Publications

2009
J. N. Munday, Federico Capasso, and V. Adrian Parsegian. 2009. “Measured long-range repulsive Casimir-Lifshitz forces.” NATURE, 457, 7226, Pp. 170-173.Abstract
Quantum fluctuations create intermolecular forces that pervade macroscopic bodies(1-3). At molecular separations of a few nanometres or less, these interactions are the familiar van der Waals forces(4). However, as recognized in the theories of Casimir, Polder and Lifshitz(5-7), at larger distances and between macroscopic condensed media they reveal retardation effects associated with the finite speed of light. Although these long- range forces exist within all matter, only attractive interactions have so far been measured between material bodies(8-11). Here we show experimentally that, in accord with theoretical prediction(12), the sign of the force can be changed from attractive to repulsive by suitable choice of interacting materials immersed in a fluid. The measured repulsive interaction is found to be weaker than the attractive. However, in both cases the magnitude of the force increases with decreasing surface separation. Repulsive Casimir - Lifshitz forces could allow quantum levitation of objects in a fluid and lead to a new class of switchable nanoscale devices with ultra-low static friction(13-15).
2009_Munday_et_al_Nature
Christine Y. Wang, Lyuba Kuznetsova, V. M. Gkortsas, L. Diehl, F. X. Kaertner, M. A. Belkin, A. Belyanin, X. Li, D. Ham, H. Schneider, P. Grant, C. Y. Song, S. Haffouz, Z. R. Wasilewski, H. C. Liu, and Federico Capasso. 2009. “Mode-locked pulses from mid-infrared Quantum Cascade Lasers.” OPTICS EXPRESS, 17, 15, Pp. 12929-12943.Abstract
In this study, we report the unequivocal demonstration of mid-infrared mode-locked pulses from quantum cascade lasers. The train of short pulses was generated by actively modulating the current and hence the gain of an edge-emitting quantum cascade laser (QCL). Pulses with duration of about 3 ps at full-width-at-half-maxima and energy of 0.5 pJ were characterized using a second-order interferometric autocorrelation technique based on a nonlinear quantum well infrared photodetector. The mode-locking dynamics in the QCLs was modeled based on the Maxwell-Bloch equations in an open two-level system. Our model reproduces the overall shape of the measured autocorrelation traces and predicts that the short pulses are accompanied by substantial wings as a result of strong spatial hole burning. The range of parameters where short mode-locked pulses can be formed is found. (C) 2009 Optical Society of America
2009_Wang_et_al_Optics_Express.pdf
Nanfang Yu, Mikhail A. Kats, Christian Pfluegl, Markus Geiser, Qijie Wang, Mikhail A. Belkin, Federico Capasso, Milan Fischer, Andreas Wittmann, Jerome Faist, Tadataka Edamura, Shinichi Furuta, Masamichi Yamanishi, and Hirofumi Kan. 2009. “Multi-beam multi-wavelength semiconductor lasers (vol 95, 161108, 2009).” APPLIED PHYSICS LETTERS, 95, 23. 2009_yu_et_al_apl.pdf
Jiming Bao, David C. Bell, Federico Capasso, Natasha Erdman, Dongguang Wei, Linus Froberg, Thomas Martensson, and Lars Samuelson. 2009. “Nanowire-induced Wurtzite InAs Thin Film on Zinc-Blende InAs Substrate.” ADVANCED MATERIALS, 21, 36, Pp. 3654+.Abstract
InAs pyramids and platelets on a zinc-blende InAs substrate are found to exhibit a wurtzite crystal structure. induced by wurtzite InAs nanowires, wurtzite InAs thin film and its associated zinc-blende/wurtzite heterocrystalline heterostructures may open up new opportunities in band-gap engineering and related device applications.
2009_bao_et_al_advancedmaterials.pdf
Elizabeth J. Smythe, Michael D. Dickey, Jiming Bao, George M. Whitesides, and Federico Capasso. 2009. “Optical Antenna Arrays on a Fiber Facet for in Situ Surface-Enhanced Raman Scattering Detection.” NANO LETTERS, 9, 3, Pp. 1132-1138.Abstract
This paper reports a bidirectional fiber optic probe for the detection of surface-enhanced Raman scattering (SERS). One facet of the probe features an array of gold optical antennas designed to enhance Raman signals, while the other facet of the fiber is used for the input and collection of light. Simultaneous detection of benzenethiol and 2-[(E)-2-pyridin-4-ylethenyl]pyridine is demonstrated through a 35 cm long fiber. The array of nanoscale optical antennas was first defined by electron-beam lithography on a silicon wafer. The array was subsequently stripped from the wafer and then transferred to the facet of a fiber. Lithographic definition of the antennas provides a method for producing two-dimensional arrays with well-defined geometry, which allows (i) the optical response of the probe to be tuned and (I!) the density of ``hot spots'' generating the enhanced Raman signal to be controlled. It is difficult to determine the Raman signal enhancement factor (EF) of most fiber optic Raman sensors featuring hot spots because the geometry of the Raman enhancing nanostructures is poorly defined. The ability to control the size and spacing of the antennas enables the EF of the transferred array to be estimated. EF values estimated after focusing a laser directly onto the transferred array ranged from 2.6 x 10(5) to 5.1 x 10(5).
