Recently, the research team of the Laboratory of Organic Nanophotonics at the Physics and Chemistry Institute of Chinese Academy of Sciences published a paper [Laser Photon. Rev. 10 (4), 665-672 (2016)] in the top optical journal "Laser & Photonics Review" ), Three-dimensional Luneburg lens at optical frequencies]. Zhao Yuanyuan, a Ph.D. candidate, is the co-author of the article's first author, Zheng Meiling Associate Researcher and Duan Xuanming Researcher. The paper pioneered the use of nanoscale 3D printing technology - super-diffractive multiphoton direct write processing polymer three-dimensional Luneburg lens device, which is only the size of the human hair diameter equivalent to 1/2, the first real three-dimensional Luneburg lens operating band from microwave to light band, making the study of three-dimensional Luneburg lens from the macro field of microwave to the optical field is a solid step forward. The research results will further promote the development of micro-optics and transform optics, and open a new application of nano-scale 3D printing technology in the field of micro-nano devices. The paper was chosen as the Front Cover Article of Laser & Photonics Review 2016 Volume 10, Number 4. The Laser & Photonics Review is a bi-monthly publication that aims to present the latest advances in theoretical and experimental research in the fields of laser physics and photonics. Each issue contains only 2-3 review papers, each edited by an editor. 2013 received a small amount of high-level original research articles published, the number of articles in 67, ranked SCI top area included in the field of photonics ranked second only to Nature Photonics industry-renowned magazine.
Micro-scale Optical Band Luneburg Lens Prepared by Femtosecond Laser Direct Writing and Characterization of Its Focusing Light Field
In recent years, the world of optics has attracted worldwide attention with a series of new achievements. One of them is the Gradient index optics that has been rapidly developed. The object of gradient index optical research is the optical phenomenon in non-uniform refractive index media. The optical phenomena that occur in inhomogeneous media are a kind of ubiquitous objective physical phenomena in nature. As early as the year 100 AD, people have observed miracles such as "mirage" and "desert god springs", all of which are miracles that refraction of the ground scenery due to local uneven changes of atmospheric refractive index. Through the observation and study of these natural phenomena, people gradually realized that the non-uniform refractive index of materials can lead to the optical properties that some homogeneous media do not have. In 1944, RKLuneburg proposed a spherical symmetry spherical lens model with graded refractive index distribution, n (r) = [2- (r / R) 2] 1/2, whose refractive index is calculated from the center Position 2 2 decreases radially to 1, parallel rays incident on the Luneburg lens focus to a point on the spherical surface without aberration, so Luneburg lenses achieve ideal aberration-free imaging or ideal focus. The traditional spherical lens due to the existence of aberrations, and can not achieve the ideal light focus.
Although the research results of Luneburg lens with graded index (GRIN) material both at home and abroad have been reported, there are still many problems to be solved. At present, Luneburg lens research and experimental realization based on graded-index optics of micro / nano structure has been mainly focused on two-dimensional or quasi-three-dimensional (column symmetry) structure, and its application potential is far from being developed. Due to the fact that the point source emits spherical waves, designing and producing a real three-dimensional Luneburg lens in the optical band and studying its ideal imaging capabilities is essential for truly ideal imaging and for truly exploiting the wide field of view of Luneburg lenses. However, the preparation techniques of GRIN optical Luneburg lens reported at present are mainly based on the processing technology of planar devices such as standard electron beam lithography and ion beam etching, which make it difficult to realize the real three-dimensional gradient refractive index device in the optical wavelength band.
Multi-photon laser direct writing processing technology is a low-cost, fast, high-precision 3D micro / nano structure preparation technology, which can break through the limitation of optical diffraction limit, the light reaction area is limited to the focal spot center of the smallest three-dimensional space ~ l3), to achieve any complex 3D micro-nano photonic structure processing. When the structure size of the micro-nano photonic structure is much smaller than the wavelength, that is, in the metamaterial area, the photonic structure can be regarded as an equivalent medium with a certain refractive index. When adjusting the duty cycle or period length at different locations in the micro / nano structure, complex GRIN media can be obtained. In 2010, Wegner's research group realized quasi-three-dimensional stealth carpet structure in the wavelength range of 1.5-2.6mm through laser direct writing on the structure of polymer. Based on the inspiration of this work, the research group of Physics and Chemistry made use of femtosecond laser to write, design and process Based on the three-dimensional Luneburg lens with gradient media metamaterials, COMSOL simulation results show that the operating band (> 6mm) is located in the mid-infrared band. On this basis, relevant experimental verification is carried out. The focusing properties of three-dimensional Luneburg lens under incident plane wave are characterized by near field optical microscope (SNOM) of Neaspec Company in Germany. The measured optical field intensity distribution shows a half-height and full-width FWHM) of 1/2 of the spot morphology validates the Luneburg lens's ideal 3D focus performance.
Based on multi-photon laser direct writing technology, the research team has made a series of research results in recent years, such as high resolution 3D hydrogel structure (J. Mater. Chem. B 2, 4318-4323, 2014; 3, 8486-8491 Phys. Lett. 108, 221104, 2016), a highly transparent ordered ordered metal grid electrode structure (Appl. Phys. Lett. And is invited to write a review article at Chem. Soc. Rev. (Chem. Soc. Rev. 44, 5031-5039, 2015). The relevant research work was supported by the Nano-Research Major Research Program (973) of the Ministry of Science and Technology, the Key Research Fund for National Natural Science Foundation of China, and the National Natural Science Foundation of China.