Long wavelength quantum cascade lasers based on high strain composition

Abstract

An improved longwave infrared quantum cascade laser. The improvement includes a strained InxGa1-xAs/AlyIn1-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 μm, an energy spacing E54 equal to or greater than 50 meV, an energy spacing EC4 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 2·1016 cm−3. The optical waveguide also has a top InP layer with a doping level of about 5·1016 cm−3 and a bottom InP layer with a doping level of about 5 1016 cm−3. Additionally, the optical waveguide has a plasmon layer with a doping level of about 8·1018 cm−3.

Description

TECHNICAL FIELD

This invention relates to quantum cascade lasers.

BACKGROUND ART

Quantum cascade lasers (QCLs) of the type currently known in the art are described, among other places, in U.S. Pat. Nos. 7,903,704; 8,014,430; and 8,121,164,

Driven by a strong demand for a number of commercial and defense applications, research on midwave infrared (MWIR) QCLs emitting in the first atmospheric window (3.5-4.8 μm) have resulted in a significant progress in laser performance over the last several years. (See [Ref. 1] and [Ref 2].) However, since room temperature QCL characteristics could not be fully described by practical models that would not rely on computation-intensive numerical simulations, MWIR. QCL development was mostly guided by general principles, without a systematic analysis of relative contribution of different laser design parameters to overall laser performance. This likely will hinder further progress in laser performance.

While some success has been achieved in calculating threshold current density and its temperature dependence (see [Ref 3]), there is still a significant discrepancy between theoretical and experimental data for slope efficiency of MWIR QCLs.

In a simple model based on the rate equations, slope efficiency can be presented in the following form:

$\begin{array}{cc}\frac{dP}{dI}\approx \frac{h\vartheta }{q}{N}_s\frac{{\alpha }_m}{{\alpha }_m+{\alpha }_w}\frac{1}{1+{\tau }_3/{\tau }_4}{\eta }_i& \left(\mathrm{Equation}1\right)\end{array}$ where N_S is the number of cascade stages, α_m are the mirror losses, α_w are the waveguide losses, τ₄ is the upper laser level lifetime, τ₃ is the lower laser level lifetime, and η_i is the injection efficiency, which is usually determined by fitting the results of Equation 1 to experimental data. Injection efficiency for MWIR QCLs is typically reported to be in the range of 50% to 60%. (See [Ref, 4] and [Ref. 5].)

The root cause of the problem why simple models do not adequately describe room temperature laser characteristics is that the injection efficiency term is a function of carrier leakage from the upper laser level that is very difficult to fully account for. As a consequence, unintentional changes in injection efficiency often mask targeted changes in laser design. The best approach to study this term would be first designing a structure with nearly ideal injection efficiency and then modifing the structure by changing, for example, band offset to study corresponding changes in injection efficiency in a controllable manner.

Large laser transition energy for MWIR QCLs leads to a high position of the upper laser level, close to the top of the Γ-valley barriers and bottom of indirect-valley quantum wells. As a consequence, it is difficult to entirely suppress these leakage paths in MWIR QCLs. in addition, it is difficult to evaluate individual contributions of the two types of carrier leakage, i.e. leakage through continuum and indirect states.

The situation is more favorable in the case of longwave infrared (MIR) QCLs emitting in the second atmospheric window (8-12 μm). Since laser transition is much smaller, it is easier to confine carriers on the upper laser level.

LWIR QCLs are traditionally designed using lattice matched AlInAs/InGaAs composition that has a relatively small band offset of 520 meV. For emission wavelength of ˜9 μm, this band offset results in ˜250 meV energy spacing between the upper laser level and the continuum states located above the barriers, similar to that of MWIR QCLs. Therefore, the band offset of the lattice matched composition is not sufficient for taking full advantage of smaller transition energy of LWIR QCLs.

The main reason for using the lattice matched composition is that linewidth of the laser transition is expected to increase with increase in band offset, i.e. with increase in strain, which, turn, reduces material . However, we experimentally showed recently that highly strained QCL designs can have line width similar to that of designs based on significantly to lower strain composition. (See [Ref. 1].) Employment of high strain to LWIR QCL design therefore presents a promising way of improving laser performance and studying carrier leakage in QCL structures.

