Single-Mode Fabry-Pérot Quantum Cascade Lasers at λ ~10.5 μm

In this paper, we report a single-mode Fabry-Pérot long wave infrared quantum cascade lasers based on the double phonon resonance active region design. For room temperature CW operation, the wafer with 35 stages was processed into buried heterostructure lasers. For a 4 mm long and 13 μm wide laser with high-reflectivity (HR) coating on the rear facet, continuous wave output power of 43 mW at 288 K and 5 mW at 303 K is obtained with threshold current densities of 2.17 and 2.7 kA/cm 2 . The lasing wavelength is around 10.5 μm. Single mode emission was observed for this particular device over the whole investigated current and temperature range.


Introduction
In the last 20 years quantum cascade lasers (QCLs) have received a great deal of attention because of their potential advantages for use in a wide range of areas, including infrared countermeasures, environmental monitoring, free-space optical communications, and optical gas sensing [1] [2] [3]. Among them, the long-wave infrared (LWIR, λ = 8 -12 μm) QCLs are particularly important due to low atmosphere absorption loss and the rich variety of molecular species have their "fingerprint" absorption in this spectrum range [4]. To date, watt-level outputs at wavelengths in the middle-wave infrared (MWIR, λ = 3 -5 μm) range have been obtained [5]. However, because of the limitation of the intrinsic technological characteristic of long-wave devices (such as increased free-electron optical losses at longer wavelengths, the lower intersubband gain, the decreased optical confinement), the progress of LWIR QCLs had been slower than the MWIR QCLs [6] [7] [8]. The room temperature continuous wave operation of LWIR QCL can be obtained by few groups [9] [10] [11]. Therefore, the further study on LWIR QCLs is necessary.
The first room temperature continuous wave LWIR QCLs was demonstrated using a double phonon resonance structure by Mattias Beck [12]. Base on the same design, continuous wave (CW) CW output power of 45mW at 10˚C, and wavelength ~9.4 μm have been demonstrated by Chuncai Hou [13]. However, the room temperature (RT) continuous wave (CW) operation of single mode LWIR QCL can be obtained by few groups, especially when the wavelength is longer than 10 μm.
In this letter, we present a single mode LWIR QCL with a continuous wave (CW) operating temperature up to 303 K. The active region is designed with a double phonon resonance and grown with strain-compensation technology. For a 4 mm long and 13 μm wide QCL with high-reflectivity (HR) coating on the rear facet, CW output power of 43 mW at 288 K and 5 mW at 303 K is obtained, at a lasing wavelength of ~10.5 μm.
Then the wafer was processed in a narrow-stripe, buried heterostructure by photolithography and wet chemical etching. After etching, in order to confinement of carriers and improve radiation efficiency, the semi-insulating InP (Fe-doped) was grown by metal organic chemical vapor deposition (MOCVD). Next, a 450-nm thick SiO 2 layer was deposited by plasma enhanced chemical vapor deposition (PECVD) for electrical insulation, and Ti/Au layer was growthed by e-beam evaporation to realize the electrical contact. In order to reduce thermal resistance, an additional 5-µm-thick Au layer was subsequently electroplated. With thinning and annealing, the wafer was then cleaved into 4-mm-long laser bars and mounted epilayer side down on the copper heat sink with indium solder.  was observed, and the laser exhibited optical output power of 43 mW and a slope efficiency dP/dI of 118 mW/A. When the temperature higher, the threshold current increased to ~2.7 kA/cm 2 , and while still more than 5 mW of output power was emitted at 303K. The pulsed peak output power of 125mW was obtained at 293 K with a repetition frequency of 5 kHz and a pulse width of 2 μs, while still more than 88 mW of output power was emitted at 308 K, as shown in Figure 1(b).

Results and Discussion
At 293 K, the threshold current of pulsed mode is 0.8 A (1.5 kA/cm 2 ). The slope efficiency of the device can be well calculated by the following model shown as equation: where hν is the photon energy, e is the elemental electronic charge, N p is number of cascade period, and η i is the internal quantum efficiency of each period. According to this equation, the internal quantum efficiency is around 38% per cascade period at 288 K. The performance of the LWIR QCL shows good performance.
The spectrum characteristic of LWIR QCL is shown in Figure 2. The emission frequency ν of a QCL can be tunedover a small range by changing the current and temperature. Figure 2 In order to investigate the thermal behavior, we measured the threshold current characteristic at the different heatsink temperatures in CW and pulsed mode shown in Figure 3. The red line fits with the exponential function J th = J 0 exp(T/T 0 ), where J th is the threshold current density, J 0 is the constant and T 0 is the characteristic temperature. The T 0 is 132 K for the pulsed mode. Generally, due to the low heat conductivity of InGaAs/InAlAs ultrathin layer, the heat dissipation for the active region in CW operation mode is poor, thus the core temperature T act of the QCL is much higher than the heatsink temperature T sink . As a result, the threshold current increases more rapidly with a higher value in CW mode than in pulsed mode as the temperature is increased. From 288 to 303 K, T 0 decreases to 71 K for CW mode. For high temperature and high power CW operation the lower threshold power density and weaker temperature dependence are required.
The far-field measurement was done by mounting the laser on a computer controlled rotational stage with a step resolution of 0.05˚. A room temperature operation HgCdTe detector was located 35 cm away from the QCL to collect the lasing light. Figure 4 shows the measured lateral far-field radiation patterns of the LWIR QCL with the black dots and the fitted result of Gauss function with the red line. The measured full width at half maximum (FWHM) of the far-field pattern is 27.64˚, which can be explained by the diffraction limit formula sinθ = 1.22 λ/D, where θ is the diffraction angle, λ is the lasing wavelength and D is the width of the waveguide.

Conclusion
In conclusion, a single mode QCL emitting at 10.5 μm has been demonstrated based on the double phonon resonance active region design. A CW output power of 43 mW was demonstrated at 288 K with a laser chip which has a 4-mm-long cavity and a 13-μm-wide stripe. The threshold current density is measured as 2.17 kA/cm 2 at 288 K and the far-field pattern shows normal single-lobed distribution. Single mode emission was observed for the device over the whole investigated current and temperature range, which shows a good potential for practical applications.