Robert W. Adams, Karun Vijayraghavan, Qijie Wang, Jonathan Fan, Federico Capasso, Suraj P. Khanna, A. Giles Davies, Edmund H. Linfield, and Mikhail A. Belkin. 2010. “GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers with double-phonon resonant depopulation operating up to 172 K.” APPLIED PHYSICS LETTERS, 97, 13.Abstract
We report the design and performance of GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers with double-phonon resonant depopulation and a vertical lasing transition. Devices were processed into gold-clad double-metal waveguides. Lasing at 3 THz was observed up to a heat-sink temperature of 172 K, which compares favorably with the performance of single-phonon resonant depopulation devices based on vertical lasing transitions. These results demonstrate that terahertz quantum cascade lasers based on double-phonon depopulation designs may be a viable alternative to single-phonon depopulation designs for achieving high-temperature operation. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3496035]
Markus Geiser, Christian Pfluegl, Alexey Belyanin, Qijie Wang, Nanfang Yu, Tadanaka Edamura, Masamichi Yamanishi, Hirofumi Kan, Milan Fischer, Andreas Wittmann, Jerome Faist, and Federico Capasso. 2010. “Gain competition in dual wavelength quantum cascade lasers.” OPTICS EXPRESS, 18, 10, Pp. 9900-9908.Abstract
We investigated dual wavelength mid-infrared quantum cascade lasers based on heterogeneous cascades. We found that due to gain competition laser action tends to start in higher order lateral modes. The mid-infrared mode with the lower threshold current reduces population inversion for the second laser with the higher threshold current due to stimulated emission. We developed a rate equation model to quantitatively describe mode interactions due to mutual gain depletion. (C) 2010 Optical Society of America
A. Lyakh, R. Maulini, A. Tsekoun, R. Go, S. Von der Porten, C. Pfluegl, L. Diehl, Federico Capasso, and C. Kumar N. Patel. 2010. “High-performance continuous-wave room temperature 4.0-mu m quantum cascade lasers with single-facet optical emission exceeding 2 W.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 107, 44, Pp. 18799-18802.Abstract
A strain-balanced, AlInAs/InGaAs/InP quantum cascade laser structure, designed for light emission at 4.0 mu m using nonresonant extraction design approach, was grown by molecular beam epitaxy. Laser devices were processed in buried heterostructure geometry. An air-cooled laser system incorporating a 10-mm x 11.5-mu m laser with antireflection-coated front facet and high-reflection-coated back facet delivered over 2 W of single-ended optical power in a collimated beam. Maximum continuous-wave room temperature wall plug efficiency of 5.0% was demonstrated for a high-reflection-coated 3.65-mm x 8.7-mu m laser mounted on an aluminum nitride submount.
Federico Capasso. 2010. “High-performance midinfrared quantum cascade lasers.” OPTICAL ENGINEERING, 49, 11.Abstract
The design and operating principles of quantum cascade lasers (QCLs) are reviewed along with recent developments in high-power cw and broadband devices. Cw power levels of several watts at room temperature have been achieved at 4.6-mu m wavelength; broadband single-mode tuning (approximate to 400 cm(-1)) has been achieved using an external-cavity QCL with a grating as a tuning element. An alternative approach, consisting of a monolithically integrated array of single-mode QCLs individually currentdriven by a microcontroller, has led to broadband single-mode tuning over a range of 200 cm(-1) without requiring the use of moving parts. This spectrometer on a chip holds promise for high-brightness compact trace-gas sensors capable of operating in harsh environments. (C) 2010 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3505844]
Mark W. Knight, Jonathan Fan, Federico Capasso, and Naomi J. Halas. 2010. “Influence of excitation and collection geometry on the dark field spectra of individual plasmonic nanostructures.” OPTICS EXPRESS, 18, 3, Pp. 2579-2587.Abstract
Dark field microspectroscopy is the primary method for the study of plasmon modes of individual metallic nanostructures. Light from a plasmonic nanostructure typically scatters with a strong angular and modal dependence, resulting in significant variations in the observed spectral response depending on excitation and collection angle and polarization of incident light. Here we examine how spectrally dependent radiation patterns arising from an individual plasmonic nanoparticle, positioned on a dielectric substrate, affect the detection of its plasmon modes. Careful consideration of excitation and collection geometry is of critical concern in quantitative studies of the optical response of these nanoparticle systems. (C) 2010 Optical Society of America
