Pulsed Threshold Current Density Research Articles

High-performance quantum cascade lasers at λ ∼ 9 µm grown by MOCVD.

We demonstrate a high power InP-based quantum cascade laser (QCL) (λ ∼ 9 µm) with high characteristic temperature grown by metalorganic chemical vapor deposition (MOCVD) in this article. A 4-mm-long cavity length, 10.5-µm-wide ridge QCL with high-reflection (HR) coating demonstrates a maximum pulsed peak power of 1.55 W and continuous-wave (CW) output power of 1.02W at 293 K. The pulsed threshold current density of the device is as low as 1.52 kA/cm2. The active region adopted a dual-upper-state (DAU) and multiple-lower-state (MS) design and it shows a wide electroluminescence (EL) spectrum with 466 cm-1 wide full-width at half maximum (FWHM). In addition, the device performance is insensitive to the temperature change since the threshold-current characteristic temperature coefficient, T0, is as high as 228 K, and slope-efficiency characteristic temperature coefficient, T1, is as high as 680 K, over the heatsink-temperature range of 293 K to 353 K.

Room temperature continuous wave operation of InAs/AlSb‐based quantum cascade laser at λ ∼11 µm

The InAs/AlSb quantum cascade lasers (QCLs) operating ∼11 µm in the continuous wave (cw) regime at room temperature are reported. The lasers with cleaved Fabry–Perot resonators demonstrated pulsed threshold current densities below 1 kA/cm2 at 300 K and operated in a cw up to 320 K that is the maximum temperature of such regime of operation for InAs-based QCLs. Distributed feedback lasers fabricated from the same wafer exhibited single frequency emission near 11 µm and operated in the cw regime up to 295 K.

MBE-grown long-wavelength interband cascade lasers on InAs substrates

An interband cascade (IC) laser structure with an emission wavelength designed to be near 11μm was grown by molecular beam epitaxy on an InAs substrate. Defects and surface smoothness, as well as the overall crystalline quality, were characterized by optical microscopy, atomic force microscopy and X-ray diffraction. The maximum operating temperatures of a broad-area IC laser device in CW and pulsed modes were 97K and 130K, respectively, at an emission wavelength of 11μm, which is the longest wavelength among interband lasers based on III–V semiconductor materials. The pulsed threshold current density at 80K was measured to be 72A/cm2. The performance of this IC laser device may be limited by intersubband absorption loss in the active region and an unidentified carrier leakage channel.

High power, continuous wave, room temperature operation of λ ∼ 3.4 μm and λ ∼ 3.55 μm InP-based quantum cascade lasers

We report two highly strain-balanced InP-based AlInAs/GaInAs quantum cascade lasers emitting near 3.39 and 3.56 μm. A pulsed threshold current density of only 1.1 kA/cm2 has been achieved at room temperature for both lasers with characteristic temperatures (T0) of 166 K and 152 K, respectively. The slope efficiency is also relatively temperature insensitive with characteristic temperatures (T1) of 116 K and 191 K, respectively. Continuous wave powers of 504 mW and 576 mW are obtained at room temperature, respectively. This was accomplished without buried ridge processing.

HIGH-PERFORMANCE INTERBAND CASCADE LASERS FOR λ = 3-4.5 μm

We discuss the single-mode performance characteristics of midwave-infrared interband cascade lasers. Broad-area devices with 5 active stages display pulsed threshold current densities as low as 375 A/cm2 and threshold power densities as low as 920 W/cm2 at room temperature, owing in part to the suppression of Auger recombination. Narrow ridges were processed using optical lithography to incorporate a periodically-corrugated pattern into the sidewalls. The corrugations are intended to suppress lasing in higher-order lateral modes, and also provide a 4th-order grating for distributed feedback. A corrugated-sidewall device operating at T = -20° C produced more than 46 mW of cw output in a single spectral mode, with a peak wallplug efficiency of 7.6%. The device maintains single-mode operation at current densities up to 10 times the lasing threshold, and the single-mode tuning range is 26 nm if both current and temperature are varied. Another device operating at -23°C produces up to 31 mW of single-mode power in the 3.3142-3.3164 μm range that spans several of the strongest absorption signature lines for methane.

Mid-infrared interband cascade lasers operating at ambient temperatures

We discuss the state-of-the-art performance of interband cascade lasers emitting in the 3–5 μm spectral band. Broad-area devices with five active stages display pulsed threshold current densities as low as 400 A cm−2 at room temperature. Auger decay rates are extracted from the analysis of threshold current densities and differential slope efficiencies of nearly 30 lasers, and found to be significantly lower than was anticipated based on prior information. New designs also produce ICLs with room-temperature internal losses as low as ≈6 cm−1. The combination of these advances with improvements to the processing of narrow ridges has led to the fabrication of a 4.4-μm-wide ridge emitting at 3.7 μm that lased to 335 K in continuous mode. This is the highest continuous-wave (cw) operating temperature for any semiconductor laser in the 3.0–4.6 μm spectral range. A 10-μm-wide ridge with high-reflection and anti-reflection facet coatings produced up to 59 mW of cw power at 298 K, and displayed a maximum wall-plug efficiency of 3.4%.

