Optics is generally interpreted as a world of electric fields and electric dipoles because magnetic-dipole transitions at optical (visible) frequencies are considered forbidden. Can magnetic dipole resonance serve as active element in nano-optics similar to the electric dipole resonance? The magnetic dipole resonance can then serve as probe of magnetic fields similar to the electric dipole resonances that are being used as probes to study electric light-matter interactions. Moreover, by spectrally overlapping the electric and magnetic dipole resonances, one can realizes novel optical phenomena, such as Invisibility Cloak, Unidirectional Scattering, Fano Resonances, Dark Mode, etc.

We take two different approaches to address this question. (i) We create light with cylindrical shape, and use this light to enhance and control magnetic dipole resonance in the optical frequencies. (ii) We design materials with structural symmetry, and excite them with cylindrical beam to enhance and control magnetic dipole transitions in the optical frequencies.

We collaborate with Prof. Hiroshi Sugimoto at the Mesocopic Materials Laboratory at the Kobe University, and Prof. Mahua Biswas at Millkin University.

CYLINDRICAL VECTOR BEAMS:


The state of polarization of scalar beams (e.g., linearly, elliptically, and circularly polarized light) does not depend on the spatial location over the beam cross-section; hence, they have spatially homogeneous states of polarization. On the other hand, Cylindrical Vector Beams (CVBs) are solutions of Maxwell’s equations that possess spatially varying polarization with cylindrical symmetry in both amplitude and phase. Such beams can have their electric (magnetic) field aligned in azimuthal orientations while their magnetic (electric) field is aligned in radial directions with respect to the optical axis, known as azimuthally (radially) polarized beam. The figure above shows a schematic of an azimuthally polarized beam.

Ref: D. G. Hall, “Vector-beam solutions of Maxwell's wave equation”, Opt. Lett. 21, 9-11 (1996)


OPTICAL SPECTROSCOPY SET-UP:

We use a
spatially (but not temporally) coherent (broadband) white light continuum (400-2700 nm) to illuminate the sample. The white light
continuum is coupled to an inverted optical microscope (Olympus IX-71) equipped with a 100X objective with NA = 1.4. A polarization converter that converts linearly polarized light to vector beam is placed in the beam path to produce an azimuthally or a radially polarized light. The back-scattered images of the sample plane are recorded by a sCMOS array detector (Andor Neo) connected to the trinoc eye-piece of the microscope and scattering spectra are acquired by an EM-CCD (Andor Newton) connected to an imaging spectrometer coupled to the side port of the microscope via a home-built achromatic 4f relay system.
The set-up has the capability to measure optical response (scattering, extinction, absorption, transmission) of single nano-structures with dimension of few tens of nanometers.