Publications

2008
Ertugrul Cubukcu, Nanfang Yu, Elizabeth J. Smythe, Laurent Diehl, Kenneth B. Crozier, and Federico Capasso. 2008. “Plasmonic Laser Antennas and Related Devices.” IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 14, 6, Pp. 1448-1461.Abstract
This paper reviews recent work on device applications of optical antennas. Localized surface plasmon resonances of gold nanorod antennas resting on a silica glass substrate were modeled by finite difference time-domain simulations. A single gold nanorod of length 150 or 550 nm resonantly generates enhanced near fields when illuminated with light of 830 nm wavelength. A pair of these nanorods gives higher field enhancements due to capacitive coupling between them. Bowtie antennas that consist of a pair of triangular gold particles offer the best near-field confinement and enhancement. Plasmonic laser antennas based on the coupled nanorod antenna design were fabricated by focused ion beam lithography on the facet of a semiconductor laser diode operating at a wavelength of 830 nm. An optical spot size of few tens of nanometers was measured by apertureless near-field optical microscope. We have extended our work on plasmonic antenna into mid-infrared (mid-IR) wavelengths by implementing resonant nanorod and bowtie antennas on the facets of various quantum cascade lasers. Experiments show that this mid-IR device can provide an optical intensity confinement 70 times higher than that would be achieved with diffraction limited optics. Near-field intensities similar to 1 GW / cm(2) were estimated for both near-infrared and mid-IR plasmonic antennas. A fiber device that takes advantage of plasmonic resonances of gold nanorod arrays providing a high density of optical ``hot spots'' is proposed. Results of a systematic theoretical and experimental study of the reflection spectra of these arrays fabricated on a silica glass substrate are also presented. The family of these proof-of-concept plasmonic devices that we present here can be potentially useful in many applications including near-field optical microscopes, high-density optical data storage, surface enhanced Raman spectroscopy, heat-assisted magnetic recording, and spatially resolved absorption spectroscopy.
A. Lyakh, C. Pfluegl, L. Diehl, Q. J. Wang, Federico Capasso, X. J. Wang, J. Y. Fan, T. Tanbun-Ek, R. Maulini, A. Tsekoun, R. Go, and C. Kumar N. Patel. 2008. “1.6 W high wall plug efficiency, continuous-wave room temperature quantum cascade laser emitting at 4.6 mu m.” APPLIED PHYSICS LETTERS, 92, 11.Abstract
A strain-balanced, InP-based quantum cascade laser structure designed for light emission at 4.6 mu m was grown by metal-organic chemical vapor deposition. A maximum total optical power of 1.6 W was obtained in continuous-wave mode at 300 K for uncoated devices processed in buried heterostructure geometry with stripe dimensions of 5 mm by 9.5 mu m. Corresponding maximum wall plug efficiency and threshold current density were measured to be 8.8% and 1.05 kA/cm(2), respectively. Fully hermetically packaged laser of identical dimensions produced in excess of 1.5 W under the same conditions. (C) 2008 American Institute of Physics.
1.2899630.pdf
Michael D. Dickey, Emily A. Weiss, Elizabeth J. Smythe, Ryan C. Chiechi, Federico Capasso, and George M. Whitesides. 2008. “Fabrication of arrays of metal and metal oxide nanotubes by shadow evaporation.” ACS NANO, 2, 4, Pp. 800-808.Abstract
This paper describes a simple technique for fabricating uniform arrays of metal and metal oxide nanotubes with controlled heights and diameters. The technique involves depositing material onto an anodized aluminum oxide (AAO) membrane template using a collimated electron beam evaporation source. The evaporating material enters the porous openings of the AAO membrane and deposits onto the walls of the pores. The membrane is tilted with respect to the column of evaporating material, so the shadows cast by the openings of the pores onto the inside walls of the pores define the geometry of the tubes. Rotation of the membrane during evaporation ensures uniform deposition inside the pores. After evaporation, dissolution of the AAO in base easily removes the template to yield an array of nanotubes connected by a thin backing of the same metal or metal oxide. The diameter of the pores dictates the diameter of the tubes, and the incident angle of evaporation determines the height of the tubes. Tubes up to similar to 1.5 mu m in height and 20-200 nm in diameter were fabricated. This method is adaptable to any material that can be vapor-deposited, including indium-tin oxide (ITO), a conductive, transparent material that is useful for many opto-electronic applications. An array of gold nanotubes produced by this technique served as a substrate for surface-enhanced Raman spectroscopy: the Raman signal (per molecule) from a monolayer of benzenethiolate was a factor of similar to 5 x 105 greater than that obtained using bulk liquid benzenethiol.
