1. Introduction

Circularly polarized light (CPL) refers to an electromagnetic wave where the electric field rotates in a constant magnitude in a plane perpendicular to the wave direction. The direction of rotation classifies CPL as left-handed circularly polarized light (LCP) or right-handed circularly polarized light (RCP), corresponding to the two spin states of photons. CPL has found a variety of important application fields, including optical communication [1], astronomical imaging [2], plasmonic sensing [3], and quantum information processing [4–6], due to the robustness of the circular polarization state during propagation [7], asymmetric interaction with chiral substances [8], and quantum information [9] carried by photon spin states. For all these applications, circular polarization detection is a pivotal technology. With the miniaturization of optoelectronic devices and systems, compact on-chip circular polarization detectors are pursued to replace traditional devices relying on separate polarizers and wave plates. Advances in micro-nano fabrication technologies provide new opportunities to integrate chiral plasmonic antennas directly on the detection material to realize compact on-chip circular polarization detectors [10,11]. The chiral plasmonic antennas selectively couple LCP or RCP light into photonic modes by resonance [12–15], and reflect the CPL with the opposite handedness at a high rate, leading to enhanced photoresponse for the principle circular polarization state and inhibited photoresponse for that of the opposite handedness [16–18]. Most researches on compact on-chip circular polarization detectors focus on the visible to mid-infrared range, while the terahertz band received much less attention. The terahertz band have been extensively explored for imaging, spectroscopy, and wireless communication due to the unique properties of THz waves, such as low photon energy [19,20], formidable penetration capability [21,22], large communication bandwidth [23,24] and superior spatial and temporal resolution [25–27] compared to microwave. Circular polarized THz waves have been considered promising to improve these applications [28]. Concerning the bulky sizes of non-integrated THz circular polarization detectors and the difficulty in finding proper polarization optics in the THz band, the development of compact on-chip THz circular polarization detectors is highly desired.

In this work, we present a compact on-chip THz circular polarization detector based on the composite structure of quantum well (QW) infrared detection material sandwiched by a chiral plasmonic antenna array and a metal plane. The composite structure can efficiently couple the incident circular polarized wave of a chosen handedness into a cavity enhanced surface plasmon polariton (SPP) wave for enhanced light absorption in the QWs, and reflect the circular polarized wave of the opposite handedness at a high rate for inhibited light absorption in the QWs, leading to a discrimination of circular polarization states. In addition to the cavity coupled chiral antenna array, the GaAs/AlGaAs QWs that only absorbs the light with an electric field component perpendicular to the QWs performs a second polarization selection [29], resulting in a circular polarization extinction ratio (CPER) as high as 25, exceeding other reported integrated terahertz circular polarization detection-type devices [30]. The CPER denotes the ratio of the absorption of CPL with a chosen handedness compared to that of CPL with the opposite handedness. Due to the intensified SPP wave at the QWs, the absorption for the principle circular polarization is 15 times higher than the standard 45° edge facet coupled device. By adjusting the structural parameters, we demonstrate that the absorption peak wavelength can be tuned over the range from 6.41 to 6.56 THz, while considerable absorption enhancement and high circular polarization discrimination are preserved even under non-normal incidence. The proposed device is compact in size and suitable for integration. The designed chiral antenna is considered as in-plane two-dimensional chiral structures that can be obtained easily by electron beam exposure and peeling processes. This innovative could open a new avenue to the development of high-performance compact on-chip THz circular polarization detectors.

