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Abstract
Distributed feedback quantum cascade lasers emitting at a wavelength of 6.12 µm are reported. Benefitted from the optimized materials epitaxy and the modified bound to continuum transition active region design along with three pairs of phonon scattering, high device performance is achieved. For a 2-mm-long, 8.4-µm-wide device, the threshold current is as low as 130 mA, the corresponding threshold current density is only 0.77 kA/cm2, and the optical output power is 69 mW at 20 °C in continuous wave mode. The temperature of continuous wave operation can reach 100 °C, where the optical output power is still more than 8 mW. In addition, it maintains a stable single mode operation from 20 to 100 °C without mode hopping, corresponding to a total wavelength shift of 41 nm. Such low-threshold quantum cascade lasers are highly beneficial to portable and highly integrated system sensor applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Quantum cascade lasers (QCLs) have attracted much attention for the applications of trace gas-sensing, remote sensing, and high-resolution spectroscopy owing to its highlighted features of a wide wavelength covering range, compact size, high output power after its first demonstration [1–4]. For practical applications, much progress has been made in single-mode emission and output power improvement [5–7]. However, in terms of portable and highly integrated systems sensor applications, low threshold and electrical power consumption are highly desired [8–10]. Some meritorious results have been made, single-mode lasers with low continuous wave (cw) threshold below 100 mA emitting at λ ∼ 4.5 µm, 5.26 µm, 7.85 µm have been reported at 20 °C, yet the corresponding threshold current densities were all above 1.2 kA/cm2 [11]. Lower threshold current of 16 mA (current density of 1.6 kA/cm2) was also achieved in 2016 at shorter wavelength of 3.36 µm at -10 °C under pulsed operation with 2% duty-cycle by optimizing the device size, but the performance of devices was rapidly degraded under cw operation, and the limited operating temperature was 15 °C in cw operation [12]. Further efforts including optimizations of the active region design, epitaxial growth and fabrication process are devoted to reducing the cw power consumption. As a result, a low threshold current of 57.4 mA distributed feedback (DFB) QCL was obtained emitting at λ ∼ 4.89 µm along with low threshold current density of 0.64 kA/cm2 at 15 °C [13]. Moreover, an ultralow power consumption of 0.26 W QCL emitting at λ ∼ 4.6 µm with high operating temperature in cw mode was also achieved based on modified two-phonon resonant active region design and optimized device size, the corresponding threshold current is as low as 22 mA at 10 °C in cw mode [14]. Recently, the threshold power consumption was reduced further as low as 143 mW based on phase front engineering [15]. However, the reported achievements of QCLs focused on the two atmospheric windows of 3 µm to 5 µm and 8 µm to 12 µm. In terms of 6 µm QCLs, related report was few [16,17], which is also important wavelength range for certain gas detection such as NO2 gas sensing [18]. Here, we designed and fabricated a high performance DFB QCL emitting at λ ∼ 6.12 µm aiming for developing portable and highly integrated NO2 gas sensing systems based on a modified bound to continuum transition active region structure with three pairs of phonon scatter. For this purpose, the active region material system is designed based on a strain-compensated In0.42Al0.58As/In0.61Ga0.39As structure. Furthermore, the strong coupled DFB grating is also fabricated to improve device threshold performance. As a result, a low threshold of QCL emitting at λ ∼ 6.12 µm with high operating temperature in cw mode is achieved. The threshold current is as low as 130 mA at 20 °C in cw mode for a 2-mm-long and 8.4-µm-wide device with high-reflection (HR) coating on back facet. The corresponding threshold current density is only 0.77 kA/cm2. At the same time, the devices maintain a stable single mode emission from 20 to 100 °C without mode hopping, corresponding to a total wavelength shift of 41 nm. The maximum cw output power of device is 69 mW at 20 °C and still above 8 mW at 100 °C.
