Being able to probe light polarization states is crucial for both civilian and military applications. Currently, the successful commercialization of state-of-the-art linear polarimeters relies on bulky and complicated optics composed spatially separated polarizers and cameras. Instead, on-chip polarization-sensitive photodetectors offer unique opportunities for next-generation ultra-compact polarimeters [Science 362, 750-751 (2018)]. Thus far, the implementation of such devices has been realized by harnessing anisotropic photoactive semiconductors. However, limited by the inherent anisotropy of these materials, the obtained dichroic ratios (DRs) are at a low level of typically smaller than 10, which is insufficient for practical uses. Though a few works have incorporated photoactive crystals into heterostructures, ferroelectrics, and an external amplification circuitry for enhanced DRs of ~102, an effective and general strategy for polarization sensitivity amplification in single-component photodetectors remains elusive thus far, posing fundamental constraints to the promotion of simplified polarimetry for practical applications.
Recently, our research team led by Prof. Jiansheng Jie, Prof. Xiujuan Zhang, and Prof. Xiaohong Zhang in the Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University have proposed an anisotropic photocurrent amplification strategy to overcome the restriction of inherent anisotropy of photoactive crystals, realizing more than 2,000-fold enhanced polarization sensitivity in phototransistors. Specifically, we unites the advantages of light sensitivity enhancement in phototransistors and anisotropic light absorption of photoactive crystals. It is anticipated that if abundant trap sites exist in a phototransistor composed of a photoactive crystal, the number of trapped photogenerated charge carriers should be polarization-dependent (Fig. 1a). These trapped charge carriers can induce an additional polarization-dependent localized electric field (photo-induced gate bias), causing polarization-dependent onset/threshold voltage shift in phototransistors and amplifying the photocurrent to different extent (Fig. 1b). Theoretical estimations unveil the striking enhancement of DRs by several orders of magnitude, which far exceeds the records of 2D material-based polarization-sensitive photodetectors and reaches over those of commercial polarizers (Fig. 1c).
Using small-molecule organic semiconductor as an example. We fabricated organic phototransistors (OPTs) based on well-aligned C8-BTBT crystal array (Fig. 2a-d). Thanks to the highly anisotropic nature of C8-BTBT crystals and the considerable charge trapping capability of SiO2 dielectrics, the OPT exhibited remarkable transfer curve drift under polarized ultraviolet (UV) light with a maximum DR of ＞ 104 (Fig. 2e-g), more than 2 orders of magnitude higher than the largest value ever reported in 2D material-based heterojunction photodiodes. Notably, our strategy enables a facile fabrication technique and is also applicable to different organic material systems and low-power consumption devices to realize ultrahigh polarization sensitivity, thus providing a robust, general, and scalable solution for developing ultrasensitive polarimetry with simple device structure. Looking forward, we will be very excited if our proposed strategy could be implemented in chiral organic semiconductors for the ultrasensitive detection of circularly polarized light, or further be extended to inorganic material systems in a series of follow-up works.
Intriguingly, we note that the as-fabricated OPTs based on C8-BTBT crystal array are endowed with the visible-blind UV photoresponse nature, which are especially applicable in bio-inspired polarization navigation. Because UV polarized light maintains most reliable under complex weather conditions compared with that in visible region, desert ant Cataglyphis uses two sets of orthogonally aligned UV photoreceptors in its ommatidia to sense the e-vector (E) of skylight at zenith for navigation (Fig. 3a,b). Conventional bionic polarization navigation sensors consist of bulky and spatially separated UV light filter, polarizer, and polarization-insensitive photodiode (Fig. 3c), while our OPT enables a filterless, polarizer-free, and miniaturized route towards polarization navigation (Fig. 3d). Based on the robust skylight polarization mode where the polarization direction of scattered sunlight is symmetrical about the solar meridian (SM, Fig. 3e,f), we demonstrated a novel on-chip celestial compass (Fig. 3g), which can truly reflect the skylight polarization direction in different weather conditions (Fig. 3h-m).
We believe that these findings not only lay the foundation for the design of next-generation ultrasensitive polarimeters, but also provide fresh perspectives for the realization of highly compact optoelectronic systems for real applications.
For more details of our work, please refer to "https://doi.org/10.1038/s41467-022-34421-3".