Publications

2010
Hyunyong Choi, Vasileios-Marios Gkortsas, Laurent Diehl, David Bour, Scott Corzine, Jintian Zhu, Gloria Hoefler, Federico Capasso, Franz X. Kaertner, and Theodore B. Norris. 2010. “Ultrafast Rabi flopping and coherent pulse propagation in a quantum cascade laser.” NATURE PHOTONICS, 4, 10, Pp. 706-710.Abstract
Pulse propagation phenomena are central to ultrashort pulse generation and amplification in lasers(1-5). In the coherent regime, the phase relationship between the pulse and the material transition is preserved, allowing both optical fields and material states to be controlled(6). The most prominent form of coherent manipulation is Rabi flopping(7), a phenomenon well established in few-level absorbers, including atoms and single quantum dots(8-19). However, Rabi flopping is generally much weaker in semiconductors because of strong dephasing in the electronic bands, in contrast to discrete-level systems. Although low-density induced coherent oscillations have been observed in semiconductor absorbers(11,13-20), coherent pulse propagation phenomena in active semiconductor devices have not been observed. In this Letter, we explore coherent pulse propagation in an operating quantum cascade laser and directly observe Rabi flopping and coherent pulse reshaping. This work demonstrates the applicability of few-level models for quantum cascade lasers and may stimulate novel approaches to short pulse generation(21,22).
nphoton.2010.205.pdf
Nanfang Yu and Federico Capasso. 2010. “Wavefront engineering for mid-infrared and terahertz quantum cascade lasers.” JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, 27, 11, Pp. B18-B35.Abstract
We review our recent work on beam shaping of mid-infrared (mid-IR) and terahertz (THz) quantum cascade lasers (QCLs) using plasmonics. Essentials of QCLs are discussed; these include key developments, the operating principle based on quantum design, and beam quality problems associated with laser waveguide design. The bulk of the present paper is focused on the use of surface plasmons (SPs) to engineer the wavefront of QCLs. This is achieved by tailoring the SP dispersion using properly designed plasmonic structures, in particular, plasmonic Bragg gratings, designer (spoof) surface plasmon structures, and channel polariton structures. Using mid-IR and THz QCLs as a model system, various functionalities have been demonstrated, ranging from beam collimation, polarization control, to multibeam emission and spatial wavelength demultiplexing. Plasmonics offers a monolithic, compact, and low-loss solution to the problem of poor beam quality of QCLs and may have a large impact on applications such as sensing, light detection and ranging (LIDAR), free-space optical communication, and heterodyne detection of chemicals. The plasmonic designs are scalable and applicable to near-infrared active or passive optical devices. (C) 2010 Optical Society of America
josab-27-11-b18.pdf
Qijie Wang, Changling Yan, Nanfang Yu, Julia Unterhinninghofen, Jan Wiersig, Christian Pfluegl, Laurent Diehl, Tadataka Edamura, Masamichi Yamanishi, Hirofumi Kan, and Federico Capasso. 2010. “Whispering-gallery mode resonators for highly unidirectional laser action.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 107, 52, Pp. 22407-22412.Abstract
Optical microcavities can be designed to take advantage of total internal reflection, which results in resonators supporting whispering-gallery modes (WGMs) with a high-quality factor (Q factor). One of the crucial problems of these devices for practical applications such as designing microcavity lasers, however, is that their emission is nondirectional due to their radial symmetry, in addition to their inefficient power output coupling. Here we report the design of elliptical resonators with a wavelength-size notch at the boundary, which support in-plane highly unidirectional laser emission from WGMs. The notch acts as a small scatterer such that the Q factor of the WGMs is still very high. Using midinfrared (lambda similar to 10 mu m) injection quantum cascade lasers as a model system, an in-plane beam divergence as small as 6 deg with a peak optical power of similar to 5 mW at room temperature has been demonstrated. The beam divergence is insensitive to the pumping current and to the notch geometry, demonstrating the robustness of this resonator design. The latter is scalable to the visible and the near infrared, thus opening the door to very low-threshold, highly unidirectional microcavity diode lasers.
