Advanced Optical Materials

Light‐emitting carbon dots (CDots) are widely investigated due to their distinct merits. However, it is still a challenge to modulate their bandgap emissions and conquer their aggregation‐induced luminescence quenching to achieve full‐color highly emissive CDot‐based phosphors. Herein, this study proposes an approach toward realization of full‐color emissive CDots from two common precursors, citric acid and urea, through employing three different solvents (water, glycerol, and dimethylformamide) and their combinations in a solvothermal synthesis. Employing sodium silicate solution, this study further demonstrates the microwave‐assisted method allowing to incorporate CDots into a silica network, which effectively prevents aggregation of CDots and results in strongly luminescent full‐color inorganic CDot phosphors with photoluminescence quantum yields of 30–40%. Through deposition of the red‐ and green‐emitting CDot phosphors on blue‐emitting InGaN chips, white‐light‐emitting diodes are fabricated with Commission Internationale de L'Eclairage of (0.34, 0.31) and the color rendering index of 82.4, indicating their promising application for solid‐state lighting.

For a donor–acceptor (D–A) molecule, there are three possible cases for its low‐lying excited state (S₁): a π–π* state (a localized electronic state), a charge‐transfer (CT) state (a delocalized electronic state), and a mixed or hybridized state of π–π* and CT (named here as the hybridized local and charge transfer (HLCT) state). The HLCT state is an important excited state for the design of next‐generation organic light‐emitting diode (OLED) materials with both high photoluminescence (PL) efficiency and a large fraction of singlet exciton generation in electroluminescence (EL). According to the principle of state mixing in quantum chemistry, a series of twisting D–A molecules are designed and synthesized, and their HLCT state characters are verified by both fluorescent solvatochromic experiments and quantum chemical calculations. The CT components in the HLCT state, which greatly affect the molecular optical properties, are found to be enhanced with a decrease of the twist angle of the D–A segment or an increase of the D–A intensity in these twisting D–A molecules. In OLEDs, using these HLCT compounds as the emitting layer, the maximum exciton utilization efficiency is harvested up to 93%. Surprisingly, an exception of Kasha's rule is revealed in some HLCT compounds: restricted internal‐conversion (IC) from the high‐lying triplet state (T₂) to the low‐lying triplet T₁, and a reopened path of reverse intersystem crossing (RISC) from T₂to S₁or S₂, based on the analysis of the excited‐state energy levels and the measurement of the low‐temperature spectrum. RISC from T₂to S₁(S₂) as a “hot exciton” channel is believed to contribute to the large proportion of the radiative singlet excitons.

Ultrasmall black phosphorus quantum dots (BPQDs) with an average size of 2.1 ± 0.9 nm are synthesized by using a solvothermal method in a N‐methyl‐2‐pyrrolidone solution. Verified by femto‐second laser Z‐scan measurement, BPQDs exhibit excellent nonlinear optical response with a modulation depth of about 36% and a saturable intensity of about 3.3 GW cm⁻². By using BPQDs as optical saturable absorber, the ultrashort pulse with a pulse duration of about 1.08 ps centered at a wavelength of 1567.5 nm is generated in mode‐locked fiber laser. These results suggest that BPQDs may be developed as another kind of promising nanomaterial for ultrafast photonics.

Solid‐state lighting has made tremendous progress this past decade, with the potential to make much more progress over the coming decade. In this article, the current status of solid‐state lighting relative to its ultimate potential to be “smart” and ultra‐efficient is reviewed. Smart, ultra‐efficient solid‐state lighting would enable both very high “effective” efficiencies and potentially large increases in human performance. To achieve ultra‐efficiency, phosphors must give way to multi‐color semiconductor electroluminescence: some of the technological challenges associated with such electroluminescence at the semiconductor level are reviewed. To achieve smartness, additional characteristics such as control of light flux and spectra in time and space will be important: some of the technological challenges associated with achieving these characteristics at the lamp level are also reviewed. It is important to emphasise that smart and ultra‐efficient are not either/or, and few compromises need to be made between them. The ultimate route to ultra‐efficiency brings with it the potential for smartness, the ultimate route to smartness brings with it the potential for ultra‐efficiency, and the long‐term ultimate route to both might well be color‐mixed RYGB lasers.

