Chapter Conclusions and Suggested Future Work Chapter Conclusions and Suggested Future Work 6.1 Conclusions The main focus of this work was to design and fabricate saturable absorbers and SESAMs operating in the blue region. GaN-based quantum structures, comprising of quantum wells and quantum dots, were studied and applied as the nonlinear absorption materials. To fabricate the GaN-based SESAM, a non-monolithic SiO2/Si3N4 DBR was chosen in order to overcome the limitations of monolithic GaN-based DBRs in terms of their narrow stopbands and unsatisfactory maximum reflectance. Because this SESAM structure suffered from severe interference-induced reflectance fluctuations, further modifications on the structure were necessary to suppress the interference. The modifications of the SESAM structure were first simulated and then shown by experiments to be an effective way to improve the properties of GaN-based SESAM. The major findings in this work are summarized below. 175 Chapter Conclusions and Suggested Future Work 6.1.1 Saturable absorbers with GaN-based quantum structures In this work, saturable absorbers fabricated with GaN-based quantum structures, comprising of quantum wells and quantum dots, operating in the blue wavelength region were investigated. For InGaN/GaN quantum well saturable absorbers, a novel and convenient method to reduce the absorption recovery time was presented. The recovery time was effectively reduced by introducing dislocations and defects into the active region through buffer thickness engineering. An absorption recovery time as short as 34 ps was achieved by an InGaN/GaN quantum well saturable absorber with a 15-nm LT GaN buffer and a 500-nm HT GaN buffer. The modulation depth was also reduced due to the degraded crystal quality. It was also found that the quantum well samples with reduced buffer thicknesses exhibited high density of large V-pits, which could be related with their ultra-short absorption recovery time. So the experimental evidences from SEM, TEM and PL were investigated. Besides the normal c-plane MQWs, MQWs grown on the {10 1} and {11 2m } ( m ≥ ) faceted sidewalls of the V-pits were observed and contributed to different PL emission bands. For quantum dot saturable absorbers, in this work, a 3-layer InGaN quantum dot sample was fabricated and its nonlinear property as a saturable absorber was investigated. Because of the increased electron-hole wave-function overlap in the quantum dots with three-dimensional confinements, an absorption recovery time of 60 ps was demonstrated. The modulation depth of this quantum dot saturable absorber was as large as 20 %. The results show that using quantum dots as the active region of the saturable absorber is an easier way to achieve 176 Chapter Conclusions and Suggested Future Work the ultra-short absorption recovery time without sacrificing the crystal quality as well as the nonlinear property. 6.1.2 Design & fabrication of GaN-based SESAM To fabricate the GaN-based SESAM, a non-monolithic SiO2/Si3N4 dielectric DBR was used instead of the traditional monolithic DBR. This is because the monolithic GaN-based DBRs suffer from narrow stopbands and unsatisfactory maximum reflectance. A 90-nm-wide stopband with the maximum reflectance approaching 100% was achieved with a dielectric DBR consisting of 15 pairs of SiO2/Si3N4, deposited by PECVD after the MOCVD growth of quantum wells. It was also observed that this GaN-based SESAM suffered from severe interference-induced reflectance fluctuations in the stopband. This interference is mainly caused by the as-grown thick high-temperature grown GaN buffer, and it might cause the instability in the mode-locking by such a saturable absorber. Modifications on the SESAM structure were therefore required before the SESAM can be used for passive mode-locking. 6.1.3 Optimization of GaN-based SESAM The severe interference-induced reflectance fluctuations in the stopband of the GaN-based SESAM were suggested to be due to the as-grown thick high-temperature grown GaN buffer, and this was further confirmed by simulation. In order to suppress the interference, this thick GaN buffer needs to be thinned down. According to simulation, wafer bonding and laser lift-off of the sapphire substrate 177 Chapter Conclusions and Suggested Future Work were conducted to expose the GaN buffer. The GaN buffer was then thinned down by ICP etching till there was only 300-nm GaN buffer left. An AR coating was finally deposited by PECVD to reduce the surface reflectance. Characterization results on the SESAM after modifications show that the interference effect was effectively suppressed without producing adverse effects on the device quality. The GaN-based SESAM after modifications can thus be used to passively mode-lock blue semiconductor lasers to generate ultra-short blue pulses. 6.2 Suggested future work In this work, saturable absorbers and SESAMs operating in the blue region have been successfully fabricated. This is the first demonstration of such devices in the short-wavelength region. Considering that the research in this area is still at an infant stage, several key aspects are recommended for future research. The buffer thickness engineering method has successfully reduced the absorption recovery times of InGaN/GaN quantum well saturable absorbers. Although this method requires no post-growth treatment or optimization and therefore is simple, it is still worthwhile to study the applications of other traditional recovery time reduction methods, such as low temperature growth [Gupta1992], ion implantation [Delpon1998] and proton bombardment [Gopinath2001], in GaN-based saturable absorbers so as to analyze the advantages and disadvantages of each method. The nonlinear property and recovery time results of the InGaN/GaN quantum well saturable absorbers fabricated by different methods should be studied and compared 178 Chapter Conclusions and Suggested Future Work with the present results using the buffer thickness engineering method. Such studies would provide a better understanding of the properties of InGaN/GaN quantum well saturable absorbers in terms of the relationships between crystal quality, absorption recovery time and nonlinear property. GaN-based quantum dots have been incorporated into the absorption regions of GaN-based saturable absorbers. A novel growth technique to fabricate the multi-layer InGaN quantum dot structure was presented. Because of the unique zero-dimensional properties of quantum dots, enhanced nonlinear property as well as largely reduced absorption recovery time has been achieved in this quantum dot saturable absorber. Additional work is required in the future to improve this quantum dot fabrication technique. For example, the GaN barrier thickness could be reduced from 15 nm in this work to around 10 nm, so that the strain created by the formation of quantum dots in the first InGaN layer could be well transmitted into the third InGaN layer. Also, the formation of quantum dots is largely dependent on the growth temperature. A small variation on the growth temperature could result in quantum dots with different sizes and densities. It would be interesting to investigate the effects of quantum dot size and density on the nonlinear property and recovery dynamics of the saturable absorbers. This is because the size of the quantum dots may affect the electron-hole wave-function overlap, and therefore the carrier transition probability. Quantum dots with different densities may also result in the different nonlinear property. With the knowledge of these relationships, special growth control or post-growth treatments can therefore be designed to further modify or improve the 179 Chapter Conclusions and Suggested Future Work properties of quantum dot saturable absorbers according to different applications. While the properties of GaN-based saturable absorbers and SESAMs can still be further explored and improved, it should be highlighted that the present GaN-based SESAMs are already suitable for the passive mode-locking applications. More studies on the optical system designs are needed to realize the passive mode-locking. If successfully demonstrated, the ultra-short blue pulse generation by direct passive mode-locking would represent a milestone in the fields of ultra-fast optics and data storage. The research on passive mode-locking by these GaN-based SESAMs will in turn facilitate the study on GaN-based saturable absorbers and promote further improvement in the SESAM design. 180 . 1 76 6.1.1 Saturable absorbers with GaN- based quantum structures In this work, saturable absorbers fabricated with GaN- based quantum structures, comprising of quantum wells and quantum. saturable absorbers and SESAMs operating in the blue region. GaN- based quantum structures, comprising of quantum wells and quantum dots, were studied and applied as the nonlinear absorption materials observed and contributed to different PL emission bands. For quantum dot saturable absorbers, in this work, a 3-layer InGaN quantum dot sample was fabricated and its nonlinear property as a saturable