We report on the performance improvement of long-wave infrared quantum cascade lasers (LWIR QCLs) by studying and optimizing the anti-reflection (AR) optical facet coating. Compared to the Al2O3 AR coating, the Y2O3 AR coating exhibits higher catastrophic optical mirror damage (COMD) level, and the optical facet coatings of both material systems have no beam steering effect. A 3-mm-long, 9.5-μm-wide buried-heterostructure (BH) LWIR QCL of λ ~ 8.5 μm with Y2O3 metallic high-reflection (HR) and AR of ~ 0.2% reflectivity coating demonstrates a maximum pulsed peak power of 2.19 W at 298 K, which is 149% higher than that of the uncoated device. For continuous-wave (CW) operation, by optimizing the reflectivity of the Y2O3 AR coating, the maximum output power reaches 0.73 W, which is 91% higher than that of the uncoated device.
【AIGC One Sentence Reading】:研究优化长波红外量子级联激光器的腔面增透膜,采用Y2O3膜系提升器件性能。实验显示,脉冲峰值功率和连续波输出功率均显著提高,且该膜系无光束转向效应。
【AIGC Short Abstract】:本研究通过优化长波红外量子级联激光器的光学腔面增透膜,提升了器件性能。使用Y2O3材料的光学膜,有效提高了腔面光学灾变水平且无光束转向效应。在脉冲和连续波操作下,分别显著提升了器件的峰值功率和输出功率,展现了镀膜技术在改善激光器性能方面的巨大潜力。
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As a significant candidate for highly efficient and compact mid-infrared (MIR) to terahertz light sources,quantum cascade lasers (QCLs) have shown tremendous potential in applications such as chemical and biological sensing[1-2],free space optical communication[3],and infrared countermeasures (IRCM)[4]. Among them,more and more attentions have been devoted to the development of long-wave infrared (LWIR,λ ≈ 8-12 μm) QCLs with high output power,high wall-plug efficiency (WPE) and good beam quality[5-7]. However,some problems still need to be solved,which brings greater challenges to the performance improvement of LWIR QCLs. First,additional waveguide designs or thicker epitaxial structures need to be considered in order to maintain optical confinement factor of LWIR QCLs[8]. Otherwise,the reduction in mode gain,the increase in free carrier absorption losses (proportional to ) and the enhancement of plasmonic mode coupling at the metal-semiconductor interface will significantly deteriorate the device performance[9]. Second,optical facet coating,as a key technology,has important applications in power extraction[10],self-lasing suppression of single-mode lasers[11],etc. Nevertheless,the cavity facet coating of LWIR QCLs requires not only increasing thickness of anti-reflection (AR) coating to match the target wavelength[12],but also selecting an appropriate coating material to reduce the facet optical absorption,thereby improving the level of catastrophic optical mirror damage (COMD)[13]. Additionally,it is necessary to avoid deterioration of beam quality due to the beam steering effect[14]. Metallic high-reflection (HR) coatings have been commonly used and studied in LWIR QCLs to improve one facet output power[15-17]. Currently,there are relatively few reports on the optimization of AR coatings.
In this letter,two types of optical facet coatings (Al2O3 and Y2O3) were demonstrated and compared on 8.5 μm LWIR QCLs for the selection of appropriate coating material. By optimizing a single-layer Y2O3 AR coating on the 3-mm-long buried-heterostructure (BH) LWIR QCLs,the continuous-wave (CW) and pulsed performance have been significantly improved. Corresponding deterioration of far-field characteristics after coating was not observed.
1 Wafer structure, fabrication and testing
The LWIR QCL wafer,lasing wavelength close to 8.5 μm,is based on a single phonon resonance-continuum depopulation structure with 35 cascade stages. The active region consists of lattice-matched In0.53Ga0.47As/In0.52Al0.48As layers as previously shown in Ref. [18]. The double channel ridge structure with an average ridge width of 9.5 μm was first fabricated by contact photolithography and wet chemical etching. Then,semi-insulating InP:Fe was grown by MOCVD in order to confine the carriers and reduce the sidewall optical losses. A 400-nm-thick SiO2 layer was deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) for insulation,and a Ti/Au top contact layer was deposited by e-beam evaporation for electrical contacts. An additional 5-μm-thick gold layer was subsequently electroplated to further improve the heat dissipation. After thinned to 120 µm,an AuGeNi/Au bottom contact layer was deposited on the substrate. The processed QCL wafer was cleaved into bars with a cavity length of 3 mm for facet coating.
The uncoated and coated devices were mounted epi-side down on diamond submounts with indium for higher heat dissipation efficiency. The CW and pulsed (5 kHz,1 μs) performance were characterized by a Thorlabs S450C calibrated thermopile power meter and an IS50R Fourier Transform Infrared (FTIR) spectrometer. We used the rotation method to test the far-field divergence angle and built an automated scanning system.
