Study On Whispering Gallery Mode Microcavities Based On Femtosecond Laser Direct Writing

When light travels at the periphery of a cavity by total internalreflection (TIR), standing wave oscillation can be formed when theoptical path meets an integer multiple of the wavelength. The standingwave pattern in the cavity is usually called whispering gallery mode (WGM)and the cavity that hold the mode is called whispering gallery moderesonator or whispering gallery mode microcavity. Whispering gallerymode microcavity possess two main characteristic which are ultra-highquality factor (Q-factor) and small mode volume. This makes it very usefulin ultra-sensitive sensing, cavity quantum electrodynamics (CQED) etc.Since Lord Rayleigh propose the concept of whispering gallery mode in1911, a great number of researchers begin to study on it. The main workin this thesis is the fabrication of whispering gallery mode microcavityby femtosecond laser direct writing (FsLDW) via two-photon inducedphotopolymerization and applications of whispering gallery modemicrocavity.The main contents of this thesis include:1. Fabrication of whispering gallery mode microdisk lasers. In thiswork, we firstly compound the photoresist with gain material. SU8photoresist is diluted with cyclopentanone and Rhodamine B (RhB) isdissolved in ethanol solution. By uniformly mixing the two together, RhBdoped SU8photoresist is achieved with RhB1%in weight. Secondly, weestablished a picosecond laser pumping system. A picosecond laser ofwavelength532nm (frequency doubled from a1064nm laser), pulse widthof15ps, and a repetition rate of50KHz is chosen for the system. Wefound that the threshold of the microdisk laser is low which is less than 1mw while the full width at the half maxima (FWHM) is quite narrow whichis only0.38nm. Moreover, lasing action is also achieved in microringresonator, microellipse resonator, polygon resonator etc.2. Single mode lasing in double disk coupled whispering gallery modelasers. In the first part of the thesis, we propose a low threshold, highQ factor laser. However, there are a lot of different azimuthal numbermode existing in the microdisk, this is very useful in practicalapplications. It is quite easy for a microstructure to obtain single modelasing. Still researchers get single mode lasing via diminishing the sizeof the cavity or using distributed Bragg reflectors (DBRs) structure.But each has drawbacks. Here we demonstrate an edge-alignment-coupledwhispering gallery mode microdisk laser which consists of two differentdiameters mcirodisk and shows single mode lasing. Only wavelength thatsimultaneously satisfy the resonance condition of both microdisk canoscillate in the coupled microcavity. This kind of single mode laser issimple in structure, small in size, ease of integration, and the Q-factoris high. The FWHM is only0.25nm.3. Sensor based on whispering gallery mode microcavity. This partof work can be divided into two parts. The first part of this sectionis whispering gallery mode single microdisk sensor. When light propagateinside the whispering gallery mode microcavity, there is still a partof light existing out of the cavity in the form of evanescent field. Whenthe refractive index of the surrounding environment changes, theevanescent field will interact with the environment. This will make themode in the microcavity slightly changed. By detecting the change in themode, we can detection the change in the surrounding environment.Experiment results show that the sensitivity of the single microdiskcavity is30nm/RIU. As the whispering gallery mode microcavity hasultra-high Q, under this sensitivity, the detection limit will be quite low. It is reported that the unlabeled whispering gallery modemicrocavity can detect single molecule. The second part of this sectionis double microdisk coupled whispering gallery mode sensor. Just assingle microdisk, the coupled microcavity contact the environmentthrough evanescent field. However the two microdisk that make up thesystem can be coupled to each other. Similar to the working principleof a vernier caliper, by using the difference of the spectrum changesin the two microdisk when the surrounding environment change, the coupledmicrodisks greatly increase the sensitivity. The experiment shows thatthe sensitivity of the coupled microcavity reaches500nm/RIU which isan order of magnitude larger compared with a single microdisk.4. Theory and simulation. In the second chapter of this thesis, weexplained in detail the use of geometrical optics and electromagnetictheory. We calculated the field distribution in a whispering gallery modemicrocavity and the characteristic of the whispering gallery mode. Inaddition, we use Rosft software to simulate the microdisk laser,whispering gallery mode double microdisk coupled single mode lasing, andsensing in a single microdisk. Experiment, theory and simulation are ingood agreement with each other.