Generalization of the optical theorem: experimental proof for radially polarized beams

The optical theorem,which is a consequence of the energy conservation in scattering processes,directly relates the forward scattering amplitude to the extinction cross-section of the object.Originally derived for planar scalar waves,it neglects the complex structure of the focused beams and the vectorial nature of the electromagnetic field.On the other hand,radially or azimuthally polarized fields and various vortex beams,essential in modern photonic technologies,possess a prominent vectorial field structure.Here,we experimentally demonstrate a complete violation of the commonly used form of the optical theorem for radially polarized beams at both visible and microwave frequencies.We show that a plasmonic particle illuminated by such a beam exhibits strong extinction,while the scattering in the forward direction is zero.The generalized formulation of the optical theorem provides agreement with the observed results.The reported effect is vital for the understanding and design of the interaction of complex vector beams carrying longitudinal field components with subwavelength objects important in imaging,communications,nanoparticle manipulation,and detection,as well as metrology.

All-optical switching in silicon photonic waveguides with an epsilon-near-zero resonant cavity [Invited]

Strong nonlinearity of plasmonic metamaterials can be designed near their effective plasma frequency in the epsilon-near-zero(ENZ) regime. We explore the realization of an all-optical modulator based on the Au nonlinearity using an ENZ cavity formed by a few Au nanorods inside a Si photonic waveguide. The resulting modulator has robust performance with a modulation depth of about 30 dB/μm and loss less than 0.8 dB for switching energies below 600 fJ. The modulator provides a double advantage of high mode transmission and strong nonlinearity enhancement in the few-nanorod-based design.