2. Device structure

The physical mechanism of photo-detection of the device is explained in the manuscript, and several performance indexes of the device are estimated. Quantum well infrared detectors belong to the photoconductive detectors, and their fundamental physical mechanism for photoelectric detection arises from the difference in energy bandgaps between two III-V semiconducting materials, GaAs and AlGaAs. When these materials are stacked together, a heterojunction is formed [31]. In very thin stacks of this composite material, a quantum well is created, leading to the formation of subbands within the well. When the energy gap between these subbands matches the energy of incident photons, the incoming light can excite inter-subband transition from the ground state to the first excited state at the top of the quantum well, contributing to photocurrent. Figure 1(a) shows a single period of the chiral-antenna-QW composite structure. As shown in Fig. 1(b), the QW detection material contains 3 stacks of Al_0.04Ga_0.96As (75 nm)/GaAs (15 nm) quantum wells with an additional barrier layer (Al_0.04Ga_0.96As (75 nm)), a top contact layer, and a bottom contact layer. The 3 stacks of quantum wells with an additional barrier layer form the 345 nm thick photosensitive region. The top or the bottom contact layer, made of heavily doped GaAs, has a thickness of 0.5 µm or 2.43 µm. The III-V semiconductor layers are grown epitaxially and integrated with a metal reflective layer in a flip-chip manner, commonly used for focal plane arrays [29,32–35]. The reflective layer is obtained through an electron beam vapor deposition process. As shown in Fig. 1(c), the chiral plasmonic antenna has a two-dimensional periodic structure in the x-y plane. The period along the x-axis (P_x) is 20 µm, and that along the y-axis (P_y) is 24.6 µm. The antenna is a branch-shaped grating with the geometric parameters w₁= 6.13 µm, w₂= 2.3 µm, L = 15.2 µm, and θ = 50°. Figure 1(d) shows a schematic of the device in the oblique incidence case.

 

Fig. 1. (a) 3D schematic of the device designed. (b) schematic cross-sectional view along the z axis of the device contained in the black dashed frame of (a). (c) top view illustration of the device designed. (d) The definition of the tilt angle for oblique incidence. (e) dielectric constant of the QW layer along the z-direction. (f) absorption spectrum of the standard 45° edge facet coupled device. Sketch inserted shows the basic structure of the device.

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The QWs only absorb light with a z-component electric field due to the selection rule of the intersubband transition. Consequently, they can be considered as a uniaxial effective medium with a permittivity tensor like ɛ_QW= diag(ɛ_x, ɛ_y, ɛ_z), where ɛ_x= ɛ_y= ɛ_GaAs and ${\varepsilon _z} = {\varepsilon _{GaAs}} + \frac{{{\varepsilon _{GaAs}}{f_{12}}\omega _p^2}}{{\omega _{12}^2 - {\omega ^2} - i\omega \gamma }}$ [36]. For the light polarized within the x-y plane, the effective medium of QWs is similar to GaAs, that is a transparent in the long-wave infrared range. For the light polarized in the z-direction, ɛ_z follows the Lorentz form. Here, ɛ_GaAs represents the permittivity of GaAs, f₁₂ is the oscillator strength, ω_p is the two-dimensional effective plasma frequency, ω₁₂ is the transition frequency in optics, and γ is the relaxation frequency [37]. Figure 1(e) shows Re (ɛ_z) and Im (ɛ_z) of the QWs in a typical THz quantum well infrared photodetector (THz QWIP) with a peak detection frequency of 6.52 THz (λ = 46 µm). Since the QWs are unresponsive to normally incident light, they are typically made into a 45° edge facet coupled device for photoresponse characterization [38]. The absorptance of the QWs in a standard 45° edge facet coupled device (Fig. 1(f)) is considered a standard reference. The absorption peak at 6.52 THz is 0.67%. The Drude model is used to describe the dielectric constant of Au [39].

