臺大光電所所訊

High Efficiency Laser Phosphor Lighting with Diffractive Optical Elements design for Beam Shaping

Professor Hoang-Yan Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所林晃巖教授

The ways for creating an efficient white light source are developed with challenges since several years ago. In solid-state lighting (SSL), laser diodes (LDs) offer a better efficiency at high power density applications compared to other lighting sources, such as, light emitting diode and organic light emitting devices. Therefore, the hot spot on phosphor plate which is produced by laser beam, critically influence the performance of laser phosphor lighting systems. Diffractive optical element (DOE) was introduced to homogenize the optical beam from LDs. Design of a DOE for beam shaping with uniform illumination has been widely employed for several applications, such as, laser cutting, laser drilling, and material ablation, etc.

In such uniform illumination processing, strong zero-order patterns frequently arise owing to the manufacturing error, which drastically reduces the uniformity of the diffraction pattern. Some methods for zero-order suppression have been reported, e.g., adjusting the amplitude and phase profiles by using a spatial light modulator (SLM) and using an off-axis system or additional elements. Estimation of the manufacturing error is indispensable for numerically calculating the phase profile of transmission functions of DOEs. By taking a simulated algorithm of laser beam shaping, the optical properties for the laser phosphor lighting system are improved with a beam shaping DOE.

The method we propose in this paper provides a high-power conversion efficiency for high power density laser lighting system. This approach might be beneficial for enhancing the lifetime and efficiency of the phosphor plate, which are hampered by the thermal issue due to hot spot from laser focusing, and improving the performance of laser phosphor lighting systems.

Fig. 1.Radiation spectra of laser phosphor lighting system with laser optical power of 2W (a) without DOE beam shaping (b) with DOE beam shaping.

Ref. Min-Chian Wu, Yao Lee, Hoang Yan Lin, Chih-Hsuan Tsuei, Chih-Sheng Jao, Kuo-Hsiang Chien, High Efficiency Laser Phosphor Lighting with Diffractive Optical Elements design for Beam Shaping, SID 2019.

A New Analytic Formula for Minority Carrier Decay Length Extraction from Scanning Photocurrent Profiles in Ohmic-Contact Nanowire Devices

Professor Ming-Hua Mao’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所毛明華教授

Spatially resolved current measurements such as scanning photocurrent microscopy (SPCM) as shown in Fig. 1(a) have been extensively applied to investigate carrier transport properties in semiconductor nanowires. In this work, we derive an analytic formula for scanning photocurrent profiles in widely used ohmic-contact nanowire devices where the common assumption of carrier diffusion dominance is invalid. Under uniform applied electric fields, weak optical excitation, and the influence of photo-carrier-induced electric field, the scanning photocurrent profile and the carrier spatial distribution strikingly do not share the same functional form. Instead, a surprising new analytic relation between the scanning photocurrent profile and the minority carrier decay length was established. Figure 1(b) shows the calculated scanning photocurrent profiles using analytical model and numerical simulation. The analytic formula is applied in SPCM experiments for an n-InAs nanowire, where electric-field-dependent decay length is found as shown in Fig. 1(c,d).This analytic formula provides a new fitting method for SPCM profiles to correctly determine the minority carrier decay length, which allows us to quantitatively evaluate the performance of nanowire-based devices. This work has been published: Chu, C.-H., Mao, M.-H., Yang, C.-W. and Lin, H.-H.,Scientific Reports 9, 9426, (2019). doi:10.1038/s41598-019-46020-2

Fig. 1. (a) Schematic of a typical SPCM setup. (b) Calculated scanning photocurrent profiles using analytical model and numerical simulation. (c) Measured scanning photocurrent profiles with varied applied electric field. (d) The fitted and corrected hole decay lengths from the measured scanning photocurrent profiles as a function of the applied electric field.