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Abstract
Multi-resonance light coupling management is a promising way to expand the operating spectral ranges of optoelectronic devices. The classical strategies are either lack of independent tunability for each resonance or involved with complex fabrication. Here, we propose a new scheme for expanding the operating spectral range of an optoelectronic device through a dual-color active material integrated with a simple resonant waveguide structure. The TM waveguide mode and the SPP mode of the resonant waveguide structure are regulated to match the two active regions of the dual-color material both spectrally and spatially. Applying this scheme to a long-wavelength infrared quantum well photodetector, the absorption efficiencies at the two peak detection wavelengths of the dual-color quantum wells are both enhanced by more than 10 times compared with the case of a standard 45° edge facet coupled device with the same detection material. The simple light coupling structure is easy to accomplish and compatible with focal plane arrays. For thermal radiation detection, the absorption efficiency of the 300 K blackbody radiation by our dual-color detector is 83.8% higher than that by a single-color detector with the optimized structural parameters. Moreover, either polarization sensitive or polarization insensitive detection could be achieved in this dual-color infrared quantum well photodetector by using anisotropic or isotropic gratings.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Light coupling management is one of the most important concerns of optoelectronic devices [1–8]. Especially for those based on low-dimensional active materials, integrated photonic structures are employed to generate locally intensified light fields at the active materials by resonances [5–9]. However, a single resonance usually corresponds to a limited operating spectral range. A lot of multi-resonance light couplers or absorbers have been proposed to enlarge operating spectral ranges [10–16], but achieving such a photonic structure still remains challenging, both in terms of design and fabrication. There are two classical strategies. One is to utilize the multiple orders of resonances of a photonic structure [16], but it is difficult to independently control the resonant wavelengths and field distributions of those resonances. The other is to combine multiple photonic structures together either horizontally or vertically [10–15], but it inevitably increases fabrication complexity and device sizes. Here, we report a new scheme of enlarging the operating spectral range of an optoelectronic device through a dual-color active material enhanced by independently tunable double resonances of a simple resonant waveguide structure.
The optoelectronic device addressed detailedly in this study is a quantum well infrared photodetector (QWIP) working in the long-wavelength infrared (LWIR) range. Long-wavelength infrared detection is important for surveillance, industrial process control, medical imaging, and astronomy [17–19]. QWIPs have become a leading candidate for sensitive and cost-effective LWIR staring arrays [20,21] due to the mature processing techniques for III-V semiconductors, high uniformity of the epitaxial materials, tailorable detection wavelengths, radiation hardness [22,23]. However, the inherent narrow-band photoresponse is a shortcoming of QWIPs, especially for detecting thermal radiation, which is broadband. Thus, a detection band expansion of QWIPs becomes an appealing research topic [24–26]. Multicolor quantum well stacks with different photoresponse spectra provide a way for QWIPs to cover a wider spectral range [27,28]. How to efficiently couple the incident light of a specific wavelength to the corresponding quantum wells in the multicolor stacks remains a bottleneck problem [29,30]. Grating diffraction is commonly used in QWIPs, especially in QWIP focal plane arrays (FPAs) [17]. It is known that GaAs/AlGaAs QWIPs are only responsive to the light field with an electric component perpendicular to the GaAs/AlGaAs multiple layers [17–19]. Grating couplers usually deflect the incident light for an electric field component perpendicular to the QWs so that the light could be effectively absorbed by the QWs. Meanwhile, since the optical path of the deflected light is enlarged, the absorption efficiency of the QWs is enhanced. For multicolor QW stacks, higher-order diffractions of a grating coupler are exploited for shorter-wavelength QWs [16]. However, there are several shortcomings of this method: 1) The diffraction efficiency quickly decreases with diffraction order, so the QWs with shorter detection wavelengths cannot be effectively enhanced. 2) The wavelengths of different diffraction orders cannot be independently tuned to match the QWs of different colors. 3) In absence of resonances, the local field at the QWs is not intensified. In this work, we propose to use a waveguide mode and a surface plasmon polariton (SPP) mode supported by a simple resonant waveguide structure to enhance the absorption efficiency of an LWIR dual-color QWIP. In principle, the two peak detection wavelengths of the dual-color quantum wells can be arbitrarily set in the long wavelength infrared range. The light coupling structure is easy to fabricate and it is compatible with focal plane arrays. The semiconductor layer contains two QW stacks of different detection wavelengths (labeled as blue and red). Then the waveguide mode and the SPP mode can be independently manipulated to match the dual-color QW stacks both spectrally and spatially. As a result, the absorption efficiencies of the dual-color QW stacks are both enhanced by more than 10 times compared to the case of a 45° edge facet coupled QWIP (45°-QWIP) with the same detection material. Then, the detection band is effectively expanded. Concerning thermal radiation detection, the absorption efficiency of the 300 K blackbody radiation by our dual-color QWIP is 83.8% higher than that by a single-color QWIP with the optimized structural parameters. Moreover, either polarization sensitive or polarization insensitive detection could be achieved in the dual-color QWIPs by using anisotropic or isotropic gratings.