3. Result and analysis

As shown in Fig. 2(a) and (b), the absorptance for LCP light significantly surpasses that for RCP light in the frequency range from 6.37 to 6.68 THz. Especially, at the frequency of 6.52 THz, the CPER reaches a peak of 25, and the absorptance for LCP light reaches a peak of 0.1. As shown in Fig. 2(c), LCP light resonantly excites an intensified local field between the chiral antenna and the metal plane. This local field with a significant E_z component overlaps the QWs as the photosensitive region, leading to a prominent enhancement in the absorptance. This local field is attributed to an SPP wave in the plasmonic waveguide formed between the chiral antenna and the bottom metal plane. In contrary, as shown in Fig. 2(d), RCP light does not effectively excite the SPP wave, leading to pretty low field intensity at the QWs and thus low absorptance. The sharp contrast between the absorption of LCP and RCP light causes high circular polarization discrimination. In addition, the absorptance for LCP is over 15 times higher than that of a standard 45° edge facet coupled device. Thus, our device not only has a high circular polarization discrimination ability but also has an enhanced absorption for the principle circular polarization.

 

Fig. 2. (a) shows the absorption spectrum in the QW layer under the conditions of both LCP and RCP incidence. (b) depicts the CPER spectrum of the designed device. (c) and (d) display the field distribution in the xy section at the center of the quantum well layer and yz section along the center of the structural unit under incident conditions of LCP and RCP.

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Regarding to the estimation of the detection performance, under low-temperature conditions, the simplified formula for noise, ${i_{noise}}$, is given by ${i_{noise}} = \sqrt {4{i_{dark}}eg\Delta f}$, which is essentially an approximation of the G-R noise [40], and it applies to photoconductive detectors. This is because the capture probability ${P_c}$ is much less than 1 (photoconductive devices have a high electron emission probability, resulting in a low capture probability). In this equation, ${i_{dark}}$ represents the dark current, e is the electron charge, $g = {1 / {N{P_c}}}$ is the photoconductive gain, and ${P_c}$ is typically taken as 0.1. The detectivity, ${D^\ast }$, is given by ${D^\ast } = {{R\sqrt A } / {{i_{noise}}}}$, where A is the photosensitive area, and R is the responsivity. Based on prior research [41], performance estimates for the detector can be made. For the chosen quantum well material, the standard device responsivity is 0.05 A/W. By incorporating the designed antenna and a bottom metal reflector, the responsivity can be enhanced to 0.7 A/W, with a typical dark current of 3 × 10⁻³ A/cm², resulting in a noise current of 8.25 × 10⁻⁹ A/Hz^1/2 Consequently, the detector's responsivity can reach 1.97 × 10⁸cm Hz^1/2 W⁻¹.

Conventionally, the excitation of an SPP wave does not require the incident light to be LCP. Once the incident light gains an extra momentum of ${{2\pi } / {{P_y}}}$ from the grating structure, and it is equal to the propagation constant of the SPP wave ${\beta _{spp}} = ({{{2\pi } / {{\lambda_0}}}} )\sqrt {{{{\varepsilon _m}{\varepsilon _d}} / {({{\varepsilon_m} + {\varepsilon_d}} )}}}$ at a specific frequency, the SPP wave is excited. ${\lambda _0}$ denotes the free-space wavelength of the incident light; ${\varepsilon _m}$ denote the permittivity of metallic material and ${\varepsilon _d}$ denote the permittivity of dielectric medium. The contrast between the LCP and RCP light for the excitation of the SPP wave is attributed to the interference of the co-polarized and cross-polarized radiation field. The circular polarization selectivity of asymmetric metamaterials can be analyzed by studying the interference of co-polarized and cross-polarized components in the reflection spectrum [29]. Circularly polarized light can be decomposed into two orthogonal electric fields (E_x and E_y) with a phase difference of π/2. When a linearly polarized light field (E_x or E_y) is incident on asymmetric metamaterials, the resulting reflection field contains both co-polarized fields (r_xxE_x or r_yyE_y) and cross-polarized fields (r_yxE_x or r_xyE_y). The coefficients r_xx and r_yy denote the reflection coefficients for co-polarization, while r_yx and r_xy denote the reflection coefficients for cross polarization. The reflection can be calculated using the Jones matrix.

