Publications by Year: 2014

Lulu Liu, Alexander Woolf, Alejandro W. Rodriguez, and Federico Capasso. 12/13/2014. “Absolute position total internal reflection microscopy with an optical tweezer.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 111, 52, Pp. E5609-E5615. Publisher's VersionAbstract
A noninvasive, in situ calibration method for total internal reflection microscopy (TIRM) based on optical tweezing is presented, which greatly expands the capabilities of this technique. We show that by making only simple modifications to the basic TIRM sensing setup and procedure, a probe particle's absolute position relative to a dielectric interface may be known with better than 10 nm precision out to a distance greater than 1 mu m from the surface. This represents an approximate 10x improvement in error and 3x improvement in measurement range over conventional TIRM methods. The technique's advantage is in the direct measurement of the probe particle's scattering intensity vs. height profile in situ, rather than relying on assumptions, inexact system analogs, or detailed knowledge of system parameters for calibration. To demonstrate the improved versatility of the TIRM method in terms of tunability, precision, and range, we show our results for the hindered near-wall diffusion coefficient for a spherical dielectric particle.
Henning Galinski, Antonio Ambrosio, Pasqualino Maddalena, Iwan Schenker, Ralph Spolenak, and Federico Capasso. 12/2/2014. “Instability-induced pattern formation of photoactivated functional polymers.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 111, 48, Pp. 17017-17022. Publisher's VersionAbstract
Since the pioneering work of Turing on the formation principles of animal coat patterns [Turing AM (1952) Phil Trans R Soc Lond B 237(641): 37-72], such as the stripes of a tiger, great effort has been made to understand and explain various phenomena of self-assembly and pattern formation. Prominent examples are the spontaneous demixing in emulsions, such as mixtures of water and oil [Herzig EM, et al. (2007) Nat Mater 6: 966-971]; the distribution of matter in the universe [Kibble TWB (1976) J Phys A: MathGen 9(8): 1387]; surface reconstruction in ionic crystals [Clark KW, et al. (2012) Nanotechnol 23(18): 185306]; and the pattern formation caused by phase transitions in metal alloys, polymer mixtures and binary Bose-Einstein condensates [Sabbatini J, et al. (2011) Phys Rev Lett 107: 230402]. Photoactivated pattern formation in functional polymers has attracted major interest due to its potential applications in molecular electronics and photoresponsive systems. Here we demonstrate that photoactivated pattern formation on azobenzene-containing polymer films can be entirely explained by the physical concept of phase separation. Using experiments and simulations, we show that phase separation is caused by an instability created by the photoactivated transitions between two immiscible states of the polymer. In addition, we have shown in accordance with theory, that polarized light has a striking effect on pattern formation indicated by enhanced phase separation.
Shuyan Zhang, Mikhail A. Kats, Yanjie Cui, You Zhou, Yu Yao, Shriram Ramanathan, and Federico Capasso. 11/25/2014. “Current-modulated optical properties of vanadium dioxide thin films in the phase transition region.” APPLIED PHYSICS LETTERS, 105, 21, Pp. 211104. Publisher's VersionAbstract
Vanadium dioxide (VO2) is a correlated electron material which undergoes an insulator-metal transition proximal to room temperature. The large change of optical properties across this phase transition is promising for tunable optical and optoelectronic devices especially at infrared frequencies. We demonstrate the ability to locally tune the optical properties on the micron scale through a simple design consisting of two electrodes patterned on a VO2 thin film. By current injection between the electrodes, a localized conducting path (metallic phase) can be formed within the insulating background. The width of the conducting path can be controlled by varying the applied current. Fourier transform infrared imaging shows that this current-modulated reflectance changes significantly over a distance on the order of the wavelength in the mid-infrared spectral range. (C) 2014 AIP Publishing LLC.
Patrick Rauter, Jiao Lin, Patrice Genevet, Suraj P. Khanna, Mohammad Lachab, A. Giles Davies, Edmund H. Linfield, and Federico Capasso. 11/18/2014. “Electrically pumped semiconductor laser with monolithic control of circular polarization.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 111, 52, Pp. E5623-E5632. Publisher's VersionAbstract
We demonstrate surface emission of terahertz (THz) frequency radiation from a monolithic quantum cascade laser with built-in control over the degree of circular polarization by `` fishbone'' gratings composed of orthogonally oriented aperture antennas. Different grating concepts for circularly polarized emission are introduced along with the presentation of simulations and experimental results. Fifth-order gratings achieve a degree of circular polarization of up to 86% within a 12 degrees-wide core region of their emission lobes in the far field. For devices based on an alternative transverse grating design, degrees of circular polarization as high as 98% are demonstrated for selected far-field regions of the outcoupled THz radiation and within a collection half-angle of about 6 degrees. Potential and limitations of integrated antenna gratings for polarization-controlled emission are discussed.