Self‐powered photodetectors are considered as a new type of photodetectors enabling self‐powered photodetection without external power. The excellent photoresponsivity, fast photoresponse rate, low dark current, and large light on/off ratio of these photodetectors have attracted wide interest among scholars. 2D materials are widely used in self‐powered photodetectors due to their excellent optical and electrical properties, unique 2D structures, and their capabilities to exhibit excellent photodetection performance. According to the self‐driving mechanism of 2D material‐based self‐powered photodetectors, they are divided into three categories: p–n junction photodetectors, Schottky junction photodetectors, and photoelectrochemical photodetectors. From these three perspectives, the research progress of 2D material‐based self‐powered photodetectors is summarized in detail here. Research reports indicate that 2D material‐based self‐powered photodetectors have excellent self‐powered photoresponse behavior, good light on/off characteristics, and wideband spectral response ranges. The excellent photoresponse performance of 2D material‐based self‐powered photodetectors facilitates their potential applications in the field of optoelectronic devices. In particular, self‐powered photodetectors have great potential as novel emerging self‐driven optoelectronic devices. Finally, directions for the further development of 2D material‐based self‐powered photodetectors are anticipated.

The efficient generation of white light by phosphor‐converted LEDs (pcLEDs) suffers from a trade‐off between high color rendition, low correlated color temperatures, and luminous efficacy. While this is partially an inherent problem, it is also caused by the spectral efficiency of the materials used. The particular challenges for materials research lie, amongst the demanding general requirements, in finding very narrow‐band or line‐emitting materials: excitable with blue light, emitting in the near red. A way to design Mn(IV) activated line emitters is proposed, and methods for high‐throughput combinatorial syntheses are specified.

Arbitrary control of terahertz (THz) waves remains a significant challenge although it promises many important applications. Here, a method to tailor the reflection and scattering of THz waves in an anomalous manner by using 1‐bit coding metamaterials is presented. Specific coding sequences result in various THz far‐field reflection and scattering patterns, ranging from a single beam to two, three, and numerous beams, which depart obviously from the ordinary Snell's law of reflection. By optimizing the coding sequences, a wideband THz thin film metamaterial with extremely low specular reflection, due to the scattering of the incident wave into various directions, is demonstrated. As a result, the reflection from a flat and flexible metamaterial can be nearly uniformly distributed in the half space with small intensity at each specific direction, manifesting a diffuse reflection from a rough surface. Both simulation and experimental results show that a reflectivity less than −10 dB is achieved over a wide frequency range from 0.8 to 1.4 THz, and it is insensitive to the polarization of the incident wave. This work reveals new opportunities arising from coding metamaterials in effective manipulation of THz wave propagation and may offer widespread applications.

The rampant appearance of counterfeiting has a serious adverse effect on every aspect in the global markets. Thus, the development of high‐tech security strategies has become an urgent challenge. Recently, lanthanide ions (Ln³⁺) doped materials open new avenues for concealing factual data and shield against counterfeiting because of their unique optical characteristics of color‐tunable emissions under near‐infrared excitation. The present review surveys the recent advances in Ln³⁺‐doped upconversion crystals (UCCs) as fluorescent functional inks toward designable and high‐level anti‐counterfeiting patterns and graphical encoding. The great achievements of fabrication of versatile security patterns can be ascribed to the combination of the single or multiple colorful UCCs with polymers, additives, and solvents. Moreover, the crucial factors including underlying mechanisms, synthetic methods of upconversion fluorescence materials, and the diverse printing technologies employed for patterning the fluorescence images have been highlighted, and the corresponding challenges and opportunities in this promising research area are presented. This review will help providing a fundamental understanding and guidance to rational design fluorescence optical labels based on Ln³⁺‐doped UCCs for broadening their applications in high‐level security fields.