Carbon Nanotube Far Infrared Detectors with High Responsivity and Superior Polarization Selectivity Based on Engineered Optical Antennas.

Single-wall carbon nanotube (SWCNT) thin films are promising for sensitive uncooled infrared detection based on the photothermoelectric effect. The SWCNT film is usually shaped into a belt and diversely doped to form a p-n junction at the center. Under the illumination of a focused incident light, the temperature gradient from the junction to the contacts leads to photoresponse. When the SWCNTs are aligned in one direction, the photoresponse becomes polarization selective. Although a typical bowtie antenna can improve the responsivity and polarization extinction ratio by deep-subwavelength light focusing, the absolute absorptance of the junction region is only 0.6%. In this work, the antenna was engineered for a higher light coupling efficiency. By integrating a bottom metal plane at a specific distance from the SWCNT film and optimizing the antenna geometries, we achieved ultra-efficient impedance matching between the antenna and the SWCNTs, thus the absorptance of the junction region was further enhanced by 21.3 times and reached 13.5%, which is more than 3 orders of magnitude higher than that of the device without the engineered antenna. The peak responsivity was further enhanced by 19.9 times and responsivity reached 1500 V/W at 1 THz. The resonant frequency can be tuned by changing the size of the antenna. Over the frequency range of 0.5 THz to 1.5 THz, the peak responsivity was further enhanced by 8.1 to 19.9 times, and the polarization extinction ratio was enhanced by 2.7 to 22.3 times. The highest polarization extinction ratio reached 3.04 × 105 at 0.5 THz. The results are based on the numerical simulations of the light and the thermal fields.

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.

Highly polarization-sensitive far infrared detector based on an optical antenna integrated aligned carbon nanotube film.

Polarization detection is another important way to characterize a light field in addition to intensity and spectrum. This is required for high-fidelity information acquisition and high precision target recognition in the infrared detection region. Single-wall carbon nanotube (SWCNT) films have been investigated for infrared detection with considerable sensitivity at room temperature based on the photothermoelectric effect. In this work, a bowtie antenna integrated aligned SWCNT film is proposed for highly polarization sensitive, far infrared detection. The SWCNT film is shaped into a belt and doped with diverse agents to form a p-n junction at the center. The SWCNTs are arranged perpendicular to the electronic transportation direction. The antenna is aligned at the junction and along the SWCNTs. Based on the following four factors: (1) deep-subwavelength light concentration at the junction, (2) alignment between the SWCNTs and the antenna, (3) anisotropic heat transfer in the aligned SWCNT film, and (4) light field reduction within the gap of the bowtie antenna for the polarization perpendicular to the antenna axis, the ratio between the responsivities for the polarizations parallel and perpendicular to the SWCNTs could be higher than 13 600. By changing the size of the antenna, the resonant frequency could be tuned. Over the range from 0.5 to 1.5 THz, the peak polarization extinction ratios at different resonant frequencies are all bigger than 700, and they are 16 to 320 times higher than that of the aligned SWCNT belt without the antenna. Moreover, the integration of the antenna and the aligned SWCNT belt also enhances the responsivity by 1 to 2 orders of magnitude. Compared to an aligned multi-wall carbon nanotube (MWCNT) film, an aligned SWCNT film integrated with an optical antenna is more favorable for highly polarization sensitive, far infrared detection. The result is based on the numerical simulations of the light and the thermal fields.