Short Wavelength Quantum Cascade Lasers Research Articles

Coupled‐Waveguides for Dispersion Compensation in Semiconductor Lasers (Laser Photonics Rev. 12(5)/2018)

Semiconductor Lasers In article number 1700323, Yves Bidaux and co-workers present a dual-waveguide for intracavity dispersion compensation in semiconductor lasers and apply it to a short-wavelength quantum cascade laser. The fundamental mode of the laser cavity leaks into a second, passive waveguide and creates a new coupled mode. The chromatic dispersion of the coupled mode can be tailored in order to allow for frequency comb operation of the device.

Design considerations for λ ∼ 3.0- to 3.5-μm-emitting quantum cascade lasers on metamorphic buffer layers

Quantum cascade lasers (QCLs) that employ metamorphic buffer layers as substrates of variable lattice constant have been designed for emission in the 3.0- to 3.5-μm wavelength range. Theoretical analysis of the active-region (AR) energy band structure, while using an 8-band k•p model, reveals that one can achieve both effective carrier-leakage suppression as well as fast carrier extraction in QCL structures of relatively low strain. Significantly lower indium-content quantum wells (QWs) can be employed for the AR compared to QWs employed for conventional short-wavelength QCL structures grown on InP, which, in turn, is expected to eliminate carrier leakage to indirect-gap valleys (X, L). An analysis of thermo-optical characteristics for the complete device design indicates that high-Al-content AlInAs cladding layers are more effective for both optical confinement and thermal dissipation than InGaP cladding layers. An electroluminescence-spectrum full-width half-maximum linewidth of 54.6 meV is estimated from interface roughness scattering and, by considering both inelastic and elastic scattering, the threshold-current density for 3.39-μm-emitting, 3-mm-long back-facet-coated QCLs is projected to be 1.40 kA/cm2.

Quantum Cascade Lasers in the InAs/AlSb Material System

We review the current state of the InAs/AlSb quantum cascade laser (QCL) technology. These materials have brought significant progress in short wavelength QCL due to the high-conduction band offset. The first QCLs emitting below 3 μm have been demonstrated in this system. The short wavelength limit of QCL operation has further been moved down to 2.6 μm. This system is also well suited for far-infrared QCLs because high intersubband gain can be achieved at low transition energies due to the small electron effective mass in InAs. Room temperature operation has been demonstrated in InAs-based QCLs to a wavelength of 21 μm, for the first time for any semiconductor laser emitting above 16 μm. Both short- and long-wavelength single-frequency distributed feedback InAs/AlSb QCLs have been realized, as well as widely tunable external cavity sources for the 3–3.5 μm spectral range.

Thermally activated leakage current in high-performance short-wavelength quantum cascade lasers

The threshold condition for a 4-level quantum cascade laser (QCL)-active region is formulated to include thermally activated leakage of charge carriers from active region confined states into states with higher energy. A method is described and demonstrated to extract the associated thermal escape current density from measurements at laser threshold. This current is modeled by including both the temperature dependent subband-distribution of charge carriers and longitudinal optical-phonon probability. The method is used to analyze the thermally activated leakage of charge carriers in two short-wavelength strain-compensated InGaAs/InAlAs QCL-structures. The energies of the higher-lying states extracted from the model are in good agreement with the values calculated numerically within the effective-mass approximation. The estimated scattering time for the thermal activation process agrees with the expected value as well. Our approach offers a straightforward and accurate method to analyze and troubleshoot thermally activated leakage in new QCL-active region designs.

