Design and Optimization of GaN-based Semiconductor Saturable Absorber Mirror Operating at Around 415 nm
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Design and Optimization of GaN-based Semiconductor Saturable Absorber Mirror Operating at Around 415 nm Fen LIN1, Ning XIANG1, Xin Cai WANG2, Jesudoss AROKIARAJ3, Wei LIU3, Hong Fei LIU1, and Soo Jin CHUA1,3 1 Dept of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore, 117576, Singapore 2 Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore, 638075, Singapore 3 Institute of Materials Research and Engineering, 3 Reseacrh Link, Singapore, 117602, Singapore
ABSTRACT A broadband GaN-based semiconductor saturable absorber mirror (SESAM) with a dielectric SiO2/Si3N4 distributed Bragg reflector (DBR) operating at wavelength around 415 nm was fabricated. Serious oscillation fringes due to the light interference were observed in the SESAM’s reflectance spectrum. Such oscillation in reflectivity can impede the function of the saturable absorber. Simulations showed that by removing the sapphire substrate and thinning the GaN buffer layer, oscillation fringes could be significantly reduced. Experiments were carried out and the results agreed well with the simulation prediction. INTRODUCTION Remarkable progresses have been made in the ultrashort pulse technology with the development of semiconductor saturable absorber mirrors (SESAMs) [1-7]. For data storage applications, in order to achieve high speed operation as well as high density storage, high repetition rate ultrashort optical pulses in blue or UV region become indispensable. So far, the blue/UV ultrashort optical pulses have been obtained mainly by frequency conversion methods from infrared solid state lasers, such as Ti: sapphire lasers [8] or Cr:LiSAF lasers [9]. The direct generation of ultrashort blue/UV optical pulses by passive modelocking has not been reported yet. One of the major reasons is the lack of suitable SESAM due to the lack of lattice-matched distributed Bragg reflector (DBR) materials in blue/UV wavelength region. To overcome the limit of the epitaxially grown DBR, we have recently reported the fabrication of a broadband SESAM with SiO2/Si3N4 dielectric DBR [10]. Over 100 nm high reflectivity stopband was achieved with this SESAM. However, this non-monolithic SESAM suffers from a serious oscillation in reflectivity within the high reflection stopband resulting from the light interferences. The interference fringes can cause about 6-7% reflectivity fluctuations (see Fig. 5 in Ref. [10]) which is comparable to the modulation depth of the SESAM. This would make it difficult to achieve stable modelocking. Therefore, it is necessary to suppress the interference-induced oscillation fringes by further optimization processes.
In this paper, we first study the interference phenomenon in the GaN-based SESAM structure by simulation. A series of experiments have been conducted according to the simulated optimization processes. Experimental results are shown and discussed. EXPERIMENT The SESAM structure (Fig. 1) before the optimization processes consists of 8 InGaN/GaN quan
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