Abstract
Low-dimensional material embedded cavities have been widely used in nano-lasers and detectors etc. The effects of embedded materials on the cavity resonant mode need to be intensively studied for achieving the efficient coupling between the gain material and the cavity. The influences of embedded material thickness and position, cavity layer thickness and the number of distributed Bragg reflector pairs on the cavity resonant mode are discussed in this work. Results show that the cavity resonant mode changes periodically with different embedded positions and there is a maximum peak shift within a period of λ/2 optical path. The maximum peak shift decreases with increasing cavity thickness and is proportional to the embedded material thickness. The number of distributed Bragg reflector pairs does not affect the cavity resonant mode. These results provide guidance on the optical device design and the analysis of experimental phenomena, which can be applied to different wavelength ranges of distributed Bragg reflector cavity structures.
The light-matter interaction plays a vital role in modern science and technology, which has been used in lasers, detectors, spectroscopy and so on. An optical cavity can be exploited to limit the photons at resonant wavelengths and tailor their interactions with the matter via regulated local density state
The rapid development of low-dimensional (LD) materials, especially two-dimensional (2D) materials has been widely used in nano-lasers and detectors due to their excellent photoelectric propertie
In the previous work, the interaction between the optical microcavity and black phosphorus was demonstrated experimentally. The stimulated emission at 3611 nm was achieved when the black phosphorus (BP) was embedded in dielectric DBR structures. And the lasing wavelength can be tuned from 3425 nm to 4068 nm by varying the thickness of BP laye
In this research, the interaction between light and LD material in the DBR cavity is investigated by the transfer matrix method (TMM) systematically, with the embedded position, number of DBR pairs, cavity layer thickness and embedded material thickness. The law or trend drawn can be used to guide the design of the enhanced interaction structure between optical microcavity and LD gain materials such as BP and 2D materials, for example, by choosing the critical position of the cavity to embed.

Fig. 1 The schematic diagram of the DBR cavity and its equivalent interface
图1 分布式布拉格反射镜腔的原理示意图及其等效界面示意图
Any optical multilayer can be replaced by a virtually equivalent interface, as shown on the right of
, | (1) |
Let, , , | (2) |
So, , | (3) |
, | (4) |
, , , are the transmission coefficients and reflection coefficients of the equivalent interfaces, respectively. , are the reflection phases of the equivalent interfaces and is the propagation phase.
Let’s simplify it further,
, ,, |
, | (5) |
. | (6) |
The position of the maximum transmittance, also the central wavelength, is determined by the following equation:
, | (7) |
, | (8) |
and are the reflection phases of the top and bottom equivalent interfaces. So according to
For the reflection phase of the top interface, it can be calculated according to TMM as follow
Supposing that the top DBR and the top cavity layer compose a j-th layer film, the admittance of the lower surface of the top equivalent interface can be written as follows according to the boundary conditions of plane electromagnetic wave propagation at the interface,
, | (9) |
and are the amplitudes of the electric and magnetic fields of the incident wave, respectively. is the phase thickness of the film layer. is the admittance of the -th layer film and K is the number of films. There are only forward waves and no backward waves, corresponding to /=. So,
. | (10) |
Let B= and C= represent the normalized electric and magnetic fields of the equivalent interface, then one can get:
. | (11) |
For - polarized waves and- polarized waves, the phase thickness of the film layer is
, | (12) |
the angle of refraction is determined by the refraction theorem. and are the refractive index and physical thickness of the -th layer film, respectively. is the admittance of the incident medium, which is considered a positive real number. Then there is
, | (13) |
and of
The transmission spectra of embedded material at different positions are studied first. In this part, the only variable is the position of the embedded material. Other parameters such as the number of DBR pairs, embedded material thickness and cavity layer thickness are fixed. The specific structure is (LH

Fig. 2 (a) The transmission spectra of embedded materials with the same thickness (50 nm) at different locations, (b) the variations of resonant modes with the same thickness (50 nm) at different embedded locations
图2 (a) 相同厚度 (50 nm) 的嵌埋材料在不同嵌埋位置的透射谱,(b) 相同厚度 (50 nm) 的嵌埋材料在不同嵌埋位置的共振模式的变化

Fig. 3 (a) The electric field distribution without embedding (λ = 3.6 μm), (b) the electric field distribution embedded in 4L (λ = 3.64 μm), (c) the electric field distribution embedded in 3L (λ = 3.65 μm), (d) the electric field distribution embedded in 3L (λ = 3.6 μm)
图3 (a) 没有嵌埋时电场分布 (λ = 3.6 μm),(b) 材料嵌埋在4L处的电场分布 (λ = 3.64 μm),(c) 材料嵌埋在3L处的电场分布 (λ = 3.65 μm),(d) 材料嵌埋在3L处的电场分布 (λ = 3.6 μm)
The number of DBR pairs is a key parameter for DBR performance, therefore we discuss the changes of the resonant mode under different DBR pairs as shown in

Fig. 4 Transmission spectra of the cavity under different numbers of DBRs with the cavity layer thickness 8L and the embedded material thickness 50 nm
图4 在腔长厚度为8L以及嵌埋材料厚度50 nm时不同DBR对数下腔的透射谱
The cavity layer thickness is important to the localization of the electric field and the resonant mode can be changed by tuning the cavity layer thickness. In this part, the influence of the cavity layer thickness on the resonant mode under two extreme positions is investigated. Other parameters such as the embedded material thickness and the number of DBR pairs are fixed. The specific structure is (LH

Fig. 5 (a) The cavity transmissivity with different cavity thicknesses, (b) the maximum peak shifts vary with different cavity thicknesses
图5 (a) 不同腔层厚度条件下腔的透过率,(b) 不同腔层厚度下的最大峰位移变化
The influence of the embedded material thickness is also important on the resonant mode. Therefore, it is studied theoretically since single or low layer material is very difficult to be controlled freely in experiments. The variable here is mainly the embedded material thickness under two extreme embedded positions. Other parameters such as the cavity layer thickness and the number of DBR pairs are fixed. The specific structure is (LH

Fig. 6 The MPS of the cavity mode varies with the thickness of the embedded material
图6 最大偏移量随嵌埋材料厚度的变化
In summary, the effects of the embedded materials on the cavity resonant mode in DBR cavities have been studied intensively. The embedded position, number of DBRs pairs, cavity layer thickness and embedded material thickness are four key parameters that affect the resonant mode selection and coupled electric field distribution. The cavity resonant mode changes periodically with the embedded positions with a change period of 2L. Therefore, there is an MPS with a period that decreases with the increase of the cavity layer thickness. It is also proportional to the embedded material thickness when it is thin. One can control the material embedded in different positions of the cavity or the cavity layer thickness to tune the resonant wavelength and achieve the strongest light-matter interaction. Although the number of DBR pairs does not affect the resonant mode, it still plays an important role in the localization of the light field. These results provide guidance for precise optical devices and analysis of experimental phenomena, which can be applied to different wavelength ranges of DBR cavity structures.
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