2. Results and discussions
As shown in Fig. 1(a), the dual-color QWIP consists of an etching stop layer (200 nm AlGaAs), a bottom contact layer (600 nm GaAs), 14 stacks of GaAs (5 nm)/Al0.24Ga0.76As (50 nm) (blue QWs), 14 stacks of GaAs (6 nm)/Al0.18Ga0.82As (50 nm) (red QWs), and a top contact layer (580 nm GaAs). A light coupling grating is created by etching the top contact layer and coating the surface with metal. The grating period (Λ) is 3.2 µm, the grating width (g) is 1.5 µm, and the depth (h) is 380 nm. Then, the semiconductor layers together with the grating form a resonant waveguide. The air and the metal serve as the cladding, and the semiconductor part serves as the core. The thickness of the waveguide core (d) is defined as the thickness of all the semiconductor layers minus h. Figure 1 (a) shows the flipped-chip structure of the dual-color QWIP that is ready for connection with readout circuits. The blue and red QW stacks have two different detection bands in the LWIR range. The multiple QWs could be regarded as an effective uniaxial medium with a diagonal dielectric constant tensor [17,33,34]. $\varepsilon _{{\rm QW}} = {\rm diag}\,\left( {\varepsilon _x,\,\varepsilon _y,\varepsilon _z} \right)$, where $\varepsilon _z = \varepsilon _{GaAs} + 0.5\omega \gamma /\left( {\omega _0^2 -\omega ^2-i\omega \gamma } \right)$. The x-y plane is parallel to the multilayers, and the z-axis is perpendicular to them. For the light field polarized parallel to the x-y plane, the effective medium of the QWs behaves like GaAs, which is a dielectric with a slowly varying real dielectric constant within the LWIR range. For the light field polarized along the z-axis, the intersubband transition is active so the effective medium behaves as a Lorentz oscillator. ${\hbar}$ω0 corresponds to the intersubband energy difference. γ is the damping frequency and is estimated to be 0.05ω0 in our case [24,31,32]. Assuming the peak absorption efficiency wavelengths of the blue and the red QWs are 8.5 µm and 10.5 µm, respectively, the real and imaginary parts of the wavelength dependent z-component dielectric constants (εz) are shown in Fig. 1(b). The refractive index of GaAs takes 3.27. The thickness of the GaAs top contact (excluding the grating) is 200 nm and that of the bottom contact is 600 nm. The permittivity of gold follows the Drude model. Figure 1 (c) shows the absorption efficiency of the 45° edge facet coupled dual-color QWIP (45°-QWIP).
Fig. 1. (a) Schematic of the resonant waveguide integrated dual-color long-wavelength QWIP. db = 0.8 µm, t2 = 0.8 µm, t1 = 0.8 µm, dt = 0.58 µm, h = 0.38 µm, $\Lambda$ = 3.2 µm, g = 1.5 µm. The right panel is the band diagram of the QWs and contacts. (b) Real and imaginary parts of εz. (c) Absorption efficiency (ηa) of the 45°-QWIP. The two absorption efficiency peaks correspond to the two peak detection wavelengths of the dual-color QWs.
