As a coherent light source, quantum cascade lasers (QCLs) have been extensively investigated over the past 20 years [1]. QCLs can cover a broad wavelength range from middle-wave infrared to terahertz range by changing the thickness, the compositions, and the layer numbers of superlattices, as compared to conventional interband transition semiconductor lasers [2–4]. They also have the characteristics of other semiconductor lasers, such as low power consumption, miniature size, and high reliability [5]. QCLs show the huge potential for applications like environmental monitoring, free-space optical communications, and trace gas analysis due to these advantages [6,7]. Great progress has been made on QCL design and fabrication in the range of 3–11 μm, and room temperature continuous-wave (CW) and watt-level light power have been obtained driven by these requirements [8–11]. Recently, the development of 12–16 μm bands has become a new focus direction in the research on QCLs [12]. However, in comparison with short-wavelength QCLs, there are more intrinsic technological challenges in design, especially for λ>14μm, including the dropped lifetime of the upper laser level, the increased nonradiative leakage of the injection carrier, the lower voltage efficiency, and the increased waveguide losses [13–15]. Therefore, the key issues in the design of long-wavelength QCLs are to increase the carrier’s injection efficiency of the upper laser level, improve the carrier’s extraction efficiency, and prevent the lower carrier’s thermal backfilling.

InAs-based InAs/AlSb and InP-based InGaAs/InAlAs are two kinds of material systems suitable to fabricate QCLs operating above 14 μm. Because of the small electron effective mass of InAs, an InAs/AlSb material system can obtain a larger QCL gain and has realized room temperature CW operation [16,17]. However, it is difficult to design low-loss waveguides since the refractive index of the InAs substrate is larger than that of the superlattice active region. On the contrary, InP is the natural choice for waveguide material for the InGaAs/InAlAs active region, which helps to design high-power QCLs. Based on an InGaAs/InAlAs material system, QCLs with a chirped superlattice active region have demonstrated operation at long wavelengths above 17 μm, but they were limited to operation temperatures below 240 K [15]. Using a bound-to-continuum (BTC) structure, an efficient resonant tunneling injection into the upper-bounded state was established, and room temperature operation was realized [12,14]. Another effective way to improve injection efficiency is the indirect pumping mechanism through longitudinal optical (LO) phonon resonance [13]. By further optimization, a diagonal optical transition and a two-phonon-continuum extraction scheme were employed in active region design, and high-performance lasers were demonstrated [18]. In this structure, the nonlocalized upper level greatly improves the carrier injection efficiency. Furthermore, the two-phonon-continuum depletion scheme realizes effective lower-level carrier extraction while reducing the thermal backfilling. However, the restrictive two-phonon resonance condition requires relatively strict structural parameters, which may reduce the flexibility of structure design [8].

In this paper, we demonstrate a high-average-power long-wavelength QCL emitting at ∼14μm, optimized by employing a diagonal transition and nonresonant extraction active region structure. A 4 mm long, 40 μm wide QCL with high-reflection (HR) coating shows a low threshold current density of 3.13kA/cm2 at room temperature. At 293 K, the maximum peak power and average power are up to 830 and 75 mW, respectively, which is a significant improvement over earlier reports.

We present a long-wavelength QCL structure design combined with the diagonal optical transition [18] and nonresonant extraction [8] mechanism. In this way, the diagonal transition reduces carrier injection leakage, and the two-phonon resonance condition is removed without sacrificing the efficient carrier extraction from the lower laser level. A schematic conduction band diagram at an applied electric field of 28 kV/cm is shown in Fig. 1. The design emission wavelength at room temperature is 14.2 μm. The main points of the active region design include the following aspects. I.

Efficient electron injection into the upper laser level is guaranteed. The upper laser level is nonlocalized, and its wave function overlaps strongly with the injector miniband (the width is ∼61meV). This leads to strong coupling of the injector ground state with the laser upper level, ℏΩ∼5.8meV. Electron resonant tunneling takes place from the injector ground state to upper laser level 4. At the same time, phonon-assisted scattering enables direct electron injection from high injector levels. The energy spacing between the upper laser level and the active region level located above it, E54, is 57.3 meV. It helps keep a lower electron thermal activation leakage into the continuum.

The QCL structure was grown on n-doped (Si, 2×1017cm−3) InP substrate by solid-source molecular beam epitaxy (MBE). The Si doping level in the active region was empirically adjusted to 2.3×1017cm−3 to get a high dynamic range. The complete structure along the growth direction includes a 4 μm thick lower InP cladding layer (Si, 3×1016cm−3), a 0.3 μm thick n-In0.53Ga0.47As layer (Si, 4×1016cm−3), 55 stages of the active/injector region, a 0.3 μm thick n-In0.53Ga0.47As layer (Si, 4×1016cm−3), a 5 μm thick upper cladding layer (Si, 3×1016cm−3), a 0.15 μm gradually doped layer (Si, from 1×1017 to 3×1017cm−3), and a 0.8 μm highly doped cladding layer (Si, 5×1018cm−3). After growth, the epitaxial materials were characterized by double crystal X-ray diffraction (XRD) and transmission electron microscopy (TEM).

