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Selected papers from “International Quantum Cascade Laser School and Workshop 2018”

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A special issue of Photonics (ISSN 2304-6732).

Deadline for manuscript submissions: closed (31 October 2019) | Viewed by 27224

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Special Issue Editors

Dr. Sukhdeep Dhillon

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Laboratoire Pierre Aigrain, Département de physique de l’ENS, École normale supérieure, PSL Research University, Université Paris Diderot, Sorbonne Paris Cité, Sorbonne Universités, UPMC Université Paris 06, CNRS, Paris, France
Interests: optoelectronics; photonics; terahertz; mid-infrared; ultrafast spectroscopy; nonlinear optics

Dr. Sushil Kumar

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Department of Electrical and Computer Engineering, Lehigh University, Bethlehem, PA 18015, USA

Prof. Dr. Angela Vasanelli

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Université Paris Diderot-Paris7, Laboratoire Matériaux et Phénomènes Quantiques, Bâtiment Condorcet, Case courrier 7021, F-75205 PARIS CEDEX 13, France

Special Issue Information

Dear Colleagues,

IQCLSW 2018 will be the 8^th conference in a series of international meetings on quantum cascade lasers (QCLs), and will be held in Cassis (France), 2–7 September, 2018. QCLs are unipolar optoelectronic devices that exploit optical transitions between engineered electronic subbands in semiconductor quantum wells. This concept has provided outstanding laser performance across the mid-infrared and terahertz (THz) spectral ranges, where there is a lack of practical photonic sources. IQCLSW is the main biennial event for the QCL community, divided into a school and workshop environment. The school provides an overview of the main developments and concepts in the field through courses from world leading researchers in the domain. The second workshop part presents the latest developments in the community, from fundamental physics to the exploitation and applications of this technology.

We are very glad to serve as Guest Editors of this Special Issue to be published in Photonics that will contain a selection of papers submitted and accepted at IQCLSW 2018. Its main scope is to provide a timely and broad collection of the most innovative topics discussed at the latest edition of the school and workshop related to mid-infrared and THz photonics and optoelectronics. We warmly invite researchers to submit their contributions to this Special Issue. Potential topics include, but are not limited to:

Theory and design of quantum cascade lasers
New breakthroughs in quantum cascade laser performance
Novel and new material systems
Nonlinear effects in quantum cascade lasers
Broadband gain media design and growth
Quantum cascade laser photonic integrated circuits
Novel waveguides for functionalised emission
Strong light-matter interactions
New detection schemes and methods (coherent detection, near-field etc.)
Chemical/biomedical applications for cascade lasers
Communication applications for cascade lasers
Active imaging applications for cascade lasers
Defense/homeland security applications for cascade lasers

Please note that for this Special Issue on the IQCLSW, MDPI will waive the Article Processing Charges (APC) for submissions from this conference.

Prof. Sukhdeep Dhillon
Dr. Sushil Kumar
Prof. Angela Vasanelli
Guest Editor

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Keywords

Quantum Cascade Lasers
Mid-infrared and Terahertz Photonics
Novel optical effects
Quantum and Waveguide Engineering
Light-matter interactions
New material systems
Quantum Electronics
Semiconductor Lasers

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Research

13 pages, 359 KiB

Open AccessArticle

Quantum Theory of Multisubband Plasmon– Phonon Coupling

by Sofia Ribeiro, Angela Vasanelli, Yanko Todorov and Carlo Sirtori

Photonics 2020, 7(1), 19; https://doi.org/10.3390/photonics7010019 - 20 Feb 2020

Cited by 5 | Viewed by 3410

Abstract

We present a theoretical description of the coupling between longitudinal optical phonons and collective excitations of a two-dimensional electron gas. By diagonalizing the Hamiltonian of the system, including Coulomb electron–electron and Fröhlich interactions, we observe the formation of multisubband polarons, mixed states partially [...] Read more.

