Temperature CW Operation Research Articles

Fundamental mode emission in tapered quantum cascade lasers for high brightness

Currently, the highest power quantum cascade lasers (QCLs) include (i) an efficient active region design, which minimizes the energy fraction transferred into heat, (ii) a relatively large number (40) of cascades, which increases the mode confinement factor, Γ, and (iii) narrow buried-heterostructure waveguides, which efficiently cool the active region through efficient lateral heat flow. This combination allows watt-level continuous wave (cw) emission powers at room temperature. Moreover, narrow waveguides favor the fundamental TM00 lateral mode, resulting in a nearly Gaussian beam. To the extent that power efficiency is already largely optimized, power scaling in narrow-stripe QCLs is only possible through increasing the laser length, which is limited by the needs for robust construction and transparent antireflection coatings. Broad-area (BA) QCLs with a small number of cascades can also deliver watt-level cw powers at room temperature, profiting from the enhanced vertical heat flow. Power can be scaled for these lasers also via the laser width and is nearly unlimited. The usual drawback of the BA lasers is poor beam quality. We discuss the use of tapered QCLs and the first demonstration of room temperature cw operation of the BA QCL in fundamental TM00 mode. A stable and reproducible transverse optical mode is achieved using a double-taper waveguide geometry. The laser emits at 4.6 μm and has a demonstrated room temperature cw power of 100 mW.

Single-Mode Fabry-Pérot Quantum Cascade Lasers at λ~10.5 μm

In this paper, we report a single-mode Fabry-Pérot long wave infrared quantum cascade lasers based on the double phonon resonance active region design. For room temperature CW operation, the wafer with 35 stages was processed into buried heterostructure lasers. For a 4 mm long and 13 μm wide laser with high-reflectivity (HR) coating on the rear facet, continuous wave output power of 43 mW at 288 K and 5 mW at 303 K is obtained with threshold current densities of 2.17 and 2.7 kA/cm2. The lasing wavelength is around 10.5 μm. Single mode emission was observed for this particular device over the whole investigated current and temperature range.

Power scaling in quantum cascade lasers using broad-area stripes with reduced cascade number

For many applications, optical emission power of many watts in the midinfrared part of the optical spectrum is required. Quantum cascade lasers for these applications typically contain 30 to 40 cascades and have laser ridge widths of about 10 μm. These lasers cannot, however, be readily scaled up in power. This paper demonstrates a path for further power scaling that is based on broad stripes with fewer than 15 cascades. Although the confinement factor Γ for this design approach is lower than for designs with 30 to 40 cascades, the ability to efficiently extract heat from the top and bottom of the lasers more than offsets the Γ disadvantage. We expect this approach to deliver tens of watts of power with high beam quality leading to high brilliance. The paper discusses the design approach, thermal conduction considerations, and confinement factor. Room temperature cw operation of broad-area lasers based on this approach is demonstrated and described.

Low Power Consumption Distributed-Feedback Quantum Cascade Lasers Operating in Continuous-Wave Mode above 90°C at λ ∼ 7.2 μm**Supported by the National Basic Research Program of China under Grant No 2013CB632801, the National Key Research and Development Program under Grant No 2016YFB0402303, the National Natural Science Foundation of China under Grant Nos 61435014, 61627822, 61574136 and 61306058, the Key Projects of

We report on the design and fabrication of λ ∼ 7.2 μm distributed feedback quantum cascade lasers for very high temperature cw operation and low electrical power consumption. The cw operation is reported above 90°C. For a 2-mm-long and 10-μm-wide laser coated with high-reflectivity on the rear facet, more than 170 mW of output power is obtained at 20°C with a threshold power consumption of 2.4 W, corresponding to 30mW with a threshold power consumption of 3.9 W at 90°C. Robust single-mode emission with a side-mode suppression ratio above 25 dB is continuously tunable by the heat sink temperature or injection current.