Room Temperature CW Operation of Short Wavelength Quantum Cascade Lasers Made of Strain Balanced Ga $_{\bm x}$In$_{\bm {1-x}}$As/Al$_{\bm y}$ In$_{\bm {1-y}}$As Material on InP Substrates

We present our recent development of short wavelength quantum cascade lasers (QCLs) made of strain balanced GaxIn1-xAs/AlyIn1-yAs material on InP substrates. We demonstrate room temperature continuous-wave (CW) lasing of the fundamental lateral mode at four wavelengths of 4.6, 4.0, 3.8, and 3.5 μm. We obtained 60-mW CW output power at 10 °C and 3.55-μm wavelength, which is the shortest CW lasing wavelength demonstrated at room temperature by a QCL, to the best of our knowledge. We also performed a life test on λ = 4.6 μm QCL chips. To date, we have accumulated the life test data for more than 11 000 and 4100 h under two aging conditions, 20°C and 0.85-A constant current, and 60°C and 1-A constant current, respectively.

Room-Temperature $\lambda\approx 2.7~\mu{\rm m}$ Quantum Cascade Laser Sources Based on Intracavity Second-Harmonic Generation

We present experimental results on a short-wavelength quantum cascade laser based on intracavity second-harmonic generation. Our devices consist of an injectorless quantum cascade laser, which serves as a pump, and a transversely integrated passive two-well nonlinear section. The presented devices demonstrate above 1 mW output power at 2.65 μm emission wavelength with a record high peak conversion efficiency of 278 mW/W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 80 K and still around 10 μW at 2.73 μm emission wavelength with 770 μW/W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> conversion efficiency at room temperature. Experimental threshold current densities are demonstrated to be as low as 1.5 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at room temperature.

Room-temperature operation of 3.6 μm In0.53Ga0.47As/Al0.48In0.52As quantum cascade laser sources based on intracavity second harmonic generation

We discuss a design of short-wavelength quantum cascade laser sources based on intracavity second harmonic generation. A passive heterostructure tailored for giant optical nonlinearity is integrated on top of an active region and patterned for quasiphase matching. We demonstrate operation of λ≈3.6 μm lattice-matched InGaAs/AlInAs/InP sources with approximately 6 μW of second-harmonic output at room temperature and conversion efficiency of approximately 130 μW/W2. Threshold current densities of devices with a nonlinear section were similar to that of the reference lasers without the nonlinear section.

Short Injector Quantum Cascade Lasers

We report our study on the effects of shortened quantum cascade (QC) laser injector regions. While conventional short-wavelength QC lasers typically have around seven or more injector region quantum wells, we investigate QC structures with three and two injector wells. Improvements in threshold currents, output powers, and wall-plug efficiencies are expected for fundamental reasons. At heat sink temperatures near 80 K, we observe threshold current densities less than 0.5 kA/cm2, nearly 4 W peak output power, and wall-plug efficiencies in excess of 20%. At room temperature, we see threshold current densities around 2.3 kA/cm2, output powers in excess of 1 W, and wall-plug efficiencies around 7.6%. We also observe new effects in midinfrared QC lasers, such as a pronounced negative differential resistance, pulse instabilities, and multiple and varied turn-off mechanisms. These effects result from the greatly abbreviated injector regions with highly discrete states.

Grating-coupled external-cavity short-wavelength (λ ≈ 3.9 μm) InGaAs/InAlAs/AlAs quantum-cascade lasers

In this work, the tunability properties of short-wavelength (λ ~ 3.9 μm) quantum-cascade lasers (QCLs) were studied, which is a first in the world at such short wavelengths. The experimental setup of an external cavity (EC) QCL was arranged in a Littrow configuration. A tuning range over 75 cm−1 has been achieved by using an uncoated 23 μm stripe-width QCL at room temperature. A single-mode operation could be obtained at 2527 and 2544 cm−1 at different grating angles by using an anti-reflection (AR) coated 23 μm ridge. A 5 μm stripe-width QCL without an AR coating could be tuned over 160 cm−1.

Impact of doping on the performance of short-wavelength InP-based quantum-cascade lasers

The effect of doping concentration on the performance of short-wavelength quantum-cascade lasers based on the strain-compensated InGaAs/InAlAs/AlAs heterostructure on InP, emitting at 3.8 μm, is investigated for average doping concentrations between 0.3 and 3.9×1017 cm−3 (sheet densities between 1.6 and 20.9×1011 cm−2). Although the threshold current density is rather independent of doping concentration, the maximum current density increases with doping and exhibits a saturation for the highest doping level. Other important performance characteristics such as differential quantum efficiency, peak optical emission power, slope efficiency, and maximum operating temperature are observed to be maximized for structures with an average doping of 2−3×1017 cm−3, corresponding to a sheet density of about 1.5×1012 cm−2.

