🔬 mesoscale physics

Gain and Threshold Improvements of 1300 nm Lasers based on InGaAs/InAlGaAs Superlattice Active Regions

This article demonstrates that the use of highly strained InGaAs/InAlGaAs superlattice active regions significantly improves the gain, efficiency, and high-temperature performance of 1300-nm lasers by achieving low internal losses and enhanced characteristic temperatures, indicating great potential for VCSEL applications.

Original authors: Andrey Babichev, Evgeniy Pirogov, Maksim Sobolev, Sergey Blokhin, Yuri Shernyakov, Mikhail Maximov, Andrey Lutetskiy, Nikita Pikhtin, Leonid Karachinsky, Innokenty Novikov, Anton Egorov, Si-Cong Tian

Published 2026-04-30

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Andrey Babichev, Evgeniy Pirogov, Maksim Sobolev, Sergey Blokhin, Yuri Shernyakov, Mikhail Maximov, Andrey Lutetskiy, Nikita Pikhtin, Leonid Karachinsky, Innokenty Novikov, Anton Egorov, Si-Cong Tian, Dieter Bimberg

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a very efficient, high-speed flashlight that emits a specific color of invisible light (infrared) used for things like sensing the world around us or transmitting data between computers. The "engine" in this flashlight is a tiny laser chip. The problem is that when these chips get hot, they often stutter, become inefficient, or require too much power to function.

This article is about a team of scientists who attempted to redesign the "engine" of these 1300-nanometer lasers to make them cooler, stronger, and more efficient, especially when they get warm.

Here is a breakdown of their work using simple analogies:

1. The Problem: The "Leaky Bucket"

Imagine the active region of the laser (where the light is generated) as a bucket containing water (electrons). To generate light, you must fill this bucket.

Old Design: You used standard "quantum wells." Imagine these as flat, wide bowls. When the temperature rises, the water (electrons) easily splashes over the rim. This is called "thermal escape." To keep the laser functioning, you must pour in much more water (current), which wastes energy and generates more heat.
The Goal: You wanted to build a bucket that holds the water even when the room gets hot.

2. The Solution: The "Superlattice" Staircase

Instead of a single flat bowl, the team built a superlattice.

The Analogy: Imagine replacing one large bowl with a stack of many tiny, flat steps (like a staircase) made from different materials (InGaAs and InAlGaAs).
How it helps: In this staircase design, the "floor" where the electrons sit is lower than in the old design. It is like digging a deeper hole for your water. Even when the temperature rises and the water gets agitated, it is much harder to jump out of this deeper hole. This keeps the electrons trapped where they need to be to generate light.

3. The Experiment: Testing Three Different Buckets

The scientists grew three slightly different versions of this "staircase" to see which worked best:

Version 1: A standard design.
Version 2: A design with high "strain" (the materials are slightly stretched) and thinner steps.
Version 3: A design with even thinner steps but different barrier materials.

They turned these into broad-area lasers (essentially flat, wide lasers used to test the engine before it is installed into a tiny VCSEL device) and measured their performance.

4. The Results: The Winner

Version 2 was the clear champion. Here is what they found, translated into everyday terms:

Less Friction (Internal Losses): The laser lost very little energy as heat inside the chip. It was like driving a car with a perfectly lubricated engine compared to a rusty one.
Easier to Start (Threshold): It required much less current to start glowing. They measured a "transparency current" of about 500 A/cm², which is very low. Imagine it as the car needing only a tiny push to get moving.
Stronger Light (Gain): Once running, it produced a large amount of light power relative to the current used.
Heat Resistance: This is the big win. They measured how the laser performed as the temperature rose from 20°C to 80°C.
- The "characteristic temperature" (a value for thermal stability) rose to 76 K for the threshold current and 100 K for efficiency.
- The Metaphor: If the old lasers were like ice cream melting quickly in the sun, this new design is like an ice block that stays solid much longer in the same heat.

5. Why This Matters (According to the Article)

The article states that these results provide a "roadmap" for building better VCSELs (Vertical-Cavity Surface-Emitting Lasers).

VCSELs are the tiny, efficient lasers used in sensors, 3D face scans, and high-speed data centers.
The team found that by using this "superlattice" staircase instead of the old "quantum well" bowls, they could potentially:
- Reduce the current required to start the laser by about 23%.
- Increase the speed at which the laser can be turned on and off (differential gain) by at least 33%.
- Make the laser much more stable when it gets hot.

Summary

The scientists replaced a simple, flat bowl with a complex, deep staircase made of materials. This new design captures energy better, requires less power to start, and does not give up when the temperature rises. This proves that this specific "staircase" design is a superior engine for the next generation of 1300-nm lasers used in sensing and communication.

1. Problem Statement

Short-wave infrared (SWIR) Vertical-Cavity Surface-Emitting Lasers (VCSELs) operating at 1300 nm are critical for sensing, data center connectivity, and 3D imaging. However, the development of high-performance 1300 nm VCSELs faces significant challenges:

Thermal Instability: Conventional compressively strained InAlGaAs quantum wells (QWs) suffer from high Shockley-Read-Hall (SRH) recombination and thermal carrier escape, leading to high threshold currents and poor temperature stability.
Material Limitations: Approaches using dilute nitrides encounter growth difficulties and reliability issues.
Gain and Efficiency: Existing strain-balanced QW designs often exhibit low modal gain and high internal losses, limiting output power and modulation bandwidth.
Optimization Need: There is a necessity to improve the active region design to reduce transparency current density, increase differential gain, and enhance characteristic temperatures ( $T_0$ and $T_1$ ) for high-temperature operation.

