Herman Gudjonson, Mikhail A. Kats, Kun Liu, Zhihong Nie, Eugenia Kumacheva, and Federico Capasso. 2/11/2014. “Accounting for inhomogeneous broadening in nano-optics by electromagnetic modeling based on Monte Carlo methods.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 111, 6, Pp. E639-E644. Publisher's VersionAbstract
Many experimental systems consist of large ensembles of uncoupled or weakly interacting elements operating as a single whole; this is particularly the case for applications in nano-optics and plasmonics, including colloidal solutions, plasmonic or dielectric nanoparticles on a substrate, antenna arrays, and others. In such experiments, measurements of the optical spectra of ensembles will differ from measurements of the independent elements as a result of small variations from element to element (also known as polydispersity) even if these elements are designed to be identical. In particular, sharp spectral features arising from narrow-band resonances will tend to appear broader and can even be washed out completely. Here, we explore this effect of inhomogeneous broadening as it occurs in colloidal nanopolymers comprising self-assembled nanorod chains in solution. Using a technique combining finite-difference time-domain simulations and Monte Carlo sampling, we predict the inhomogeneously broadened optical spectra of these colloidal nanopolymers and observe significant qualitative differences compared with the unbroadened spectra. The approach combining an electromagnetic simulation technique with Monte Carlo sampling is widely applicable for quantifying the effects of inhomogeneous broadening in a variety of physical systems, including those with many degrees of freedom that are otherwise computationally intractable.
STEVEN J. BYRNES, Romain Blanchard, and Federico Capasso. 2/3/2014. “Harvesting renewable energy from Earth's mid-infrared emissions.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 111, 11, Pp. 3927-3932. Publisher's VersionAbstract
It is possible to harvest energy from Earth's thermal infrared emission into outer space. We calculate the thermodynamic limit for the amount of power available, and as a case study, we plot how this limit varies daily and seasonally in a location in Oklahoma. We discuss two possible ways to make such an emissive energy harvester (EEH): A thermal EEH (analogous to solar thermal power generation) and an optoelectronic EEH (analogous to photovoltaic power generation). For the latter, we propose using an infrared-frequency rectifying antenna, and we discuss its operating principles, efficiency limits, system design considerations, and possible technological implementations.
Meinrad Sidler, Patrick Rauter, Romain Blanchard, Pauline Metivier, Tobias S. Mansuripur, Christine Wang, Yong Huang, Jae-Hyun Ryou, Russell D. Dupuis, Jerome Faist, and Federico Capasso. 2/3/2014. “Mode switching in a multi-wavelength distributed feedback quantum cascade laser using an external micro-cavity.” APPLIED PHYSICS LETTERS, 104, 5, Pp. 051102. Publisher's VersionAbstract
We demonstrate a multi-wavelength distributed feedback (DFB) quantum cascade laser (QCL) operating in a lensless external micro-cavity and achieve switchable single-mode emission at three distinct wavelengths selected by the DFB grating, each with a side-mode suppression ratio larger than 30 dB. Discrete wavelength tuning is achieved by modulating the feedback experienced by each mode of the multi-wavelength DFB QCL, resulting from a variation of the external cavity length. This method also provides a post-fabrication control of the lasing modes to correct for fabrication inhomogeneities, in particular, related to the cleaved facets position. (C) 2014 AIP Publishing LLC.
