Person:

Capasso, Federico

We report the experimental realization of deformed microcavity quantum cascade lasers (QCLs) with a Limaçon-shaped chaotic resonator. Directional light emission with a beam divergence of (\theta_{|} \approx 33^{\circ}) from QCLs emitting at λ ≈ 10µm was obtained in the plane of the cavity for deformations in the range 0.37 < ε < 0.43. An excellent agreement between measured and calculated far-field profiles was found. Both simulations and experiments show that the Limaçon-shaped microcavity preserves whispering gallery-like modes with high Q-factors for low deformations (ε < 0.50). In addition, while the measured spectra show a transition from whispering gallery-like modes to a more complex mode structure at higher pumping currents, we observed ‘universal far-field behavior’ for different intracavity mode distributions in the Limaçon microcavity, which can be explained by the distribution of unstable manifolds in ray optics simulations. Furthermore, the performance of the deformed microcavity lasers is robust with respect to variations of the deformation near its optimum value ε = 0.40, which implies that this structure reduces the requirements on photolithography fabrication. The successful realization of these microcavity lasers may lead to applications in optoelectronics.

For more than three decades, research on tunneling through planar barriers has focused principally on processes that conserve momentum parallel to the barrier. Here we investigate transport in which scattering destroys lateral momentum conservation and greatly enhances the tunneling probability. We have measured its energy dependence using capacitance spectroscopy, and we show that for electrons confined in a quantum well, the scattering enhancement can be quenched in an applied magnetic field, enabling this mechanism to function as an external probe of the origin of the quantum Hall effect.

A strain-balanced, InP-based quantum cascade laser structure, designed for light emission at 4.6 (\mu)m using a new nonresonant extraction design approach, was grown by molecular beam epitaxy. Removal of the restrictive two-phonon resonant condition, currently used in most structure designs, allows simultaneous optimization of several design parameters influencing laser performance. Following the growth, the structure was processed in buried heterostructure. Maximum single-ended continuous-wave optical power of 3 W was obtained at 293 K for devices with stripe dimensions of 5 mm 11.6 (\mu)m. Corresponding maximum wallplug efficiency and threshold current density were measured to be 12.7% and 0.86 kA/cm(^{2}).

Multibeam emission and spatial wavelength demultiplexing in semiconductor lasers by patterning their facets with plasmonic structures is reported. Specifically, a single-wavelength laser was made to emit beams in two directions by defining on its facet two metallic gratings with different periods. The output of a dual-color laser was spatially separated according to wavelength by using a single metallic grating. The designs can be integrated with a broad range of active or passive optical components for applications such as interferometry and demultiplexing.

Metallic components such as plasmonic gratings and plasmonic lenses are routinely used to convert free-space beams into propagating surface plasmon polaritons and vice versa. This generation of couplers handles relatively simple light beams, such as plane waves or Gaussian beams. Here we present a powerful generalization of this strategy to more complex wavefronts, such as vortex beams that carry orbital angular momentum, also known as topological charge. This approach is based on the principle of holography: the coupler is designed as the interference pattern of the incident vortex beam and focused surface plasmon polaritons. We have integrated these holographic plasmonic interfaces into commercial silicon photodiodes, and demonstrated that such devices can selectively detect the orbital angular momentum of light. This holographic approach is very general and can be used to selectively couple free-space beams into any type of surface wave, such as focused surface plasmon polaritons and plasmonic Airy beams.

This paper describes the fabrication of arrays of nanostructures (rings, crescents, counterfacing split rings, cylinders, coaxial cylinders, and other structures) by a four-step process: (i) molding an array of epoxy posts by soft lithography, (ii) depositing thin films on the posts, (iii) embedding the posts in epoxy, and (iv) sectioning in a plane parallel to the plane defined by the array of posts, into slabs, with an ultramicrotome (“nanoskiving”). This work demonstrates the combination of four capabilities: (i) formation of structures that are submicrometer in all dimensions; (ii) fabrication of 3D structures, and arrays of structures, with gradients of height; (iii) patterning of arrays containing two or more materials, including metals, semiconductors, oxides, and polymers; and (iv) generation of as many as 60 consecutive slabs bearing contiguous arrays of nanostructures. These arrays can be transferred to different substrates, and arrays of gold rings exhibit plasmonic resonances in the range of wavelengths spanning 2−5 μm.

We show that the surface plasmon polariton (SPP) radiation patterns of point-dipole emitters in the vicinity of a metal-dielectric interface are generally asymmetric with respect to the location of the emitter. In particular rotating dipoles, which emit elliptically polarized light, produce highly asymmetric SPP radiation fields that include unidirectional emission. Asymmetric SPP radiation patterns also result when a dipole oscillates tilted with respect to the plane of the interface and optical losses or gains are present in the materials. These effects can be used to directionally control SPP emission and absorption, as well as to study emission and scattering processes close to metal-dielectric interfaces. Possible implementations of asymmetrically emitting SPP sources are discussed.

By analogy to the three dimensional optical bottle beam, we introduce the plasmonic bottle beam: a two dimensional surface wave which features a lattice of plasmonic bottles, i.e. alternating regions of bright focii surrounded by low intensities. The two-dimensional bottle beam is created by the interference of a non-diffracting beam, a cosine-Gaussian beam, and a plane wave, thus giving rise to a non-diffracting complex intensity distribution. By controlling the propagation constant of the cosine-Gauss beam, the size and number of plasmonic bottles can be engineered. The two dimensional lattice of hot spots formed by this new plasmonic wave could have applications in plasmonic trapping.

Convenient and inexpensive methods to pattern the facets of optical fibers with metallic nanostructures would enable many applications. This communication reports a method to generate and transfer arrays of metallic nanostructures to the cleaved facets of optical fibers. The process relies on nanoskiving, in which an ultramicrotome, equipped with a diamond knife, sections epoxy nanostructures coated with thin metallic films and embedded in a block of epoxy. Sectioning produces arrays of nanostructures embedded in thin epoxy slabs, which can be transferred manually to the tips of optical fibers at a rate of approximately 2 min−1, with 88% yield. Etching the epoxy matrices leaves arrays of nanostructures supported directly by the facets of the optical fibers. Examples of structures transferred include gold crescents, rings, high-aspect-ratio concentric cylinders, and gratings of parallel nanowires.

Enhancing nonlinear processes at the nanoscale is a crucial step toward the development of nanophotonics and new spectroscopy techniques. Here we demonstrate a novel plasmonic structure, called plasmonic nanocavity grating, which is shown to dramatically enhance surface nonlinear optical processes. It consists of resonant cavities that are periodically arranged to combine local and grating resonances. The four-wave mixing signal generated in our gold nanocavity grating is enhanced by a factor up to ≈2000, 2 orders of magnitude higher than that previously reported.