Room 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.

Development of Surface Grating Distributed Feedback Quantum Cascade Laser for High Output Power and Low Threshold Current Density

We report on the room-temperature cw operation of a surface grating distributed feedback (DFB) quantum cascade laser (QCL) at λ ∼4.7 μm. Both grating design and material optimization are used to decrease the threshold current density and to increase the output power. For a high-reflectivity-coated 13-μm-wide and 4-mm-long laser, high wall-plug efficiency of 6% is obtained at 20°C from a single facet producing over 1 W of cw output power. The threshold current density of DFB QCL is as low as 1.13 kA/cm2 at 10°C and 1.34 kA/cm2 at 30°C in cw mode. Stable single-mode emission with a side-mode suppression ratio of about 30 dB is observed in the working temperature range of 20–50°C.

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)

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)

Gain and Losses and Room-Temperature Operation in Interband Cascade Lasers

Interband cascade laser structures with a different number of cascades and doping types and levels in the separate confinement and in the hole-injection regions were grown. The temperature dependence of the gain and loss coefficients was measured. Although the gain dropped appreciably with increasing temperature, the internal losses remained nearly constant. Internal losses as low as 4.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> were measured at room temperature in an optimized design. Internal quantum efficiency per cascade of 86% is achieved at 240 K. Room-temperature CW operation with a front facet output power of 42 mW was obtained in a 5-cascade structure with a 54% reduced size injection region. The laser operated at 3.8 μm.

Room-Temperature Continuous-Wave Operation of a Tunable External Cavity Quantum Cascade Laser

A room-temperature cw operation of a tunable external cavity (EC) quantum cascade laser (QCL) at an emitting wavelength of 4.6 μm is presented. Strain-compensation combined with two-phonon resonance in an active region design promises low threshold current density. A very low threshold current density of 1.47kA/cm2 for an EC-QCL operated in cw mode is realized. Single-mode cw operation with a side-mode suppression ratio of 20 dB and a wide tuning range of over 110cm−1 are achieved. Moreover, an even wider tuning range of over 135cm−1 is obtained in pulsed mode at room temperature.

Room-temperature continuous wave operation of distributed feedback quantum cascade lasers with watt-level power output

We demonstrate surface-grating distributed feedback quantum cascade lasers (QCLs) with a watt-level power output at 4.75 μm. A device with a 5 mm cavity length exhibits an output power of 1.1 W in room-temperature cw operation. Single-mode operation with a side mode suppression ratio of 30 dB is obtained in the working temperature of 15–105 °C. A double-lobed far field with negligible beam steering is observed. The significance of this demonstration lies in its simplicity and readiness to be applied to standard QCL wafers with the promise of high-power performances.

Room-Temperature Continuous-Wave Operation of InGaN-Based Blue-Violet Laser Diodes with a Lifetime of 15.6 Hours

We report our recent progress of investigations on InGaN-based blue-violet laser diodes (LDs). The room-temperature (RT) cw operation lifetime of LDs has extended to longer than 15.6 h. The LD structure was grown on a c-plane free-standing (FS) GaN substrate by metal organic chemical vapor deposition (MOCVD). The typical threshold current and voltage of LD under RT cw operation are 78 mA and 6.8 V, respectively. The experimental analysis of degradation of LD performances suggests that after aging treatment, the increase of series resistance and threshold current can be mainly attributed to the deterioration of p-type ohmic contact and the decrease of internal quantum efficiency of multiple quantum well (MQW), respectively.

Continuous-wave laser action around 2-μm in Ho^3+:Lu_2SiO_5

Continuous-wave (cw) laser action around 2 microm in Ho(3+)-doped Lu(2)SiO(5) (LSO) was demonstrated in this paper. Cryogenically cooled by liquid nitrogen, a 10-mm long Tm-sensitized (6% at.) Ho(0.4% at.):LSO produced a maximum output power of 3 W at 2.07 microm for incident diode power of 11 W at 786 nm, and a slope efficiency of 35% with respect to incident pump power. To achieve room-temperature operation of Tm, Ho:LSO laser, a 1-mm long microchip crystal was pumped by a high brightness diode, generating an output power of greater than 80 mW and a slope efficiency of 26% at 2.08 microm. Using a 1.91 microm Tm:YLF laser as an in-band pump source, room-temperature cw operation of singly-doped Ho: Lu(2)SiO(5) laser at 2106 nm was attained with a maximum output power of 2.8 W and a slope efficiency of 35% corresponding to absorbed pump power.

