Alejandro W. Rodriguez, Federico Capasso, and Steven G. Johnson. 2011. “The Casimir effect in microstructured geometries.” Nature Photonics, 5, 4, Pp. 211-221.Abstract
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.
Alejandro W. Rodriguez, David Woolf, Pui-Chuen Hui, Eiji Iwase, Alexander P. McCauley, Federico Capasso, Marko Loncar, and Steven G. Johnson. 2011. “Designing evanescent optical interactions to control the expression of Casimir forces in optomechanical structures.” Applied Physics Letters, 98, 19.Abstract
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]
J-P Tetienne, R. Blanchard, N. YU, P. Genevet, M. A. Kats, J. A. Fan, T. Edamura, S. Furuta, M. Yamanishi, and F. Capasso. 2011. “Dipolar modeling and experimental demonstration of multi-beam plasmonic collimators.” New Journal of Physics, 13.Abstract
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.
Anish K. Goyal, Melissa Spencer, Oleg Shatrovoy, Benjamin G. Lee, Laurent Diehl, Christian Pfluegl, Antonio Sanchez, and Federico Capasso. 2011. “Dispersion-compensated wavelength beam combining of quantum-cascade-laser arrays.” Optics Express, 19, 27, Pp. 26725-26732.Abstract
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
Jonathan A. Fan, Yu He, Kui Bao, Chihhui Wu, Jiming Bao, Nicholas B. Schade, Vinothan N. Manoharan, Gennady Shvets, Peter Nordlander, David R. Liu, and Federico Capasso. 2011. “DNA-Enabled Self-Assembly of Plasmonic Nanoclusters.” Nano Letters, 11, 11, Pp. 4859-4864.Abstract
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.
Mikhail A. Kats, Nanfang Yu, Patrice Genevet, Zeno Gaburro, and Federico Capasso. 2011. “Effect of radiation damping on the spectral response of plasmonic components.” Optics Express, 19, 22, Pp. 21748-21753.Abstract
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
Patrice Genevet, Jean-Philippe Tetienne, Romain Blanchard, Mikhail A. Kats, J. P. BALTHASAR MUELLER, Marlan O. Scully, and Federico Capasso. 2011. “Enhancement of optical processes in coupled plasmonic nanocavities [Invited].” Applied Optics, 50, 31, Pp. G56-G62.Abstract
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
R. Blanchard, S. Menzel, C. Pfluegl, L. Diehl, C. Wang, Y. Huang, J-H Ryou, R. D. Dupuis, L. Dal Negro, and F. Capasso. 2011. “Gratings with an aperiodic basis: single-mode emission in multi-wavelength lasers.” New Journal of Physics, 13.Abstract
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.
Nanfang Yu, Patrice Genevet, Mikhail A. Kats, Francesco Aieta, Jean-Philippe Tetienne, Federico Capasso, and Zeno Gaburro. 2011. “Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction.” Science, 334, 6054, Pp. 333-337. Publisher's VersionAbstract
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.
Romain Blanchard, Svetlana V. Boriskina, Patrice Genevet, Mikhail A. Kats, Jean-Philippe Tetienne, Nanfang Yu, Marlan O. Scully, Luca Dal Negro, and Federico Capasso. 2011. “Multi-wavelength mid-infrared plasmonic antennas with single nanoscale focal point.” Optics Express, 19, 22, Pp. 22113-22124.Abstract
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
Aleksander K. Wojcik, Nanfang Yu, Federico Capasso, and Alexey Belyanin. 2011. “Nonlinear optical interactions of laser modes in quantum cascade lasers.” Journal of Modern Optics, 58, 9, Pp. 727-742. Publisher's VersionAbstract
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.
Yong Huang, Jae-Hyun Ryou, Russell D. Dupuis, Christian Pfluegl, Federico Capasso, Kewei Sun, Alec M. Fischer, and Fernando A. Ponce. 2011. “Optimization of growth conditions for InGaAs/InAlAs/InP quantum cascade lasers by metalorganic chemical vapor deposition.” Journal of Crystal Growth, 316, 1, Pp. 75-80. Publisher's VersionAbstract
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.
Darren J. Lipomi, Ramses V. Martinez, Mikhail A. Kats, Sung H. Kang, Philseok Kim, Joanna Aizenberg, Federico Capasso, and George M. Whitesides. 2011. “Patterning the Tips of Optical Fibers with Metallic Nanostructures Using Nanoskiving.” Nano Letters, 11, 2, Pp. 632-636.Abstract
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.
