Combined role of polarization matching and critical coupling in enhanced absorption of 2D materials based on metamaterials.

Since 2D materials are typically much more efficient to absorb in-plane polarized light than out-of-plane polarized light, keeping the light polarization in-plane at the 2D material is revealed to be a crucial factor other than critical coupling in light absorption enhancement in a 2D material integrated with a light coupling structure. When the composite of a metal-insulator-metal structure and a 2D material changes from the magnetic resonator form to the metasurface Salisbury screen one, the field polarization at the 2D material changes from a mainly out-of-plane status to a mainly in-plane status. As a result, for graphene, the absorptance enhancement is increased by 1.6 to 4.2 times, the bandwidth enlarged by 3.6 to 6.4 times, and the metal loss suppressed by 7.4 to 24 times in the mid- to far-infrared range, leading to the absorptance of graphene approaching 90% in the mid-infrared regime and 100% in the THz regime. For monolayer black phosphorus, the absorptance enhancement at the wavelength of 3.5 µm is increased by 5.4 times, and the bandwidth enlarged by 1.8 times. For monolayer MoS2, the averaged absorptance in the visible-near infrared range is enhanced by 4.4 times from 15.5% to 68.1%.

Absorption enhancement in all-semiconductor plasmonic cavity integrated THz quantum well infrared photodetectors.

The light coupling properties of all-semiconductor plasmonic cavity integrated THz quantum well infrared photodetectors were studied for absorption enhancement of the quantum wells. The all-semiconductor plasmonic cavity is constructed by heavily doped GaAs with a plasmonic behavior in the THz regime. The plasmonic behavior of GaAs was thoroughly studied by taking into account the carrier density dependent effective mass of electrons. An optimal doping level for GaAs to be the most metallic is selected since the plasma frequency of the doped GaAs varies nonmonotonically with the carrier density. By tuning the absorption competition between the quantum wells and the doped GaAs meanwhile keeping the system at a critical coupling status, the absorptance of the quantum wells is prominently enhanced by 13.2 times compared to that in a standard device. The all-semiconductor plasmonic cavity integrated quantum well photodetectors can be polarization sensitive (polarization extinction ratio > 900) when the plasmonic cavity is shaped into an anisotropic form. The good tolerance of the incident angle is favored for wide-field infrared detection. The GaAs plasmonic cavities are demonstrated to be effective when integrated at a pixel level, indicating a good compatibility with focal plane arrays.

HgCdTe mid-Infrared photo response enhanced by monolithically integrated meta-lenses.

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.