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Developing fluorescent materials that can achieve efficient upconversion (UC) emission under low power density excitation has always been a great challenge, especially for Tm3+ ions. Here, Tm3+‐doped Cs2NaYbCl6 double perovskite single‐crystals (DPSCs) are prepared successfully. The DPSCs exhibit efficient UC emission under 980‐nm laser excitation, and the optimum UC efficiency reaches 9.48%. It is a recorded value. More importantly, its efficient UC emission can be achieved under low‐power density of 0.117 W cm−2 excitation. Moreover, the luminescence color of the DPSCs can be regulated from blue to red by increasing the doping concentration of Tm3+ ions. This is because the severe cross‐relaxation occurs between Tm3+ ions at high doping concentrations. The Cs2NaYb0.98Tm0.02Cl6 DPSCs with efficient UC emission are coupled to the CsPbI3 perovskite photodetector, achieving good visible and near‐infrared (NIR) detection without an external bias. It is worth emphasizing that the detectivity for 980 nm light can reach 1.26 × 1011 Jones, which is the highest value for self‐powered NIR photodetectors. Finally, given the dual functional characteristics of the device's response to broadband visible and narrowband NIR light, the dual encryption function is implemented during information transmission. This work will open new chances for applications in advanced integrated photonics devices. [ABSTRACT FROM AUTHOR]

The development of rare‐earth‐doped luminescent materials with single component, multicolor, and multimode still remains a challenge. Here, rare‐earth‐based double perovskite single crystals (Cs2NaYbCl6 DPSCs) are fabricated with low phonon energy of 134 cm−1, which offers great potential to serve as highly efficient luminescent host materials. Subsequently, Er3+, Tm3+, and Ho3+ are successfully incorporated into Cs2NaYbCl6 host to form Cs2NaLnxYb1−xCl6 (Ln = Er, Tm, and Ho) DPSCs, realizing efficient dual‐mode polychromatic emission. Under 980 nm laser excitation, Cs2NaLnxYb1−xCl6 DPSCs show high upconversion (UC) efficiencies of 12.9%, 7.2%, and 3.1% for Cs2NaEr0.3Yb0.7Cl6, Cs2NaTm0.02Yb0.98Cl6, and Cs2NaHo0.3Yb0.7Cl6, respectively. Meanwhile, Cs2NaErxYb1−xCl6 DPSCs exhibit the adjustable size (0.5–10 mm) and tunable UC or down‐shifting emission colors from yellow, green to red upon 980, 1540, or 380 nm excitation. Moreover, the DPSCs demonstrate weak concentration and thermal quenching effect, and excellent resistance against oxygen, heat and humidity. Finally, the anti‐counterfeiting and information encryption applications based on multicolor emitting DPSCs are designed and realized. [ABSTRACT FROM AUTHOR]