2. Laser design and experiment
The realization of low threshold single mode operation depended on optimizing active region design, epitaxial growth and strongly coupled grating. The epitaxial growth started with a buffer layer growth on an n-InP (Si, 1 × 1017 /cm3) substrate using metal organic vapor phase epitaxy (MOVPE). The active region sandwiched between two lattice-matched InGaAs confinement layers was grown by solid-source molecular beam epitaxy (MBE), which could guarantee that the epitaxial interface is sharp for low interface scattering loss. The epitaxial layer sequence was as follows: 1.34 µm InP buffer layer (Si, 3 × 1016 /cm3), 0.2-µm-thick n-In0.53Ga0.47As lower confinement layer (Si, 4 × 1016 /cm3), 40 stages active region, 0.3-µm-thick n-In0.53Ga0.47As upper confinement layer (Si, 4 × 1016 /cm3) and 4-µm-thick upper waveguide layer. The active region structure was based on bound to continuum transition design along with three pairs of phonon scattering, consisting of strain-compensated In0.42Al0.58As/In0.61Ga0.39As material system. The specific layer sequence of active region in one stage (layer thickness is in angstrom) is as follows: 47/13/42/15/32/17/30/18/28/23/26/34/24/40/15/10/48/12, where In0.42Al0.58As barrier layers are in bold font, and n-doped layers (Si, 1.7 × 1017 /cm3) are underlined. The conduction band diagram of the active region is shown in Fig. 1(a). Radiative transition is between levels 5 and 4, and then the electron of the lower laser level 4 can be extracted rapidly and injected into the next active region by the three pairs of phonon ladders of 4 (4’) to 3 (3’), 3 (3’) to 2 (2’) and 2 (2’) to 1 (1’), where the bottom of phonon ladder of 1’ is also the ground injector energy level of the next stage. The electron lifetime τi for an energy level i can be written as:
where τif is the scattering time from the energy level i to a final level f and the summation is over all possible final states. Consequently, the calculated efficient lifetime of the lower laser level and upper laser level are 0.16 ps and 0.7 ps, respectively. It can be found that the design of multi-phonon steps depletes efficiently the lower laser level, which is beneficial to population inversion. At the same time, a large voltage defect of 122 meV is obtained, which can restrain efficiently the thermally activated backfilling of the electrons from the active region ground state into the lower laser level, so the threshold performance of device can be improved. Moreover, the overall transition is more vertical than the high efficiency designs [7] and higher laser transition matrix element z54 of 1.8 nm is obtained, which would be beneficial to high gain and low threshold operation. The observed narrow electro luminescence (EL) spectra with a full-width-at-half-maximum (FWHM) of 19.2 meV given in Fig. 1(b) also reveals the high-gain vertical transition design.
Fig. 1. a: Schematic conduction band diagram of one stage of the active region structure under an applied electric field of 67 kV/cm. b: The electroluminescence spectra of the wafer.
The DFB grating was patterned on the upper InGaAs confinement layer by holographic lithography combined wet etching. After that, the upper InP layer included a 3-µm-thick low-doped (Si, 2 × 1016 /cm3) InP layer, a 0.15-µm-thick gradual-doped (Si, 1.5 × 1017 /cm3) InP layer, and a 0.85-µm-thick high-doped (Si, 5 × 1018 /cm3) InP layer was regrown by MOVPE. Then the wafer was processed into double-channel ridge waveguide laser with an average core region width of 8.4 µm, where the channels were filled with semi-insulating InP:Fe cladding. After the standard window opening and metallization process, the waveguides were cleaved into 2-mm-long laser bars, and the back facet of device was deposited the HR coating consisting of Al2O3/Ti/Au/Ti/Al2O3 by electron beam evaporation and the front facet of device was without any coating. Finally, the lasers were mounted epi-side down on diamond heat sinks with indium solder, which were subsequently soldered on copper heat sinks.
The scanning electron microscope (SEM) of DFB grating is shown in Fig. 2(a). The grating period and etching depth are 961 nm and 180 nm, respectively. Figure 2(b) displays the calculated grating coupling coefficient as a function of the grating etching depth assuming a grating duty cycle of 50%. The values of coupling coefficient of κ can be calculated from the following equation derived from coupled mode theory [19,20]:
where the Δλ0, λB and neff are the stop band width, the Bragg wavelength and effective refractive index. As is shown in Fig. 2(b), the coupling coefficient of κ increases as the etching depth of grating becomes deep; as a result, the corresponding coupling strength of κ·L also increases, where L is the cavity length of device. The strong coupling grating could be chosen for the purpose of improving the threshold of device. However, as the coupling strength growing, more light would be confined in the center of the device, resulting in the low optical output power. In this work, the grating etching depth is defined as 180 nm and the corresponding coupling strength κ·L is about 6.3.Fig. 2. a: SEM image of the buried grating. b: The grating coupling coefficient as a function of the grating etching depth.