1015386107.full_.pdf
2009
A. Lyakh, R. Maulini, A. Tsekoun, R. Go, C. Pfluegl, L. Diehl, Q. J. Wang, Federico Capasso, and C. Kumar N. Patel. 2009. “3 W continuous-wave room temperature single-facet emission from quantum cascade lasers based on nonresonant extraction design approach.” APPLIED PHYSICS LETTERS, 95, 14.Abstract
A strain-balanced, InP-based quantum cascade laser structure, designed for light emission at 4.6 mu m using a new nonresonant extraction design approach, was grown by molecular beam epitaxy. Removal of the restrictive two-phonon resonant condition, currently used in most structure designs, allows simultaneous optimization of several design parameters influencing laser performance. Following the growth, the structure was processed in buried heterostructure. Maximum single-ended continuous-wave optical power of 3 W was obtained at 293 K for devices with stripe dimensions of 5 mm x 11.6 mu m. Corresponding maximum wallplug efficiency and threshold current density were measured to be 12.7% and 0.86 kA/cm(2). (C) 2009 American Institute of Physics. [doi: 10.1063/1.3238263]
2009_Lyakh_et_al_APL.pdf
Ertugrul Cubukcu, Fatih Degirmenci, Coskun Kocabas, Mariano A. Zimmler, John A. Rogers, and Federico Capasso. 2009. “Aligned carbon nanotubes as polarization-sensitive, molecular near-field detectors.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 106, 8, Pp. 2495-2499.Abstract
Near-field scanning optical microscopes are widely used in imaging of subwavelength features in various material systems and nanostructures. For a variety of applications, polarization-sensitive near-field probes can provide valuable information on the nature and symmetry of the imaged nanoparticles and emitters. Conventional near-field optical microscopy lacks in-plane polarization sensitivity. Here, we use aligned single-wall carbon nanotubes as polarization-sensitive molecular scale probes to image the transverse near-field components of an optical Hertzian dipole antenna. Because of the Raman ``antenna effect'' in carbon nanotubes, only the near-field components along the nanotube axis are detected. These findings demonstrate that aligned carbon nanotubes can be used as polarization-sensitive near-field detectors.
2009_Cubukcu_et_al_PNAS.pdf
Martina Hentschel, Tae-Yoon Kwon, Mikhail A. Belkin, Ross Audet, and Federico Capasso. 2009. “Angular emission characteristics of quantum cascade spiral microlasers.” OPTICS EXPRESS, 17, 12, Pp. 10335-10343.Abstract
We perform ray and wave simulations of passive and active spiral-shaped optical microcavities, comparing our results to experimental data obtained with mid-infrared quantum cascade spiral microlasers. Focusing on the angular emission characteristics, we find that both ray and wave simulations are consistent with the experimental data, showing richly-featured, multidirectional far-field emission patterns in the case of uniform pumping and TM-polarized light. Active cavity simulations using the Schrodinger-Bloch model indicate that selective pumping of the quantum cascade spiral microlasers near the resonator boundary will yield unidirectional laser emission. (c) 2009 Optical Society of America
2009_Hentschel_et_al_OpticsExpress.pdf
Benjamin G. Lee, Jan Kansky, Anish K. Goyal, Christian Pfluegl, Laurent Diehl, Mikhail A. Belkin, Antonio Sanchez, and Federico Capasso. 2009. “Beam combining of quantum cascade laser arrays.” OPTICS EXPRESS, 17, 18, Pp. 16216-16224.Abstract
Wavelength beam combining was used to co-propagate beams from 28 elements in an array of distributed-feedback quantum cascade lasers (DFB-QCLs). The beam-quality product of the array, defined as the product of near-field spot size and far-field divergence for the entire array, was improved by a factor of 21 by using wavelength beam combining. To demonstrate the applicability of wavelength beam combined DFB-QCL arrays for remote sensing, we obtained the absorption spectrum of isopropanol at a distance of 6 m from the laser array. (C) 2009 Optical Society of America
2009_Lee_et_al_OpticsExpress.pdf
Benjamin G. Lee, Haifei A. Zhang, Christian Pfluegl, Laurent Diehl, Mikhail A. Belkin, Milan Fischer, Andreas Wittmann, Jerome Faist, and Federico Capasso. 2009. “Broadband Distributed-Feedback Quantum Cascade Laser Array Operating From 8.0 to 9.8 mu m.” IEEE PHOTONICS TECHNOLOGY LETTERS, 21, 13, Pp. 914-916.Abstract
An ultra-broadband distributed-feedback quantum cascade laser array was fabricated, using a heterogeneous cascade based on two bound-to-continuum designs centered at 8.4 and 9.6 mu m. This array emitted in a range over 220 cm(-1) near a 9-mu m wavelength, operated in pulsed mode at room temperature. The output power of the array varied between 100- and 1100-mW peak intensity.