Patrice Genevet, Jean-Philippe Tetienne, Evangelos Gatzogiannis, Romain Blanchard, Mikhail A. Kats, Marlan O. Scully, and Federico Capasso. 2010. “Large Enhancement of Nonlinear Optical Phenomena by Plasmonic Nanocavity Gratings.” NANO LETTERS, 10, 12, Pp. 4880-4883.Abstract
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 nanocavtiy 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 approximate to 2000 2 orders of magnitude higher than that previously reported
A. L. Rakhmanov, V. A. Yampol'skii, J. A. Fan, Federico Capasso, and Franco Nori. 2010. “Layered superconductors as negative-refractive-index metamaterials.” PHYSICAL REVIEW B, 81, 7.Abstract
We analyze the use of layered superconductors as strongly anisotropic metamaterials, which can possess negative-refractive-index in a wide frequency range. Superconductors are of particular interest because they have the potential to support low losses, which is critical for applications such as super-resolution imaging. We show that low-T(c) (s-wave) superconductors can be used to construct layered heterostructures with low losses for T << T(c). However, the real part of their in-plane effective permittivity is very large, making coupling into the structure difficult. Moreover, even at low temperatures, layered high-T(c) superconductors have a large in-plane normal conductivity, producing large losses (due to d-wave symmetry). Therefore, it is difficult to enhance the evanescent modes in either low-T(c) or high-T(c) superconductors.
Aleksander K. Wojcik, Nanfang Yu, Laurent Diehl, Federico Capasso, and Alexey Belyanin. 2010. “Nonlinear coupling of transverse modes in quantum cascade lasers.” OPTICAL ENGINEERING, 49, 11.Abstract
We study the multimode operation regimes of midinfrared quantum cascade lasers (QCLs), taking into account nonlinear phase-sensitive interactions between transverse modes. We show the possibility of the coherent coupling of several transverse modes, which results in a number of interesting effects including frequency and phase locking between transverse modes, bistability, and beam steering. We present an analytical model for the modal dynamics and its numerical analysis. Effects of amplitude and phase fluctuations on the modal stability are explored. The theoretical results are in agreement with our experimental measurements of buried heterostructure QCLs. (C) 2010 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3498773]
Aleksander K. Wojcik, Nanfang Yu, Laurent Diehl, Federico Capasso, and Alexey Belyanin. 2010. “Nonlinear dynamics of coupled transverse modes in quantum cascade lasers.” JOURNAL OF MODERN OPTICS, 57, 19, SI, Pp. 1892-1899.Abstract
We analyze the dynamics of broad-area mid-infrared quantum cascade lasers (QCLs). We show the possibility of the coherent coupling of several transverse modes which results in several interesting effects including frequency and phase locking between transverse modes, bistability, and beam steering. We present an analytical model for the modal dynamics and its numerical analysis. Effects of amplitude and phase fluctuations on the modal stability are explored. We compare our theoretical results with our experimental measurements of buried heterostructure QCLs.
Alejandro W. Rodriguez, Alexander P. McCauley, David Woolf, Federico Capasso, J.D. Joannopoulos, and Steven G. Johnson. 2010. “Nontouching Nanoparticle Diclusters Bound by Repulsive and Attractive Casimir Forces.” PHYSICAL REVIEW LETTERS, 104, 16.Abstract
We present a scheme for obtaining stable Casimir suspension of dielectric nontouching objects immersed in a fluid, validated here in various geometries consisting of ethanol-separated dielectric spheres and semi-infinite slabs. Stability is induced by the dispersion properties of real dielectric (monolithic) materials. A consequence of this effect is the possibility of stable configurations (clusters) of compact objects, which we illustrate via a molecular two-sphere dicluster geometry consisting of two bound spheres levitated above a gold slab. Our calculations also reveal a strong interplay between material and geometric dispersion, and this is exemplified by the qualitatively different stability behavior observed in planar versus spherical geometries.
