mea-sured for an ensemble. We derived the radiative and non-radiative decay rates for single q-dots from the fluorescence quantum efficiency and total decay rate of its ex-cited state. We demonstrated that fluorescence quantum efficiency of single q-dots degraded in time. This degradation in the fluorescence quantum efficiency of single q-dots may limit the applications in quantum information science. In order to pre-vent such degradation, one possible idea is to coat the nanofiber with a polymer after depositing the q-dots.
We have experientally investigated the effect of q-dot azimuthal position on the estimation of the channeling efficiency for various fiber diameters, especially thicker fiber diameters.
7.2. Future Prospects 73
FBG
High quantum efficiency q-dot
Figure 7.1: Schematic diagram of the experimental setup. A high quantum efficiency q-dot is deposited in the nanofiber cavity. FBG denotes fiber Bragg grating.
However, when we combine a q-dot with a nanofiber cavity, a narrow emission spectrum would be necessary in order to realize enhancement. As we demonstrated in Chapter 3, one can readily measure emission spectrum of single q-dots using a nanofiber technique. In the experiments performed so far, we excited the q-dot well above the band edge (excitation wavelength=640 nm, emission wavelength=800 nm).
The measured spectrum width was about 52 nm. The mechanism of the broadening may be understood to be due to spectral diffusion and exciton-phonon interactions.
In order to get narrow linewidth, one promising way is to excite the q-dot at its band edge. This kind of approach should be investigated systematically. Furthermore, a nanofiber-cavity system combined with advanced quantum emitters, such as un-blinking color centers in nanodiamonds [95, 96], may provide a new route to achieve on-demand single-photon generation into single-mode optical fibers.
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Appendix A
Degradation of Single Quantum Dots
0 10 20 30
Time [s]
0 100 200 300 400
Ph ot on c ou nt r at e [k cp s]
Figure A.1: Typical fluorescence photon-count rate as a function of time before degra-dation of the q-dot.
Figure A.1 shows a typical measurement of the fluorescence photon-count rate as a function of time before the single q-dot degraded. In order to protect the q-dot from oxidation, we flow nitrogen gas continuously for the whole experiment period.
However, the sudden drop in the fluorescence count might be explained by the forma-87
tion of lattice defects, which creates additional non-radiative recombination pathways (permanent quenching states). Due to photoinduced quenching states, at a high laser power q-dot degradation is expected [97].
Appendix B
Certificate of Analysis
For the experiments in this thesis, we use a carboxyl colloidal CdSeTe/ZnS q-dots [72, 88]. The followings are Certificate of Analysis (CoA) provided by the Invitrogen company.
B.1 CoA for Lot Number: 834674
Catalog Number: Q21371MP
Product Name: Qdot 800 ITK carboxyl quantum dots “8 µM solution”
Appearance: black suspension
Medium: 50 mM borate buffer, pH 9.0 Lot Number: 834674
89
LOT DATA SPECIFICATION ABSORBANCE
Concentration 7.9 µM 7.5-8.5 µM
FLUORESCENCE
Emission Maximum 792 nm 795 (±10) nm
Relative Quantum Yield1 72% ≥35%
Full Width at Half Maximum 82 nm ≤100 nm
1. Quantum yield determined relative to rhodamine 101.
B.2 CoA for Lot Number: 898274
Catalog Number: Q21371MP
Product Name: Qdot 800 ITK carboxyl quantum dots “8 µM solution”
Appearance: black suspension
Medium: 50 mM borate buffer, pH 9.0 Lot Number: 898274
LOT DATA SPECIFICATION
ABSORBANCE
Concentration 8.4 µM 7.5-8.5 µM
FLUORESCENCE
Emission Maximum 797 nm 795 (±10) nm
Relative Quantum Yield1 56% ≥35%
Full Width at Half Maximum 66 nm ≤100 nm
1. Quantum yield determined relative to rhodamine 101.
Acknowledgements
I would like to express my sincere thanks to many people for their continuous support during my journey of research in the last four years. First and foremost, I would like to sincerely thank to my Ph.D supervisor Professor Kohzo Hakuta (Hakuta sensei) for his valuable guidance and giving me interesting projects to work on with constant encouragement in his Laboratory. Hakuta sensei trained me in all possible direc-tions in the research with his experience, knowledge and talent. For example, logical thinking, writing scientific articles and presenting skills. I should mention here that, inspite of his very busy schedule, we are always welcome in his room to discuss re-search problem, without any prior appointments. As if now, I have learned so much of knowledge and experience by working in his Laboratory and discussing with sensei.
I hope that my hunger for learning ever last.
Here, I would like to express my thanks to my laboratory seniors and colleagues, who helped me in the experiments during my work. Here the first person comes in line, I would like to sincerely thank to my super senior Dr. Kali Prasanna Nayak, who helped me during my entire research period. I am very fortunate enough that his stay in our laboratory overlapped with mine. During Kali stay in the Laboratory, I have learned many experimental technique, mainly how to fix the problem. I would like to sincerely thank to Prof. Fam Le Kien, who helped me the theoretical part under-standing of our nanofiber experiments. I would like to sincerely thank to Morinaga Sensei of Institute of Laser Science in our University to teach me various experimen-tal details and software skills. Here, I should mention that I used Morinaga sensei C program entire my thesis. I would like to sincerely thank my senior Dr. Manoj Das, who helped me not only in the experimental room but also in general life in Japan. I would like to sincerely thank to Dr. Mark Sadgrove for his help during final days of my thesis writing. I would like to thank my laboratory members Oyamoto, Oyama, Yuto, Kojima, Yamashita, Sakaguchi and Yazawa.
I would like to express my sincere thank to the members of review committee for my Ph.D. work. I would like to thank Prof. Kohzo Hakuta, Prof. Kouichi Yamaguchi,
san. I am indebted to my Japanese language teachers Ikeda sensei, Tanaka sensei, Miyoshi sensei and Ooki sensei who taught me the basic Japanese languages and introduce me to the Japanese culture. I am also grateful to my other Japanese language teacher Oohori Sensei, who taught me Japanese language in a local volunteer society.
I would also like to acknowledge my friends here, Raju, Manasa, Maruthi Manoj, Srinu, Sanjay, Rakesh, Jameesh and Vikram.
Finally, this thesis would not have been possible without the support of my Family, especially my elder Father. I am deeply grateful to my Mother, Father, elder Father and Sisters for their love, care and all the sacrifice they made for my education.