We demonstrate that tunable attractive (bonding) and repulsive (anti-bonding) forces can arise in highly asymmetric structures coupled to external radiation, a consequence of the bonding/anti-bonding level repulsion of guided-wave resonances that was first predicted in symmetric systems. Our focus is a geometry consisting of a photonic-crystal (holey) membrane suspended above an unpatterned layered substrate, supporting planar waveguide modes that can couple via the periodic modulation of the holey membrane. Asymmetric geometries have a clear advantage in ease of fabrication and experimental characterization compared to symmetric double-membrane structures. We show that the asymmetry can also lead to unusual behavior in the force magnitudes of a bonding/antibonding pair as the membrane separation changes, including nonmonotonic dependences on the separation. We propose a computational method that obtains the entire force spectrum via a single time-domain simulation, by Fourier-transforming the response to a short pulse and thereby obtaining the frequency-dependent stress tensor. We point out that by operating with two, instead of a single frequency, these evanescent forces can be exploited to tune the spring constant of the membrane without changing its equilibrium separation. (C) 2011 Optical Society of America
In 1948, Hendrik Casimir predicted that a generalized version of van der Waals forces would arise between two metal plates due to quantum fluctuations of the electromagnetic field. These forces become significant in micromechanical systems at submicrometre scales, such as in the adhesion between movable parts. The Casimir force, through a close connection to classical photonics, can depend strongly on the shapes and compositions of the objects, stimulating a decades-long search for geometries in which the force behaves very differently from the monotonic attractive force first predicted by Casimir. Recent theoretical and experimental developments have led to a new understanding of the force in complex microstructured geometries, including through recent theoretical predictions of Casimir repulsion between vacuum-separated metals, the stable suspension of objects and unusual non-additive and temperature effects, as well as experimental observations of repulsion in fluids, nonadditive forces in nanotrench surfaces and the influence of new material choices.
We propose an optomechanical structure consisting of a photonic-crystal (holey) membrane suspended above a layered silicon-on-insulator substrate in which resonant bonding/antibonding optical forces created by externally incident light from above enable all-optical control and actuation of stiction effects induced by the Casimir force. In this way, one can control how the Casimir force is expressed in the mechanical dynamics of the membrane, not by changing the Casimir force directly but by optically modifying the geometry and counteracting the mechanical spring constant to bring the system in or out of regimes where Casimir physics dominate. The same optical response (reflection spectrum) of the membrane to the incident light can be exploited to accurately measure the effects of the Casimir force on the equilibrium separation of the membrane. (C) 2011 American Institute of Physics. [doi:10.1063/1.3589119]
We designed a new class of plasmonic gratings that generate multiple free-space beams in arbitrary directions from a point source of surface waves, using a phenomenological model that accurately predicts their far-field, in amplitude, phase and polarization. We fabricated such gratings on the facets of semiconductor lasers. The plasmonic gratings proposed here are generally relevant to the interfacing of nanoscale optical components to free-space beams. The model introduced here can be used to design general two-dimensional plasmonic gratings.
A multiwavelength array of distributed feedback (DFB) quantum cascade lasers (QCLs) that spans lambda = 8.28 to 9.62 mu m is wavelength beam combined (WBC) using both single-grating and dual-grating designs. WBC with a single grating results in a pointing error of 3-times the beam divergence for a single laser and arises from the nonlinear dispersion of the grating. By adding a second grating to compensate for the nonlinear dispersion, the pointing error is reduced to only 13% of the beam divergence for a single laser. A transceiver based on the dual-grating-WBC QCL was used to measure the transmittance of a polymer sheet placed between itself and a retroreflector over a round-trip distance of 70 meters. (C) 2011 Optical Society of America
DNA nanotechnology provides a versatile foundation for the chemical assembly of nanostructures. Plasmonic nanoparticle assemblies are of particular interest because they can be tailored to exhibit a broad range of electromagnetic phenomena. In this Letter, we report the assembly of DNA-functionalized nanoparticles into heteropentamer clusters, which consist of a smaller gold sphere surrounded by a ring of four larger spheres. Magnetic and Fano-like resonances are observed in individual clusters. The DNA plays a dual role: it selectively assembles the clusters in solution and functions as an insulating spacer between the conductive nanoparticles. These particle assemblies can be generalized to a new class of DNA-enabled plasmonic heterostructures that comprise various active and passive materials and other forms of DNA scaffolding.