Mariano A. Zimmler, Federico Capasso, Sven Mueller, and Carsten Ronning. 2010. “Optically pumped nanowire lasers: invited review.” SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 25, 2, SI.Abstract
This paper reviews several topics related to optically pumped ZnO nanowire lasers. A systematic study of the various properties of a device as it evolves from the regime of amplified spontaneous emission to laser oscillation above threshold is presented. The key dependence of the laser threshold on nanowire diameter is demonstrated and explained by means of a thorough study of guided modes in semiconducting nanowires for a nanowire-on-substrate geometry. A `head on' detection geometry is used to measure the far-field profile of a nanowire laser and thus identify the modes responsible for lasing. Finally, the temperature behavior of a nanowire laser is reported, and possible mechanisms that may be responsible for gain are suggested.
Nanfang Yu, Romain Blanchard, Jonathan Fan, Qijie Wang, Christian Pfluegl, Laurent Diehl, Tadataka Edamura, Shinichi Furuta, Masamichi Yamanishi, Hirofumi Kan, and Federico Capasso. 2010. “Plasmonics for Laser Beam Shaping.” IEEE TRANSACTIONS ON NANOTECHNOLOGY, 9, 1, Pp. 11-29.Abstract
This paper reviews our recent work on laser beam shaping using plasmonics. We demonstrated that by integrating properly designed plasmonic structures onto the facet of semiconductor lasers, their divergence angle can be dramatically reduced by more than one orders of magnitude, down to a few degrees. A plasmonic collimator consisting of a slit aperture and an adjacent 1-D grating can collimate laser light in the laser polarization direction; a collimator consisting of a rectangular aperture and a concentric ring grating can reduce the beam divergence both perpendicular and parallel to the laser polarization direction, thus achieving collimation in the plane perpendicular to the laser beam. The devices integrated with plasmonic collimators preserve good room-temperature performance with output power comparable to that of the original unpatterned lasers. A collimator design for one wavelength can be scaled to adapt to other wavelengths ranging from the visible to the far-IR regimes. Plasmonic collimation offers a compact and integrated solution to the problem of laser beam collimation and may have a large impact on applications such as free-space optical communication, pointing, and light detection and ranging. This paper opens up major opportunities in wavefront engineering using plasmonic structures.
Zhihong Liu, Jiandong Huang, Pooran C. Joshi, Apostolos T. Voutsas, John Hartzell, Federico Capasso, and Jiming Bao. 2010. “Polarity-controlled visible/infrared electroluminescence in Si-nanocrystal/Si light-emitting devices.” APPLIED PHYSICS LETTERS, 97, 7.Abstract
We report the demonstration of a room-temperature visible/infrared color-switchable light-emitting device comprising a Si nanocrystal-embedded silicon oxide thin film on a p-type Si substrate. The device emits band-edge infrared light from the silicon substrate when the substrate is positively (forward) biased with respect to the Si-nanocrystal film. Under reverse bias, visible emission from the Si-nanocrystal film is observed. Compared to the photoluminescence of the Si-nanocrystal film, the visible electroluminescence is broader and blueshifted to shorter wavelength, and is ascribed to impact ionization in the Si-nanocrystal/SiO2 film. (C) 2010 American Institute of Physics. [doi:10.1063/1.3480403]
Fedenco Capasso. 2010. “Q & A Professor Federico Capasso Harvard University.” ELECTRONICS LETTERS, S, Pp. S51. 05692942.pdf
Robert F. Curl, Federico Capasso, Claire Gmachl, Anatoliy A. Kosterev, Barry McManus, Rafal Lewicki, Michael Pusharsky, Gerard Wysocki, and Frank K. Tittel. 2010. “Quantum cascade lasers in chemical physics.” CHEMICAL PHYSICS LETTERS, 487, 1-3, Pp. 1-18.Abstract
In the short space of 15 years since their first demonstration, quantum cascade lasers have become the most useful sources of tunable mid-infrared laser radiation. This Letter describes these developments in laser technology and the burgeoning applications of quantum cascade lasers to infrared spectroscopy. We foresee the potential application of quantum cascade lasers in other areas of chemical physics such as research on helium droplets, in population pumping, and in matrix isolation infrared photochemistry. (C) 2010 Elsevier B.V. All rights reserved.
