高激发功率密度与真空条件下的反斯托克斯发光过程

Anti-Stokes Emission under High Power Density Excitation in Vacuum

摘要: 通常上转换机制大体可归纳为三类：1) 基态与激发态的步进吸收过程；2) 掺杂离子间的能量传递型上转换；3) 光子雪崩型上转换，这些跃迁过程都发生在带间掺杂稀土离子的4fN~4fN组态之间。在高激发功率密度与真空条件下，稀土氧化物发生了一类特殊的上转换过程，在激发发射机制上不同于上述已知的上转换形式。光子数拟合和光电导测量结果表明这是一种带隙激发过程，激发过程始于稀土离子基态，经多光子过程到达导带；电子由导带的退激发过程导致了起源于电荷迁移态、带间能级辐射跃迁、激子复合等多种发光过程，加上光致黑体辐射形成了具有多种发光起源的混合反斯托克斯发射，在光谱上表现为宽带连续强发射。因此发光机制是掺杂离子(激发态过程)与基质(带隙激发)体系的整体协同激发发射过程，分立发光与复合发光两类不同的发光机制中的各种形式，都参与了这种特殊的上转换激发发射过程。激发功率密度、激活离子掺杂浓度、样品所处环境的真空度以及基质带隙宽度，是实现这种近红外激发下的反斯托克斯发射的关键实验条件。在功率密度为50 W/cm2的1 W 980 nm近红外激发下，能量转化率可达10%以上，亮度可达100,000 cd/m2，流明效率达到15 lm/W，色度可通过对真空度、激发功率密度及掺杂浓度等实验参数来连续调配。这类特殊的反斯托克斯过程不仅在机制分析和新掺杂体系尝试两方面有大量的理论和实验工作值得深入挖掘拓展，在应用方面还有开发特种需求的高亮度白光点光源，非接触型气压传感器的潜在价值。

Abstract: Usually, the mechanisms of upconversion are mainly based on excitated state absorption, energy transfer upconversion and photon avalanche, where 4fN transitions are included in bandgap of host. Here, we present a special upconversion of lanthanide oxide under high power density excitation in vacuum that differs from three known mecha-nisms. Upconversion intensity-power fitting and photon conductance measuring demanstrate it is a bandgap excitation process from rare earth ion ground state to conduction band through a multi-photon process. The following deexcitation process from conduction band leads to various luminescence processes that arise from charge transfer band, radiative energy level transitions, exciton recombination and so on, together with thermal blackbody emission, forming a strong continue anti-stokes emission. Therefore this process is a collective interaction of the whole system for doping ion and substrate, and has luminescence mechanisms for both discrete luminescence centre and recombination of electrons and holes. Excitation power density, concentration of activated ion, pressure and bandgap of substrate are key experiment conditions to fulfill this anti-Stokes emission. Under 1 W excitation power with 50 W/cm2 power density of 980 nm NIR, it can achieve more than 10% energy transfer efficiency, 100,000 cd/m2 illumination, and 15 lm/W luminous effi-ciency and its chrominance can be continually tuned by pressure, power density and doping concentration. This kind of special anti-Stokes is not only worth investigating its mechanism and trying various systems in both theory and experi-ment, but also has potential applications in high luminosity point white light source, non-contact pressure meter and so on.