Publications

2004
A Soibel, F. Capasso, C. Gmachl, ML Peabody, AM Sergent, R Paiella, DL Sivco, AY Cho, and HC Liu. 2004. “Stability of pulse emission and enhancement of intracavity second-harmonic generation in self-mode-locked quantum cascade lasers.” IEEE JOURNAL OF QUANTUM ELECTRONICS, 40, 3, Pp. 197-204.Abstract
We report the observation of stable pulse emission and enhancement of intracavity second-harmonic generation (SHG) in self-mode-locked quantum cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an RF signal allows the direct observation of the pulsed laser emission in the time domain and reveals a stable train of pulses characteristic of mode-locked lasers. The onset of self-mode locking in QC lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. A pulse duration of similar to12 ps is estimated from the measured increase of the SHG signal in pulsed emission compared to the power expected for the SHG signal in CW emission. This value is in good agreement with the pulse duration deduced from the optical spectral width.
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C. Gmachl, N Owschimikow, A Belyanin, AM Sergent, DL Sivco, ML Peabody, AY Cho, and F. Capasso. 2004. “Temperature dependence and single-mode tuning behavior of second-harmonic generation in quantum cascade lasers.” APPLIED PHYSICS LETTERS, 84, 15, Pp. 2751-2753.Abstract
Second-harmonic generation (SHG) is reported in quantum cascade (QC) lasers with active regions that also support nonlinear cascades with large second order nonlinear susceptibility. SHG has been measured from 10 up to 250 K heat sink temperature, with about 1 muW of nonlinear power at 10 K and about 50 nW at 250 K. Single-mode and tunable SHG at 3.5 mum wavelength has been measured from single-mode QC distributed feedback lasers operating at the fundamental pump wavelength of 7.0 mum. Thermal tuning results in a tuning rate for the SHG emission of similar to0.2 nm/K for temperatures above similar to100 K. (C) 2004 American Institute of Physics.
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J. Kim, M Lerttamrab, SL Chuang, C. Gmachl, DL Sivco, F. Capasso, and AY Cho. 2004. “Theoretical and experimental study of optical gain and linewidth enhancement factor of type-I quantum-cascade lasers.” IEEE JOURNAL OF QUANTUM ELECTRONICS, 40, 12, Pp. 1663-1674.Abstract
A theoretical and experimental study of the optical gain and the linewidth enhancement factor (LEF) of a type-I quantum-cascade (QQ laser is reported. QC lasers have a symmetrical gain spectrum because the optical transition occurs between conduction subbands. According to the Kramers-Kronig relation, a zero LEF is predicted at the gain peak, but there has been no experimental observation of a zero LEF. There are other mechanisms that affect the LEF such as device self-heating, and the refractive index change due to other transition states not involved in lasing action. In this paper, the effects of these mechanisms on the LEF of a type-I QC laser are investigated theoretically and experimentally. The optical gain spectrum and the LEF are measured using the Hakki-Paoli method. Device self-heating on the wavelength shift in the Fabry-Perot modes is isolated by measuring the shift of the lasing wavelength above the threshold current. The band structure of a QC laser is calculated by solving the Schrodinger-Poisson equation self-consistently. We use the Gaussian lineshape function for gain change and the confluent hypergeometric function of the first kind for refractive index change, which satisfies the Kramers-Kronig relation. The refractive index change caused by various transition states is calculated by the theoretical model of a type-I QC laser. The calculated LEF shows good agreement with the experimental measurement.
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2003
MO Scully, VV Kocharovsky, A Belyanin, E Fry, and F Capasso. 2003. “Enhancing acceleration radiation from ground-state atoms via cavity quantum electrodynamics.” PHYSICAL REVIEW LETTERS, 91, 24.Abstract
When ground-state atoms are accelerated through a high Q microwave cavity, radiation is produced with an intensity which can exceed the intensity of Unruh acceleration radiation in free space by many orders of magnitude. The reason is a strong nonadiabatic effect at cavity boundaries and its interplay with the standard Unruh effect. The cavity field at steady state is still described by a thermal density matrix under most conditions. However, under some conditions gain is possible, and when the atoms are injected in a regular fashion, squeezed radiation can be produced.