2009_smythe_et_al_nanoletters.pdf
Ertugrul Cubukcu and Federico Capasso. 2009. “Optical nanorod antennas as dispersive one-dimensional Fabry-Peacuterot resonators for surface plasmons.” APPLIED PHYSICS LETTERS, 95, 20.Abstract
Resonant optical nanoantennas exhibit a different length scaling due to the surface plasmons compared to their radio frequency counterparts. In this letter, we address this difference by calculating the wavelength-dependent effective mode index n(eff) for a cylindrical one-dimensional gold nanowire waveguide. Our results show that nanorod optical antennas act as dispersive and lossy Fabry-Peacuterot resonators for surface plasmons.
2009_cubukcu_et_al_apl.pdf
Nanfang Yu, Qijie Wang, Christian Pflugl, Laurent Diehl, Federico Capasso, Tadataka Edamura, Shinichi Furuta, Masamichi Yamanishi, and Hirofumi Kan. 2009. “Semiconductor lasers with integrated plasmonic polarizers.” APPLIED PHYSICS LETTERS, 94, 15.Abstract
The authors reported the plasmonic control of semiconductor laser polarization by means of metallic gratings and subwavelength apertures patterned on the laser emission facet. An integrated plasmonic polarizer can project the polarization of a semiconductor laser onto other directions. By designing a facet with two orthogonal grating-aperture structures, a polarization state consisting of a superposition of a linearly and right-circularly polarized light was demonstrated in a quantum cascade laser; a first step toward a circularly polarized laser.
2009_yu_et_al_apl_01.pdf
Elizabeth J. Smythe, Michael D. Dickey, George M. Whitesides, and Federico Capasso. 2009. “A Technique to Transfer Metallic Nanoscale Patterns to Small and Non-Planar Surfaces.” ACS NANO, 3, 1, Pp. 59-65.Abstract
Conventional lithographic methods (e.g., electron-beam lithography, photolithography) are capable of producing high-resolution structures over large areas but are generally limited to large (> 1 cm(2)) planar substrates. Incorporation of these features on unconventional substrates (i.e., small (< 1 mm(2)) and/or non-planar substrates) would open possibilities for many applications, including remote fiber-based sensing, nanoscale optical lithography, three-dimensional fabrication, and integration of compact optical elements on fiber and semiconductor lasers. Here we introduce a simple method in which a thin thiol-ene film strips arbitrary nanoscale metallic features from one substrate and is then transferred, along with the attached features, to a substrate that would be difficult or impossible to pattern with conventional lithographic techniques. An oxygen plasma removes the sacrificial film, leaving behind the metallic features. The transfer of dense and sparse patterns of isolated and connected gold features ranging from 30 nm to 1 mu m, to both an optical fiber facet and a silica microsphere, demonstrates the versatility of the method. A distinguishing feature of this technique is the us? of a thin, sacrificial film to strip and transfer metallic nanopatterns and its ability to directly transfer metallic structures produced by conventional lithography.
2009_smytheet_al_acs_nano.pdf
Daniel Recht, Federico Capasso, and Michael J. Aziz. 2009. “On the temperature dependence of point-defect-mediated luminescence in silicon.” APPLIED PHYSICS LETTERS, 94, 25.Abstract
We present a model of the temperature dependence of point-defect-mediated luminescence in silicon derived from basic kinetics and semiconductor physics and based on the kinetics of bound exciton formation. The model provides a good fit to data for W line electroluminescence and G line photoluminescence in silicon. Strategies are discussed for extending luminescence to room temperature.
2009_recht_et_al_apl.pdf
Hyunyong Choi, Laurent Diehl, Zong-Kwei Wu, Marcella Giovannini, Jerome Faist, Federico Capasso, and Theodore B. Norris. 2009. “Time-Resolved Investigations of Electronic Transport Dynamics in Quantum Cascade Lasers Based on Diagonal Lasing Transition.” IEEE JOURNAL OF QUANTUM ELECTRONICS, 45, 4, Pp. 307-321.Abstract
In this study, the nature of electronic transport in quantum cascade lasers (QCLs) has been extensively investigated using an ultrafast time-resolved, degenerate, pump-probe optical technique. Our investigations enable a comprehensive understanding of the gain recovery dynamics in terms of a coupling of the electronic transport to the oscillating intracavity laser intensity. In QCLs that have a lasing transition diagonal in real space, studies of the near-threshold reveal that the transport of electrons changes bias region from phonon-limited relaxation (tens of picoseconds) below threshold to photon-driven transport via stimulated emission (a few picoseconds) above threshold. The gain recovery dynamics in the photon-driven regime is compared with conventional four-level lasers such as atomic, molecular, and semiconductor interband lasers. The depopulation dynamics out of the lower lasing state is explained using a tight-binding tunneling model and phonon-limited relaxation. For the superlattice relaxation, it is possible to explain the characteristic picosecond transport via dielectric relaxation; Monte Carlo simulations with a simple resistor model are developed, and the Esaki-Tsu model is applied. Subpicosecond dynamics due to carrier heating in the upper subband are isolated and appear to be at most about 10% of the gain compression compared with the contribution of stimulated emission. Finally, the polarization anisotropy in the active waveguide is experimentally shown to be negligible on our pump-probe data, supporting our interpretation of data in terms of gain recovery and transport.