The double channel waveguides with a ridge width of 40 μm were fabricated by photolithography and wet chemical etching. Then, a 450 nm thick SiO2 layer was deposited around the ridges by plasma enhanced chemical vapor deposition (PECVD) for electrical insulation. A 200 nm thick Ti/Au layer was deposited by e-beam evaporation to realize the electrical contact after opening a 20 μm wide window through the insulation layer for current injection. In order to reduce thermal resistance, an additional 5 μm thick Au layer was subsequently electroplated. After substrate thinning and backside alloying, the wafer was cleaved into 4 mm long lasers, and subsequently HR coated for the rear facets. Finally, the chips were mounted with the epilayer side down on the copper heat sink with indium solder for characterization.

The lasing spectra were recorded through a Fourier-transform infrared spectrometer at a resolution of 0.5cm−1 in rapid scan mode. Figure 2 displays the lasing spectra of the laser with 1.1 times threshold current at different heat sink temperatures ranging from 293 to 353 K. The emission wavelength is around 14 μm, which is consistent with the design. The peak wavelengths show a slight redshift with increase of the temperature from 14.0 μm at 293 K to 14.02 μm at 353 K. The inset of Fig. 2 shows the lateral (slow axis) far-field characteristics of the laser operating under an injection current of 7.0 A at 293 K. The far-field pattern shows normal single-lobed distribution, which originates from the fundamental lateral mode emission. The full width at half-maximum (FWHM) of the far-field patterns is 17.95°.

The optical output power was collected by an air-cooled thermocouple detector. Figure 3 shows the typical power-current-voltage (PIV) characteristics at different heat sink temperatures for an HR-coated 4 mm long and 40 μm wide laser. The device is driven with a 2 μs long current pulse with a repetition rate of 5 kHz. At 293 K, the threshold current density is 3.13kA/cm2. The higher doping level in the active region used to achieve a larger dynamic range resulted in a relatively higher threshold current density compared to the similar long-wavelength QCLs [18]. The maximum peak output power reaches 830 mW at the rollover current of 9.4 A. The slope efficiency is 271 mW/A. When the temperature increases to 353 K, the threshold current density increases to ∼3.62kA/cm2, and the peak power is still more than 400 mW. The internal quantum efficiency of the device can be calculated by the following equation: η=hνeNpαmαw+αmηi,(1)where η, hν, e, Np, αm, αw, and ηi represent slope efficiency, photon energy, elemental electronic charge, number of cascade period, mirror loss, waveguide loss, and internal quantum efficiency of each period. By an optical simulation, the calculated effective complex refractive index neff=n+κi=3.2181+0.0003517i. The waveguide loss αw can be calculated as αw=(4π/λ)×κ=3.157cm−1. The mirror loss αm=1.675cm−1 is calculated with the formula αm=−(1/2L)ln(R1R2), where L is the cavity length, and R1 and R2 are facet reflectivities. According to Eq. (1), the internal quantum efficiency ηi is calculated to 16% per cascade period at 293 K.

Based on the measured PIV data, the threshold current density and slope efficiency of the device as functions of temperature are shown in Fig. 4. The pulsed characteristic temperatures T0 and T1 of the laser device can be fitted with usual exponential function as follows: Jth=J0exp(T/T0),(2)η=η0exp(−T/T1),(3)where J_th is the threshold current density. The dashed curve is the exponential fit to the threshold current density and the slope efficiency. As a result, the characteristic temperature extracted from this fit turns out to be T0=395K and T1=119K. This quite high T0 is one of the best results for QCLs operating at similar wavelengths [13,18]. This indicates that the electron thermal backfilling into lower laser level 3 and thermal activation leakage from upper laser level 4 to much higher continuum states are efficiently suppressed by employing a diagonal transition and nonresonant extraction design.

The average power under different duty cycle (1%–15%) operation from 293 to 353 K is shown in Fig. 5. The average power reaches a maximum of 75 mW with a duty cycle of 15% at 293 K. At 353 K, the maximum average power is still above 16 mW. To our knowledge, this is the highest average power reported to date for a QCL with an emission wavelength >14μm, which is 1 order of magnitude higher than ever reported to our knowledge [12,18]. The thermal resistance (Rth) of the lasers can be roughly estimated by the following equation [19]: Rth=T0ln(Iavg,maxdmaxI0)−TsinkVIavg,max,(4)where Tsink is the heat sink temperature, Iavg,max is the average current, and dmax is the maximum allowable duty cycle at which the average power goes to zero. Limited by the maximum cooling capacity of the test system, the maximum allowable duty cycle at 293 K cannot be measured. At 353 K, the maximum operating duty cycle of the device is ∼18%. For the 40 μm wide laser, I0=2.36A, T0=395K, and V≈15.1V, a thermal resistance of 6.7 K/W was obtained. It should be mentioned here that further improved performance can be expected if a buried-ridge design is used in the laser fabrication.

In conclusion, a high-average-power QCL emitting at 14 μm has been demonstrated based on the diagonal transition and nonresonant extraction active region design. At 293 K, an 830 mW peak power was obtained for a double channel ridge waveguide, HR-coated, 55 period quantum cascade laser. The threshold current density is measured as 3.13kA/cm2 at a duty cycle of 1%, and the maximum average power is achieved up to 75 mW at a duty cycle of 15%. Over a temperature range from 293 to 353 K, a high characteristic temperature T0 of 395 K was achieved.

Acknowledgment. The authors thank Ping Liang and Ying Hu for their help in processing.