David Woolf, Mikhail A. Kats, and Federico Capasso. 2/1/2014. “Spoof surface plasmon waveguide forces.” OPTICS LETTERS, 39, 3, Pp. 517-520. Publisher's VersionAbstract
Spoof surface plasmons (SP) are SP-like waves that propagate along metal surfaces with deeply sub-wavelength corrugations and whose dispersive properties are determined primarily by the corrugation dimensions. Two parallel corrugated surfaces separated by a sub-wavelength dielectric gap create a ``spoof'' analog of the plasmonic metal-insulator-metal waveguides, dubbed a ``spoof-insulator-spoof'' (SIS) waveguide. Here we study the optical forces generated by the propagating ``bonding'' and ``anti-bonding'' waveguide modes of the SIS geometry and the role that surface structuring plays in determining the modal properties. By changing the dimensions of the grooves, strong attractive and repulsive optical forces between the surfaces can be generated at nearly any frequency. (C) 2014 Optical Society of America
Nanfang Yu and Federico Capasso. 1/23/2014. “Flat optics with designer metasurfaces.” NATURE MATERIALS, 13, 2, Pp. 139-150. Publisher's VersionAbstract
Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed `metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.
Vishal Jain, Ali Nowzari, Jesper Wallentin, Magnus T. Borgstrom, Maria E. Messing, Damir Asoli, Mariusz Graczyk, Bernd Witzigmann, Federico Capasso, Lars Samuelson, and Hakan Pettersson. 1/21/2014. “Study of photocurrent generation in InP nanowire-based p(+)-i-n(+) photodetectors.” NANO RESEARCH, 7, 4, Pp. 544-552. Publisher's VersionAbstract
We report on electrical and optical properties of p(+)-i-n(+)photodetectors/solar cells based on square millimeter arrays of InP nanowires (NWs) grown on InP substrates. The study includes a sample series where the p(+)-segment length was varied between 0 and 250 nm, as well as solar cells with 9.3% efficiency with similar design. The electrical data for all devices display clear rectifying behavior with an ideality factor between 1.8 and 2.5 at 300 K. From spectrally resolved photocurrent measurements, we conclude that the photocurrent generation process depends strongly on the p(+)-segment length. Without a p(+)-segment, photogenerated carriers funneled from the substrate into the NWs contribute strongly to the photocurrent. Adding a p(+)-segment decouples the substrate and shifts the depletion region, and collection of photogenerated carriers, to the NWs, in agreement with theoretical modeling. In optimized solar cells, clear spectral signatures of interband transitions in the zinc blende and wurtzite InP layers of the mixed-phase i-segments are observed. Complementary electroluminescence, transmission electron microscopy (TEM), as well as measurements of the dependence of the photocurrent on angle of incidence and polarization, support our interpretations.
Mikhail A. Kats, Romain Blanchard, Shriram Ramanathan, and Federico Capasso. 1/2014. “Thin-Film Interference in Lossy, Ultra-Thin Layers.” Optics and Photonics News 25 (1), Pp. 40-47. Publisher's VersionAbstract
Although much thinner than conventional optical interference coatings, nanometer-thick films made of optically absorbing materials can display strong interference effects. This new class of coatings shows promise for coloring and labeling, optical filters, tunable absorbers and emitters, and energy harvesting.
Yu Yao, Mikhail A. Kats, Raji Shankar, Yi Song, Jing Kong, Marko Loncar, and Federico Capasso. 2014. “Wide Wavelength Tuning of Optical Antennas on Graphene with Nanosecond Response Time.” NANO LETTERS, 14, 1, Pp. 214-219. Publisher's VersionAbstract
Graphene is emerging as a broadband optical material which can be dynamically tuned by electrostatic doping. However, the direct application of graphene sheets in optoelectronic devices is challenging due to graphene's small thickness and the resultant weak interaction with light. By combining metal and graphene in a hybrid plasmonic structure, it is possible to enhance graphene-light interaction and thus achieve in situ control of the optical response. We show that the effective mode index of the bonding plasmonic mode in metal-insulator-metal (MIM) waveguides is particularly sensitive to the change in the optical conductivity of a graphene layer in the gap. By incorporating such MIM structures in optic antenna designs, we demonstrate an electrically tunable coupled antenna array on graphene with a large tuning range (1100 nm, i.e., 250 cm(-1), nearly 20% of the resonance frequency) of the antenna resonance wavelength at the mid-infrared (MIR) region. Our device exhibits a 3 dB cutoff frequency of 30 MHz, which can be further increased into the gigahertz range. This study confirms that hybrid metal-graphene structures are promising elements for high-speed electrically controllable optical and optoelectronic devices.
