(Invited) One-Dimensionally Aligned Quantum Rods for Generation of Highly-Pure Circularly Polarized Light with High Light Intensity
Authors: Okazaki Yutaka , Kimura Misaki , Sagawa Takashi 
Published: July 2022

Converting non-polarized visible light (natural light) into circular polarized (CP) light is currently an important issue because of the attractive properties of CP light such as acceleration of photosynthesis and enhancement of conversion efficiency of photovoltaics[1,2]. Until now, various methods for creating CP visible light, such as filtration by circular polarizer, selective reflection by chiral liquid crystal structures, and circular polarized luminescence (CPL) from chiral luminophores[3], have been reported. However, creation of highly-pure CP visible light with high light intensity is still considered as challenging issue. In this presentation, we propose an approach for creating highly-pure CP visible light with high light intensity by converting linearly polarized luminescence (LPL) using quarter-wave plate.
In this work, we put spotlight on colloidal semiconductor quantum rods (QRs) because of its unique advantages described as follows: (1) Absorption coefficient and emission quantum yield are relatively high. (2) Sharp emission peaks are expected. (3) Variety of QRs with different emission colors can be excited by irradiation of single-wavelength light. We selected CdSe/CdS core/shell QRs as LPL generating 1D nanostructure. CdSe/CdS core/shell QRs were synthesized by hot-injection method. As shown in Figure 1a, the obtained suspension showed orange-colored strong emission under UV light irradiation. TEM image of the dried sample of the QRs toluene suspension indicates that the average length, average width of the obtained rod-like nanostructures were about 30 nm and 4.5 nm, respectively. After QRs/toluene suspension was mixed with poly(ethylene-co-vinyl acetate) (EVA)/toluene solution, the mixture was casted on glass slide and dried it under air. Then, the prepared composite polymer film was stretched in one direction and was attached on glass slide. When the luminescence from the QRs/EVA stretched film was observed under the linear polarizer placed in parallel direction to the stretching direction, the observed light was brighter than that of perpendicular direction (Figure 1c). By measuring the photoluminescence of parallel (I//) and perpendicular (I⊥) components under depolarized excitation at 450 nm, the QRs/EVA stretched film showed clear LPL property with constant ratio (CR) = 5.4, defined as CR = I// / I⊥ (Figure 1b). Furthermore, when the QRs/EVA stretched film was combined with a quarter-wave plate placed at 45° to the stretching direction and was observed through the right-handed (RH) or left-handed (LH) circular polarizer, brighter light was observed through LH circular polarizer (Figure 1d). By quantitative evaluation of RH- and LH-CP light components, total emission includes 82% of LH-CP light and 18% of RH-CP light (Figure 1e). These results indicate that the combination of the QRs/EVA stretched film and quarter-wave plate generate highly-pure CP light which can be distinguished by the naked eye (Figure 2d). Interestingly, the handedness of CP light can be switched by simply changing the angle between fast axis of the quarter-wave plate and stretching direction of LPL films (Figure 1f). In this presentation, we will also discuss about multiplexing of optical information by using CP light conversion.
[1] W. Qin et. al., J. Phys. Chem., 2018, 122, 12566-12571. [2] B. Hu et. al., ACS Photonics, 2017, 4, 2821-2827. [3] P. Duan, M. Liu et. al., Adv. Mater. 2020, 32, 1900110.

Figure 1