Gradient-induced long-range optical pulling force based on photonic band gap

Optical pulling provides a new degree of freedom in optical manipulation.It is generally believed that long-range optical pulling forces cannot be generated by the gradient of the incident field.Here,we theoretically propose and numerically demonstrate the realization of a long-range optical pulling force stemming from a self-induced gradient field in the manipulated object.In analogy to potential barriers in quantum tunnelling,we use a photonic band gap design in order to obtain the intensity gradients inside a manipulated object placed in a photonic crystal waveguide,thereby achieving a pulling force.Unlike the usual scattering-type optical pulling forces,the proposed gradient-field approach does not require precise elimination of the reflection from the manipulated objects.In particular,the Einstein-Laub formalism is applied to design this unconventional gradient force.The magnitude of the force can be enhanced by a factor of up to 50 at the optical resonance of the manipulated object in the waveguide,making it insensitive to absorption.The developed approach helps to break the limitation of scattering forces to obtain longrange optical pulling for manipulation and sorting of nanoparticles and other nano-objects.The developed principle of using the band gap to obtain a pulling force may also be applied to other types of waves,such as acoustic or water waves,which are important for numerous applications.

Topological transformation and free-space transport of photonic hopfions

Structured light fields embody strong spatial variations of polarization,phase,and amplitude.Understanding,characterization,and exploitation of such fields can be achieved through their topological properties.Three-dimensional(3D)topological solitons,such as hopfions,are 3D localized continuous field configurations with nontrivial particle-like structures that exhibit a host of important topologically protected properties.Here,we propose and demonstrate photonic counterparts of hopfions with exact characteristics of Hopf fibration,Hopf index,and Hopf mapping from real-space vector beams to homotopic hyperspheres representing polarization states.We experimentally generate photonic hopfions with on-demand high-order Hopf indices and independently controlled topological textures,including Néel-,Bloch-,and antiskyrmionic types.We also demonstrate a robust free-space transport of photonic hopfions,thus showing the potential of hopfions for developing optical topological informatics and communications.

Experimental demonstration of linear and spinning Janus dipoles for polarisation-and wavelength-selective near-field coupling

The electromagnetic field scattered by nano-objects contains a broad range of wavevectors and can be efficiently coupled to waveguided modes.The dominant contribution to scattering from subwavelength dielectric and plasmonic nanoparticles is determined by electric and magnetic dipolar responses.Here,we experimentally demonstrate spectral and phase selective excitation of Janus dipoles,sources with electric and magnetic dipoles oscillating out of phase,in order to control near-field interference and directional coupling to waveguides.We show that by controlling the polarisation state of the dipolar excitations and the excitation wavelength to adjust their relative contributions,directionality and coupling strength can be fully tuned.Furthermore,we introduce a novel spinning Janus dipole featuring cylindrical symmetry in the near and far field,which results in either omnidirectional coupling or noncoupling.Controlling the propagation of guided light waves via fast and robust near-field interference between polarisation components of a source is required in many applications in nanophotonics and quantum optics.

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.

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.