\sim

40 %

, of the phonon energy. We demonstrate that multisubband plasmons and longitudinal optical phonons are in the ultra-strong coupling regime in several III–V and II–VI material systems. Full article

(This article belongs to the Special Issue Selected papers from “International Quantum Cascade Laser School and Workshop 2018”)

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8 pages, 2790 KiB

Open AccessEditor’s ChoiceArticle

RF Injection of THz QCL Combs at 80 K Emitting over 700 GHz Spectral Bandwidth

by Andres Forrer, Lorenzo Bosco, Mattias Beck, Jérôme Faist and Giacomo Scalari

Photonics 2020, 7(1), 9; https://doi.org/10.3390/photonics7010009 - 16 Jan 2020

Cited by 14 | Viewed by 4860

Abstract

We report about RF injection locking of an homogeneous THz quantum cascade laser operating at 3 THz central frequency. The extremely diagonal nature of the optical transition, combined with low-loss copper-based double-metal waveguides, allow CW operation up to 105 K and CW power in excess of 5.6 mW measured at 80 K. Terahertz emission spanning up to 600 GHz, together with a narrow beatnote, indicate comb operation at 80 K, and strong RF injection clearly modifies the laser spectrum up to 700 GHz spectral bandwidth making these devices ideal candidates for an on-chip dual comb spectrometer. Full article

(This article belongs to the Special Issue Selected papers from “International Quantum Cascade Laser School and Workshop 2018”)

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Figure 1

Figure 1
(a) LIV Pulsed (2 % duty cycle at 132 kHz repetition rate) and CW of a 4 mm long and 86 <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m wide device. For visibility, only CW IV curves are presented. Pulsed power is detected by the Absolut THz Power Meter by Thomas Keating Ltd and in CW by the 3A-P-THz by Ophir. CW power is corrected for collection losses and atmospheric absorption in the setup to be compared with the pulsed measurements. (b) Threshold Current Density vs Temperature of a 4 mm long and 64 <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m wide Cu–Cu device and a 1.45 mm long and 65 <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m wide Au-Au device. Inset: Increased CW performance of the same Cu–Cu device up to 105 K before having nickel evaporated on the top contact setback, i.e., having side-absorbers. Full article ">Figure 2
Spectra of a 4 mm long and 64 <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m wide device as a function of increasing injected currents at 80 K in CW. Full article ">Figure 3
Beatnote maps at 20 K, 50 K and 80 K of the 4 mm long and 64 <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m wide copper device. White dotted line indicates the lasing region and the pink blocks indicate regions with a single narrow beatnote around 9.9 GHz. Full article ">Figure 4
(a) Low power (−10 dBm at synthesizer) injection map at 80 K (714 mA, 9.69 V). Injection is via a neighbouring laser acting as an antenna without bias applied. (b) Spectra of the free running and injected laser at 80 K (590 mA, 9.12 V). Minor changes due to injection are observed in the spectrum. (c) Low-frequency components and narrow (<2 kHz) free running BN at 80K corresponding to the spectra in (b). Low-frequency components are unchanged and suggest that injection is not destabilizing the comb state. Full article ">Figure 5
(a) Free running (single beatnote regime, 714 mA) and corresponding strong injected spectra at 80 K. Significant change in the modes as well as generation of additional modes due to injection which leads to a spectral bandwidth from 600 GHz to 700 GHz. (b) Mode spacing extracted from (a) showing a quantitative agreement of the equally spaced modes. Full article ">

8 pages, 397 KiB

Open AccessArticle

Short Barriers for Lowering Current-Density in Terahertz Quantum Cascade Lasers

by Liang Gao, John L. Reno and Sushil Kumar

Photonics 2020, 7(1), 7; https://doi.org/10.3390/photonics7010007 - 8 Jan 2020

Cited by 8 | Viewed by 4153

Abstract

Scattering due to interface-roughness (IR) and longitudinal-optical (LO) phonons are primary transport mechanisms in terahertz quantum-cascade lasers (QCLs). By choosing GaAs/Al_0.10Ga_0.90As heterostructures with short-barriers, the effect of IR scattering is mitigated, leading to low operating current-densities. A series of resonant-phonon terahertz QCLs developed over time, achieving some of the lowest threshold and peak current-densities among published terahertz QCLs with maximum operating temperatures above 100 K. The best result is obtained for a three-well 3.1 THz QCL with threshold and peak current-densities of 134 A/cm² and 208 A/cm² respectively at 53 K, and a maximum lasing temperature of 135 K. Another three-well QCL designed for broadband bidirectional operation achieved lasing in a combined frequency range of 3.1–3.7 THz operating under both positive and negative polarities, with an operating current-density range of 167–322 A/cm² at 53 K and maximum lasing temperature of 141 K or 121 K depending on the polarity of the applied bias. By showing results from QCLs developed over a period of time, here we show conclusively that short-barrier terahertz QCLs are effective in achieving low current-density operation at the cost of a reduction in peak temperature performance. Full article