nn800036r.pdf
Benjamin J. Wiley, Darren J. Lipomi, Jiming Bao, Federico Capasso, and George M. Whitesides. 2008. “Fabrication of surface plasmon resonators by nanoskiving single-crystalline gold microplates.” NANO LETTERS, 8, 9, Pp. 3023-3028.Abstract
This paper demonstrates the sectioning of chemically synthesized, single-crystalline microplates of gold with an ultramicrotome (nanoskiving) to produce single-crystalline nanowires; these nanowires act as low-loss surface plasmon resonators. This method produces collinearly aligned nanostructures with small, regular changes in dimension with each consecutive cross-section: a single microplate thus can produce a number of ``quasi-copies'' (delicately modulated variations) of a nanowire. The diamond knife cuts cleanly through microplates 35 mu m in diameter and 100 nm thick without bending the resulting nanowire and cuts through the sharp edges of a crystal without deformation to generate nanoscale tips. This paper compares the influence of sharp tips and blunt tips on the resonator modes in these nanowires.
nl802252r.pdf
Hyunyong Choi, Theodore B. Norris, Tobias Gresch, Marcella Giovannini, Jerome Faist, Laurent Diehl, and Federico Capasso. 2008. “Femtosecond dynamics of resonant tunneling and superlattice relaxation in quantum cascade lasers.” APPLIED PHYSICS LETTERS, 92, 12.Abstract
Time-resolved mid-infrared pump-probe measurements are performed on a quantum cascade laser below and above the threshold. The gain recovery is determined by the electron transport through the cascade heterostructure. Subpicosecond resonant tunneling injection from the injector ground state into the upper lasing state is found to be incoherent due to the strong dephasing in the active subband. The gain recovery due to transport through superlattice is interpreted in terms of dielectric relaxation within the superlattice miniband. (C) 2008 American Institute of Physics.
1.2898518.pdf
Hyunyong Choi, Laurent Diehl, Zong-Kwei Wu, Marcella Giovannini, Jerome Faist, Federico Capasso, and Theodore B. Norris. 2008. “Gain recovery dynamics and photon-driven transport in quantum cascade lasers.” PHYSICAL REVIEW LETTERS, 100, 16.Abstract
Quantum cascade lasers are semiconductor devices based on the interplay of perpendicular transport through the heterostructure and the intracavity lasing field. We employ femtosecond time-resolved pump-probe measurements to investigate the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery is determined by the time-dependent transport of electrons through both the active regions and the superlattice regions connecting them. As the laser approaches and exceeds threshold, the component of the gain recovery due to the nonzero lifetime of the upper lasing state in the active region shows a dramatic reduction due to the onset of quantum stimulated emission; the drift of the electrons is thus driven by the cavity photon density. The gain recovery is qualitatively different from that in conventional lasers due to the superlattice transport in the cascade.
physrevlett.100.167401.pdf
Mariano A. Zimmler, Jiming Bao, Federico Capasso, Sven Mueller, and Carsten Ronning. 2008. “Laser action in nanowires: Observation of the transition from amplified spontaneous emission to laser oscillation.” APPLIED PHYSICS LETTERS, 93, 5.Abstract
Direct evidence of the transition from amplified spontaneous emission to laser action in optically pumped zinc oxide (ZnO) nanowires, at room temperature, is presented. The optical power evolves from a superlinear to a linear regime as the pump power exceeds threshold, concomitant with a transition to directional emission along the nanowire and the emergence of well defined cavity Fabry-Perot modes around a wavelength of approximate to 385 nm, the intensity of which exceeds the spontaneous emission background by orders of magnitude. The laser oscillation threshold is found to be strongly dependent on nanowire diameter, with no laser oscillation observed for diameters smaller than similar to 150 nm. Finally, we use an alternative ``head on'' detection geometry to measure the output power of a single nanowire laser. (c) 2008 American Institute of Physics.
