Study On Laser Tuning Of Cholesteric Liquid Crystal Microdroplet Resonator

The Whispering Gallery Mode(WGM)optical microcavity has been widely studied in many fields due to its simple structure,easy manufacturing,high quality factor,small mode volume,and some other advantages.Among all research fields,the most prominent field is its application in sensing and micro laser.As a unique material with liquid fluidity and crystal anisotropy,liquid crystal has good response in many aspects such as temperature,electric field,magnetic field,etc.,which makes it occupy a very important position in optical research,especially in the display field.The fluidity of liquid crystal makes its application in WGM optical microcavities possible.A WGM resonant cavity manufactured using liquid crystal droplets on the basis of a droplet WGM microcavity can have the advantages of both WGM and liquid crystal,which provides a potential method for realizing a microsensor or microlaser with simple structure,low cost,and sensitive response.In this paper,cholesteric liquid crystal is used as the research object.The liquid crystal droplets are used as WGM resonators and their stimulated emission characteristics are studied.It is verified that dye-doped cholesteric liquid crystals can simultaneously excite two lasers with different mechanisms.The temperature and electric field response tests were performed on the corresponding lasers,and it was proved that temperature and electric field can tune the two lasers.The specific research content is as follows:(1)A liquid crystal microdroplet resonator with a good cholesteric structure is realized by the tools produced by the flame heated taper-drawing method.The WGM laser and photonic band gap(PBG)laser emission were successfully obtained simultaneously under the532 nm pulsed laser pump,and the thresholds of both lasers were measured.The promotion effect of the diglycerol layer on the formation of cholesteric phase was verified by comparing the emission spectrum of the microresonator with or without the diglycerol layer.Next,the size dependence and spectral resonance mode of the WGM spectrum are studied.The experimental results show that the free spectral space(FSR)of the WGM is inversely proportional to the size of the liquid crystal droplets,and the WGM resonance in the spectrum is the transverse magnetic mode(TM mode).Finally,the temperature response of the two lasers was mainly studied.The wavelengths of the WGM and PBG lasers showed relativelysensitive linear responses to temperature changes.This verifies the tuning effect of temperature on the wavelengths of these two lasers.In addition,some details were studied and discussed as well.(2)The effect of electric field on laser wavelength was further studied based on temperature response experiments.Firstly,the doping concentration of chiral agent in cholesteric liquid crystal was adjusted to change the initial wavelengths of the two laser modes,so as to better distinguish the two lasers and reduce the interference between them.In this part of the study,the theoretical prediction of the electric field response was first made,and then the electric field responses of the PBG laser and WGM laser were actually measured through experiments.The observed change in the wavelength of the WGM laser with the electric field intensity shows a non-linear response which is in line with theoretical expectations.While the drift of PBG laser shows a overall linear trend with the increase of the electric field intensity.Due to the difference between the experimental phenomenon of PBG laser and the expected phenomenon,repeated experiments were performed to verify the accuracy of the experimental results;and based on this,a low-refractive index liquid crystal was used to suppress the WGM laser to conduct experiments on pure PBG lasers.The results confirmed that the electric field response of the PBG laser wavelength does show an overall linear trend.The experimental results in this part demonstrate the effect of the electric field on the wavelength tuning of WGM lasers and PBG lasers.The experimental results show that the cholesteric liquid crystal droplet resonator proposed in this paper can simultaneously generate two kinds of lasers with different generation mechanisms.Both laser wavelengths will respond to changes in temperature and electric field,thereby achieving wavelength tuning.Therefore,this miniature resonator has potential application value in the fields of micro sensors and micro lasers.