Open Access
Add to BibSonomy
Abstract
Many molecules have broad fingerprint absorption spectra in mid-wave infrared range which requires broadly tunable lasers to cover the interested spectrum in one scan. We report a strain-balanced, InAlAs/InGaAs/InP quantum cascade laser structure based on diagonal transition active region with high output power and and wide tuning range at λ ∼ 8.9 µm. The maximum pulsed optical power and the wall-plug efficiency at room temperature are 4 W and 11.7%, respectively. Maximum continuous wave double-facet power is 1.2 W at 25 °C for a 4 mm by 9 µm laser mounted epi-side down on a diamond/copper composite submount. The maximum pulsed and continuous wave external-cavity tuning range are from 7.71 µm to 9.15 µm and from 8 µm to 8.9 µm, respectively. The continuous wave power of the external cavity mode exceeds 200 mW across the entire spectrum.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Driven by the applications in spectroscopy, sensing, communications, and medical diagnosis, research on quantum cascade lasers (QCLs) [1] has gained in significant progress in the laser performance of power, efficiency, and tuning range during the past few years [2–7]. Although the continuous wave (CW) output power over 5 W has been achieved for mid-wave infrared (MWIR) QCLs [8], CW power is limited to watt-level in the long-wave infrared (LWIR) band due to lower optical confinement factor, decreased gain coefficient and increased free-carrier absorption. Besides, thicker active region will lead to worse thermal performance compared with MWIR QCLs. At present, watt-level CW power at the wavelength of 9 µm has been reported in literature [9–11]. For LWIR active region, strong diagonal transition is a popular scheme especially for high-CW-power LWIR QCLs because of its long lifetimes for upper-laser levels [7,10].
Many molecules have broad absorption spectra in MWIR range. For instance, nitrous oxide and methane exhibits characteristic absorption peaks between 1200 cm-1 and 1300 cm-1, ester possesses an asymmetric stretching vibration at around 1200 cm-1, and carbonyl groups in proteins and lipids have salient vibration absorption peaks between 1159 cm-1 and 1174 cm-1. Widely tunable LWIR QCLs are capable of covering the interested spectral range in one scan, which would be mostly desired for detection of various gas molecules featuring broad absorption spectra. Wide spectral tuning is normally achieved by external cavity scheme, which requires not only precise optical collimation, high collection efficiency of lens and higher first-order diffraction efficiency of blazed gratings, but also a broad gain from the QCL gain medium. QCLs designs featuring multiple upper or lower laser levels have used for wide tuning because the transition among different energy levels can broaden the gain spectrum [12,13]. On the other hand, widely tunable external cavity QCLs based on multi-core designs have been reported [14–16]. Although multi-core designs have been shown superior performance in tuning range in recent years, high-CW-power QCLs based on these active region designs have rarely been reported. The peak gain of such QCLs is insufficient, which is mainly owing to the number of periods of individual sub-cores. The threshold current density is thus higher than that of the single-stack structure. However, for some special applications, QCLs need to have a wide tuning range, but also have a high output power. For instance, laser spots with low power density are difficult to detect by conventional detectors after a long -distance travelling, which limits the long-distance stand-off application of LWIR QCLs. When detecting cells or proteins, low power beams may result in a poor resolution or limited penetration into the sample.
In this paper, a diagonal transition active region scheme is proposed to achieve a desired output power and tuning range. It can widen the gain spectrum and therefore tuning range for many laser transitions between the upper laser level and the lower energy levels when the lower energy levels constitute a lower active-region miniband [17]. The maximum pulsed optical power and the wall-plug efficiency (WPE) at room temperature are 4 W and 11.7%, respectively. A pulsed tuning range of 204 cm-1 is demonstrated. A 4-mm-long laser emits 1.2 W at room temperature in CW operation with a maximum wall-plug efficiency of 6%. A maximum CW tuning range of 126 cm-1 and output power over 200 mW is achieved.