By engineering the boundary conditions of electromagnetic fields between material interfaces, one can dramatically change the Casimir-Lifshitz force between surfaces as a result of the modified zero-point energy density of the system. Repulsive interactions between macroscopic bodies occur when their dielectric responses obey a particular inequality, as pointed out by Dzyaloshinskii, Lifshitz, and Pitaevskii. We discuss experimental verification of this behavior as well as a description of how this can be used to develop a scheme for quantum levitation. Based on these concepts, we discuss the possible development of a new class of devices based on ultra-low static friction and the ability to sort objects based on their dielectric functions.
K. J. Russell, F. Capasso, V. Narayanamurti, H. Lu, J. M. O. Zide, and A. C. Gossard. 2010. “Scattering-assisted tunneling: Energy dependence, magnetic field dependence, and use as an external probe of two-dimensional transport.” PHYSICAL REVIEW B, 82, 11.Abstract
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.
Jonathan A. Fan, Chihhui Wu, Kui Bao, Jiming Bao, Rizia Bardhan, Naomi J. Halas, Vinothan N. Manoharan, Peter Nordlander, Gennady Shvets, and Federico Capasso. 2010. “Self-Assembled Plasmonic Nanoparticle Clusters.” SCIENCE, 328, 5982, Pp. 1135-1138.Abstract
The self-assembly of colloids is an alternative to top-down processing that enables the fabrication of nanostructures. We show that self-assembled clusters of metal-dielectric spheres are the basis for nanophotonic structures. By tailoring the number and position of spheres in close-packed clusters, plasmon modes exhibiting strong magnetic and Fano-like resonances emerge. The use of identical spheres simplifies cluster assembly and facilitates the fabrication of highly symmetric structures. Dielectric spacers are used to tailor the interparticle spacing in these clusters to be approximately 2 nanometers. These types of chemically synthesized nanoparticle clusters can be generalized to other two- and three-dimensional structures and can serve as building blocks for new metamaterials.
Joerg Heber and Federico Capasso. 2010. “The staircase to flexibility.” NATURE MATERIALS, 9, 5, Pp. 374-375. nmat2758.pdf
N. YU, Q. J. Wang, M. A. Kats, J. A. Fan, F. Capasso, S. P. Khanna, L. Li, A.G. Davies, and E. H. Linfield. 2010. “Terahertz plasmonics.” ELECTRONICS LETTERS, 46, 26, S, Pp. S52-S57.Abstract
Semiconductor microstructures can be used to tailor the dispersion properties of surface plasmon polaritons in the terahertz (THz) frequency range, and therefore can be used as important building blocks for terahertz optical devices. The physical principles of three structures are discussed: plasmonic second-order gratings, designer (spoof) surface plasmon polariton structures, and channel polariton structures. The effectiveness of these structures is demonstrated by utilising them to improve power throughput and to reduce the beam divergence of edge-emitting THz quantum cascade lasers. Plasmonics promises compact and low-loss solutions for manipulating light at THz wavelengths, and will have a large impact on applications such as imaging, light detection and ranging (LIDAR), and the heterodyne detection of chemicals.