222–282 nm AlGaN and InAlGaN‐based deep‐UV LEDs fabricated on high‐quality AlN on sapphire

AbstractWe demonstrate 222–282 nm AlGaN and InAlGaN‐based deep ultraviolet (DUV) light‐emitting diodes (LEDs) fabricated on low threading dislocation density (TDD) AlN template. Low TDD AlN templates were realized by using ammonia (NH3) pulse‐flow multilayer (ML) growth technique. The edge‐ and screw‐type dislocation densities of AlN layer were reduced to 7.5 × 108 and 3.8 × 107, respectively. Single‐peaked electroluminescence (EL) were obtained for 222– 273 nm AlGaN multi quantum well (MQW) DUV‐LEDs. We obtained the maximum output power of 1.1 mW and 4.0 mW for the AlGaN‐QW LEDs with wavelengths of 241 nm, 256 nm, respectively, under room temperature (RT) CW operations. The maximum output power of 227 nm and 222 nm AlGaN‐QW were 0.15 mW and 0.014 mW, respectively, under RT pulsed operation. The maximum external quantum efficiency (EQE) of the 227 nm and 250 nm AlGaN LEDs were 0.2% and 0.43%, respectively. We also fabricated 280 nm band quaternary InAlGaN‐MQW DUV‐LEDs with n‐type and p‐type InAlGaN layers on ML‐AlN templates. We demonstrated extremely high internal quantum efficiency (IQE) of 284 nm InAlGaN‐QW emission, which was confirmed by the fact that the ratio of the integrated intensity of the RT‐PL against the 77 K‐PL was 86%. The maximum output power and EQE of the 282 nm InAlGaN LED were 10.6 mW and 1.2%, respectively, under RT CW operation. (© 2009 WILEY‐VCH Verlag GmbH &amp; 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 &amp; 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 &amp; Co. KGaA, Weinheim)

Quaternary InAlGaN quantum‐dot ultraviolet light‐emitting diode emitting at 335 nm fabricated by anti‐surfactant method

AbstractIn order to realize high‐efficiency ultraviolet (UV) light‐emitting diodes (LEDs) or low threshold current UV laser diodes (LDs), group‐III nitride quantum dots (QDs) are very attractive. We fabricated quaternary InAlGaN quantum dots (QDs) using an anti‐surfactant method. The InAlGaN‐QDs were grown by low‐pressure metal‐organic chemical‐vapor deposition (MOCVD) on an InAlGaN buffer surface treated by a silicon anti‐surfactant. The lateral size and height of the self‐assemble InAlGaN QDs were 8‐18 nm and 5‐8 nm respectively as observed by the atomic force microscope (AFM). We observed intense photoluminescence (PL) from the InAlGaN QDs at room temperature (RT). We also fabricated an InAlGaN‐QD UV‐LED on a high‐quality AlN/AlGaN buffer template grown on a sapphire substrate. We obtained current injection emission of the InAlGaN‐QD UV‐LED with the wavelength of 335nm under RT CW operation. (© 2008 WILEY‐VCH Verlag GmbH &amp; 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 &amp; 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 &amp; 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 &amp; Co. KGaA, Weinheim)

227-261nm AlGaN-based Deep Ultraviolet Light-emitting Diodes Fabricated on High-quality AlN Buffer on Sapphire

We demonstrated AlGaN multi-quantum well (MQW) deep ultraviolet (UV) light-emitting diodes (LEDs) with wavelength in the range of 227-261 nm fabricated on high-quality AlN buffers on sapphire substrates. We achieved crack-free, thick AlN buffer on sapphire with low threading dislocation density (TDD) and atomically flat surface by introducing an ammonia (NH3) pulse-flow multi-layer (ML) growth method through metal-organic chemical vapor deposition (MOCVD). The edge- and screw-type dislocation densities of AlGaN layer on AlN buffer were reduced to 7.5×108 and 3.8×107 cm-2, respectively, by using a ML-AlN buffer. We achieved single-peaked high-brightness operations of AlGaN deep-UV LEDs by fabricating them on the ML-AlN buffers on sapphire substrates. The maximum output power and external quantum efficiency (EQE) of the 261 nm and 227.5 nm LEDs were 1.65 mW and 0.23% under room-temperature (RT) CW operation, and 0.15 mW and 0.2%, under RT pulsed operation, respectively.