2009_Lee_et_al_IEEE_Photonics_Technology_Letters.pdf
Nanfang Yu, Laurent Diehl, Ertugrul Cubukcu, David Bour, Scott Corzine, Gloria Hoefler, Aleksander K. Wojcik, Kenneth B. Crozier, Alexey Belyanin, and Federico Capasso. 2009. “Coherent Coupling of Multiple Transverse Modes in Quantum Cascade Lasers.” PHYSICAL REVIEW LETTERS, 102, 1.Abstract
Quantum cascade lasers are a unique laboratory for studying nonlinear laser dynamics because of their high intracavity intensity, strong intersubband optical nonlinearity, and an unusual combination of relaxation time scales. Here we investigate the nonlinear coupling between the transverse modes of quantum cascade lasers. We present evidence for stable phase coherence of multiple transverse modes over a large range of injection currents. We explain the phase coherence by a four-wave mixing interaction originating from the strong optical nonlinearity of the gain transition. The phase-locking conditions predicted by theory are supported by spectral data and both near- and far-field mode measurements.
2009_Yu_et_al_PRL.pdf
Nanfang Yu, Alexey Belyanin, Jiming Bao, and Federico Capasso. 2009. “Controlled modification of erbium lifetime by near-field coupling to metallic films.” NEW JOURNAL OF PHYSICS, 11.Abstract
Systematic measurements of the photoluminescence lifetime of the 1.54 mu m transition of erbium implanted at different energies in SiO2 films with different metallic overlayers are reported. The lifetime shows a strong reduction up to a factor of 20 with decreasing distance between the erbium and the metal overlayer. The reduction of lifetime is mainly due to a near-field interaction between the erbium ions and the metal overlayers through generation of surface plasmon polaritons at the metal/SiO2 interface and direct generation of heat in the metal. These experiments combined with rigorous theoretical modeling demonstrate that a high degree of control over the radiative properties of erbium can be achieved in erbium-implanted materials in a wide range of implantation energies. The experiments also allow us to determine the radiative efficiency of erbium in bulk SiO2.
2009_Yu_et_al_New_Journal_Of_Physics.pdf
Qijie Wang, Changling Yan, Laurent Diehl, Martina Hentschel, Jan Wiersig, Nanfang Yu, Christian Pfluegl, Mikhail A. Belkin, Tadataka Edamura, Masamichi Yamanishi, Hirofumi Kan, and Federico Capasso. 2009. “Deformed microcavity quantum cascade lasers with directional emission.” NEW JOURNAL OF PHYSICS, 11.Abstract
We report the experimental realization of deformed microcavity quantum cascade lasers (QCLs) with a Lima on-shaped chaotic resonator. Directional light emission with a beam divergence of theta(parallel to) approximate to 33 degrees from QCLs emitting at lambda approximate to 10 mu m was obtained in the plane of the cavity for deformations in the range 0.37 < epsilon < 0.43. An excellent agreement between measured and calculated far-field profiles was found. Both simulations and experiments show that the Lima on-shaped microcavity preserves whispering gallery-like modes with high Q-factors for low deformations (epsilon < 0.50). In addition, while the measured spectra show a transition from whispering gallery-like modes to a more complex mode structure at higher pumping currents, we observed `universal far-field behavior' for different intracavity mode distributions in the Lima on
2009_Jie_Wang_et_al_new_Journal_of_Physics.pdf
Benjamin G. Lee, Mikhail A. Belkin, Christian Pfluegl, Laurent Diehl, Haifei A. Zhang, Ross M. Audet, Jim MacArthur, David P. Bour, Scott W. Corzine, Gloria E. Hoefler, and Federico Capasso. 2009. “DFB Quantum Cascade Laser Arrays.” IEEE JOURNAL OF QUANTUM ELECTRONICS, 45, 5-6, Pp. 554-565.Abstract
DFB quantum cascade laser (DFB-QCL) arrays operating between 8.7 and 9.4 mu m are investigated for their performance characteristics-single-mode selection of the DFB grating, and variability in threshold, slope efficiency, and output power of different lasers in the array. Single-mode selection refers to the ability to choose a desired mode/frequency of laser emission with a DFB grating. We apply a theoretical framework developed for general DFB gratings to analyze DFB-QCL arrays. We calculate how the performance characteristics of DFB-QCLs are affected by the coupling strength kappa L of the grating, and the relative position of the mirror facets at the ends of the laser cavity with respect to the grating. We discuss how single-mode selection can be improved by design. Several DFB-QCL arrays are fabricated and their performance examined. We achieve desired improvements in single-mode selection, and we observe the predicted variability in the threshold, slope efficiency, and output power of the DFB-QCLs. As a demonstration of potential applications, the DFB-QCL arrays are used to perform infrared absorption spectroscopy with fluids.