Short-Injector Quantum Cascade Laser Emitting at 8-$\mu$m Wavelength With High Slope Efficiency

This letter presents experimental results of a three-alloy-based short-injector quantum cascade laser (QCL). The investigated 4-mm-long device shows a pulsed threshold current density of 1.24 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , a slope efficiency of 1.4 W/A, a characteristic temperature above 250 K, and a peak average output power above 726 mW at room temperature. A good high-temperature performance is attributed to the diagonal transition design and better depopulation of the lower laser levels at higher temperatures. The laser emission wavelength at room temperature is 8 mum, resulting in a low voltage defect of 71 meV per period for the QCL structure.

High-performance injectorless quantum cascade lasers emitting below 6 μm

An injectorless quantum cascade laser emitting around 5.7 μm with more than 1 W of output power and a wall-plug efficiency of 3.6% at room temperature in pulsed operation is presented. The active region of the device is a design that picked up the idea of the four-alloy structure, which has already lead to very low threshold current densities. The pulsed threshold current densities were measured as low as 1.1 kA/cm2 at room temperature for an as-cleaved 3.6 mm long device, and 0.93 kA/cm2 for a high-reflection-coated 4 mm long device.

Interband Cascade Lasers with Wavelengths Spanning 3.2–4.2 μm

We report a detailed experimental investigation of five interband cascade lasers with five active stages each and emitting at wavelengths between 3.2 μm and 4.2 μm at room temperature. Pulsed threshold current densities as low as 394 A/cm2 and voltage efficiencies as high as 76% are obtained at 300 K. The low pulsed threshold power densities (0.9–1.6 kW/cm2 at 300 K) imply that ambient-temperature cw operation should be possible over the entire spectral band once optimized narrow ridges can be fabricated.

Band anti‐crossing and carrier recombination in dilute nitride phosphide based lasers and light emitting diodes

AbstractWe use high pressure techniques to investigate the properties of two classes of “dilute‐nitride‐phosphide”‐based devices; GaNP/GaP light emitting diodes for yellow–amber–red display applications and GaNAsP/GaP lasers, a potential route to producing lasers monolithically on silicon. Based upon high pressure electroluminescence measurements we find that the band anti‐crossing (BAC) model reasonably describes the GaN(As)P system based on an average of the nitrogen states. In terms of device characteristics, we find that carrier leakage into the X‐minima of GaP reduces the efficiency of GaNP/ GaP LEDs with increasing pressure. Lasing has been observed in GaNAsP/GaP devices with a pulsed threshold current density of 2.5 kA/cm2 at 80 K (λ = 890 nm). A weak increase in threshold current with hydrostatic pressure indicates that a carrier leakage path that does not involve the GaP X ‐minima is the dominant carrier recombination mechanism in these devices, in contrast to the LEDs. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

GaAs-based 1.53 [micro sign]m GaInNAsSb vertical cavity surface emitting lasers

Electrically-pumped GaAs-based 1.53 µm vertical cavity surface emitting lasers operating in pulsed mode at room temperature and continuous wave (CW) up to 20°C are reported for the first time. The lasers employ a GaInNAsSb/GaNAs multiple quantum well active region, a selectively oxidised AlAs aperture and p- and n-doped Al(Ga)As/GaAs distributed Bragg reflectors. Typical devices have room-temperature pulsed threshold current densities of 8.3 kA/cm2 and threshold voltages of 5.5 V. CW threshold currents as low as 2.87 mA for a 7 µm aperture device were observed at 15°C.

Temperature dependence and physical properties of Ga(NAsP)/GaP semiconductor lasers

We report on the properties of GaNAsP/GaP lasers which offer a potential route to producing lasers monolithically on silicon. Lasing has been observed over a wide temperature range with pulsed threshold current density of 2.5 kA/cm2 at 80 K (λ=890 nm). Temperature dependence measurements show that the radiative component of the threshold is relatively temperature stable while the overall threshold current is temperature sensitive. A sublinear variation of spontaneous emission versus current coupled with a decrease in external quantum efficiency with increasing temperature and an increase in threshold current with hydrostatic pressure implies that a carrier leakage path is the dominant carrier recombination mechanism.

High-power, narrow-ridge, mid-infrared interband cascade lasers

Two ten-stage interband cascade laser structures were grown by solid-source molecular beam epitaxy. The wafers were processed into both broad-area ridges for threshold characterization and Au-electroplated narrow ridges for high-temperature cw operation. Pulsed threshold current densities in the broad-area devices were as low as 3.8 A/cm2 at 78 K and 590 A/cm2 at 300 K. An 11-μm-wide ridge exhibited a new maximum cw operating temperature of 288 K at λ=4.1 μm. An even longer-wavelength device emitted at 5.1 μm with a maximum cw operating temperature of 229 K.

