Research On The Mode-interactions In Whispering Gallery Mode Optical Microcavity And The Applications

Compared to other microcavity resonances,the Whisperin-Gallery-Modes(WGM)have extraordinarily high quality(Q)factor and small mode volumes,which lead to diverse applications in linear and nonlinear optics as well as quantum optics,and effective manipulation of cavity resonant modes is crucial for emission control in laser physics and applications.In general,laser cavities support a large number of closely spaced modes because their dimensions are typically much larger than an optical wavelength.The presence of competing modes can be detrimental to beam quality and spectral purity,thus leading to spatial as well as temporal fluctuations in the emitted radation.During recent decades,effective mode manipulation and selection strategies have been intensively explored to achieve mode modulation with both spatial and spectral controllability.In traditional research,obtaining mode modulation depends on sufficiently modulated gain and loss,but such modulation is impeded by factors such as inhomogeneous gain saturation.Several approaches have been developed that make use of coupling to detuned external cavities or by including intracavity dispersive elements,and spatially modulating the optical pump.However,these approaches are applicable to specific configurations,what is desired is a general design concept with flexible control of cavity modes.To address this problem,through the derivation of the mode interaction in the cavity,we design and demonstrate a series mechanism of mode modulation experimentally,including single mode operation,ultrafast interaction induced mode switching,and the modulation of mode chirality.These mechanism could help the development of the next generation of optical communication,optical sensing and photon computing and storage.The thesis is organized as follows:(1)In theory,based on the requirement for enhanced laser performance with higher monochromaticity,less mode competition,and better beam quality,we demonstrate a general mechanism to achieve single-mode lasing in WGM microcavity.We consider a so called quasi-parity-time system.The theoretical studies show that the external mode coupling plays an essential role in wavelength selection.This external mode coupling mechanism is verified with the two-dimensional numerical simulation using finite-element method,and we find significant gain enhancement for selected modes by external coupling.(2)In experiment,the size-mismathed photonic molecule is fabricate from negative photoresist SU8 doped with 1 wt% laser dye Rhodamine B and shaped by standard photolithography.With optical excitation,the experiments have confirmed the resulting single-mode laser emission in size-mismatched photonic molecules,when only one constituent cavity is pumped.This behavior persists for a wide range of pump power.In addition,the output intensity of such single mode lasers also displays enhancement when compared with the sample under uniformaly pumping.This new mechanism supports mass production of devices using standard lithography,can be advance the understanding of different coupling scenarios in coupled cavities and improve the characteristics of onchip laser sources for practical application.(3)We further studied the key role of modal interaction in the modulation of laser actions.Single mode laser emission was obtained in two coupled cavities,unlike the conventional single-mode lasers,a deterministic mode switching take place as the pump power was increased: the onset of a new lasing mode switched off the initial one via a negative power slope,while the characteristics of the laser were barely changed.The whole process was reversed when the pump power was reduced.The experiment and simulation verification of the essential role of modal interactions via cross-gain saturation,and intrinsically different from the predictions from the recently developed partiy-time symmetric lasers and bi-stability caused mode switching.In additional to the size-mismatched PM,in lead halide perovskite microrods also observed this modal interaction induced mode switching,making the perovskite microlasers dynamically switchable for the first time,and the switching time is faster than 70 ps.These phenomenon indicates that the interaction-induced mode switching is robust for different materials and cavity shapes,extending microlasers to previously inaccessible areas,e.g.,optical memory,flip-flop,and ultrafast switches etc.(4)Chirality in optical microcavity has shown great potentials in optical sensing.The abundant phase space structures of deformed microcavity are applied to study the physical phenomena around exceptional point,especially mode chirality.The key of our finding is the recent development in heteronuclear diatomic photonic molecules(HDPM).By placing a spiral cavity and a limacon cavity in proximity,chiral resonances has been demonstrated experimentally.The changes in chirality and in far field pattern were found to be closely correlated.Consequently,the internal chirality of limacon cavity can be simply detected by its far field angular distributions for the first time.Once a nanoparticle is attached to the limacon microdisk,the asymmetrical backscattering at the notch of the spiral can be averaged by the symmetrical scattering on the nanoparticle.Consequently,the internal chirality and the corresponding far-field patterns are changed.By measuring a far-field directional laser emission,single nanoparticle can be successfully detected and sized without employing any spectral information.Moreover,the resonances and scattered waves in chaotic cavities have a higher number of angular momentum,thus the dependence of chirality on the position of the nanoparticle can be diminished.This will shed light on the practical applications of chiral photonic devices.In summary,this thesis takes optical microcavity as a research platform,especially photonic molecule coupled microcavities structure,study the mode interaction in it.Compared with traditional research,these mode modulation are more general.What's more,they all show a high tolerance for fabrication imprecisions,reduce the cost of device fabrication,and enable large-scale production.In addition,we experimentally simplifies the characterization of mode chirality,provides a new path to cost-effective,portable,highly sensitive optical sensing.The researches in this thesis promotes the development of WGM optical microcavity in the important practice application fields such as integrated photonic devices and provides an ideal platform for fundamental quantum mechanics research.