2. Lasers design and fab
The active region is based on a strain-balanced In0.58Ga0.42As/In0.365Al0.635As material systems. Although many structures near 9 µm are based on lattice matched AlInAs/InGaAs composition, we adopted a diagonal-transition design with poor overlap between the upper-laser level wavefunction and the wavefunctions of high-energy active-region levels in order to suppress carrier leakage [6]. A conduction band diagram of the active region is shown in Fig. 1 under an electric field of 48 kV/cm. Radiative transition occurs between levels 3, and the energy states between levels 2 and 1. Energy spacing E43 is designed to be about 60 meV. The voltage defect approximately is 95 meV, which is beneficial to a lower threshold voltage operation and therefore higher WPE. The dipole matrix element of radiative transition between levels 3 and 2 is 1.4 nm. The lifetimes of upper and lower laser levels are 1.4 ps and 0.21 ps, respectively. Carrier lifetimes were only calculated taking into account absorption emission with longitudinal optical phonons at 298 K. The energy difference between energy 3 and 2 is about 140 meV, corresponding to the wavelength of 8.9 µm. The transitions between the upper laser level and low laser levels can broaden the gain spectrum and therefore the tuning range.
Fig. 1. Band structure and wave functions of relevant energy levels of a high power and wide tuning range LWIR QCL active region with strain-balanced In0.58Ga0.42As/In0.365Al0.635As material emitting at the wavelength of 8.9 µm. The upper (level 3) and lower laser levels (level 2) are shown in red solid line. The active region is under an applied electric field of 48 kV/cm.
The epitaxial layer sequence from the n-substrate starts with an InP buffer layer (Si, ∼3.5 × 1016 cm−3, 3.5 µm), 50-stage active region design described above (Si, the thicknesses of the two doped layers in the injector regions are 34.8 Å and 31.0 Å, thus the sheet-doping density is ∼1.0 × 1011 cm−2), InGaAs layer (Si, ∼4 × 1016 cm-3, 0.3 µm) to improve the optical confinement of active regions, InP cladding layer (Si, ∼3.5 × 1016 cm-3, 4 µm), graded doped InP layer (Si, ∼1-2 × 1017 cm−3, 0.15 µm) and highly doped InP cap layer (Si, ∼5 × 1018 cm−3, 0.85 µm) to decouple the optical mode from the lossy top metal contact. The active region and InGaAs layer are grown on an (n-type) InP buffer layer through molecular beam epitaxy in a single growth step. Metal organic chemical vapor deposition is adopted to grow the InP buffer layer, InP cladding layer, graded doped InP layer and highly doped InP cap layer. The wafer is then processed into a buried ridge with a ridge width of 9 µm. Semi-insulated InP:Fe is grown on either side of the ridge to reduce the temperature of the active region through metal organic chemical vapor deposition. The laser is mounted epi-side down on a diamond submount with indium soft solder for pulsed and CW operation. The diamond submount is then mounted on a copper heatsink.
3. Lasers characterization
The experimental and simulated x-ray diffraction (XRD) spectra for the 50-stage laser core are shown in Fig. 2 (a). Excellent agreement between the experimental and simulated XRD curves is obtained. The intensity and position of the peaks relate to the layer thicknesses and the material composition of the heterostructure. The broadening of satellite peaks indicates that interfaces scattering is not negligible, which will lead to a higher threshold current density on the one hand, but also an increased gain bandwidth and tuning range on the other.
Fig. 2. (a) Experimental and simulated X-ray diffraction spectra of the 8.9 µm laser. (b) Electroluminescence spectra in pulsed operation (1 µs, 100 kHz) at room temperature. The rear cavity surface is destroyed to suppress resonance. Inset in (b): The FWHM of Electroluminescence spectra related to different voltage. FWHM refers to the full width at half maximum.
The electroluminescence (EL) spectra of a 1-mm-long mesa, where the rear cavity surface is destroyed to suppress resonance, are measured at pulsed operation (1 µs, 100 kHz) at room temperature as shown in Fig. 2(b). The full width at half maximum (FWHM) of the EL spectra are shown in the inset at different voltages. As for a diagonal transition design [18,19], blueshift is normally reported due to Stark effect. However, the EL spectrum peaks with respect to the increase of voltage showing no significant shifting are observed in the present work. This perhaps is because the high duty-cycle and wide pulsed width, used in this work results in a non-negligible thermal effect inside the devices which would redshift the EL spectra. The overall Stark and thermal effects could compensate each other or show redshift when thermal effect overpowers Stark effect, as observed this work. Meanwhile, the FWHM of EL spectrum can still reach 30 meV at 10 V voltage.