Two‐color lasing in a coupled multilayer cavity with InAs quantum dots by optical pumping

AbstractTwo‐color lasing in a GaAs/AlAs coupled multilayer cavity has been studied for a novel type of terahertz emitting devices. In the structure, two cavity layers based on GaAs were connected by the intermediate GaAs/AlAs distributed Bragg reflector multilayer and three layers of self‐assembled InAs quantum dots (QDs) were inserted only in the top‐side cavity layer as optical gain materials. The broadened QD emissions around 1.25 µm are very useful to realize the optical gain because they could cover the wide range of the mode frequency difference. Two mode emissions with an optical frequency difference of 3.2 THz were successfully demonstrated under room temperature cw operation by optical pumping. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Continuous-Wave Operation of Terahertz Quantum Cascade Lasers at 3.2 THz

We demonstrate continuous-wave (cw) operation of terahertz (THz) quantum cascade lasers emitting at 3.2 THz based on bound-to-continuum active region and semi-insulating surface-plasmon waveguide design. Optical power of 62 mW with a threshold current density of 285 A/cm2 is obtained at 10 K from a 130-μm-wide and 1.5-mm-long laser in cw operation. Maximum cw operation temperature is up to 60 K. In pulsed mode, peak optical power more than 100 mW at 10 K and 2.1 mW at 85 K are observed from a 230-μm-wide and 2-mm-long device.

Growth of flat p‐GaN contact layer by pulse flow method for high light‐extraction AlGaN deep‐UV LEDs with Al‐based electrode

AbstractImprovement of light extraction efficiency (LEE) is strongly demanded in an AlGaN‐based deep ultraviolet (DUV) light‐emitting diode (LED), because the LEE of AlGaN DUV LED is low (usually below 10%) due to the light absorption at around p‐GaN contact layer and p‐type electrode. We have demonstrated improvement of LEE of AlGaN DUV LEDs by using thin p‐GaN contact layer and using Al‐based high‐reflectivity p‐electrode. It was difficult to grow thin and flat p‐type GaN layers with heavy Mg‐doping on high‐Al‐content AlGaN layers. We demonstrated the growth of flat and thin p‐GaN contact layer by using ‘an NH3 pulse‐flow growth method’. We investigated the growth conditions for thin p‐GaN contact layer with high‐hole‐concentration using the NH3 pulse‐flow growth method. The surface roughness of the p‐GaN layer on high‐Al‐content p‐AlGaN was markedly improved by using an NH3 pulse‐flow growth method. We fabricated 250 nm‐band AlGaN quantum well (QW) DUV LEDs with thin p‐GaN contact layers grown by an NH3 pulse‐flow growth method. We observed single‐peaked operation spectrum for the 251 nm AlGaN MQW LED. The maximum output power and external quantum efficiencies (EQE) were 17.7 mW and 1.32%, respectively, measured under room temperature cw operation. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

2.0 μm room temperature CW operation of InGaAsSb/AlGaAsSb laser with asymmetric waveguide structure

The GaInAsSb/AlGaAsSb double quantum well lasers with an emission wavelength 2.0 μm, using the separate-confinement asymmetric waveguide, have been designed and fabricated, showing high quantum efficiency and high power conversion efficiency at continuous-wave operation mode. The threshold current density of the device is as low as 92 A/cm2. The internal loss coefficient and the internal quantum efficiency are 1.0 cm−1 and 86.1%, respectively. The 35% maximum power conversion efficiency (PCE) and narrow far-field patterned were achieved.

