Dynamic Beam Steering from a Subwavelength Slit by Selective Excitation of Guided Modes

Dynamic control of the direction of radiation of the light emanating from a subwavelength slit carved out of a thin metal film is experimentally demonstrated. This is achieved by selective excitation of the individual guided modes in the slit by setting the phase of three coherent laser beams. By changing the voltage across a piezoelement, we obtain unprecedented directional steering, without relying on any mechanical alignment of optical elements. The angular range over which this maximum can be swept is determined by the intensity setting of one of the incident beams. Through simulations, we show that this method can also be applied to steer the radiation from a square hole in two independent directions. Our method can be applied to create a directional nanoemitter which can selectively address one or more detectors, or as an optical switch in photonic circuits.

Figure 1

A narrow slit in a gold film supports only two TM modes at $\lambda =632.8\mathrm{nm}$, a symmetric ${\mathrm{TM}}_0$ mode (green curve), and an antisymmetric ${\mathrm{TM}}_1$ mode (red curve). Three coherent beams $A$, $-A$ (with opposite angle of incidence compared to $A$ and $\pi$-phase shifted), and $B$ are incident on the slit from the glass substrate. By varying the voltage across the piezoelement, the angle of maximum radiation can be varied dynamically. This can be used, for example, to selectively address detector $D_1$ or $D_2$.

Figure 2

Simulation of the radiation patterns for two different ratios of the modal amplitudes $C_1/C_0=1.0$ (dashed curves $a$, $b$, and $c$) and $C_1/C_0=2.4$ (solid curves $d$, $e$, and $f$). For the red, black, and blue curves, the phase difference $\delta =-\pi /2$, $\pi /2$, and 0, respectively. Zero degrees indicates the $z$ direction, normal to the metal film.

Figure 3

Measurement of two maximally steered radiation patterns. For two different voltage settings, the peak intensity occurs at $\theta \approx -11°$ (blue curve) or at $\theta \approx +9°$ (red curve). The narrow peaks at $-21°$, 0°, and 21° are due to direct transmission of the three incident beams.

Figure 4

Measurement of the radiation pattern for a voltage in between those of Fig. 3. The maximum intensity occurs near $\theta \approx 0°$.

Figure 5

Polar plot of three-dimensional radiation patterns showing the four extreme directions into which the radiation from a small square aperture can be steered. In all four cases, the intensity peaks at an angle of 18° with the forward direction. In this example, $C_{01}=1$, $C_{02}=0.6$, $C_{11}=0.7$, and $a=1.01\lambda$. The phase ${\delta }_{02}=0$ (yellow and blue patterns) or $\pi$ (red and green patterns), whereas ${\delta }_{11}=0$ (yellow and red patterns) or $\pi$ (blue and green patterns).