(This article belongs to the Special Issue Selected papers from “International Quantum Cascade Laser School and Workshop 2018”)

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Figure 1

Figure 1
(a) Plot of magnitude-squared wavefunctions computed in a tight-binding formalism for a typical three-well resonant-phonon terahertz quantum cascade laser (QCL) design at the bias corresponding to peak-gain. A QCL module is divided into two sub-modules at barriers affecting transport of electrons via resonant-tunneling (RT). The radiative transition <math display="inline"><semantics> <mrow> <mn>4</mn> <mo>→</mo> <mn>3</mn> </mrow> </semantics></math> occurs in the sub-module consisting of quantum-wells QW 1 and QW 2, whereas the injector and extraction subbands 2 and 1 respectively are localized in the sub-module comprising of quantum-well QW 3. (b) Theoretically calculated gain spectra using a simplified density-matrix transport model for the three-well resonant-phonon designs RTRP3W198 (with short Al-10% barriers) and the design from Fathololoumi et al. [<a href="#B5-photonics-07-00007" class="html-bibr">5</a>] (with taller Al-15% barriers) respectively, at the bias corresponding to peak-gain. In the model, the electron and lattice temperatures are set to 50 K and the inter-module electron transport occurs via RT whereas intra-module transport is modeled by electron-LO-phonon scattering and interface-roughness scattering. Full article ">Figure 2
(a) One module conduction band diagram of the short-barrier three-well resonant-phonon QCL design named RTRP3W198 at the peak-gain bias. Starting from injector barrier, layer thicknesses in monolayers (ML) are (with barriers indicated in bold face) 23/31/14/30/22/61 where the center of the widest well is n-doped with sheet-density of 2.8 × 1010 cm−2. The radiative transition is between <math display="inline"><semantics> <mrow> <mn>4</mn> <mo>→</mo> <mn>3</mn> </mrow> </semantics></math> where E43 = 12.4 meV (∼3 THz) and the radiative oscillator strength f43 = 0.41. <math display="inline"><semantics> <msub> <mo>Δ</mo> <mi>mn</mi> </msub> </semantics></math> is the energy splitting between subbands m and n when they are aligned for optimal resonant-tunneling. <math display="inline"><semantics> <msub> <mo>Δ</mo> <mn>32</mn> </msub> </semantics></math> = 3.86 meV. (b) Experimental light-current-voltage (L-I-V) characteristics from a 3.2 mm × 120 μm ridge laser with metal-metal cavity. A threshold current-density of 134 A/cm2 and a maximum of 208 A/cm2 was measured at 53 K in pulsed mode of operation with 300 ns pulses repeated at 100 kHz. Insets show the representative spectra of the QCL measured at 53 K. The QCL emits in the frequency range of 3.0–3.2 THz. Full article ">Figure 3
(a) and (b) Conduction band diagrams for a bidirectional three-well resonant-phonon terahertz QCL design named BIDR3W198 at peak-gain bias corresponding to positive and negative polarity operation respectively. In contrast to RTRP3W198 of <a href="#photonics-07-00007-f002" class="html-fig">Figure 2</a>, the bidirectional design BIDR3W198 is characterized by injection and extraction barriers of same thickness. Starting from injector barrier, layer thicknesses in monolayers (ML) (where barriers are indicated in bold face) are 23/31/12/30/23/60 where the center of the widest well is n-doped with a sheet-density of 2.8 × 1010 cm−2. Key design parameters are indicated alongside the band diagrams. (c) and (d) Experimental light-current-voltage (L-I-V) characteristics from a 1.6 mm × 120 μm ridge laser with metal-metal cavities biased under positive and negative polarity respectively. The threshold current-densities at positive and negative polarity bias are 167 A/cm2 and 217 A/cm2 respectively at 53 K, and the corresponding maximum operating temperatures are 141 K and 121 K in pulsed mode of operation. Pulsed L-I-V measurement was done with 300 ns pulses repeated at 100 kHz. The QCL radiates in the frequency range 3.1–3.4 THz under positive polarity and 3.3–3.7 THz under negative polarity bias respectively. Representative spectra measured at 53 K are shown the insets of the corresponding figures. Full article ">