We explore the relationship between the near-field enhancement, absorption, and scattering spectra of localized plasmonic elements. A simple oscillator model including both internal and radiative damping is developed, and is shown to accurately capture the near-and far-field spectral features of linear optical antennas, including their phase response. At wavelengths away from the interband transitions of the metal, we expect the absorption of a plasmonic element to be red-shifted relative to the scattering, and the near-field to be red-shifted relative to both. (C) 2011 Optical Society of America
We present detailed experimental and numerical investigations of resonances in deep nanogroove gratings in metallic substrates. These plasmonic nanocavity gratings feature enhanced fields within the grooves that enable a large enhancement of linear and nonlinear optical processes. This enhancement relies on both localized and propagating surface plasmons on the nanopatterned surface. We show that the efficiency of optical processes such as Raman scattering and four-wave mixing is dramatically enhanced by plasmonic nanocavity gratings. (C) 2011 Optical Society of America
We propose a new class of gratings having multiple spatial frequencies. Their design relies on the use of small aperiodic grating sequences as unit cells whose repetition forms a superlattice. The superlattice provides well-defined Fourier components, while the choice of the unit cell structure enables the selection, modulation or suppression of certain Fourier components. Using these gratings to provide distributed feedback in mid-infrared quantum cascade lasers, we demonstrate simultaneous lasing on multiple well-defined and isolated longitudinal modes, each one having a sidemode suppression ratio of about 20 dB.
Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat's principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.
We propose and demonstrate a novel photonic-plasmonic antenna capable of confining electromagnetic radiation at several mid-infrared wavelengths to a single sub-wavelength spot. The structure relies on the coupling between the localized surface plasmon resonance of a bow-tie nanoantenna with the photonic modes of surrounding multi-periodic particle arrays. Far-field measurements of the transmission through the central bow-tie demonstrate the presence of Fano-like interference effects resulting from the interaction of the bow-tie antenna with the surrounding nanoparticle arrays. The near-field of the multi-wavelength antenna is imaged using an aperture-less near-field scanning optical microscope. This antenna is relevant for the development of near-field probes for nanoimaging, spectroscopy and biosensing. (C) 2011 Optical Society of America
We overview the results of recent experimental and theoretical studies of nonlinear dynamics of mid-infrared quantum cascade lasers (QCLs) associated with nonlinear interactions of laser modes. Particular attention is paid to phase-sensitive nonlinear mode mixing which turns out to be quite prominent in QCLs of different kinds and which gives rise to frequency and phase locking of laser modes. Nonlinear phase coupling of laser modes in QCLs leads to a variety of ultrafast and coherent phenomena: synchronization of transverse modes, beam steering, the RNGH multimode instability, and generation of mode-locked ultrashort pulses.
We investigate the growth conditions for lattice-matched InGaAs/InAlAs/InP quantum cascade lasers (QCLs) by metalorganic chemical vapor deposition (MOCVD). Effect of substrate misorientation, growth temperature, and V/III ratios of InGaAs and InAlAs layers on the surface morphology, optical quality, and impurity incorporation were systematically studied. It was found that epitaxial layers and multiquantum-well structures grown at 720 degrees C with V/Ill ratios of 116 for InGaAs and 21 for InAlAs on InP substrates with an off-cut angle of similar to 0.06 degrees exhibit a stable step-flow growth and low oxygen and carbon contamination. Using these conditions, a similar to 11.3-mu m-thick QCL with an emission wavelength at similar to 9.2 mu m was grown and fabricated, which demonstrated excellent structural quality and operated at room temperature in pulsed mode with a threshold current density of 2.0 kA/cm(2) and a slope efficiency of 550 mW/A. (C) 2010 Elsevier B.V. All rights reserved.
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.
Photoconductivity is studied in individual ZnO nanowires. Under ultraviolet (UV) illumination, the induced photocurrents are observed to persist both in air and in vacuum. Their dependence on UV intensity in air is explained by means of photoinduced surface depletion depth decrease caused by oxygen desorption induced by photogenerated holes. The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase. After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained. Once vacuum is broken and air is let in, the photocurrent quickly decays down to the typical air-photoresponse values. The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity. The adsorption-desorption balance is fully recovered after the ZnO surface is exposed to air, which suggests that under UV illumination, the ZnO surface is actively ``breathing'' oxygen, a process that is further enhanced in nanowires by their high surface to volume ratio.
We report quantum cascade laser (QCL) master-oscillator power-amplifiers (MOPAs) at 300 K reaching output power of 1.5 W for tapered devices and 0.9 W for untapered devices. The devices display single-longitudinal-mode emission at lambda = 7.26 mu m and single-transverse-mode emission at TM00. The maximum amplification factor is 12 dB for the tapered devices. (C) 2011 Optical Society of America
We predict and confirm experimentally the regime of complete synchronization between lateral modes in a quantum cascade laser, when frequency combs belonging to different lateral modes merge into a single comb. The synchronization occurs through the transition from multistability to a single stable state and is accompanied by phase locking and beam steering effects.
We describe the properties of guided modes in metallic parallel plate structures with subwavelength corrugation on the surfaces of both conductors, which we refer to as spoof-insulator-spoof (SIS) waveguides, in close analogy to metal-insulator-metal (MIM) waveguides in plasmonics. A dispersion relation for SIS waveguides is derived, and the modes are shown to arise from the coupling of conventional waveguide modes with the localized modes of the grooves in the SIS structure. SIS waveguides have numerous design parameters and can be engineered to guide modes with very low group velocities and adiabatically convert light between conventional photonic modes and plasmonic ones. (C) 2011 Optical Society of America