Interband Cascade Lasers with Wavelengths Spanning 2.9 μm to 5.2 μm

We report an experimental investigation of four interband cascade lasers with wavelengths spanning the mid-infrared spectral range, i.e., 2.9 μm to 5.2 μm, near room temperature in pulsed mode. One broad-area device had a pulsed threshold current density of only 3.8 A/cm2 at 78 K (λ = 3.6 μm) and 590 A/cm2 at 300 K (λ = 4.1 μm). The room-temperature threshold for the shortest-wavelength device (λ = 2.6 μm to 2.9 μm) was even lower, 450 A/cm2. A␣cavity-length study of the lasers emitting at 3.6 μm to 4.1 μm yielded an internal loss varying from 7.8 cm−1 at 78 K to 24 cm−1 at 300 K, accompanied by a decrease of the internal efficiency from 77% to 45%.

Room-temperature continuous-wave 1.55 [micro sign]m GaInNAsSb laser on GaAs

The first low-threshold 1.55 µm lasers grown on GaAs are reported. Lasing at 1.55 µm was observed from a 20×2400 µm as-cleaved device with a room-temperature continuous-wave threshold current density of 579 A/cm2, external efficiency of 41%, and 130 mW peak output power. The pulsed threshold current density was 550 A/cm2 with &gt;600 mW peak output power.

High-performance 1.5 [micro sign]m GaInNAsSb lasers grown on GaAs

Substantially reduced threshold current density and improved efficiency in long-wavelength (&gt;1.4 µm) GaAs-based lasers are reported. A 20×1220 µm as-cleaved device showed a room temperature continuous-wave threshold current density of 580 A/cm2, external efficiency of 53%, and 200 mW peak output power at 1.5 µm. The pulsed threshold current density was 450 A/cm2 with 1145 mW peak output power.

Ultraviolet InAlGaN multiple-quantum-well laser diodes

We report on ultraviolet InGaN, GaN and InAlGaN multiple-quantum-well (MQW) laser diodes grown on sapphire substrates. Pulsed threshold current densities near 5 kA/cm 2 have been achieved for InGaN MQW laser diodes with emission wavelength between 368 nm and 378 nm and continuous-wave operation of ridge-waveguide devices with threshold currents of 85 mA and output power of more than 3 mW. We also demonstrate room-temperature pulsed operation of InAlGaN MQW laser diodes emitting at a wavelength of 359.9 nm. Light output powers greater than 80 mW under pulsed current-injection conditions (pulse duration 100 ns, repetition frequency 5 kHz) and differential quantum efficiencies of 4.2% have been achieved.

1.5 μm GaInNAs(Sb) lasers grown on GaAs by MBE

We demonstrate the first 1.5 μm GaInNAsSb laser grown on GaAs. It exhibits much improved threshold current density as compared with previously reported GaInNAs lasers at 1.52 μm. A 1.465 μm laser with far superior performance is also demonstrated. This device exhibits a pulsed threshold current density of 930 A/cm 2 per quantum well, a differential quantum efficiency of 0.30 W/A (both facets), an external quantum efficiency of 35%, and peak power above 70 mW. Additionally, the use of antimony allows for a decrease in the bandgap out to 1.6 μm, while still preserving luminescence efficiency as compared to 1.3 μm GaInNAs material.

Low-threshold oxide-confined GaInNAs long wavelength vertical cavity lasers

We report, for the first time, room temperature continuous-wave (CW) operation of GaInNAs vertical-cavity surface-emitting laser diodes emitting at a wavelength of 1.2 μm and grown all-epitaxially in a single step on a GaAs substrate. Oxide-apertured devices demonstrated CW threshold currents as low as 1 mA, slope efficiency above 0.045 W/A, and thermal impedance of 1.24 K/mW. Larger sized devices exhibited a pulsed threshold current density of 2-2.5 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and slope efficiency above 0.09 W/A.

Performance and optical gain characteristic of InGaN MQW laser diodes

This paper discusses the performance of InGaN multi-quantum well (MQW) laser diodes and the effects of composition fluctuations on the optical gain characteristics. Pulsed threshold current densities as low as 4.8 kA/cm 2 have been observed for ridge-waveguide laser diodes allowing room-temperature continuous-wave (cw) operation with threshold currents of 125 mA and operating voltages of 6.5 V. The effects of composition fluctuations in the InGaN alloy were investigated by comparing theoretical and experimental gain spectra obtained from true spontaneous emission measurements, cavity length studies and cw laser emission spectra. We find that the distributed losses in our structures are of the order of 45–50 cm −1. Using a simple logarithmic gain–current relationship, we were able to describe the experimental results with a characteristic gain g 0=72 cm −1 and a transparency current density J trans=2.5 kA/cm 2. Optical gain calculations have been performed for InGaN quantum-well structures, based on band structures generated with an effective-mass Hamiltonian and taking pseudomorphic strain into account. We find that a very modest amount of composition fluctuation (standard deviation in the In content <0.008), which produces inhomogeneous broadening, is consistent with the experimental observations. This is consistent with TEM structural studies, which show that there is negligible phase separation in In x Ga 1− x N MQW with indium content smaller than x=0.1.