2 Coating material processing and comparison
Dielectric material used as LWIR QCLs facet coating must have low absorption in the LWIR range and mature fabrication process to ensure that the AR coating does not suffer from COMD. For this purpose,Al2O3 and Y2O3 facet coating material systems were studied from the same wafer with 3 mm cavity length. The metallic HR coating consisting of M2O3/Ti/Au/M2O3 was coated on the back facet,as shown in Fig. 1(a). The AR coating was the single-layer of M2O3 coated on the front facet,where M2O3 represented Al2O3 or Y2O3. A dielectric material M2O3 was deposited on the back facet using e-beam evaporation to provide electrical insulation from Au. After that,a thin layer of Ti was sputtered to increase the adhesion between M2O3 and Au. Next,100 nm of Au with a high refractive index was used to achieve a high reflection of nearly 100% for the LWIR QCLs. Finally,another layer of dielectric material M2O3 was deposited to complete the HR coating to avoid short-circuiting of the devices in the subsequent packaging process. E-beam evaporation was chosen to deposit a single-layer of M2O3 AR coating. The AR reflectivity between 27% (uncoated) and close to 0% (quarter-wave layer) can be adjusted by simply adjusting the coating thickness[10]. The AR and HR scanning electron microscope (SEM) images of Y2O3 material system are shown in Figs. 1(b) and 1(c).
Figure 1.Schematic diagram of the LWIR QCLs:(a) schematic diagram of the LWIR QCLs with AR and HR coatings applied to the front and back facets; scanning electron microscopy (SEM) images of coatings on (b) front and (c) back facet
Simulation based on characteristic transfer matrix was derived to determine the coating thickness at the corresponding reflectivity. Reflectivity measurements were performed on an FTIR spectrometer with an MCT detector. Figure 2 shows the optical curves of the simulations and measurements of the Al2O3 and Y2O3 AR coatings. The wavelength for an uncoated LWIR QCL is shown in the inset. The theoretical simulations are in good agreement with the experimental measurements.
Figure 2.Reflectivity curves:experimental and theoretically simulated reflectivity curves of (a) Al2O3 and (b) Y2O3 AR coatings,the inset shows the lasing spectrum of the LWIR QCL at 298 K
For the Al2O3 material system,the thickness at quarter-wave layer exceeds 2 μm,which poses great difficulties in manufacturing. Therefore,in the initial stage,we selected a single-layer Al2O3 AR coating with a reflectivity of ~ 8.0% by reducing the thickness,and the HR coating was Al2O3/Ti/Au/Al2O3. The CW light-current-voltage (L-I-V) and WPE curves of uncoated and HR-AR (≈8%) coated devices operating at 298 K are shown in Fig. 3(a). Among them,the solid lines in the black and red circles represent the voltage and optical power curves,respectively,while the dashed lines in the blue circle represent the WPE curve. Compared with uncoated and coated devices,the slope efficiency () has been improved from 0.42 W/A to 0.7 W/A,while WPE has been increased from 1.6% to 2.7%. Moreover,the maximum output power () increased from 0.25 W to 0.33 W. However,the coated devices using Al2O3 exhibit COMD at the power density level of ~ 1.73 MW/cm2 before thermal inversion,as shown in Fig. 4(a). This also shows that Al2O3 coating has high absorption in the LWIR range[19],with an absorption coefficient as high as 755.2 cm-1 at 8.5 μm,which is not suitable for power extraction of high-performance LWIR QCLs.
Figure 3.L-I-V and WPE curves of the QCLs at 298 K, (a) CW operation of uncoated and HR-AR (Al2O3 coating); (b) pulsed and (c) CW operation of uncoated, HR-only and HR-AR (Y2O3 coating); (d) measured lateral far-field profiles
In comparison,the Y2O3 material system exhibits better performance. Since Y2O3 has a higher refractive index in the LWIR range,the thickness of the AR coating is initially set to quarter-wave layer with reflectivity of ~ 0.2%. The pulsed L-I-V and WPE curves of uncoated,HR-only and HR-AR coated devices operating at 298 K are shown in Fig. 3(b). is from 0.65 W/A for uncoated,0.88 W/A for HR-only,to a higher value of 1.63 W/A for HR-AR device. WPE is from 4.0%,5.8% to 8.8% and from 0.88 W,1.34 W to 2.19 W. The HR-AR coated device shows 120% and 149% improvements in WPE and ,respectively,compared to uncoated device under 298 K in pulsed mode. Since the absorption coefficient of Y2O3 at 8.5 μm is only 8.3 cm-1[20-21]. The cavity facet remained normally after the test,as shown in Fig. 4(b). The COMD has not occurred until the power density reached 11.5 MW/cm2. The L-I-V and WPE curves of these devices under 298 K CW operation are shown in Fig. 3(c),representing an improvement of from 0.50 W/A for uncoated,0.68 W/A for HR-only,to 1.07 W/A for HR-AR laser,WPE from 2.2%,3.5% to 3.3% and from 0.35 W,0.54 W to 0.54 W.