Yu Yao, Raji Shankar, Mikhail A. Kats, Yi Song, Jing Kong, Marko Loncar, and Federico Capasso. 10/13/2014. “Electrically Tunable Metasurface Perfect Absorbers for Ultrathin Mid-Infrared Optical Modulators.” NANO LETTERS, 14, 11, Pp. 6526-6532. Publisher's VersionAbstract
Dynamically reconfigurable metasurfaces open up unprecedented opportunities in applications such as high capacity communications, dynamic beam shaping, hyperspectral imaging, and adaptive optics. The realization of high performance metasurface-based devices remains a great challenge due to very limited tuning ranges and modulation depths. Here we show that a widely tunable metasurface composed of optical antennas on graphene can be incorporated into a subwavelength-thick optical cavity to create an electrically tunable perfect absorber. By switching the absorber in and out of the critical coupling condition via the gate voltage applied on graphene, a modulation depth of up to 100% can be achieved. In particular, we demonstrated ultrathin (thickness < lambda(0)/10) high speed (up to 20 GHz) optical modulators over a broad wavelength range (5-7 mu m). The operating wavelength can be scaled from the near-infrared to the terahertz by simply tailoring the metasurface and cavity dimensions.
Mikhail A. Kats and Federico Capasso. 9/30/2014. “Ultra-thin optical interference coatings on rough and flexible substrates.” APPLIED PHYSICS LETTERS, 105, 13, Pp. 131108. Publisher's VersionAbstract
Recently demonstrated ultra-thin optical coatings comprising nanometer-thick highly absorbing films on top of reflecting substrates can display strong optical interference effects, resulting in structural colors and absorption enhancement. Here, we demonstrate that these optical interference effects persist when the films are deposited on substrates that have a large degree of roughness and inhomogeneity on micro- and nano-scales. In particular, we deposited films of gold and amorphous germanium onto paper which serves as a rough and flexible substrate and observe matte interference colors that vary as a function of the germanium thickness. (C) 2014 AIP Publishing LLC.
J. P. BALTHASAR MUELLER, KRISTJAN LEOSSON, and Federico Capasso. 8/25/2014. “Polarization-Selective Coupling to Long-Range Surface Plasmon Polariton Waveguides.” NANO LETTERS, 14, 10, Pp. 5524-5527. Publisher's VersionAbstract
We demonstrate polarization-selective coupling from an optical fiber to long-range surface plasmon polariton waveguide modes using plasmonic antenna arrays. The arrays allow the sorting of two distinct (not necessarily orthogonal) polarizations to counter-propagating waveguide modes. The polarization-selective behavior of the devices is described by a compact formalism based on Stokes vectors that offers a clear graphical representation of the response. We experimentally observe polarization-controlled switching and unidirectional coupling with extinction ratios greater than 30 dB and coupling efficiencies comparable to those of a conventional grating coupler.
