Yongge Cao

We report in detail the defect dynamics in the active region by monitoring the external quantum efficiency (EQE) – injection current curves, I-V curves, and electroluminescence spectra during the ageing test, under a forward current of 850 mA (85 A/cm²), room temperature. We apply a two-level model to analyze the EQE curves and the electroluminescence spectra. The results suggest that high injection density during the ageing may reduce the density of the Shockley-Reed-Hall nonradiative recombination centers and enhance the carrier mobility and diffusion length. The former effect would directly lead to initial surge of EQE, whereas the latter would enhance the effect of extended defects which leads to reduction in peak EQE and increase in EQE droop rate.

We demonstrated the efficient guided laser action in a diode-pumped YAG/Nd:YAG/YAG ceramic planar waveguide produced by tape casting and vacuum sintering technology for the first time to the best of our knowledge. In the regime of continuous wave operation, a maximum output power of 840 mW corresponding to the slope efficiency of 65% was achieved. During passively Q-switched operation, by replacing the dichroic mirror with graphene-oxide based output coupler, we obtained the stable pulse trains with the shortest pulse duration of 179 ns at a pulse repetition rate of 930 kHz which resulted in the single pulse energy of 221 nJ.

We dope Gd³⁺ ions into Ce: YAG ceramics to induce red-shift in the photoluminescence, of which the degree is proportional to the Gd³⁺ doping concentration. This kind of ceramic, when collaborating with InGaN blue chips, proves to be a promising fluorescent material of white light-emitting diodes, for not only its high in-line transmittance and decent quantum yield, but also the improvement in color rendering ability brought by the red-shift, which enhances the ratio of red portion in spectra. We demonstrate that 10% is the optimized value of Gd³⁺ doping concentration, as it provides the maximum color rendering index of ~78 with luminous efficiency as high as 128 lm/W.

We introduce a new fabrication technique of phosphor in glass (PiG) for light-emitting diodes (LEDs) employing the tape-casting. Through the detailed process described herein and the measurement results, it is clear that the PiG-on-glass not only share the same characteristic of those obtained from other techniques or the bulk PiG, but with more precisely controlled width from a few to hundreds micrometers. The samples are mounted on blue InGaN LED chips to test the color properties of the white light. Besides, we established an empirical model that could predict the final color properties of LEDs solely by the phosphor concentration of phosphor glass under certain conditions. This model would greatly facilitate the design of PiG-based LEDs.

The intrinsic spectrally resolved sensitivity (ISRS) of color rendering indices (CRIs) is investigated by using spectral loss simulations. It is demonstrated that R_a exhibits large sensitivities around 444, 480, 564, and 622 nm, while for R₉ the sensitivity peaks are around 461, 581 and 630 nm, which all shift slightly with the correlated color temperature. If considering the ISRS as a bridge between the spectral power distribution of LED and its CRI, one could obtain a high CRI by minimizing the deviation between the shapes of the illuminant spectrum and the reference spectrum, both after modulations by the ISRS as a weighting function. This approach, recommended as a guideline for the spectra design aiming at a high CRI, is described and justified in depth via a mathematical model. This method is spectra-oriented and could largely facilitate the spectra design.

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Fifteen layers’ composite YAG/Yb:YAG/YAG laser ceramic with grain size of about 5 μm was fabricated through tape casting process together with vacuum sintering technology, and its microstructure, optical properties, and laser performance were investigated. The experimental results showed that annealing in oxygen atmosphere could remove the lattice distortion and improve the optical transmittance. The in‐line transmittance was 83% at 800 nm. The laser study indicated that the threshold pump power was 5.5 W with output coupler transmission of 6% and slope efficiency achieved 12%. The maximum output power of 0.53 W was achieved at the absorbed pump power of 10 W.