Milliwatt power 270 nm‐band AlGaN deep‐UV LEDs fabricated on ELO‐AlN templates

AbstractWe demonstrated CW milliwatt power operations of 270 nm‐band AlGaN multi‐quantum well (MQW) deep‐ultraviolet (DUV) light‐emitting diodes (LEDs) fabricated on epitaxial lateral overgrowth (ELO) AlN templates on sapphire. An initial AlN stripe layer was grown directly on sapphire by using ammonia (NH3) pulse‐flow multilayer (ML) growth method. An AlN stripe structure with a width of 5 μm and a spacing of 3 μm was completely coalesced after the growth of an approximately 15 μm‐thick ELO‐AlN layer grown by low‐pressure metal‐organic chemical vapor deposition (LP‐MOCVD). The edge‐type threading dislocation density (TDD) of the wing region of the ELO‐AlN layer was 3×108 cm‐2, as observed by cross‐sectional transmission electron microscope (TEM) image. The maximum output power of 2.7 mW was obtained from an AlGaN MQW LED with emission wavelength at 273 nm fabricated on ELO‐AlN template under room temperature (RT) CW operation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

226–273 nm AlGaN deep‐ultraviolet light‐emitting diodes fabricated on multilayer AlN buffers on sapphire

AbstractWe demonstrated AlGaN multi‐quantum‐well (MQW) deep‐ultraviolet (UV) light‐emitting diodes (LEDs) with wavelengths in the range of 226–273 nm on sapphire substrates. We achieved low threading dislocation density (TDD) AlN templates, formed through an ammonia (NH3) pulse‐flow multilayer (ML) growth technique, which were suitable for deep UV LEDs. The edge and screw‐type dislocation densities in these AlGaN buffer layers on ML‐AlN were 7.5×108 and 3.8×107 cm–2, respectively, as observed from cross‐sectional transmission electron microscope (TEM) images. We obtained single‐peaked operations of the LEDs with emission wavelengths of between 227‐273 nm. The output powers of the 253 nm and 261 nm LEDs were 0.4 mW and 1.0 mW respectively with injection currents of 100 mA, while maximum powers of 1.04 mW and 1.65 mW respectively were obtained under room temperature (RT) CW operation. The output power of a 227.5 nm LED was 0.11 mW with an injection current of 10 mA, and a maximum power of 0.15 mW was obtained under RT pulsed operation. The maximum external quantum efficiencies of the 261 nm and 227.5 nm LEDs were 0.23 and 0.2%, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement of surface roughness and reduction of threading dislocation density in AlN/AlGaN templates on sapphire by employing trimethylaluminum pulsed supply growth

AbstractWe fabricated high‐quality AlN/AlGaN templates, which are applicable to ultraviolet (UV) emitters on sapphire, by using a trimethylaluminum (TMAl) pulsed‐supply growth method. We succeeded in achieving a dramatic improvement in the surface roughness of AlN/AlGaN layer by introducing a short‐period pulsed supply of TMAl during the growth intervals of a ‘standard’ AlGaN growth regime. We speculate that the improvement in surface roughness is due to the realization of stable group III (+c) polarity by the growth of an Al‐rich surface. In addition, we have found that this method is effective for reducing threading dislocation densities (TDD). The TDD was reduced from 2.9×109 to 1.1×109 cm–2 by introducing the TMAl pulsed supply growth. We fabricated a 350 nm‐band InAlGaN multi‐quantum well (MQW) UV light‐emitting diode (LED) on an AlN/AlGaN template formed by the TMAl pulsed supply method, and obtained an output power of 1.95 mW under room temperature (RT) CW operation. The output power of the LED was increased by a factor of 1.7 by introducing the TMAl pulsed supply method. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Realization of 340 nm‐band high‐power InAlGaN‐based ultraviolet light‐emitting diodes by the suppression of electron overflow

Abstract(In)AlGaN alloys are attracting much attention as candidate materials for realizing deep ultraviolet (UV) light‐emitting diodes (LEDs). In this study, we demonstrated high‐power CW operations of 340 nm‐band InAlGaN‐based UV‐LEDs by suppressing electron overflow using optimized electron injection structures. LED layer structure consisting of quaternary InAlGaN quantum wells (QWs), Mg‐doped InAlGaN electron blocking layer (EBL) and Mg‐doped InAlGaN p‐type layers was grown on sapphire/AlN/AlGaN template by low‐pressure metal‐organic chemical‐vapor deposition (LP‐MOCVD). The output power was dramatically improved by increasing the potential barrier height of EBL. Furthermore, the output power was improved by increasing the depth of the quantum well. These tendencies agree well with the theoretical analysis calculated by a device simulator. The maximum output power was 8.4 mW under room temperature (RT) CW operation with peak emission wavelength at 346 nm. These results indicate that the suppression of electron overflow is quite important for the realization of high‐efficiency (In)AlGaN‐based UV‐LEDs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