DISCLOSURE OF INVENTION

One purpose of this invention is to improve LWIR. QCL performance by employing a high strain active region composition, To do so, the invention provides an improved longwave infrared quantum cascade laser. The improvement includes a strained In_xGa_1-xAs/Al_yIn_1-yAs composition, an active region emitting at a wavelength equal to or greater than 8 μm, an energy spacing E₅₄ equal to or greater than 50 meV, an energy spacing E_C4 equal to or greater than 250 meV gird an optical waveguide with a cladding layer on each side of the a active region. In the composition formula, x and y are each between 0.53 and 1. Each cladding layer has a doping level of about 2·10¹⁶ cm⁻³. The optical waveguide also has a top InP layer with a doping level of about 5·10¹⁶ cm⁻³ and a bottom InP layer with a doping level of about 5·10¹⁶ cm⁻³. Additionally, the optical waveguide has a plasmon layer with a doping level of about 8·10¹⁸ cm⁻³.

In a version of the invention, the composition is a highly strained In_0.5840Ga_0.4160As/Al_0.6417In_0.3583As composition, and the active region has 45 stages emitting at a wavelength of about 9 μm. In a version of the invention, the cladding layers are each about 3 μm thick, the top InP layer of the optical waveguide is about 4 μm thick, the bottom InP layer of the optical waveguide is about 2 μm thick, and the plasmon layer is about 1 μm thick. This optical waveguide design results a free-carrier waveguide loss of 2.1 cm⁻¹ and a mode overlap factor with the active region of 52%.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a band diagram of a quantum cascade laser structure based on In_0.5840Ga_0.4160As/Al_0.6417In_0.3583As composition and designed using non-resonant extraction principle for light emission at λ˜9 μm.

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 μm laser (λ˜9 μm) 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

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.

The new 9 μm active region design presented here was based on a highly strained In_0.5840Ga_0.4160As/Al_0.6417In_0.3583As composition. A conduction band diagram of two gain stages of the new design is shown in FIG. 1. Energy spacings E₅₄ and E_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_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.

The optical waveguide was designed to achieve low free-carrier optical losses. To reduce these losses, the doping level was kept low (2·10¹⁶ cm⁻³) in the 3 μm-thick cladding layers adjacent to the 45-stage active region design described above. The rest of the waveguide structure consists of 4 μm-thick (top) and 2 μm-thick (bottom) low doped (5·10¹⁶ cm⁻³) InP layers and a highly doped (8·10¹³ cm⁻³) 1 μm-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 α_fc=2.1 cm⁻¹ and mode overlap factor with the active region of 52%.

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 (2·10¹⁷ cm⁻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.

Pulsed mode spectrum for the grown structure was centered at ˜9.2 μm. 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 μm 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/cm², 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.

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.

Improved injection efficiency led to a. much better correspondence between measured pulsed slope efficiency and its predicted value calculated using Equation 1. If α_m=4.2 cm⁻¹, α_w=α_fc=2.1 cm⁻¹, τ₄=1.22 ps, τ₃=0.25 ps, and η_i=1 are used in Equation 1, corresponding slope efficiency equals ˜3.3 cm⁻¹, very close to its pulsed measured value of 2.8 cm⁻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.

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⁻¹. Taking into account that α_m=4.2 cm⁻¹ and α_fc=2.1 cm⁻¹, the upper limit on combination of all the other losses, such roughness scattering and intersubband losses, is 1.2 cm⁻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 α_wα_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.

In conclusion, we have presented experimental data on 9 μm 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 μm 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.

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

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

REFERENCES

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

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

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

[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).

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

Claims

1. An improved longwave infrared quantum cascade laser, the improvement comprising: (a) a highly strained In0.5840Ga0.4160As/Al0.6417In0.3583As composition;(b) an active region having 45 stages and emitting at a wavelength of 9 μm;(c) an energy spacing E54 of 60 meV;(d) an energy spacing EC4 of 430 meV; and(e) an optical waveguide with 3 μm thick cladding layers adjacent the active region, the cladding layers having a doping level of 2·1016 cm−3, the optical waveguide further having a 4 μm thick top InP layer with a doping level of 5·1016 cm−3 and a 2 μm thick bottom InP layer with a doping level of 5·1016 cm−3, the optical waveguide further having a 1 μm thick plasmon layer with a doping level of 8·1018 cm−3, where the optical waveguide results a free-carrier waveguide loss of 2.1 cm−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 InxGa1-xAs/AlyIn1-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 μm;(c) an energy spacing E54 equal to or greater than 50 meV;(d) an energy spacing EC4 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 2·1016 cm−3, the optical waveguide further having a top InP layer with a doping level of 5·1016 cm−3 and a bottom InP layer with a doping level of 5·1016 cm−3, the optical waveguide further having a plasmon layer with a doping level of 8·1018 cm−3, where x=0.5840 and y=0.6417.