The resonant waveguide with a metal-coated grating can hold waveguide modes and an SPP mode. It is known that the excitation of a TM waveguide mode or an SPP mode that spectrally matches the photosensitive spectral range of the QWs can prominently enhance the absorption efficiency of the QWs since the local field with a z-direction electric component in the QWs is resonantly intensified [19,24–26]. Here, in the dual-color QWIP, there are two stacks of QWs with two different detection wavelengths. And we propose to utilize the 1st TM mode and the SPP mode to enhance the absorption efficiency in the blue and red QW stacks, respectively. The key issue is to ensure that these two light modes match the two stacks of QWs both spectrally and spatially. The excitation of either the 1st TM mode or the SPP mode requires that the lateral wave vector of the incident light (kx) equals the propagation constant (βTM1 or βspp). Concerning normal incidence, with the help of the grating, kx equals ±2mπ/Λ. Here, we only consider the first order diffraction of the grating (m = 1) since higher order diffractions typically correspond to lower efficiency. Thus, with a single grating structure, the propagation constants of the 1st TM mode and the SPP mode that can be excited by a normally incident light fulfills the equation βTM1 = βspp = ±2π/Λ. Then, the resonant wavelengths of these two modes are determined by the following equations:
The absorption efficiency enhancement of the dual-color QWIP is not only attributed to the matching of λTM (λSPP) and λblue (λred), but also to the fact that the light fields of the 1st TM mode and the SPP mode are mainly distributed in the blue QW stack and the red QW stack, respectively. Since the semiconductor layers of the dual-color QWIP are grown by epitaxy, the stacking order of these layers can be manipulated. In our device, the blue QW stack is placed in the middle of the waveguide layer, corresponding to the local field of the 1st TM mode. The red QW stack is placed closer to the Au/GaAs interface since the light field of the SPP mode is mainly distributed in the vicinity of the metal surface. During the material growth process, the blue QW stack should be grown before the red one, which can be easily realized by a typical molecular beam epitaxy system. As shown in Fig. 3 (a), for the incident light with a wavelength of 8.5 µm, the 1st TM mode is excited in the resonant waveguide integrated dual-color QWIP. The local field of the 1st TM mode is mainly concentrated at the blue QW stack. Since the GaAs/AlGaAs QWs only absorb the light component polarized along the z-direction, the absorption efficiency of the QWs is proportional to |Ez|2, and thus |Ez|2 becomes an indicator of the light coupling condition. To quantitatively describe the light coupling efficiency in the QWs, the z-dependent absorbable field enhancement, defined as
Fig. 2. (a), (c) and (d) Grating period dependent spectra of ηa-blue (left) and ηa-red (right) of three grating coupled dual-color QWIPs. (a): d = 2.6 µm, Λ = 3.2 µm, g = 1.5 µm, dt = 0.58 µm, h = 0.38 µm, t2 = 0.8 µm, t1 = 0.8 µm, db = 0.8 µm, λblue = 8.5 µm, and λred = 10.5 µm. (c): d = 3.8 µm, Λ = 3.2 µm, g = 1.9 µm, dt = 0.73 µm, h = 0.53 µm, t2 = 1.3 µm, t1 = 1.5 µm, db = 0.8 µm, λblue = 9.2 µm, and λred = 10.2 µm. (d): d = 2.2 µm, Λ = 3.8 µm, g = 2.25 µm, dt = 0.58 µm, h = 0.38 µm, t2 = 0.8 µm, t1 = 0.95 µm, and db = 0.25 µm, λblue = 8.2 µm, and λred = 12.2 µm. (b) Dispersion curves of the SPP mode and the 1st TM mode of the grating coupled dual-color QWIPs with different d values. The lines are predicted by Eq. (1). The scatters are based on the simulation results in (a), (c), and (d). (e) Spectra of the total absorption efficiency (ηa) of the three QWIPs in (a), (c) and (d) with the optimized structure parameters, by which the peak wavelengths of the three ηa spectra match the three sets of λblue and λred, respectively.
Fig. 3. (a) and (c) Simulated |Ez|2/|E0|2 distributions in the x-z plane of the dual-color QWIP device ($\Lambda$ = 3.2 µm, g = 1.5 µm, dt = 0.58 µm, h = 0.38 µm, t2 = 0.8 µm, t1 = 0.8 µm, and db = 0.8 µm) at the wavelengths of 8.5 µm and 10.5 µm, respectively. (b) and (d) Field enhancement factor as a function of the distance (z) from the grating at 8.5 µm and 10.5 µm, respectively.
The remarkably expanded detection range in the LWIR regime is favored for room temperature thermal radiation detection. The thermal radiation arriving at the detector at normal incidence is expressed as:
where $\varepsilon$ denotes the emissivity of this object, and B ($\lambda$, T) denotes the blackbody radiation energy density (Fig. 4 (a) black line). When the temperature is 300 K, the peak wavelength of blackbody radiation is about 9.7 µm, and the FWHM is 11.59 µm. Then, the absorption efficiency of the blackbody radiation in the QWs writesFig. 4. Blackbody radiation energy density at 300 K (black line) and absorption efficiency of our dual-color detector (pink line) and the single-color detector (red line).
To get a high responsivity, a broad and high ηa (λ) is pursued. Concerning a typical single color LWIR QWIP with an absorption peak at the wavelength of 9.7 µm, which is consistent with the peak wavelength of blackbody radiation at 300 K, the detection range is about 0.72 µm, only 6.2% of the FWHM of the blackbody radiation spectrum. Thus, 93.8% of the blackbody thermal radiation signal is wasted even though it reaches the detector. This single color QWIP consists of 28 periods of quantum wells, whose total thickness is 1.6 µm. The QWIP is integrated with a typical grating coupler ($\varLambda$ = 3.0 µm, g = 1.8 µm, dt = 0.58µm, h = 0.38 µm, t2 = 0.8 µm, t1 = 0.8 µm, and db = 0.8 µm). With these optimized structure parameters, the peak absorption efficiency at 9.7 µm reaches 73.7%.
