Intermittency in second-harmonic radiation from plasmonic hot spots on rough silver films

Surface enhancement of electromagnetic fields in plasmonic hot spots formed on rough silver films enables the observation of second-harmonic generation (SHG) from single metal nanoparticles. Nonlinear light scattering from these particles exhibits blinking in analogy to luminescence from single quantum dots, molecules and atoms; and fluctuations in single molecule surface-enhanced Raman scattering. Hot spots also display multiphoton white light emission besides SHG. In contrast to SHG, white light emission is stable with time, demonstrating that it is not the plasmonic field enhancement which fluctuates but the nonlinear polarizability $\left({\chi }^{\left(2\right)}\right)$ of the emitting species.

Figure 1

(Color online) Nonlinear optical emission from a fractal silver nanoparticle film. (A) Real-space image of surface-plasmon localization and hot spot emission (seen as diffraction-limited spots) under pulsed excitation at 1070 nm. Placing a slit in the image (dashed lines) and dispersing the light through a spectrometer provides spatially resolved spectral information along the vertical axis (B). (C) Hot spot emission spectrum, truncated at 660 nm by a short-pass filter. The spectrum consists of a broad CE and a narrow peak at half the excitation wavelength which is due to SHG. A scanning electron micrograph of a typical nanoparticle sample is shown in the inset, as are the nonlinear excitation power dependencies of CE ($\propto {\text{I}}^{2.6}$, gray) and SHG ($\propto {\text{I}}^2$, blue/black).

Figure 2

(Color online) Spectrally resolved polarization anisotropy of the excitation and emission of single hot spots. (A) Modulation of the polarization of the exciting laser leads to a periodic change of both CE (broad) and SHG (537 nm) intensity (5 s time resolution). (B) The emission of the hot spot is dipolar, following a similar modulation as in excitation. (C) CE and SHG must arise from the same hot spot. We propose that the two nonlinear optical phenomena originate from a small particle located within the hot spot.

Figure 3

(Color online) Temporal fluctuations of SHG from a single hot spot. (A) Emission trace recorded with 5 s resolution, showing that the SHG peak at 532 nm is not constant in intensity with time, reminiscent of blinking in the emission of single molecules or nanoparticles. (B) Temporal evolution of the SHG (blue/black) and CE (gray) intensity from the same hot spot.

Figure 4

Polarization dependence of SHG intermittency and CE from a single hot spot. The parallel (A) and orthogonal (B) polarization components of SHG with respect to the incident laser polarization (black arrows) are only weakly correlated, particularly in the shaded regions (measurement resolution 10 s). The simultaneous CE does not display any substantial fluctuations in either polarization channel (C and D), besides a gradual intensity decay due to photodegradation. A complete analysis of the polarization state is given in the supporting information (Ref. 23).