Jiming Bao, Ilan Shalish, Zhihua Su, Ron Gurwitz, Federico Capasso, Xiaowei Wang, and Zhifeng Ren. 2011. “Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires.” Nanoscale Research Letters, 6.Abstract
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.
Stefan Menzel, Laurent Diehl, Christian Pfluegl, Anish Goyal, Christine Wang, Antonio Sanchez, George Turner, and Federico Capasso. 2011. “Quantum cascade laser master-oscillator power-amplifier with 1.5 W output power at 300 K.” Optics Express, 19, 17, Pp. 16229-16235.Abstract
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
Aleksander K. Wojcik, Nanfang Yu, Laurent Diehl, Federico Capasso, and Alexey Belyanin. 2011. “Self-Synchronization of Laser Modes and Multistability in Quantum Cascade Lasers.” Physical Review Letters, 106, 13.Abstract
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.
Mikhail A. Kats, David Woolf, Romain Blanchard, Nanfang Yu, and Federico Capasso. 2011. “Spoof plasmon analogue of metal-insulator-metal waveguides.” Optics Express, 19, 16, Pp. 14860-14870.Abstract
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
Alejandro W. Rodriguez, David Woolf, Alexander P. McCauley, Federico Capasso, John D. Joannopoulos, and Steven G. Johnson. 2010. “Achieving a Strongly Temperature-Dependent Casimir Effect.” PHYSICAL REVIEW LETTERS, 105, 6.Abstract
We propose a method of achieving large temperature T sensitivity in the Casimir force that involves measuring the stable separation between dielectric objects immersed in a fluid. We study the Casimir force between slabs and spheres using realistic material models, and find large >2 nm/K variations in their stable separations (hundreds of nanometers) near room temperature. In addition, we analyze the effects of Brownian motion on suspended objects, and show that the average separation is also sensitive to changes in T. Finally, this approach also leads to rich qualitative phenomena, such as irreversible transitions, from suspension to stiction, as T is varied.
Christian Pfluegl, Laurent Diehl, Arkadiy Lyakh, Qijie Wang, Richard Maulini, Alexei Tsekoun, C. Kumar N. Patel, Xiaojun Wang, and Federico Capasso. 2010. “Activation energy study of electron transport in high performance short wavelengths quantum cascade lasers.” OPTICS EXPRESS, 18, 2, Pp. 746-753.Abstract
We present a method to study current paths through quantum cascade lasers (QCLs). The temperature dependence of the current is measured at a fixed voltage. At low temperatures we find activation energies that correspond to the energy difference between the injector ground state and the upper laser level. At higher temperatures additional paths with larger activation energies are found. Application of this method to high performance QCLs based on strained InGaAs/InAlAs quantum wells and barriers with different band-offsets allows us to identify individual parasitic current paths through the devices. The results give insight into the transport properties of quantum cascade lasers thus providing a useful tool for device optimization. (C)2010 Optical Society of America
Nanfang Yu, Qijie Wang, Mikhail A. Kats, Jonathan A. Fan, Suraj P. Khanna, Lianhe Li, A. Giles Davies, Edmund H. Linfield, and Federico Capasso. 2010. “Designer spoof surface plasmon structures collimate terahertz laser beams.” NATURE MATERIALS, 9, 9, Pp. 730-735.Abstract
Surface plasmons have found a broad range of applications in photonic devices at visible and near-infrared wavelengths. In contrast, longer-wavelength surface electromagnetic waves, known as Sommerfeld or Zenneck waves(1,2), are characterized by poor confinement to surface and are therefore difficult to control using conventional metallo-dielectric plasmonic structures. However, patterning the surface with subwavelength periodic features can markedly reduce the asymptotic surface plasmon frequency, leading to `spoof' surface plasmons(3,4) with subwavelength confinement at infrared wavelength and beyond, which mimic surface plasmons at much shorter wavelength. We demonstrate that by directly sculpting designer spoof surface plasmon structures that tailor the dispersion of terahertz surface plasmon polaritons on the highly doped semiconductor facets of terahertz quantum cascade lasers, the performance of the lasers can be markedly enhanced. Using a simple one-dimensional grating design, the beam divergence of the lasers was reduced from similar to 180 degrees to similar to 10 degrees, the directivity was improved by over 10 decibels and the power collection efficiency was increased by a factor of about six compared with the original unpatterened devices. We achieve these improvements without compromising high-temperature performance of the laser.