4. Lasers performance
Single-mode spectra under different grating angles are obtained by employing the Littrow configuration, as shown in Fig. 3 (a). Anti-reflective coating is applied on the front facet of a 2-mm-long lasers to lower the reflectivity. A wide tuning range of 1297 cm-1 to 1093 cm-1 (from 7.71 µm to 9.15 µm) is achieved in pulsed operation (1 µs, 40 kHz) at room temperature. At the bias voltage of 11.5 V, the FWHM of EL spectrum is more than 20 meV (from 1110 cm-1 to 1280 cm-1), which is almost consistent with the pulsed tuning range. The wide tuning range in pulsed operation is due to broadening of the gain spectrum caused by scattering of interface roughness and many transitions from the upper laser level to the levels in lower minibands of the active region. The multi-layer coating (alumina and germanium) has a strong absorption for LWIR in this paper. Therefore, the pulsed tuning range can be further improved if the suitable anti-reflection coating like ZnS∕YF3, is coated on the front cavity surface [20].
Fig. 3. (a) Tuning performance of the Littrow EC-QCL, for a 2-mm-long cavity with an anti-reflective coating, is taken at room temperature with a duty cycle of 1% (500 ns, 20 kHz). The wavelength tuning range is of 204 cm-1(from 1297 cm-1 to 1093 cm-1). AR refers to anti-reflective coating. (b) Wavelength tuning range of 104 cm-1 (from 1250 cm-1 to 1123 cm-1) is in CW operation at 25 °C without an anti-reflective coating for a 4-mm-long cavity. The CW power is plotted as a function of the emission wavelength. CL refers to an as-cleaved facet of the cavity. Inset in (b): The CW spectra are in 0.53 A and 1.02 A, respectively, in 25 °C for a 2-mm-long laser without coating.
Figure 3 (b) presents the EC mode spectra corresponding to the CW power. A wide tuning range of 1250 cm-1 to 1123 cm-1 (from 8 µm to 8.9 µm) is achieved at 25 °C for laser without an anti-reflective coating. The light power exceeds 200 mW over the entire spectral and the maximum CW power is 260 mW at a wavelength of 8.46 µm. In the case of Littrow configuration under CW operation, the tuning range and output power are limited by the low heat dissipation of our experimental setup. Specifically, both the front and rear facets of the laser need lens collimation, thus the length of a diamond heatsink and a copper heatsink needs to be the same as the cavity length, which greatly limits the heat dissipation capacity. As a consequence, the leakage of electrons is increased so that the power and dynamic range of the laser are deteriorated. Although the CW tuning range is narrower than that of the dual-upper-state design [21,22], the output power in this work is higher thus the lasers can thus be used in the specific situation that high output power is required.
Figure 4(a) shows the pulsed (0.5 µs, 40 kHz) power-current-voltage (PIV) characteristics for lasers with 4 mm and 6 mm length, respectively. The maximum peak power is more than 3 W while the maximum WPE is 11.7%. For a 6-mm-long laser, the maximum peak power and the WPE are 4.25 W and 10.4% at room temperature, respectively. As shown in Fig. 4(b), the differential gain Gd is 6.65 cm/kA, which is no significant deterioration compared with Ref. [7]. Thus lower voltage defects still can suppress carrier backfilling to the lower lasing levels. Figure 4(c) shows CW PIV characteristics for a 9-µm-wide, 4-mm-long, buried-ridge laser mounted epi-side down on a diamond heatsink. The laser is fixed on a thermoelectric cooler for the temperature control and operated at 25 °C. The output power is measured using a pyroelectric power meter. The maximum double-facet CW power at 298 K is 1.2 W.
Fig. 4. (a) The experimental results of Power-current-voltage (PIV) characteristics and WPE are for 4 mm and 6 mm lasers, respectively, at room temperature in pulsed operation (500 ns, 40 kHz). CL refers to an as-cleaved facet of the cavity. (b) The external quantum efficiency and threshold current density are measured at room temperature for 3 mm, 4 mm and 6 mm, respectively. (c) PIV characteristics and WPE are for 4 mm cavity in CW operation. The maximum double-facet power and WPE are 1.2 W and 6%, respectively. (d) The threshold current density and the slope efficiency are measured at temperatures from 293 to 353 K at intervals of 10 K. The characteristic temperature T0 and T1 are 217 K and 529 K, respectively.
