Long wavelength quantum cascade lasers based on high strain composition

Abstract

An improved longwave infrared quantum cascade laser. The improvement includes a strained In.sub.xGa.sub.1-xAs/Al.sub.yIn.sub.1-yAs composition, with x and y each between 0.53 and 1, an active region emitting at a wavelength equal to or greater than 8 m, an energy spacing E.sub.54 equal to or greater than 50 meV, an energy spacing E.sub.C4 equal to or greater than 250 meV, and an optical waveguide with a cladding layer on each side of the active region. Each cladding layer has a doping level of about 2.Math.10.sup.16 cm.sup.3. The optical waveguide also has a top InP layer with a doping level of about 5.Math.10.sup.16 cm.sup.3 and a bottom InP layer with a doping level of about 510.sup.16 cm.sup.3. Additionally, the optical waveguide has a plasmon layer with a doping level of about 8.Math.10.sup.18 cm.sup.3.

Claims

1. An improved longwave infrared quantum cascade laser, the improvement comprising: (a) a highly strained In.sub.0.5840Ga.sub.0.4160As/Al.sub.0.6417In.sub.0.3583As composition; (b) an active region having 45 stages and emitting at a wavelength of 9 m; (c) an energy spacing E.sub.54 of 60 meV; (d) an energy spacing E.sub.C4 of 430 meV; and (e) an optical waveguide with 3 m thick cladding layers adjacent the active region, the cladding layers having a doping level of 2.Math.10.sup.16 cm.sup.3, the optical waveguide further having a 4 m thick top InP layer with a doping level of 5.Math.10.sup.16 cm.sup.3 and a 2 m thick bottom InP layer with a doping level of 5.Math.10.sup.16 cm.sup.3, the optical waveguide further having a 1 m thick plasmon layer with a doping level of 8.Math.10.sup.18 cm.sup.3, where the optical waveguide results a free-carrier waveguide loss of 2.1 cm.sup.1 and a mode overlap factor with the active region of 52%.

2. An improved longwave infrared quantum cascade laser, the improvement comprising: (a) a strained In.sub.xGa.sub.1-xAs/Al.sub.yIn.sub.1-yAs composition, where x is between 0.53 and 1 and y is between 0.53 and 1; (b) an active region emitting at a wavelength equal to or greater than 8 m; (c) an energy spacing E.sub.54 equal to or greater than 50 meV; (d) an energy spacing E.sub.C4 equal to or greater than 250 meV; and (e) an optical waveguide with a cladding layer on each side of the active region, each cladding layer having a doping level of 2.Math.10.sup.16 cm.sup.3, the optical waveguide further having a top InP layer with a doping level of 5.Math.10.sup.16 cm.sup.3 and a bottom InP layer with a doping level of 5.Math.10.sup.16 cm.sup.3, the optical waveguide further having a plasmon layer with a doping level of 8.Math.10.sup.18 cm.sup.3, where x=0.5840 and y=0.6417.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a band diagram of a quantum cascade laser structure based on In.sub.0.5840Ga.sub.0.4160As/Al.sub.0.6417In.sub.0.3583As composition and designed using non-resonant extraction principle for light emission at 9 m.

(2) FIG. 2 is a comparison between pulsed and CW optical power vs. current and voltage vs. current characteristics measured at 293K for an uncoated 3 mm by 10 m laser (9 m) mounted epi-down on a AlN/SiC composite submount. Total Optical Power is shown with the dashed line, Voltage is shown in the mid-weight line, and Wallplug Efficiency is shown in the heavy-weight line.

BEST MODE FOR CARRYING OUT THE INVENTION

(3) The detailed description set forth below in connection with the appended drawings is intended as a description of presently-preferred embodiments of the invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description meta forth the functions and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments. However, it is to be understood that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention.

(4) The new 9 m active region design presented here was based on a highly strained In.sub.0.5840Ga.sub.0.4160As/Al.sub.0.6417In.sub.0.3583As composition. A conduction band diagram of two gain stages of the new design is shown in FIG. 1. Energy spacings E.sub.54 and E.sub.3-inj were designed to be approximately 60 meV and 135 meV, respectively. Energy spacing between the upper laser level and top of the barriers, E.sub.C4, was increased from 250 meV typically reported for WMIR and LWIR QCLs up to 430 meV. Calculated laser transition matrix element and upper laser lifetime for this design were 2.44 nm and 1.22 ps, respectively.

(5) The optical waveguide was designed to achieve low free-carrier optical losses. To reduce these losses, the doping level was kept low (2.Math.10.sup.16 cm.sup.3) in the 3 m-thick cladding layers adjacent to the 45-stage active region design described above. The rest of the waveguide structure consists of 4 m-thick (top) and 2 m-thick (bottom) low doped (5.Math.10.sup.16 cm.sup.3) InP layers and a highly doped (8.Math.10.sup.13 cm.sup.3) 1 m-thick plasmon layer, which helps to decouple the optical mode from the lossy metal contact. This waveguide design resulted in calculated free-carrier waveguide losses of .sub.fc=2.1 cm.sup.1 and mode overlap factor with the active region of 52%.