The amplitude and phase of calculated reflection coefficients are depicted in Fig. 3(a) and 3(b), respectively. The interference between co-polarized and cross-polarized reflected fields for LCP light and RCP light is analyzed through vector field diagrams (Fig. 3(c)–(f). At resonant frequency of the SPP wave, the co-polarized reflected field (r_xxE_x or r_yyE_y) is out of phase with the cross-polarized reflected field (r_yxE_x or r_xyE_y) for LCP light (Fig. 3(c)–(d), causing a decrease in the total reflection (E_x,sum or E_y,sum). The decrease in reflection means that more power of the incident light flows into the SPP wave, and thus enhances the local field at the QWs. For RCP incident light, the co-polarized field (r_xxE_x or r_yyE_y) is in phase with the cross-polarized field (r_yxE_x or r_xyE_y) (Fig. 3(e)–(f), resulting in an increase in the total reflection (E_x,sum or E_y,sum). The increase in reflection means that the incident light cannot efficiently excite the SPP wave, and thus the local field at the QWs becomes low. The bottom metal plane and the chiral antennas form a cavity, which further enhances the contrast between the LCP and RCP light in terms of the interference between the co-polarized and the cross-polarized field [29]. To absorb RCP light and reflect LCP light, a right-handed chiral antenna can be employed. Such a structure is essentially a mirror image of the left-handed chiral antenna.

 

Fig. 3. (a) and (b) represent the amplitude and phase of the reflection coefficients, respectively. (c-f) depict vector plots of destructive (c,d) and constructive (e,f) interference between the unconverted reflected field (r_xxE_x or r_yyE_y) and the converted reflected field (r_xyE_y or r_yxE_x) at resonant frequency 6.52 THz.

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The resonant frequency for excitation of the SPP wave can be tuned by adjusting and P_y. As shown in Fig. 4, increasing P_x leads to a red shift in the absorption peak under LCP illumination. This behavior can be explained by the effective medium theory if we treat the layer chiral antenna array located as a mixture of Au and air [42]. As P_x increases, the chiral antenna layer becomes less metallic since the percentage of Au (f) becomes smaller. Thus, the permittivity of this layer ${\varepsilon _{eff}} = f{\varepsilon _{Au}} + ({1 - f} ){\varepsilon _{air}}$ becomes more positive. As a result, in order to fulfill the momentum matching condition that ${\beta _{spp}} = ({{{2\pi } / {{\lambda_0}}}} )\sqrt {{{{\varepsilon _{eff}}{\varepsilon _d}} / {({{\varepsilon_{eff}} + {\varepsilon_d}} )}}} = {{2\pi } / {{P_y}}}$, ${\lambda _0}$ becomes larger. On the other hand, increasing P_y also red-shifts the absorption peak [43]. This behavior can also be explained by the requirement of momentum matching. As P_y increases, the lateral momentum gained from the grating decreases, and it can only excite the SPP wave with a small propagation constant. Since increasing P_y does not affect the percentage of Au in the chiral antenna array layer, ${\varepsilon _{eff}}$ almost remains unchanged. Thus, a smaller propagation constant corresponds to a longer free space wavelength. It is worth noting that the period of the grating structure plays a crucial role in determining the excitation wavelength of SPP.

 

Fig. 4. The absorption and CPER spectra were obtained by varying the period of the structure in two directions. (a-c) show the results of changing the period P_x in the x direction, while (d-f) depict the results of changing the period P_y in the y direction.

