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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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]
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.
G. Wysocki, R. Lewicki, R. F. Curl, F. K. Tittel, L. Diehl, F. Capasso, M. TROCCOLI, G. Hofler, D. Bour, S. Corzine, R. Maulini, M. Giovannini, and J. Faist. 2008. “Widely tunable mode-hop free external cavity quantum cascade lasers for high resolution spectroscopy and chemical sensing.” APPLIED PHYSICS B-LASERS AND OPTICS, 92, 3, Pp. 305-311.Abstract
Recent progress in the development of room temperature, continuous wave, widely tunable, mode-hop-free mid-infrared external cavity quantum cascade laser (EC-QCL) spectroscopic sources is reported. A single mode tuning range of 155 cm(-1) (similar to 8% of the center wavelength) with a maximum power of 11.1 mW and 182 cm(-1) (similar to 15% of the center wavelength) with a maximum power of 50 mW was obtained for 5.3 and 8.4 mu m EC-QCLs respectively. This technology is particularly suitable for high resolution spectroscopic applications, multi species trace-gas detection and spectroscopic measurements of broadband absorbers. Several examples of spectroscopic measurements performed using EC-QCL based spectrometers are demonstrated.
Jonathan A. Fan, Mikhail A. Belkin, Federico Capasso, Suraj P. Khanna, Mohamed Lachab, A. Giles Davies, and Edmund H. Linfield. 2008. “Wide-ridge metal-metal terahertz quantum cascade lasers with high-order lateral mode suppression.” APPLIED PHYSICS LETTERS, 92, 3.Abstract
Terahertz quantum cascade lasers with wide-ridge metal-metal waveguides are prone to lasing in high-order lateral modes, which reduce the maximum light output power from these devices. We have demonstrated, theoretically and experimentally, that the output power can be improved severalfold by introducing ``side absorbers'' into the waveguide structure, which enforce lasing in the TM(00) mode with minor temperature performance deterioration. Lasers without side absorbers operate up to 168 K, a current record for devices processed using indium/gold wafer bonding.
Nanfang Yu, Ertugrul Cubukcu, Laurent Diehl, David Bour, Scott Corzine, Jintian Zhu, Gloria Hoefler, Kenneth B. Crozier, and Federico Capasso. 2007. “Bowtie plasmonic quantum cascade laser antenna.” Optics Express, 15, 20, Pp. 13272-13281. Publisher's VersionAbstract
We report a bowtie plasmonic quantum cascade laser antenna that can confine coherent mid-infrared radiation well below the diffraction limit. The antenna is fabricated on the facet of a mid-infrared quantum cascade laser and consists of a pair of gold fan-like segments, whose narrow ends are separated by a nanometric gap. Compared with a nano-rod antenna composed of a pair of nano-rods, the bowtie antenna efficiently suppresses the field enhancement at the outer ends of the structure, making it more suitable for spatially-resolved high-resolution chemical and biological imaging and spectroscopy. The antenna near field is characterized by an apertureless near-field scanning optical microscope; field confinement as small as 130 nm is demonstrated at a wavelength of 7.0 mu m. (C) 2007 Optical Society of America.
Federico Capasso, Jeremy N. Munday, Davide Iannuzzi, and H. B. Chan. 2007. “Casimir forces and quantum electrodynamical torques: Physics and nanomechanics.” IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 13, 2, Pp. 400-414.Abstract
This paper discusses recent developments on quantum electrodynamical (QED) phenomena, such as the Casimir effect, and their use in nanomechanics and nanotechnology in general. Casimir forces and torques arise from quantum fluctuations of vacuum or, more generally, from the zero-point energy of materials and their dependence on the boundary conditions of the electromagnetic fields. Because the latter can be tailored, this raises the interesting possibility of designing QED forces for specific applications. After a concise review of the field in an historical perspective, high precision measurements of the Casimir force using microelectromechanical systems (MEMS) technology and applications of the, latter to nonlinear oscillators are presented, along with a discussion of its use in nanoscale position sensors. Then, experiments that have demonstrated the role of the skin-depth effect in reducing the Casimir force are presented. The, dielectric response of materials enters in a nonintuitive way in the modification of the Casimir-Lifshitz force between dielectrics through the dielectric function at imaginary frequencies epsilon(i xi). The latter is illustrated in a dramatic way by experiments on materials that can be switched between a reflective and a transparent state (hydrogen switchable mirrors). Repulsive Casimir forces between solids separated by a fluid with epsilon(i xi) intermediate between those of the solids over a large frequency range is discussed, including ongoing experiments aimed at its observation. Such repulsive forces can be used to achieve quantum floatation in a virtually frictionless environment, a phenomenon that could be exploited in innovative applications to nanomechanics. The last part of the paper deals with the elusive QED torque between birefringent materials and efforts to observe it. We conclude by highlighting future important directions.
Christine Y. Wang, L. Diehl, A. Gordon, C. Jirauschek, F. X. Kartner, A. Belyanin, D. Bour, S. Corzine, G. Hofler, M. TROCCOLI, J. Faist, and Federico Capasso. 2007. “Coherent instabilities in a semiconductor laser with fast gain recovery.” Physical Review A, 75, 3. Publisher's VersionAbstract
We report the observation of a coherent multimode instability in quantum cascade lasers (QCLs), which is driven by the same fundamental mechanism of Rabi oscillations as the elusive Risken-Nummedal-Graham-Haken (RNGH) instability predicted 40 years ago for ring lasers. The threshold of the observed instability is significantly lower than in the original RNGH instability, which we attribute to saturable-absorption nonlinearity in the laser. Coherent effects, which cannot be reproduced by standard laser rate equations, can play therefore a key role in the multimode dynamics of QCLs, and in lasers with fast gain recovery in general.