Nonlinear Nanophotonic Devices Based On Cadmium Sulfide Nanowires

In recent years,with the trend towards miniaturization and integration of photonic devices,more and more attention has been focused on micro/nanophotonics.As an excellent optical waveguide,semiconductor nanowires have attracted much attention due to their remarkable optical properties,such as ultralow propagating loss,strong evanescent fields,tight optical confinement and strong optical nonlinearity,and have been widely used in micro/nanophotonic devices.The intensive research of semiconductor nanowires is of great significance in enriching the types of micro/nanophotonic devices,improving their performance,and realizing miniaturization and integration.In this work,we studied the characteristics of transverse second harmonic generation(TSHG)and photo-induced Varshni shift in single cadmium sulfide(CdS)nanowires and then demonstrated single-nanowire Sagnac-loop optical correlator and single-nanowire thermo-optic(TO)modulator.Benefiting from the strong nonlinearlity of semiconductor materials,semiconductor nanowires showed great potential in nonlinear micro/nanophotonic applications.The main contents of this dissertation are as follows:In Chapter 1 of this thesis,we reviewed the growth method of semiconductor nanowires,optical near-field coupling technology and their applications in micro/nanophotonic devices.In particular,some typical nonlinear micro/nanophotonic applications based on semiconductor nanowires were introduced.In Chapter 2,CdS nanowires with different diameters and lengths were synthesised by chemical vaper deposision process.Their morphology and waveguiding properties were characterized by optical microscopy,scanning electron microscopy(SEM)and photoluminescence(PL)spectrum.We introduced the numerical methods of propagation constant,calculated the bending loss and group index of the fundamental mode,measured the PL and propagation loss of PL in nanowires.In Chapter 3,we demonstrated single CdS nano wire Sagnac-loop optical correlator.The principle of TSHG in nonlinear optical waveguides and the mechanism of Sagnac-loop optical correlator were firstly introduced.A single CdS nanowire was assembled into a Sagnac loop by using a fiber probe which was also used to launch a train of femtosecond(fs)pulse into the loop,and the autocorrelation measurment with only one input port and no delay lines was realized.Benefiting from the high refractive index and high flexibility of CdS nanowires,the footprint was only?46×120 ?m2.We measured the autocorrelation of fs pulses with different widths,and compared with the results obtained from a commercial autocorrelator,which showed a good consistency.By replacing CdS nanowires with different diameters,we also realized the autocorrelation measurements in telecommunication band.In Chapter 4,we demonstrated single CdS nanowire TO modulator.By using two fiber probes,single CdS nanowire with 13 ?m in length and 170 nm in diameter was suspended in air,and the optical signals were launched into the nanowire.Photo-induced Varshni shift in nanowires was studied and single CdS nanowire TO modulators were demonstrated.Compared with other TO modulators,single nanowire TO modulators show at least 2 orders of magnitude reduction in the footprint(?0.17 × 13.5 ?m2)and have a low power consumption(91 ?m).At the same time,their response time(<20 ?s)is 1 or 2 orders of magnitude shorter than all-optical TO modulators based on 2D material-coated optical fibers and of the same order with SOI-based(Silicon-On-Insulator)TO modulators.By using semiconductor nano wires with different band gaps,the operation wavelength of the single-nanowire modulators can be extended to the common telecommunication and they have a potential to be integrated with SOI platforms.In chapter 5,we summaried the works and innovations of the work and gave an outlook regarding the future development.