Publications

2011
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
2011_kats_et_al_optics_express.pdf
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
2011_genevet_et_al_applied_optics.pdf
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.
2011_blanchard_et_al_new_journal_physics.pdf
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.
2011_yu_et_al._science.pdf
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
2011_blanchard_et_al_optics_express.pdf
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.
2011_wojcik_et_al.pdf
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.
2011_huang_et_al.pdf
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.
2011_lipomi_et_al_nanoletters.pdf
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.
2011_bao_et_al_nano_express.pdf
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
2011_menzel_et_al_optics_express.pdf
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.
2011_wojcik_et_al_physical_review_letters.pdf
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
2011_kats_et_al_optics_express_spoof.pdf
2010
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.
physrevlett.105.060401.pdf
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
oe-18-2-746.pdf
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.
nmat2822.pdf
V. -M. Gkortsas, C. Wang, L. Kuznetsova, L. Diehl, A. Gordon, C. Jirauschek, M. A. Belkin, A. Belyanin, F. Capasso, and F. X. Kaertner. 2010. “Dynamics of actively mode-locked Quantum Cascade Lasers.” OPTICS EXPRESS, 18, 13, Pp. 13616-13630.Abstract
The impact of upper state lifetime and spatial hole burning on pulse shape and stability in actively mode locked QCLs is investigated by numerical simulations. It is shown that an extended upper state lifetime is necessary to achieve stable isolated pulse formation per roundtrip. Spatial hole burning helps to reduce the pulse duration by supporting broadband multimode lasing, but introduces pulse instabilities which eventually lead to strongly structured pulse shapes that further degrade with increased pumping. At high pumping levels gain saturation and recovery between pulses leads to suppression of mode locking. In the absence of spatial hole burning the laser approaches single-mode lasing, while in the presence of spatial hole burning the mode locking becomes unstable and the laser dynamics does not reach a steady state anymore. (C) 2010 Optical Society America
oe-18-13-13616.pdf
Martina Hentschel, Qijie Wang, Changling Yan, Federico Capasso, Tadataka Edamura, and Hirofumi Kan. 2010. “Emission properties of electrically pumped triangular shaped microlasers.” OPTICS EXPRESS, 18, 16, Pp. 16437-16442.Abstract
We study the emission properties of electrically pumped triangular-shaped microlasers with rounded corners. We find no signs of directional emission for the relatively large cavities (dimension similar to 100 mu m) used in our experiments, in full agreement with ray simulation results. The broad emission characteristics that we observe can be fine-tuned by adjusting the resonator geometry as is verified through simulations which might prove useful for applications in optical devices. (C) 2010 Optical Society of America
oe-18-16-16437.pdf
Darren J. Lipomi, Mikhail A. Kats, Philseok Kim, Sung H. Kang, Joanna Aizenberg, Federico Capasso, and George M. Whitesides. 2010. “Fabrication and Replication of Arrays of Single- or Multicomponent Nanostructures by Replica Molding and Mechanical Sectioning.” ACS NANO, 4, 7, Pp. 4017-4026.Abstract
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 mu m.
nn100993t.pdf
J. Britt Lassiter, Heidar Sobhani, Jonathan A. Fan, Janardan Kundu, Federico Capasso, Peter Nordlander, and Naomi J. Halas. 2010. “Fano Resonances in Plasmonic Nanoclusters: Geometrical and Chemical Tunability.” NANO LETTERS, 10, 8, Pp. 3184-3189.Abstract
Clusters of plasmonic nanoparticles and nanostructures support Fano resonances. Here we show that this spectral feature, produced by the interference between bright and dark modes of the nanoparticle cluster, is strongly dependent upon both geometry and local dielectric environment. This permits a highly sensitive tunability of the Fano dip in both wavelength and amplitude by varying cluster dimensions, geometry, and relative size of the individual nanocluster components. Plasmonic nanoclusters show an unprecedented sensitivity to dielectric environment with a local surface plasmon resonance figure of merit of 5.7. the highest yet reported for localized surface plasmon resonance sensing in a finite nanostructure.
nl102108u.pdf
Jonathan A. Fan, Kui Bao, Chihhui Wu, Jiming Bao, Rizia Bardhan, Naomi J. Halas, Vinothan N. Manoharan, Gennady Shvets, Peter Nordlander, and Federico Capasso. 2010. “Fano-like Interference in Self-Assembled Plasmonic Quadrumer Clusters.” NANO LETTERS, 10, 11, Pp. 4680-4685.Abstract
Assemblies of strongly interacting metallic nanoparticles are the basis for plasmonic nanostructure engineering. We demonstrate that clusters of four identical spherical particles self-assembled into a close-packed asymmetric quadrumer support strong Fano-like interference. This feature is highly sensitive to the polarization of the incident electric held due to orientation-dependent coupling between particles in the cluster. This structure demonstrates how careful design of self-assembled colloidal systems can lead to the creation of new plasmonic modes and the enabling of interference effects in plasmonic systems.
nl1029732.pdf

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