Nate Lawrence

Aperiodic arrays of active nanopillars for radiation engineering

We engineer aperiodic nanostructures for enhanced omnidirectional light extraction and coupling of 1.55 μm radiation to distinctive optical resonances carrying of orbital angular momentum (OAM) using light emitting Si-based materials. By systematically studying nanopillar arrays with varying pillar separations and increasing degree of rotational symmetry in Fourier space, we show that omnidirectional extraction is achieved with circularly symmetric Fourier space, leading to best light emission enhancement from planar devices such as LEDs or lasers. To demonstrate the potential of active aperiodic structures with azimuthally isotropic k-space, we fabricate nanopillar arrays of erbium doped silicon-rich nitride using electron beam lithography and reactive ion etching. Experimental results obtained using leaky-mode photoluminescence spectroscopy prove over 10 times extraction enhancement at 1.55 μm from aperiodic golden angle spirals (GA spirals), in good agreement with design based on analytical Bragg scatt...

Analytical light scattering and orbital angular momentum spectra of arbitrary Vogel spirals.

In this paper, we present a general analytical model for light scattering by arbitrary Vogel spiral arrays of circular apertures illuminated at normal incidence. This model suffices to unveil the fundamental mathematical structure of their complex Fraunhofer diffraction patterns and enables the engineering of optical beams carrying multiple values of orbital angular momentum (OAM). By performing analytical Fourier-Hankel decomposition of spiral arrays and far field patterns, we rigorously demonstrate the ability to encode specific numerical sequences onto the OAM values of diffracted optical beams. In particular, we show that these OAM values are determined by the rational approximations (i.e., the convergents) of the continued fraction expansions of the irrational angles utilized to generate Vogel spirals. These findings open novel and exciting opportunities for the manipulation of complex OAM spectra using dielectric and plasmonic aperiodic spiral arrays for a number of emerging engineering applications in singular optics, secure communication, optical cryptography, and optical sensing.

Control of optical orbital angular momentum by Vogel spiral arrays of metallic nanoparticles

In this Letter, we experimentally demonstrate structured light carrying multiple values of orbital angular momentum (OAM) in the farfield scattering region of Vogel spiral arrays of metallic nanoparticles. Using Fourier-Hankel mode decomposition analysis and interferometric reconstruction of the complex amplitude of scattered waves, we show the ability to encode well-defined numerical sequences, determined by the aperiodic spiral geometry, into azimuthal OAM values, in excellent agreement with analytical scattering theory. The generation of azimuthal sequences of OAM values by light scattering from engineered aperiodic surfaces is relevant to a number of device applications for secure optical communication, classical cryptography, and quantum cryptography.

Radiation Rate Enhancement in Subwavelength Plasmonic Ring Nanocavities

We demonstrate 25 times radiation rate and 2 times quantum efficiency enhancement of Er ions in metal-insulator-metal (MIM) ring nanocavities at room temperature. In particular, using time-resolved photoluminescence spectroscopy in partnership with full-vector numerical simulations based on the finite difference time domain (FDTD) method, we design, fabricate, and systematically investigate the photonic density of states, the quantum efficiency, and the 1.55 μm radiation dynamics of cavities with varying nanoscale active regions. Our experimental findings demonstrate that the engineering of deep subwavelength gap plasmon modes leads to dramatic Purcell enhancement even at modest cavity Q factors. Finally, we discuss the possibility of achieving lasing due to the enhancement of stimulated emission rate achievable in ring nanocavities, and we provide a perspective for Si-compatible plasmon-enhanced nanolasers.

Light scattering, field localization and local density of states in co-axial plasmonic nanowires.

Based on analytical scattering theory, we develop a multipolar expansion method to investigate systematically the near-field enhancement, far-field scattering and Local Density of States (LDOS) spectra in concentric metal-insulator-metal (MIM) cylindrical nanostructures, or coaxial plasmonic nanowires (CPNs). We demonstrate that these structures support distinctive plasmonic resonances with strongly reduced scattering in the far-field zone and significant electric field enhancement in deep sub-wavelength dielectric regions. Additionally, we study systematically the effects of geometrical parameters and dielectric index on the near-field and far-field plasmonic response of CPNs in the visible and near infrared spectral range. Finally, we demonstrate that CPNs provide a convenient approach for engineering strong (almost three orders of magnitude) LDOS enhancement in sub-wavelength dielectric gaps at multiple frequencies. These results enable the engineering of multiband optical detectors and CPNs-based light emitters with simultaneously enhanced excitation and emission rates for nanoplasmonics.

Near-field distribution and propagation of scattering resonances in Vogel spiral arrays of dielectric nanopillars

In this work, we employ scanning near-field optical microscopy, full-vector finite difference time domain numerical simulations and fractional Fourier transformation to investigate the near-field and propagation behavior of the electromagnetic energy scattered at 1.56µm by dielectric arrays of silicon nitride nanopillars with chiral 1-Vogel spiral geometry. In particular, we experimentally study the spatial evolution of scattered radiation and demonstrate near-field coupling between adjacent nanopillars along the parastichies arms. Moreover, by measuring the spatial distribution of the scattered radiation at different heights from the array plane, we demonstrate a characteristic rotation of the scattered field pattern consistent with net transfer of orbital angular momentum in the Fresnel zone, within a few micrometers from the plane of the array. Our experimental results agree with the simulations we performed and may be of interest to nanophotonics applications.

Nanopatterning of optically-active silicon nanowires

We demonstrate broadband photoluminescence enhancement up to approximately one order of magnitude from Silicon Nanowires prepared by Metal-Assisted Chemical Etching and nanopatterned into periodic and aperiodic array geometries using top-down planar processing techniques.

Engineering photonic-plasmonic aperiodic surfaces for optical biosensing

The ability to reproducibly and accurately control light matter interaction on the nanoscale is at the core of the field of optical biosensing enabled by the engineering of nanophotonic and nanoplasmonic structures. Efficient schemes for electromagnetic field localization and enhancement over precisely defined sub-wavelength spatial regions is essential to truly benefit from these emerging technologies. In particular, the engineering of deterministic media without translational invariance offers an almost unexplored potential for the manipulation of optical states with vastly tunable transport and localization properties over broadband frequency spectra. In this paper, we discuss deterministic aperiodic plasmonic and photonic nanostructures for optical biosensing applications based on fingerprinting Surface Enhanced Raman Scattering (SERS) in metal nanoparticle arrays and engineered light scattering from nanostructured dielectric surfaces with low refractive index (quartz).