DM Tennant, R Colombelli, K Srinivasan, M Troccoli, O Painter, C Gmachi, F Capasso, AM Sergent, DL Sivco, and AY Cho. 2003. “Fabrication methods for a quantum cascade photonic crystal surface emitting laser.” JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B, 21, 6, Pp. 2907-2911.Abstract
Conventional quantum cascade (QC) lasers are intrinsically edge-emitting devices with mode confinement achieved via a standard mesa stripe configuration. Surface emission in edge emitting QC lasers has therefore necessitated redirecting the waveguided laser emission using a second order grating. This paper describes the methods used to fabricate a 2D photonic crystal (PC) structure with or without a central defect superimposed on an electrically pumped QC laser structure with the goal of achieving direct surface emission. A successful systematic study of PC hole radius and spacing was performed using e-beam lithography. This PC method offers the promise of a number of interesting applications, including miniaturization and integration of QC lasers. 2003 American Vacuum Society.
S Danworaphong, IG Calasso, A Beveridge, GJ Diebold, C Gmachl, F Capasso, DL Sivco, and AY Cho. 2003. “Internally excited acoustic resonator for photoacoustic trace detection.” APPLIED OPTICS, 42, 27, Pp. 5561-5565.Abstract
The quantum-cascade laser can be used as an infrared source for a small portable photoacoustic trace gas detector. The device that we describe uses a quantum-cascade laser without collimating optics mounted inside an acoustic resonator. The laser is positioned in the center of a longitudinal resonator at a pressure antinode and emits radiation along the length of the resonator exciting an axially symmetric longitudinal acoustic mode of an open-ended cylindrical resonator. Experiments are reported with an 8-mum, quasi-cw-modulated, room-temperature laser used to detect N2O. (C) 2003 Optical Society of America.
M Lerttamrab, SL Chuang, C Gmachl, DL Sivco, F Capasso, and AY Cho. 2003. “Linewidth enhancement factor of a type-I quantum-cascade laser.” JOURNAL OF APPLIED PHYSICS, 94, 8, Pp. 5426-5428.Abstract
Experimental results using the amplified spontaneous emission spectroscopy of a type-I quantum-cascade laser are presented. Using the Hakki-Paoli method, the optical gain spectra of the laser are extracted for the wavelength of 8.2 mum at various subthreshold current levels. The change in refractive index with increased bias current is obtained from the peak wavelength shifts of the Fabry-Perot spectrum. A low value of -0.5 for the linewidth enhancement factor is found. A group index of around 3.47 has also been determined from Fabry-Perot modal spacings. (C) 2003 American Institute of Physics.
M Troccoli, F Capasso, J Chen, ML Peabody, C Gmachl, DL Sivco, CH Chen, and AY Cho. 2003. “Midinfrared electroluminescence in quantum cascade structures with InP/InGaAs active regions.” JOURNAL OF APPLIED PHYSICS, 94, 11, Pp. 7101-7104.Abstract
We report on the midinfrared emission from electroluminescent devices with quantum cascade active regions based on InGaAs/InP heterostructures. We observe emission at lambdasimilar to12 mum from two different structures and compare their emission characteristics based on the different band structure designs. Their relevance in view of the realization of InP-based quantum cascade lasers with aluminum-free waveguides is discussed. (C) 2003 American Institute of Physics.
A Soibel, C Gmachl, DL Sivco, ML Peabody, AM Sergent, AY Cho, and F Capasso. 2003. “Optimization of broadband quantum cascade lasers for continuous wave operation.” APPLIED PHYSICS LETTERS, 83, 1, Pp. 24-26.Abstract
We have optimized the design of the broadband quantum cascade laser for cw operation. The improved design leads to a gain ripple of only about 4 cm(-1) over more than a 0.5-mum spectral range. Simultaneous cw emission at several wavelengths spanning the range between 6.7 and 7.4 mum has been achieved in a temperature interval from 20 to 77 K. (C) 2003 American Institute of Physics.
C Gmachl, A Belyanin, DL Sivco, ML Peabody, N Owschimikow, AM Sergent, F Capasso, and AY Cho. 2003. “Optimized second-harmonic generation in quantum cascade lasers.” IEEE JOURNAL OF QUANTUM ELECTRONICS, 39, 11, Pp. 1345-1355.Abstract
Optimized second-harmonic generation (SHG) in quantum cascade (QC) lasers with specially designed active regions is reported. Nonlinear optical cascades of resonantly coupled intersubband transitions with giant second-order nonlinearities were integrated with each QC-laser active region. QC lasers with three-coupled quantum-well (QW) active regions showed up to 12 muW of SHG light at 3.75 mum wavelength at a fundamental peak power and wavelength of 1 W and 7.5 mum, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 muW/W-2. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 And 4.55 mum, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 muW/W-2. Full theoretical treatment of nonlinear light generation in QC lasers is given, and excellent agreement with the experimental results is obtained. For the best structure, a second-order nonlinear susceptibility of 4.7 x 10(-5) esu (2 x 10(-4) pmN) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.