Nanfang Yu, Mikhail A. Kats, Patrice Genevet, Francesco Aieta, Romain Blanchard, Guillaume Aoust, Zeno Gaburro, and Federico Capasso. 2013. “Controlling Light Propagation with Interfacial Phase Discontinuities.” In ACTIVE PLASMONICS AND TUNEABLE PLASMONIC METAMATERIALS, edited by AV Zayats and SA Maier, Pp. 171-217. OSNEY MEAD, OXFORD OX2 0EL, ENGLAND: BLACKWELL SCIENCE PUBL.
Nanfang Yu, Patrice Genevet, Francesco Aieta, Mikhail A. Kats, Romain Blanchard, Guillaume Aoust, Jean-Philippe Tetienne, Zeno Gaburro, and Federico Capasso. 2013. “Flat Optics: Controlling Wavefronts With Optical Antenna Metasurfaces.” IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 19, 3.Abstract
Conventional optical components rely on the propagation effect to control the phase and polarization of light beams. One can instead exploit abrupt phase and polarization changes associated with scattered light from optical resonators to control light propagation. In this paper, we discuss the optical responses of anisotropic plasmonic antennas and a new class of planar optical components (''metasurfaces'') based on arrays of these antennas. To demonstrate the versatility of metasurfaces, we show the design and experimental realization of a number of flat optical components: 1) metasurfaces with a constant interfacial phase gradient that deflect light into arbitrary directions; 2) metasurfaces with anisotropic optical responses that create light beams of arbitrary polarization over a wide wavelength range; 3) planar lenses and axicons that generate spherical wavefronts and nondiffracting Bessel beams, respectively; and 4) metasurfaces with spiral phase distributions that create optical vortex beams of well-defined orbital angular momentum.
C. A. Wang, A. K. Goyal, S. Menzel, D. R. Calawa, M. Spencer, M. K. Connors, D. McNulty, A. Sanchez, G. W. Turner, and F. Capasso. 2013. “High power ( > 5 W) lambda similar to 9.6 mu m tapered quantum cascade lasers grown by OMVPE.” JOURNAL OF CRYSTAL GROWTH, 370, Pp. 212-216.Abstract
AlInAs/GaInAs superlattices (SLs) with barrier and well layers of various thicknesses were grown by organometallic vapor phase epitaxy to optimize growth of quantum cascade lasers (QCLs). High-resolution x-ray diffraction data of nominally lattice-matched SLs show a systematic shift toward more compressively strained SLs as the barrier/well layer thicknesses are decreased below about 10 nm. This shift is attributed to In surface segregation in both AlInAs and GaInAs. This shift is compensated for in the growth of ultra-thin layers in QCL structures. QCLs with tapered gain regions and emitting at 9.6 mu m are demonstrated with peak power as high as 5.3 W from one facet at 20 degrees C. (C) 2012 Elsevier BY. All rights reserved.
B. Meng, M. Yamanishi, C. Pflügl, K. Fujita, F. Capasso, and Q. J. Wang. 2013. “Investigation of Tunable Single-Mode Quantum Cascade Lasers Via Surface-Acoustic-Wave Modulation.” IEEE Journal of Quantum Electronics, 49, 12, Pp. 1053-1061.Abstract
A novel distributed feedback quantum cascade laser (DFB-QCL) modulated by traveling surface acoustic wave (SAW) is proposed and theoretically studied. The device is based on a highly piezoelectric Zinc Oxide (ZnO) thin film applied directly on top of the QCL to enhance the SAW modulation of the device. To increase the coupling efficiency between the optical lasing mode and the SAW-induced DFB grating, air-waveguide and surface plasmon waveguide structures with two-section active regions are proposed. Simulation results show that a moderate coupling coefficient of   2.5 cm-1 can be achieved for the structures, assuming high quality piezoelectric material and device fabrication can be achieved. The proposed scheme may provide a potentially alternative approach to achieve single-mode, tunable QCLs.
