Research On Metalens Based On Controllable Materials

With the rapid development of science and technology and the constant pursuit for quality of life,optical imaging technology plays an important role in all walks of life.As the key component of optical imaging system,lens is an indispensable part of modern scientific and technological products such as telescope,microscope,digital camera and wavefront detector,etc.In order to achieve these sophisticated functions,multiple traditional lenses are often used in combination,which inevitably increases the volume and complexity of optical imaging system.Meantime,it also limits imaging resolution and application scopes of optical imaging system to a certain degree.All these shortcomings are not in line with the development trend of miniaturization and intelligence of modern optical systems.As a kind of artificial two-dimensional metamaterials,metasurfaces set off a wide range of research upsurge in recent years.Benefitting from the development of nano-fabrication technology,metasurfaces have shown great potential in many fields,such as beam control,metalenses,electromagnetic cloaking,holographic imaging,optical computing and so on.Among these applications,the studies of metalenses,which can solve the problems of large volume and difficult integration of traditional lenses,provide a good solution for the lightweight and miniaturization of optical systems.Moreover,the metalenses expand the application scopes of lenses further,making them play a huge role in the fields of microbial imaging,medical imaging,portable imaging and so on.At present,the researches of metalenses mainly focus on high efficiency imaging,diffraction limit imaging,large field of view imaging,wide spectrum imaging.At the same time,some researches have been carried out in three-dimensional imaging,vitual reality,augmented reality and other front applications.Compared with the conventional lenses,ordinary metalenses have a huge improvement in performance and own smaller size.Unfortunately,once they are fabricated,their functions are also fixed just as the conventional lenses,which results in the relatively rigid function of optical imaging systems.Obviously,the ordinary metalenses can not meet the intelligent concept of “one machine serves several purposes” for the modern optical systems.Therefore,more and more researchers focus their attentions on the dynamic tunable characteristics of optical elements.In this paper,the controllable materials?phase change materials?are adopted to realize the dynamic control of metalenses without breaking a series of excellent performance of metalenses.Through the artificial control of the controllable materials,the designed metalenses can be used to realize different optical functions,which may has some application potential in adaptive optics,integrated optics,three-dimensional imaging,optical coding,dynamic display,augmented reality and other fields.The main contents of this paper include:?1?The research background,significance of metalenses and several examples of active metalenses are introduced systematically.The commonly used electromagnetic field calculation methods and abrupt phase control methods are also listed.The adjustable principle and control methods are analyzed.?2?By combing the principle of geometric phase and phase change material Ge₂Sb₂Te₅?GST?,a tunable transmission metalens in communication range is designed.In the near-infrared range,the design method of using the phase change material as the phase control unit cell and state control unit cell simultaneously is adopted for the first time,which simplifies the design and fabrication complexity of metalens further.Compared with ordinary metalens,the designed tunable metalens not only has ultra-high resolution close to diffraction limit,but also owns different optical performances by reversibly adjusting the state of GST.At the designed wavelength of 1.55 ?m,the ratio of the focusing efficiency in amorphous state to the focusing efficiency in crystalline state is as high as 16 times,which can be regard as a switch.It is also worth mentioning that the metalens can be tuned in the whole range from 1.49 ?m to 1.55 ?m,which extends its application range.At the same time,the structure has great robustness making the devices more stable and practicable.?3?Four tunable duplex metalenses are designed.Firstly,two different unit cells are specially designed to modulate the different incident lights in the communication range.Each of them only responses to its corresponding wavelength and has little impact on each other,which is the basics of the tunable duplex metalenses.Then,according to the phase distribution of the metalenses,different unit cells are properly integrated and arranged in the same metalens by quadrant division method,which results that the metalens can simultaneously operate at different wavelengths?only the two wavelengths of 1.55 ?m and 1.31 ?m are presented in this paper,and the same design method can be used at other wavelengths as required?.Most importantly,by tuning the state of GST,the designed metalenses can achieve various optical functions under the single wavelength or multi wavelengths incident conditions.The designed metalenses may offer some new opportunities in the fields of radial shearing interference technique,space wavelength duplex,three-dimensional imaging,optical dynamic coding and so on.?4?An actively tunable metalens array for mid infrared range is designed for the first time.Compared with microlens array,the metalens array has smaller single sub-aperture size,which means higher cutting accuracy.At the same time,the resolution of single sub-metalens is also better than microlens,showing its great optical performance and important application value.The focusing efficiency of metalens array based on phase change materials GST stays above 50% in the whole range from 4.5 ?m to 5.2 ?m,and it can up to 80.2% at the designed wavelength of 4.6 ?m.Moreover,the metalens array can display different patterns on the focal plane via tuning the state of different sub apertures of metalens array,which is expected to be applied in the field of ultra-high speed and high precision dynamic display.In addition,due to the special design of the unit cell,it can control the incident light of different polarization states at different phase states.The metalens array can achieve the effect of imaging lens and transparent medium at the same time,and it can be applied to some frontier fields such as virtual reality and augmented reality.At last,the metalens array based on the phase change material VO₂ has the super efficiency adjustment ability in the transformation between the insulator state and the metal state.The adjustment range can reach 66 times,which enhances the switch effect further.