340 nm‐band high‐power InAlGaN quantum well ultraviolet light‐emitting diode using p‐type InAlGaN layers

AbstractIn order to achieve high‐efficiency ultraviolet (UV) light‐emitting diodes (LEDs), p‐type layers with sufficient hole concentration is quite important to suppress the electron overflow. We introduced Mg‐doped InAlGaN for p‐type layers of the UV‐LEDs for the purpose of obtaining higher hole concentration, and achieved high power CW operations of the UV‐LEDs. 340 nm‐band InAlGaN‐based multi‐quantum‐well (MQWs) UV‐LEDs with Mg‐doped InAlGaN electron‐blocking layer (EBL) and Mg‐doped InAlGaN layers were grown on sapphire substrates by metal‐organic chemical vapor deposition (MOCVD). The output power of UV‐LED with p‐InAlGaN layers was approximately 4.7 times larger than that with p‐AlGaN layers. Maximum output power and the external quantum efficiency (EQE) of the LED with p‐InAlGaN layers were 8.4 mW and 0.9%, respectively, at emission wavelength of 346 nm under room temperature (RT) CW operation. From these results, p‐InAlGaN is shown to be useful for the realization of high‐efficiency UV‐LEDs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Room temperature continuous wave operation of single-mode, edge-emitting photonic crystal Bragg lasers

We report the first room temperature CW operation of two dimensional single-mode edge-emitting photonic crystal Bragg lasers. Single-mode lasing with single-lobed, diffraction limited far-fields is obtained for 100microm wide and 550microm long on-chip devices. We also demonstrate the tuning of the lasing wavelength by changing the transverse lattice constant of the photonic crystal. This enables a fine wavelength tuning sensitivity (change of the lasing wavelength/change of the lattice constant) of 0.072. This dependence proves that the lasing mode is selected by the photonic crystal lattice.

High performance GaAsSb∕GaAs quantum well lasers

Ga As Sb ∕ Ga As quantum wells (QWs) with 1.3μm light emission are grown using solid-source molecular beam epitaxy. The growth temperature is optimized based on photoluminescence (PL) linewidth and intensity and edge-emitting laser (EEL) threshold current density; these measurements concur that the optimal growth temperature is ∼490°C (∼500°C) for GaAsSb∕GaAs QWs grown with (without) GaAsP strain compensation. High performance EELs and vertical-cavity surface-emitting lasers (VCSELs) are demonstrated using the GaAsSb∕GaAs∕GaAsP strain compensated active region. One EEL achieved an output power up to 0.9W with thresholds as low as 356A∕cm2 under room temperature pulsed operation, while another achieved continuous-wave (cw) operation at temperatures up to 48°C for wavelengths as long as 1260nm. A set of VCSELs achieved room temperature cw operation with output powers from 0.03to0.2mW and lasing wavelengths from 1240to1290nm. The temperature characteristics of these devices indicate that the optimal gain-peak cavity-mode tuning for pulsed operation specifies a room temperature PL peak redshift of 20–30nm relative to the cavity mode.

Quaternary AlInGaN multiple quantum well 368 nm light-emitting diode

We report high output power from AlInGaN multiple quantum well (MQW) ultraviolet light-emitting diodes. The high Al containing cladding layer, electron blocking layer and p-contact layer were chosen for transparency at 365 nm to reduce the internal absorption, reduce electron overflow and enhance output power. The UV output power was as high as 1.52 mW at 20 mA at 368 nm under room temperature CW operation. The maximum output power in CW operation is 11 mW at 170 mA. In this letter, we demonstrate a helpful and easy way to measure and calculate the junction temperature of AlInGaN UVLEDs.