Wondwosen Metaferia, Bouzid Simozrag, Carl Junesand, Yan-Ting Sun, Mathieu Carras, Romain Blanchard, Federico Capasso, and Sebastian Lourdudoss. 8/14/2014. “Demonstration of a quick process to achieve buried heterostructure quantum cascade laser leading to high power and wall plug efficiency.” OPTICAL ENGINEERING, 53, 8, Pp. 087104. Publisher's VersionAbstract
Together with the optimal basic design, buried heterostructure quantum cascade laser (BH-QCL) with semi-insulating regrowth offers a unique possibility to achieve an effective thermal dissipation and lateral single mode. We demonstrate here the realization of BH-QCLs with a single-step regrowth of highly resistive (>1 x 10(8) ohm . cm) semi-insulating InP:Fe in <45 min for the first time in a flexible hydride vapor phase epitaxy process for burying ridges etched down to 10 to 15 mu m depth, both with and without mask overhang. The fabricated BH-QCLs emitting at similar to 4.7 and similar to 5.5 mu m were characterized. 2-mm-long 5.5-mu m lasers with a ridge width of 17 to 22 mu m, regrown with mask overhang, exhibited no leakage current. Large width and high doping in the structure did not permit high current density for continuous wave (CW) operation. 5-mm-long 4.7-mu m BH-QCLs of ridge widths varying from 6 to 14 mu m regrown without mask overhang, besides being spatially monomode, TM00, exhibited wall plug efficiency (WPE) of similar to 8 to 9% with an output power of 1.5 to 2.5 W at room temperature and under CW operation. Thus, we demonstrate a quick, flexible, and single-step regrowth process with good planarization for realizing buried QCLs leading to monomode, high power, and high WPE. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
Yu Yao, Raji Shankar, Patrick Rauter, Yi Song, Jing Kong, Marko Loncar, and Federico Capasso. 6/18/2014. “High-Responsivity Mid-Infrared Graphene Detectors with Antenna-Enhanced Photocarrier Generation and Collection.” NANO LETTERS, 14, 7, Pp. 3749-3754. Publisher's VersionAbstract
Graphene is an attractive photoconductive material for optical detection due to its broad absorption spectrum and ultrashort response time. However, it remains a great challenge to achieve high responsivity in graphene detectors because of graphene's weak optical absorption (only 2.3% in the monolayer graphene sheet) and short photocarrier lifetime (<1 ps). Here we show that metallic antenna structures can be designed to simultaneously improve both light absorption and photocarrier collection in graphene detectors. The coupled antennas concentrate free space light into the nanoscale deep-subwavelength antenna gaps, where the graphene light interaction is greatly enhanced as a result of the ultrahigh electric field intensity inside the gap. Meanwhile, the metallic antennas are designed to serve as electrodes that collect the generated photocarriers very efficiently. We also elucidate the mechanism of photoconductive gain in the graphene detectors and demonstrate mid-infrared (mid-IR) antenna-assisted graphene detectors at room temperature with more than 200 times enhancement of responsivity (similar to 0.4 V/W at lambda(0) = 4.45 mu m) compared to devices without antennas (<2 mV/W).
Anish Goyal, Travis Myers, Christine A. Wang, Michael Kelly, Brian Tyrrell, B. Gokden, Antonio Sanchez, George Turner, and Federico Capasso. 6/4/2014. “Active hyperspectral imaging using a quantum cascade laser (QCL) array and digital-pixel focal plane array (DFPA) camera.” OPTICS EXPRESS, 22, 12, Pp. 14392-14401. Publisher's VersionAbstract
We demonstrate active hyperspectral imaging using a quantum-cascade laser (QCL) array as the illumination source and a digital-pixel focal-plane-array (DFPA) camera as the receiver. The multi-wavelength QCL array used in this work comprises 15 individually addressable QCLs in which the beams from all lasers are spatially overlapped using wavelength beam combining (WBC). The DFPA camera was configured to integrate the laser light reflected from the sample and to perform on-chip subtraction of the passive thermal background. A 27-frame hyperspectral image was acquired of a liquid contaminant on a diffuse gold surface at a range of 5 meters. The measured spectral reflectance closely matches the calculated reflectance. Furthermore, the high-speed capabilities of the system were demonstrated by capturing differential reflectance images of sand and KClO3 particles that were moving at speeds of up to 10 m/s. (C) 2014 Optical Society of America
Patrice Genevet and Federico Capasso. 2/25/2014. “Breakthroughs in Photonics 2013: Flat Optics: Wavefronts Control With Huygens' Interfaces.” IEEE PHOTONICS JOURNAL, 6, 2, Pp. 0700404. Publisher's VersionAbstract
Recent progress in the fields of nanophotonics and metamaterials has enabled the development of ultrathin and flat optical components, providing physicists and optical engineers a new method to control light. According to the Huygens-Fresnel principle, light gradually propagates step by step by exciting secondary waves that then reradiate to form the next wavefront; the phase and amplitude of these secondary waves are intimately related to the incoming optical wavefront. By using the response of nanoengineered subwavelength optical resonators at interfaces, it is now possible to engineer Huygens' interfaces to achieve an unprecedented control of the wavefront over large bandwidths and subwavelength propagation distances.
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.