Design of InxGa1−xAs1−yNy∕AlAs quantum cascade structures for 3.4μm intersubband emission

We report the design of an active region of InxGa1−xAs1−yNy∕AlAs quantum cascade laser structure emitting in the near infrared wavelength range based on an eight-band k∙p model. The InxGa1−xAs1−yNy∕AlAs heterostructure system is of significant interest for the development of short wavelength quantum cascade lasers due to its large conduction band discontinuity (∼1.5eV) and compatibility with the mature GaAs fabrication process. A detailed analysis of the intersubband transition energy within the conduction band as a function of layer thickness, composition, electric field, and temperature has been carried out. Finally, an optimized combination of In0.2Ga0.8As0.97N0.03∕AlAs three-coupled-well structure has been obtained. Under an applied field of 100kV∕cm and at room temperature, a shortest wavelength of 3.4μm has been achieved by making use of this system without introducing an upper lasing level beyond the X minima of the AlAs barrier.

In As ∕ Al Sb quantum cascade lasers emitting below 3μm

Quantum cascade lasers emitting below 3μm are demonstrated. The lasers based on the InAs∕AlSb material system emit at 2.95–2.97μm in pulsed mode with threshold current densities near 3kA∕cm2 at 84K and operate up to room temperature. No degradation has been observed in laser performances compared with InAs∕AlSb quantum cascade lasers emitting at 3.14–3.35μm. The obtained results show no influence of the L valley in InAs on operation of these short wavelength quantum cascade lasers.

Short-wavelength (λ≈3.05μm) InP-based strain-compensated quantum-cascade laser

The design and implementation of a short-wavelength quantum-cascade laser based on the strain-compensated In0.73Ga0.27As–In0.55Al0.45As–AlAs heterosystem on InP is described. Lasers with a reduced level of doping in the active region require a larger bias voltage and emit at shorter wavelength; the emission wavelength is 3.05μm at T≈80K. The lasers operate up to T≈150K and electroluminescence persists up to room temperature, where the peak position is close to 3.3μm. The short-wavelength limit of such lasers is evaluated based on the dependence of their maximum operation temperatures and on the probable energies of the indirect valleys in the active region.

Short-Wavelength $(\lambda\approx 3.6\ \mu{\hbox {m}})$${\hbox {In}}_{0.73}{\hbox {Ga}}_{0.27}{\hbox {As–AlAs–InP}}$ Quantum-Cascade Laser

We describe the design and implementation of a short-wavelength quantum-cascade laser emitting in the range of 3.6-3.7 mum at 77 K and in the range of 3.8-3.9 mum at 300 K. The shortest wavelength laser emission observed at 77 K is lambda=3.62 mum. The active region is based on the strain-compensated In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.73</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.27</sub> As-AlAs heterosystem on InP. The laser operates in pulsed mode up to 300 K, and in continuous-wave mode at 77 K. Laser threshold current densities are as low as 600 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 10 K, 800 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 77 K, and 3.8 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 300 K in pulsed mode. The temperature coefficient T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is as high as 143 K. Driven with 50% duty cycle at 77 K, lasers deliver up to 70 mW average power per facet and the differential quantum efficiency is as high as 14% per cascade

Short-wavelength (λ≈3.3μm) InP-based strain-compensated quantum-cascade laser

The authors describe the design and implementation of a short-wavelength quantum-cascade laser emitting at approximately 3.3μm at 80K. The active region is based on the strain-compensated In0.73Ga0.27As–In0.55Al0.45As–AlAs heterosystem on InP. The band structure and the strain are controlled through the use of both composite barriers as well as composite wells. The structure is designed so the transition resulting in laser emission is very spatially diagonal; the upper laser state is primarily located in a thick In0.55Al0.45As layer in the injector while the lower laser state is in an In0.73Ga0.27As well. This design allows the lasing transition to bypass (in energy–growth-coordinate space) the lowest indirect X and L valleys of In0.73Ga0.27As, and population inversion is achieved in spite of the upper laser state reaching the energy of the indirect X- and L-valley edges of the adjacent In0.73Ga0.27As well.

