Fu Xiang Wang

Multipolar tensor analysis of second-order nonlinear optical response of surface and bulk of glass.

We use two-beam second-harmonic generation to perform a quantitative tensor analysis of the effective dipolar surface nonlinearity and the separable multipolar bulk nonlinearity for BK7 glass. The most straightforward, self-consistent interpretation of the results is obtained when the effective surface response is assumed to have approximate Kleinman symmetry and the bulk contribution is dominated by magnetic, rather than quadrupole, effects.

Enhancement of bulk-type multipolar second-harmonic generation arising from surface morphology of metals

Gold films of 20 and 150 nm nominal thickness were characterized by two-beam second-harmonic generation to address their second-order nonlinear optical responses of surface (dipolar) and bulk (higher multipolar) origin. The surface response is enhanced by ~20% in the case of the 20 nm film, as expected due to its higher surface roughness. Surprisingly, the bulk-type response is enhanced to a greater extent, exceeding 80%, and this can be explained by effective quadrupolar nonlinearity arising from the field interaction with the local nonlinearity of nanoscale surface features.

Suppression of nonlinear optical signals in finite interaction volumes of bulk materials

We show that nonlinear optical signals generated by non-phase-matched interactions are strongly suppressed when the interaction volume is finite and localized deep inside the bulk of a homogeneous material, as opposed to the case where the interaction volume extends across a boundary of the material. The suppression in the bulk originates from destructive interference between the signals generated in the two regions where the interaction is gradually turned on and off and depends on the ratio of the coherence length to the characteristic length of the interaction volume.

Multipolar contributions to the second-order nonlinearity of gold

We use two-beam second-harmonic generation to separate the surface (electric-dipole) and bulk (magnetic-dipole and electric-quadrupole)-type contributions to the second-order nonlinear optical response of gold films of different thicknesses. Surprisingly, the bulk-type response is more sensitive to surface morphology than the surface-type response.

Enhanced quadrupolar second-harmonic generation from rough gold films

The nonlinear optical response of gold films of different thicknesses is investigated by two-beam second-harmonic generation to address the role of surface and bulk effects in their second-order nonlinear optical response. Preliminary results suggest that both types of effects contribute to the measured second-harmonic signals. Furthermore, both effects are found to be enhanced for the thin film with a higher level of nanoscale surface roughness.

Surface and bulk contributions to the second-order nonlinearity of gold

We use two-beam second-harmonic generation to separate the surface and bulk contributions to the second-order nonlinearity of gold. The results provide direct evidence of bulk contributions, nevertheless, surface-like contributions dominate the measured signals.

Second-Order Nonlinear Optical Response of Gold

We use two-beam second-harmonic generation to separate the surface and bulk contributions to the nonlinearity of gold for different metal-dielectric interfaces. One of the surface parameters is shown to arise predominantly from the bulk response.

Calibration of multipolar second-order response of isotropic bulk materials

The bulk response for second-harmonic generation (SHG) is best accessed in a geometry with two incident fundamental beams, which intersect in the sample. The finite overlap of the two beams and the resulting varying interaction strength along the beam propagation direction precludes reliable calibration of the multipolar nonlinearity using the traditional Maker fringe technique. In this paper, we present a novel technique that takes advantage of the two-beam geometry and allows absolute calibration of the multipolar response against a known reference material.

Suppression of non-phase-matched nonlinear optical signals in homogeneous bulk materials

Nonlinear optical signals generated by non-phase-matched interactions localized in the bulk of homogeneous materials are suppressed. Even when the response has bulk origin, the strongest signals are obtained near the boundaries of the material.