17 pages, 2328 KiB

Open AccessFeature PaperArticle

Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models

by Chiara Ciano, Michele Virgilio, Luigi Bagolini, Leonetta Baldassarre, Andrea Rossetti, Alexej Pashkin, Manfred Helm, Michele Montanari, Luca Persichetti, Luciana Di Gaspare, Giovanni Capellini, Douglas J. Paul, Giacomo Scalari, Jèrome Faist, Monica De Seta and Michele Ortolani

Photonics 2020, 7(1), 2; https://doi.org/10.3390/photonics7010002 - 18 Dec 2019

Cited by 5 | Viewed by 4454

Abstract

n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron–phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter- and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1→3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3. Full article

(This article belongs to the Special Issue Selected papers from “International Quantum Cascade Laser School and Workshop 2018”)

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Figure 1
(a) A sketch of the saturable absorption process: electrons are excited with an intense optical source, at the E13 ISB energy, from level 1 to level 3, and the output signal (Dout) is measured as a function of the variable incoming signal (Din). The most relevant wavefunctions and the potential profile reported in the panel are obtained from calculations based on a Poisson–Schrödinger solver. (b) A sketch of a single-pass waveguide allowing optical coupling to the quantum well (QW) region. The TM-polarized electric field direction is indicated with a double-headed arrow. (c) The absorbance spectra of the two investigated samples S1 and S6, featuring two clear peaks corresponding to the 1→2 and 1→3 ISBTs: Absorbance of sample S6 is less intense, as expected from being less doped. The region of the pump photon energies E13 used for the experiment is reported as a green-shaded area, which is just outside the energy range at which the most relevant transitions of the Si wafer impurities are present (grey-shaded area). In the calculations, we used a lattice temperature TL = 15 K and an electron temperature (calculated from the typical optical pump power) Te = 65 K. Full article ">Figure 2
(a) The transmittance measured as a function of the pump intensity for both the investigated samples at the three photon energies, as reported in the legend: Symbols are the experimental data, and the continuous curves are the results of the fitting function to the data. (b) Same quantity as <a href="#photonics-07-00002-f002" class="html-fig">Figure 2</a>a for S1 at <math display="inline"><semantics> <mrow> <mo>ℏ</mo> <msub> <mstyle mathvariant="bold" mathsize="normal"> <mi mathvariant="bold-sans-serif">ω</mi> </mstyle> <mstyle mathvariant="bold" mathsize="normal"> <mi>p</mi> </mstyle> </msub> <mo> </mo> </mrow> </semantics></math>= 41.9 meV (dark blue curve) and <math display="inline"><semantics> <mrow> <mo>ℏ</mo> <msub> <mstyle mathvariant="bold" mathsize="normal"> <mi mathvariant="bold-sans-serif">ω</mi> </mstyle> <mstyle mathvariant="bold" mathsize="normal"> <mi>p</mi> </mstyle> </msub> </mrow> </semantics></math> = 48.1 meV (light blue curve) to show how the (c) transmission spectrum at 6 K changes at Ip→0 and Ip,max. <a href="#photonics-07-00002-f002" class="html-fig">Figure 2</a>b,c has the same vertical scale, and the dotted lines are a guide to connect the transmittance at Ip→0 and Ip,max for these two photon energies. (d) A sketch of the setup used for the experiment: the intensity level is set by metal mesh attenuators placed before the beam splitter (BS). (e) The experimental and (f) theoretical relative transmittance variations for sample S1 at 6 K: For experimental determination, the value T0 is the FTIR transmittance at zero pump intensity, which has been subtracted to the data in <a href="#photonics-07-00002-f002" class="html-fig">Figure 2</a>b. Full article ">Figure 3
The net non-radiative rate for the 2→1 transition in a rectangular QW made of a polar (blue curves) and a nonpolar (red curves) semiconductor heterostructure as a function of the energy separation: (a) The empty-band approximation and (b) a situation in which the 2DEG distribution of electrons in the subbands with a sheet carrier density of 1011 cm-2 is. Note that the values reported for the nonpolar lattice have been multiplied to a factor 10 in both panels. Calculations are performed at TL = 15 K and Te = 65 K. Full article ">Figure 4
The simulated electron population dynamics ni = Ni/Ntot for samples S1 and S6 (a,b) obtained with the full-subband numerical model at a lattice temperature set to TL = 6 K. (c) The dynamics of S1 is simulated with the three-discrete-energy level analytical model. The time envelope of the pump pulse used in the simulations is also reported as a dotted line. Full article ">Figure 5
The simulated absorption coefficient (upper panels) and the relative transmittance variation (lower panels) as a function of the pump intensity: (a) The full-subband-model has been used to simulate sample S1 (blue curves) and S6 (red curves). For comparison, the lower signal for the less-doped S6 has been multiplied by 3. The temperature has been set to 6 K. (b) The discrete-energy-level analytical model has been used for simulating sample S1 (light-blue curve). To favor the direct comparison of the value for Ip,sat, the same curves of <a href="#photonics-07-00002-f005" class="html-fig">Figure 5</a>a for sample S1 (blue dots) have been repeated here and the intensity range has been reduced. The two values Ip,sat,num and Ip,sat,theo are marked with a black and a purple vertical line, respectively. In the inset, the relative transmittance variation is reported for the two models in the entire range of pump intensities used for the simulations. Full article ">