1.2965797.pdf
J. N. Munday, Federico Capasso, V. Adrian Parsegian, and Sergey M. Bezrukov. 2008. “Measurements of the Casimir-Lifshitz force in fluids: The effect of electrostatic forces and Debye screening.” PHYSICAL REVIEW A, 78, 3.Abstract
We present detailed measurements of the Casimir-Lifshitz force between two gold surfaces (a sphere and a plate) immersed in ethanol and study the effect of residual electrostatic forces, which are dominated by static fields within the apparatus and can be reduced with proper shielding. Electrostatic forces are further reduced by Debye screening through the addition of salt ions to the liquid. Additionally, the salt leads to a reduction of the Casimir-Lifshitz force by screening the zero-frequency contribution to the force; however, the effect is small between gold surfaces at the measured separations and within experimental error. An improved calibration procedure is described and compared with previous methods. Finally, the experimental results are compared with Lifshitz's theory and found to be consistent for the materials used in the experiment.
physreva.78.032109.pdf
Ariel Gordon, Christine Y. Wang, L. Diehl, F. X. Kaertner, A. Belyanin, D. Bour, S. Corzine, G. Hoefler, H. C. Liu, H. Schneider, T. Maier, M. TROCCOLI, J. Faist, and Federico Capasso. 2008. “Multimode regimes in quantum cascade lasers: From coherent instabilities to spatial hole burning.” PHYSICAL REVIEW A, 77, 5, B.Abstract
A theoretical and experimental study of multimode operation regimes in quantum cascade lasers (QCLs) is presented. It is shown that the fast gain recovery of QCLs promotes two multimode regimes: One is spatial hole burning (SHB) and the other one is related to the Risken-Nummedal-Graham-Haken instability predicted in the 1960s. A model that can account for coherent phenomena, a saturable absorber, and SHB is developed and studied in detail both analytically and numerically. A wide variety of experimental data on multimode regimes is presented. Lasers with a narrow active region and/or with metal coating on the sides tend to develop a splitting in the spectrum, approximately equal to twice the Rabi frequency. It is proposed that this behavior stems from the presence of a saturable absorber, which can result from a Kerr lensing effect in the cavity. Lasers with a wide active region, which have a weaker saturable absorber, do not exhibit a Rabi splitting and their multimode regime is governed by SHB. This experimental phenomenology is well-explained by our theoretical model. The temperature dependence of the multimode regime is also presented.
physreva.77.053804.pdf
Jiming Bao, David C. Bell, Federico Capasso, Jakob B. Wagner, Thomas Martensson, Johanna Tragardh, and Lars Samuelson. 2008. “Optical properties of rotationally twinned InP nanowire heterostructures.” NANO LETTERS, 8, 3, Pp. 836-841.Abstract
We have developed a technique so that both transmission electron microscopy and microphotoluminescence can be performed on the same semiconductor nanowire over a large range of optical power, thus allowing us to directly correlate structural and optical properties of rotationally twinned zinc blende InP nanowires. We have constructed the energy band diagram of the resulting multiquantum well heterostructure and have performed detailed quantum mechanical calculations of the electron and hole wave functions. The excitation power dependent blue-shift of the photoluminescence can be explained in terms of the predicted staggered band alignment of the rotationally twinned zinc blende/wurzite InP heterostructure and of the concomitant diagonal transitions between localized electron and hole states responsible for radiative recombination. The ability of rotational twinning to introduce a heterostructure in a chemically homogeneous nanowire material and alter in a major way its optical properties opens new possibilities for band-structure engineering.
nl072921e.pdf
Mark B. Romanowsky and Federico Capasso. 2008. “Orientation-dependent Casimir force arising from highly anisotropic crystals: Application to Bi2Sr2CaCu2O8+delta.” PHYSICAL REVIEW A, 78, 4.Abstract
We calculate the Casimir interaction between parallel planar crystals of Au and the anisotropic cuprate superconductor Bi2Sr2CaCu2O8+delta (BSCCO), with BSCCO's optical axis either parallel or perpendicular to the crystal surface, using suitable generalizations of the Lifshitz theory. We find that the strong anisotropy of the BSCCO permittivity gives rise to a difference in the Casimir force between the two orientations of the optical axis, which depends on distance and is of order 10-20 % at the experimentally accessible separations 10 to 5000 nm.