(6) The 45-stage quantum cascade laser active region, along with the waveguide and contact layers was grown by molecular beam epitaxy on a low doped (2.Math.10.sup.17 cm.sup.3) InP substrate. The wafer was then processed into a buried heterostructure geometry and cleaved into individual laser chips. Finally, the laser chips were mounted epi-side down on AlN/SiC composite submounts for pulsed and continuous wave (CW) characterization. Pulsed testing was performed with 500 ns pulses and 0.5% duty cycle.

(7) Pulsed mode spectrum for the grown structure was centered at 9.2 m. FIG. 2 shows a comparison between pulsed and CW optical power vs. current (LI) and voltage vs. current (IV) characteristics at 293 K for a 3 mm by 10 m uncoated laser. Threshold current density, slope efficiency, maximum wallplug efficiency (WPE) and maximum total optical power in pulsed/CW modes were measured to be 2.1/2.5 kA/cm.sup.2, 2.8/2.1 W/A, 16/10%, and 4.4/2.0 W, respectively. Both optical power and efficiency in pulsed/CW mode are the highest values reported at this wavelength.

(8) An important aspect of the LIV curves shown in FIG. 2 is the behavior at bias above Li curve rollover. The pulsed LI curve experiences a very abrupt decrease in optical power, while the pulsed IV curve shows signs of negative differential resistance (NDR), which has not been demonstrated for QCLs at room temperature yet. NDR was more pronounced for some of the other chips processed from the same wafer. This behavior in the vicinity of the rollover condition demonstrates that carrier tunneling from the injector to the active region states other than the upper laser level is suppressed. In other words, these results indicate improved injection efficiency for the upper laser level.

(9) Improved injection efficiency led to a. much better correspondence between measured pulsed slope efficiency and its predicted value calculated using Equation 1. If .sub.m=4.2 cm.sup.1, .sub.w=.sub.fc=2.1 cm.sup.1, .sub.4=1.22 ps, .sub.3=0.25 ps, and .sub.i=1 are used in Equation 1, corresponding slope efficiency equals 3.3 cm.sup.1, very close to its pulsed measured value of 2.8 cm.sup.1. The importance of this result is that it was obtained using standard waveguide simulations with well-studied input parameters for bulk refractive indices and free-carrier losses and carrier lifetimes that are routinely calculated in laser active region design. As a consequence, slope efficiency can be predicted to a high degree of accuracy based on a relatively simple device modeling.

(10) By using either injection efficiency or waveguide losses in Equation 1 as the only fitting parameter for experimental slope efficiency, we can estimate two important limits. First, if injection efficiency is assumed to be 100%, then total losses have to be 7.5 cm.sup.1. Taking into account that .sub.m=4.2 cm.sup.1 and .sub.fc=2.1 cm.sup.1, the upper limit on combination of all the other losses, such roughness scattering and intersubband losses, is 1.2 cm.sup.1. This shows that free carrier losses have the highest contribution to waveguide losses for this laser design and need to be further reduced to improve laser performance. On the other hand, assuming that waveguide losses are entirely determined by free-carrier absorption, i.e. that .sub.w.sub.fc in Formula 1, and using injection efficiency as the only fitting parameter, we obtain the lower limit on the injection efficiency equal to 85%. The latter result is especially important since it shows that injection efficiency for LWIR QCLs based on high strain composition is approaching its upper limit of 100%. Therefore, these devices are especially interesting for systematic study of QCL operational characteristic as argued above.

(11) In conclusion, we have presented experimental data on 9 m QCLs with active region design based on a high strain composition. Record-high pulsed/CW WPE of 16%/10% and optical power of 4.4 W/2.0 W were demonstrated at 293 K for an uncoated 3 mm by 10 m laser mounted on AlN/SiC submounts. Very good correspondence was demonstrated between experimental data for pulsed slope efficiency and its value calculated using the expression in Equation 1 derived from the three-level model assuming 100% injection efficiency and that waveguide losses are dominated by free carrier losses.

(12) While the present invention has been described with regards to particular embodiments, it is recognized that additional variations of the present invention may be devised without departing from the inventive concept.

INDUSTRIAL APPLICABILITY

(13) This invention may be industrially applied to the development, manufacture, and use of quantum cascade lasers.

REFERENCES

(14) [Ref. 1] A. Lyakh, R. Maulini, A. Tsekoun, R. Go, and C. K. N. Patel, Optics Express 20, 4382 (2012).

(15) [Ref. 2] Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, Appl. Phys. Lett. 98, 181102 (2011).

(16) [Ref. 3] D. Botez, S. Kumar, J. C. Shin, L. I Mawst, I. Vurgaftman and J. R. Meyer, Appl. Phys. Lett. 97, 071101 (2010).

(17) [Ref. 4] R. Maulini, A. Lyakh, A. Tsekoun, R. Go, C. Pflugl, L. Diehl, F. Capasso, and C. K. N. Patel, Appl. Phys. Lett. 95, 151112 (2009).

(18) [Ref. 5] Y. Bai, S. Slivken, S. R. Darvish, and M. Razeghi, Proc. SPIE 7608, 7608F-1 (2010).