10 pages, 1798 KiB

Open AccessArticle

Evidence of Intersubband Linewidth Narrowing Using Growth Interruption Technique

by Ngoc Linh Tran, Giorgio Biasiol, Arnaud Jollivet, Alberto Bertocci, François H. Julien, Jean-Michel Manceau and Raffaele Colombelli

Photonics 2019, 6(2), 38; https://doi.org/10.3390/photonics6020038 - 1 Apr 2019

Cited by 4 | Viewed by 3878

Abstract

We report on the systematic study of two main scattering mechanisms on intersubband transitions, namely ionized impurity scattering and interface roughness scattering. The former mechanism has been investigated as a function of the dopants position within a multiple GaAs/AlGaAs quantum well structure and compared to the transition of an undoped sample. The study on the latter scattering mechanism has been conducted using the growth interruption technique. We report an improvement of the intersubband (ISB) transition linewidth up to 11% by interrupting growth at GaAs-on-AlGaAs interfaces. As a result, the lifetime of intersubband polaritons could be improved up to 9%. This leads to a reduction of 17% of the theoretical threshold intensity for polaritonic coherent emission. This work brings a useful contribution towards the realization of polariton-based devices. Full article

(This article belongs to the Special Issue Selected papers from “International Quantum Cascade Laser School and Workshop 2018”)

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Figure 1

Figure 1
(a) (upper panel) Schematic of the absorption spectroscopy of Mid-Infrared (MIR) photons within a doped Quantum Well (QW). The 1st subband (conduction band) is filled with electrons and MIR photons are absorbed by the quantized states. (lower panel) Schematic of the experimental approach with a 45° facets multipass prism. The sample is probed in both Transverse Electric (TE) polarization and Transverse Magnetic polarization (TM) (b) (Upper Panel) Schematic representation of the photo-induced absorption spectroscopy with nominally undoped QW. Electrons are promoted from the valence band up to the conduction band using a continuous wave laser at 532 nm. MIR photons are then absorbed by the quantized states. (lower panel) Schematic of the experimental approach with a 45° facets multipass prism illuminated by a CW laser. Full article ">Figure 2
(a) Transmittance spectra of the sample doped within the well (upper panel) and the modulation doped sample at 78 K (lower panel). The red curves are the Voigt fitting functions. (b) The photo-induced absorption spectrum of the nominally undoped sample at 78 K with its Voigt fit (red). Full article ">Figure 3
(a) ISB transmission spectra of the samples where the growth interruption was applied on the inverted interfaces (0, 30, 60, and 120 s.). (b) ISB transmission spectra of the samples, where the growth interruption was applied on the normal interfaces (0, 30, 60, and 120 s.). (c) The dependence of the ISBT linewidth on the growth interruption time. Full article ">Figure 4
(a) Transmission spectrum at 300 K of the sample HM4045. The sample was grown with 120 s interruption time on the inverted interfaces. (b) The schematic of the cavity for operation in strong light matter coupling, and its angle-resolved measurement approach (c) The reconstructed angle-resolved dispersion of the HM4045 sample in the metal-metal cavity configuration. The lower and upper polaritons reflectivity minima are depicted in black and blue dots, respectively. The dashed black line shows the central frequency of the ISBT. (d) The reflectivity spectrum acquired at an angle of incidence of 31° (50%-50% light-matter fraction). The polariton linewidths are estimated using a Lorentzian fitting procedure. Full article ">