2009_Lee_et_al_IEEE_JQE
Changling Yan, Qijie Wang, Laurent Diehl, Martina Hentschel, Jan Wiersig, Nanfang Yu, Christian Pfluegl, Federico Capasso, Mikhail A. Belkin, Tadataka Edamura, Masamichi Yamanishi, and Hirofumi Kan. 2009. “Directional emission and universal far-field behavior from semiconductor lasers with limaccedilon-shaped microcavity.” APPLIED PHYSICS LETTERS, 94, 25.Abstract
We report experimental demonstration of directional light emission from limaccedilon-shaped microcavity semiconductor lasers. Quantum cascade lasers (QCLs) emitting at lambda approximate to 10 mu m are used as a model system. Both ray optics and wave simulations show that for deformations in the range 0.37
2009_Yan_et_al_APL
I. Shalish, G. Seryogin, W. Yi, J. M. Bao, M. A. Zimmler, E. Likovich, D.C. Bell, F. Capasso, and V. Narayanamurti. 2009. “Epitaxial Catalyst-Free Growth of InN Nanorods on c-Plane Sapphire.” NANOSCALE RESEARCH LETTERS, 4, 6, Pp. 532-537.Abstract
We report observation of catalyst-free hydride vapor phase epitaxy growth of InN nanorods. Characterization of the nanorods with transmission electron microscopy, and X-ray diffraction show that the nanorods are stoichiometric 2H-InN single crystals growing in the [0001] orientation. The InN rods are uniform, showing very little variation in both diameter and length. Surprisingly, the rods show clear epitaxial relations with the c-plane sapphire substrate, despite about 29% of lattice mismatch. Comparing catalyst-free with Ni-catalyzed growth, the only difference observed is in the density of nucleation sites, suggesting that Ni does not work like the typical vapor-liquid-solid catalyst, but rather functions as a nucleation promoter by catalyzing the decomposition of ammonia. No conclusive photoluminescence was observed from single nanorods, while integrating over a large area showed weak wide emissions centered at 0.78 and at 1.9 eV.
2009_Shalish_et_al_Nanoscale_Res_Lett.pdf
Mariano A. Zimmler, Tobias Voss, Carsten Ronning, and Federico Capasso. 2009. “Exciton-related electroluminescence from ZnO nanowire light-emitting diodes.” APPLIED PHYSICS LETTERS, 94, 24.Abstract
The authors study the microscopic origin of the electroluminescence from zinc oxide (ZnO) nanowire light-emitting diodes (LEDs) fabricated on a heavily doped p-type silicon (p-Si) substrate. By comparing the low-temperature photoluminescence and electroluminescence of a single nanowire LED, bound- and free-exciton related recombination processes, together with their longitudinal-optical phonon replicas, can be identified as the origin of both electroluminescence and photoluminescence.
2009_Zimmler_et_al_APL
Markus C. Amann, Federico Capasso, Anders Larsson, and Markus Pessa. 2009. “Focus on advanced semiconductor heterostructures for optoelectronics.” NEW JOURNAL OF PHYSICS, 11.Abstract
Semiconductor heterostructures are the basic materials underlying optoelectronic devices, particularly lasers and light-emitting diodes (LEDs). Made from various III-V-, II-VI-, SiGe- and other compound semiconductors, modern semiconductor devices are available for the generation, detection and modulation of light covering the entire ultra-violet to far-infrared spectral region. Recent approaches that introduced multilayer heterostructures tailored on the lower nanometre scale made possible artificial semiconductors with new properties, such as extended wavelength coverage, that enabled new applications. Together with ongoing progress on wide-gap semiconductors, the optical wavelengths accessible by semiconductor devices are steadily expanding towards the short-wavelength ultra-violet regime, as well as further into the far-infrared and terahertz spectral regions. It is the aim of this focus issue to present cutting-edge research topics on the most recent optoelectronic material and device developments in this field using advanced semiconductor heterostructures.
