| Due to its high integration,low threshold and low power consumption,semiconductor micro-disk lasers based on GaN-based structures have broad application prospects in many fields such as integrated optics and new optoelectronic devices.An optical microcavity with a whispering gallery mode has a very high quality factor Q and a very small mode volume V,and can strongly localize light in a very small spatial region,so that the spontaneous emission properties of the atoms are regulated and thus greatly reduced the lasing threshold.This dissertation focuses on the characteristics of whispering gallery modes in different diameters of GaN-based microdisk.First,we studied the basic theory and characterization of the whispering gallery mode in microcavity,and then studied the mode spectrum of the microdisk with a diameter of 2 ?m to 50 ?m in the blue wavelength band(450 nm-470 nm)using the FDTD(finite-difference time-domain method,FDTD).The intensity and distribution of the model were studied,and the quality factor Q value of the model was calculated.The specific research process and conclusions are as follows:(1)By studying the influence of the position and number of dipoles on the intensity and distribution of resonant modes,a dipole arrangement that can compare the distribution of high-density exciton within the microdisk is designed to enable it to excite all optical modes in the disk.At the same time,this arrangement also satisfies the requirement of equal density,and can be used to study the relative strength of resonance of different diameter microdisks.(2)Using this excitation source setting,the relationship between the maximum value of the resonance mode in the microdisk and the number of effective modes with the disc diameter was studied.The results showed that the maximum intensity of the mode showed a tendency of fluctuation and decrease with the increase of the disc diameter,and the number of effective modes was Showing the opposite trend.Among them,when the microdisk diameter is 3 ?m,6 ?m,and 10 ?m,the maximum electric field intensity reaches a maximum value,and the number of effective modes also reaches a minimum value at the same time.At the same time,the maximum electric field strength in the intradiscal excitation is highly dependent on the microdisk geometry.When the disk diameter is 2-3 ?m and 5-7 ?m,the maximum resonant intensity is stronger,and the light field is also more restricted;when the disk diameter is 2-7 ?m,the number of effective modes is 2-5.It is beneficial to reduce the influence of multimode suppression,thereby reducing the threshold value of laser lasing.In addition,the maximum resonant intensity also exhibits a negative correlation with the number of effective modes(3)The resonance spectrum and mode distribution of the microdisks with diferent diameters were studied.The results show that different microdisks have different competitive advantages when they are excited.When the disk diameter is 2 ?m,the third-order mode WG20.3 is more dominant,its resonant wavelength is 461.22 nm,its Q value is greater than 116000,the maximum electric field area is distributed near the center of the disk and is about 0.4 ?m from the disk edge;when the disk diameter is 3 ?m,the first-order whispering gallery mode WG28,1 is more dominant,with a resonant wavelength of 459.25 nm and a Q value greater than 338,000.The strongest electric field is distributed near the edge of the disk and its mode volume is small.The radial order q of all WGM modes in both diameter microdisks is not more than 3 When the disk diameter is 4 ?m,the maximum intensity of its mode decreases sharply.The modes corresponding to the effective frequency are mostly third-order or more WGM,the electric field distribution is very close to the center of the disk,and the strongest electric field distribution is about 1/2 radius of the disk;when the diameter is 5-7 ?m,the mode corresponding to its effective frequency is the fundamental mode of the whispering gallery mode,and the mode distribution is close to the edge of the disc.The Q value of the effective mode in the disk with diameter of 6 ?m is greater than 1.2x107.The mode intensity is smaller when the diameter is 8-9 ?m,and the high-order whispering gallery mode dominates.After the diameter is larger than 10 ?m,the first-order whispering gallery mode takes the absolute advantage,and the pattern is distributed close to the edge of the disc.(4)The statistical method was used to superimpose nearly all the modes in the disc,and the distribution of the mode light field when the micro disc was excited was approximated.There are differences in the light field distribution of microdisks of different diameters.The high-order and low-order whispering wall modes in the micro-disc occupies different advantages in the mode competition.When the diameter D is 2-3 ?m,the light field distribution width is about 0.4 ?m,the maximum light field intensity is distributed in the center of the broadening,and the mode volume is larger;when D=4 ?m,the light field distribution width is about 1 ?m,the maximum light field intensity Near the center of the disk,its mode volume is relatively larger;the light field width is 0.2-0.4 ?m when D=5-7 ?m,the light field distribution width is small and the maximum light field intensity is close to the disk edge,and its mode volume is small;D=8 At-9 ?m,the light field distribution width is about 1.5 ?m,the maximum light field distribution is close,and the mode volume is larger;after D is larger than 10 ?m,the light field distribution width is very small,about 0.1 ?m,and the maximum light field distribution is close to the disk edge.The volume of the body is small,and as the diameter of the disk increases,the distribution of the light field is closer to the edge and the volume of the pattern is smaller. |
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