手性相位成像元传感器

2.1 Metasurface design

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Here, r _ξ and r _ψ are the amplitude reflection coefficients for linearly polarized light along the two axes of the rectangular NP, and α is the NP orientation angle relative to the sides of the square unit cell (Figure 2a). If the NPs are designed so that r ξ + r ψ ≪ r ξ − r ψ , most of the incident RCP light experiences a reversal in the direction of field rotation accompanied by a total phase shift φ t o t = arg r ξ − r ψ + 2 α . Similarly, the LCP component is scattered with the same resonance phase φ r e s = arg r ξ − r ψ but equal and opposite PB phase φ _PB = −2α. The selective detection of either state of circular polarization in the proposed PIMSs is enabled by this sign difference for the PB phase contribution.

Figure 2:

Design simulations. (a) Schematic top-view image of a meta-unit. (b) Calculated scattering efficiency 1 4 r ξ − r ψ 2 versus resonance phase φ r e s = arg r ξ − r ψ for all simulated NPs. The red circles indicate the NPs used in the devices reported in this work. (c) Total scattering phase φ _tot = φ _res + φ _PB of all the NPs in the metasurface of device R versus NP number for RCP (red) and LCP (blue) incident light. Device L features the same scattering phase profiles with the two states of circular polarization interchanged.

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For the detection of light with k _‖ along the x direction and angle of incidence θ _p (e.g., point A in the reciprocal-space diagram of Figure 1c), this phase matching condition becomes Δ k t o t = k 0 n S P P − sin θ p , where k ₀ = 2π/λ ₀ and n _SPP is the SPP effective index. To enforce this condition for RCP light, we select the NP dimensions and orientations such that Δφ _res and Δα satisfy

With these prescriptions, the resonance and PB phase produce equal contributions to the RCP in-plane wavevector shift Δ k t o t = Δ φ r e s + 2 Δ α / Δ x , which add up to each other to bridge the k _‖ mismatch k 0 n S P P − sin θ p between the incident light and SPPs (black arrows in Figure 1c). In contrast, under LCP illumination the resonance and PB contributions of the same metasurface become equal and opposite (due to the sign change in the latter), and therefore cancel each other to yield Δk _tot = 0. As a result, scattering of LCP light by the metasurface simply results in specular reflection without SPP excitation. Similarly, if Δα in Eq. (2) is replaced by −Δα (i.e., the NPs are rotated by the same angle as in the previous design but in the opposite direction), the metasurface will selectively couple LCP light incident at θ _p to SPPs while at the same time reflecting the RCP component.

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2.2 Device fabrication and characterization

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Figure 3:

Measurement results. (a) Top-view SEM images of the metasurfaces in the two devices reported in this work, illustrating their chiral relationship. (b), (c) Responsivity of device R versus polar θ and azimuthal ϕ angles of incidence for RCP (b) and LCP (c) light. (d) Horizontal line cuts of the color maps of (b) and (c) (red and blue traces, respectively). (e), (f), (g) Same as (b), (c), (d) for device L. All the data presented in these plots were measured at 1,550-nm wavelength and were normalized to the responsivity of a similar device without any metasurface.

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All the data presented in these figures are normalized to the measured normal-incidence responsivity of a similar device without any metasurface (29 mA/W/V). The resulting peak values observed in Figure 3d and g [about 29 % and 27 % for the RCP and LCP responsivity of devices R and L, respectively] are reasonably consistent with expectations. Specifically, from the ratio between the peak metasurface transmission computed in Figure 1b (28 %) and the Fresnel transmission of the uncoated Ge surface in the reference sample (62 %), the expected normalized peak responsivity is 45 %. The difference between the measured and calculated values can be primarily ascribed to SPP scattering by surface roughness in the experimental samples, which is also responsible for the observed decrease in peak-to-background ratio in Figure 3d and g compared to Figure 1b.

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Figure 4:

Wavelength dependence. (a) Absolute value of the normal-incidence responsivity slope under RCP (red squares) and LCP (blue circles) illumination measured as a function of wavelength with device R. (b) Same as (a) for device L. All responsivity values are normalized as in Figure 3. The double arrows indicate the full width at half maximum of their respective traces.

2.3 Computational imaging results

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Figure 5:

Chiral phase imaging system. The sensor array is partitioned into blocks of four adjacent pixels coated with the metasurfaces of devices R, L, and their replicas rotated by 180° (labeled R ̄ and L ̄ ). In this example, the incident light consists of a superposition of RCP and LCP components with equal magnitudes and different phase profiles. The RCP (LCP) phase profile can be reconstructed from the readout signals of pixels R and R ̄ (L and L ̄ ). The experimental RCP and LCP angular response maps of all four pixels in each block are also shown for θ ≤ 6°. The black circle in each color map indicates the pupil-function cutoff frequency of the imaging optics, corresponding to a maximum angle of incidence θ _c = 2.3°.

The imaging system of Figure 5 can therefore be used to record, in a single shot, two independent differential-phase-contrast images of the RCP and LCP incident wavefronts, i.e., S R = I R ̄ − I R / I R ̄ + I R and S L = I L ̄ − I L / I L ̄ + I L . Specifically, the readout signal S _R computed from the photocurrents I _R and I R ̄ of each superpixel is zero, positive, or negative if the local angle of incidence θ of the RCP component is zero, positive, or negative, respectively, regardless of the total incident intensity and of the direction of propagation of the LCP component (and similarly for S _L with the two states of circular polarization interchanged). Since local angle of incidence is proportional to transverse phase gradient, the resulting edge-enhanced images can then be numerically inverted to reconstruct the underlying RCP and LCP phase distributions.

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Here F − 1 denotes the inverse spatial Fourier transform evaluated at the pixel location, Δ k is the phase difference between the two transfer functions t R C P k and t L C P k , and E R C P k and E L C P k are the Fourier transforms of the incident RCP and LCP optical fields on the sensor array. Physically, the two terms in the curly brackets of eq. (3) correspond to the SPPs excited by the Fourier components of in-plane wavevector k of the RCP and LCP incident waves, which add up coherently to one another before being collected at the slits. To estimate Δ k , we have measured the angle-resolved photocurrent of our experimental samples under s and p linearly polarized illumination, and then we have used eq. (3) [with the data of Figure 3 for R R C P k and R L C P k ] to fit the resulting responsivity curves. This procedure (described in more detail in Supplementary material, Section S5) yields accurate fits with Δ ≈ 43° across the pupil-function passband of the envisioned microscope.

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Figure 6:

Chiral phase imaging simulation results. (a) Horizontal line cut through the middle of the differential-phase-contrast image S R r recorded by devices R and R ̄ for the phase object of Figure 5. (b) Horizontal line cut of the differential-phase-contrast image S L r simultaneously recorded from the readout signals of devices L and L ̄ . (c) Phase profile φ R C P r of the RCP component of the incident light (grey trace) and computationally reconstructed image (red trace). (d) Phase profile φ L C P r of the LCP component of the incident light (grey trace) and computationally reconstructed image (blue trace).

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