Investigation On Whispering Gallery Mode Lasing Of All-optical Tunable Composite Microcavity

Whispering gallery mode(WGM)microcavities usually possess small mode volumes and high quality(Q)factors.The application of WGM microresonators in the field of tunable microelectronic devices has attracted great interest from researchers in various fields.In recent years,various technologies supporting resonant wavelength tuning have been confirmed.From heating,applying electric field,mechanical bending,changing surface tension,injecting liquid and other traditional tuning methods,to using the photothermal effect of nanoparticles,they all have their own advantages.In many tuning schemes,microscopic photothermal control using nanocrystals(NCs)or photoabsorbing molecules as heat sources is expected to achieve a wide tunable wavelength range and reduce damage to the cavity due to its excellent photothermal effect and non-contact property.Because Nd³⁺has a large absorption cross-section and a large number of closely adjacent energy levels,the neodymium heavily doped NCs(Nd³⁺:NaGdF₄)synthesized by an improved thermal decomposition method has a higher photothermal conversion efficiency(>80%).The nanoheater-doped functional WGM microcavity derived from the combination of WGM optical microcavity and NCs combines the advantages of both.On the basis of the typical WGM laser radiation obtained by the pump light source exciting the functional cavity,the particle doping integration method supported by the core-shell structure composite microcavity and the efficient light absorption of the particles ensure an effective heat transfer rate and large tunability.In the paper,a systematic study on the composite WGM microcavity integrated with photothermal materials is carried out.The experimental results verify that NCs can be used as an efficient heating source for microstructure.The laser-assisted tuning of WGM resonance in the composite WGM microcavity is explored.At the same time,the wide all-optical tuning range of the WGM microcavity based on hollow and coated composite microspheres is realized by doping nanoparticles.The specific research content includes:(1)The tip of the copper rod dipped in hydrofluoric acid is close to the hollow glass microsphere(HGM)until a microhole is formed on the surface.Then,the NaNdF₄/dye co-doped liquid was injected into the HGM using the tapered capillary microtube prepared by the heat drawing method to prepare the liquid-filled hollow microresonator.The WGM resonance was obtained under the excitation of 532 nm pulsed laser.The essence of WGM lasing is verified by experiment and simulation:the electric field distribution in the cavity is verified by establishing a three-dimensional model,which shows the WGM resonance behavior in the liquid-filled microcavity;the Q-factor,free spectrum range(FSR)and resonance peak wavelength of lasing spectrum are studied experimentally,and it is confirmed that all parameters conform to the characteristics of WGM lasing.In addition,a second 793 nm laser is irradiated into the hollow microcavity to trigger the photothermal effect of NaNdF₄ in the liquid core,and the influence of doping concentration of NaNdF₄ nanocrystals on the tuning range and tuning sensitivity in the power intensity range of 0-1.68 W/mm² is investigated experimentally.The maximum values of 4.95 nm/(W·mm^-2)and 2.95 nm/(W·mm^-2)are obtained.(2)Silica microsphere resonators of various sizes were prepared by applying arc discharge to the tip of a tapered optical fiber fixed in a fusion splicer.The spherical end surface connected to the tapered area is repeatedly immersed in the pre-prepared NaNdF₄/dye co-doped polymethyl methacrylate(PMMA)mixed solution to prepare a polymer-coated silica microsphere cavity.By analyzing the influence of Nd³⁺doping concentration on the photothermal conversion efficiency of NCs,the maximum photothermal conversion efficiency achieved in Nd³⁺doped NaGdF₄ is over 80%.Here,we obtained the WGM lasing from the prepared polymer-coated silica microcavity under a 532 nm pulsed laser.The second 793 nm continuous wave laser was used to trigger the photothermal effect of the particles,so as to realize all-optical tuning of the resonant wavelength by changing the effective refractive index of the microcavity.By measuring the shift of lasing modes on exposure to CW laser with different power intensities,the maximum tuning range and sensitivity of 2 nm and 1.96nm/(W·mm^-2)were achieved as the power intensity increased from 0 to 1.02 W/mm².