近红外波段偏振无关型反射超透镜的设计 下载: 551次
0 Introduction
Metamaterials have many unusual phenomena, including negative refractive index, giant chirality, and indefinite permittivity[1-4], which made them widely investigated in recent years. Metasurfaces, the two dimensional versions of metamaterials, have been demonstrated to be a novel approach to control electromagnetic wavefronts[5-6]. Since Capasso et al. first investigated beam steering in 2011[7], many kinds of structures have been reported on metasurfaces to achieve the functions of focusing lens[8-10] and anomalous reflection and refraction[11]. Among many kinds of metasurfaces, metalens can achieve the function of focusing and anomalous reflection by configuring different materials and shapes of units. Lots of models like nanoslit[12], nanohole[13-14], graphene ribbons[15-19] are introduced in metalens design, where 2π phase shift resulted by the designed antennas is needed in controlling the wavefront. Although much structures have been proposed in past reports, the function of metalens have not been fully investigated.
In this paper, we proposed a new model based on Au and MgF2, which is independence of polarization[20-21]. x-polarized and y-polarized incidence are both have the same focusing effect. The proposed metalens can work well in the range of 700 - 850 nm for one-focus, and by configuring the resonance antenna, the proposed metalens can also achieves dual focus and three focus. When we use the incidence of 800 nm wavelength, the focal length of one focus, dual focus and three focus are 9.6, 6.6 and 4.7 μm, respectively. The focal length will decrease as the focus number increases. We believe that our findings are beneficial in designing new function controlling devices.
1 Structure design
图 1.
Fig. 1. Schematic of proposed metalens. (a) Fragment; (b) Cross section view ; (c) Top view of proposed metalens
Generally, the resonant antenna is a circle, it has the same effect on different polarized light. To explore this, we choose the working wavelength as 800 nm. It shows the phase shift and reflectance of the reflected light for x-polarization incidence in
图 2.
Fig. 2. Phase shift and reflectance of reflected wave of (a) x -polarization incidence and (b) y -polarization incidence
2 Results and discussion
To design metalens to focus incident light, the phase profile of the metalens should follow the expression[24]:
where x is the horizontal position from the center of metalens;
图 3.
Fig. 3. Proposed metalens working for one focus. (a) Relationship between the position x and radius of each unit for x -polarization; (b) Simulated Poynting vector distributions for metalens for x -polarization; (c) Intensities of the focusing spots along x direction for x -polarization; (d) Relationship between position x and the radius of each unit for y -polarization; (e) Simulated Poynting vector distributions for the metalens for y -polarization; (f) Intensities of the focusing spots along the x direction for y -polarization
According to the expression, we selected the radius of the resonant antenna from
Then we simulated the proposed metalens in other incident wavelengths for x-polarization, thats 650, 700, 750, 850 and 900 nm. Simulated results are shown in
图 4.
Fig. 4. Simulated Poynting vector distributions for the metalens with incidence wavelength of (a) 650 nm, (b) 700 nm, (c) 750 nm, (d) 900 nm, (e) 850 nm
图 5.
Fig. 5. Intensities of the focusing spots along the x direction with the incidence wavelength of (a) 650 nm, (b) 700 nm, (c) 750 nm, (d) 850 nm, (e) 900 nm
Further more, the proposed metalens can achieve dual focus by configuring the resonance antenna according to the expression:
where ɑ(x) is the nearest amplitude, d is the distance between the two focals and ɑ1=ɑ2=0.5. We designed two focus locate at x=−3, x=3 and the focal lengths are both 10 μm. The relationship between the position x and the radius of each unit is shown in
图 6.
Fig. 6. Proposed metalens working for daul focus. (a) Relationship between the position x and the radius of each unit; (b) Simulated Poynting vector distributions for the metalens; (c) Intensities of the focusing spots along the x direction
Then we designed three focus by the proposed metalens. According to the expression:
where the a(x) is the nearest amplitude, a1=a2=a3=1/3. The designed three focus locate at x=−4,x=0, x=4 and the focal lengths are all 10 μm. We selected the radius of the resonant antenna according to
图 7.
Fig. 7. Proposed metalens working for three focus. (a) Relationship between the position x and the radius of each unit; (b) Simulated Poynting vector distributions for the metalens; (c) Intensities of the focusing spots along the x direction
3 Conclusion
In summary, we proposed the metalens based on Au and MgF2, which is independence of polarization. We simuated by using FDTD method. According to the simulated results, the proposed metalens can work well in the range of 700 nm to 850 nm and work best in the range of 750 nm to 800 nm for one focus. By configuring the resonance antenna, the proposed metalens can also work well for multi-focus. When the incidence wavelength is chosen as 800 nm, the focal length of one focus, dual focus and three focus are 9.6 μm, 6.6 μm and 4.7 μm, respectively. In conclusion, we can control the focus of the proposed metalens according to our requirements. We believe that our findings are beneficial in designing new function controlling devices.
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张玉慧, 杨博伟, 李轶庭, 赵元埴, 付跃刚. 近红外波段偏振无关型反射超透镜的设计[J]. 红外与激光工程, 2020, 49(6): 20200048. Zhang Yuhui, Yang Bowei, Li Yiting, Zhao Yuanzhi, Fu Yuegang. Design of polarization-independent reflective metalens in near infrared waveband[J]. Infrared and Laser Engineering, 2020, 49(6): 20200048.