Near-Field Second-Harmonic Generation Induced by Local Field Enhancement

Abstract

The field near a sharp metal tip can be strongly enhanced if irradiated with an optical field polarized along the tip axis. We demonstrate that the enhanced field gives rise to local second-harmonic (SH) generation at the tip surface thereby creating a highly confined photon source. A theoretical model for the excitation and emission of SH radiation at the tip is developed and it is found that this source can be represented by a single on-axis oscillating dipole. The model is experimentally verified by imaging the spatial field distribution of strongly focused laser modes. [3][4][5]. The enhancement originates from a combination of the electrostatic lightning-rod effect, which is due to the geometric singularity of sharply pointed structures, and localized surface plasmon resonances which depend sensitively on the excitation wavelength. In most studies, the field enhancement factor is either deduced from theoretical calculations or from ensemble averaged measurements. However, recent spatially resolved SERS measurements on single molecules have shown that the field enhancement may vary by many orders of magnitude between different sites of a colloidal metal surface [6,7]. In order to understand which structures render particularly strong enhancements it is necessary to perform measurements on a single particle level. In this Letter, we demonstrate that local SH generation provides a direct means to determine the field enhancement near a nanoscale structure such as a sharp metal tip.SH generation in the context of near-field optical microscopy has been discussed by various authors [8][9][10][11] but the strong dependence on local field enhancement has not been analyzed. Here, we show that the field enhancement effect localizes SH generation to the very end of a metal tip thereby creating a highly confined photon source. We experimentally demonstrate the strong light confinement and investigate the radiation properties of the SH light. We also show that the tip acts as a local probe for fields polarized along the tip axis (longitudinal fields). This unique property can be used for the mapping of field distributions in optical fields as demonstrated recently with trapped ions [12] and single molecules [13,14].

Applications of field-enhanced near-field optical microscopy

“…The nonlinearity can be accounted for by a second-order nonlinear susceptibility of gold which gives rise to a surface nonlinear polarization at the second-harmonic frequency [14]. It has been shown that second-harmonic generation at a sharp gold tip can be modelled by a vertical on-axis oscillating dipole [15]. The surface morphology of the tip strongly influences the magnitude of the second-harmonic signal.…”

Extreme-ultraviolet light generation in plasmonic nanostructures

“…[24], where higher diffraction orders are absent. Second, the photon count rates in our experiments did not exceed several thousand per second using an imaging detector and obtaining a signal to noise ratio of~10 3 . In contrast, ref.…”

“…For noble gases and infrared light wavelengths, this situation corresponds to intensities in the excess of 10 13 W/cm 2 , which are usually reached using moderately focused femtosecond pulses from Ti:sapphire laser amplifiers. A microscopic examination within the semi-classical simple man's model [20] leads to a coarse description of HHG without the need for a detailed quantum mechanical treatment 3 . Figure 1.6 illustrates the particular sequences of the simple man's model within a half-cycle of the driving laser field (red curve).…”

“…This allows for the study of numerous linear and nonlinear optical processes such as surface-enhanced Raman scattering [1,2], second [3,4] and third [5,6] harmonic generation, and multiphoton photoemission [7,8] using low-energy laser pulses with high (MHz) repetition rates. The rapidly increasing number of reports about the enhancement of optical phenomena in nanoantennas, -waveguides, -tips, -spheres, -particles, and rough surfaces impressively illustrate the growing importance of nano-optics in the natural sciences.…”

Direct Near-Field Optical Imaging of Higher Order Plasmonic Resonances

“…The probing AFM tip and the demodulation are not included in the simulation. Instead, we construct the detected signal as produced by a point-like nanoantenna with a highly anisotropic dipolepolarizability 27 that probes the electric field (normal to the surface) at an effective average distance of 30 nm to the sample, and then re-emits p-polarized radiation toward the detector. The striking resemblance of experimentally recorded ( Figure 2) and simulated near-field images ( Figure 3) both in amplitude and phase justifies our modeling assumptions.…”