Quantum Dot Lasing In Different Distributed Feedback Cavities

Lasing properties of the distributed feedback(DFB)lasers can be engineered either by replacing gain medium(responsible for the stimulated emission process)or by periodic structures(necessary for feedback mechanism).Similarly,among other fluorescent materials(including,dyes,polymers,and bulk semiconductors)colloidal quantum dots offer inexpensive wet-chemical processing syntheses and seamless tunability of the optoelectronic properties over the whole visible and infra-red region.The extremely small dimension(few nanometers across)of the CQDs allow one to access the quantum confinement effect regime,where materials properties become dimension-dependent.Over the past few decades,CQDs have adjusted a firm status in fluorescent materials and find several applications in optoelectronic devices,laser systems,displays,and solar energy conversion panels.This thesis mainly focuses on the design,fabrication,and characterization of the CQDs lasing in complex DFB cavities.Here,we explored rectangular and quasicrystal cavity fabricated via multi-exposure two-beam interference technique and holographic lithography.The spin coating technique was applied to cover the respective complex DFB cavities.Further,the multi-wavelength and tunable emission wavelength realized in the complex cavities were well explained by developing an analytical model based on cavity mode coupling effect,simulation software's(Matlab and Comsol)and compared with the experimental results.In DFB lasers a thin film of the waveguide(gain medium)is deposited on the periodic grating structures.So the light propagating in the waveguide is Bragg scattered from the periodic structures in some new direction with discrete sets of wavelengths in different directions.Similarly,the resonant cavity in DFB lasers arranges the feedback mechanism and also defines some key features e.g.discrete sets of emission wavelengths,spatial characteristics,and power characteristics.Among them,2^nd order DFB lasers provide a surface-emitting coherent output.However,most of the reported studies demonstrate the lasing performance of 2^nd order DFB laser including simple grating structures and organic fluorescent materials(dyes,polymers,perovskites,etc)as a gain medium.Therefore,our research includes a comprehensive study of multi-wavelength colloidal quantum dots lasing in 2^nd order complex(periodic and quasiperiodic)DFB cavities.The main research contents and innovations of this thesis are as follows:1.Lasers with multi-wavelength colloidal quantum dots can be achieved using rectangular micro-cavity and flexible substrate.The structure includes graduated periods and rectangular cavities fabricated through two-beam single and multi-exposure interference lithography,which acts as the DFB cavity.A layer of densely packed CQD(core-shell-shell)film is deposited on the cavity via the spin coating technique.The fabricated CQD lasers on glass and PET substrate are excited by a femtosecond laser having 400 nm wavelength,1 k Hz repetition rate,and 200 fs pulse width.Similarly,CQD lasing was achieved in one-dimension(1D)DFB cavities at different grating periods(395 nm,400 nm,and 405 nm,respectively),in which laser wavelength redshifted with increasing the grating periods.The typical lasing spot color changes with the tunning emission wavelength.The 1D CQD laser with a smaller grating period(395 nm)shows approximately three times less lasing threshold as compared to other larger periods.Multi-wavelength lasing in a single CQD DFB laser is realized by introducing a rectangular cavity on a glass substrate.The periods of the rectangular cavity are changed to study their effect on emission wavelengths.Due to the efficient feedback mechanism in two-dimension(2D)CQD lasers,the laser threshold dropped five times as compared to 1D DFB lasers.The output beam from the rectangular cavity can be regarded as a linear combination of two 1D cavities.Furthermore,the 1D CQD DFB laser was transferred to the PET substrate and mechanical tuning of laser wavelength was achieved.2.The physics of periodic structures plays a dominant role in DFB lasers and results in various interesting mechanism that manages wave transport and optical interference.As periodic structure has found numerous applications but deviation from the periodicity can lead to high complexity and give rise to a variety of astonishing results.Photonic quasicrystals contain such types of deviations in which microstructures are arranged in well-defined long-range patterns but lack translational symmetry.Among them,2D quasicrystals are more efficient in providing coherent lasing due to radiation feedback,high-quality-factor optical mode,and long-range rotational symmetry.So,we have fabricated a 2D quasicrystal exhibiting 10-fold rotational symmetry by using a specially design pentagonal prism in the optical setup of a simple and low-cost holographic lithography.We developed a general analytical model based on the cavity coupling effect,which can be used to explain the underlying mechanism responsible for the multi-wavelength lasing in the fabricated 2D CQDs holographic photonic quasicrystal.The multi-wavelength surface-emitting lasers such as?₀=629.27 nm,?₁=629.85 nm,?_-1=629.06 nm,?₂=630.17 nm,and?_-2=628.76 with a coupling constant=0.38 achieved from the 2D holographic photonic quasicrystal are approximately the same as the developed analytical model based on the cavity mode coupling effect.Moreover,the lasing patterns of the 2D CQDs photonic quasicrystal laser exhibit asymmetrical polarization effect by rotating the axis of polarization with a difference of 120~0 angle in a round trip.We expect that our findings will provide a new approach to customize the 2D CQDs holographic photonic quasicrystal lasers in the field of optoelectronic devices and miniature lasing systems.