R Colombelli, K Srinivasan, M Troccoli, O Painter, CF Gmachl, DM Tennant, AM Sergent, DL Sivco, AY Cho, and F Capasso. 2003. “Quantum cascade surface-emitting photonic crystal laser.” SCIENCE, 302, 5649, Pp. 1374-1377.Abstract
We combine photonic and electronic band structure engineering to create a surface-emitting quantum cascade microcavity laser. A high-index contrast two-dimensional photonic crystal is used to form a micro-resonator that simultaneously provides feedback for laser action and diffracts light vertically from the surface of the semiconductor surface. A top metallic contact allows electrical current injection and provides vertical optical confinement through a bound surface plasmon wave. The miniaturization and tailorable emission properties of this design are potentially important for sensing applications, while electrical pumping can allow new studies of photonic crystal and surface plasmon structures in nonlinear and near-field optics.
N Owschimikow, C Gmachl, A Belyanin, V Kocharovsky, DL Sivco, R Colombelli, F Capasso, and AY Cho. 2003. “Resonant second-order nonlinear optical processes in quantum cascade lasers.” PHYSICAL REVIEW LETTERS, 90, 4.Abstract
We demonstrate an efficient intracavity nonlinear interaction of laser modes in a specially adapted quantum cascade laser. A two-wavelength quantum cascade laser structure emitting at wavelengths of 7.1 and 9.5 mum included cascaded resonant optical intersubband transitions in an intracavity configuration leading to resonantly enhanced sum-frequency and second-harmonic generation at wavelengths of 4.1, 3.6, and 4.7 mum, respectively. Laser peak optical powers of 60 and 80 mW resulted in 30 nW of sum-frequency signal and 10-15 nW of second-harmonic signal, both in good agreement with theoretical calculations.
V Tamosiunas, R Zobl, J Ulrich, K Unterrainer, R Colombelli, C Gmachl, K West, L Pfeiffer, and F Capasso. 2003. “Terahertz quantum cascade lasers in a magnetic field.” APPLIED PHYSICS LETTERS, 83, 19, Pp. 3873-3875.Abstract
We have investigated the behavior of a terahertz quantum cascade laser in an external magnetic field. A reduction of the threshold current density and a simultaneous enhancement of the laser emission intensity are observed. Although several mechanisms can induce this effect, the suppression of nonradiative Auger-intersubband transitions through Landau quantization of the in-plane electron motion is the most probable candidate. In addition, the injection rate via resonant inter-Landau-level transfer and the waveguide properties are modulated by the field. We also observed clear shifts of the emission spectra when the external magnetic field is applied, while operating the device at constant voltage or current. (C) 2003 American Institute of Physics.
2002
A Belyanin, V Kocharovsky, V Kocharovsky, and F Capasso. 2002. “Coherent radiation from neutral molecules moving above a grating.” PHYSICAL REVIEW LETTERS, 88, 5.Abstract
We predict and study the effect of parametric self-induced excitation of a molecule moving above the dielectric or conducting medium with periodic grating. In this case the radiation reaction force modulates the molecular transition frequency which results in a parametric instability of dipole oscillations even from the level of quantum or thermal fluctuations. The present mechanism of instability of electrically neutral molecules is different from that of the well-known Smith-Purcell and transition radiation in which a moving charge and its oscillating image create an oscillating dipoles We show that parametrically excited molecular bunches can produce an easily detectable coherent radiation flux of up to a microwatt.
AA Kosterev, FK Tittel, W Durante, M Allen, R Kohler, C Gmachl, F Capasso, DL Sivco, and AY Cho. 2002. “Detection of biogenic CO production above vascular cell cultures using a near-room-temperature QC-DFB laser.” APPLIED PHYSICS B-LASERS AND OPTICS, 74, 1, Pp. 95-99.Abstract
We report the first application of pulsed, near-room-temperature quantum cascade laser technology to the continuous detection of biogenic CO production rates above viable cultures of vascular smooth muscle cells. A computer-controlled sequence of measurements over a 9-h period was obtained, resulting in a minimum detectable CO production of 20 ppb in a 1-m optical path above a standard cell-culture flask. Data-processing procedures for real-time monitoring of both biogenic and ambient atmospheric CO concentrations are described.
