Optical coatings are enabling technology for modern optical systems, they are almost applied on every surface of the optical components. However, due to the uniform structure of the optical coatings, optical coatings have limited capability manipulating electromagnetic characteristics. Optical metasurfaces can locally manipulate optical field and enhance light–matter interactions, thus offering fascinating possibilities to control various properties of light, such as amplitude, phase, and polarization. While many new physical effects and applications were demonstrated based on metasurfaces, their practical application still faces challenges of low optical efficiency. Here, we propose the quasi-three-dimensional subwavelength structures, consisting of optical coatings and metasurfaces, to promote the efficiency of metasurfaces. We will present our recent advances in high-efficiency quasi-three-dimensional subwavelength structure devices. Our results pave the way to realizing optical meta-devices facing strict efficiency requirements in realistic applications.
Multilayer dielectric gratings (MDGs) have been widely used in spectral beam combining, chirped pulse amplification, and external cavity diode lasers systems. However, the one-dimensional MDGs suffer from the polarization-sensitive properties due to the modulation in only one direction. Two-dimensional (2D) MDGs have more flexible control ability to realize polarization-independent properties but are rarely reported. Here, we demonstrate the realization of polarizationindependent broadband MDGs via 2D grating array. The 2D Si:H MDG exhibits more than 97% non-polarized diffraction efficiency over 90 nm. Further, the influence of the refractive index on high-efficiency bandwidth of the MDGs was investigated via a comparative study using Si:H and TiO2, materials with a large difference in refractive indexes. The high-efficiency bandwidth of Si:H MDGs is near 1.4 times the breadth of TiO2 MDGs. Our results provide a new platform and deeper understanding to realize polarization-independent broadband MDGs.
Planarization of nodular defects was investigated in order to improve the laser-induced damage threshold (LIDT) of high-reflection coatings. Monodisperse SiO2 microspheres were first deposited on the substrate surface by a spin coating process. Using a dual ion beam sputtering system, these engineered seeds were used to create artificial nodules in 1064nm HfO2/SiO2 high-reflection coatings and Ta2O5/SiO2 high-reflection coatings. These SiO2 microspheres were then smoothed by a single thick SiO2 planarization layer, where the relationship between the thickness of the planarization layer and the size of the microspheres was investigated. When the planarization layer (etching layer) thickness is slightly larger than the diameter of the seeds, the seeds could be completely planarized to obtain smooth thin films. In addition, the LIDT of the high-reflection coatings with different coating materials and different planarization layer thicknesses were tested. The results showed that the nodular defects planarization could noticeably improve the damage resistance of high- reflection coatings. In addition, the surface roughness of Ta2O5/SiO2 high-reflection coatings was shown to decrease after the planarization, while the surface roughness of the 1064nm HfO2/SiO2 high-reflection coatings was shown to increase.
Efficiently controlling the propagation of light is a most important issue in the optics. The traditional optical elements manipulate light by phase accumulation through light propagation. Metasurfaces, serving as nanoscale phase shift, have attracted the interest because they possess the ability offering fascinating possibilities to realize unprecedented photonic phenomena while interact with light over the scale of the wavelength. Although kinds of novel functionalities are reported, monolayer metasurfaces and hybrid metasurfaces are limited by their less geometric structure parameters. Since the hybrid multilayer metasurfaces have more adjustable parameters, expanding the capacity to control the light beam, we use the hybrid multilayer metasurfaces to realize polarization-insensitive anomalous reflection and polarization-sensitive focusing. The gradient metasurface exhibits high efficiency anomalous reflection for both x-polarized and y-polarized light. The metalens has opposite focus for right-handed circularly polarized and left-handed circularly polarized incident light. The results may offer some help to find various potential applications in nanophotonics.
Optical metasurfaces, two-dimensional counterparts of the metamaterials, are comprised of arrays of subwavelength engineered inclusions that can locally modify optical field and light–matter interactions, thus offering fascinating possibilities to realize unprecedented photonic phenomena. Here, a micro-structured optical coating or a hybrid multilayer structure is proposed. It is easy to achieve phase control over 2π by changing the geometric parameter, which is used to design gradient metasurface exhibiting anomalous reflection for linearly polarized light (1μm). By arranging the arrays elaborately, the efficiency can reach as high as 93.6%. Besides, reflective metalens taking advantage of the Pancharatnam-Berry phase is demonstrated. The electric field distribution unambiguously indicates that the metalens can focus the plane wave with high efficiency (60.3%). Such structure may find various potential applications in nanophotonics because of their high freedom to design metasurfaces.
For the sol-gel method, it is still challenging to achieve excellent spectral performance when preparing antireflection (AR) coating by this way. The difficulty lies in controlling the film thickness accurately. To correct the thickness error of sol-gel coating, a hybrid approach that combined conventional sol-gel process with ion-beam etching technology was proposed in this work. The etching rate was carefully adjusted and calibrated to a relatively low value for removing the redundant material. Using atomic force microscope (AFM), it has been demonstrated that film surface morphology will not be changed in this process. After correcting the thickness error, an AR coating working at 1064 nm was prepared with transmittance higher than 99.5%.
It has been verified that a broadband high-reflection (HR) film could restrain electric-field intensity (EFI) enhancement effect in the nodular defects at normal incidence. However, it’s impossible to design an omnidirectional HR coatings to avoid the light penetration from all incident angles at oblique incidence. In this paper, the EFI enhancement is simulated by using a three-dimensional finite-difference time-domain (FDTD) code. Two types of polarizers that prevent light penetration at low and high incident angular range (IAR) are proposed to explore the influence of transmission band at different angles in the case of oblique incidence. The damage morphologies of nodules initiating from different diameter silica microspheres in polarizers reproduce the simulated EFI distributions very well. These results indicate that light penetration at high IAR rather than at low IAR contributes mostly to EFI enhancement. Then, the conclusion is proved further by the films with low and high IAR at normal incidence. Controlling the angle position of transmission band at small angle can reduce the EFI enhancement in the usual case and increase the laser-induced damage threshold (LIDT) of films.
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