For Fig. 4(d), the threshold current density and the slope efficiency is measured at temperatures from 293 to 353 K at intervals of 10 K. T1 is 529 K in pulsed operation (0.5 µs, 40 kHz), indicating the low temperature dependence of the slope efficiency, which is mainly attributed to the high ΔE34. The characteristic temperature T0 is an important parameter to measure the high temperature characteristics of QCLs. Compared with mid-wave QCLs, the thickness of the active region of long-wave QCLs is thicker, thus the difficulty of heat dissipation in the active region is further increased [23,24]. T0 of our laser with 50 stages is 217 K, as shown in Fig. 4(d), which shows that the leakage and backfilling current are not sensitive to the rise of temperature. As shown in Fig. 4(b), slope efficiency and threshold current density is measured for 3 mm, 4 mm and 6 mm cavity in pulsed operation (0.5 µs, 40 kHz). The increased waveguide loss (1.62 cm-1) is due to the increased free carrier absorption with the increasing of laser wavelength compared with Ref. [7]. The decrease of differential gain is due to many lasing transitions to the lower levels of the active region in miniband. This is designed to increase the width of the gain spectra for a wider tuning range. Watt-level CW power is mainly achieved through the active region design of the laser. A high ΔE34 value and poor wavefunction overlap between 3 and 4 greatly reduces the shunt-type leakage [25], which is the main leakage channel within the active region. Under this leakage channel, electrons are excited into the energy levels above the upper laser level via thermal activation.
5. Conclusion
In conclusion, we have demonstrated high power widely tunable QCLs with diagonal transition active region design at λ∼8.9 µm. For a 4 mm laser, maximum pulsed double-facet power and wall-plug efficiency at room temperature are 3 W and 11.7%, respectively. Maximum continuous wave double-facet power is 1.2 W at 25 °C for a 4 mm by 9 µm laser mounted epi-side down on a diamond/copper composite submount. A broad external cavity tuning range from 7.71 µm to 9.15 µm is friendly to trace gas sensing for nitrous oxide and methane. The continuous wave external cavity tuning range from 8 µm to 8.9 µm over 200 mW has potential to detect the vibration absorption peaks of carbonyl groups in proteins and lipids.
Funding
National Key Research and Development Program of China (2021YFB3201901); National Natural Science Foundation of China (61974141, 61991430, 62104019, 62174158, 62235016, 62274014); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2021107, Y2022046); Beijing Municipal Science and Technology Commission, Adminitrative Commission of Zhongguancun Science Park (Z221100002722018).
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 that support the findings of this study are available from the corresponding authors upon reasonable request.
References
1. J. Faist, F. Capasso, D. Sivco, et al., “Quantum cascade laser,” Science 264(5158), 553–556 (1994). [CrossRef]
2. R. Maulini, A. Lyakh, A. Tsekoun, et al., “λ∼7.1 µm quantum cascade lasers with 19% wall-plug efficiency at room temperature,” Opt. Express 19(18), 17203–17211 (2011). [CrossRef]
3. T. Fei, S. Q. Zhai, J. C. Zhang, et al., “High power ∼ 8.5 µm quantum cascade laser grown by MOCVD operating continuous-wave up to 408 K,” J. Semicond. 42(11), 112301 (2021). [CrossRef]
4. S. Slivken, A. Evans, W. Zhang, et al., “High-power, continuous-operation intersubband laser for wavelengths greater than 10 µm,” Appl. Phys. Lett. 90(15), 151115 (2007). [CrossRef]
5. H. Wang, J. Zhang, F. Cheng, et al., “Broad gain, continuous-wave operation of InPbased quantum cascade laser at λ ∼11.8 µm,” Chin. Phys. B 30(12), 124202 (2021). [CrossRef]
6. D. Botez, J. D. Kirch, C. Boyle, et al., “High-efficiency, high-power mid-infrared quantum cascade lasers [Invited],” Opt. Mater. Express 8(5), 1378–1398 (2018). [CrossRef]
7. W. Zhou, Q.-Y. Lu, D.-H. Wu, et al., “High-power, continuous-wave, phase-locked quantum cascade laser arrays emitting at 8 µm,” Opt. Express 27(11), 15776–15785 (2019). [CrossRef]
8. Y. Bai, N. Bandyopadhyay, S. Tsao, et al., “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. 98(18), 181102 (2011). [CrossRef]
9. A. Lyakh, R. Maulini, A. Tsekoun, et al., “Multiwatt long wavelength quantum cascade lasers based on high strain composition with 70% injection efficiency,” Opt. Express 20(22), 24272–24279 (2012). [CrossRef]
10. F. Wang, S. Slivken, and M. Razeghi, “High-brightness LWIR quantum cascade lasers,” Opt. Lett. 46(20), 5193–5196 (2021). [CrossRef]
11. Leo Missaggia, Christine Wang, Michael Connors, et al., “Thermal management of quantum cascade lasers in an individually addressable monolithic array architecture,” Proc. SPIE 9730, Components and Packaging for Laser Systems II, 973008 (2016).