High‐efficiency 350 nm‐band quaternary InAlGaN‐based UV‐LED on GaN/sapphire template

We have demonstrated high-efficiency ultraviolet (UV) light-emitting diode (LED) with emission wavelength at 349 nm using quaternary InAlGaN multiple-quantum-well (MQW) fabricated on a GaN/sapphire template. The threading dislocation density of the GaN/sapphire template was approximately 1 × 109 cm–2. The maximum UV output power obtained was as high as 4.1 mW with an injection current of 160 mA under room temperature (RT) CW operation. The maximum external quantum efficiency (EQE) was 1.02% with an injection current of 60 mA, which is higher than the EQE obtained for a 350 nm-band AlGaN-based QW LED fabricated on GaN substrate. From these result, we confirmed the advantage of the use of quaternary InAlGaN for 350 nm-band UV emitters in comparison with the use of AlGaN. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Milliwatt Power 350 nm-band Quaternary InAlGaN UV-LEDs on GaN Substrates

We have demonstrated milliwatt output power from quaternary InAlGaN-based multi-quantum-well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) with an emission wavelengths of 350–360 nm. GaN substrates were used in order to reduce the density of threading dislocations. By examining a cathode luminescence (CL) image, we confirmed that the threading dislocation density of the quaternary InAlGaN MQW layer was reduced to as little as 1 × 107 cm–2 by using GaN substrate. A significant increase in the UV output power was achieved by increasing the Al content of the p-AlxGa1–xN electron blocking layer up to 28%. As a result, the maximum UV output power obtained was as high as 3.8 mW at 351 nm and 6.3 mW at 358 nm under room temperature (RT) CW operation. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Short Wavelength and High-Efficiency Operation of Deep UV LED Using Quaternary InAlGaN

High-efficiency ultraviolet (UV) light sources, i.e., UV light-emitting diodes (LEDs) or laser diodes (LDs), are very attractive for application to white lighting, high-density memories, medical fields, etc. We have demonstrated that 300-350 nm UV emission is considerably enhanced by the introduction of several percent of In into AlGaN due to an In-segregation effect. We fabricated InAlGaN-based deep UV LEDs on sapphire substrate that had an emission wavelength at 308-314 nm, and we obtained sub-milliwatt output power under pulsed operation. We also fabricated InAlGaN-based quantum-well (QW) LEDs on GaN substrates in order to eliminate the effects of threading dislocations. We achieved a remarkable increase in output power by using a GaN substrate instead of sapphire substrates. As a result, the maximum UV output power obtained was as high as 3.8 mW at 351nm and 6.3 mW at 358 nm under room temperature (RT) CW operation.

Recent progress in the development of type II interband cascade lasers

Type-II interband cascade lasers combine the advantage of an interband optical transition with interband tunneling to enable the cascading of type-II quantum well active regions as is done in type-I quantum cascade laser. The relatively high radiative efficiency resulting from interband optical transitions translates into very low-threshold current densities, and when combined with the high quantum efficiency of cascade lasers, this diode laser design has the potential to operate under cw conditions at room temperature with high output power. Experimental results have already demonstrated some of this potential including high differential external quantum efficiency (>600%), high peak output power ( ∼6 W/facet at 80 K ), high cw power conversion efficiency (>17% at 80 K ), and operation at 300 K under pulsed conditions. Recent work aimed at reducing device thermal resistance and increasing cw operating temperature is reviewed including the demonstration of significant reductions in thermal resistance (averaging 25 K/W or 40% for 1-mm-long devices), 80 K cw operation at 3.4 μm with high-power conversion efficiency (23%) and high differential external quantum efficiency (532%), and cw operation up to 214 K .