Abstract
Polarization-independent dielectric meta-lens is proposed to monolithically integrate with a HgCdTe infrared photodetector to concentrate power flux into a reduced photosensitive area for performance enhancement. Although a reduction in photosensitive area could suppress the dark current, the more seriously reduced light absorptance would degrade the specific detectivity D*. The integration of the meta-lens could reverse the situation by improving the absorptance of the photosensitive region. The meta-lens composed of an array of nano-pillars with varying diameters is formed by carving the CdZnTe substrate of the HgCdTe detector so that the integration could be accomplished in situ. The meta-lens focuses the incident light through the CdZnTe medium and at the HgCdTe photosensitive region. The focal spot is about the wavelength size and the focusing efficiency is above 63%. Concerning a HgCdTe detector with a pitch size of 40 μm × 40 μm, when the photosensitive area is reduced to 5 μm × 5 μm, the meta-lens could still keep the light absorptance above 50%, which is 49 times higher than that of the device without the meta-lens. The dark current reduces with the decreasing photosensitive area in a linear manner. When the photosensitive area shrinks from 40 μm × 40 μm to 10 μm × 10 μm or 5 μm × 5 μm, the dark current reduces by 16 or even 64 times. Compared to the pristine device, the employment of the meta-lens together with the reduction in photosensitive area could enhance D* by 5.5 times for the photosensitive area as 5 μm × 5 μm. Further, the meta-lens exhibits a good dispersion tolerance over the wavelength range from 3.3 μm to 5 μm. The averaged detectivity enhancement over this spectrum range is around 3 times for the photosensitive area as 5 μm × 5 μm. The angular response of the meta-lens integrated detector depends on the focal length. For a focal length of 73 µm or 38 µm, the angle of view for a 5 μm × 5 μm photosensitive area is 4.0° or 7.7°. For the inter-pillar distance to be 2 µm in our design, the influence of the coupling effect between the nano-pillars on the performance of the meta-lens is little.
Highlights
Polarization-independent dielectric meta-lens is proposed to monolithically integrate with a HgCdTe infrared photodetector to concentrate power flux into a reduced photosensitive area for performance enhancement
The meta-lens is based on the all-CdZnTe meta-surface, which could be directly fabricated on the backside of the HgCdTe infrared detector by standard photolithography and etching process
With the help of the meta-lens, the absorptance of the photosensitive region remains above 50%, even though the photosensitive area is reduced by 64 times to 5 μm × 5 μm
Summary
Figure 3[a,b] present how the incident light is focused into the CdZnTe substrate by the meta-lens. In the absence of the meta-lens, the light absorptance of the photosensitive region and the responsivity Ri decrease with Sm linearly In this case, the detectivity D⁎ = Ri S ⋅ Δf /In as a figure of merit for an infrared detector degrades with the decreasing Sm (Fig. 4[b] red line), because the responsivity Ri is proportional to Sm while the noise current In = 2eIdΔf is proportional to the square root of Sm52. Shortening the focal length of a meta-lens is to rearrange the distribution of the nano-pillars, while shortening the focal length of a traditional micro-lenses needs to enlarge the surface curvature, which takes more efforts in fabrication Another method to alleviate the degradation with incident angle is to enlarge the active region. A more detailed comparison can be found in the Supplementary Material
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