12. H. Wang, J. Zhang, F. Cheng, et al., “Watt-level, high wall plug efficiency, continuous-wave room temperature quantum cascade laser emitting at 7.7 µm,” Opt. Express 28(26), 40155–40163 (2020). [CrossRef]
13. Y. Yao, X. Wang, J.-Y. Fan, et al., “High performance “continuum-to-continuum” quantum cascade lasers with a broad gain bandwidth of over 400 cm−1,” Appl. Phys. Lett. 97(8), 081115 (2010). [CrossRef]
14. N. Bandyopadhyay, Y. Bai, S. Slivken, et al., “High power operation of λ ∼ 5.2-11 µm strain balanced quantum cascade lasers based on the same material composition,” Appl. Phys. Lett. 105(7), 071106 (2014). [CrossRef]
15. N. Bandyopadhyay, M. Chen, S. Sengupta, et al., “Ultra-broadband quantum cascade laser, tunable over 760 cm−1, with balanced gain,” Opt. Express 23(16), 21159–21164 (2015). [CrossRef]
16. A. Hugi, R. Terazzi, Y. Bonetti, et al., “External cavity quantum cascade laser tunable from 7.6 to 11.4 µm,” Appl. Phys. Lett. 95(6), 061103 (2009). [CrossRef]
17. A. Wittmann, Y. Bonetti, J. Faist, et al., “Intersubband linewidths in quantum cascade laser designs,” Appl. Phys. Lett. 93(14), 141103 (2008). [CrossRef]
18. J. Faist, M. Beck, T. Aellen, et al., “Quantum-cascade lasers based on a bound-to-continuum transition,” Appl. Phys. Lett. 78(2), 147–149 (2001). [CrossRef]
19. A. Bismuto, R Terazzi, M. Beck, et al., “Electrically tunable, high performance quantum cascade laser,” Appl. Phys. Lett. 96(14), 141105 (2010). [CrossRef]
20. W. Zhou, D. Wu, R. McClintock, et al., “High performance monolithic, broadly tunable mid-infrared quantum cascade lasers,” Optica 4(10), 1228–1231 (2017). [CrossRef]
21. T. Dougakiuchi, K. Fujita, T. Edamura, et al., “Broadband continuous-wave tuning of external cavity anticrossed dual-upper-state quantum cascade lasers,” Proc. SPIE 8277, 82770V (2012). [CrossRef]
22. T. Dougakiuchi, K. Fujita, A. Sugiyama, et al., “Broadband tuning of continuous wave quantum cascade lasers in long wavelength (> 10 µm) range,” Opt. Express 22(17), 19930–19935 (2014). [CrossRef]
23. Kazuue Fujita, Shinich Furuta, Atsushi Sugiyama, et al., “High Performance λ ∼ 8.6 µm Quantum Cascade Lasers With Single Phonon- Continuum Depopulation Structures,” IEEE J. Quantum Electron. 46(5), 683–688 (2010). [CrossRef]
24. Y. Bai, N. Bandyopadhyay, S. Tsao, et al., “Highly temperature insensitive quantum cascade lasers,” Appl. Phys. Lett. 97(25), 251104 (2010). [CrossRef]
25. D. Botez, C.-C. Chang, and L. J. Mawst, “Temperature sensitivity of the electro-optical characteristics for mid-infrared (λ = 3-16 µm)-emitting quantum cascade lasers,” J. Phys. D: Appl. Phys. 49(4), 043001 (2016). [CrossRef]
