Nanoplasmonic near-field synthesis

The temporal response of resonances in nanoplasmonic structures typically converts an incoming few-cycle field into a much longer near-field at the spot where nonlinear physical phenomena including electron emission, recollision, and high-harmonic generation can take place. We show that for practically useful structures pulse shaping of the incoming pulse can be used to synthesize the plasmon-enhanced field and enable single-cycle-driven nonlinear physical phenomena. Our method is demonstrated for the generation of an isolated attosecond pulse by plasmon-enhanced high harmonic generation. We furthermore show that optimal control techniques can be used even if the response of the plasmonic structure is not known a priori.

Figure 1

Frequency-dependent intensity enhancement (blue) and phase shift of $E_z$ (red) at the center of the structure. The insets show the spatial dependence of enhancement and phase shift for three frequencies: In the quasistatic regime, close to the resonance peak, and on the blue side of the resonance.

Figure 2

Optimized isolated attosecond pulse power $P_{as}$ for HHG from the plasmon-enhanced near-field transient for different filter settings. In all cases, the total intensity and CEP have been optimized. The noise $N$ is fixed to $2\%$, while the peak intensity $I_0$ of the incoming pulse is varied. For details, see text.

Figure 3

Different synthesized pulses. In each row from left to right: Filter settings $f_n$ and electric field amplitudes $E_n$ (absolute value and phase are plotted separately), plasmon-enhanced electric field in the hot spot and filtered high harmonic radiation generated by this pulse (inset: incoming pulse), and time-frequency distribution of unfiltered high harmonic radiation. The incoming pulse has a maximum intensity of $4\times {10}^{11}\text{W}/{\text{cm}}^2$ before filtering. The rows, from top to bottom, show: (a) Fourier-limited incoming pulse with only amplitude and CEP adjusted to optimize single as pulse production in the hot spot. (b) Manually optimized pulse chosen to produce a few-cycle Gaussian pulse in the hot spot as well as possible, amplitude and CEP optimized. (c) Pulse obtained after numerical optimization (see text). The numerically optimized pulse features a phase jump before the “spike” producing the attosecond pulse, effectively suppressing HHG from the previous half-cycle of the field.