Laser diodes (LDs) can not only be used for solid-state illumination, but also have broad applications in visible light communication (VLC) due to large modulation bandwidth. In order to realize the bifunctional applications of both laser illumination and visible light communication, the red-green-blue (RGB) LDs of synthetic white light should have the same shape and uniform intensity in the illumination region. Here, we propose and demonstrate the beam shaping and uniformity function at red-green-blue wavelengths of 450 nm, 532 nm, and 625 nm by a single two-dimensional diffractive optical element (DOE). By using the iterative algorithm in conjunction with the optimization algorithm and variable weight, the proposed device can achieve an average energy efficiency of 81.27% and an average light spot uniformity of 91.64% under RGB trichromatic lighting conditions. Experimental fabrication of the proposed DOE was fabricated using UV laser direct writing photolithography, and the experimental results show that an average energy efficiency of 60.25% and an average uniformity of 83.76% of a rectangular spot at three wavelengths of RGB can be obtained, and excellent beam shaping and homogenization under composite white light illumination can also be achieved by the fabricated two-dimensional DOE.
Micro-LED has been widely studied due to its strong performance and has applications in fields such as display, biology, and communication. However, as the size of the micro-LED decreases, so does the light extraction efficiency and top emission, which affects its development and application. The improvement of light extraction efficiency is crucial. Surface nanostructures and chip design are utilized to enhance the top emission efficiency of micro-LEDs in this work. Top emission percentage and top emission efficiency of 10 μm GaN-based vertically flip-chip micro-LED are studied in relation to their structural parameters using the three-dimensional time-domain finite-difference method (3D FDTD) simulation. The results show a clear relationship between the structural parameters and the top emission efficiency. Surface micro-LED structures with a period ranging from 0 to 1200 nm are designed and prepared. The sidewall inclination is also optimized to achieve the optimal top emission efficiency. Full-structure simulations of spectral distributions and dipole source polarization are performed to obtain accurate results. The top emission percentage and top light emission efficiency are improved from 22.0% and 6.5% to 78.0% and 26.5%, respectively, compared to the conventional 10 μm micro-LED structures.
Semiconductor doping strongly influences its electrical and optical properties. The transition metal Fe doping can induce the deep levels to compensate residual donors to generate semi-insulating GaN, and make carrier lifetime short due to carrier trapping, which thus has potential applications in ultrafast optical and optoelectronic devices. For such applications, it is critical to understand the effect of Fe doping on optical nonlinearities and ultrafast carrier dynamics in GaN crystals. In this paper, we studied the dependence of GaN ultrafast nonlinearities on Fe-related intermediate states using femtosecond two-photon Z-scan and pump-probe with phase object (PO) techniques. In particular, we investigated the nonlinearities around 458 nm (2.71 eV) and 540 nm (2.29 eV) where it shows dips in transmission spectra. It is found that the two-photon absorption coefficient has a minimum value at the peak of 458 nm, in contrast, the coefficient has a maximum value at the peak of 540 nm and enhances around 1.5 times more. It thus indicates that these two absorption resonances are attributed to different physical reasons. We further investigated the ultrafast carrier dynamics by the time-resolved PO pump-probe techniques with 190 fs laser pulses. We observed that the transient refraction curves are recovered once pump and probe pulses are separated in time delay. It is considered that carrier lifetime is dramatically reduced due to Fe trapping centers. Our finding that Fe-doped semi-insulating GaN has ultrafast carrier lifetime expect GaN potential applications in ultrafast detectors and all-optical switches.
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