Room-Temperature Electroluminescence from Single-Period (CdF2/CaF2) Inter-Subband Quantum Cascade Structure on Si substrate

Near-infrared electroluminescence from a single-period (CdF2/CaF2) inter-subband quantum cascade structure on a Si substrate is reported. The CdF2/CaF2 heterostructure is a good candidate for realizing a Si-based short-wavelength quantum cascade laser because of its large conduction band discontinuity of 2.9 eV and small lattice mismatch with the Si substrate. In the experiment, an active region consisting of (CdF2/CaF2) heterostructures was grown epitaxially on the Si substrate by molecular beam epitaxy, and an Au/Al electrode was evaporated on the active region. The wafer was polished mechanically and cleaved. Electroluminescence from the device was observed in the near-infrared region at room temperature for the first time.

Short wavelength intersubband emission from InAs∕AlSb quantum cascade structures

The InAs∕AlSb material system is a promising candidate for the development of short wavelength quantum cascade lasers because of the large conduction band offset of 2.1 eV. In this letter, we present a study of room temperature electroluminescence of InAs∕AlSb quantum cascade structures as a function of the emission wavelength. Intersubband emission with a transition energy of 500 meV (λ=2.5μm) has been obtained.

High-quality GaInAs/AlAsSb quantum cascade lasers grown by molecular beam epitaxy in continuous growth mode

Short wavelength ( λ<5 μm) quantum cascade lasers are of current interest as they operate in the technologically important atmospheric 3–5 μm transparency window. For this wavelength range the GaInAs/AlAsSb-on-InP material system offers the advantage of a large conduction band offset of about 1.6 eV over the well-established lattice-matched GaInAs/AlInAs-on-InP material combination with a band offset of only 0.5 eV. In this paper, we report on molecular beam epitaxial growth and subsequent structural and compositional analysis of GaInAs/AlAsSb quantum wells as well as quantum cascade laser structures. Special emphasis has been laid on establishing a growth procedure which allows the growth of these structures without growth interruption at the GaInAs/AlAsSb interfaces. The epitaxial layer sequences were analyzed by high-resolution X-ray diffraction, secondary ion mass spectrometry, and transmission electron microscopy. Finally, mesa waveguide GaInAs/AlAsSb QC lasers were fabricated emitting around 4.5 μm, which could be operated in pulsed mode up to 400 K.

λ∼4–5.3 μ m intersubband emission from InGaAs–AlAsSb quantum cascade structures

The In0.53Ga0.47As–AlAs0.56Sb0.44 materials system, lattice matched to InP, is an attractive candidate for short wavelength quantum cascade lasers due to the very large conduction band discontinuity (∼1.6 eV) and compatibility with well established quantum cascade laser waveguide design and fabrication technology. In this letter we report the operation of In0.53Ga0.47As–AlAs0.56Sb0.44 quantum cascade structures emitting in the wavelength range λ∼4–5.3 μm. Clear intersubband electroluminescence peaks are observed close to the design wavelengths, with full widths at half maximum in the range of ∼30–40 meV.

Short wavelength (λ∼3.4 μm) quantum cascade laser based on strained compensated InGaAs/AlInAs

Growth of quantum cascade lasers based on strain-compensated InxGa1−xAs/InyAl1−yAs and operating at a wavelength shorter than 4 μm is reported. Pulsed mode operation of these lasers up to T=280 K is reported with a high T0. Continuous wave powers as high as 120 mW are reported at cryogenic temperatures (15 K).