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As shown in Fig. 5, the θ-dependent absorption spectra of the composite structure under LCP illumination exhibit a non-monotonic behavior in the range of θ from 35° to 90°. The absorption increases initially, reaches a maximum around θ = 50°, and decreases after that. This phenomenon is attributed to the interference between the principle polarization radiation and the cross polarization radiation. As stated in the main text, the efficient in-coupling of LCP is due to the destructive interference between the principle polarization radiation and the cross polarization radiation. The relative phase and amplitude relationship between the principle polarization radiation and the cross polarization radiation is mainly influenced by the chiral shape of the plasmonic antenna. At θ = 50°, the principle polarization radiation and the cross polarization radiation have similar amplitudes and a π phase difference so that the reflection is effectively suppressed, and the light power is efficiently coupled into the SPP wave. Based on the similar mechanism, the absorption of RCP light decreases at first, reaches a minimum around θ = 50°, and increases at larger θ angles. At θ = 50°, the principle polarization radiation and the cross polarization radiation experience sufficiently constructive interference so that the reflection reaches a maximum and absorption reaches a minimum. As a result, the θ-dependent CPER also shows a non-monotonous behavior and reaches a maximum at θ = 50°. In addition, peak absorption wavelength under LCP illumination is red-shifted as θ increases from 35° to 90°. The resonant behavior is caused by the excitation of a SPP wave along the metal strip in the y-direction. The metal strip together with the inclining branch acts as a composite plasmonic strip waveguide. As θ increases, the effective width of the plasmonic strip waveguide increases and thus the resonant wavelength for exciting the SPP wave is red-shifted. Based on analytical model for plasmonic strip waveguides [44], ${k_x}W + {\phi _r} = m\pi$, where ${k_x}$ is the wave vector in the x-direction, W is the effective width of the strip waveguide, ${\phi _r}$ is the phase of the reflection coefficient at the interface, m = 1, 2, 3… represents the mode order. And ${k_x}^2 + {\beta ^2} = ({{{2\pi } / \lambda }} )\cdot ({{{{\varepsilon_m}{\varepsilon_d}} / {({{\varepsilon_m} + {\varepsilon_d}} )}}} )$, where β denotes the propagation constant in the y-direction, ${\varepsilon _m}$ and ${\varepsilon _d}$ denote the dielectric constants of metal and dielectric layers. At resonant excitation, $\beta = {{2\pi } / {{P_y}}}$. Thus, $\lambda = 2\pi \sqrt {{{({{{{\varepsilon_m}{\varepsilon_d}} / {({{\varepsilon_m} + {\varepsilon_d}} )}}} )} / {[{{{({{{({m\pi - {\phi_r}} )} / W}} )}^2} + {{({{{2\pi } / {{P_y}}}} )}^2}} ]}}}$. Therefore, as θ increases, the effective width of the strip plasmonic waveguide increase, and then the absorption peak wavelength is red-shifted.

 

Fig. 5. (a) θ-dependent absorption spectra for LCP and RCP light. (b) θ-dependent CPER spectra.

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Furthermore, we investigated the impact of the remaining physical parameters of the antenna, namely, w₁, w₂, and L, on the absorption and CPER. Figure 6(a) and (b) present the influence of w₁ on the absorption spectra for LCP and RCP light and on the CPER. Based on the plasmonic strip waveguide model, the increase in w₁ increases the waveguide width and thus red-shifts the resonant peak in the absorption spectrum. Concerning the CPER, the increase in w₁ makes the structure less chiral and thus decreases the CPER. When w₁ increases to $L \cdot \cos (\theta )$, the structure becomes achiral and the CPER reduces to 1. For w₂, the situation is similar (Fig. 6(c) and (d)). The increase in w₂ red-shifts the resonant peak in the absorption spectrum and reduces the CPER. The branch length L influences the cross polarization radiation and thus influences the CPER. Basically, the cross polarization radiation comes from the oscillatory current flowing in the inclining branch acting as a dipole antenna. Therefore, the amplitude and phase of the cross polarization radiation is naturally affected by the dipole antenna length, especially by the relationship between L and the wavelength. At a specific branch length (L = 16 µm), the cross polarization radiation sufficiently destructively interferes with the principle polarization radiation for LCP light, and constructively interferes with the principle polarization radiation for RCP light, leading to a maximum CPER. In addition, as L increases, the effective width of the strip plasmonic waveguide increases, resulting in the red-shift of the peak wavelength.

