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The Study Of Ultrafast Polarization Optical Switch And Slow Light Based On Quantum Dots

Posted on:2014-01-15Degree:DoctorType:Dissertation
Country:ChinaCandidate:C C YuFull Text:PDF
GTID:1268330398985710Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
All-optical communication has been the dream of the people for a long time. Now, some electronic devices have to be used in the optical communication and optical communication speed is limited by them, i.e. electronic bottleneck, and all-optical communication can break through the restriction. In the all-optical communication, there are two technical difficulties:all-optical switching and all-optical buffer. The emergence of quantum dots, offers the possibility for people to solve the above problem. In this thesis, based on the strong restriction and strong nonlinearity in the quantum dots, we design ultrafast all optical switching and study the slow light in order to propose possible options to overcome the electronic bottleneck in the optical communication. The main content is as follows.(1) The quantum dot metal organic chemical vapor deposition (MOCVD) growth technique is researched. The growth method and craft are studied. The growth conditions including growth temperature, deposition velocity, deposition thickness, V/III ratio, covering layer etc., are discussed. The growth of the quantum dot is optimized through the experiments. Finally, we preliminarily get the optimal growth condition:growth temperature of500℃, deposition velocity of0.074ML/s, deposition thickness of1.7ML, V/III ratio of10. Quantum dot density is about5×109/cm2.(2) We design a type of ultrafast polarization all optical switching based on quantum dot. How the optical switching works is studied. The reflection spectrum and the contrast ratio of the optical switching are calculated by the transfer matrix method. The relation between the contrast ratio and the control light intensity is researched. The quantum dot relaxation rate, i.e. temperature influence on the optical switching is discussed. The optical switching dependence on the quantum dot inhomogeneous broadening is studied. Without considering the inhomogeneous broadening, for100periods of the Bragg-spaced structure contained uniform quantum dots, under the operation of control light intensity0.5MW/cm2, the optical switching contrast ratio can reach930(30dB). When considering the inhomogeneous broadening of20meV, for200periods of the Bragg-spaced structure contained uniform quantum dots, under the operation of control light intensity 0.5MW/cm2, the optical switching contrast ratio is up to350(25dB). The theory optical switching opening time is up to the order of ps. The room temperature operation stability, operation power, contrast ratio etc. are superior to the same type of quantum well optical switching.(3) We study the slow light in the quantum dot based on electromagnetically induced transparency (EIT). Firstly, we calculated the slow light based on EIT in the quantum dot biexciton-exciton cascade system. By solving the density matrix equation in the steady state, we have obtained the EIT spectral hole burning in the quantum dot biexciton-exciton cascade system. We discuss the pump pulse intensity and biexciton relaxation rate influence on the absorption spectrum. We calculate the refractive index dispersion of the system. Finally, we consider the biexciton energy renormalization influence on the system absorption and the slow light. Without considering the renormalization, the slow factor is up to3000; when considering the renormalization, the slow factor is reduced to2500, and at the same time, it needs more pump pulse power. The study has shown that due to large confinement, long lifetime and low relaxation rate of the exciton and biexciton in the quantum dot, the system temperature stability is superior to the quantum well. It is expected to observe the phenomenon in the quantum dot at room temperature. It is another way to realize the slow light at room temperature and maybe used in the optical buffer. Secondly, we also theoretically study the slow light in the double quantum dots Y-shape level structure by the density matrix theory. The most prominent feature of the slow light is the double window transparency. We get0.002c and0.01c respectively in each window. What is more, we can tune the slow light factor and bandwidth by the applied voltage. In addition to application in the optical buffer, it can also be used in the tunable optical switch and tunable optical notch filter. It has a good development prospects.(4) We study the slow light not based on the EIT. Firstly, under the resonant excitation of the control light, we find that we can achieve continuous conversion from fast to slow light by change of the control light intensity. We create a simple two-level structure model. We use the density matrix method to calculate the first-order and third-order absorption and refractive index of the InGaAs/InGaAsP quantum dot. We study the slow and fast light near the quantum dot resonance energy0.85eV. When the incident light intensity is less than the critical intensity, quantum dots show fast light properties, which originate from the first-order absorption of the quantum dots. When the incident light intensity exceeds the critical intensity, quantum dots show slow light properties due to the presence of large third-order absorption. We demonstrate the fast light and slow light effect in quantum dots. It may be applied in the optical communication. Secondly, we study slow light in the asymmetry double quantum dots based on double resonance. In the middle of the two quantum dots resonance energy, it shows relatively low refractive index dispersion, large bandwidth and suitable for optical communication. The difference of the two quantum dots resonance energy determines the absorption, slow factor and bandwidth. The system bandwidth can reach60G, and the signal pulse in the1mm dispersion material can be delayed for many pulse widths than in vacuum. We simulate the pulse transmits the dispersion material. The delay of the signal light can be modulated by the pump light intensity.In a word, we study ultrafast polarization all optical switch and slow light based on quantum dot. It may be a consultation for the future all optical communication.
Keywords/Search Tags:All optical switching, Slow light, Quantum dot, Electromagnetically induced transparency, Inhomogeneous broadeningGroup velocity, Biexciton-exciton cascade, Third-order nonliear
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