2009_Amann_et_al_NJP.pdf
David Woolf, Marko Loncar, and Federico Capasso. 2009. “The forces from coupled surface plasmon polaritons in planar waveguides.” OPTICS EXPRESS, 17, 22, Pp. 19996-20011.Abstract
We analytically investigate the forces due to Surface Plasmon Polariton (SPP) modes between finite and infinitely thick metal slabs separated by an air gap. Using the Drude model and experimentally determined values of the dielectric functions of gold and silver, we study how frequency dispersion and loss in the metals affects the behavior of the SPP modes and the forces generated by them. We calculate the force using the Maxwell Stress Tensor for both the attractive and repulsive modes. (C) 2009 Optical Society of America
2009_Woolf_et_al_Optics_Express.pdf
Qijie Wang, Christian Pflugl, Laurent Diehl, Federico Capasso, Tadataka Edamura, Shinichi Furuta, Masamichi Yamanishi, and Hirofumi Kan. 2009. “High performance quantum cascade lasers based on three-phonon-resonance design.” APPLIED PHYSICS LETTERS, 94, 1.Abstract
A quantum cascade laser structure based on three-phonon-resonance design is proposed and demonstrated. Devices, emitting at a wavelength of 9 mu m, processed into buried ridge waveguide structures with a 3 mm long, 16 mu m wide cavity and a high-reflection (HR) coating have shown peak output powers of 1.2 W, slope efficiencies of 1 W/A, threshold current densities of 1.1 kA/cm(2), and high wall-plug efficiency of 6% at 300 K. A 3 mm long, 12 mu m wide buried-heterostructure device without a HR coating exhibited continuous wave output power of as high as 65 mW from a single facet at 300 K.
2009_Wang_et_al_APL
Richard Maulini, Arkadiy Lyakh, Alexei Tsekoun, Rowel Go, Christian Pfluegl, Laurent Diehl, Federico Capasso, and C. Kumar N. Patel. 2009. “High power thermoelectrically cooled and uncooled quantum cascade lasers with optimized reflectivity facet coatings.” APPLIED PHYSICS LETTERS, 95, 15.Abstract
We present a method of preserving the device wall-plug efficiency by adjusting mirror losses with facet coatings for longer cavity quantum cascade lasers. An experimental study of output power and wall-plug efficiency as functions of mirror losses was performed by varying the front facet coating reflectivity with a high-reflectivity-coated rear facet. The use of optimized reflectivity coatings on 7-mm-long chips resulted in continuous-wave output power of 2.9 W at 293 K for thermoelectrically cooled devices mounted on AlN submounts and average and continuous-wave output power in excess of 1 W for uncooled devices emitting at 4.6 mu m. (C) 2009 American Institute of Physics. [doi:10.1063/1.3246799]
2009_Maulini_et_al_APL
Mikhail A. Belkin, Qijie Wang, Christian Pfluegl, Alexey Belyanin, Suraj P. Khanna, Alexander Giles Davies, Edmund Harold Linfield, and Federico Capasso. 2009. “High-Temperature Operation of Terahertz Quantum Cascade Laser Sources.” IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 15, 3, Pp. 952-967.Abstract
Terahertz (THz) quantum cascade lasers (QCLs) are currently the most advanced electrically pumped semiconductor lasers in the spectral range 1-5 THz. However, their operation at room temperature is still an unresolved challenge. In this paper, we discuss our efforts to improve the temperature performance of these devices. In particular, we present THz QCLs that approach thermoelectric cooled operation and discuss factors that limit their high-temperature performance. We also discuss a different type of THz QCL source that produces coherent THz radiation without population inversion across the THz transition. These devices are based on intracavity difference-frequency generation in dual-wavelength mid-IR QCLs, and can now provide microwatt levels of coherent THz radiation up to room temperature. We discuss how the output power of these devices can be further improved to produce milliwatts of THz radiation at room temperature.
2009_Belkin_et_al_IEEE_JSTQE.pdf

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