8 pages, 1886 KiB 17 µm) Distributed Feedback Quantum Cascade Lasers Operating in a Continuous Wave at Room Temperature" data-journal="photonics">

Open AccessEditor’s ChoiceArticle

Long Wavelength (λ > 17 µm) Distributed Feedback Quantum Cascade Lasers Operating in a Continuous Wave at Room Temperature

by Hoang Nguyen Van, Zeineb Loghmari, Hadrien Philip, Michael Bahriz, Alexei N. Baranov and Roland Teissier

Photonics 2019, 6(1), 31; https://doi.org/10.3390/photonics6010031 - 21 Mar 2019

Cited by 21 | Viewed by 5629

Abstract

The extension of the available spectral range covered by quantum cascade lasers (QCL) would allow one to address new molecular spectroscopy applications, in particular in the long wavelength domain of the mid-infrared. We report in this paper the realization of distributed feedback (DFB) QCLs, made of InAs and AlSb, that demonstrated a continuous wave (CW) and a single mode emission at a wavelength of 17.7 µm, with output powers in the mW range. This is the longest wavelength for DFB QCLs, and for any QCLs or semiconductor lasers in general, operating in a CW at room temperature. Full article

(This article belongs to the Special Issue Selected papers from “International Quantum Cascade Laser School and Workshop 2018”)

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Graphical abstract

Graphical abstract
Full article ">Figure 1
Band diagram of a portion of the studied active region. The laser transition is between the red (up) and green (down) states. Two quantum wells in the middle of the injector are doped with Si for an electron sheet density of 0.3 × 1011 cm−2 per stage. Full article ">Figure 2
Room temperature electrical and optical characteristics of a typical FP quantum cascade laser (QCL) fabricated from the studied wafer. The laser is 3.6 mm long and 16 µm wide and driven with 330 ns current pulses at a repetition rate of 12 kHz. Inset: the pulsed emission spectrum of the laser. Full article ">Figure 3
(a) Emission spectra in pulsed mode at 25 °C of DFB lasers with a different period of the DFB grating (Λ). The linewidth is limited by the resolution of the FTIR spectrometer. (b) Peak wavelength of the tested devices as a function of the period of the DFB grating. Full article ">Figure 4
(a) Comparison of the pulsed characteristics of FP and DFB QCLs with different grating periods. The FP laser is 3.6 mm long and 16 µm wide; the DFB laser with Λ = 2.666 µm is 3.4 mm long and 15 µm wide; the laser with Λ = 2.590 µm is 3.0 mm long and 14 µm wide. (b) Threshold current densities of the studied devices as a function of operating temperature. The open symbols are for pulsed operation, and the solid symbols are for CW operation. Full article ">Figure 5
(a) CW characteristics of a 3.4-mm-long and 14-µm-wide DFB QCL with Λ = 2.666 µm. The optical power is the power collected from one facet with a f/1 off-axis parabolic mirror without any correction for the collection efficiency. (b) Emission spectra in a CW as a function of the sample holder temperature for a current of 700 mA. The linewidth is limited by the resolution of the FTIR spectrometer. Full article ">Figure 6
(a) CW characteristics of a 3.0-mm-long and 14-µm-wide DFB QCL with Λ = 2.590 µm, mounted in a Peltier cooled module. The optical power is the power collected from one facet with a f/1 off-axis parabolic mirror without any correction for the collection efficiency. (b) Emission spectra in a CW as a function of the sample holder temperature, for a set of currents starting at the maximum current of 580 mA and decreasing in 20 mA steps. The linewidth is limited by the resolution of the FTIR spectrometer. Full article ">

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