physreva.78.042110.pdf
Nanfang Yu, Romain Blanchard, Jonathan Fan, Qijie Wang, Christian Pfluegl, Laurent Diehl, Tadataka Edamura, Masamichi Yamanishi, Hirofumi Kan, and Federico Capasso. 2008. “Quantum cascade lasers with integrated plasmonic antenna-array collimators.” OPTICS EXPRESS, 16, 24, Pp. 19447-19461.Abstract
We demonstrated in simulations and experiments that by defining a properly designed two-dimensional metallic aperture-grating structure on the facet of quantum cascade lasers, a small beam divergence angle can be achieved in directions both perpendicular and parallel to the laser waveguide layers (denoted as theta(perpendicular to) and theta(parallel to), respectively). Beam divergence angles as small as theta(perpendicular to)=2.7 degrees and theta(parallel to)=3.7 degrees have been demonstrated. This is a reduction by a factor of similar to 30 and similar to 10, respectively, compared to those of the original lasers emitting at a wavelength of 8.06 mu m. The devices preserve good room temperature performance with output power as high as similar to 55% of that of the original unpatterned lasers. We studied in detail the trade-off between beam divergence and power throughput for the fabricated devices. We demonstrated plasmonic collimation for buried heterostructure lasers and ridge lasers; devices with different waveguide structures but with the same plasmonic collimator design showed similar performance. We also studied a device patterned with a ``spider's web'' pattern, which gives us insight into the distribution of surface plasmons on the laser facet. (C) 2008 Optical Society of America
oe-16-24-19447.pdf
Mikhail A. Belkin, Federico Capasso, Feng Xie, Alexey Belyanin, Milan Fischer, Andreas Wittmann, and Jerome Faist. 2008. “Room temperature terahertz quantum cascade laser source based on intracavity difference-frequency generation.” APPLIED PHYSICS LETTERS, 92, 20.Abstract
We report on our progress in the development of a terahertz quantum cascade laser source based on intracavity terahertz difference-frequency mixing in a dual-wavelength mid-infrared quantum cascade laser with the active region engineered to possess giant second-order nonlinear susceptibility. In this letter, we demonstrate devices that operate in mid-infrared at lambda(1)=8.9 mu m and lambda(2)=10.5 mu m and produce terahertz output at lambda approximate to 60 mu m via difference-frequency generation with 7 mu W output power at 80 K, 1 mu W output at 250 K, and still approximately 300 nW output at 300 K. (c) 2008 American Institute of Physics.
1.2919051.pdf
Mariano A. Zimmler, Daniel Stichtenoth, Carsten Ronning, Wei Yi, Venkatesh Narayanamurti, Tobias Voss, and Federico Capasso. 2008. “Scalable fabrication of nanowire photonic and electronic circuits using spin-on glass.” NANO LETTERS, 8, 6, Pp. 1695-1699.Abstract
We present a method which can be used for the mass-fabrication of nanowire photonic and electronic devices based on spin-on glass technology and on the photolithographic definition of independent electrical contacts to the top and the bottom of a nanowire. This method allows for the fabrication of nanowire devices in a reliable, fast, and low cost way, and it can be applied to nanowires with arbitrary cross section and doping type (p and n). We demonstrate this technique by fabricating single-nanowire p-Si(substrate)-n-ZnO(nanowire) heterojunction diodes, which show good rectification properties and, furthermore, which function as ultraviolet light-emitting diodes.
nl080627w.pdf
Nanfang Yu, Romain Blanchard, Jonathan Fan, Federico Capasso, Tadataka Edamura, Masamichi Yamanishi, and Hirofumi Kan. 2008. “Small divergence edge-emitting semiconductor lasers with two-dimensional plasmonic collimators.” APPLIED PHYSICS LETTERS, 93, 18.Abstract
Using quantum cascade lasers with a two-dimensional metallic aperture-grating structure defined on the facet the authors demonstrate a collimated laser beam with small divergence angle perpendicular and parallel to the laser waveguide layers (2.7 degrees and 3.7 degrees, respectively). These values represent a reduction by a factor of similar to 30 and similar to 10, respectively, compared to those of the original 8.06-mu m- wavelength laser without plasmonic collimation. The devices preserve good room temperature performance with output power as high as 53% of that of the original unpatterned lasers.