The double-resonance-enhanced dual-color QWIP provides a new opportunity. As shown in Fig. 4, the double-peak ηa spectrum leads to a total absorption efficiency much higher than that of the single color QWIP. As listed in Table 1, the absorption efficiency of the 300 K thermal radiation in the dual-color QWIP is 83.8% higher than that of the single color QWIP. As the temperature of the thermal radiation increases, the peak wavelength of the radiation is blue-shifted, so the absorption efficiency enhancement ${{({{A_{\textrm{QW-dual}}} - {A_{\textrm{QW-single}}}} )} / {{A_{\textrm{QW-single}}}}}$ provided by the dual-color QWIP slightly decreases. But it is still as high as 83.4% at the thermal radiation temperature of 500 K. AQW-dual denotes the blackbody radiation absorption efficiency of the dual-color detector (λblue = 9.2 µm, λred = 10.2 µm). AQW-dual denotes the blackbody radiation absorption efficiency of the single color detector with a peak absorption efficiency of 9.7 µm.
Table 1. Absorption efficiency of thermal radiation in the dual-color QWIP and the single-color QWIP at different temperatures
The dual-color infrared detector based on the double resonances of the resonant waveguide structure could be either polarization sensitive or insensitive by using anisotropic or isotropic coupling gratings. As shown in Fig. 5 (a), the polarization extinction ratio ${{{\eta _\alpha }_{ - TM}} / {{\eta _\alpha }_{ - TE}}}$ of the dual-color QWIP with a 1D grating (Fig. 5 (c)) is larger than 1900 at both the wavelengths of 8.5 µm and 10.5 µm. The high polarization discrimination is attributed to the combined effect of the intrinsic polarization selectivity of the QWs and the polarization dependent coupling of the 1D grating [6,9,32]. Since GaAs/AlGaAs QWIPs are only responsive to the light field with an electric component perpendicular to the GaAs/AlGaAs multiple layers [17–19], only the TM modes and the SPP mode can be absorbed by the QWs, while the TE modes cannot. Moreover, the 1D grating serves as a polarization selective coupler. The TM modes and the SPP mode can only be excited by the incident light with an electric component along the x-direction, while the y-polarized incident light can only excite TE modes that cannot be absorbed by the QWs. Once the grating is changed into a 2D form (Fig. 5 (d)), the x-polarized and the y-polarized incident light are the same for the detector. As shown in Fig. 5 (b), the incident light from the two polarization directions, and the absorption efficiency spectrum of QWIP is exactly the same. Both of them can excite the TM modes and the SPP mode.
Fig. 5. (a)-(b) Absorption efficiency spectra of the double-resonance-enhanced dual-color QWIP with a 1D grating (a) and a 2D grating (b) under illumination of TE and TM waves. (c)-(d) Sketches of the double-resonance-enhanced dual-color QWIP with a 1D grating (c) and a 2D grating (d).
3. Conclusion
In summary, we propose a new scheme of using independently tunable guided mode resonance and SPP resonance to enhance a dual-color LWIR QWIP for detection band expansion. The double resonances are supported by a simple resonant waveguide structure. In the frequency domain, the resonant waveguide mode and the SPP mode can be separately tuned to match the two detection wavelength ranges of the two QW stacks, respectively. In the spatial domain, the two light modes are mainly distributed in the corresponding QW stacks. In this way, the absorption efficiencies of the two QW stacks are both enhanced by more than 10 times and thus the detection band is effectively expanded. Besides, the device structure is easy to fabricate, and compatible with focal plane arrays. Concerning the detection of room temperature thermal radiation, the double-resonance-enhanced dual-color QWIP absorbs 83.8% more power than a grating coupled single-color QWIP with the optimized structural parameters. Further, the device could be highly polarization discriminative or polarization insensitive by switching between one- or two-dimensional grating structures.
Funding
National Key Research and Development Program of China (2018YFA0306200); National Natural Science Foundation of China (61874126, 61975223, 91850208, 61991442); Hundred Talents Program of the Chinese Academy of Sciences (No. 20181214); Key Deployment Projects of the Chinese Academy of Sciences (ZDRW-XH-2021-7-1); Program of Shanghai Academic Research Leader (22XD1424400); Shanghai Municipal Science and Technology Major Project (Grant No.2019SHZDZX01).
Acknowledgments
This work was supported by the Shanghai Tech University Quantum Device Lab (SQDL).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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