 

Fig. 6. (a,b) w₁-dependent absorption and CPER spectra for LCP and RCP light. (c,d) w₂-dependent absorption and CPER spectra for LCP and RCP light. (e,f) L-dependent absorption and CPER spectra for LCP and RCP light.

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The circular polarization selectivity of the designed structure was examined at oblique incident angles. Figure 7 shows that the performance of the device remains excellent within an oblique incidence angle range of up to 15°, with a circular polarization extinction ratio higher than 10. At oblique incidence, the incident light has a lateral wave vector, corresponding to a lateral momentum. Then, the momentum matching condition becomes ${{2n\pi } / {{P_y}}} + {k_y} = {\beta _{spp}}$, where $n ={\pm} 1,\textrm{ }2,\textrm{ }3\ldots $ The oblique incident in the x-z plane does not affect the momentum matching condition, so the absorption peak remains the same as the incident angle changes from 0° to 30°. When the oblique incident occurs in the y-z plane, the forward propagating SPP (${{2\pi } / {{P_y}}} + {k_y} = {\beta _{spp}}$) and backward propagating SPP ($- {{2\pi } / {{P_y}}} + {k_y} = {\beta _{spp}}$) appear at different frequencies as determined by the momentum matching condition. Therefore, the absorption peak splits into two peaks as the incident angle ${\alpha _z}$ increases.

 

Fig. 7. The absorption and CPER spectra of the designed structure are analyzed under oblique incidence conditions. (a-c) present the results for an incident angle inclined towards the x-axis direction, while (d-f) show the results when the incident angle is inclined towards the y-axis direction.

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4. Conclusion

In summary, we proposed a compact on-chip THz circular polarization detector based on the composite structure of quantum well (QW) infrared detection material sandwiched by a chiral plasmonic antenna array and a metal plane. The composite structure can efficiently couple the incident circular polarized wave of a chosen handedness into a cavity enhanced surface plasmon polariton (SPP) wave for enhanced light absorption in the QWs, and reflect the circular polarized wave of the opposite handedness at a high rate for inhibited light absorption in the QWs, leading to a discrimination of circular polarization states. In addition to the cavity coupled chiral antenna array, the GaAs/AlGaAs QWs that only absorbs the light with an electric field component perpendicular to the QWs performs a second polarization selection, resulting in a CPER as high as 25. Due to the intensified SPP wave at the QWs, the absorption for the principle circular polarization is 15 times higher than the standard 45° edge facet coupled device. By adjusting the structural parameters we demonstrate that the absorption peak wavelength can be tuned over the range from 6.41 to 6.56 THz, while considerable absorption enhancement and high circular polarization discrimination are preserved even under non-normal incidence. We believe that our findings hold great potential for the advances of high-performance compact on-chip THz circular polarization detectors.

Methods. The performance of the proposed structure is characterized by using the three-dimensional finite difference time domain (FDTD) method from Lumerical Inc [45] and COMSOL Multiphysics [46]. For the simulation of the unit cell, periodic boundary conditions are applied along the x and y axis and perfectly matched layers (PML) is applied along the z axis. The simulated total area of the model unit is 20 × 24.6 µm². The proposed structure is composed of three distinct layers. The top layer, which is responsible for chiral light absorption, has a thickness of 50 nm. At the bottom of the structure lies a metal reflector with a thickness of 2 µm. Sandwiched between these two layers is an absorption layer comprising an upper electrode layer measuring 0.5 µm in thickness, a three-period quantum well layer measuring 0.345 µm in thickness, and a lower electrode layer measuring 2.4 µm in thickness. The quantum well layer is composed of three 15-nm GaAs potential wells and four 75-nm Al_0.04Ga_0.96As barriers. The upper and lower electrode layers function to provide a means of applying an external electric field to the quantum well layer, allowing for the manipulation of its optical properties.