Under CW operation,the WPE and of the HR-AR coated device increased by 50% and 54% respectively,compared to that of the uncoated device. However,it showed no significant improvement compared to the only-HR device. The threshold current density of LWIR QCLs is defined as[22]:
,
where is the transparency current, is the internal loss of the laser waveguide, is the overall mirror loss, is the length of the cavity,and is the differential modal gain. increases significantly from 2.2 cm-1 for the only-HR device to 10.4 cm-1 for the HR-AR (≈0.2%) device. According to Eq. (1),the of CW operation significantly increases from 2.50 kA/cm2 to 3.82 kA/cm2. This substantial increase in the threshold current density brings additional self-heating effects to the devices[15],which appears to impede the AR coating from extracting further power.
One concern with optical coatings is the beam steering effect[12,14]. Conversely,the lateral far-field divergence angle of the HR+AR devices with Al2O3 and Y2O3 coatings shown in Fig. 3(d) is consistent with that of the uncoated device and shows no shift or beam steering effect. This can be explained as the negligible thickness of the coating compared to the length of the cavity.
3 Coating design and results
Despite the AR coating with near 0% reflectivity achieved significant improvements in output performance under pulsed operation. Nevertheless,for LWIR QCLs,it is necessary to optimize the reflectivity for ensuring further improvement of and WPE under CW operation. The slope efficiency of LWIR QCLs is defined as[23]:
,
where takes into account the unequal power distribution from asymmetry in facet reflectivity[24], is the number of stages,and is the internal quantum efficiency per stage. and are the reflectivity of the back and front facets. According to Eqs. (1) and (2),we can derive the internal parameters of the laser through the changes in and of devices with different coating states (different ) under pulsed operation,as follows:=1.43 kA/cm2,=2.1 cm-1,=8.8 cm/kA[10]. By introducing the additional resonant waveguide loss ==12.6 cm-1,the total waveguide loss is obtained as +=14.7 cm-1[25].
In the simple Rigrod analysis,i.e.[26],assuming uniform gain saturation,the device WPE at the roll-over current density can be expressed as[10]:
,
where is the photon energy, is the voltage at . Therefore,for LWIR QCLs,the back cavity is generally a metallic HR coating,and the appropriate is selected by adjusting the front cavity AR coating to achieve both high and WPE. Figure 5 shows the WPE and AR coating thickness as a function of predicted by Eq. (3) using the above internal parameter. The inset shows the reflectivity curves for different Y2O3 AR coating thicknesses,where increases from 0.2% to 27% by reducing the coating thickness,starting from a quarter-wave thickness. The measured WPE is approximately 18% lower than the model predictions due to the reduced slope efficiency between the threshold and the roll-over[10]. Although the pulsed WPE is maximum at =0.2% coincidentally and then decreases with increasing front cavity reflectivity,=0.2% is not conducive to power extraction for the CW operation. Therefore,in order to ensure a high WPE and prevent the additional self-heating effect caused by the increase in ,the 3-mm-long device was chosen with the same =4.4 cm-1 as the uncoated device. Finally,an 850 nm single-layer Y2O3 AR coating with a reflectivity of about 7.5% was applied by adjusting the AR coating thickness.
Figure 5.Predicted WPE and AR coating thickness as a function of front facet AR reflectivity,inset:the reflectivity curves for different Y2O3 AR coating thicknesses
The L-I-V and WPE curves of the HR-AR (≈7.5%) coated device under 298 K pulsed and CW operation are shown in Figs. 6 (a) and 6(b) respectively. The threshold current density under pulsed operation is =1.79 kA/cm2,along with = 1.15 W/A,WPE=7.20% and =1.65 W,and the threshold current density under CW operation is =2.71 kA/cm2,along with = 0.93 W/A,WPE=4.18% and =0.73 W. After the optimization of AR coating reflectivity,compared to the uncoated device,the HR-AR device showed 80% and 88% improvements in WPE and under pulsed operation,respectively,while significant improvements of 106% and 91% were also obtained under CW operation. A more suitable reflectivity may exist at =0.2% to 7.5%,allowing the power extraction under CW operation to be further increased. Furthermore,utilizing a device of the same size as in Ref. [18] along with Y2O3 coatings with optimized reflectivity,the output performance will be significantly improved.
Figure 6.L-I-V and WPE curves of the QCL with optimized front facet Y2O3 AR coating reflectivity under (a) pulsed and (b) CW operation at 298 K
In summary,we compared the application of Al2O3 and Y2O3 coatings in LWIR QCLs without beam steering effect,and proved that Y2O3 was more suitable for power extraction of high-performance LWIR QCLs compared to Al2O3. The LWIR QCL with λ~8.5 μm was applied with a Y2O3 coating with a reflectivity close to 0%,which was suitable for power extraction under pulsed operation and obtained a peak power of 2.19 W and WPE of 8.8%. Considering self-heating effects,the Y2O3 AR coated device with a reflectivity of 7.5% showed a maximum pulsed peak power of 1.65 W and CW output power of 0.73 W at 298 K,which were 88% and 91% higher than uncoated lasers,respectively. In the future,by improving the epitaxial performance and combining the power extraction of Y2O3 AR coating,higher performance LWIR QCLs will be obtained.