C. J. Regan

Imaging nanoscale features with plasmon-coupled leakage radiation far-field superlenses

Optical images from nano-scale features were obtained by collection of leakage radiation coupled to surface plasmon polaritons excited by near-field fluorescence. Plasmonic crystals with spatial periods as small as 190 nm and non-periodic features separated by 80 nm, corresponding to ~λ/7, were clearly visible in the real plane images using this far-field technique. We show that the leaked light from the investigated samples carries detailed information to the far-field which is not present in the images obtained with conventional optical microscopy.

Direct observation of photonic Fermi surfaces by plasmon tomography

Light propagation in dielectric plasmonic crystals with different parameters and symmetries was investigated by plasmon tomography. We show that the photonic Fermi surfaces at the crystal’s reciprocal lattice space can be observed directly from the Fourier plane images. Directional gaps were observed where the isofrequency wavevectors of the propagating surface plasmon mode intersect the first Brillouin zone of the plasmonic crystal structures. We determined that the angular magnitude of the directional gaps depends strongly on the crystal symmetry and the lattice period.

Far-field optical superlenses without metal

The spatial resolution in traditional optical microscopy is limited by diffraction. This prevents imaging of features with dimensions smaller than half of the wavelength (λ) of the illumination source. Superlenses have been recently proposed and demonstrated to overcome this issue. However, its implementation often involves complex sample fabrication and lossy metal layers. Alternatively, a superlens without metals can be realized using surface waves as the illumination source at the interface between two dielectrics, at the total internal reflection condition, where one of the dielectrics is doped with a fluorescent material. Non-scanning far-field images with resolution of ∼λ/5 and without the need of any post-processing or image reconstruction can be achieved with this approach.

Fundaments of optical far-field subwavelength resolution based on illumination with surface waves

We present a general discussion about the fundamental physical principles involved in a novel class of optical superlenses that permit to realize in the far-field direct non-scanning images with subwavelength resolution. Described superlenses are based in the illumination of the object under observation with surface waves excited by fluorescence, the enhanced transmission of fluorescence via coupling with surface waves, and the occurrence of far-field coherence-related fluorescence diffraction phenomena. A Fourier optics description of the image formation based on illumination with surface waves is presented, and several recent experimental realizations of this technique are discussed. Our theoretical approach explains why images with subwavelength resolution can be formed directly in the microscope camera, without involving scanning or numerical post-processing. While resolution of the order of λ/7 has been demonstrated using the described approach, we anticipate that deeper optical subwavelength resolution should be expected.

Equifrequency curve dispersion in dielectric-loaded plasmonic crystals

We present a quantitative description of the momentum-space dispersion and directivity of light propagation in dielectric-loaded plasmonic crystals. In our analysis the three-dimensional lossy plasmonic crystals are modeled by two-dimensional lossless dielectric crystals with real effective refractive indexes. Simulated equifrequency curves are in excellent agreement with measured Fourier plane images obtained from dye-doped dielectric-loaded plasmonic crystals with different lattice geometries and parameters. Our results provide fundamental information about the origin of the directional bandgaps in these structures.

Study of plasmonic crystals using Fourier-plane images obtained with plasmon tomography far-field superlenses

We explore the use of surface plasmon polariton (SPP) tomography far-field superlenses for quantitative characterization of plasmonic crystals with sub-wavelength features. Essential information concerning the dependence of the effective refractive index on the hole diameter and the filling factor was obtained from the Fourier-plane images of the fabricated plasmonic crystals. We also provide a comprehensive discussion on the influence of hole diameters on the formation of directional stop-bands in plasmonic crystals.

SPP tomography: a simple wide-field nanoscope.

We explore the wide-field optical nanoimaging capabilities of the surface plasmon polariton (SPP) tomography technique. We show that nanofeatures with lateral dimensions smaller than λ/20 can be observed in the surface emission (SE) images of plasmonic crystals with a period of 300 nm. Moreover, as a proof-of-concept, we demonstrate that SPP tomography permits to resolve two single objects with a center-to-center separation of 200 nm and edge-to-edge separation as small as λ/7. We present a comprehensive discussion about the nanoimaging capabilities of the SPP tomography technique. In contrast to other optical subwavelength resolution techniques, in our approach for imaging nanosize features, enhanced evanescent waves are coupled to the far-field via leakage radiation associated with SPPs excited by near-field fluorescence; therefore wide-field images, which are not out-of-plane diffraction-limited, are formed directly in the microscopes camera. We also discuss additional imaging processing capabilities associated with the fact that SPP tomography SE images are formed by the microscope lenses through an analog tomography process.

Resonant coupling in dielectric loaded plasmonic waveguides

Light propagation in dielectric loaded surface plasmon polariton waveguide (DLSPPW) resonant coupling devices operating at visible frequencies was experimentally investigated. The transmission characteristics of these devices were studied by leakage radiation microscopy. We show that a strong coupling between DLSPPWs can be achieved with nanoscale gaps. We demonstrate the operation of compact DLSPPW linear couplers and 3 dB power splitters. The performances of micro-DLSPPW racetrack resonators and signal drop filter are also discussed.

Probing photonic Bloch wavefunctions with plasmon-coupled leakage radiation

We demonstrate theoretically and experimentally the direct observation of photonic Bloch wavefunctions in dielectric loaded plasmonic crystals. The ultimate ability to observe the Bloch wavefunctions in the surface emission images depends not on the light diffraction through the holes but on the strength of the in-plane light scattering from the individual lattice features and the presence of the metal layer which allows the light propagating within the crystal to be imaged in the far-field. Experimental results are in excellent agreement with simulated surface emission and back focal plane images of plasmonic crystals.

Two-dimensional Bessel-like surface plasmon-polariton beams

We present experimental evidence of non-diffracting two-dimensional Bessel-like surface plasmon-polariton (SPP) beams using the simultaneous excitation of two grating couplers forming an angle. The Bessel-like SPP beam properties were verified experimentally using the plasmon-coupled leakage radiation microscopy technique. The good agreement between simulations and measured intensity beam profiles confirms the effectiveness of the grating-coupler approach. Our results revealed that the spreading and propagation length characteristics of the Bessel-like SPP beams are primarily influenced by the angle between the grating couplers.