3. Result and discussion
The lasers were mounted on a holder containing a thermoelectric cooler (TEC) for testing. The spectra were measured by a Fourier transform infrared (FTIR) spectrometer with a resolution of 0.25 cm-1 in rapid scan mode. The emitted optical power was measured with the calibrated thermopile detector placed in front of the laser facet without any correction.
Figure 3(a) shows the typical current-voltage-power characteristics of an 8.4-µm-wide and 2-mm-long device changing with temperature from 20 to 100 °C. In Fig. 3(b) the optical power is plotted as a function of the injected electrical power at different heat-sink temperatures. At 20 °C, the cw threshold current and threshold current density are 130 mA and 0.77 kA/cm2, respectively. The corresponding threshold power consumption is 1.3 W, which can be further reduced by optimizing the reflectivity of front facet and size of device for the applications of portable and highly integrated system sensor. The optical output power is 69 mW at the injection current of 280 mA at 20 °C, and there is still output power of 8.6 mW even though the heat-sink temperature reaches 100 °C in cw mode.
Fig. 3. a: Current-voltage-power characteristics at different heat-sink temperatures for a 2-mm-long, 8.4-µm-wide device. b: Optical power vs electrical power consumption at different heat-sink temperature.
Figure 4 shows the cw emission spectra of the device at different heat-sink temperatures from 20 to 100 °C with a step of 10 °C above threshold and different injection currents at 20 °C with a step of 30 mA, and the inset shows the molecular absorption peaks of NO2. The peak emission spectrum shifts from 6123 nm to 6164 nm at around threshold as heat-sink temperatures increasing, leading to a total wavelength tuning range of 41 nm in cw mode. In addition, we also measured the cw emission spectra of the device from 20 to 100 °C with a step of 10 °C at the same injection currents, corresponding to a total wavelength shift of 34 nm. The emission wavelength was designed in the center of strongest absorption peaks of NO2. Stable single mode emission is maintained without any mode hopping, and the side mode suppression radio (SMSR) is more than 23 dB in both temperature tuning and current tuning.
Fig. 4. a: The cw emission spectra of device at different heat-sink temperatures from 20 to 100 °C above threshold. b: The characteristics of emission spectra changing with injection current from 130 mA to 280mA with a step of 30 mA at 20 °C, and the inset shows the molecular absorption peaks of NO2.
The far-field profile along ridge-width direction was measured under pulsed condition at room temperature. As shown in Fig. 5, the dots are the measured data which is fitted with the Gauss function showing as the red solid line, where the single-lobe far-field distribution along ridge-width direction is obtained with the full width at half maximum (FWHM) of 29°. In the inset of Fig. 5, the picture of light beam spot of the device after collimating by the aspheric lens is displayed with a pyroelectric camera placed 50 cm away from the laser, which proves that the lasing mode is fundamental transverse mode.
Fig. 5. The far-field profile along ridge-width direction. The inset shows the light beam spot of the device after collimating by the aspheric lens.
4. Conclusion
In conclusion, low-threshold DFB QCLs with high operating temperature and stable single mode operation in cw mode have been obtained emitting at 6.12 µm for the purpose of NO2 gas-sensing application based on optimized active region structure of modified bound to continuum transition along with the three pairs of phonon scattering steps. The 2-mm-long, 8.4-µm-wide devices exhibit a threshold current density of 0.77 kA/cm2 at 20 °C in cw mode with an optical output power of 69 mW. It maintains a stable single-mode operation from 20 to 100 °C without mode hopping, corresponding to a total wavelength tuning range of 41 nm. Output power of 8.6 mW is even achieved for the high heat-sink temperature of 100 °C in cw mode.
Funding
National Basic Research Program of China (2018YFB2200500); National Natural Science Foundation of China (61991430, 61790583, 61674144, 61774150); Beijing Municipal Science & Technology Commission (Z201100004020006); Key Projects of the Chinese Academy of Sciences (2018147, YJKYYQ20190002, QYZDJ-SSW-JSC027, XDB43000000).
Acknowledgments
The authors would like to thank Ping Liang and Ying Hu for their help in device processing.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented herein are not publicly available currently but can be obtained from the authors upon reasonable request.
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