Long-wavelength and cascaded-transition quantum lasers

Quantum cascade lasers (QCLs) are semiconductor lasers that typically produce IR radiation. Since most important chemical trace gases have strong absorption lines in this spectral region, QCLs have become important laser sources for trace-gas sensing,¹ thanks to their unique wavelength versatility and high output power. Sensing techniques based on QCLs in the λ∼3.5–12μm wavelength range have been widely applied in different areas, ranging from environmental monitoring to medical diagnostics. However, there is still a strong need for high-performance long-wavelength (λ∼12–16μm) QCLs, which are crucial devices for improving the detection sensitivity for important gases including harmful BTEX (benzene, toluene, ethylbenzene, and xylenes) compounds or uranium hexafluoride.

Long-wavelength QCLs have a few challenges compared with shorter-wavelength devices. These include the short lifetime of the upper laser level (the energy level where electrons can emit photons by transitioning to a lower laser level), a low voltage efficiency, and high waveguide loss.² In earlier attempts, these issues lead λ∼12–16μm QCLs at room temperature to emit only at current densities above 3.5kA/cm², a threshold level higher than that of short-wavelength QCLs.^3–5

We developed two design approaches to address the above issues. One focuses on optimizing the optical transition and electron transport leading to low threshold (2.0kA/cm²) and temperature-insensitive performance.² The other employs a novel same-wavelength cascaded-transition scheme, meaning each electron experiences two subsequent optical transitions at the same wavelength, emitting two identical photons. This technique enhances the ‘re-use’ efficiency of electrons (the number of times each electron is used for optical transitions) within each stage of the QCL.⁶

We used a diagonal optical transition in our lasers, which resulted in a reduction in the overlap between the upper and lower laser levels, leading to a long lifetime (more than 2.0ps) for the upper level. We achieved depopulation of the lower level by employing a ‘two-phonon-continuum’ scheme. The lower laser level was depleted by two consecutive phonon scatterings via a single level ‘ll’—see Figure 1(a)—and the lower energy level continuum that spans from the bottom of the active region to the following injector, enabling fast electron extraction to the next stage of injector and active region.

Figure 1. (a) Portion of the conduction band diagram of the λ∼14μm QCL structure with the moduli squared of the relevant wave functions at an electric field of 35kV/cm. The optical transition is marked by the red arrow. The red indicated levels ‘u’ and ‘l’ are the upper and lower laser levels, respectively. The ‘two-phonon-continuum’ depletion is achieved through the green marked level ‘ll’ and the level continuum below it. The blue marked level ‘uu’ is ∼ 63meV above the upper laser level. (b) Laser spectra of our 3.8mm-long, 38μm-wide QCL with an applied current density of 1.1 times the threshold density, from 80K to 370K. (c) Pulsed light-current-voltage characteristics for our QCL at the indicated heat-sink temperatures, from 80K to 390K. The pulse width is 100ns and the repetition rate is 4kHz.² a.u.: Arbitrary units.

We grew this QC structure using a method for depositing single crystals called molecular beam epitaxy (MBE), and we manufactured our lasers using a standard wet-etching fabrication process.² Our 2.8mm-long, 38μm-wide QCL emited at ∼ 14μm—see Figure 1(b)—and showed a record-low threshold current density of 2.0kA/cm², an output power of 336mW, and a slope efficiency of 375mW/A at 300K, as well as temperature-insensitive performance with a high characteristic temperature: see Figure 1(c).²

Conventional QCLs (see Figure 1) are based on intersubband transitions (i.e. between two subbands in the conduction band) in repeated stages of precisely engineered coupled quantum wells, with one electron emitting at most one photon in each stage. To further improve the ‘re-use’ efficiency of electrons, we built a same-wavelength (∼ 14.2μm) cascaded-transition QC structure⁶ with two subsequent cascaded optical transitions per stage: see Figure 2(a). We designed these two transitions at the same wavelength to interact with a single-wavelength optical field and to contribute to its gain together. Our approach increases the quantum efficiency—by using each electron at most twice for optical transitions in each stage—and the voltage efficiency—since the voltage drop due to optical transitions is doubled in each stage. In addition, our devices have novel physical effects: unique light-intensity-dependent characteristics in population inversion, gain, pumping and depopulation rates.⁶

Figure 2. (a) Portion of the conduction-band diagram of the same-wavelength cascaded-transition QC structure at an applied electric field (E) of 70kV/cm. The conduction-band energy potential is plotted with black lines. |3〉, |2〉, and |1〉 are upper, middle, and lower laser levels, respectively, with the moduli of corresponding wave functions plotted in red, green, and blue. Other levels are in grey. The two red arrows indicate the optical transitions, both designed with λ∼ 14.2μm. (b) Light output of a 3.4mm-long, 23μm-wide ridge laser at 80K plotted with the lasing spectra at different injected currents of 5A, 6A and 7.1A (before, in, and after the quasi-saturation region, respectively). Single-facet-output slope efficiency in the linear regions before and after the quasi-saturation region are indicated in green and blue, respectively.⁶

Figure 2(b) shows the light-current characteristics of a same-wavelength cascaded-transition QC laser with an unusual saturation region. The lasing spectra before, in, and after the saturation region had the same center emission wavelength of 703cm⁻¹, as well as similar spectral shape. However, the slope efficiency after the quasi-saturation was 46% higher than that before it. Further simulations, which took into account possible electron injections into the higher laser levels |3〉 and |2〉—see Figure 2(a)—showed that electrons are injected to level |2〉 and that only the lower optical transition contributes to lasing before the saturation region.⁶ In contrast, after the saturation region, electrons were injected to level |3〉. Both cascaded optical transitions contributed to lasing together. Therefore, the saturation region is a transition region from operation with a single lower transition (like a conventional QCL) to operation with two cascaded transitions at the same wavelength to enhance efficiency.

The success of our approach broadens our perspective on engineering QCLs, not only limited to wavelength, gain, and electron transport, but also leading to novel and interesting interactions between light and matter. Applications of our high-performance long-wavelength QCL currently under investigation include toluene sensing aiming at the parts-per-billion concentration level. We continue to explore the potential nonlinear process in the cascaded-transition QC structure, which is of interest to the quantum-electronics community for its potential applications, such as high precision measurements, laser gyroscope, and quantum communication.⁷

This work is supported in part by the National Science Foundation's Engineering Research Center MIRTHE, Mid-Infrared Technologies for Health and the Environment.