Quantum Cascade Laser

The mid-infrared region (approximately 3–20 μm) is often referred to as the “molecular fingerprint” region, as it contains the fundamental vibrational absorption bands of most molecules. This makes it a highly valuable spectral range for chemical composition analysis and molecular detection. A prominent class of semiconductor lasers operating in this region is the quantum cascade laser (QCL). Despite their compact size—often no larger than a fingernail—QCLs can generate powerful mid-infrared output, enabling applications that were previously difficult or unfeasible using conventional sources.

Unipolar Cascade Mechanism

Unlike conventional semiconductor lasers that rely on electron–hole recombination, QCLs are unipolar devices that utilize intersubband transitions of electrons within multiple quantum well structures. Electrons cascade down through dozens of precisely engineered active regions, emitting a photon at each step. This cascading mechanism allows a single electron to generate multiple photons, resulting in high quantum efficiency and strong optical output power.

Tunable Mid-IR Emission

One of the most significant advantages of QCLs is their wavelength tunability. Since the emission wavelength is determined by the quantum well structure rather than the material bandgap, it can be precisely engineered for specific target molecules. QCLs have demonstrated emission capabilities ranging from 2.75 μm up to 161 μm (1.9 THz), making them highly versatile mid-infrared sources for applications including gas sensing, spectroscopy, medical diagnostics, and defense technologies.

Episolution specializes in the fabrication of QCL epi-wafers by stacking III–V semiconductor thin films—such as InGaAs, InAlAs, and InGaAsP—on InP substrates. Our core growth technique is metal-organic chemical vapor deposition (MOCVD), a widely adopted process in the semiconductor industry known for its high reproducibility and ability to produce multiple wafers with uniform quality in a single run.

Nanometer-Scale Lattice Matching

Our QCL structures typically comprise active and barrier layers made of In₀.₅₃Ga₀.₄₇As and In₀.₅₂Al₀.₄₈As, precisely lattice-matched to InP substrates. We specialize in achieving nanometer-scale precision, as even a 0.1% lattice mismatch across hundreds of stacked layers can lead to dislocations that degrade device performance. Through iterative calibration and in-situ process feedback, we finely control layer composition and thickness to ensure structural coherence throughout the entire device.

After growth, we utilize high-resolution X-ray diffraction (HRXRD) to verify layer thickness and crystal quality, photoluminescence (PL) mapping to evaluate optical uniformity, and L-I-V (light–current–voltage) characterization to assess electrical performance.

Process Verification & Feedback Loop

Furthermore, TEM analysis confirmed that the calibrated layer thickness based on HRXRD closely matched the target value within a deviation of less than 5%. This demonstrates the critical importance of precise thickness control for ensuring QCL device performance, and highlights the high reproducibility and reliability of our fabrication process.

These measurement results are incorporated into the process feedback loop, enabling continuous quality improvement and stable epitaxial growth for high-performance QCL devices.

Mid-infrared (mid-IR) quantum cascade lasers(QCLs) have revolutionized sensing and imaging due to two key advantages:
1. atmospheric transparencyat specific mid-IR wavelengths
2. strong vibrational absorptionsof many molecules in this spectral range
These properties make QCLs ideal for precise detection and identification of chemical species in various fields such as:

• Threshold Current Density
  Threshold current density refers to the minimum current per unit area required for a laser to initiate stimulated emission and achieve lasing. Below this threshold, only spontaneous emission occurs; above it, the optical gain surpasses losses, enabling laser action. A lower threshold current density results in reduced power consumption and heat generation, making it especially important for high-temperature operation, low-power devices, and compact or portable laser systems.


• Wall-Plug Efficiency
  Wall-plug efficiency describes how effectively a laser converts electrical input power into emitted optical power. It plays a critical role in determining overall system efficiency. Lasers with high wall-plug efficiency consume less electrical power and generate less heat, thereby reducing the demands on power supplies and thermal management systems.


• Output Power
  Output power refers to the total mid-infrared optical energy emitted during operation and serves as a key indicator of device performance. High output power directly enhances signal quality, detection range, and measurement precision in a wide range of QCL applications such as long-range gas sensing, infrared countermeasures (IRCM), spectroscopy, and medical diagnostics.


• Slope Efficiency
  Slope efficiency refers to the rate at which the optical output power increases with respect to the input drive current, typically expressed as dP/dI. Unlike conventional diode lasers, QCLs rely on intersubband transitions in a cascade of quantum wells, where the overall efficiency is determined by photon extraction per electron, internal optical losses, and waveguide confinement.


• Emission Wavelength
  The emission wavelength of a Quantum Cascade Laser (QCL) is determined by the engineered energy spacing between quantum states within the active region, rather than by the material bandgap as in conventional diode lasers. This enables precise tailoring of the lasing wavelength across a wide portion of the mid-infrared (mid-IR) and terahertz (THz) spectrum—typically from 3 µm to over 12 µm, and even into the THz range.


• Operation Mode
  Quantum Cascade Lasers (QCLs) can operate in either continuous wave (CW) mode or pulsed mode, depending on the design and application requirements.
  
  1) Continuous Wave (CW) Mode
  In CW mode, the QCL emits a stable and continuous laser beam as long as the drive current exceeds the lasing threshold.
  
  2) Pulsed Mode
  In pulsed mode, the QCL is driven by periodic current pulses, which reduces thermal load and lowers average power consumption.