2009_choi_et_al_ieee_jqe.pdf
R. Maulini, I. Dunayevskiy, A. Lyakh, A. Tsekoun, C. K. N. Patel, L. Diehl, C. Pfluegl, and F. Capasso. 2009. “Widely tunable high-power external cavity quantum cascade laser operating in continuous-wave at room temperature.” ELECTRONICS LETTERS, 45, 2, Pp. 107-U30.Abstract
A grating-coupled external cavity quantum cascade laser operating in continuous-wave at room temperature is reported. Single-frequency operation tunable over more than 160 cm(-1) around the centre wave-length of 4.6 mu m has been observed at a chip temperature of 300 K. The maximum optical power at the gain peak was 300 mW, corresponding to a wall-plug efficiency of 6%. Observed power output at the gain bandwidth edges was in excess of 125 mW.
2009_maulini_et_al_electronics_letters.pdf
2008
Benjamin G. Lee, Mikhail A. Belkin, and Federico Capasso. 2008. “Array of tiny quantum cascade lasers provides tunable mid-IR output.” PHOTONICS SPECTRA, 42, 5, Pp. 70+.
Qijie Wang, Christian Pfluegl, William F. Andress, Donhee Ham, Federico Capasso, and Masamichi Yamanishi. 2008. “Gigahertz surface acoustic wave generation on ZnO thin films deposited by radio frequency magnetron sputtering on III-V semiconductor substrates.” JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B, 26, 6, Pp. 1848-1851.Abstract
The authors demonstrate 1.6 GHz surface acoustic wave (SAW) generation using interdigital transducers patterned by e-beam lithography on a thin ZnO piezoelectric film deposited on an InP substrate. The highly oriented, dense, and fine-grain ZnO film with high resistivity was deposited by radio frequency magnetron sputtering and was characterized by x-ray diffraction, scanning electron microscopy, atomic force microscopy, and a four-point probe station. The acoustic wavelength of the 1.6 GHz SAW generated by exciting the interdigital transducer on ZnO/InP with a microwave signal is 1.6 mu m. This SAW filter device could be monolithically integrated with optoelectronic devices, opening new opportunities to use SAWs for applications such as gigahertz-frequency filters on optoelectronic devices and novel widely tunable quantum cascade lasers.
Mariano Troccoli, Laurent Diehl, David P. Bour, Scott W. Corzine, Nanfang Yu, Christine Y. Wang, Mikhail A. Belkin, Gloria Hoefler, Rafal Lewicki, Gerard Wysocki, Frank K. Tittel, and Federico Capasso. 2008. “High-Performance Quantum Cascade Lasers Grown by Metal-Organic Vapor Phase Epitaxy and Their Applications to Trace Gas Sensing.” JOURNAL OF LIGHTWAVE TECHNOLOGY, 26, 21-24, Pp. 3534-3555.Abstract
We present an overview of our results on the design, material growth, device characterization, and spectroscopic applications of MOVPE-grown quantum cascade lasers (QCLs). These devices are capable of room-temperature (RT) continuous-wave operation and high power emission. The first section focuses on growth of laser material, device fabrication, and quantum design The second section discusses RT pulsed operation, in particular the doping dependence of laser performance and broadband emission. Near-field measurements performed on the devices' facets correlating lateral modes to device size are also discussed. Section III deals with continuous-wave high-temperature operation from lasers with different active region designs, including their spectral characteristics and in the emergence of coherent phenomena at high power levels. Section IV analyses the devices thermal dissipation capabilities, while in Section V we report reliability data. The final section focuses on spectroscopic applications and tunability. Optofluidic narrow ridge lasers and their application to chemical sensing are reported along with recent data on a broadband on chip spectrometer consisting of individually addressable distributed feedback QCLs. Various spectroscopic techniques and in particular quartz-enhanced photoacoustic absorption spectroscopy and its use in gas sensing systems are discussed. Finally, optofluidic narrow ridge lasers and their applications to fluid sensing are presented.
Q. J. Wang, C. Pflugl, L. Diehl, F. Capasso, S. Furuta, and H. Kan. 2008. “High-power long-wavelength room-temperature MOVPE-grown quantum cascade lasers with air-semiconductor waveguide.” ELECTRONICS LETTERS, 44, 8, Pp. 525-527.Abstract
Quantum cascade lasers grown by metal organic vapour phase epitaxy (MOVPE) with high peak output power of 1.3 W at 300 K emitting a wavelength of 9.8 mu m are reported. The devices are processed in wide ridge waveguide structures with an air-semiconductor interface to confine the laser optical mode. This design increases the optical overlap factor and reduces waveguide losses.
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

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