R Colombelli, F Capasso, A Straub, C Gmachl, MI Blakey, AM Sergent, DL Sivco, AY Cho, KW West, and LN Pfeiffer. 2002. “FIR quantum cascade lasers at lambda > 20 pm and THz emitters at lambda=80 mu m.” PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 13, 2-4, Pp. 848-853.Abstract
Quantum cascade lasers operating at lambda > 20 mum wavelength are reported. Pulsed operation as obtained up to 140 K with a peak power of few milliwatts at cryogenic temperatures, Laser action originates from interminiband transitions in ``chirped'' superlattice active regions. The waveguides are based on surface-plasmon modes confined at a metal semiconductor interface, but we also report on a 21.5 mum wavelength laser based on a double-sided interface-plasmon waveguide. This latter, contrary to the single-sided surface plasmon, is a viable waveguiding solution in the THz range, i.e. at wavelengths between 60 and 100 mum. Finally, intersubband electroluminescence is reported in a quantum cascade structure based on asymmetric superlattice active regions and designed for emission in the THz range at lambda approximate to 80 mum. (C) 2002 Published by Elsevier Science B.V.
TL Myers, RM Williams, MS Taubman, C Gmachl, F Capasso, DL Sivco, JN Baillargeon, and AY Cho. 2002. “Free-running frequency stability of mid-infrared quantum cascade lasers.” OPTICS LETTERS, 27, 3, Pp. 170-172.Abstract
The intrinsic frequency fluctuations of two single-mode quantum cascade (QC) distributed-feedback lasers operating continuously at a wavelength of 8.5 mum are reported. A Doppler-limited rovibrational resonance of nitrous oxide is used to transform the frequency noise into measurable intensity fluctuations. The QC lasers, along with recently improved current controllers, exhibit a free-running frequency stability of 150 kHz over a 15-ms time interval. (C) 2002 Optical Society of America.
R Martini, C Bethea, F Capasso, C Gmachl, R Paiella, EA Whittaker, HY Hwang, DL Sivco, JN Baillargeon, and AY Cho. 2002. “Free-space optical transmission of multimedia satellite data streams using mid-infrared quantum cascade lasers.” ELECTRONICS LETTERS, 38, 4, Pp. 181-183.Abstract
Experimental results for an optical free-space high-speed link using direct modulated mid-infrared (lambda = 8.1 mum) quantum cascade lasers are presented, A total of 800 digitally encoded multimedia channels were transmitted. The reliability of the system against weather influence (fog) was experimentally compared to that of a near-infrared (lambda=0.85 mum) link.
MS Taubman, TL Myers, BD Cannon, RM Williams, F Capasso, C Gmachl, DL Sivco, and AY Cho. 2002. “Frequency stabilization of quantum-cascade lasers by use of optical cavities.” OPTICS LETTERS, 27, 24, Pp. 2164-2166.Abstract
We report a heterodyne beat with a linewidth of 5.6 +/- 0.6 Hz between two cavity-stabilized quantum-cascade lasers operating at 8.5 mum. We also present a technique for measuring this beat that avoids the need for extreme isolation of the optical cavities from the environment, that of employing a third servo loop with low bandwidth to force one cavity to track the slow drifts and low-frequency fluctuations of the other. Although it is not fully independent, this technique greatly facilitates heterodyne beat measurements for evaluating the performance of cavity-locked lasers above the bandwidth of the third loop. (C) 2002 Optical Society of America.
G Gagliardi, S Viciani, M Inguscio, P De Natale, C Gmachl, F Capasso, DL Sivco, JN Baillargeon, AL Hutchinson, and AY Cho. 2002. “Generation of tunable far-infrared radiation with a quantum cascade laser.” OPTICS LETTERS, 27, 7, Pp. 521-523.Abstract
We demonstrate the generation of cw tunable far-infrared radiation by mixing a quantum cascade laser and a CO2 laser in a W-Ni metal-insulator-metal diode. The first known spectroscopic application to the recording of an H Br-79 transition near 4.47 THz is reported. (C) 2002 Optical Society of America.

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