Deterministic spatiotemporal control of optical fields in nanoantennas and plasmonic circuits

We show that laser pulse shaping techniques can be applied to tailor the ultrafast temporal response of localized and propagating optical near fields in resonant optical antennas (ROAs) and plasmonic transmission lines, respectively. Using finite-difference time-domain simulations followed by Fourier transformation, we obtain the impulse response of a nanostructure in the frequency domain, which allows obtaining its temporal response to any arbitrary pulse shape. To illustrate the potential of the method we demonstrate deterministic optimal temporal pulse compression in ROAs with reduced symmetry, in a plasmonic two-wire transmission line, and in a prototype plasmonic circuit combining propagation effects and local resonances. The method described here will be of importance for the coherent control of field propagation in nanophotonic structures and light-induced processes in nanoscopic volumes.

Figure 1

(Color online) Time-dependent electric fields of the default excitation source (black, top) and the corresponding near-field response (red, bottom) at the feed gap center of a dipole antenna with a displaced gap. Antenna dimensions are $20\times 20\times 170{\text{nm}}^3$ including a 10 nm gap, which is shifted 30 nm away from the antenna center. The displaced gap results in arm lengths of 110 and 50 nm as sketched in the inset.

Figure 2

(Color online) (a) Source-independent impulse spectra (black solid line) and spectral phase (red dashed line) of isolated 50 nm and 110 nm gold nanorods recorded at a point on the long axis of the rod, 5 nm from its end in air. (b) Source-independent impulse spectrum (black solid line) and spectral phase (red dashed line) of the respective displaced-gap antenna consisting of the two rods in (a), recorded at the center of the 10 nm feed gap. The amplitude scale in (b) is about 30 times larger than in (a) due to antenna enhancement. (c) Spectrum (black solid line) and phase (red dashed line) of a new source pulse. The new phase is shaped as the opposite of the displaced-gap antenna spectral phase shown in (b). All amplitudes are normalized to unity.

Figure 3

(Color online) (a) New response spectrum (black solid line) and spectral phase (red dashed line). The amplitude is normalized to unity. (b) Temporal intensity profiles of the original response (black dotted line) and of the new response evaluated directly from Eq. (3) followed by an inverse Fourier transformation (blue dashed line). For comparison, we also plot the square of the real part of the electric field (red solid line) obtained from a full FDTD simulation with the shaped source shown in Fig. 2c. All intensities are normalized to the maximum intensity of the original response.

Figure 4

(Color online) Left panel: sketch of the transmission line and the observation points. Two long gold nanowires with $20\times 20{\text{nm}}^2$ cross section and 10 nm separation are attached to two arms of a $20\times 20\times 190{\text{nm}}^3$ antenna. The red dashed circle denotes the excitation area. Observation: point A is located at the feed gap center, while points B–D are shifted in the $+y$ direction from point A by 1000, 2000, and 3000 nm, respectively. Right panel: (a) normalized temporal intensity profiles recorded at points A (red), B (blue), C (green), and D (pink) obtained with the default source (black); (b) normalized original (black thin line) and new (red thick line) temporal intensity profiles of the source (dashed) and the response (solid) at point D.

Figure 5

(Color online) Left panel: sketch of an optical circuit and the observation points. The circuit consists of three antennas and a finite-length plasmonic transmission line with a $T$ branching. All wires and antennas have a $20\times 20{\text{nm}}^2$ quadratic cross section and 10 nm gap. Dimensions are indicated in nanometers. The red-dashed circle denotes the excitation area. Observation: points E and F are located at the feed gap centers of the left and right antennas, respectively. Right panel: normalized original (black solid line) and new (red dashed line) temporal intensity profiles of signals recorded at (a) point E and (b) point F.