordinary component diffracting along $z$. The beams appear wider than they are as the images result from collection of scattered light.

Figure 4

Evolution of the red light emitted by the dye molecules and trapped in the soliton-induced waveguide. In (a) through (f) the corresponding input powers at ${\lambda }_{\mathrm{blue}}=442\mathrm{nm}$ are those indicated in Fig. 3. Since $({\lambda }_{\mathrm{red}}{)}^2\gg ({\lambda }_{\mathrm{blue}}{)}^2$, Rayleigh scattering [($\propto {\lambda }^{-2}$) in NLCs] is much lower than at ${\lambda }_{\mathrm{blue}}$ (see Fig. 3).

Figure 5

(a) Measured and (b) calculated soliton trajectories in $yz$ for various excitations (we used $I_{\mathrm{sat}}=0.5\mathrm{mW}/\mu {\mathrm{m}}^2$ and ${\eta }_0=50$). The slight discrepancy between (a) and (b) in the transition region ($z<100\mu \mathrm{m}$) at high powers is due to the tilt of optic axis out of $yz$, as witnessed by the appearance of an ordinary component. (c) Measured (symbols) and calculated angle (solid line) $\varphi$ in $z=0.3\mathrm{mm}$ versus $P_0$. (d) Fit-coefficient used in (b): the decrease for $P_0>300\mu \mathrm{W}$ is due to saturation.