Broadband mid-infrared quantum cascade lasers (QCLs) are essential devices for enabling cutting-edge applications, including widely tunable mid-infrared sources, ultrashort pulse generation, and mid-infrared optical frequency combs. However, conventional mid-infrared QCLs typically exhibit electroluminescence spectral bandwidths of merely ~100 cm⁻¹ at room temperature, rendering them inadequate for simultaneous multi-species gas detection. For QCL-based frequency combs, such limited spectral coverage also poses significant challenges for carrier-envelope offset (CEO) frequency stabilization. Constrained by the inherent complexity of active-region band structure engineering, the electroluminescence full width at half maximum (FWHM) of state-of-the-art long-wave infrared (LWIR) single-active-region QCLs remains generally below 500 cm⁻¹ (corresponding to a wavelength span of <1 μm), severely restricting their potential for broadband spectroscopic manipulation and precision measurement applications.
Currently, three predominant active-region design strategies have been pursued to achieve spectral broadening: the double-upper-level (DAU) design, the bound-to-continuum (BTC) design, and the continuum-to-continuum (CTC) design. Among these, the BTC configuration, serving as a classical approach, suffers from limited accessible upper-state populations, yielding a typical FWHM of only ~300 cm⁻¹. The DAU design mitigates this limitation by incorporating multiple upper states to expand the available transition channels, thereby increasing the FWHM to approximately 500 cm⁻¹; nevertheless, it remains constrained by insufficient transition channel multiplicity, resulting in modest spectral coverage (~0.5 μm). While the CTC architecture further alleviates the mismatch between upper- and lower-state densities, it introduces pronounced gain nonuniformity due to significant variations in transition dipole moments across different channels, achieving an FWHM of merely ~450 cm⁻¹ with limited improvement in spectral breadth (~0.75 μm).
An alternative mainstream strategy employs vertically stacked multiple-active-region architectures, which achieve spectral broadening through the integration of multiple active regions with complementary center wavelengths. However, this approach inevitably induces elevated threshold current densities, intensified mode competition, and vertically nonuniform gain distribution, making it exceedingly difficult to obtain flat, continuous, and high-power broadband emission. In summary, the development of novel single-active-region mid-infrared QCL architectures that simultaneously achieve broad spectral coverage and high gain uniformity represents a critical scientific challenge that demands urgent attention in this field.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Bo Meng from State Key Laboratory of Luminescence Science and Technology, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences and co-workers have developed a diagonal multi-state-to-continuum active region design, implemented within a strain-compensated material system. As illustrated in Figure 1, optimized band structure engineering enables strong coupling between two injector states (levels 4 and 6) and the upper lasing level 5 within the active region. Electrons undergo diagonal transitions from three upper lasing levels (levels 4–6) to three lower lasing levels (levels 1–3), with multiple transition channels simultaneously contributing to the optical gain, thereby substantially broadening the gain bandwidth of the active region.
The epitaxial structures were grown by metal-organic chemical vapor deposition (MOCVD). Both mesa-geometry and Fabry–Pérot (FP) cavity quantum cascade laser devices were fabricated from the grown material. The FP ridge waveguide devices had a ridge width of 12 µm and a cavity length of 5 mm. Electroluminescence (EL) measurements were performed on the mesa devices under pulsed operation with a repetition rate of 100 kHz and a pulse width of 300 ns, across a bias voltage range of 11–16 V; the results are shown in Figure 2. At a heat-sink temperature of 298 K and a bias voltage of 11 V, the EL spectrum exhibited a maximum full width at half maximum (FWHM) of 75.6 meV, representing the broadest EL linewidth achieved in the long-wave infrared spectral region to date.
Figure 3(a) presents the lasing spectra of the FP device as a function of injection current across its operational dynamic range. Near threshold, the device exhibits narrowband lasing with a center wavelength of 8.8 µm. As the injection current increases to 2.5 A, the spectral width gradually broadens while the center wavelength redshifts to 9.2 µm. Upon further increasing the current to the rollover point, the lasing spectrum continues to broaden, reaching a width of 1.2 µm, with the center wavelength blueshifting and eventually stabilizing at 8.0 µm. This bias-dependent center wavelength shift reflects the characteristic signature of diagonal transitions. The observed spectral tuning behavior arises from the interplay between the Stark effect and multi-level transitions. The 1.2 µm spectral width represents, to the best of our knowledge, the broadest lasing spectrum achieved at room temperature for single-active-region quantum cascade lasers operating at comparable wavelengths. At 80 K, the same device achieved a lasing spectral width of 1.93 µm, as shown in Figure 3(b).
These results successfully address the long-standing challenge of spectral bandwidth limitation in single-active-region QCLs, opening new avenues for integrated, high-efficiency mid-infrared sources targeting optical frequency combs, high-precision broadband sensing, imaging, and free-space optical communications.