Maria Becker, Wayne Cheng-Wei Huang, Herman Batelaan, Elisabeth J. Smythe, and Federico Capasso. 2013. “Measurement of the ultrafast temporal response of a plasmonic antenna.” ANNALEN DER PHYSIK, 525, 1-2, SI, Pp. L6-L11.Abstract
A measurement on the temporal response of a plasmonic antenna at the femtosecond time scale is reported. The antenna consists of a square array of nanometer-size gold rods. The far-field dispersion of light reflected from the plasmonic antenna is found to be less than that of a 1.2 mm thick glass slide. Assuming a simple oscillating dipole model this implies that the near-field of the antenna may be used as an electron switch that responds faster than 20 fs. Alternatively, ultrafast electron diffraction may be used to investigate the near-field dynamics of the plasmonic antenna.
Jiao Lin, J. P. BALTHASAR MUELLER, Qian Wang, Guanghui Yuan, Nicholas Antoniou, Xiao-Cong Yuan, and Federico Capasso. 2013. “Polarization-Controlled Tunable Directional Coupling of Surface Plasmon Polaritons.” SCIENCE, 340, 6130, Pp. 331-334.Abstract
Light can be coupled into propagating electromagnetic surface waves at a metal-dielectric interface known as surface plasmon polaritons (SPPs). This process has traditionally faced challenges in the polarization sensitivity of the coupling efficiency and in controlling the directionality of the SPPs. We designed and demonstrated plasmonic couplers that overcome these limits using polarization-sensitive apertures in a gold film. Our devices enable polarization-controlled tunable directional coupling with polarization-invariant total conversion efficiency and preserve the incident polarization information. Both bidirectional and unidirectional launching of SPPs are demonstrated. The design is further applied to circular structures that create radially convergent and divergent SPPs, illustrating that this concept can be extended to a broad range of applications.
Francesco Aieta, Patrice Genevet, Mikhail Kats, and Federico Capasso. 2013. “Aberrations of flat lenses and aplanatic metasurfaces.” OPTICS EXPRESS, 21, 25, Pp. 31530-31539.Abstract
A study of optical aberrations for flat lenses based on phase discontinuities is reported. The wave aberration function and the analytical expression of the aberrations up to the 4th order are derived to describe the performance of both ideal and practical flat lenses. We find that aberration-free focusing is possible under axial illumination but off-axis aberrations appear when the excitation is not normal to the interface. An alternative design for an aplanatic metasurface on a curved substrate is proposed to focus light without coma and spherical aberrations. (C) 2013 Optical Society of America
Alejandro W. Rodriguez, M. T. Homer Reid, Jaime Varela, John D. Joannopoulos, Federico Capasso, and Steven G. Johnson. 2013. “Anomalous Near-Field Heat Transfer between a Cylinder and a Perforated Surface.” PHYSICAL REVIEW LETTERS, 110, 1.Abstract
We predict that the near-field radiative heat-transfer rate between a cylinder and a perforated surface depends nonmonotonically on their separation. This anomalous behavior, which arises due to evanescent-wave effects, is explained using a heuristic model based on the interaction of a dipole with a plate. We show that nonmonotonicity depends not only on geometry and temperature but also on material dispersion-for micron and submicron objects, nonmonotonicity is present in polar dielectrics but absent in metals with small skin depths. DOI: 10.1103/PhysRevLett.110.014301
J. P. BALTHASAR MUELLER and Federico Capasso. 2013. “Asymmetric surface plasmon polariton emission by a dipole emitter near a metal surface.” PHYSICAL REVIEW B, 88, 12.Abstract
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.