1.3009599.pdf
Nanfang Yu, Jonathan Fan, Qijie Wang, Christian Pfluegl, Laurent Diehl, Tadataka Edamura, Masamichi Yamanishi, Hirofumi Kan, and Federico Capasso. 2008. “Small-divergence semiconductor lasers by plasmonic collimation.” NATURE PHOTONICS, 2, 9, Pp. 564-570.Abstract
Surface plasmons offer the exciting possibility of improving the functionality of optical devices through the subwavelength manipulation of light. We show that surface plasmons can be used to shape the beams of edge- emitting semiconductor lasers and greatly reduce their large intrinsic beam divergence. Using quantum cascade lasers as a model system, we show that by defining a metallic subwavelength slit and a grating on their facet, a small beam divergence in the laser polarization direction can be achieved. Divergence angles as small as 2.4 degrees are obtained, representing a reduction in beam spread by a factor of 25 compared with the original 9.9-mu m-wavelength laser used. Despite having a patterned facet, our collimated lasers do not suffer significant reductions in output power (similar to 100 mW at room temperature). Plasmonic collimation provides a means of efficiently coupling the output of a variety of lasers into optical fibres and waveguides, or to collimate them for applications such as free-space communications, ranging and metrology.
nphoton.2008.152.pdf
Alejandro W. Rodriguez, J. N. Munday, J.D. Joannopoulos, Federico Capasso, Diego A. R. Dalvit, and Steven G. Johnson. 2008. “Stable Suspension and Dispersion-Induced Transitions from Repulsive Casimir Forces Between Fluid-Separated Eccentric Cylinders.” PHYSICAL REVIEW LETTERS, 101, 19.Abstract
We numerically demonstrate a stable mechanical suspension of a silica cylinder within a metallic cylinder separated by ethanol, via a repulsive Casimir force between the silica and the metal. We investigate cylinders with both circular and square cross sections, and show that the latter exhibit a stable orientation as well as a stable position, via a method to compute Casimir torques for finite objects. Furthermore, the stable orientation of the square cylinder undergoes a 45 degrees transition as the separation length scale is varied, which is explained as a consequence of material dispersion.
physrevlett.101.190404.pdf
Christian Pfluegl, Mikhail A. Belkin, Qijie Wang, Markus Geiser, Alexey Belyanin, Milan Fischer, Andreas Wittmann, Jerome Faist, and Federico Capasso. 2008. “Surface-emitting terahertz quantum cascade laser source based on intracavity difference-frequency generation.” APPLIED PHYSICS LETTERS, 93, 16.Abstract
We report a surface-emitting terahertz source based on intracavity difference-frequency generation in dual-wavelength midinfrared quantum cascade lasers with integrated giant second-order nonlinear susceptibility. The terahertz light is coupled out of the waveguide by a second-order grating etched into the laser ridges. In contrast to sources where the difference-frequency radiation is extracted from the facet, this approach enables extraction of the terahertz emission from the whole length of the device even when the coherence length is small. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3009198]
1.3009198.pdf
D.C. Bell, J. Bao, F. Capasso, J Wagner, T Martensson, J Trägårdh, and L Samuelson. 2008. “TEM Correlation between the Structural and Optical Properties Of Rotationally Twinned InP Nanowires.” Microscopy and Microanalysis, 14, S2, Pp. 414–415. tem_correlation_between_the_structural_and_optical_properties_of_rotationally_twinned_inp_nanowires.pdf
Mikhail A. Belkin, Jonathan A. Fan, Sahand Hormoz, Federico Capasso, Suraj P. Khanna, Mohamed Lachab, A. Giles Davies, and Edmund H. Linfield. 2008. “Terahertz quantum cascade lasers with copper metal-metal waveguides operating up to 178 K.” OPTICS EXPRESS, 16, 5, Pp. 3242-3248.Abstract
We report terahertz quantum cascade lasers operating in pulsed mode at an emission frequency of 3 THz and up to a maximum temperature of 178 K. The improvement in the maximum operating temperature is achieved by using a three-quantum-well active region design with resonant-phonon depopulation and by utilizing copper, instead of gold, for the cladding material in the metal-metal waveguides. (c) 2008 Optical Society of America.
oe-16-5-3242.pdf

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