2. Methodology

The authors investigated the influence of active region design on laser performance by fabricating and testing three different broad-area (BA) edge-emitting laser structures using Molecular Beam Epitaxy (MBE).

Active Region Design: All structures utilized InGaAs/InAlGaAs Superlattices (SLs) instead of single quantum wells. The SL approach was chosen to lower the miniband energy position compared to thin QWs, thereby improving carrier confinement and temperature stability.
Comparison of Three Structures:
1. Sample 1: 21 periods of In $_{0.60}$ Ga $_{0.40}$ As (1.3 nm) / In $_{0.53}$ Al $_{0.25}$ Ga $_{0.22}$ As (2 nm). Strain mismatch ( $\epsilon$ ) $\approx$ 0.48 %.
2. Sample 2: 23 periods of In $_{0.74}$ Ga $_{0.26}$ As (1.0 nm) / In $_{0.53}$ Al $_{0.25}$ Ga $_{0.22}$ As (2 nm). High strain mismatch ( $\epsilon$ = 1.44 %).
3. Sample 3: 27 periods of In $_{0.74}$ Ga $_{0.26}$ As (0.6 nm) / In $_{0.53}$ Al $_{0.20}$ Ga $_{0.27}$ As (2 nm). High strain mismatch with thinner wells and different barrier composition.
Fabrication & Testing:
- The structures were grown on n-doped InP substrates with Separate Confinement Heterostructure (SCH) layers.
- Broad-area lasers (400 $\mu$ m wide, 100 $\mu$ m pumped) with varying cavity lengths were fabricated.
- Measurements: Light-Current-Voltage (LIV) characteristics were measured in pulsed operation (300 ns, 4 kHz) over a temperature range of 20°C to 80°C.
- Analysis: Parameters such as internal loss ( $\alpha_i$ ), internal efficiency ( $\eta_i$ ), transparency current density ( $J_{tr}$ ), threshold modal gain ( $\Gamma \cdot G_0$ ), and characteristic temperatures ( $T_0, T_1$ ) were extracted using standard linear approximation methods.

3. Key Contributions

Superlattice Optimization: It was demonstrated that InGaAs/InAlGaAs superlattices can effectively replace traditional QWs for 1300 nm lasers, offering superior carrier confinement through miniband formation.
Strain Engineering: It was shown that increasing the strain mismatch to 1.44 % (Sample 2) significantly reduces transparency current density without compromising crystal quality.
Differential Gain Analysis: Experimental evidence was provided that SL-based active regions offer significantly higher differential gain compared to conventional InAlGaAs QWs, a critical factor for high-speed modulation in VCSELs.
Temperature Stability: A correlation between miniband depth and characteristic temperature was established, proving that deeper minibands (achieved in Sample 2) lead to superior high-temperature performance.

4. Key Results

The study yielded the following quantitative results, with Sample 2 (highly strained In $_{0.74}$ Ga $_{0.26}$ As/In $_{0.53}$ Al $_{0.25}$ Ga $_{0.22}$ As SL) emerging as the optimal design:

Parameter	Sample 1 (Low Strain)	Sample 2 (High Strain)	Sample 3 (Thin Wells)
Internal Loss ( $\alpha_i$ )	8 cm $^{-1}$	6 cm $^{-1}$ (Lowest)	10 cm $^{-1}$
Transparency Current ( $J_{tr}$ )	590 A/cm $^2$	500 A/cm $^2$ (Lowest)	640 A/cm $^2$
Internal Efficiency ( $\eta_i$ )	54 %	53 %	43 %
Modal Gain ( $\Gamma \cdot G_0$ )	45 cm $^{-1}$	46 cm $^{-1}$	49 cm $^{-1}$
Characteristic Temp. ( $T_0$ )	68 K	76 K	62 K
Characteristic Temp. ( $T_1$ )	N/A	100 K	N/A
Max. External QE (1mm)	31 %	37 %	N/A

Low Losses & High Efficiency: Sample 2 achieved an internal loss of ~6 cm $^{-1}$ (25 % lower than previous QW reference values) and a maximum external quantum efficiency of 37 % for 1 mm long cavities.
Threshold Reduction: The transparency current density was reduced to ~500 A/cm $^2$ , corresponding to an improvement of ~23 % compared to standard InAlGaAs QW designs.
Temperature Performance: Characteristic temperatures reached $T_0 = 76$ K and $T_1 = 100$ K, indicating robust operation at elevated temperatures.
Differential Gain: The differential gain ($dg/dj$) for Sample 2 was determined to be 33 % higher near threshold and 53 % higher near roll-over current compared to reference InAlGaAs QW lasers.

5. Significance and Impact

VCSEL Advancement: Although this study used broad-area edge emitters, the results are directly applicable to 1300 nm VCSELs. The improved gain, lower threshold, and higher differential gain suggest that SL-based active regions can overcome the limitations of current strain-balanced QW VCSELs.
High-Speed Applications: The significant increase in differential gain is crucial for achieving higher modulation bandwidths (potentially >25 Gbps) required for next-generation data center connectivity.
Energy Efficiency: Lower threshold currents and internal losses lead to higher wall-plug efficiency, which is critical for reducing heat dissipation in high-density laser arrays used in 3D sensing and LiDAR.
Scalability: The use of MBE-grown superlattices offers a reliable alternative to the difficult growth of dilute nitrides and could enable mass production of high-performance SWIR devices in the 1300–2000 nm range.

In summary, the paper demonstrates that optimizing the strain and composition of InGaAs/InAlGaAs superlattices creates a superior active region for 1300 nm lasers, paving the way for more efficient, powerful, and faster SWIR VCSELs.