Yu Yao, Mikhail A. Kats, Patrice Genevet, Nanfang Yu, Yi Song, Jing Kong, and Federico Capasso. 2013. “Broad Electrical Tuning of Graphene-Loaded Plasmonic Antennas.” NANO LETTERS, 13, 3, Pp. 1257-1264.Abstract
Plasmonic antennas enable the conversion of light from free space into subwavelength volumes and vice versa, which facilitates the manipulation of light at the nanoscale. Dynamic control of the properties of antennas is desirable for many applications, including biochemical sensors, reconfigurable meta-surfaces and compact optoelectronic devices. The combination of metallic structures and graphene, which has gate-voltage dependent optical properties, is emerging as a possible platform for electrically controlled plasmonic devices. In this paper, we demonstrate in situ control of antennas using graphene as an electrically tunable load in the nanoscale antenna gap. In our experiments, we demonstrate electrical tuning of graphene-loaded antennas over a broad wavelength range of 650 nm (similar to 140 cm(-1), similar to 10% of the resonance frequency) in the mid infrared (MIR) region. We propose an equivalent circuit model to quantitatively analyze the tuning behavior of graphene-loaded antenna pairs and derive an analytical expression for the tuning range of resonant wavelength. In a separate experiment, we used doubly resonant antenna arrays to achieve MIR optical intensity modulation with maximum modulation depth of more than 30% and bandwidth of 600 nm (similar to 100 cm(-1), 8% of the resonance frequency). This study shows that combining graphene with metallic nanostructures provides a route to electrically tunable optical and optoelectronic devices.
Ali Kabiri, Emad Girgis, and Federico Capasso. 2013. “Buried Nanoantenna Arrays: Versatile Antireflection Coating.” NANO LETTERS, 13, 12, Pp. 6040-6047.Abstract
Reflection is usually a detrimental phenomenon in many applications such as flat-panel-displays, solar cells, photodetectors, infrared sensors, and lenses. Thus far, to control and suppress the reflection from a substrate, numerous techniques including dielectric interference coatings, surface texturing, adiabatic index matching, and scattering from plasmonic nanoparticles have been investigated. A new technique is demonstrated to manage and suppress reflection from lossless and lossy substrates. It provides a wider flexibility in design versus previous methods. Reflection from a surface can be suppressed over a narrowband, wideband, or multiband frequency range. The antireflection can be dependent or independent of the incident wave polarization. Moreover, antireflection at a very wide incidence angle can be attained. The reflection from a substrate is controlled by a buried nanoantenna array, a structure composed of (1) a subwavelength metallic array and (2) a dielectric cover layer referred to as a superstrate. The material properties and thickness of the superstrate and nanoantennas' geometry and periodicity control the phase and intensity of the wave circulating inside the superstrate cavity. A minimum reflectance of 0.02% is achieved in various experiments in the mid-infrared from a silicon substrate. The design can be integrated in straightforward way in optical devices. The proposed structure is a versatile AR coating to optically impedance matches any substrate to free space in selected any narrow and broadband spectral response across the entire visible and infrared spectrum.
Robert Roeder, Marcel Wille, Sebastian Geburt, Jura Rensberg, Mengyao Zhang, Jia Grace Lu, Federico Capasso, Robert Buschinger, Ulf Peschel, and Carsten Ronning. 2013. “Continuous Wave Nanowire Lasing.” NANO LETTERS, 13, 8, Pp. 3602-3606.Abstract
Tin-doped cadmium sulfide nanowires reveal donor-acceptor pair transitions at low-temperature photoluminescence and furthermore exhibit ideal resonator morphology appropriate for lasing at continuous wave pumping. The continuous wave lasing mode is proven by the evolution of the emitted power and spectrum with increasing pump intensity. The high temperature stability up to 120 K at given pumping power is determined by the decreasing optical gain necessary for lasing in an electron-hole plasma.